| 1. |
- Delczeg-Czirjak, Erna-Krisztina, 1978-
(författare)
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Energy relavant materials: Investigations based on first principles
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Energy production, storage and efficient usage are all crucial factors for environmentally sound and sustainable future technologies. One important question concerns the refrigeration industry, where the energy efficiency of the presently used technologies is at best 40% of the theoretical Carnot limit. Magnetic refrigerators offer a modern low-energy demand and environmentally friendly alternative. The diiron phosphide-based materials have been proposed to be amongst the most promising candidates for working body of magnetic refrigerators. Hydrogen is one of the most promising sources of renewable energy. Considerable international research focuses on finding good solid state materials for hydrogen storage. On the other hand, hydrogen gas is obtained from hydrogen containing chemical compounds, which after breaking the chemical bonds usually yield to a mixture of different gases. Palladiumsilver alloys are frequently used for hydrogen separation membranes for producing purified hydrogen gas. All these applications need a fundamental understanding of the structural, magnetic, chemical and thermophysical properties of the involved solid state materials. In this thesis ab initio electronic structure methods are used to study the magnetic and crystallographic properties of Fe2P based magneto-caloric compounds and the thermophysical properties of Pd-Ag binary alloys. The nature of magnetism and the strong sensitivity of the Curie temperature of the Fe2P1−xTx (T = boron, silicon and arsenic) are investigated. Using first principles theory, the change in the average magnetic exchange interactions upon doping is decomposed into chemical and structural contributions, the latter including the c/a and vol-ume effects. It is demonstrated that for the investigated alloys the structural effect can´be ascribed mainly to the c/a ratio that strengthens the magnetic exchange interactions between the two Fe sublattices. On the other hand, it is shown that the two types of Fe atoms have a very complicated co-dependency, which manifests in a metamagnetic behavior of the FeI sublattice. This behavior is strongly influenced by doping the P sites. Lattice stability of pure Fe2P and the effect of Si doping on the phase stability are pre-sented. In contrast to the observation, for the ferromagnetic state the hexagonal structure (hex, space group P62m) has no the lowest energy. For the paramagnetic state, the hex structure is shown to be the stable phase and the computed total energy versuscomposition indicates a hex to bco (body centered orthorhombic, space group Imm2)crystallographic phase transition with increasing Si content. The mechanisms responsi-ble for the structural phase transition are discussed in details. The magnetic properties of Fe2P can be subtly tailored by Mn doping. It was shown experimentally that Mn atoms preferentially occupy one of the two different Fe sites of Fe2P. Theoretical results for the Mn site occupancy in MnFeP1−xSix are presented. The single crystal elastic constants, the polycrystalline elastic moduli and the Debye temperature of disordered Pd1−xAgx binary alloys are calculated for the whole range of concentration, 0 ≤ x ≤ 1. It is shown that the variation of the elastic parameters of Pd-Ag alloys with chemical composition strongly deviates from the simple linear trend. The complex electronic origin of these anomalies is demonstrated. The effect of long range order on the single crystal elastic constants of Pd0.5Ag0.5 alloy is also investigated. Within this thesis most of the calculations were performed using the Exact Muffin-Tin Orbitals method. The chemical and magnetic disorder are treated via the Coherent Potential Approximation. The paramagnetic phase is modeled by the Disordered Local Magnetic Moments approach.
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| 2. |
- Delczeg-Czirjak, Erna-Krisztina, 1978-
(författare)
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Energy Relevant Materials: Investigations Based on First Principles
- 2010
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Energy production, storage and efficient usage are all crucial factors for environmentally sound and sustainable future technologies. One important question concerns the refrigeration industry, where the energy efficiency of the presently used technologies is at best 40% of the theoretical Carnot limit. Magnetic refrigerators offer a modern low-energy demand and environmentally friendly alternative. Iron phosphide based materials have been proposed to be amongst the most promising candidates for working body of magnetic refrigerators. Hydrogen is one of the central elements on the most promising sources of renewable energy. Considerable international research focuses on finding good solid state materials for hydrogen storage. On the other hand, hydrogen gas is obtained from hydrogen containing chemical compounds, which after breaking the chemical bounds usually yield to a mixture of different gases. Palladium-silver alloys are frequently used for hydrogen separation membranes for producing purified hydrogen gas. All these applications need a fundamental understanding of the structural, magnetic, chemical and thermophysical properties of the involved solid state materials. In the present thesis ab initio electronic structure methods are used to study the crystallographic and magnetic properties of Fe2P based magneto-caloric compounds and the thermophysical properties of Pd-Ag binary alloys. Lattice stability of pure Fe2P and the effect of Si doping on the phase stability are presented. In contrast to the observation, for the ferromagnetic state the body centered orthorhombic structure (bco, space group Imm2) is predicted to have lower energy than the hexagonal structure (hex, space group P62m). The zero-point spin fluctuation energy difference is found to be large enough to stabilize the hex phase. For the paramagnetic state, the hex structure is shown to be the stable phase and the computed total energy versus composition indicates a hex to bco crystallographic phase transition with increasing Si content. The magneto-structural effects and the mechanisms responsible for the structural phase transition are discussed in details. The magnetic properties of Fe2P can be subtly tailored by Mn doping. It has been shown experimentally that Mn atoms preferentially occupy one of the two different Fe sites of Fe2P. Theoretical results for the Mn site occupancy in MnFeP1-xSix are presented. The single crystal and polycrystalline elastic constants and the Debye temperature of Pd1-xAgx binary alloys are calculated for the whole range of concentration, 0≤x≤1. It is shown that the variation of the elastic parameters of Pd-Ag alloys with chemical composition strongly deviates from the simple expected trend. The complex electronic origin of these anomalies is demonstrated. Within the present thesis, all relaxed crystal structures are obtained using the Projector AugmentedWave full-potential method. The chemical and magnetic disorder is treated using the Exact Muffin-Tin Orbitals method in combination with the Coherent Potential Approximation. The paramagnetic phase is modeled by the Disordered Local Magnetic Moments approach.
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| 3. |
- Dong, Zhihua
(författare)
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Temperature dependent mechanical properties of as-cast steels : Experimental and theoretical studies
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The temperature-dependent mechanical properties of steels are important to avoid processing defects, to understand and to improve the high-temperature performance. At the same time, having access to thermal properties gives us opportunity to assess the first-principles theoretical predictions at elevated temperatures. These properties are directly bound up with the performance of individual phase and also the evolving microstructure states at different thermalmechanical processes. In the present thesis, the temperature-dependent mechanical properties of continuously cast steels and iron are investigated using experimental and theoretical methods. Experimental studies are performed centering on the influence of thermal cycles occurring in secondary cooling.The temperature reversion in secondary cooling makes the hot ductility trough occurring at higher temperatures with greater depth. Increasing the reversion rate, the low temperature end of the ductility trough slightly extends to lower temperatures. As indicated by microstructure examinations, the intergranular fracture contributed from the thin film-like ferrite and (Fe,Mn)S particles slightly changes with the varying thermal cycles; however, the widmanstatten ferrite observed in the temperature reversion process seriously deteriorates the ductility. Due to the temperature reversion process, the peak stress slightly declines and the peak of strain to peak stress moves to higher temperatures. On the other hand, the sequential formations of ferrite and pearlite in the austenite transformation are indicated by two distinct peaks on the thermal expansion coefficient. By applying the developed concise model, the volume fractions of ferrite, pearlite, and austenite are quantitatively monitored in the phase transformation. Either increasing the cooling rate or the content of austenite stabilizing atoms Ni and Cu, the austenite transformation occurs at relatively low temperatures and indicates a greater phase transformation rate for both ferrite and pearlite. In addition, the final fraction of ferrite/pearlite increases/decreases with increasing the cooling rate, increasing the alloying atoms like Ni, Cr and Cu or lowering the carbon content.The temperature dependence of the polycrystalline Young’s modulus and the tetragonal shear modulus c0 of iron is predicted using ab initio calculations within the exact muffin-tin orbitals formalism. The dependence exhibits a good consistency with that of the peak stress observed in the experiments for the commercial steel. Despite the significant effects of magnetic sate and crystal structure on the elastic property of iron, the magneto-volume coupling primarily determines the temperature dependence for the single phase. In contrast, the dominant role of the volume expansion is observed for both the paramagnetic (PM) face centered cubic (fcc) and body centered cubic (bcc) Fe, although they show different magneto-elastic behaviors. Based on the theoretically predicted thermal expansion for PM bcc Fe, both the lattice vibrations and the magnetic evolution contribute to the thermal expansion, and the former is dominant.
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| 4. |
- Li, Wei
(författare)
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First-principles description of planar faults in metals and alloys
- 2014
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Phase interface and stacking fault are two common planar defects in metallic materials. In the present thesis, the interfacial energy and the generalized stacking fault energy of random alloys are investigated using density functional theory formulated within the exact muffin-tin orbitals (EMTO) method in combination with the coherent-potential approximation (CPA).The interfacial energy is one of the key physical parameters controlling the formation of the Cr-richα’ phases during the phase decomposition in Fe-Cr ferrite stainless steels. This decomposition is believed to cause the so-called“475°C embrittlement”. Aluminum addition to ferritic stainless steels was found to effectively suppress the deleterious 475oC embrittlement. The effect of Al on the interfacial energy and the formation energy of Fe-Cr solid solutions are studied in this thesis. The interface between the decomposed Fe-rich α and Cr-rich α phases carries a positive excess energy, which represents a barrier for the process of phase separation. Our results show that for the α-Fe70Cr20Al10/α0-Fe100−x−yCryAlx(0≤x≤10, 55≤y≤80) interface, the Al content(x) barely changes the interfacial energy. However, when Al is partitioned only in the alpha phase, i.e. for the α-Fe100−x−yCryAlx/α0-Fe10Cr90(0≤x≤10,0≤y≤25) interface, the interfacial energy increases with Al concentration due to the variation of the formation energies of the Fe-Cr alloys upon Al alloying. The intrinsic energy barriers (IEBs) of the γ surface (also called generalized stacking fault energy, GSFE) provide fundamental physics for understanding the plastic deformation mechanisms in face-centred cubic metals and alloys. In this thesis, the GSFEs of the disordered Cu-X (X=Al, Zn, Ga, Ni) and Pd-X (X=Ag,Au) alloys are calculated. Studying the effect of segregation of the solutes to the stacking fault planes shows that only the local chemical composition affects the GSFEs. Based on the calculated GSFEs values, the previously revealed “universal scaling law” between these IEBs is demonstrated to be well obeyed in random solid solutions. This greatly simplifies the calculations of the twinning parameters or the critical twinning stress. Adopting two twinnability measure parameters derived from the IEBs, we find that in binary Cu alloys, Al, Zn and Ga increase the twinnability, while Ni decreases it. Aluminum and gallium yield similar effects on the twinnability. Our theoretical predictions are in line with the available experimental data. These achievements open new possibilities in understanding and describing the plasticity of complex alloys.
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| 5. |
- Lorand, Delczeg, 1980-
(författare)
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Density functional study of mono-vancacies in metals and austenitic steel alloys
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Trough the following pages a comprehensive study of open structures will be shown, including mono-vacancy calculations and open surfaces. These are electronic structure calculations using density functional theory within the exact muffin tin method.First I investigate the accuracy of five common density functional approximations for the theoretical description of the formation energy of mono-vacancies in three close packed metals. Besides the local density approximation (LDA), I consider two generalized gradient approximation developed by Perdew and co-workers (PBE and PBEsol) and two gradient-level functionals obtained within the subsystem functional approach (AM05 and LAG). As test cases, I select aluminium, nickel and copper, all of them adopting the face centered cubic crystallographic structure.This investigation is followed by a performance comparison of the three common gradient level exchange-correlation functionals for metallic bulk, surface and vacancy systems. I find that approximations which by construction give similar results for the jellium surface, show large deviations for realistic systems. The particular charge density and density gradient dependence of the exchange-correlation energy densities is shown to be the reason behind the obtained differences. Our findings confirm that both the global (total energy) and the local (energy density) behavior of the exchange-correlation functional should be monitored for a consistent functional design.I also calculate the vacancy formation energies of paramagnetic face centered cubic (fcc) Fe-Cr-Ni alloys as a function of chemical composition. These alloys are well known model systems for low carbon austenitic stainless steels. The theoretical predictions obtained for homogeneous chemistry and relaxed nearest neighbor lattice sites are in line with the experimental observations. In particular, Ni is found to decrease and Cr increase the vacancy formation energy of the ternary system. The results are interpreted in terms of effective chemical potentials. The impact of vacancy on the local magnetic properties of austenitic steel alloys is also investigated.I made a performance comparison of local density and generalized gradient level approach on substitutional defects in five light actinides. This is a complex test for high density calculations to check the weaknesses of the local density approximation against gradient level ones. I believe the existing other gradient level approaches fit our error bar in the obtained data and shows similar trends against the very limited number of experimental data. Based on our ab initio results, I predict that vacancies are more easily formed (more stable) in the fcc(bcc) lattice for U, Np and Pu and in the bcc(fcc) lattice for Th and Pa.
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| 6. |
- Lukinov, Tymofiy, 1982-
(författare)
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Atomistic modeling of materials under extreme pressure
- 2014
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is dealing with simulation of polycrystalline materials underthe conditions of anisotropic pressure and temperature. Work has been carriedout in three steps:ˆ Research of the inuence of point defects and grain boundaries on theprocess of melting. The inuence of defects' concentration, grain sizeand lattice direction mismatch on the superheating temperature wasstudied.ˆ Investigation of the boundaries of application of the metadynamics methodto the simple atomic model with Buckingham interaction. The solidsolidphase transition, where one of the phases has temperature inducedstability was conrmed. We found the optimal size of simulation boxto study the solid-solid phase transitions using the metadynamics technique.ˆ A model of polycrystalline materials based on macroscopic approximationwas formulated. This model was applied to the model of the polycrystallinematerial using cellular automata. Using this approximationthe eect of anisotropic stress caused by anisotropic heating was studied.
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| 7. |
- Nordström, Joakim, 1971-, et al.
(författare)
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Temperature study of deformation twinning behaviour in Nickel-base Superalloy 625
- 2024
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Ingår i: Materials Science & Engineering. - 0921-5093 .- 1873-4936.
-
Tidskriftsartikel (refereegranskat)abstract
- Deformation behaviour in the Nickel-base superalloy 625 has been studied by tensile testing at four temperatures: 295, 223, 173 and 77 K. The microstructure has been investigated using TEM, FIB-SEM, EBSD and ECCI techniques. Deformation in the alloy turns out to be a competitive course of events between at least two deformation mechanisms, namely dislocation slip and deformation twinning. Slip is the predominant deformation mechanism at higher temperatures. While at 77 K, deformation induced twinning gives an extra degree of freedom as one of the main deformation mechanisms, i.e., the material shows a twin induced plasticity, TWIP, behaviour. Ab initio calculations indicate that the influence of cryogenic/sub-zero temperatures on the stacking fault energy of this alloy can be limited and that the formation of deformation twins cannot be determined solely by the stacking fault energy. The results implies that it is the critical strain and strain hardening rate that influences the deformation twinning onset and twinning rate.
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| 8. |
- Nordström, Joakim, 1971-, et al.
(författare)
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Temperature study of deformation twinning behaviour in nickel-base Superalloy 625
- 2024
-
Ingår i: Materials Science & Engineering. - : Elsevier BV. - 0921-5093 .- 1873-4936. ; 907
-
Tidskriftsartikel (refereegranskat)abstract
- Deformation behaviour in the Nickel-base superalloy 625 has been studied by tensile testing at four temperatures: 295, 223, 173 and 77 K. The microstructure has been investigated using TEM, FIB-SEM, EBSD and ECCI techniques. Deformation in the alloy turns out to be a competitive course of events between at least two deformation mechanisms, namely dislocation slip and deformation twinning. Slip is the predominant deformation mechanism at higher temperatures. While at 77 K, deformation induced twinning gives an extra degree of freedom as one of the main deformation mechanisms, i.e., the material shows a twin induced plasticity, TWIP, behaviour. Ab initio calculations indicate that the influence of cryogenic/sub-zero temperatures on the stacking fault energy of this alloy can be limited and therefore the formation of deformation twins cannot be determined solely by the stacking fault energy. The results implies that critical strain and strain hardening rate influences the deformation twinning onset and twinning rate.
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| 9. |
- Sun, Xun, 1992-
(författare)
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Ab initio Investigation of Al-doped CrMnFeCoNi High-Entropy Alloys
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High-entropy alloys (HEAs) represent a special group of solid solutions containing five or more principal elements. The new design strategy has attracted extensive attention from the materials science community. The design and development of HEAs with desired properties have become an important subject in materials science and technology. For understanding the basic properties of HEAs, here we investigate the magnetic properties, Curie temperatures, electronic structures, phase stabilities, and elastic properties of paramagnetic (PM) body-centered cubic (bcc) and face-centered cubic (fcc) AlxCrMnFeCoNi (0 ≤ x ≤ 5, in molar fraction) HEAs using the first-principles exact muffin-tin orbitals (EMTO) method in combination with the coherent potential approximation (CPA) for dealing with the chemical and magnetic disorder.Whenever possible, we compare the theoretical predictions to the available experimental data in order to verify our methodology. In addition, we make use of the previous theoretical investigations carried out on AlxCrFeCoNi HEAs to reveal and understand the role of Mn in the present HEAs. The theoretical lattice constants are found to increase with increasing x, which is in good agreement with the available experimental data. The magnetic transition temperature for the bcc structure strongly decreases with x, whereas that for the fcc structure shows a weak composition dependence. Within their own stability fields, both structures are predicted to be PM at ambient conditions. Upon Al addition, the crystal structure changes from fcc to bcc with a broad two-phase field region, in line with the observations. Bain path calculations suggest that within the duplex region both phases are dynamically stable.Comparison with available experimental data demonstrates that the employed approach describes accurately the elastic moduli of the present HEAs. The elastic parameters exhibit complex composition dependences, although the predicted lattice constants increase monotonously with Al addition. The elastic anisotropy is unusually high for both phases. The brittle/ductile transitions formulated in terms of Cauchy pressure and Pugh ratio become consistent only when the strong elastic anisotropy is accounted for. The negative Cauchy pressure of CrMnFeCoNi is found to be due to the relatively low bulk modulus and C12 elastic constant, which in turn are consistent with the relatively low cohesive energy. Our findings in combination with the experimental data suggest anomalous metallic character for the present HEAs system.The work and results presented in this thesis give a good background to go further and study the plasticity of AlxCrMnFeCoNi type of HEAs as a function of chemistry and temperature. This is a very challenging task and only a very careful pre-study concerning the phase stability, magnetism and elasticity can provide enough information to turn my plan regarding ab initio description of the thermo-plastic deformation mechanisms in AlxCrMnFeCoNi HEAs into a successful research.
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| 10. |
- Sun, Xun, Bachelor of Engineering, 1992-
(författare)
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Ab initio Investigation of Face-centered cubic High-Entropy Alloys
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High-entropy alloys (HEAs) represent a special group of solid solutions containing five or more principal elements. The new design strategy has attracted extensive attention from the materials science community. The design and development of HEAs with desired properties have become an important subject in materials science and technology. Herein this case, I investigate the basic properties of paramagnetic (PM) HEAs, including the magnetic properties, Curie temperatures, electronic structures, phase stabilities, and elastic properties using the first-principles exact muffin-tin orbitals (EMTO) method in combination with the coherent potential approximation (CPA) for dealing with the chemical and magnetic disorder. To understand and model the mechanical properties of face-centered cubic (fcc) HEAs, I also study the generalized stacking fault energy (GSFE), negative stacking fault energy (SFE) and twinning mechanism of various HEAs. Thesis focuses mainly on AlxCrMnFeCoNi (0 ≤ x ≤ 5, in molar fraction) and related HEAs.Whenever possible, I compare the theoretical predictions to the available experimental data in order to verify the employed ab initio methodology. I make use of the previous theoretical investigations carried out on AlxCrFeCoNi HEAs to reveal and understand the role of Mn in AlxCrMnFeCoNi HEAs. The theoretical lattice constants are found to increase with increasing x, which is in good agreement with the available experimental data. The magnetic transition temperature for the body-centered cubic (bcc) structure strongly decreases with x, whereas that for the fcc structure shows a weak composition dependence. Within their own stability fields, both structures are predicted to be PM at ambient conditions. Upon Al addition, the crystal structure changes from fcc to bcc with a broad two-phase field region, in line with the observations. Bain path calculations suggest that within the duplex region both phases are dynamically stable.Comparison with available experimental data demonstrates that the employed approach describes accurately the elastic moduli of the present HEAs. The elastic parameters exhibit complex composition dependences and the elastic anisotropy is unusually high for both cubic phases. The brittle/ductile transitions formulated in terms of Cauchy pressure and Pugh ratio become consistent only when the strong elastic anisotropy is accounted for. The negative Cauchy pressure of CrMnFeCoNi is found to be due to the relatively low bulk modulus and C12 elastic constant, which in turn are consistent with the relatively low cohesive energy. Our findings in combination with the experimental data suggest anomalous metallic character for the present HEAs system.The negative SFE of fcc medium-entropy alloys (MEAs) and HEAs originate from the metastable character of the fcc phase. I argue that the common models underlying the experimental measurements of SFE fail in metastable alloys. Considering various metals including concentrated solid solutions, I demonstrate that in contrast to the experimentally measured SFEs, the SFEs obtained by DFT calculations correlate well with the primary deformation mechanisms observed experimentally in these alloys. In the case of negative SFE (or in metastable fcc alloys), the transformation-mediated twinning (TMT) is the predominant mechanism instead of the layer-by-layer twinning mechanism. It provides a continuous avenue for strain accommodation and strain hardening, realizing the joint transformation-induced plasticity and twinning-induced plasticity in the same system, and thus enabling the simultaneous improvement of strength and ductility. For the fcc CrMnFeCoNi HEA, upon Al addition or temperature increase, the intrinsic and extrinsic stacking fault energies increase, whereas the hexagonal close packed (hcp)/fcc interfacial energy stays almost constant.The work and results presented in this thesis give a good background to go further and study the plasticity of fcc HEAs as a function of chemistry and temperature. This is a very challenging task and only a very careful pre-study concerning the phase stability, magnetism, elasticity and GSFE can provide enough information to turn my plan regarding ab initio description of the thermo-plastic deformation mechanisms in fcc HEAs into a successful research. The novel TMT mechanism disclosed for the first time by myself and my colleagues advances our knowledge in plasticity and paves the road to design novel alloys with outstanding mechanical properties using quantum metallurgy.
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| 11. |
- Tian, Fuyang, 1980-
(författare)
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Ab initio atomistic simulation of metals and multicomponent alloys
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Ab initio theory provides a powerful tool to understand and predict the behavior of materials. This thesis contains both of these aspects. First we use ab initio alloy theory to investigate a new kind of complex alloy (high-entropy alloy). Second we introduce a novel potential (interlayer potential), which can be extracted from ab inito total energy calculations using the Chen-Möbius inversion method.High-entropy alloys (HEAs) are composed of four or more metallic elements with nearly equimolar composition. In spite of the large number of components, most of the HEAs have a simple solid-solution phase rather than forming complex intermetallic structures. Extensive experiments have reported the unique microstructures and special properties of HEAs. Single-phase HEAs may be divided into three types, i.e. the3d-HEAs adopting the face centered cubic (fcc) phase, the refractory-HEAs with a body centered cubic (bcc) phase, and the HEAs with the duplex fcc-bcc structure. We employ the exact muffin-tin orbitals (EMTO) method in combination with the coherent potential approximation (CPA) to investigate the electronic structure, the equilibrium volume and the elastic properties of these three-type HEAs.First we compare the CPA with the super cell technique (SC) to assess the performance of the EMTO-CPA method. As typical fcc 3d-HEAs, we consider the CuNiCoFeCrTix systems in the paramagnetic state. Starting from the calculated electronic structure, we give an explanation for the observed magnetic states. Furthermore, we provide a theoretical prediction for the elastic parameters and polycrystalline elastic moduli for CuNiCoFeCrTix (x= 0.0−0.5, 1.0) and NiCoTeCrTi. A detailed comparison between the theoretical results and the available experimental data demonstrates that ab initio theory can properly describe the fundamental properties of this important class of engineering alloys.Refractory-HEAs are composed of Ti, Zr, Hf, V, Nb, Ta, Mo, and W. These HEAs have a simple bcc structure. Taking the TiZrNbMoVx and TiZrVNb HEAs as examples, we provide a detailed investigation of the effect of alloying elements on the elastic parameters and the elastic isotropy. Our results indicate that vanadium enhances the anisotropy and ductility of TiZrNbMoVx. As an application of the present theoretical database, we verify the often quoted correlation between the valence charge concentration (VEC) and the micro-mechanical properties in the case of multi-component alloys. Furthermore, we predict that the present HEAs become elastically isotropic for VEC ≃ 4.72.With increase of the aluminum content, phase transformations (fcc→(fcc+bcc)→bcc) occur in NiCoFeCrAlx HEAs. Our ab initio results predict that at room temperature the paramagnetic NiCoFeCrAlx HEAs adopt the fcc structure for x ≤ 0.60 and the bcc structure for x ≥ 1.23, with an fcc-bcc duplex region in between the two pure phases. The calculated single- and polycrystal elastic parameters exhibit strong composition and crystal structure dependence. Based on the present theoretical findings, it is concluded that alloys around the equimolar NiCoFeCrAl composition have superior mechanical performance as compared to the single-phase regions.Many modern materials and material systems are layered. The properties related to layers are connected to interactions between atomic layers. We introduce the interlayer potential (ILP), a novel model potential which fully describes the interaction between layers. The ILPs are different from the usual interatomic potentials which present inter- action between atoms. We use the Chen-Möbius inversion method to extract the ILPs from ab initio total energy calculations. The so obtained ILPs can be employed to investigate several physical parameters connected with the particular set of atomic layers, e.g. surface energy, stacking fault energy, elastic parameters, etc.As an application, we adopt the supercell method and the axial interaction model in connection with the ILPs to calculate the stacking fault energy along the fcc ⟨111⟩ direction, including the intrinsic stacking fault energy, extrinsic stacking fault energy and twin stacking fault energy as well as the interactions between the intrinsic stacking faults. We find that the data derived from ILPs are consistent with those obtained in direct ab initio calculations. Along the fcc ⟨111⟩ direction, we study the surface energy and surface relaxation using the ILPs. The phonon dispersions are also described.Our conclusions are as followsthe EMTO-CPAab initioalloy theory can be used to understand and predict the fundamental properties of multicomponent alloys.the interlayer potentials based on the Chen-Möbius inversion method may provide a new way to investigate the properties related to layers in layered materials,the EMTO-CPA alloy theory combined with the Chen-Möbius inversion method offers a powerful technique to study the properties of complex alloys.
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| 12. |
- Wang, Guisheng, 1985-
(författare)
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Ab initio prediction of the mechanical properties of alloys
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- At the time of the 50th anniversary of the Kohn-Sham method, ab initio calculations based on density functional theory have formed an accurate, efficient, and reliable method to work on the properties of engineering materials. In this thesis, we use the exact muffin-tin orbitals method combined with the coherent-potential approximation to study the mechanical properties of high-technology materials. The thesis includes two parts: a study of long-range chemical order effects and a study of alloying effects on the mechanical properties of alloys. In the first part, we concentrate on the impact of chemical ordering on the mechanical properties. The long range order effect on the elastic constants behaves in a very different way for non-magnetic materials and ferromagnetic materials. For a non-magnetic Cu3Au, the long-range order effect on the elastic constants is very small. The Debye temperature does not show a strong chemical order dependence either. For a ferromagnetic material, on the other hand, the long-range chemical order produces considerable influence on C' in the ferromagnetic state, but negligible effect on C' in the paramagnetic state. The lattice parameter, bulk modulus $B$, and shear elastic constant C44 change slightly with the degree of long-rang order for both magnetic states. The Young's modulus E and the shear modulus G increase significantly with the degree of order in the ferromagnetic state, but the effect becomes weak as the system approaches the random regime.In the second part, the alloying effect of Mn/Ni on the lattice parameter, elastic constants, surface energy, and unstable stacking fault energy of bcc Fe is examined. The calculated results show that the lattice parameter of ferrite Fe is slightly altered upon Ni/Mn alloying the trend of which can be explained by the magnetism-induced pressure. Nickel addition decreases C' but has a negligible effect on C44, whereas manganese addition increases C44 and has a weak influence on C'. In both systems, the bulk modulus B shows a smooth second order variation. On the other hand, the surface energy and the unstable stacking fault (USF) energy decrease by adding Mn or Ni to Fe. For both planar fault energies, Ni shows a stronger effect than Mn. Segregation seems to have a minor effect on the surface and USF energies for dilute Fe-Ni and Fe-Mn alloys. The ductility can be estimated using available physical parameters via traditional phenomenological criteria like the Pugh ratio B/G, the Poisson ratio ν, the Cauchy pressure C12-C44, and the Rice ratio γs/γu .According to dislocation theory, a dislocation can not cross a grain boundary. Therefore, the study of dislocations is assumed to be limited to single-crystals. Several theoretical studies indicate that the cleavage plane is {001} in bcc crystals. Based on these information, we suggest that the resolved single-crystal tensile strength E[001] and the resolved single crystal shear strength G{110}<111> should be used to describe brittle cleavage and dislocation movement rather than polycrystalline parameters such as B and G. We demonstrate that all shear moduli G{lmn}<111> associated with the <111> Burgers vector take the same value 3C44C'/(C'+2C44), which could in fact explain the observed multiple slip in bcc systems. Due to the discrepancy between the resolved single-crystal elastic constants and the averaged polycrystalline elastic constants, the Pugh ratio B/G and the traditional criteria based on polycrystalline elastic constants lead to large differences for magnetic systems. Finally, we propose a new measure of the ductile-bittle behavior based on the ratio σclevage/Gresolved which gives the right experimental trend for Fe-Mn and Fe-Ni system.
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| 13. |
- Xiaoqing, Li, 1984-
(författare)
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Mechanical Properties of Transition Metal Alloys from First-PrinciplesTheory
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aim of the thesis is to investigate the alloying and temperature effects on the mechanical properties of body-centered cubic (bcc) random alloys. We employ the all-electron exact muffin-tin orbitals method in combination with the coherent-potential approximation. The second-order elastic constants reflect the mechanical properties of materials in the small deformation region, where the stress-strain relations are linear. Beyond the small elastic region, the mechanical properties of defect-free solids are described by the so called ideal strength. These two sets of physical quantities are the major topic of my investigations.In part one (papers I and II), the elastic constants and the ideal tensile strengths (ITS) are investigated as a function of Cr and Ti for the bccV-based random solid solution. We find that alloys along the equi-composition region exhibit the largest shear modulus and Young’s modulus, which is a resultof the opposite alloying effects obtained for the two cubic shear elastic constants C′ and C44. The classical Labusch-Nabarro solid-solution hardening (SSH) model extended to ternary alloys predicts a larger hardening effect in V-Ti than in V-Cr alloy. By considering a phenomenological expression for the ductile-brittle transition temperature (DBTT) in terms of Peierls stress and SSH, we show that the present theoretical results can account for the observed variations of DBTT with composition. Under uniaxial [001] tensile loading, the ITS of V is 12.4 GPa and the lattice fails by shear. Assuming isotropic Poisson contraction, the ITSs are 36.4 and 52.0 GPa for V in the [111] and [110] directions, respectively. For the V-based alloys, Cr increases and Ti decreases the ITS in all principal directions. Adding the same concentration of Cr and Ti to V leads to ternary alloys with similar ITS values as that of pure V. We show that the ITS correlates with the fcc-bcc structural energy difference and explain the alloying effects on the ITS based on electronic band structure theory.In part two (paper III), the alloying effect on the ITS of four bcc refractory HEAs based on Zr, V, Ti, Nb, and Hf is studied. Starting from ZrNbHf, we find that the ITS decreases with equimolar Ti addition. On the other hand, if both Ti and V are added to ZrNbHf, the ITS is enhanced by about 42%. An even more captivating effect is the ITS increase by about 170%, if Ti and V are substituted for Hf. We explain the alloying effect on the ITS based on the d-band filling. We explore an intrinsic brittle-to-ductile transition, which arises due to an alloying-driven change of the failure mode under uniaxial tension. Our results indicate that intrinsically ductile HEAs with high intrinsic strength can be achieved by controlling the proportion of group four elements to group five elements.In part three (papers IV and V), the ITS of bcc ferromagnetic Fe-based random alloys is calculated as a function of compositions. The ITS of Fe is calculated to be 12.6 GPa under [001] direction tension, which is in good agreement with the available theoretical data. For the Fe-based alloys, we predict that V, Cr, and Co increase the ITS, while Al and Ni decrease it. Manganese yields a weak non-monotonic alloying behavior. We show that the previously established ideal tensile strengths model based on structural energy differences for the nonmagnetic V-based alloys is of limited use in the case of Fe-bases alloys, which is attributed to the effect of magnetism. We find that upon tension all investigated solutes strongly alter the magnetic response of the Fe host from the unsaturated towards a stronger ferromagnetic behavior.In part four (paper VI), the temperature effect on the ITS of bcc Fe and Fe0.9Co0.1alloy is studied. We find that the ITS of Fe is only slightly temperature dependent below∼500K but exhibits large thermal gradients at higher temperatures. Thermal expansion and electronic excitations have an overall moderate effect, but magnetic disorder reduces the ITS with a pronounced 90% loss in strength in the temperature interval∼500 - 920K. Such a dramatic temperature effect far below the magnetic transition temperature has not been observed for other micro-mechanical properties of Fe. We demonstrate that the strongly reduced Curie temperature of the distorted Fe lattices compared to that of bcc Fe is primarily responsible for the onset of the drop of the intrinsic strength. Alloying additions, which have the capability to partially restore the magnetic order in the strained Fe lattice, push the critical temperature for the strength-softening scenario towards the magnetic transition temperature of the undeformed lattice. This can result in a surprisingly large alloying-driven strengthening effect at high temperature as illustrated in our work in the case of Fe-Co alloy
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| 14. |
- Östlin, Andreas, 1985-
(författare)
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Electronic structure studies and method development for complex materials
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Over the years electronic structure theory has proven to be a powerful method with which one can probe the behaviour of materials, making it possible to describe and predict material properties. The numerical tools needed for these methods are always in need of development, since the desire to calculate more complex materials pushes this field forward. This thesis contains work on both this implementational and developmental aspects.It begins by reviewing density functional theory and dynamical mean field theory, with the aim of merging these two methods. We point out theoretical and technical issues that may occur while doing this. One issue is the Padé approximant, which is used for analytical continuation. We assess the approximant and point out difficulties that can occur, and propose and evaluate methods for their solution.The virial theorem is assessed within the framework of density functional theory merged with many-body methods. We find that the virial theorem is extended from its usual form, and confirm this by performing practical calculations.The unified theory of crystal structure for transition metals has been established a long time ago using early electronic structure calculations. Here we implement the first- principles exact muffin-tin orbitals method to investigate the structural properties of the 6d transition metals. The goal of our study is to verify the existing theory for the mostly unknown 6d series and the performance of the current state-of-the art in the case of heavy d metals. It is found that these elements behave similarly to their lighter counterparts, except for a few deviations. In these cases we argue that it is relativistic effects that cause this anomalous behaviour. Palladium is then studied, taking many-body effects into account. We find that we can reproduce experimental photoemission spectra by these methods, as well as the Fermi surface.The thesis ends with an investigation of the stacking fault energies of the strongly correlated metal cerium. In addition to providing the first ab-initio stacking fault data for the two cubic phases of Ce, we discuss how these results could have an impact on the interpretation of the phase diagram of cerium
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| 15. |
- Östlin, Andreas, 1985-
(författare)
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Electronic structure studies and method development for complex materials
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Over the years electronic structure theory has proven to be a powerful method with which one can probe the behaviour of materials, making it possible to predict properties that are difficult to measure experimentally. The numerical tools needed for these methods are always in need of development, since the desire to calculate more complex materials pushes this field forward. This thesis contains work on both this implementational and developmental aspects. In the first part we investigate the structural properties of the 6d transition metals using the exact muffin-tin orbitals method. It is found that these elements behave similarly to their lighter counterparts, except for a few deviations. In these cases we argue that it is relativistic effects that cause this anomalous behaviour. In the second part we assess the Padé approximant, which is used in several methods where one wants to include many-body effects into the electronic structure. We point out difficulties that can occur when using this approximant, and propose and evaluate methods for their solution.
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| 16. |
- Delczeg, Lorand, 1980-
(författare)
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Ab-initio description of mono-vacancies in metals and alloys
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Trough the following pages a comprehensive study of open structures will be shown, including mono-vacancy calculations and open surfaces. These are electronic structure calculations using density functional theory within the exact muffin tin method. First we investigate the accuracy of five common density functional approximations for the theoretical description of the formation energy of mono-vacancies in three closepacked metals. Besides the local density approximation (LDA), we consider two generalized gradient approximation developed by Perdew and co-workers (PBE and PBEsol) and two gradient-level functionals obtained within the subsystem functional approach (AM05 and LAG). As test cases, we select aluminium, nickel and copper, all of them adopting the face centered cubic crystallographic structure. This investigation is followed by a performance comparison of the three common gradientlevel exchange-correlation functionals for metallic bulk, surface and vacancy systems. We find that approximations which by construction give similar results for the jellium surface, show large deviations for realistic systems. The particular charge density and density gradient dependence of the exchange-correlation energy densities is shown to be the reason behind the obtained differences. Our findings confirm that both the global (total energy) and the local (energy density) behavior of the exchange-correlation functional should be monitored for a consistent functional design. Last we show the vacancy formation energies of paramagnetic Fe-Cr-Ni alloys as a function of chemical composition. The theoretical predictions obtained for homogeneous chemistry and relaxed nearest-neighbors are in line with the experimental observation. In particular, Ni is found to decrease and Cr increase the vacancy formation energy of the ternary system.
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| 17. |
- Huang, Shuo
(författare)
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Theoretical Investigations of High-Entropy Alloys
- 2017
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High-entropy alloys (HEAs) are composed of multi-principal elements with equal or near-equal concentrations, which open up a vast compositional space for alloy design. Based on first-principle theory, we focus on the fundamental characteristics of the reported HEAs, as well as on the optimization and prediction of alternative HEAs with promising technological applications.The ab initio calculations presented in the thesis confirm and predict the relatively structural stability of different HEAs, and discuss the composition and temperature-induced phase transformations. The elastic behavior of several HEAs are evaluated through the single-crystal and polycrystalline elastic moduli by making use of a series of phenomenological models. The competition between dislocation full slip, twinning, and martensitic transformation during plastic deformation of HEAs with face-centered cubic phase are analyzed by studying the generalized stacking fault energy. The magnetic moments and magnetic exchange interactions for selected HEAs are calculated, and then applied in the Heisenberg Hamiltonian model in connection with Monte-Carlo simulations to get further insight into the magnetic characteristics including Curie point. The Debye-Grüneisen model is used to estimate the temperature variation of the thermal expansion coefficient.This work provides specific theoretical points of view to try to understand the intrinsic physical mechanisms behind the observed complex behavior in multi-component systems, and reveals some opportunities for designing and optimizing the properties of materials
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| 18. |
- Li, Changle, 1992-
(författare)
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Ab Initio Investigation of Interfacial and Grain Boundary Properties of Metals and Alloys
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Phase interface (IF) and grain boundary (GB) are both common yet critical planar defects influencing the mechanical and physical properties of polycrystalline materials. Due to the complex nature of IFs and GBs in terms of structure and chemistry, determining the accurate excess energies associated with these defects is challenging for both experimental measurements and theoretical simulations. In this thesis, using first-principles methods, I make efforts towards establishing an efficient and robust model for predicting the IF and GB properties in elemental metals and complex multi-component alloys including the temperature and composition dependences.First I focus on the temperature dependent interfacial energy (IFE) for Cu-Co alloys. IFE plays a critical role in determining the nucleation and precipitate coarsening thermodynamics and kinetics. I start with assessing the phase diagram of Cu-Co alloys by ab initio calculations, which is used for establishing the composition-temperature relationship of precipitates and matrix. Calculations of the physical and thermodynamic properties for the ferromagnetic (FM) and paramagnetic (PM) $\ce{Cu}_{1-x}\ce{Co}_{x}$ solid solutions are performed using the exact muffin-tin orbital (EMTO) method in combination with the coherent potential approximation (CPA) for dealing chemical and magnetic randomness. This study demonstrates that the equilibrium volumes and magnetic states are crucial for a quantitative description of the thermodynamics of the Cu-Co system at temperatures up to 1400 K. The predicted ab initio Cu-Co phase diagram is in good agreement with the measurements and CALPHAD data. Then, the composition and magnetic dependent IFEs for a coherent $\ce{Cu}_{1-x}\ce{Co}_{x}$/$\ce{Cu}_{x}\ce{Co}_{1-x}$ interface are investigated at various magnetic states including FM, PM, and the mixed PM+FM states to account for the magnetic state change at different temperatures. Then, I translate the composition dependence of the IFE to the temperature dependence. The obtained results are in reasonable agreement with those obtained by experiments and thermodynamic calculations. The first part of the thesis provides an ab initio database for the IFEs in Cu-Co system and emphasizes the importance of understanding and properly describing various physical and thermodynamic quantities in different materials modeling approaches.The second focus of this thesis is on grain boundary energy (GBE). We calculate the GBEs for ten face-centered cubic (fcc) and seven body-centered cubic (bcc) metals. Various types of symmetric tilt GB structures ranging from twin boundary up to $\Sigma$19 coincident site lattice (CSL) boundaries are studied using the Vienna Ab initio Simulation Package (VASP). Ab initio results show a correlation between the GBEs of the same grain boundary structure in different fcc and bcc metals. Importantly, I show that the correlation factor is best determined by the ratio of the low-index surface energy. By using this correlation, the general GBEs of fcc and bcc metals are predicted at 0 K. Furthermore, using the Foiles's method, which assumes that the general GBE has a similar temperature dependence as the elastic modulus $c_{44}$, the general GBEs at elevated temperatures are predicted. The so obtained theoretical results show a good agreement with the available experimental data. Finally, the proposed method for predicting the general GBEs is applied to complex multicomponent alloys (austinite Fe-Cr-Ni and ferritic Fe-Cr alloys), yielding a parameterized prediction of the composition and temperature dependent GBE. After examining two common experimental methods used for determining the general GBEs, it is concluded that the two sets of experimental GBEs for fcc metals agree well with each other, while for bcc metals they correspond to different grain boundary structures and differ by a factor of 2. This part of the thesis introduces an effective and robust model for predicting the general GBEs of metals and alloys, facilitating grain boundary engineering for advanced alloy design. The third focus is on alloying GB segregation in complex alloys. Manganese (Mn) and Nickel (Ni) segregation behaviors at bcc Fe-Cr grain boundaries are investigated. In this segregation study, three GB structures, namely, $\Sigma$3(111), $\Sigma$9(114), and $\Sigma$11(332), are considered. First, a systematic comparison of the theoretical segregation energies for Mn and Ni solutes in pure Fe GBs is conducted between VASP and EMTO calculations. The EMTO results agree reasonably with VASP and previous theoretical data, indicating a reliable potential for capturing the solute segregation behaviors. Next, the Mn and Ni segregation energies at bcc Fe-Cr solid solution GBs are determined at various concentrations of the matrix and at the FM and PM states to account for the temperature effects on the magnetic state using the EMTO-CPA method. Strong magnetic effects on the segregation energy are observed. Particularly, it is found that the magnetic states of Mn atoms depend strongly on local chemical and structural environment, which has a remarkable effect on the segregation energy. It is found that Mn and Ni show different segregation tendencies at FM and PM states. This part of the thesis puts forward an attempt to investigate the solute segregation properties in complex solid solutions as compared to pure metal or dilute alloys, and improves our understanding of GB segregation in engineering alloys, like steels.
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| 19. |
- Li, Chun-Mei, 1979-
(författare)
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Elastic properties and phase stability of shape memory alloys from first-principles theory
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Ni-Mn-Ga and In-Tl are two examples of shape memory alloys. Their shape memory effect is controlled by the martensitic transformation from the high temperature cubic phase to the low temperature tetragonal phase. Experimentally, it was found that the martensitic transformation, related to the elastic properties, is highly composition-dependent.In order to better understand the phase transition and facilitate the design of new materials with improved shape memory properties, the atomic scale description of the thermophysical properties of these alloys is needed. Therefore, in the presen tthesis, the elastic properties and phase stability of Ni-Mn-Ga and In-Tl shape memory alloys are investigated by the use of first-principles exact muffin-tin orbitals method in combination with the coherent-potential approximation. We present a theoretical description of the equilibrium properties of pure In and standard stoichiometric Ni2MnGa alloy with both cubic and tetragonal structures. In In-Tl alloys, all the calculated composition-dependent thermophysical properties: lattice parameter c/a, tetragonal shear modulus C" = (C11 - C12)/2, energy difference between the austenitic and martensitic phases, as well as electronic structures are shown to be in line with the experimentally observed lowering of the martensitic transition temperature TM with the addition of Tl. For most of the off-stoichiometric Ni2MnGa, the excess atoms of the rich component prefer to occupy the sublattice of the deficient one, except for the Ga-rich alloys, where the excess Ga atoms have strong tendency to take the Mn sublattice irrespective of the Mn occupation. In Ni-Mn-Ga-X (X=Fe, Co, and Cu) quarternary alloys, Fe atom prefers to occupy the Mn and Ni sublattices even in Ga-deficient alloys; Co has strong tendency to occupy the Ni-sublattice in all types of alloys; Cu atoms always occupy the sublattice of the host elements in deficiency. For most of the studied Ni-Mn-Ga and Ni-Mn-Ga-X alloys with stable site-occupations, the shear modulus C" can be considered as a predictor of the composition dependence of TM of the alloys: the alloy with larger C" than that of the perfect Ni2MnGa generally possesses lower TM except for Ni2Mn1+xGa1-x and Ni2Mn1-xGaFex. The failure of C" as a factor of TM in these two types of alloys may be ascribed that the compositiondependentmagnetic interactions and the temperature-dependent C0, which also playan important role on the martensitic transformation in these alloys. Furthermore, wedemonstrate that a proper account of the temperature and composition dependence ofC0 gives us reasonable theoretical TM(x) values in Ni2+xMn1-xGa alloys. Also in this type of Ni-rich and Mn-deficient alloys, by using the Heisenberg model in combination with the mean-field approximation, the abnormal trend of experimental magnetic transition temperature TC(x) with respect to the composition x is shown to be well captured by the theory.
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| 20. |
- Li, Chun-Mei
(författare)
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Elastic properties and phase stability of shape memory alloys from first-principles theory
- 2010
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Ni-Mn-Ga and In-Tl are two examples of shape memory alloys. Their shape memory effect is controlled by the martensitic transformation from high temperature face-centered-cubic (fcc) phase to the low temperature face-centered-tetragonal (fct) phase. Experimentally, it was found that the martensitic transformation is related to the elastic properties. In order to better understand the phase transition and facilitate the design of new materials with improved shape memory properties, the atomic scale description of the thermophysical properties of these alloys is needed. In the present thesis, the elastic properties and phase stability of Ni-Mn-Ga and In-Tl shape memory alloys are investigated by the use of first-principles exact muffin-tin orbital method in combination with coherent-potential approximation.It is shown that the theoretical lattice parameters and elastic constants of stoichiometric Ni2MnGa and pure In agree well with the available theoretical and experimental data, indicating that the employed theoretical approach is suitable to study the elastic properties of both cubic and tetragonal crystals. For most of the off-stoichiometric Ni2MnGa, the excess atoms of the rich component prefer to occupy the sublattice of the deficient one, except for the Ga-rich alloys, where the excess Ga atoms have strong tendency to take the Mn sublattice irrespective of the Mn occupation. With increasing e/a ratio (the number of valence electrons per atom), it is found that the theoretical bulk modulus B and the shear constant C44 increase but the tetragonal elastic constant C′ decreases. Except for Mn-rich Ga-deficient alloys, C′ is generally inversely proportional and the energy difference between parent and martensitic phases is directly proportional to the martensitic tansformation temperature TM. For In1-xTlx alloys, the tetragonal lattice parameter c/a and the shear modulus C′ in the fct phase and the total energy difference between the fcc and fct phases decrease with Tl addition, whereas the negative C′ of the fcc phase increases with x turning positive around x=0.35. All of these composition dependent thermophysical properties can be understood by investigating the electronic structure of In and In-Tl alloys and they are in line with the experimentally observed lowering of TM with addition of Tl.
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| 21. |
- Li, Ruihuan, 1986-
(författare)
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First-principles study of the multiple He trapping in defects in vanadium and SiC
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In fusion environment, large amount of helium (He) atoms are produced by transmutation along with structural damage in the structural materials, causing materials swelling and degrading of physical properties. In this thesis, using first-principles method, I examined the microscopic mechanism of He trapping in vacancies and voids in structural materials (vanadium solid and 6H–SiC composites). In vanadium, a single He atom located in the tetrahedral interstitial site (TIS) turned out to be more stable than that in the octahedral interstitial site (OIS). Helium atoms were placed one by one into the vacancy defects (monovacancy and void) from the remote TISs, and we calculated the trapping energies as a function of the number of He atoms inside the vacancy defects. We found that, the monovacancy and void (about 0.6 mn in diameter) can host up 18 and 66 He atoms, respectively, in vanadium solid. The induced internal pressure by He bubbles in monovacancy and small void increased up to 7.5 GPa and 19.3 GPa, respectively. In vacancy defect, the He–He equilibrium distances decreased with the amount of He atoms incorporated in monovacancy and small void, and the host lattice expanded dramatically. The atomic structures of selected He clusters trapped in vacancies were compared with the gas-phase clusters. In complex 6H–SiC, there are ten kinds of interstitial sites for a single He atom. According to the calculated formation energy, the most stable site is the. R site. [1], where R site alternates with hexagonal interstitial sites. We explored the interactions between an interstital He atom and HenVam (Va stands for vacancy) clusters (n, m = 1 – 4). We found that the binding energy between He and the HenVam clusters increases with the number of vacancies (e.g., the binding energy is 1.3 eV for He2Va3, and 1.7 eV for He2Va4, respectively). The small void (about 0.55 nm in diameter) in 6H–SiC can accommodate up to 14 He atoms and the corresponding internal pressure is estimated to be 2.5 GPa. The maximum density of He atoms in a small He bubble is about 50 atoms/nm3, which is of the same magnitude as the experimental value 10 atoms/nm 3. Compared to vanadium, a small nanosized void in the 6H–SiC host lattice has a weak tendency for trapping He. When trapped seventy He atoms in small void in vanadium, the nearest vanadium bond expands 22–28 %, and the volume of the void expands by 80%. At the same time, with fourteen atoms encapsulated in a small void in 6H–SiC, the local Si–C bonds explans 1–5%, and the volume of the small void expands about 7%. We suggest that the differences in the cohesive energies in these two systems are responsible for the different He trapping behavior.
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| 22. |
- Li, Wei, 1984-
(författare)
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First-principles description of planarfaults in metals and alloys
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Phase interface and stacking fault are two common planar defects in metallic materials. In the present thesis, the interfacial energy and the generalized stacking fault energy of random alloys are investigated using density functional theory formulated within the exact muffin-tin orbitals (EMTO) method in combination with the coherent-potential approximation (CPA).The interfacial energy is one of the key physical parameters controlling the formation of the Cr-richα’ phases during the phase decomposition in Fe-Cr ferrite stainless steels. This decomposition is believed to cause the so-called “475°Cembrittlement”. Aluminum addition to ferritic stainless steels was found to effectively suppress the deleterious 475oC embrittlement. The effect of Al on the interfacial energy and the formation energy of Fe-Cr solid solutions are studied in this thesis. The interface between the decomposed Fe-rich α and Cr-richα0 phases carries a positive excess energy, which represents a barrier for the process of phase separation. Our results show that for the α-Fe70Cr20Al10/α0-Fe100−x−yCryAlx (0≤x≤10, 55≤y≤80) interface, the Al con-tent(x) barely changes the interfacial energy. However, when Al is partitioned only in the alpha phase, i.e. for the α-Fe100−x−yCryAlx/α0-Fe10Cr90 (0≤x≤10, 0≤y≤25) interface, the interfacial energy increases with Al concentration due to the variation of the formation energies of the Fe-Cr alloys upon Al alloying.The intrinsic energy barriers (IEBs) on theγ−surface (also called generalized stacking fault energy, GSFE) provide fundamental physics for understanding the plastic deformation mechanisms in face-centred cubic (fcc) metals and alloys. In this thesis, the GSFEs of the disordered Cu-X (X=Al, Zn, Ga, Ni) and Pd-X (X=Ag, Au) alloys are calculated. Studying the effect of segregation of the solutes to the stacking fault planes shows that only the local chemical composition affects the GSFEs. Based on the calculated GSFE values, the previously revealed “universal scaling law” between these IEBs is demonstrated to be well obeyed in random solid solutions. This greatly simplifies the calculations of the twinning parameters or the critical twinning stress. Adopting two twinnability measure parameters derived from the IEBs, we find that in binary Cu alloys, the addition of Al, Zn and Ga increases the twinnability, while adding Ni decreases it. Aluminum and gallium yield similar effects on the twinnability. Our theoretical predictions are in line with the available experimental data. These achievements open new possibilities in understanding and describing the plasticity of complex alloys.We investigate theγ-surface of paramagneticγ-Fe as a function of temperature. At ambient conditions, the fcc lattice is thermodynamically unstable with respect to the hexagonal close-packed (hcp) lattice, resulting in negative intrinsic stacking fault energy (ISF). However, the unstable stacking fault energy (USF), representing the energy barrier along theγ-surface connecting the ideal fcc and the intrinsic stacking fault positions, is large and positive. The ISF is calculated to have a strong positive temperature coefficient, while the USF decreases monotonously with temperature. According to the recently developed plasticity theory, the overall effect of temperature is to move the plastic deformation mode of the paramagnetic fcc Fe from the stacking fault formation regime (T <<300K) towards maximum twinning (T≈300K) and finally to a dominating full-slip regime (T >>300K). Our predictions are discussed in connection with the available experimental observations.The same methodology is used to establish theγ-surface of Fe-Cr-Ni alloys as a function of chemical composition and temperature. We fix the concentration of Cr at 20 at.%. Nickel is found to increase the intrinsic stacking fault (SFE), unstable stacking fault (USF) and unstable twin fault (UTF) energies. The theoretical SFE versus chemistry and temperature trends agree well with experiments. Both USF and UTF decrease with increasing temperature. The calculated IEBs are used to establish the temperature and composition dependence of the deformation modes in Fe-Cr-Ni alloys. Stacking fault formation is predicted to be the leading deformation mechanism for alloys with effective SFE below∼18mJm−2, which is in good agreement with the observed upper limit of the SFE for the TRIP (transformation-induced plasticity) mechanism. Alloys with SFE above this critical value show both twinning and full slip at room temperature and surprisingly, even the SFE is very high, twinning remains a possible deformation mode even at elevated temperatures, which is in line with observations
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| 23. |
- Li, Xiaojie, 1989-
(författare)
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Materials for advanced energy technology from quantum-mechanical modeling
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The present thesis addresses promising material solutions for fusion reactors from a theoretical point of view. We focus on two specific systems: W-based alloys used as plasma-facing materials and reduced activation ferritic/martensitic (RAFM, Ferich) steels used as structural materials of breeding-blanket. We aim to systematically investigate the alloying effects on the micro-mechanical properties of these body-centered cubic (bcc) solid solutions. The all-electron exact muffin-tin orbitals (EMTO) method in combination with the coherent-potential approximation (CPA) is the main tool for our theoretical studies. The knowledge of the elastic parameters and their solute-induced changes is important for alloy design and for a multi-scale modeling approach to the mechanical properties. We also explore the planar faults in the present Fe-based alloys.In part one, the effect of neutron transmutation elements on the elastic properties of the W-based alloys are calculated. Under intensive radiation, W transforms to Re/Os and thus there is a certain degree of Re/Os doping in the base alloy. Both Re and Os solute atoms shrink the lattice constant, which lead to increasing bulk modulus as the amount of Re or Os increases. The polycrystalline shear and Young’s moduli of W1−x−yRexOsy (0 ≤ x, y ≤0.06) enhance with the addition of Re but decrease with increasing Os level. From the variations of the Cauchy pressure, Poisson’s ratio, Pugh ratio B/G, and the ratio of cleavage energy to shear modulus for the dominant slip system, we conclude that the intrinsic ductility of the alloy increases with increasing Re and Os content. We use the energy difference between the face-centered cubic (fcc) and bcc structures to estimate the alloying effect on the ideal tensile strength in the [001] direction.In part two, we choose three RAFM steels: CLAM/CLF-1, F82H, and EUROFER97 and investigate the micro-mechanical properties of the main alloy phases at low temperature (0 K). Being the main building blocks of the RAFM steels, first the lattice parameters, elastic properties, surface energy and unstable stacking fault energy of ferromagnetic α-Fe and Fe91Cr9 are calculated for reference. For quantitative understanding, we present a detailed analysis of the calculated individual alloying effects of V, Cr, Mn, and W on the elastic properties of Fe91Cr9. A linear superposition of these individual rates on the elastic properties of RAFM steels is shown to reproduce well the values from ab initio calculations. The composition dependence of the elastic constants is decomposed into electronic and volumetric contributions and they are analyzed separately. Finally, the intrinsic ductility is evaluated through Rice’s phenomenological theory by using the ratio of surface and unstable stacking fault energies. The results are consistent with those obtained by the common empirical criteria.In part three, the temperature dependence (T ≤ 1120 K) of the isothermal singlecrystal and polycrystalline elastic parameters of α-Fe and CLAM are reported by using a first-principles based modeling approach. The effect of temperature on the strongly temperature-dependent elastic constants C11 T and CT' is reproduced, as well as that on derived isotropic elastic moduli. Weak changes in C12 T and C44 T with temperature are obtained. The approach is applied to predict the temperature effect on the elastic parameters of three RAFM steels. Contributions due to loss of longrange magnetic order and the combined effect of volume expansion and entropy are found to be important in determining the temperature dependence of the elastic parameters in all the materials investigated.In part four, the (100) and (110) surface energies and surface segregation energies of Fe1−xbCrxb binary alloys, xb ≤ 15 at.%, are computed. These alloys form the basic building blocks of RAFM steels and thus their surface properties are of fundamental importance for the modeling the mechanical behavior. The implications of these results for the surface alloy phase diagram are discussed. The surface chemistry of Fe-Cr(100) is characterized by a transition from Cr depletion to Cr enrichment in a critical bulk Cr composition window of 6 < xb < 9 at.%. In contrast, a nearly homogeneous Cr concentration profile is energetically favorable in Fe-Cr(110) surface. The strongly suppressed surface-layer relaxation at both surfaces is shown to be of magnetic origin. The compressive, magnetic contribution to the surface relaxation stress is found to correlate well with the surface magnetic moment squared at both surface terminations. The surface electronic structures are used to explain the stability of the Cr surface magnetic moments against bulk Cr content.
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| 24. |
- Li, Xiaoqing
(författare)
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Mechanical Properties of Transition Metal Alloys from First-Principles Theory
- 2014
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aim of the thesis is to investigate the alloying effect on the mechanical properties of random alloys using the all-electron exact muffin-tin orbitals methodin combination with the coherent-potential approximation. The second-order elastic constants describe the mechanical properties of materials in the small deformation region, where the stress-strain relations arelinear. Beyond the small elastic region, the mechanical properties of dislocation-free solids are described by the ideal strength.The elastic constants and ideal tensile strengths have been investigated as a function of Cr and Ti for the body centered cubic V-based random solidsolution. Alloys along the equi-composition region are found to exhibit the largest shear and Young’s modulus as a result of the opposite alloying effectsobtained for the two cubic shear elastic constants C' and C44.Classical solid-solution hardening (SSH) model predicts larger hardening effect in V-Ti thanin V-Cr alloy. By considering a phenomenological expression for the ductile-brittle transition temperature (DBTT) in terms of Peierls stress and SSH, itis shown that the present theoretical results can account for the variations of DBTT with composition. Under uniaxial [001] tensile loading, the ideal tensilestrength of V is 12.4 GPa and the lattice fails by shear. Assuming isotropic Poisson contraction, the ideal tensile strength are 36.4 and 52.0 GPa for V inthe [111] and [110] directions, respectively. For the V-based alloys, Cr increases and Ti decreases the ideal tensile strength in all principal directions. Addingthe same concentration of Cr and Ti to V leads to ternary alloys with similar ideal tensile strength values as that of pure V. The alloying effects on the idealtensile strength are explained using the electronic band structure.The ideal tensile strengths of bcc ferromagnetic Fe-based random alloys have been calculated as a function of compositions. The ideal tensile strength of Fe in the [001] direction is calculated to be 12. 6GPa,in agreement with the available data. For the Fe-based alloys, we predict that V, Cr, and Co increase the ideal tensile strength, while Al and Ni decrease it. Manganese yields a weak non-monotonous alloying behavior. We show that the limited use of the previouslyestablished ideal tensile strengths model based on structural energy differences in the case of Fe-bases alloys is attributed to the effect of magnetism. We find that upon tension all the investigated solutes strongly alter the magneticresponse of the Fe host from the unsaturated towards a stronger ferromagnetic behavior.
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| 25. |
- Lu, Song, 1985-
(författare)
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First-principles investigations of planar defects
- 2012
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Two types of planar defects, phase interface and stacking fault, are addressed in thisthesis. The first-principles exact-muffin orbitals method in combination with thecoherent-potential approximation is the main density functional theory (DFT) toolfor our studies. The investigation is mainly carried out for stainless steels which arefundamental materials in modern society. Ferritic and austenitic stainless steels arethe two largest subcategories of stainless steels.In ferritic stainless steels, the interface between Fe-rich α and Cr-rich α′ phasesformed during spinodal phase decomposition is studied. This decomposition isknow to increase the hardness of ferrites, making them brittle (also called the "475◦ Cembrittlement"). We calculate the interfacial energies between the Cr-rich α′ -Fex Cr1−xand Fe-rich α-Fe1−y Cry phases (0 < x, y < 0.35) and show that the formation energyis between ∼0.02 and ∼0.33 J m−2 for the ferromagnetic state and between ∼0.02and ∼0.27 J m−2 for the paramagnetic state. Although for both magnetic states,the interfacial energy follows a general decreasing trend with increasing x and y,the fine structures of the γ(x, y) maps exhibit a marked magnetic state dependence.The subtleties are shown to be ascribed to the magnetic interaction between the Feand Cr atoms near the interface. The theoretical results are applied to estimate thecritical grain size for nucleation and growth in Fe-Cr stainless steel alloys.In close-packed alloys possessing the face centered cubic crystallographic lattice ,stacking faults are very common planar defects. The formation energy of a stackingfault, named stacking fault energy, is related to a series of mechanical properties.Intrinsic stacking fault energy for binary Pd-Ag, Pd-Cu, Pt-Cu and Ni-Cu solid so-lutions are calculated using the axial interaction model and the supercell model. Bycomparing with experimental data, we show that the two models yield consistentformation energies. For Pd-Ag, Pd-Cu and Ni-Cu, the theoretical SFEs agree wellwith those from the experimental measurements. For Pt-Cu no experimental resultsare available, and thus our calculated SFEs represent the first reasonable predictions.We also discuss the correlation of the SFE and the minimum dmin in severe plasticdeformation experiments and show that the dmin values can be evaluated from firstprinciples calculations.After gaining confidence with the axial interaction model, the alloying effects of Mn,Co, and Nb on the stacking fault energy of austenitic stainless alloys, Fe-Cr-Ni withvarious Ni content, are investigated. In the composition range (cCr = 20%, 8 ≤cNi ≤ 20%, 0 ≤ cMn , cCo , cNb ≤ 8%, balance Fe) studied here, it is found that Mndecreases the SFE at 0 K, but at room temperature it increases the SFE in high-Ni (cNi16%) alloys. The SFE always decreases with increasing Co. Niobiumincreases the SFE significantly in low-Ni alloys, however this effect is strongly di-minished in high-Ni alloys. The SFE-enhancing effect of Ni usually observed inFe-Cr-Ni alloys is inverted to SFE-decreasing effect in the hypothetical alloys con-taining more than 3% Nb in solid solution. The revealed nonlinear compositionivdependencies are explained in terms of the peculiar magnetic contributions to thetotal SFE.
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| 26. |
- Song, Lu
(författare)
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First-principles investigations of planar defects
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Two types of planar defects, phase interface and stacking fault, are addressed in this thesis. The investigation is mainly carried out for stainless steels which are fundamental materials in modern society. For the phase interface, we investigate the metallic bcc/bcc and fcc/bcc phase interfaces.The bcc/bcc phase interface in ferrite steels and the stacking fault in austenite steels are studied, respectively. The interfaces between fcc and bcc phases are studied for the model Fe/Ag system, the methods used in this case may be expanded and adopted for studying the interface between ferrite and austenite in duplex stainless steels in future. Stacking faults in some binary metallic systems are also investigated. The first-principles exact-muffin orbitals method (EMTO) in combination with the coherent-potential approximation (CPA) and the Vienna Ab initio Simulation Package (VASP) are the main density functional theory (DFT) tools for our studies.In ferritic stainless steels, the interface between Fe-richand Cr-rich′phases formed during spinodal phase decomposition is studied. This decomposition is known to increase the hardness of ferrites, making them brittle (also called the ”475◦Cembrittlement”). We calculate the interfacial energies between the Cr-rich α′-FexCr1−x and Fe-rich α-Fe1−yCry phases (0−2for the ferromagnetic state and between∼0.02 and∼0.27 J m−2for the paramagnetic state. Although for both magnetic states, the interfacial energy follows a general decreasing trend with increasing x and y, the fine structures of the γ (x, y)maps exhibit a marked magnetic state dependence. The subtleties are shown to be ascribed to the magnetic interaction between the Fe and Cr atoms near the interface. The theoretical results are applied to estimate the critical grain size for nucleation and growth in Fe-Cr stainless steel alloys.For the fcc/bcc interface, because of the difficulty to model a realistic semicoherent interface with misfit dislocations, alternatively, we perform ab initio calculations to determine the lower and upper bounds of the interfacial energy and work of separation of fcc-Ag/bcc-Fe interface. The strain-free interfacial energy of the coherent interface is taken as the lower bound and the interfacial energy of the commensurate incoherent interface as the upper bound of the interfacial energy of a realistic semicoherent interface. The latter is estimated by applying an averaging scheme based on the interfacial energies obtained for the coherent interfaces. Similar calculations are performed for determining the bounds of the work of separation. We justify the use of the averaging scheme by carrying out large supercell calculations for a semicoherent interface (not realistic either). For a Fe(110)/Ag(111) semicoherent interface, we show that taking either Fe or Ag as the underlying lattice, our averaging scheme can yield a reasonable estimation of the work of separation of the semico-herent interface. However, when taking Ag as the underlying lattice the averaged interfacial energy of the semicoherent interface is significantly underestimated due to the magnetism. The structure and magnetism at the coherent and semicoherent interfaces are discussed.In close-packed alloys possessing the face centered cubic crystallographic lattic ,stacking faults are very common planar defects. The formation energy of a stacking fault, named stacking fault energy (SFE), is related to a series of mechanical properties.Intrinsic stacking fault energy for binary Pd-Ag, Pd-Cu, Pt-Cu and Ni-Cu solid solutions are calculated using the axial interaction model and the supercell model. By comparing with experimental data, we show that the two models yield consistent formation energies. For Pd-Ag, Pd-Cu and Ni-Cu, the theoretical SFEs agree well with those from the experiments. For Pt-Cu no experimental results are available, and thus our calculated SFEs represent the first reasonable predictions. We also discuss the correlation of the SFE and the minimum dmin in severe plastic deformation experiments and show that the dmin values can be evaluated from first principles calculations.After gaining confidence with the axial interaction model, the alloying effects of Mn, Co, and Nb on the stacking fault energy of austenitic stainless alloys, Fe-Cr-Ni with various Ni content, are investigated. In the composition range (cCr= 20%;8≤cNi≤20%;0≤cMn;cCo;cNb≤8%, balance Fe) studied here, it is found that Mn decreases the SFE at 0 K, but at room temperature it increases the SFE in high-Ni (cNi≥16%) alloys. The SFE always decreases with increasing Co. Niobium increases the SFE significantly in low-Ni alloys, however this effect is strongly diminished in high-Ni alloys. The SFE-enhancing effect of Ni usually observed in Fe-Cr-Ni alloys is inverted to SFE-decreasing effect in the hypothetical alloys containing more than 3% Nb in solid solution. The revealed nonlinear composition dependencies are explained in terms of the peculiar magnetic contributions to the total SFE
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| 27. |
- Tian, Liyun, 1986-
(författare)
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Density Functional Study of Elastic Properties of Metallic Alloys
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Special quasi-random structure (SQS) and coherent potential approximation (CPA) are techniques widely employed in the first-principles calculations of random alloys. The aim of the thesis is to study these approaches by focusing on the local lattice distortion (LLD) and the crystal symmetry effects. We compare the elastic parameters obtained from SQS and CPA calculations. For the CPA and SQS calculations, we employ the Exact Muffin-Tin Orbitals (EMTO) method and the pseudopotential method as implemented in the Vienna Ab initio Simulation Package (VASP), respectively. We compare the predicted trends of the VASP-SQS and EMTO-CPA parameters against composition.As a first case study, we investigate the elastic parameters of face centered cubic (fcc) Ti1−xAlx(0≤x≤100at.%) random solid solutions as a function of Al content (x). The EMTO-CPA and VASP-SQS results are in good agreement with each other. Comparing the lattice constants from SQS calculations with and without local lattice relaxations, we find that in Ti-rich (Al-rich) side the lattice constants remain almost unchanged (slightly increase) upon atomic relaxations. Taking local lattice distortions into consideration decreases the C11 and C44 elastic parameters, but their trends are not significantly affected. The C12 elastic constant, on the other hand, is almost unchanged when atomic relaxations are included. In general, the uncertainties in the elastic parameters associated with the symmetry lowering in supercell studies turn out to be superior to the differences between the two alloy techniques including the effect of LLD.We also investigate the elastic properties of random fcc Cu1−xAux(0≤x≤100 at.%) alloys as a function of Au content employing the CPA and SQS approaches. It is found that the CPA and SQS values forC11andC12 are consistent with each other no matter whether the atomic relaxations are taken into account or not. On the other hand, the EMTO-CPA values for C44 are slightly larger than those from SQS calculations especially for Cu-rich alloys which we ascribe to the differences in the DFT solvers rather than the differences between CPA and SQS.The Perdew-Burke-Ernzerhof (PBE) approximation to the exchange-correlation term in density functional theory (DFT) is a mature approach and have been adopted routinely to investigate the properties of metallic alloys. In most of the cases, PBE provides theoretical results in good agreement with experiments. However, the ordered Cu-Au system turned out to be a special case where large deviations between the PBE predictions and observations occur. In this work, we make use of a recently developed exchange-correlation functional, the so-called quasi-non-uniform exchange-correlation approximation (QNA), to calculate the lattice constants and formation energies for ordered Cu-Au alloys as a function of composition. The calculations are performed using the EMTO method. We find that the QNA functional leads to excellent agreement betweent heory and experiment. The PBE strongly overestimates the lattice constants for ordered Cu3Au, CuAu, CuAu3 compounds and also for the pure metals which is nicely corrected by the QNA approach. The errors in the formation energies of Cu3Au, CuAu, CuAu3relative to the experimental data decrease from 38-45% obtained with PBE to 5-9% calculated for QNA.
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| 28. |
- Xie, Ruiwen, 1993-
(författare)
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Defects in Austenitic Steels and Hard Metals - A DFT-based Study
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Materials are never 100% pure due to the limitation of purification method or manufacturing process. Nor are they perfect, especially under deformation. The present work aims to explore different roles played by the defects in austenitic steels and hard metals.The first focus is iron-manganese (Fe-Mn) based twinning induced plasticity(TWIP) steels which are a category of austenitic materials showing a good combination of high strength and ductility. The planar fault is fundamental for the TWIP mechanism. First, the γ-surface of pure γ-Fe (fcc-Fe) is calculated for different magnetic states. Next, the effects of alloying elements, including Mn,interstitial carbon (C) and nitrogen (N), are addressed. The γ-surface includes several prominent stacking fault energies that are fundamental for, e.g, predicting critical twinning stress and twinnability. The present work compares the γ-surface obtained at different magnetic states, including nonmagnetic (NM), paramagnetic(PM), antiferromagnetic single-layer (AFMI) and double-layer (AFMD) states. The local magnetism significantly influences the γ-surface. In addition, the existing antiferromagnetic (AFM) order results in two different deformation paths inγ-Fe, leading to the generations of superlattice intrinsic stacking fault (SISF) and complex stacking fault (CSF), respectively. The intrinsic stacking fault energy corresponding to SISF is relatively low while its corresponding unstable stacking fault energy is relatively high. The magnetic structures are investigated in the unstable stacking fault and the intrinsic stacking fault configurations via Monte Carlo (MC) simulations. The MC results show that only SISF configuration is favourable, and the two distinctive unstable stacking fault configurations may coexist.The Mn effect on the γ-surface of γ-Fe is studied at AFMI state and the crystal tetragonality is considered. The comparison with experimentally measured stacking fault energy (SFE) dependence on Mn composition shows that the AFMI results reproduce better the experimental trend in high-Mn Fe-Mn alloys than the PM results. Further, the interstitial alloying effects of C and N on the γ-surface of γ-Fe are investigated and no remarkable difference is observed betweenthe C and N impacts. The interaction between dislocation and interstitial atoms, which is fundamental to understand the phenomenon like dynamic strain ageing (DSA), is studied using the generalized stacking fault as an approximation of the partial dislocation core. The minimum migration energy path (MEP) and migration energy surface (MES) of C in the dislocation core of AFMD γ-Fe are calculated. In contrast to the common assumption that the interstitial atoms are stationary during the passage of fast-moving dislocations, the present work suggests that a pair of dislocation partials are capable of moving C atoms forward on the slip plane by one full Burgers vector. Moreover, at the stacking fault ribbon and especially near the dislocation core, the in-plane diffusion energy barriers of C are significantly reduced compared to that in the bulk, rendering a fast diffusion channel for C. The proposed mechanisms for C transport and diffusion are not decided by local magnetic order and can be used to explain the strain rate dependent formation kinetics of twinning or hexagonal close-packed (hcp) martensite in C-alloyed TWIP steels or high entropy alloys. Similarly, the ab initio results show that the diffusion energy barrier of N in the dislocation core is approximately 14.9% of that in the bulk. According to experimental observations, carbon promotes while N suppresses the DSA. However, the different C and N effects on the DSA cannot be understood from current thermodynamic investigations.The defects in the binder phase of hard metals (cemented carbides) are another important topic in this thesis. The interstitial tungsten (W) and C defects in hard metals come from the sintering process during industrial manufacturing. The cemented carbides are composite materials made of tungsten carbide (WC) grains glued together by a binder phase. Typically, the binder phase consists of ductile cobalt (Co) and some amount of dissolved W and C. The measurement ofthe magnetic saturation is one method employed for quality control of cemented carbides. Despite the great success of Co, a substitute of Co is needed due to its rising price and health threats. The substitution of a material in production processes can be complex. Ideally, manufacturing processes and quality controls should be used as usual or at least new ones have to be devised in a simpleway. The present work selects 85Ni-15Fe (85 at.% of Ni and 15 at.% of Fe) to demonstrate the relation between the magnetic saturation and the components of the binder phase of cemented carbides using ab initio method, which providesa non-destructive quality control method in cemented carbides.
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| 29. |
- Zhang, Hualei, 1981-
(författare)
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Elastic Properties of Iron Alloys from First-Principles Theory
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Accurate description of materials requires the most advanced atomic-scale techniques from both experimental and theoretical areas. In spite of numerous available techniques, however, the experimental study of the atomic-scale properties and phenomena even in simple solids is rather difficult. Iron and its alloys (including steels) are among the most important engineering materials due to their excellent mechanical properties. In these systems, the above challenges become more complex due to the interplay between the structural, chemical, andmagnetic effects. On the other hand, advanced computational methods based on density functional theory (DFT) ensure a proper platform for studying the fundamental properties of materials from first-principles theory. The present thesis belongs to the latter category. We use advanced theoretical tools to give a systematic description of Fe and a series of Fe-rich alloys in the ferromagnetic (FM) body-centered-cubic (bcc), paramagnetic (PM) bcc, and PM face-centered-cubic (fcc) structures. For solving the basic DFT equations for steel alloys, we adopt the all-electron exact muffin-tin orbitals (EMTO) method in combination with the coherent-potential approximation (CPA) and the disordered local magnetic moment (DLM) model. We start by assessing our theoretical tools in the case of Fe. For the FM state, we find that there is a magnetic transition close to the ground state volume of bcc Fe, which is explained by the peculiarmagnetic band structure. We conclude that the common equation of state functions can not capture the physics of this magnetic transition, leading to serious underestimation of theoretical bulk modulus of Fe. When the above effect is properly taken into account, theory is shown to reproduce the low-temperature experimental bulk properties (equation of state and elastic parameters) of FM bcc Fe within ∼ 1% for the volume and ∼7% for the elastic constants. Using the EMTO-CPA-DLM picture, in contrast to previous theoretical predictions, we demonstrate that the competing high-temperature cubic phases of PM Fe correspond to two distinct total energy minima in the tetragonal (Bain) configurational space. Both fcc and bcc lattices are dynamically stable, and at static conditions the fcc structure is found to be the thermodynamically stable phase. When the thermal expansion is taken into account, our theoretical bulk properties calculated for PM Fe agree well with the available experimental data. Increasing temperature is predicted to stabilize the bcc (δ) phase against the fcc (γ) one because of the shallow energy minimum around the bcc structure. The calculated composition-dependent equilibriumlattice constants, single-crystal elastic constants Cij(c) (here c stands for the amount of alloying additions), and polycrystalline elastic parameters of FM bcc Fe show good agreement with former theoretical and available experimental data, implying that the employed theoretical approach is suitable to calculate the elastic properties of FM Fe alloys. For FM bcc Fe alloys, all impurities considered in this thesis (Al, Si, V, Cr, Mn, Co, Ni, and Rh) enlarge the equilibrium lattice parameter and accordingly decrease the C11(c), C12(c), and C′(c) elastic constants. However, a peculiar phenomenon appears for C44(c). Namely, in spite of increasing volume, Al, Si, V, Cr, and Mn are found to increase C44(c), whereas the alloying effects of Co, Ni, and Rh are small. The anomalous alloying effect in C44(c) isshown to originate from the particular electronic structure of FM bcc Fe. The complex composition dependence of C44(c) is reflected in the polycrystalline properties of FM Fe as well. Unlike for FM bcc Fe, both positive and negative alloying effects appear for the theoretical equilibrium lattice parameters, single-crystal and polycrystalline elastic properties of PM bcc and fcc Fe. For many elastic parameters and binary systems considered in this thesis, alloying element induces opposite effects in fcc and bcc phases. In other words, the alloying effects on the elastic properties of PM Fe-based alloys show strong structure dependence. While neither the volume nor the electronic effect can explain the calculated trends of C′(c), we find that there is a general correlation between alloying effects on the lattice stability and C′(c). With a few exceptions, alloying elements have much larger effects on FM bcc Fe than on PM fcc Fe. A slightly larger alloying effect appears on PM fcc Fe compared to PM bcc Fe. According to the calculated fundamental properties, we also estimate the relative hardness of Fe alloys via two phenomenological solid-solution strengthening mechanisms. In those caseswhere experimental data are available, the predicted solid-solution strengthening effects are in line with the observations. The metastable Mg-doped Fe alloys surpass all rival binaries in density and solid-solution strengthening effects. The Fe-Cr and Fe-Cr-Ni alloys containing a few percent of Mg are also predicted to possess unusually high solid-solution hardening and low density compared to the host alloys. These attributes make theMg-bearing stainless steels very promising candidates for many applications, such as the high-strength and light-weight designs desired by for example the automotive industry.
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