| 1. |
- Li, Pei, et al.
(author)
-
Adaptive Volume Control in Titanium Alloy for High Temperature Performance
- 2019
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In: Materials. - : MDPI. - 1996-1944. ; 12:23
-
Journal article (peer-reviewed)abstract
- With the increase of time, the shrinkage of materials at fixed temperature could enhance the failure of fasteners. We report a potential way to alter the volume/length of alloy automatically through isothermal aging due to pseudospinodal decomposition mechanism. The volume of Ti-10V-2Fe-3Al alloy first shrunk and then expanded during isothermal aging at 550 degrees C. It can fit tightly and make up for volume loss. Transmission electron microscopy observation exhibits no obvious coarsening of intragranular alpha phase with the increasing time. However, composition evolution with time shows a gradual change through energy dispersive spectrometer analysis. The result shows that beta stabilizers, V and Fe, are prone to diffuse to the beta matrix, while alpha stabilizers, Al, prefer to segregate to the alpha phase. First principle calculations suggest that the structure transition for beta to alpha cause the first decrease of volume, and the diffusion of V, Fe and Al is the origin of the later abnormal increase of volume.
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| 2. |
- Li, Pei, et al.
(author)
-
Secondary hardening behavior in Ti alloy
- 2019
-
In: Materials Science & Engineering. - : Elsevier. - 0921-5093 .- 1873-4936. ; 759, s. 640-647
-
Journal article (peer-reviewed)abstract
- We report the existence of secondary hardening behavior in Ti-10V-2Fe-3Al (wt.%) (Ti-1023) for the first time. Through controlling the ageing temperature window between 550 degrees C and 575 degrees C, alloys are found to show the existence of two hardness peaks with aging time. This heat treatment with secondary hardening phenomenon exhibits unusual increase of hardness and strength. Further experimental observations show that the first hardness peak corresponds with the well precipitated alpha phase at very short time. Further increase of aging time does not influence the morphology of these intragranular alpha precipitates, but accompanying with a continuous composition change, which could be the origin of the secondary hardness peak. This structure change followed by composition diffusion is the typical characteristic of pseudospinodal mechanism. Phase field and ab-initio calculations based on pseudospinodal mechanism analyze the detailed microstructure/composition evolution and related solid solution strength, which confirm the role of composition on the appearance of secondary hardening behavior.
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| 3. |
- Sun, Xun, et al.
(author)
-
Can experiment determine the stacking fault energy of metastable alloys?
- 2021
-
In: Materials & design. - : Elsevier Ltd. - 0264-1275 .- 1873-4197. ; 199
-
Journal article (peer-reviewed)abstract
- Stacking fault energy (SFE) plays an important role in deformation mechanisms and mechanical properties of face-centered cubic (fcc) metals and alloys. In many concentrated fcc alloys, the SFEs determined from density functional theory (DFT) calculations and experimental methods are found having opposite signs. Here, we show that the negative SFE by DFT reflects the thermodynamic instability of the fcc phase relative to the hexagonal close-packed one; while the experimentally determined SFEs are restricted to be positive by the models behind the indirect measurements. We argue that the common models underlying the experimental measurements of SFE fail in metastable alloys. In various concentrated solid solutions, we demonstrate that the SFEs obtained by DFT calculations correlate well with the primary deformation mechanisms observed experimentally, showing a better resolution than the experimentally measured SFEs. Furthermore, we believe that the negative SFE is important for understanding the abnormal behaviors of partial dislocations in metastable alloys under deformation. The present work advances the fundamental understanding of SFE and its relation to plastic deformations, and sheds light on future alloy design by physical metallurgy.
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| 4. |
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| 5. |
- Laukkanen, P., et al.
(author)
-
Formation and destabilization of Ga interstitials in GaAsN : Experiment and theory
- 2012
-
In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 86:19, s. 195205-
-
Journal article (peer-reviewed)abstract
- Using first-principles total energy calculations we have found complex defects induced by N incorporation in GaAsN. The formation energy of the Ga interstitial atom is very significantly decreased due to local effects within the defect complex. The stability of the Ga interstitials is further increased at surfaces. The present results suggest that the energetically favorable Ga interstitial atoms are much more abundant in GaAsN than the previously considered N defects, which have relatively large formation energies. Our synchrotron radiation core-level photoemission measurements support the computational results. The formation of harmful Ga interstitials should be reduced by incorporating large group IV B atoms in GaAsN.
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| 6. |
- Li, Xiaoqing, et al.
(author)
-
Elastic properties of vanadium-based alloys from first-principles theory
- 2012
-
In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 86:1, s. 014105-
-
Journal article (peer-reviewed)abstract
- The effect of Cr and Ti on the fundamental mechanical properties of V-Cr-Ti alloys has been investigated using the all-electron exact muffin-tin orbitals method in combination with the coherent-potential approximation. The static lattice constant and elastic parameters have been calculated for the body-centered-cubic V1-x-yCrxTiy (0 <= x,y <= 0.1) random solid solution as a function of composition. Our theoretical predictions are in good agreement with the available experimental data. Alloys along the equicomposition region are found to exhibit the largest shear and Young's modulus as a result of the opposite alloying effects obtained for the two cubic shear elastic constants. The classical solid-solution hardening (SSH) model predicts larger strengthening effect in V1-yTiy than in V1-xCrx. By considering a phenomenological expression for the ductile-brittle transition temperature (DBTT) in terms of Peierls stress and SSH, it is shown that the present theoretical results can account for the variations of DBTT with composition.
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| 7. |
- Li, Xiaoqing, et al.
(author)
-
Tensile strain-induced softening of iron at high temperature
- 2015
-
In: Scientific Reports. - : Nature Publishing Group. - 2045-2322. ; 5
-
Journal article (peer-reviewed)abstract
- In weakly ferromagnetic materials, already small changes in the atomic configuration triggered by temperature or chemistry can alter the magnetic interactions responsible for the non-random atomicspin orientation. Different magnetic states, in turn, can give rise to substantially different macroscopic properties. A classical example is iron, which exhibits a great variety of properties as one gradually removes the magnetic long-range order by raising the temperature towards its Curie point of T-C(degrees) = 1043 K. Using first-principles theory, here we demonstrate that uniaxial tensile strain can also destabilise the magnetic order in iron and eventually lead to a ferromagnetic to paramagnetic transition at temperatures far below T-C(degrees). In consequence, the intrinsic strength of the ideal single-crystal body-centred cubic iron dramatically weakens above a critical temperature of similar to 500 K. The discovered strain-induced magneto-mechanical softening provides a plausible atomic-level mechanism behind the observed drop of the measured strength of Fe whiskers around 300-500 K. 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 here in the case of Fe-Co alloy.
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| 8. |
- Lu, Song, et al.
(author)
-
Magnetic effect on the interfacial energy of the Ni(111)/Cr(110) interface
- 2014
-
In: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 26:35, s. 355001-
-
Journal article (peer-reviewed)abstract
- The work of separation and interfacial energy of the Ni(1 1 1)/Cr(1 1 0) interface are calculated via first-principles methods. Both coherent and semicoherent interfaces are considered. We find that magnetism has a significant effect on the interfacial energy, i.e. removing magnetism decreases the interfacial energy of the semicoherent interface by around 50% . Electronic, magnetic and atomic structures at the interface are discussed. An averaging scheme is used to estimate the work of separation and interfacial energy of semicoherent interfaces based on the results of coherent interfaces. The limitations of the scheme are discussed.
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| 9. |
- Lu, Song, et al.
(author)
-
Theory of transformation-mediated twinning
- 2023
-
In: PNAS NEXUS. - : Oxford University Press (OUP). - 2752-6542. ; 2:1
-
Journal article (peer-reviewed)abstract
- High-density and nanosized deformation twins in face-centered cubic (fcc) materials can effectively improve the combination of strength and ductility. However, the microscopic dislocation mechanisms enabling a high twinnability remain elusive. Twinning usually occurs via continuous nucleation and gliding of twinning partial dislocations on consecutive close-packed atomic planes. Here we unveil a completely different twinning mechanism being active in metastable fcc materials. The transformation-mediated twinning (TMT) is featured by a preceding displacive transformation from the fcc phase to the hexagonal close-packed (hcp) one, followed by a second-step transformation from the hcp phase to the fcc twin. The nucleation of the intermediate hcp phase is driven by the thermodynamic instability and the negative stacking fault energy of the metastable fcc phase. The intermediate hcp structure is characterized by the easy slips of Shockley partial dislocations on the basal planes, which leads to both fcc and fcc twin platelets during deformation, creating more twin boundaries and further enhancing the prosperity of twins. The disclosed fundamental understanding of the complex dislocation mechanism of deformation twinning in metastable alloys paves the road to design novel materials with outstanding mechanical properties.
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| 10. |
- Punkkinen, Marko Patrick John, et al.
(author)
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Adhesion of the iron-chromium oxide interface from first-principles theory
- 2013
-
In: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 25:49, s. 495501-
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Journal article (peer-reviewed)abstract
- We determine the interface energy and the work of separation of the Fe/Cr2O3 interface using first-principles density functional theory. Starting from different structures, we put forward a realistic interface model that is suitable to study the complex metal-oxide interaction. This model has the lowest formation energy and corresponds to an interface between Fe and oxygen terminated Cr2O3. The work of separation is calculated to be smaller than the intrinsic adhesion energy of pure Fe or Cr2O3, suggesting that stainless steel surfaces should preferentially break along the metal-oxide interface. The relative stabilities and magnetic interactions of the different interfaces are discussed. Next we introduce Cr atoms into the Fe matrix at different positions relative to the interface. We find that metallic Cr segregates very strongly to the (FeCr)/Cr2O3 interface, and increases the separation energy of the interface, making the adhesion of the oxide scale mechanically more stable. The Cr segregation is explained by the enthalpy of formation.
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| 11. |
- Punkkinen, Marko P. J., et al.
(author)
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Structure of ordered oxide on InAs(100) surface
- 2012
-
In: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 606:23-24, s. 1837-1841
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Journal article (peer-reviewed)abstract
- It was recently found that oxygen induces ordered reconstructions on several III-V surfaces. The most oxygen-rich reconstruction shows (3x1) periodicity. Based on first-principles investigations, a detailed atomic model is presented for this reconstruction. The uncommon periodicity is attributed to the highly stable In - O - In trilayer below surface which also leads to stabilizing additional bonds within the surface layer. The strain induced by the trilayer is more effectively accommodated within the (3 x 1) reconstruction than within the competing (2 x 1) reconstruction due to smaller number of dimers. It is proposed that the experimentally found semiconductivity is reached by substitutional atoms within the surface layer. Suitable substitution preserves the magnitude of the bulk band gap. (C) 2012 Elsevier B.V. All rights reserved.
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| 12. |
- Punkkinen, M. P. J., et al.
(author)
-
Thermodynamics of the pseudobinary GaAs1-xBix (0 ≤ x ≤ 1) alloys studied by different exchange-correlation functionals, special quasi-random structures and Monte Carlo simulations
- 2015
-
In: Computational Condensed Matter. - : Elsevier. - 2352-2143. ; 5, s. 7-13
-
Journal article (peer-reviewed)abstract
- GaAs1-xBix alloys have useful properties for many optoelectronic applications. Although the crystal growth by molecular beam epitaxy is influenced by kinetics, it is also important to understand the thermodynamics of the alloys. The Gibbs formation energies and the immiscibility curve of the totally disordered GaAs1-xBix (0 ≤ x ≤ 1) are determined using special quasi-random structures (SQS) of different sizes, different exchange-correlation functionals and Monte Carlo simulations. The local density approximation gives slightly larger mixing enthalpies and smaller Bi solubilities than the generalized gradient approximation for the GaAs1-xBix alloys, if the semiconducting GaAs and metallic Ga and Bi are reference states. The 64 and 512 atom SQS give similar mixing enthalpies, except at x = 0.5, where as eight atom SQS overestimate the mixing enthalpy significantly. The disordered alloys are more stable than the most stable ordered phases except at very low temperatures. The spinodal transformation temperature corresponds approximately to the latest experimental results, if the separated systems can conform to their own lattice constants. The spinodal transformation temperature is decreased very significantly, if the decomposing phases are constrained to maintain lattice coherence with the substrate. The strain energies of the alloys and decomposing phases are large also at the surfaces.
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| 13. |
- Sun, Xun, et al.
(author)
-
Large recoverable strain with suitable transition temperature in TiNb-based multicomponent shape memory alloys : First-principles calculations
- 2021
-
In: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 221
-
Journal article (peer-reviewed)abstract
- TiNb-based shape memory alloys (SMAs) have great potentials in biomaterials. However, high transition temperature or small recoverable strain limit their application. Using first-principles method, we systematically study the recoverable strain and transition temperature of TiNb-based binary, ternary, and high-entropy alloys (HEAs), and aim to lower the transition temperature and improve the recoverable strain at the same time. We find that the employed approach describes accurately the lattice strain by comparing with the available experimental results. It is well known that there is a positive correlation between lattice strain and recoverable strain in SMAs. Thus, we have evaluated the magnitude of recoverable strain of SMAs by calculating the lattice strain. Meanwhile, we correlate the available measured martensitic transformation start temperature (M-s) with the calculated energy difference between beta and alpha'' phases in Ti-Nb binary alloys. According to this relation, we evaluate the M-s in other TiNb-based alloys. We find that Zr is a good alloying element that can decrease considerably the M-s and keep the lattice (recoverable) strain almost unchanged simultaneously. Finally, an Al-containing Ti24Nb25Zr24S24Al3 HEA has been designed to have simultaneously large recoverable strain and low transition temperature.
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| 14. |
- Sun, Xun, et al.
(author)
-
Phase selection rule for Al-doped CrMnFeCoNi high-entropy alloys from first-principles
- 2017
-
In: Acta Materialia. - : PERGAMON-ELSEVIER SCIENCE LTD. - 1359-6454 .- 1873-2453. ; 140, s. 366-374
-
Journal article (peer-reviewed)abstract
- Using ab initio alloy theory, we investigate the lattice stability of paramagnetic AlxCrMnFeCoNi (0 <= x <= 5) high-entropy alloys considering the competing body-centered cubic (bcc) and face-centered cubic (fcc) crystal structures. The theoretical lattice constants increase with increasing x, in good agreement with experimental data. Upon Al addition, the crystal structure changes from fcc to bcc with a broad two-phase field region, in line with observations. The magnetic transition temperature for the bcc structure strongly decreases with x, whereas that for the fee structure shows weak composition dependence. Within their own stability fields, both structures are predicted to be paramagnetic at ambient conditions. Bain path calculations support that within the duplex region both phases are dynamically stable. As compared to AlxCrFeCoNi, equiatomic Mn addition is found to shrink the stability range of the fcc phase and delay the appearance of the bcc phase in terms of Al content, thus favoring the duplex region in 3d-metals based high-entropy alloys.
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| 15. |
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| 16. |
- Xiong, Wei, et al.
(author)
-
Magnetic phase diagram of the Fe-Ni system
- 2011
-
In: Acta Materialia. - : PERGAMON-ELSEVIER SCIENCE LTD. - 1359-6454 .- 1873-2453. ; 59:2, s. 521-530
-
Journal article (peer-reviewed)abstract
- Magnetic phase diagrams of body-centered cubic and face-centered cubic Fe-Ni alloys were constructed using available experimental data and ab initio calculations. The results show that significant improvements in the "standard" diagrams (handbooks and CALPHAD databases) are required. The present work demonstrates that the CALPHAD magnetic model is not sophisticated enough to describe the Fe-Ni system. In addition, a new thermodynamic description of the lattice stability for pure Ni is urgently needed, since the recommended magnetic properties for CALPHAD modeling are distinct from the experimental and ab initio results. This work indicates that the construction of magnetic phase diagrams is indispensable during the phase transformation study of magnetic systems. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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| 17. |
- Yang, Yaochun, et al.
(author)
-
Ab initio study of the elastic properties of body-centered cubic Ti-Mo-based alloys
- 2020
-
In: Computational materials science. - : Elsevier. - 0927-0256 .- 1879-0801. ; 172
-
Journal article (peer-reviewed)abstract
- Using ab initio alloy theory, we systemically investigate the effect of alloying elements on the elastic properties of body-centered cubic (bcc) Ti1-x-yMoxMy (0.05 <= x <= 0.2; 0 <= y <= 0.4; M = Mg, Mn, Ni, Zr, Nb, and W) alloys. The theoretical single-crystal and polycrystalline elastic moduli of Ti1-xMox (0.05 <= x <= 0.2) agree well with the available experimental values and previous theoretical data. The lattice parameters of Ti-Mo-M ternary alloys significantly increase (decrease) with increasing Mg and Zr (Mn and Ni) contents, while remain almost constant for Nb and W additions. It is found that Mg is a promising alloying element that could decrease the Young's modulus of bcc Ti-Mo alloys, but its content should be as small as possible since the stability of the beta phase decreases with increasing Mg concentration. On the other hand, Mn, Ni, Nb, Zr, and W enhance the Young's modulus and the stability of the beta phase.
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| 18. |
- Zhang, Hualei, et al.
(author)
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Ab initio calculations of elastic properties of bcc Fe-Mg and Fe-Cr random alloys
- 2009
-
In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 79:22
-
Journal article (peer-reviewed)abstract
- Using the ab initio exact muffin-tin orbitals method in combination with the coherent-potential approximation, we have calculated the elastic parameters of ferromagnetic Fe1-mMgm (0 < m < 0.1) and Fe1-cCrc (0 < c < 0.2) random alloys in the body-centered cubic (bcc) crystallographic phase. Results obtained for Fe1-cCrc demonstrate that the employed theoretical approach accurately describes the experimentally observed composition dependence of the polycrystalline elastic moduli of Fe-rich alloys encompassing maximum similar to 10% Cr. The elastic parameters of Fe-Cr alloys are found to exhibit anomalous composition dependence around 5% Cr. The immiscibility between Fe and Mg at ambient conditions is well reproduced by the present theory. The calculated lattice parameter for the Fe-Mg regular solid solution increases by similar to 1.95% when 10% Mg is introduced in Fe, which corresponds approximately to 11% decrease in the average alloy density, in perfect agreement with the experimental finding. At the same time, we find that all of the elastic parameters of bcc Fe-Mg alloys decrease almost linearly with increasing Mg content. The present results show a much stronger alloying effect for Mg on the elastic properties of alpha-Fe than that for Cr. Our results call for further experimental studies on the mechanical properties of the Fe-Mg system.
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| 19. |
- Zhang, Hualei, et al.
(author)
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Ab initio determination of the elastic properties of ferromagnetic body-centered cubic Fe-Mn-Al alloys
- 2015
-
In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 118:10
-
Journal article (peer-reviewed)abstract
- The elastic properties of ferromagnetic Fe1-x-yMnyAlx (0 <= x <= 0.5, y = 0, 0.1, and 0.2) random solid solutions in the body-centered cubic (bcc) crystallographic phase have been investigated using the ab initio exact muffin-tin orbitals method in combination with the coherent-potential approximation. Comparison with the experimental data demonstrates that the employed theoretical approach accurately captures the observed composition dependence of the lattice parameter. The predicted elastic parameters follow complex composition dependence. The C-11, C-12, and C' = (C-11 - C-12)/2 single-crystal elastic constants, the bulk (B), shear (G), and Young's (E) moduli, and the Cauchy pressure (C-12 - C-44) mainly decrease with increasing Al content, whereas the Zener anisotropy ratio (C-44/C') strongly increases with x. C-44 exhibits a non-linear x dependence. The Poisson (v) and Pugh (B/G) ratios first decrease with x but show non-monotonous behavior in high-Al alloys. In terms of the Pugh criterion, these trends suggest an increased brittleness in Al-containing alloys. Manganese has a complex non-monotonous effect on B/G in low-Al alloys (below similar to 15 at. % Al) but enhances the brittleness of the bcc solid solution in large-Al regime. The peculiar Mn alloying effect is explained in terms of magneto-volume mechanisms.
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| 20. |
- Zhang, Hualei, et al.
(author)
-
Alloying effects on the elastic parameters of ferromagnetic and paramagnetic Fe from first-principles theory
- 2011
-
In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 110:7, s. 073707-
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Journal article (peer-reviewed)abstract
- The elastic properties of paramagnetic face-centered-cubic (fcc) Fe(1-x)M(x) (M = Al, Si, V, Cr, Mn, Co, Ni, and Rh; 0 <= x <= 0.1) random alloys are investigated using the exact muffin-tin orbitals density functional method in combination with the coherent-potential approximation. We find that the theoretical lattice parameter of fcc Fe is strongly enlarged by Al, V, and Rh and slightly reduced by Si, Cr, and Co, while it remains nearly constant with Mn and Ni. Both positive and negative alloying effects appear for the elastic constants C(ij)(x) of fcc Fe. These findings are in contrast to those obtained for ferromagnetic body-centered-cubic (bcc) Fe alloys, where all alloying elements considered here are predicted to enlarge the lattice parameter and decrease the C(11)(x) and C(12)(x) elastic constants of bcc Fe. With some exceptions, alloying has much larger effects on ferromagnetic bcc alloys than on paramagnetic fcc ones. Based on the theoretical elastic parameters of the paramagnetic fcc and ferromagnetic bcc phases, simple parameterizations in terms of chemical composition of the equilibrium lattice constants, single-crystal elastic constants, and polycrystalline elastic moduli of Fe-based alloys are presented.
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| 21. |
- Zhang, Hualei, et al.
(author)
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Anomalous elastic hardening in Fe-Co alloys at high temperature
- 2014
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 89:18, s. 184107-
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Journal article (peer-reviewed)abstract
- The elastic moduli of Fe1-cCoc (c <= 0.2) alloys are found to decrease strongly with increasing temperature, but show very weak alloying effects for both low-temperature ferromagnetic and high-temperature paramagnetic states. For temperatures slightly below and around the Curie temperature of Fe, Co addition significantly increases the elastic moduli. The variation of the tetragonal shear elastic constant upon 20% Co addition increases from a small negative value to more than 135% as the temperature rises from 0 to 1200 K. The expected elastic softening in the case of Al doping is not confirmed. Both anomalous trends are ascribed to the interplay between intrinsic chemical effects, magnetism, and temperature.
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| 22. |
- Zhang, Hualei, et al.
(author)
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Density-functional study of paramagnetic iron
- 2011
-
In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 84:14, s. 140411-
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Journal article (peer-reviewed)abstract
- By using density-functional theory in combination with the coherent-potential approximation and the disordered local magnetic moment picture, we demonstrate that the competing high-temperature cubic phases of paramagnetic Fe correspond to two distinct total energy minima in the tetragonal (Bain) configurational space. Both the face-centered-cubic (fcc) and the body-centered-cubic (bcc) lattices are dynamically stable, and at static conditions the fcc structure is found to be the thermodynamically stable phase. The theoretical bcc and fcc bulk parameters are in agreement with the experimental data. Due to the shallow energy minimum around the bcc structure, increasing temperature is predicted to stabilize the bcc (δ) phase against the fcc (γ) one.
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| 23. |
- Zhang, Hualei, et al.
(author)
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Elastic anomalies in Fe-Cr alloys
- 2013
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In: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 25:19, s. 195501-
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Journal article (peer-reviewed)abstract
- Using ab initio alloy theory, we determine the elastic parameters of ferromagnetic and paramagnetic Fe1-cCrc (0 <= c <= 1) alloys in the body centered cubic crystallographic phase. Comparison with the experimental data demonstrates that the employed theoretical approach accurately describes the observed composition dependence of the polycrystalline elastic moduli. The predicted single-crystal elastic constants follow complex anomalous trends, which are shown to originate from the interplay between magnetic and chemical effects. The nonmonotonic composition dependence of the elastic parameters has marked implications on the micro-mechanical properties of ferrite stainless steels.
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| 24. |
- Zhang, Hualei, et al.
(author)
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Elastic properties of AlxCrMnFeCoNi (0 <= x <= 5) high-entropy alloys from ab initio theory
- 2018
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In: Acta Materialia. - : PERGAMON-ELSEVIER SCIENCE LTD. - 1359-6454 .- 1873-2453. ; 155, s. 12-22
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Journal article (peer-reviewed)abstract
- Using ab initio calculations, we investigate the elastic properties of paramagnetic AlxCrMnFeCoNi (0 <= x <= 5) high -entropy alloys (HEAs) in both body-centered cubic (bcc) and face-centered cubic (fcc) structures. Comparison with available experimental data demonstrates that the employed approach describes accurately the elastic moduli. The predicted lattice constants increase monotonously with Al addition, whereas the elastic parameters exhibit complex composition dependences. 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 due to the relatively low bulk modulus and C-12 elastic constant, which in turn are consistent with the relatively low cohesive energy. The present findings in combination with the experimental data suggest anomalous metallic character for the HEAs system. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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| 25. |
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| 26. |
- Zhang, Hualei, 1981-
(author)
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Elastic Properties of Iron Alloys from First-Principles Theory
- 2011
-
Doctoral thesis (other academic/artistic)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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| 27. |
- Zhang, Hualei, et al.
(author)
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First-principles study of solid-solution hardening in steel alloys
- 2012
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In: Computational materials science. - : Elsevier BV. - 0927-0256 .- 1879-0801. ; 55, s. 269-272
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Journal article (peer-reviewed)abstract
- Materials with excellent mechanical properties, such as light mass combined with remarkable hardness and toughness, are technologically important not least for automotive and other transport applications. Solid solution strengthening, due to dislocation pinning by impurities, is an effective route to enhance the intrinsic hardness of alloys. In the present work, we use advanced quantum theory to reveal the mechanical characteristics of iron alloys within and beyond their thermodynamic stability fields. Among the considered alloying elements, magnesium strongly reduces the density of the host alloys and significantly enhances the hardness. Our findings suggest that stainless steel grades containing a few percent of magnesium are promising engineering materials for high-strength and light-weight designs.
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| 28. |
- Zhang, Hualei, et al.
(author)
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Single-crystal elastic constants of ferromagnetic bcc Fe-based random alloys from first-principles theory
- 2010
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 81:18, s. 184105-
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Journal article (peer-reviewed)abstract
- The elastic properties of ferromagnetic Fe1-xMx (M=Al, Si, V, Cr, Mn, Co, Ni, and Rh; 0 <= x <= 0.1) random alloys in the body-centered-cubic (bcc) crystallographic phase have been studied using the all-electron exact muffin-tin orbitals method in combination with the coherent-potential approximation. The theoretical lattice parameters and the single-crystal elastic constants agree well with the available experimental data. The most significant alloying effects are found for Al, Si, and Ni additions. All elements enlarge the lattice parameter and decrease the C-11, C-12, and C' elastic constants and the bulk modulus of bcc Fe. At the same time, C-44 is found to increase with Al, Si, V, Cr, or Mn and remain nearly constant with Co, Ni, and Rh. Accordingly, the elastic anisotropy of bcc Fe increases with all alloying elements considered here. The calculated alloying effects on the single-crystal elastic constants are shown to originate from volume effects in combination with the peculiar electronic structure of bcc Fe.
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| 29. |
- Zhang, Hualei, et al.
(author)
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Static equation of state of bcc iron
- 2010
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 82:13, s. 132409-
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Journal article (peer-reviewed)abstract
- Body-centered-cubic (bcc) iron is one of the most investigated solid-state systems. Using four different density-functional methods, we show that there is a magnetic transition close to the ground-state volume of bcc Fe, which originates from the particular magnetic band structure. The common equation of state functions, used to determine the basic ground-state physical quantities from the calculated total energies, cannot capture the physics of this magnetic transition leading to serious underestimation of the Fe bulk modulus. Ignorance of the magnetic transition found here is reflected by large scatter of the published theoretical bulk moduli of ferromagnetic bcc Fe. Due to the low performance of the exchange-correlation functionals, most of the erroneous results are accidentally in good agreement with the experimental values. The present finding is of fundamental importance, especially taking into account that bcc Fe is frequently used as a test system in assessing the performance of exchange-correlation approximations or total-energy methods.
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| 30. |
- Zhang, Hualei, et al.
(author)
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Theoretical elastic moduli of ferromagnetic bcc Fe alloys
- 2010
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In: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 22:27, s. 275402-
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Journal article (peer-reviewed)abstract
- The polycrystalline elastic parameters of ferromagnetic Fe1-xMx (M = Al, Si, V, Cr, Mn, Co, Ni, Rh; 0 <= x <= 0.1) random alloys in the body centered cubic (bcc) crystallographic phase have been calculated using first-principles alloy theory in combination with statistical averaging methods. With a few exceptions, the agreement between the calculated and the available experimental data for the polycrystalline aggregates is satisfactory. All additions considered here decrease the bulk modulus (B) and Poisson's ratio (nu) of bcc Fe. The complex composition dependence of the C-44 single-crystal elastic constant is reflected in the polycrystalline shear modulus (G), Young's modulus (E), and Debye temperature (Theta). The polycrystalline anisotropy of bcc Fe is increased by all additions, and Al, Si, Ni, and Rh yield the largest alloying effects.
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