| 1. |
- Al-Zoubi, Noura
(author)
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Describing Interstitials in Close-packed Lattices: First-principles Study
- 2011
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Doctoral thesis (other academic/artistic)abstract
- Describing interstitial atoms in intermetallics or simple mono-atomic close-packed metals is a straightforward procedure in common full-potential calculations. One establishes a sufficiently large supercell, introduces the interstitial impurity and performs the electronic structure and total energy calculation. Real systems, however, are rarely mono-atomic or ordered metals. In most of the cases, the matrix is a random or quasirandom mixture of several chemically and/or magnetically distinct components. Because of that a proper computational tool should incorporate advanced alloy theory and at the same time have sufficiently high accuracy to describe interstitial positions in close-packed solids. The purpose of the present thesis is to make a step towards solving this fundamental problem in computational materials science. To this end, in the first part of the thesis a prestudy on some selected metals and compounds was presented, and in the second part tools were applied to investigate the effect of interstitial carbon on the structural properties of steels. For the prestudy, the equation of state for the selected Al, Cu and Rh was investigated in two equivalent phases: in conventional face-centered-cubic lattice (fcc, str-I) and in a face-centered-cubic lattice with one atomic and three interstitial empty potentialwells per primitive cell (str-II). A proper basis set of the exact muffin-tin orbitals as well as a proper potential sphere radius were established by calculating the equilibrium Wigner-Seitz radius and bulk modulus of the above elements in str-I and str-II using the exact muffin-tin orbitals (EMTO) first-principle density functional method. It was found that for Al spd orbitals are sufficient to describe the equilibrium bulk properties in both structures, while for str-II Rh and Cu at least five orbitals (spdfg) are needed to get accurate equilibrium volume and bulk modulus. Furthermore, it was shown that in general, for the str-II type of structure (close-packed structure with interstitials) the optimized overlapping muffin-tin potential in combination with spdfg orbitals ensures well converged bulk properties. As an application of the above work in alloys, (i) the chemical reaction between hydrogen H2 molecule and ScAl1−xMgx (0≤x≤0.3) random alloys, (ii) the phase stability of the hydrogenated alloys in different structures and (iii) the hydrogen absorption/desorption temperatures were studied by calculating the Gibbs energy for the components of the reaction. Experimental and theoretical studies by Sahlberg et al . showed that the ScAl0.8Mg0.2 compound with CsCl structure absorbs hydrogen by decomposing into ScH2 with CaF2 structure and fcc Al0.8Mg0.2. This reaction was found to be very fast, even without adding catalyst, and fully reversible. The theoretical hydrogen absorption/desorption temperatures agree well with the experimental values. On the other hand, the stability field of the hydrogenated alloys was found to be strongly depends on Mg content and on the microstructure of the hydrogenated alloys. For a given microstructure, the critical temperature for hydrogen absorption/desorption increases with the Mg concentration. The second part of the thesis focused on steel materials with special emphases on the effect of interstitial carbon. Steels are considered to be one of the most important engineering materials. They are mainly composed of iron and carbon. Other alloying elements in steel are introduced to get specific properties like microstructure, corrosion resistance, hardness, brittleness, etc. In order to describe the effect of carbon interstitial in iron alloys, it is important to know how the substitutional alloying elements affect the softness and some other properties of iron alloys. For that reason, the alloying effects on the energetic and magnetic structure of paramagnetic Fe0.85Cr0.1M0.05 (M = Cr, Mn, Fe, Co and Ni) alloys along the tetragonal distortion path connecting the body centered cubic (bcc) and the face centered cubic (fcc) phases were investigated. It was shown that Cr stabilizes bcc phase and increases the energy barrier (relative to bcc phase) between fcc and bcc phases. Cobalt and Ni stabilize fcc structure. Cobalt increases whereas Ni slightly decreases the energy barrier relative to fcc structure. Manganese and iron have negligible effect on the structural energy difference as well as on the energy barrier along the Bain path. The local magnetic moments on Fe atoms have maximum values at bcc phase and minimum values at fcc phase. Cobalt atoms possess local magnetic moments only for tetragonal lattices with c/a < 1.30, and the Mn magnetic moments have almost constant value along the Bain path. The tetragonality of Fe-C martensite was discovered in 1928. Early experimental works showed that the tetragonality of Fe-C is linearly depends on C content. However, Later many observations indicated that the tetragonality of martensite is influenced also by alloying and interstitial carbon distributions. Very few ab initio studies focus on investigating the tetragonality of Fe-C based alloys. In this thesis the interstitial carbon in ferromagnetic Fe-based alloys and it is impact on the tetragonal lattice ratio of Fe matrix as well as the alloying effect on the tetragonality of Fe-C system were investigated. It was found that the ferromagnetic Fe-C system with C content ∼ 1.3 wt. % has a body-centered tetragonal (bct) structure with c/a ∼ 1.07. Alloying has an impact on the tetragonality; adding 5% Al, Co or Ni enhances while 5% Cr addition decreases the tetragonal lattice ratio. The electronic structure and total energy calculations from this thesis are based on firstprinciples exact muffin-tin orbitals method. The chemical and magnetic disorder was treated using coherent-potential approximation and the paramagnetic phase was modeled by the disordered local magnetic moments approach. Some test calculations involved also full-potential tools as implemented in Vienna ab-initio simulation package (VASP).
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| 2. |
- Choi, Young Won, et al.
(author)
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Density Functional Theory Description of Paramagnetic Hexagonal Close-Packed Iron
- 2022
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In: Materials. - : MDPI. - 1996-1944. ; 15:4
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Journal article (peer-reviewed)abstract
- The hexagonal close-packed (hcp) phase of iron is unstable under ambient conditions. The limited amount of existing experimental data for this system has been obtained by extrapolating the parameters of hcp Fe-Mn alloys to pure Fe. On the theory side, most density functional theory (DFT) studies on hcp Fe have considered non-magnetic or ferromagnetic states, both having limited relevance in view of the current understanding of the system. Here, we investigate the equilibrium properties of paramagnetic hcp Fe using DFT modelling in combination with alloy theory. We show that the theoretical equilibrium c/a and the equation of state of hcp Fe become consistent with the experimental values when the magnetic disorder is properly accounted for. Longitudinal spin fluctuation effects further improve the theoretical description. The present study provides useful data on hcp Fe at ambient and hydrostatic pressure conditions, contributing largely to the development of accurate thermodynamic modelling of Fe-based alloys.
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| 3. |
- Choi, Young Won, et al.
(author)
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Predicting the stacking fault energy of austenitic Fe-Mn-Al (Si) alloys
- 2020
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In: Materials & design. - : ELSEVIER SCI LTD. - 0264-1275 .- 1873-4197. ; 187
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Journal article (peer-reviewed)abstract
- Aluminum and silicon are common alloying elements for tuning the stacking fault energy (SFE) of high Mn steels. Today the theoretical investigations on the Fe-Mn-Al/Si systems using Density Functional Theory (DFT) are very scarce. In the present study, we employ a state-of-the-art longitudinal spin fluctuations (LSFs) model in combination with DFT for describing the magnetic effects in Fe-Mn based alloys at finite temperature. We find that the traditional DFT-floating spin results fail to explain the experimental trends. However, the DFT-LSFs approach properly captures the Al-induced increase and Si-induced decrease of the SFE of the base alloy in line with the room-temperature observations. This finding highlights the importance of LSFs in describing the Al/Si effects on the SEE of Fe-Mn based alloys. We point out that the effects of the non-magnetic Al and Si additions on the SEE are in fact determined by the magnetic state of the host matrix. In addition, we estimate the role of carbon addition in the alloying effects of Al and Si. The present results provide a convenient pathway to access the important mechanical parameters for designing advanced high-strength alloys.
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| 4. |
- Dong, Zhihua, et al.
(author)
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Plastic deformation modes in paramagnetic gamma-Fe from longitudinal spin fluctuation theory
- 2018
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In: International journal of plasticity. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0749-6419 .- 1879-2154. ; 109, s. 43-53
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Journal article (peer-reviewed)abstract
- Using an efficient first-principles computational scheme, we calculate the intrinsic stacking fault energy (gamma(isf) ) and the unstable stacking fault energy (gamma(usf)) of paramagnetic gamma-Fe as a function of temperature. The formation energies are derived from free energies accounting for thermal longitudinal spin fluctuations (LSFs). LSFs are demonstrated to be important for the accurate description of the temperature-dependent magnetism, intrinsic and unstable stacking fault energies, and have a comparatively large effect on gamma(isf) of gamma-Fe. Dominated by the magneto-volume coupling at thermal excitations, gamma(isf) of gamma-Fe exhibits a positive correlation with temperature, while gamma(usf )declines with increasing temperature. The predicted stacking fault energy of gamma-Fe is negative at static condition, crosses zero around 540 K, and reaches 71.0 mJ m(-2) at 1373 K, which is in good agreement with the experimental value. According to the plasticity theory formulated in terms of the intrinsic and unstable stacking fault energies, twinning remains a possible deformation mode even at elevated temperatures. Both the large positive temperature slope of gamma(usf) and the predicted high-temperature twinning are observed in the case of austenitic stainless steels.
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| 5. |
- Huang, Shuo, et al.
(author)
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Thermal expansion in FeCrCoNiGa high-entropy alloy from theory and experiment
- 2017
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In: Applied Physics Letters. - : AMER INST PHYSICS. - 0003-6951 .- 1077-3118. ; 110:24
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Journal article (peer-reviewed)abstract
- First-principle alloy theory and key experimental techniques are applied to determine the thermal expansion of FeCrCoNiGa high-entropy alloy. The magnetic transition, observed at 649 K, is accompanied by a significant increase in the thermal expansion coefficient. The phase stability is analyzed as a function of temperature via the calculated free energies accounting for the structural, magnetic, electronic, vibrational and configurational contributions. The single-and polycrystal elastic modulus for the ferro-and paramagnetic states of the face-centered and body-centered cubic phases are presented. By combining the measured and theoretically predicted temperature-dependent lattice parameters, we reveal the structural and magnetic origin of the observed anomalous thermal expansion behavior. Published by AIP Publishing.
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| 6. |
- Jo, Minho, et al.
(author)
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Theory for plasticity of face-centered cubic metals
- 2014
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In: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 111:18, s. 6560-6565
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Journal article (peer-reviewed)abstract
- The activation of plastic deformation mechanisms determines the mechanical behavior of crystalline materials. However, the complexity of plastic deformation and the lack of a unified theory of plasticity have seriously limited the exploration of the full capacity of metals. Current efforts to design high-strength structural materials in terms of stacking fault energy have not significantly reduced the laborious trial and error works on basic deformation properties. To remedy this situation, here we put forward a comprehensive and transparent theory for plastic deformation of face-centered cubic metals. This is based on a microscopic analysis that, without ambiguity, reveals the various deformation phenomena and elucidates the physical fundaments of the currently used phenomenological correlations. We identify an easily accessible single parameter derived from the intrinsic energy barriers, which fully specifies the potential diversity of metals. Based entirely on this parameter, a simple deformation mode diagram is shown to delineate a series of convenient design criteria, which clarifies a wide area of material functionality by texture control.
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| 7. |
- Lee, Jee-Yong, et al.
(author)
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The behaviour of stacking fault energy upon interstitial alloying
- 2017
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In: Scientific Reports. - : Nature Publishing Group. - 2045-2322. ; 7
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Journal article (peer-reviewed)abstract
- Stacking fault energy is one of key parameters for understanding the mechanical properties of face-centered cubic materials. It is well known that the plastic deformation mechanism is closely related to the size of stacking fault energy. Although alloying is a conventional method to modify the physical parameter, the underlying microscopic mechanisms are not yet clearly established. Here, we propose a simple model for determining the effect of interstitial alloying on the stacking fault energy. We derive a volumetric behaviour of stacking fault energy from the harmonic approximation to the energy-lattice curve and relate it to the contents of interstitials. The stacking fault energy is found to change linearly with the interstitial content in the usual low concentration domain. This is in good agreement with previously reported experimental and theoretical data.
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| 8. |
- Li, Ruihuan, et al.
(author)
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Stacking fault energy of face-centered cubic metals : thermodynamic and ab initio approaches
- 2016
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In: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 28:39
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Journal article (peer-reviewed)abstract
- The formation energy of the interface between face-centered cubic (fcc) and hexagonal close packed (hcp) structures is a key parameter in determining the stacking fault energy (SFE) of fcc metals and alloys using thermodynamic calculations. It is often assumed that the contribution of the planar fault energy to the SFE has the same order of magnitude as the bulk part, and thus the lack of precise information about it can become the limiting factor in thermodynamic predictions. Here, we differentiate between the interfacial energy for the coherent fcc(1 1 1)/hcp(0 0 0 1) interface and the 'pseudo-interfacial energy' that enters the thermodynamic expression for the SFE. Using first-principles calculations, we determine the coherent and pseudo-interfacial energies for six elemental metals (A1, Ni, Cu, Ag, Pt, and Au) and three paramagnetic Fe-Cr-Ni alloys. Our results show that the two interfacial energies significantly differ from each other. We observe a strong chemistry dependence for both interfacial energies. The calculated pseudo-interfacial energies for the Fe-Cr-Ni steels agree well with the available literature data. We discuss the effects of strain on the description of planar faults via thermodynamic and ab initio approaches.
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| 9. |
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| 10. |
- Li, Wei, et al.
(author)
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First-principles prediction of the deformation modes in austenitic Fe-Cr-Ni alloys
- 2016
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In: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 108:8
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Journal article (peer-reviewed)abstract
- First-principles alloy theory is used to establish the gamma-surface of Fe-Cr-Ni alloys as function of chemical composition and temperature. The theoretical stacking fault energy (SFE) versus chemistry and temperature trends agree well with experiments. Combining our results with the recent plasticity theory based on the gamma-surface, the stacking fault formation is predicted to be the leading deformation mechanism for alloys with effective stacking fault energy below similar to 18 mJ m(-2). Alloys with SFE above this critical value show both twinning and full slip at room temperature. Interestingly, twinning remains a possible deformation mode in addition to full slip even at elevated temperatures, in line with observations.
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