| 1. |
- Gangaprasad Rao, Smita, et al.
(author)
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Phase formation in CrFeCoNi nitride thin films
- 2023
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In: Physical Review Materials. - : AMER PHYSICAL SOC. - 2475-9953. ; 7:4
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Journal article (peer-reviewed)abstract
- As a single-phase alloy, CrFeCoNi is a face centered cubic (fcc) material related to the archetypical highentropy Cantor alloy CrFeCoNiMn. For thin films, CrFeCoNi of approximately equimolar composition tends to assume an fcc structure when grown at room temperature by magnetron sputtering. However, the single-phase solid solution state is typically not achieved for thin films grown at higher temperatures. The same holds true for Cantor alloy-based ceramics (nitrides and oxides), where phase formation is extremely sensitive to process parameters such as the amount of reactive gas. This study combines theoretical and experimental methods to understand the phase formation in nitrogen-containing CrFeCoNi thin films. Density functional theory calculations considering three competing phases (CrN, Fe-Ni and Co) show that the free energy of mixing, Delta G of (CrFeCoNi)(1-x)N-x solid solutions has a maximum at x = 0.20-0.25, and AG becomes lower when x < 0.20 and x > 0.25. Thin films of (CrFeCoNi)1-xNx (0.14 >= x <= 0.41) grown by magnetron sputtering show stabilization of the metallic fcc when x <= 0.22 and the stabilization of the NaCl B1 structure when x > 0.33, consistent with the theoretical prediction. In contrast, films with intermediate amounts of nitrogen (x = 0.22) grown at higher temperatures show segregation into multiple phases of CrN, Fe-Ni-rich and Co. These results offer an explanation for the requirement of kinetically limited growth conditions at low temperature for obtaining single-phase CrFeCoNi Cantor-like nitrogen-containing thin films and are of importance for understanding the phase-formation mechanisms in multicomponent ceramics. The results from the study further aid in making correlations between the observed mechanical properties and the crystal structure of the films.
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| 2. |
- Mukhamedov, Boburjon, et al.
(author)
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Machine learning prediction of thermodynamic and mechanical properties of multicomponent Fe-Cr-based alloys
- 2021
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In: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 5:10
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Journal article (peer-reviewed)abstract
- We apply machine learning algorithms to optimize thermodynamic and elastic properties of multicomponent Fe-Cr alloys with additions of Ni, Mo, Al, W, V, and Nb. The target properties are mixing enthalpy, Youngs elastic modulus, and the ratio between shear and bulk moduli, which is often used as a phenomenological criterion for a materials ductility. We thoroughly analyze the descriptors that provide the robust performance of the machine learning models. Next, the iterative active learning method is used for the optimization of the chemical composition to simultaneously improve both thermodynamic stability and the elastic properties of Fe-Cr-based alloys. As a result, we predict compositions of thermodynamically stable alloys with improved mechanical properties, demonstrating the high potential of data-driven computational design in the field of materials for nuclear energy applications.
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| 3. |
- Mukhamedov, Boburjon, et al.
(author)
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Suggesting the high-pressure route to metastable high entropy alloys with ultra-large lattice distortions
- 2024
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In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 136:4
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Journal article (peer-reviewed)abstract
- One of the key features of high entropy alloys is their severe local lattice distortions, which can lead to beneficial and unusual combinations of mechanical properties. Most reported high entropy alloys (HEAs) are made of size- and chemically similar elements, but if including a component with a distinct size difference was possible, further increase in distortions could be foreseen. However, such additions have typically been disregarded as such alloys are thought to be unmixable. Here, we predict the possibility of mixing such large elements into well-studied HEAs with the help of high or moderate pressure. Miscibility of three large elements in two alloy systems has been studied: Mg/Zr addition in CoCrFeMnNi alloy and Pb addition in MoNbTaTiV alloy. These new compositions are metastable at ambient conditions but can be stabilized with high pressures and probably quenched to ambient with suitable temperature-pressure protocols. We predict that some of the promising candidates can be thermodynamically stabilized at 15-20 GPa, or even lower pressures at elevated temperatures. If synthesized and quenched to ambient conditions, these metastable HEAs would possess ultra-large lattice distortions.
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| 4. |
- Mukhamedov, Boburjon, et al.
(author)
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Tetragonal distortion in magnetron sputtered bcc-W films with supersaturated carbon
- 2022
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In: Materials & design. - : Elsevier Science Ltd. - 0264-1275 .- 1873-4197. ; 214
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Journal article (peer-reviewed)abstract
- Carbon has a low solid solubility in bcc tungsten at equilibrium. However, metastable supersaturated solid solutions can be synthesized with magnetron sputtering. Here, we present a systematic study on the phase stability and mechanical properties of such supersaturated W-C solid solutions. H-2h scans show a split of the 200/020 and the 002 peaks for supersaturated films which is explained by a tetragonal distortion of the bcc structure. This split increases with increasing C content and is maximized at 4 at.% C, where we observe an a/b axis of 3.15-3.16 A and a c-axis of 3.21-3.22 A. We performed first-principles calculations of lattice parameters, mixing enthalpies, elastic constants and polycrystalline elastic moduli for cubic and tetragonal W-C solid solutions. Calculations show that tetragonal structure is more stable than the bcc supersaturated solid solution and the calculated lattice parameters and Youngs moduli follow the same trends as the experimental ones as a function of C concentration. The results suggest that supersaturated films with lattice distortion can be used as a design approach to improve the properties of transition metal films with a bcc structure. (c) 2022 The Authors. Published by Elsevier Ltd.
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