Abstract
Multi-component solid-solution alloys, including high-entropy alloys, experience segregation or partially-ordering as they are cooled to lower temperatures. For Ti0.25CrFeNiAlx, experiments suggest a partially-ordered B2 phase, whereas CALculation of PHAse Diagrams (CALPHAD) predicts a region of L21+B2 coexistence. We employ first-principles KKR-CPA electronic-structure to assess stability of phases with arbitrary order having a specific Bravais lattice and composition, as it handles chemical disorder by direct configurational averaging during density-functional theory charge self-consistency. In addition, KKR-CPA linear-response theory is used to predict atomic short-range order (SRO) in the disorder phase, which reveals potentially competing long-range ordered (LRO) phases. With Warren-Cowley pair correlations defined relative to the diffraction lattice, SRO can be analyzed by concentration-waves (site-occupation probabilities) for any partially-ordered cells (including estimated energy gains) that can then be assess directly by KKR-CPA calculations. Our results are in good agreement with experiments and CALPHAD in Al-poor regions (x ≤0.75) and with CALPHAD in Al-rich region ($0.75 ≤x≤1), and they suggest more careful experiments in Al-rich region are needed. Our first-principles KKR-CPA electronic-structure and SRO predictions with additional concentration-wave analysis are shown to be a powerful and fast method to identify and assess competing phases in complex solid-solution alloys.
Highlights
Multi-principal-element alloys or complex solidsolution alloys, of which high-entropy alloys (HEAs) are a subset, have established a new paradigm in alloy design [1], but there are many fundamental science questions unanswered
The short-range order (SRO) theory can predict the eigenvectors at the order-disorder transition with respect to the formation of short-wavelength concentration waves [23, 24]. These eigenvectors can be used to characterize the potential ordered structures, e.g., B2 phase in between A1 (FCC) and A2 (BCC) alloys, with site-probabilities modulated in a wave-like periodicity [23,24,25,26], all determined from the underlying electronic structure of each specific HEA composition
favorable chemical interactions amongst between A1 (FCC) is stable over BCC by 65%Al, where the alloy is in a small two-phase region until 100%Al
Summary
Department of Physics, Indian Institute of Technology, Bombay, Powai, Mumbai 400076, India Ames Laboratory, U.S Department of Energy, Iowa State University, Ames, Iowa 50011, USA Materials Science & Engineering, Iowa State University, Ames, Iowa 50011, USA arXiv:1911.01602v2 [cond-mat.mtrl-sci] 28 Feb 2020
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