High-Entropy Alloys Design and Origin of Properties

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depiction of high-entropy alloy designs
Ti0.25CrFeNiAlx high-entropy alloy: (top) BCC-FCC energy difference (stability) vs. Al content (x), BCC for x < 0.65 is more stable. (bottom) Short-Range Order pairs in BCC (peak at H), indicate B2+L21 order, as observed. Atom-probe studies on thin-film or as-cast bulk samples show same order as theory. Impact: Thin-film synthesis more efficient at screening phase formation due to faster diffusion.

Multi-principal-element alloys, including near-equiatomic high-entropy alloys, experience partially-ordering or segregation as they are cooled. Our density-functional theory (DFT) based electronic-structure and SRO predictions supported by concentration-wave analysis are shown to be a powerful method for fast assessment of competing phases and their stability in multi-principal-element alloys.  For Ti0.25CrFeNiAlx alloys, experiments suggest a partially-ordered B2 phase, whereas CALculation of PHAse Diagrams (CALPHAD) predicts a region of L21+B2 coexistence. We employ our first-principles DFT-based electronic-structure approach to assess stability of phases of alloys with arbitrary compositions and Bravais lattices. In addition, DFT-based linear-response theory is utilized to predict Warren-Cowley short-range order (SRO), revealing competing long-range ordered phases. The SRO theory is uniquely analyzed using concentration-waves theory for occupation probabilities in partially-ordered states, permitting direct DFT assessment of alloys with partial order. 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 suggest more careful experiments in Al-rich region are needed.

P. Singh, A.V. Smirnov, A. Alam, Duane. D. Johnson*, First-principles prediction of incipient order in arbitrary high-entropy alloys, Acta Materialia 189, 248-254 (2020). DOI: 10.1016/j.actamat.2020.02.063

See also joint thin-film experiments & theory:  A. Marshal, P. Singh, et al.,  JALCOM (2020).   DOI: 10.1016/j.jallcom.2020.155178