Relationship between Lattice Strain and Ordering Tendency in Medium-Entropy Alloys

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In this study, thermodynamic quantities were investigated using cluster expansion and the variational method for high-entropy and medium-entropy alloys (HEAs and MEAs, respectively). In the alloy systems combined with elements that tend to form HEAs, the absolute enthalpy value was small, and the entropy term was close to the value of an ideal solution. The tendency of ordering and the accompanying change in mean square atomic displacement (MSAD) were investigated in detail for four MEAs; namely, the FeCoNi, MnFeNi, MnCoNi, and CrCoNi systems. For these alloys, the ordering tendency and the phase-separation tendency in the low-temperature region were investigated. In addition, the presence of short-range order (SRO) in the high-temperature range was confirmed from the analysis using the Warren–Cowley order parameter. Thermal equilibrium structures were used to evaluate the MSADs at various temperatures. Long-range ordering developing at low temperature tended to reduce MSAD, although in the high temperature range the effect on MSAD from the SRO was very small.

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