Structural evolution of short-range order in CrCoNi and MnCoNi medium-entropy alloys

Short-range order (SRO) is predicted in Cr- or Mn-containing face-centered-cubic (fcc) concentrated solid-solution alloys. One possible reason for SRO is the magnetic frustration of parallel spin pairs. The presence of SRO is expected to improve physical and mechanical properties. However, changes in atomic and magnetic structures induced by SRO are not clear. In the current study, the SRO in fcc CrCoNi and MnCoNi medium-entropy alloys was investigated using first-principles-based Monte Carlo simulations. In the initial stage of SRO, ${L1}_{2}$-type ordering occurs by the formation of second nearest-neighbor (NN) Cr-Cr or Mn-Mn bonds with decreasing first NN Cr-Cr or Mn-Mn bonds. These SROs originate from the energy gain caused by the decrease in the number of Cr-Cr or Mn-Mn parallel pairs. After the initial stage of SRO in MnCoNi, Mn-rich and Mn-poor layers were formed along one of the $\ensuremath{\langle}100\ensuremath{\rangle}$ directions, leading to $L{1}_{0}$-type ordering. Antiparallel Mn-Mn pairs were formed in the Mn-rich layers. In contrast, in CrCoNi, the occupation of Cr atoms on the {110} planes in every three layers was promoted after the initial stage of SRO. The difference in the SRO after ${L1}_{2}$-type ordering was considered to arise from the lower magnetic moment of the Cr atoms in CrCoNi compared with that of the Mn atoms in MnCoNi. The energy gain owing to the formation of SRO suggests that MnCoNi possesses a larger driving force for SRO formation than CrCoNi.