Role of local chemical fluctuations in the melting of medium entropy alloy CoCrNi

Medium- to high-entropy alloys are characterized by fluctuations in chemical composition, i.e., lattice distortion (LD) and chemical short-range ordering (CSRO). However, their roles in the melting of these alloys are still unclear. Using a combination of molecular dynamics and x-ray diffraction simulations, we investigate the effects of LD and CSRO on the melting of single crystalline CoCrNi, a medium entropy alloy (MEA). LD accelerates the melting process and reduces the melting temperature by lowering the energy barrier of the nucleation of amorphous clusters and then promoting the formation of amorphization. By contrast, CSRO divides CoCrNi MEA into CoCr clusters and Ni segregated regions, localizing LD to the CoCr regions and the boundaries between CoCr clusters and Ni regions. Such a LD localization reduces the area for the nucleation of the amorphous clusters in the ordered MEA at the start of melting, resulting in a lower free energy and, thus, a much higher melting temperature than the random MEA without CSRO. As the temperature rises in the ordered MEA, the degree of CSRO decreases slightly in the beginning and then experiences a rapid reduction in the last stage of melting.