Effects of lattice distortion and chemical short-range order on creep behavior of medium-entropy alloy CoCrNi

Creep behavior of multi-principal element alloys (MPEAs) is an intriguing topic to explore for their potential high-temperature applications. The challenge on this topic is to elucidate what effect the local chemical fluctuation has on the creep behavior of MPEAs. By using large-scale molecular dynamics (MD) simulations, we investigate the creep performances of CoCrNi medium-entropy alloys (MEAs) with and without chemical short-range order (CSRO) as well as the Average-atom (A-atom) counterpart (no lattice distortion (LD)) under different uniaxial tensile stresses at various elevated temperatures. A power-law model is adopted to analyze the implicit stress exponent and activation energy, which are associated with the creep mechanism and creep resistance, respectively. The results reveal CSRO rather than LD plays an important role in creep performance. Specifically, with the introduction of CSRO, the activation energy for creep in CoCrNi MEA is significantly increased and finally close to the activation energy for the diffusion of the Cr element that is the highest among the three elements (Co, Cr, and Ni). The CoCr clusters in the MEA with CSRO make it difficult to increase the shear strain in the creep process, resulting in a much lower creep rate than that of the CoCrNi MEA without CSRO. For the CoCrNi MEA without CSRO, grain boundary diffusion is the main creep mechanism. For the MEAs with CSRO, the predominant creep mechanism depends on the applied stress. At low stress, grain boundary diffusion still prevails and there is no obvious dislocation glide or grain boundary sliding. When the applied stress is high enough, grain boundary diffusion and dislocation slip dominate the creep behavior, accompanied by the occurrence of grain boundary sliding.