나노결정립 CoCrFeMnNi 고엔트로피합금의 열처리에 따른 이차상 형성 및 나노압입 크리프 거동 변화 연구

Abstract

In this study, the effects of heat treatment on the nano-scale creep behavior of CoCrFeMnNi highentropy alloy (HEA) processed by high-pressure torsion (HPT) was investigated through nanoindentation technique. Nanoindentation experiments with a Berkovich indenter were performed on HPT-processed alloy subjected to heat treatment at 450oC, revealing that the hardness of the HPT-processed alloy (HPT sample) significantly increased with the heat treatment time. The heat treatment-induced microstructural change in HPTprocessed alloy was analyzed using transmission electron microscopy, which showed the nano-sized Cr-, NiMn-, and FeCo-rich phases were formed in the HPT-processed alloy subjected to 10 hours of heat treatment (HPT+10A sample). To compare the creep behavior of HPT and HPT+10A samples, constant load nanoindentation creep experiments were performed using spherical indentation indenters with two different radii. It was revealed that the predominant mechanism for creep highly depended on the applied stress level. At low stress level, both HPT and HPT+10A samples were dominated by Coble creep. At high stress level, however, the mechanism transformed to dislocation creep for HPT sample, but continued to be Coble creep for HPT+10A sample, leading to higher creep resistance in the HPT+10A sample.