Influence of Annealing on Microstructure and Mechanical Properties of a Nanocrystalline CrCoNi Medium-Entropy Alloy

Abstract

An equiatomic CrCoNi medium-entropy alloy was subjected to high-pressure torsion.This process led to a refinement of the microstructure to a grain size of about 50 nm, combined witha strong increase in the materials hardness. Subsequently, the thermodynamic stability of the mediumentropy alloy was evaluated by isothermal and isochronal heat treatments. Annealed sampleswere investigated by scanning and transmission electron microscopy as well as X-ray diffraction,and were subjected to tensile tests to establish microstructure-property relationships. Furthermore,a comparison of mechanical properties with a grade 316L stainless steel was performed in order toevaluate if the CrCoNi alloy is competitive with commercially available structural materials in thenanocrystalline state. A minority phase embedded in the face-centered cubic matrix of the CrCoNialloy could be observed in multiple annealed states, as well as the as-received and high-pressuretorsion processed material. For 200 h of annealing at 500 C, it was determined that the minorityphase has a hexagonal-closed-packed crystal structure. A possible explanation for the formation ofthe phase is a preferential segregation of Co to stacking faults