Microstructure analysis of a CoCrFeNi high-entropy alloy after compressive deformation

Abstract

A sharp increase in the dislocation density and in the fraction of low-angle grain boundaries (LAGBs) has often been observed during the early-stage deformation of high-entropy alloys. To study the underlying reasons for this behavior, the microstructure of plastic deformation applying at a low compressive strain of 3.3% was analyzed in the CoCrFeNi high-entropy alloy. This deformation results in a relatively higher dislocation density in the CoCrFeNi alloy in comparison to that in pure Nickel. A significantly increased dislocation density was observed near grain boundaries that is collaborated by a low value of the anisotropy factor (Az ∼ 2.37) in CoCrFeNi alloys offering favorable conditions for dislocation generation. Moreover, the low anisotropy factor in CoCrFeNi alloys appears to be caused by their strong chemical heterogeneity. The relatively easy dislocation generation provides an important feature of dislocation interactions in the CoCrFeNi alloy. Local high hardness and high Young's modulus values were observed at newly-formed LAGBs. The evolution of LAGBs strongly depends on the grain orientation and the internal strain, and LAGBs are gradually formed by the accumulation and self-organization into LAGBs at a surprisingly low strain of about 2.61%. The formation of these boundaries is intrinsically promoted by the low stacking fault energy of the CoCrFeNi alloy. An easy dislocation generation and a low strain of LAGB formation result in a high density of retained dislocations, giving rise to the observed mechanical performance of the material.