Correlation between microstructural heterogeneity and anisotropy of mechanical properties of laser powder bed fused CoCrFeMnNi high entropy alloy

Abstract

The unique microstructure of face-centered-cubic (FCC) structured alloys fabricated by laser powder bed fusion (LPBF) contributes to outstanding mechanical performances, but inevitably leads to anisotropy of mechanical properties. In this study, the relationship between the hierarchical microstructure and mechanical properties along the horizontal, diagonal, and vertical directions of the LPBF-built CoCrFeMnNi high entropy alloy (HEA) were qualitatively and quantitatively investigated. The horizontal samples exhibit the highest yield strength (YS) and the lowest elongation, while the vertical samples show the lowest YS and a slightly lower elongation than the diagonal samples, which are nearly two times the horizontal samples. Among the microstructural features at multiple length scales, both the columnar grain morphology and texture determine the anisotropy of mechanical properties. For the first time, the volume-weighted (V–W) average grain size was used to estimate the YS and critical twinning stress (CTS) of the LPBF-built CoCrFeMnNi alloy with a complex grain size distribution. The V–W average grain size exhibited high accuracy in calculating the YSs of the diagonal and vertical samples as well as CTSs along all the three directions. For the horizontal sample, the grain morphology dominates over texture during deformation, which is the same for the diagonal and vertical samples at the early stage of deformation. The texture plays a more important role at the late stage of straining.