Effect of annealing heat treatment on the microstructure and mechanical properties of laser directed energy deposited CrMnFeCoNi high entropy alloy

Abstract

In this study, a thin-wall build of CrMnFeCoNi high entropy alloy (HEA) was fabricated by laser directed energy deposition (LDED) process. The effect of high-temperature annealing heat treatment at 1000 °C on the microstructure and mechanical properties of LDEDed CrMnFeCoNi HEA was investigated. The results showed that the as-deposited microstructure of CrMnFeCoNi HEA features long-strip and coarse columnar grains with plenty of tiny pores and exhibits a homogeneous elements distribution. The microstructure shows good stability when the annealing time is within 4 h. With the further increase of annealing time, recrystallized equiaxed grains with interior twins form at the original coarse columnar grain boundaries. The dislocations distinctly pile up around recrystallized equiaxed grains accompanied by interior twins and show a cluster distribution. Precipitates including Cr-rich σ phases and Cr2MnO4 oxides form and distribute uniformly in annealed microstructure. Twin boundaries across these precipitates are distorted and correspondingly form a local high-interface-energy region around these precipitates where is prone to crack. In this study, the recommended annealing time is within 4 h to avoid the crack formation and associated dramatical degeneration of tensile properties. Our results are of great importance in understanding the microstructure evolution, as well as the effect of high-temperature annealing heat treatment, in the laser directed energy deposition of CrMnFeCoNi HEA.