Additively manufactured equiatomic CoCrFeMnNi high entropy alloy: Precipitation-induced heterogeneity by mechano-chemical coupling

Abstract

Additive manufacturing via selective laser melting of gas-atomized equiatomic powder was used to fabricate a CoCrFeMnNi high-entropy alloy. Analytical transmission electron microscopy, nanobeam diffraction, atom probe tomography and nanoindentation were employed to provide a comprehensive overview on the evolution of microstructure and nano-hardness upon annealing at a moderate temperature of 550∘C motivated by a maximum heat release at that temperature. A complex mechano-chemical coupling was observed, which leads to segregation and phase separation at grain boundaries. The as-manufactured material contained alternating regions of equiaxed and columnar grains. The corresponding microstructure is composed of high-angle grain boundaries and intrinsic dislocation networks, which displayed heterogeneous segregation of Mn and to some extent Ni. Longer annealing led to Cr enrichment at high-angle grain boundaries, and later to a phase separation with neighboring Cr-rich and MnNi-rich regions. Synergetic effects of segregation, nano-precipitation and dislocation accumulation at high-angle grain boundaries give rise to built up stresses which increase and homogenize hardness in the 3D-printed CoCrFeMnNi alloy.