Sintering mechanism and microstructure evolution of a CoCrFeNiMn high entropy alloy fabricated by metal injection molding

Abstract

CoCrFeNiMn high entropy alloy samples produced by metal injection molding were sintered in different temperatures to investigate the sintering mechanism. The sintered CoCrFeNiMn samples have a matrix with FCC structure. However, a precipitate phase, which is Cr-rich and has a BCC structure different from the σ phase discovered in previous researches, is observed in all the samples sintered at different temperatures. The precipitates are distributed in the grain boundaries. Its morphology changes from discrete particles to grain boundary allotriomorph as sintering temperature increasing. Unlike the σ phase, the precipitate produced in this study is thermodynamically stable and cannot be eliminated by water-quenching. TEM analysis indicates that the formation of the precipitate is related to the element Mn in the alloy, which segregates in the sub-grain boundaries in the CoCrFeNiMn high entropy alloy powder. In the sintering, the sub-grain boundaries serve as high-diffusivity paths, allowing for rapid diffusion of Mn and Cr from the interior to the surface of the powder. In the late sintering stage, when the surfaces of the powder turn into the grain boundaries, the Cr-rich precipitates are formed in the grain boundaries.