On the origin and evolution of cellular structures in CoCrFeMnNi high entropy alloy fabricated by laser powder bed fusion

The unique cellular structures formed in face-centered-cubic (FCC) metals fabricated via laser powder bed fusion (LPBF) lead to extraordinary mechanical properties [1–3]. However, a consensus on the formation mechanism has not been achieved thus far. In this study, a CoCrFeMnNi high entropy alloy (HEA) was employed to investigate the origin and evolution of cellular structures during LPBF. The cellular structures generated in a single-track wall and a cube were characterized and analyzed. In particular, investigation and comparison was made of the morphology, chemical segregation, misorientation, and dislocation densities of cellular structures in the last melt pool and middle region of the as-built cube. Experimental results demonstrated that chemical cells with segregation of Mn and Ni and low dislocation-density cells were generated simultaneously and overlapped with each other. The multiple thermomechanical treatments exerted by subsequent laser tracks increased the dislocation density and thickness of the cell without destroying the cellular shapes or increasing the cell-wall misorientation. At the same time, chemical cell segregation was alleviated. The morphology and distribution of cellular structures that remain stable during evolution are determined by the solidification condition. The stability of the chemical cells contributes to the unique characteristics of cellular structures relative to conventional deformation-induced dislocation structures.