The influence of columnar to equiaxed transition on deformation behavior of FeCoCrNiMn high entropy alloy fabricated by laser-based directed energy deposition

Abstract

Abstract This study investigates the columnar to equiaxed transition (CET) and deformation behavior of a FeCoCrNiMn high entropy alloy (HEA) fabricated by laser-based directed energy deposition (DED). The results show that two kinds of microstructure can be obtained in the as-built HEAs by changing laser scanning speed: one kind of microstructure is formed at a low scanning speed and composed of near fully equiaxed grains; the other is formed at a high scanning speed and composed of columnar grains. These two different microstructures are closely related to the CET, which is decided by the ratio of thermal gradient and solidification rate in the molten pool. Additionally, compared with traditional HEAs, as-built HEAs show a heterogeneous grain-size distribution and high dislocation density, as well as improved tensile properties, one of which (Yield strength = 330 MPa; Ultimate tensile strength = 630 MPa; Fracture elongation = 55%) is similar to the properties of wrought HEAs. Moreover, the as-built HEA with equiaxed grains shows a higher work hardening rate than the as-built HEA with columnar grains and the equiaxed microstructure has the characteristics of intergranular microcracks, the columnar microstructure shows the characteristics of intragranular microcracks. The deformation behavior was investigated using in-situ high-resolution digital image correlation methods, and the result shows that the difference in work hardening rate is closely related to the texture formed during deposition.