Powder bed fusion laser beam (PBF-LB) offers significant advantages in fabrication of high entropy alloys (HEAs) and medium entropy alloys (MEAs) owing to its unique characteristics such as high cooling rates which could result in formation of athermal martensite and high dislocation densities induced by thermal cycling during the layer-by-layer printing process. The technique is beneficial not only in producing the near net shape components but also helps in minimising the elemental segregation which could be detrimental for the mechanical behaviour of the alloys.

This work presents the fabrication of Co45Cr25(FeNi)30, a metastable non-equiatomic MEA using PBF-LB, and a detailed characterization of the microstructure and analysis of the mechanical behaviour of the alloy in the as-printed state. In the as-printed sample, nano-scaled athermal ε-martensite (HCP) phase formed along with the FCC phase. The HCP phase observed in the as-printed microstructure exhibited the (111)fcc//(0001)hcp and [110]fcc//[2-1-10]hcp orientation relationship to the FCC phase, which is the Shoji-Nishiyama (S-N) orientation relationship. Tensile testing revealed yield strengths of 542 MPa and 512 MPa in horizontal and vertical orientations, respectively, with respect to the building direction. In-situ high energy X-ray diffraction (HEXRD) highlights the stress-induced martensitic transformation, which occurs below the macroscopic yield strength with the HCP (10-11) reflection observed approximately 250 MPa and 375 MPa in vertical and horizontal directions, respectively. This phase transformation led to the deviation of linear response between stress and strain for the FCC phase. Further straining resulted in significant load partitioning, with the HCP phase taking over the majority of the load once it formed, significantly strain-hardening the alloy and resulting in the reduced plastic anisotropy induced by texture in the as-printed material. Rietveld refinement on the HEXRD data revealed 0.8 % and 1.5 % of athermal HCP phase in horizontal and vertical samples, respectively, in the as-printed state. At the end of the tensile test, the HCP phase fraction grew to 42% in the horizontal sample, while in the vertical sample, the observed HCP phase fraction was approximately 60%. The results indicate that the stress-induced martensitic transformation, and the accompanying HCP phase fraction evolution, lead to significant load partitioning and strain-hardening of the alloy. The findings will help to reduce plastic anisotropy induced during the PBF-LB process, and to improve the overall mechanical properties of the as-printed MEA.