Microstructural mechanisms endowing high strength-ductility synergy in CoCrNi medium entropy alloy prepared by laser powder bed fusion

Abstract

CoCrNi medium entropy alloy (MEA) exhibits significant potential as a structural component for engineering applications. However, the traditional processing methods for pursuing the superior mechanical properties of CoCrNi MEA are often time-consuming and inefficient. Laser powder bed fusion (LPBF) offers substantial processing flexibility without the need for complex secondary processing. In this study, we optimized the processing parameters for CoCrNi MEA through an orthogonal experimental design, achieving as-printed CoCrNi components with a relative density of up to 99.4 %. The microstructure of the as-printed CoCrNi revealed hierarchical features, including the molten pools, equiaxed and elongated columnar grains, sub-grain cellular microstructures, and high-density dislocation networks. The mechanical properties of as-printed CoCrNi were exceptional, demonstrating a synergistic combination of strength and ductility (yield strength: approximately 660.91 MPa, tensile strength: approximately 892.78 MPa, elongation: approximately 33.28 %, and a product of strength and elongation: 27.45 GPa %). Experimental characterization elucidated the activation and interplay of mechanisms such as dislocations, stacking faults, twinning-induced plasticity, and transformation-induced plasticity. These mechanisms collaboratively enhanced the refinement and heterogeneity of substructures, thereby endowing the as-printed CoCrNi with significant strength-ductility synergy. Molecular dynamics simulations indicated that, in the initial stages of plastic deformation, a significant activation of Shockley partial dislocations occurred, leading to the formation of stacking faults and the emergence of Hexagonal Close-Packed (HCP) phases. As plastic deformation progressed, a reverse phase transformation from HCP to Face-Centered Cubic (FCC) took place, resulting in the formation of nanotwins. This transformation generated a heterogeneous microstructure comprised of both nanotwins and HCP phases, suggesting a sequential twinning and phase transformation pathway in as-printed CoCrNi: from FCC to HCP, and subsequently to FCC nanotwins.