Anisotropic microstructure, properties and molecular dynamics simulation of CoCrNi medium entropy alloy fabricated by laser directed energy deposition

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CoCrNi medium entropy alloy additive part was fabricated by laser directed energy deposition process. Based on an optimized additive manufacturing process parameters, the microstructure, tensile properties and corrosion performance in the XOY and XOZ planes were compared to learn the anisotropic properties of the additive part. The majority of the additive parts consisted of equiaxed grains, a small proportion consisted of elongated needle-like grains. The equiaxed and elongated needle-like grains in the XOY plane were finer compared to that in the XOZ plane. The corrosion electrochemical activity is uniform in the XOY plane, but varies at different heights in the XOZ plane base on scanning vibrating electrode technique results. Electron backscatter diffraction results near the fracture indicated a larger and relatively more homogeneous residual stress peak in the XOY plane. The tensile and yield strengths in the XOZ plane were lower than those in the XOY plane by approximately 98.5 MPa and 19.1 MPa. While the elongation in the XOZ plane was more than twice as high as that in the XOY plane. The reason for the anisotropic tensile behavior was the strong texture in the XOY plane and the relatively random grain orientation in the XOZ plane. Single crystal and polycrystalline molecular dynamics were used to simulate the anisotropic tensile properties in the XOZ and XOY planes. Single crystal simulation results showed that the maximum stress was 19.0 GPa and elongation was 13.0% when the stretching was along the [110] crystal orientation. The minimum stress was 17.1 GPa and elongation is 11.1% elongation when the stretching was along the [11 1‾] crystal orientation. Anisotropy existed for CrCoNi alloys in terms of tensile properties along different orientations. Polycrystalline simulation in the XOY plane results showed that the main cause of anisotropy between them was the tangling of the 1/6<112> dislocations and the 1/6<521> dislocations, which caused obstacles to dislocation motion and thus improved the strength of CoCrNi additive parts. High tensile strength in the XOY plane, not the XOZ plane, was attributed to 1/6<112> and 1/6<521> dislocation tangles at the (1 1‾ 0)/(115) planes.