Effect of Cu-Rich Phase Precipitation on the Microstructure and Mechanical Properties of CoCrNiCu_x Medium-Entropy Alloys Prepared via Laser Directed Energy Deposition

Abstract

CoCrNiCu_x (x = 0.16, 0.33, 0.75, and 1) without macro-segregation medium-entropy alloys (MEAs) was prepared using laser directed energy deposition (LDED). The microstructure and mechanical properties of CoCrNiCu_x alloys with increasing Cu content were investigated. The results indicate that a single matrix phase changes into a dual-phase structure and the tensile fracture behaviors convert from brittle to plastic pattern with increasing Cu content in CoCrNiCu_x alloys. In addition, the tensile strength of CoCrNiCu_x alloys increased from 148 to 820 MPa, and the ductility increased from 1 to 11% with increasing Cu content. The nano-precipitated particles had a mean size of approximately 20 nm in the Cu-rich phase area, and a large number of neatly arranged misfit dislocations were observed at the interface between the two phases due to Cu-rich phase precipitation in the CoCrNiCu alloy. These misfit dislocations hinder the movement of dislocations during tensile deformation, as observed through transmission electron microscopy. This allows the CoCrNiCu alloy to reach the largest tensile strength and plasticity, and a new strengthening mechanism was achieved for the CoCrNiCu alloy. Moreover, twins were observed in the matrix phase after tensile fracture. Simultaneously, the dual-phase structure with different elastic moduli coordinated with each other during the deformation process, significantly improving the plasticity and strength of the CoCrNiCu alloy.