Phase and grain size inhomogeneity and their influences on creep behavior of Co-Cr-Mo alloy additive manufactured by electron beam melting

Co-28Cr-6Mo-0.23C-0.17N alloy cylindrical rods were fabricated by electron beam melting (EBM) with cylindrical axes along the build direction. The inhomogeneity in microstructures of the as-fabricated rods and heat-treated rods were investigated, along with the creep behavior of the heat-treated rods, focusing on the influence of microstructural inhomogeneity. Although the constituent phase varied along the build direction in the as-EBM-built rod, from single ε-hexagonal close-packed (hcp) phase in the bottom to single γ-face-centered cubic (fcc) phase in the top, the γ-fcc phase can be kept in a wide range of build height, i.e. ∼40 mm from the top finishing plane. The as-EBM-built rods consisting of both ε phase and γ phase can be transformed into single ε-hcp phase by the aging treatment at 800 °C for 24 h. However, the ε-hcp grain size in the aged rod was heterogeneous along the build height. The grain size increased along the build height at first, then decreased gradually to the position where the phase transitioned from γ-fcc to ε-hcp in the as-EBM-built rod. The grain size was nearly uniform on the top part, which used to be single γ-fcc phase. The values of stress exponent n and apparent activation energy Q were determined to be 5.0 and 365 kJ mol^-1, respectively. Intergranular fracture occurred, and fracture nearly always occurred in the homogeneous fine-grain region. The decrease in stacking fault energy with increasing temperature keeps the dislocations expanding and increases the apparent activation energy.