Build direction dependence of microstructure and high-temperature tensile property of Co-Cr-Mo alloy fabricated by electron beam melting

The microstructures and high-temperature tensile properties of a Co-28Cr-6Mo-0.23C-0.17N alloy fabricated by electron beam melting (EBM) with cylindrical axes deviating from the build direction by 0, 45, 55 and 90 were investigated. The preferred crystal orientations of the γ phase in the as-EBM-built samples with angles of 0, 45, 55 and 90 were near [0 0 1], [1 1 0], [1 1 1] and [1 0 0], respectively. M₂₃C₆ precipitates (M = Cr, Mo or Si) were observed to align along the build direction with intervals of around 3 μm. The phase was completely transformed into a single ε-hexagonal close-packed (hcp) phase after aging treatment at 800 C for 24 h, when lamellar colonies of M₂N precipitates and the ε-hcp phase appeared in the matrix. Among the samples, the one built with 55 deviation had the highest ultimate tensile strength of 806 MPa at 700 C. The relationship between the microstructure and the build direction dependence of mechanical properties suggested that the conditions of heat treatment to homogenize the microstructure throughout the height of the EBM-built object should be determined by taking into account the thermal history during the post-melt period of the EBM process, especially when the solid-solid transformation is sluggish.