Heterogeneous microstructures and corrosion resistance of biomedical Co-Cr-Mo alloy fabricated by electron beam melting (EBM)

We have investigated the spatial distribution of microstructures of a Co-Cr-Mo alloy rod fabricated by Electron Beam Melting (EBM) method along built height. The topside of the rod is rich in gamma-fcc phase and consists of fine grains with high local distortion density. The bottom part has an epsilon-hcp single phase and consists of relatively coarser grains with low local distortion density. The middle part of the rod consisted of the mixture of both phases. The mean grain size increases from 56 mu m (at the top of the rod) to 159 mu m (at the bottom), and is accompanied by a decrease in the gamma-fcc phase fraction. On the other hand, a large number of precipitates including the main M23X6 phase and minor phases (eta-phase and pi-phase) were observed. The area fraction of the precipitates increases gradually from 5.26% (at the top) to 8.73% (at the bottom), and the relative proportion of each phase fluctuates at different positions. The hardness of the top side is lower than that of the bottom side. As a result, the hardness of the samples, as well as the area fraction of precipitates formed in the samples, increases gradually from top to bottom of the rod, while corrosion resistance is uniformly high throughout the rod almost independently of the location. The mechanism behind the formation of phase distribution is discussed in terms of thermodynamic phase stability and kinetics of phase transformation accompanying the thermal history during the post-solidification process.