Effects of nitrogen on microstructural evolution of biomedical Co-Cr-W alloys during hot deformation and subsequent cooling

The present study investigated how nitrogen affected the high-temperature deformation and microstructural evolution of biomedical Ni-free Co-Cr-W alloys during hot deformation. Hot compression tests of undoped and N-doped Co-28Cr-9W-1Si-0.05C (mass%) alloys were performed at deformation temperatures ranging from 1323 to 1473K at strain rates of 10^-3 to 10s^-1. The microstructures, which were subjected to a true strain of 0.92 (60% in compression), were characterized using electron backscatter diffraction (EBSD) analysis and transmission electron microscopy (TEM). Dynamic recrystallization (DRX) was found to occur in both alloys during hot deformation. The grain size (d) decreased considerably with an increase in the Zener-Hollomon (Z) parameter. Although adding nitrogen to the alloys barely affected dynamic-recrystallization-induced grain refinement, it increased the magnitude of the flow stress and delayed static recrystallization during post-deformation cooling. Consequently, the N-doped alloy contained bulk nanostructures whose average grain size was 0.9μm.