Dynamic recrystallization of a biomedical Co-Cr-W-based alloy under hot deformation

Grain refinement in Ni-free biomedical alloy Co-28Cr-9W-1Si-0.05C (mass%) under hot deformation was studied. Hot compression tests were performed at deformation temperatures ranging from 1323 to 1473K at strain rates of 10^-3-10s^-1. The microstructures, which were subjected to true strains of up to 0.92 (60% in compression), were characterized using electron backscatter diffraction analysis and transmission electron microscopy. Dynamic recrystallization (DRX) was found to occur during hot deformation. The grain size (d) decreased considerably, from the initial 60μm-1.4μm, with an increase in the value of the Zener-Hollomon (Z) parameter. However, because of the static recrystallization that occurred after deformation, grains coarser than those predicted from the d-Z relationship for low strain rates were formed when the alloy specimens were deformed at higher strain rates. The critical strain rates corresponding to this deviation were around 10^-2-0.1s^-1, depending on the deformation conditions. The nucleation of DRX grains resulted from the bulging of the grain boundaries, and the evolution of DRX grains was limited in the vicinities of grain boundaries. The stacking fault energy of the alloy investigated, which was lower than that of conventional alloys, might be responsible for the complete replacement of the microstructure of the alloy, which occurred owing to an increase in the degree of DRX.