Development of Athermal and Isothermal epsilon-Martensite in Atomized Co-Cr-Mo-C Implant Alloy Powders

摘要

In this work, CoCr-Mo compacted powders were sintered at 900 deg C to 1300 deg C for 1 to 2 hours and conditions for total carbide dissolution in fcc cobalt were determined. Accordingly, it was found that sintering at temperatures between 900 deg C to 1100 deg C led to removal of the dendritic structure and to carbide precipitation at the grain boundaries (gbs), as well as in the bulk. Moreover, recrystallization and grain growth were always found to occur during powder sintering. At temperatures above 1100 deg C, no carbide precipitation occurred indicating that carbides were not stable at these temperatures. Hence, compact powders were annealed at 1150 deg C to promote the development of a single-phase fcc solid solution. This was followed by rapid cooling to room temperature and then aging at 800 deg C for 0 to 18 hours. Rapid cooling from 1150 deg C promoted the development of up to 64 pct athermal epsilon-martensite through the face-centered cubic (fcc) -> hexagonal crystal structure (hcp) martensitic transformation. The athermal martensite was associated with the development of a network of parallel arrays of fine straight transgranular markings within the fcc matrix. Moreover, aging at 800 deg C for 15 hours led to the development of 100 pct isothermal hcp s-martensite. From the experimental outcome, it is evident that isothermal s-martensite is the most stable form of the hcp Co phase. Apparently, during aging at 800 deg C, the excess defects expected in athermal martensite are removed by thermally activated processes and by the development of isothermal s-martensite, which has the appearance of "pearlite."