Development of athermal and isothermal ε-martensite in atomized Co-Cr-Mo-C implant alloy powders

• Published in: Metallurgical and materials transactions. A, Physical metallurgy and materials science Vol. 37; no. 11; pp. 3197 - 3204
• Main Authors: SONG, C, PARK, H, SEONG, H, LOPEZ, H. F
• Format: Journal Article
• Language: English
• Published: New York, NY Springer 01.11.2006
• Subjects: Applied sciences; Exact sciences and technology; Metals. Metallurgy; Development; Biomaterial; Implant; Powder metallurgy; Martensite; Medical application
• ISSN: 1073-5623; 1543-1940
• DOI: 10.1007/BF02586154

In this work, CoCr-Mo compacted powders were sintered at 900 DGC to 1300 DGC 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 DGC to 1100 DGC 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 DGC, no carbide precipitation occurred indicating that carbides were not stable at these temperatures. Hence, compact powders were annealed at 1150 DGC 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 DGC for 0 to 18 hours. Rapid cooling from 1150 DGC 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 DGC for 15 hours led to the development of 100 pct isothermal hcp epsilon-martensite. From the experimental outcome, it is evident that isothermal epsilon-martensite is the most stable form of the hcp Co phase. Apparently, during aging at 800 DGC, the excess defects expected in athermal martensite are removed by thermally activated processes and by the development of isothermal e-martensite, which has the appearance of "pearlite."