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

Co–Cr–Mo atomized powders containing 0.05 wt.% C were sintered at temperatures above 900 °C for 1 h and then rapidly cooled to room temperature. As a result, various amounts of athermal ε-martensite were produced which increased with increasing sintering temperatures (from 30 vol.% at 950 °C to 70 vol.% at 1250 °C). Apparently, the development of ε-embryos was strongly promoted by increasing sintering temperatures due to the development of a high density of ε-nucleation site defects. In addition, athermal martensite readily formed in these powders, suggesting that its development was strongly favored by a significant reduction in the carbon supersaturation levels from 0.25 wt.% for most commercial alloys to 0.05 wt.% C. The amounts of ε-martensite were 3–4-fold those found in conventional alloys, suggesting that the powder structure provides increasing nucleation sites for athermal ε-martensite. Apparently free surfaces and grain development at powder contact surfaces combined with recrystallization and grain growth within powder particles lead to favorable dislocation configuration arrays for the development of ε-embryos.