Kinetics of the martensitic f.c.c.→ h.c.p. transformation in CoCrMo alloy powders

Proposed mechanism of HCP → FCC phase transition in titianium through first principles calculation and experiments

By means of first principles calculation and experiments, a detailed mechanism is proposed to include the stages of slip, adjustment, and expansion for the HCP → FCC phase transformation with the prismatic relation of [Formula: see text] and [Formula: see text] in titanium. It is revealed that the formation of four FCC layers is preferable after the slip of Shockley partial dislocations of 1/6 [Formula: see text] on [Formula: see text] planes, and that the adjustment of interplanar spacing and the volume expansion are energetically favorable and could happen spontaneously without any energy barrier. It is also found that the transformed FCC lattice first follows the c/a ratio (1.583) of HCP and then becomes an ideal FCC structure (c/a = √2). The proposed mechanism could not only provide a deep understanding to the process of HCP → FCC prismatic transformation in titanium, but also clarify the controversy regarding volume expansion of HCP-FCC phase transition of titanium in the literature.

Development of athermal epsilon-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 degrees C for 1h and then rapidly cooled to room temperature. As a result, various amounts of athermal epsilon-martensite were produced which increased with increasing sintering temperatures (from 30 vol.% at 950 degrees C to 70 vol.% at 1250 degrees C). Apparently, the development of epsilon-embryos was strongly promoted by increasing sintering temperatures due to the development of a high density of epsilon-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 epsilon-martensite were 3-4-fold those found in conventional alloys, suggesting that the powder structure provides increasing nucleation sites for athermal epsilon-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 epsilon-embryos.

Microstructural effects on the wear resistance of wrought and as-cast Co-Cr-Mo-C implant alloys

The tribological behavior of two cobalt-base alloys-an as-cast high-carbon and a wrought low-carbon Co alloy-that are used as hip implant materials is examined in this work. This work discusses the experimental results of cobalt-cobalt wear pairs, in wrought and as-cast conditions, where the amount of hexagonal phase is systematically modified through an isothermal aging treatment. Fully FCC and HCP Co alloys are tested versus alloys with various volume fractions of HCP phase (0.05 to 1.0 volume fractions). Preliminary results indicate that Co-Cr-Mo/Co-Cr-Mo alloy pairs both possessing an HCP matrix microstructure tend to exhibit outstanding wear properties.