The face-centered-cubic (fcc) to hexagonal close-packed (hcp) martensitic transformation exhibited by an as-cast Co-Cr-Mo-C alloy was investigated in this work. The alloy was annealed at 1150°C, water quenched, and then isothermally aged at 700°C to 900°C. Quantitative measurements of transformed hcp martensite (also known as e-martensite) as a function of time and temperature were used in plotting C curves describing the transformation kinetics. Moreover, microstructural characterization indicated that the transformation exhibited two distinctive e-martensite morphologies. In the neighborhood of coarse carbides, multiple nucleation events were linked to the formation of e-martensite, while in the bulk of the dendrite matrix, the dominant morphology was as straight plates. Kinetic determinations of activation energies, Q, in either region indicated that in platelike e-martensite, Q = 57.24 kcal/mol, but in ''pearlite''-like morphologies, Q = 37.88 kcal/mol. Apparently, as the activation energy was reduced, multiple nucleation events were favored leading to the ''pearlitic'' appearance.

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.

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The effect of various alloy preheatings followed by full solid solution treatments on the resultant strength and ductility of as-cast Co-Cr-Mo-C alloys was investigated. Three preheating temperatures were evaluated: 815 ЊC, 950 ЊC, and 1100 ЊC for 4 hours and then solid solution treated at 1225 ЊC for 4 hours. Tensile and compressive tests were carried out on the heat-treated alloys. It was found that the strength and ductility of the heat-treated alloys exhibited significant improvements over the as-cast condition. In particular, optimum ductility of the heat-treated alloys and alloy strength were promoted by preheating at 815 ЊC. A relatively fine grained structure coupled with a uniform distribution of second-phase particles promoted homogeneous plastic deformation in the bulk. Fractographic observations indicated that the exhibited ductility was associated with the development of numerous plastic bands combined with band interlockings. Alloy preheats at 950 ЊC and 1100 ЊC prior to solutionizing lead to inferior strength and ductility. Although preheating at 1100 ЊC led to slight improvements, in both cases, the fracture path was dominated by the presence of continuous carbide films surrounding the dendritic grains. Hence, less than optimum combinations of strength and ductility were achieved by the heat treatments at the higher temperatures.

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