To obtain an intrinsic stress-strain response with no effects on stress due to the friction between the sample and the anvils or on the temperature increase due to adiabatic heating, a method of compensating for a stress-strain response was proposed. Using the stress-strain response that was compensated for, a processing map, consisting of both power dissipation efficiency and flow instability map, was constructed. On the basis of our observations using EBSD, it was found that the processing map after the compensation can be effectively used to well predict the optimum hot forging conditions.

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By using a processing map and result of FEM simulation, an artificial hip joint stem made of Co-29Cr-6Mo-0.12N alloy was fabricated using a non-isothermal hot semi-closed die forging method. The optimum hot forging conditions (i.e., initial work temperature and pressing speed) were determined using a processing map and FEM simulation. It was observed that the microstructure of the forged stem, which markedly varied from position to position, contained both the fine dynamic recrystallized grains and a deformed microstructure. The variation in the microstructure is thought to be due to a decrease in the temperature of the stem during the hot forging. It is found that the combined used of a processing map and FEM simulation is a very useful for manufacturing artificial hip joint stems made of Ni-free Co-29Cr-6Mo-0.12N alloy.

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To investigate the variations in the microstructure and mechanical properties of a biomedical Co-Cr-Mo alloy with static recrystallization, metallographic observations by EBSD(Electron backscatter diffraction) and TEM(Transmission electron microscopy) and tensile tests have been conducted. Grains are refined transiently into average size of approximately 10μm by aging at 1000°C for 7.5min after warm swaging at 750°C. At the same time, high dislocation density changes into low density, which is observed by TEM. Thus, it can be said that grain refinement is derived from static recrystallization. Although the specimen is deformed by only a 20% reduction in area by warm swaging, nuclei of recrystallization are formed uniformly in initial grains because of the uniform division of the grains caused by planar dislocations, which are dissociated into Shockley partials on the {111} plane. In addition, the grain boundary misorientation after recrystallization shows a high fraction of the Σ3 boundary. The uniform division of the grains caused by planar dislocations and the high fraction of the Σ3 boundary after recrystallization are related to the very low SFE(Stacking fault energy) of the present alloy. Mechanical properties such as 0.2% proof stress and elongation of the aged specimens change linearly from the early stage of aging.

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