Formation of hcp martensite during the isothermal aging of an fcc Co-27Cr-5Mo-0.05C orthopedic implant alloy

The evolution of the microstructure of a Co-27Cr-5Mo-0.05C alloy was investigated during isothermal aging between 650 °C and 950 °C. The main structural change observed as a result of aging was an fcc (metastable)→hcp isothermal martensitic transformation. The relationships between transformation, temperature, and time for this phase transition were determined using two different techniques: (1) room-temperature X-ray diffraction on samples aged after quenching from 1150 °C to 25 °C and (2) high-temperature in situ X-ray diffraction on samples cooled at 50 °C/min from 1150 °C to the aging temperature. The results show that the intermediate water quench significantly retards the kinetics of the phase transition by up to one order of magnitude in time. In addition, it was found that the grain size of the metastable fcc phase prior to aging does not affect the kinetics of the transformation. Age hardening resulting from this transformation varies linearly with the amount of hcp phase formed during the isothermal treatment and does not depend on the aging temperature. It is suggested that local plastic deformation, due to thermal and transformation stresses induced by quenching, reduces the number of hcp martensite embryos formed in the metastable fcc phase. This effect decreases the number of nucleation sites available for the fcc→hcp transformation during isothermal aging and leads to the slower transformation rates observed in water-quenched material.