FCC to HCP transformation kinetics in a Co–27Cr–5Mo–0.23C alloy

Abstract

In this work the kinetic aspects associated with the FCC → HCP martensitic transformation in a Co–27Cr–5Mo–0.23C alloy processed by powder metallurgy were investigated. In situ X-ray diffraction during isochronous heat treatments in a hot stage indicated that a fully metastable FCC matrix transforms rather fast at temperatures above 725 °C and reaches a maximum transformation into the HCP phase at 940 °C. Alternatively, when the matrix is HCP, some HCP martensite reverts to metastable FCC. Apparently, at low temperatures carbon excess in the HCP martensite promotes the reversal to metastable FCC. In addition, the volume percent of ε-martensite precipitated from stable FCC was determined as a function of time and temperature during isothermal aging between 675 and 900 °C. From these results, TTT diagrams were plotted for a 1% HCP transformed martensite. Maximum transformation rates were found to occur between 825 and 850 °C and activation energies, Q _s of 41–52 kcal/mol were estimated from the experimental outcome. The aged microstructures indicated that below 800 °C, the isothermal transformation was dominated by a lamellar morphology. Nevertheless, aging above 800 °C promoted carbide nucleation and coarsening along the grain boundaries independently of the FCC → HCP martensitic transformation.