Authors: R TurrubiatesEstrada A SalinasRodriguez H F Lopez
Publish Date: 2010/10/14
Volume: 46, Issue: 1, Pages: 254-262
Abstract
In this work the kinetic aspects associated with the FCC → HCP martensitic transformation in a Co–27Cr–5Mo–023C alloy processed by powder metallurgy were investigated In situ Xray 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 εmartensite 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
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