Abstract
A microanalysis study of the eutectoid decomposition of austenite to ferrite and M 2C (bainite) at the bay in Fe-0.24C-4Mo is reported. The carbon remaining in the austenite showed little variation with position at any given reaction time, which ruled out carbon diffusion as a rate-limiting process. Fine-probe EDS at bainite-austenite growth fronts revealed Mo enrichment in the interface and up to 15 nm in the austenite. EDS of extracted M 2C carbides always revealed Mo enrichment, but with a non-equilibrium Fe/Mo ratio at early reaction times. It was concluded that alloy element partition between ferrite and alloy carbides at the reaction front is largely responsible for the slow kinetics in this and related alloys. A thermodynamic analysis showed that ferrite-carbide interfacial energy and non-equilibrium carbide compositions reduce the thermodynamic driving force for diffusive processes (Mo partition) by up to 20%, further slowing the kinetics.
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