The isothermal decomposition of austenite in a high purity Iron-Chromium binary alloy

Abstract

The transformation characteristics of a series of high purity iron-chromium alloys, within the γ-loop composition range, have been studied using continuous-cooling dilatometry. An Fe-10 wt pct Cr alloy, which exhibited a relatively slow γ → α transformation, was chosen for detailed investigation by isothermal dilatometry, and by optical and transmission electron microscopy. The TTT diagram exhibited a high-temperature Ccurve in which the transformation products were equiaxed ferrite and Widmanstätten ferrite, the latter developing from perturbations on the α:γ interface. In this range, a ledge mechanism was the predominant mode of ferrite growth. A simple activation energy analysis suggests that the γ→ α reaction is interface controlled, and supports the existence of a “solute-drag” effect by carbon even at low concentrations. Direct quenching of the alloy produced martensite. In the intermediate temperature range, it is proposed that the γ→ α reaction is bainitic in character.