A JMAK-like approach for isochronal austenite–ferrite transformation kinetics in Fe–0·055 wt-%N alloy

Abstract

The kinetics of the isochronal austenite γ to ferrite α transformation of Fe–0·055 wt-%N alloy was investigated for cooling rates in the range of 5–15 K min− by high resolution dilatometry. In accordance with thermodynamic characteristics of γ→α transformation investigated in this study and previous kinetic theory, a Johnson–Mehl–Avrami–Kologoromov (JMAK)-like approach for the kinetics of isochronal phase transformations was developed that incorporates three overlapping processes: site saturation nucleation, alternate growth modes (transition from interface- to diffusion-controlled growth) as well as impingement for random distribution nuclei. The JMAK-like approach has been employed to fit the experimental results, and the fitting results show that the γ→α transformation of Fe–0·055 wt-%N alloy does have two stages: a first, short interface-controlled growth stage and a second, long diffusion-controlled growth stage. In addition, soft impingement effect has been recognised to become serious in the later part of the second stage.