Consideration of the growth mode in isochronal austenite-ferrite transformation of ultra-low-carbon Fe-C alloy

The three cooling rates of 10, 100, 200 K/min dilatometry experiments are used to investigate the kinetics of the isochronal austenite (gamma) to ferrite (alpha) transformation of Fe-0.0036wt.%C alloy. Normal transformation and abnormal transformation have both been observed for transformations at different cooling rates. In accordance with the thermodynamic characteristics of the gamma ->alpha transformation investigated here and previous kinetic considerations, a JMAK-like approach for the kinetics of isochronal phase transformations was developed that incorporates three overlapping processes: site saturation nucleation, alternate growth modes (from interface-controlled to diffusion-controlled to interface-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 for the gamma ->alpha transformation of the Fe-C alloy at all applied cooling rates, the growth mode evolves in the corresponding order: from interface-controlled to diffusion-controlled growth; from interface-controlled to diffusion-controlled to interface-controlled growth; and interface-controlled growth.