Abnormal austenite-ferrite transformation kinetics of ultra-low-nitrogen Fe-N alloy

The kinetics of the isochronal austenite (gamma)-> ferrite (alpha) transformation of ultra-low-nitrogen Fe-N alloy were investigated for cooling rates in the range of 5 to 15 Kmin(-1) by high-resolution dilatometry. The explored gamma ->alpha transformation takes place in the single alpha-phase region, and the onset temperature of the transformation decreases with increasing applied cooling rate. According to the variation of the ferrite formation rate data, an abnormal transformation mechanism, i.e., multiply peaked nature with rate maxima appearing at fixed transformed fractions, was recognized. For the later (main) part of the transformation, a phase-transformation model, incorporating site saturation, interface-controlled growth, and an appropriate impingement correction, has been employed to extract the migration velocity of the gamma/alpha interface from the measured data. The thus determined interface migration velocity is approximately constant for all applied cooling rates. The first abnormal part of the transformation was discussed in comparison to the recently observed abnormal phenomena for the gamma ->alpha transformation in pure iron and ultra-low-carbon Fe-C alloy. On the basis of the thermodynamic and kinetic results, it is shown that autocatalytic nucleation occurs in the first part of the transformation and that interface-controlled growth prevails for the entire transformation.