New insights into massive transformation: In-situ X-ray observations and diffuse interface analysis of δ to γ transformation in TRIP steels

Recent in-situ studies on carbon steel solidification have shown that the δ to γ transformation can occur at a high degree of undercooling, even in the single γ-phase region, a transformation referred to as massive or massive-like by researchers. In this study, the δ to γ phase transformation of two experimental TRIP steels was investigated using a setup that integrates X-ray imaging, Laue diffraction, and precise temperature measurement. Steel T1, a hypo-peritectic alloy (0.1% C-1.23% Mn-1.97% Si), and Steel T2, a hyper-peritectic alloy (0.23% C-1.55% Mn-1.87% Si), were compared. The results reveal that the δ to γ transformation in both steels occurred within the single γ-phase region, but at significantly different rates. Compared to carbon steels with lower Mn and Si content, these steels exhibit differences in behavior, as discussed in the results section.

The thermodynamic and interface kinetic of the δ to γ transformation were investigated to gain deeper insights into this transformation behavior. Thermodynamic analysis reveals that the transformation in both steels is strongly dependent on undercooling from the $T_0$ temperature, rather than the equilibrium peritectic temperature. Using a diffuse interface model of massive transformation, the kinetics of the δ to γ transformation were analyzed. The results showed two distinct regimes in the relationship between interface velocity and thermodynamic driving force: a low-velocity regime with partitioning of C, Mn, and Si, and a high-velocity regime with almost no partitioning of Mn and Si. The massive transformations observed in these steels occurred in the low and high-velocity regimes, respectively.