In situ observation of austenite coarsening induced by massive-like transformation during solidification in Fe-C alloys

A massive-like transformation, in which δ-ferrite massively transforms to γ-austenite in the solid state, was examined by time-resolved X-ray imaging and X-ray diffractometry on a four-dimensional-computed tomography setup. In the unidirectional solidification of a hyperperitectic steel (0.3 mass% C) at 50 μm/s, δ dendrites, fine γ grains that are produced through the massive-like transformation, and coarse γ grains grow together in the steady state. The massive-like transformation occurred commonly in Fe-based alloys with a peritectic reaction in equilibrium. Time-resolved X-ray diffraction measurements for a peritectic steel (0.18 mass% C) showed that the massive-like transformation induced strains in the γ grains. The fine γ grains coarsened and/or vanished and new γ grains formed. The induced strains were released during coarsening. The results obtained by the observations contribute to building of a physical model for γ grain coarsening.