Application of in-situ diffraction experiments to understand nonequilibrium phase transformations in structural alloys

It has been a challenge to measure in-situ phase transformation kinetics in structural alloys under rapid thermal cycles with good time resolution in the past. However, by using high-brightness synchrotron sources that deliver sufficient photon fluxes, in-situ time-resolved X-ray diffraction measurements with millisecond time-resolution are now possible. This technique was used to track nonequilibrium austenite formation in a Fe-C-Al-Mn steel weld during solidification. This solidification mode is contrary to the equilibrium ferrite solidification mode. Subsequent to solidification, splitting of austenite diffraction peaks with two different lattice parameters was observed during cooling. Continued cooling led to the decomposition of low-volume austenite into bainite first which was followed by the decomposition of large-volume austenite to martensite. A similar phenomenon of austenite splitting and subsequent transformation of low-volume austenite first was also observed in a high-carbon steel before bainitic transformation.