Unusual martensite-formation kinetics in steels: Observation of discontinuous transformation rates

In order to study the role of the potentially thermal activation of martensite formation in systems which are classically considered as “athermal”, the austenite → martensite transformation kinetics of a maraging steel (FeNiCoMo), forming lath martensite, was investigated by dilatometry and differential thermal analysis (DTA), supplemented by electron backscatter diffraction, X-ray diffraction and transmission electron microscopy analysis of the microstructure. No dependence of the transformation rate on cooling rate could be observed and thus any possible activation energy has to be very small. Dilatometry as well as DTA measurements revealed an unusual transformation behavior during the martensite formation: a train of transformation-rate maxima occurs, which was found to be reproducible for different specimens, heating rates and specimen geometries. This phenomenon was attributed to the formation of the highly hierarchical microstructure of blocks of martensite laths, grouped in packages of parallel blocks: the observed train of transformation-rate maxima is caused by the simultaneous, concerted formation of blocks in different packages in all grains. The simultaneity results from the defined formation conditions for a block as established by the formation of the preceding adjacent block within the package.