Real-time synchrotron X-ray diffraction study on the isothermal martensite transformation of maraging steel in high magnetic fields

The isothermal austenite-to-martensite transformation kinetics in a maraging steel have been studied by time-dependent microbeam diffraction measurements with high-energy X-rays. The transformation kinetics are shown to be accelerated significantly when a magnetic field of 8 T is applied. The average phase behaviour, obtained from a Rietveld refinement of the powder-averaged diffraction data, demonstrates that the martensite formation does not lead to a macroscopic strain in the austenite and martensite phases. An analysis of individual austenite reflections in the microbeam diffraction patterns, however, indicates that within the transforming austenite grains a transformation strain develops as a result of the formed martensite. The development of elastic strains during the transformation is explained by a partial strain confinement within the untransformed part of the austenite grain. The strain relaxation to the surrounding austenite grains is found to be dependent on the austenite volume. For a set of individual austenite grains the martensite nucleation is correlated with the initial austenite volume and the strain developed prior to the transformation as a result of martensite formation in the neighbouring grains.