The mechanical properties of polycrystalline materials are largely determined by the kinetics of the phase transformations during the production process. Progress in x-ray diffraction instrumentation at synchrotron sources has created an opportunity to study the transformation kinetics at the level of individual grains. Our measurements show that the activation energy for grain nucleation is at least two orders of magnitude smaller than that predicted by thermodynamic models. The observed growth curves of the newly formed grains confirm the parabolic growth model but also show three fundamentally different types of growth. Insight into the grain nucleation and growth mechanisms during phase transformations contributes to the development of materials with optimal mechanical properties.

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In situ three-dimensional (3-D) X-ray diffraction experiments have been performed at a synchrotron source on low-alloyed multiphase TRIP steels containing 0.25 wt.% Si and 0.44 wt.% Al and produced with different bainitic holding times, in order to assess the influence of the bainitic transformation on the thermal stability of individual austenite grains with respect to their martensitic transformation. A detailed characterization of the austenite grain volume distribution at room temperature was performed as a function of the prior bainitic holding time. In addition, the martensitic transformation behaviour of individual metastable grains was studied in situ during cooling to a temperature of 100 K. Both the carbon content and the grain volume play a key role in the stability of the austenite grains below 15 lm 3 , while the carbon content exerts the dominant effect in the stability of the bigger grains. Measurements also suggest that the tetragonality of the thermally formed martensite is suppressed.

The in¯uence of the microsegregation of Mn, Si, and Cr on the austenite decomposition during isothermal transformations in hot rolled medium carbon steel has been studied by neutron depolarisation, electron probe microanalysis (EPMA), and optical microscopy. Eight specimens of the same alloy were held at 1173 K for 30 min and were rapidly cooled to different isothermal transformation temperatures. Two-dimensional EPMA maps of the specimen annealed at 1013 K showed that microsegregation of alloying elements in hot rolled steel is strongly related to the ferrite/pearlite band formation. The local variations in alloying element concentration lead to variations in local transition temperatures, which were calculated with the thermodynamic database MTDATA. Similar EPMA maps for the specimen transformed at 953 K demonstrate the presence of microchemical bands, while optical microscopy reveals the absence of microstructural bands. It is shown that the formation of microchemical bands is a prerequisite for band formation, but that the kinetics of the phase transformation determines the actual formation of microstructural bands. A quantitative model has been developed, which describes the observations in terms of the relative difference between ferrite nucleation rates in regions with a high and low local undercooling and the subsequent growth of the ferrite. The isothermal transformation experiments have led to generalised nucleation and growth criteria for the formation of microstructural bands.MST/5150