Athermal martensite: Genesis of microstructure and transformation curves

The austenite partitioning concept is reviewed and modified to take into consideration the influence of the austenitic grain size on the genesis of plate martensitic microstructures. Two modes of martensitic transformation are analyzed. The “spread” mode is characterized by the initial formation of clusters of martensite plates in only some austenitic grains, with gradual spreading of plate formation into all austenitic grains. The “fill-in” mode is characterized by initial martensite plate formation uniformly in all austenite grains, with subsequent transformation proceeding with the nucleation of smaller and smaller martensite plates filling in smaller austenitic volumes created by the partitioning caused by earlier-formed plates. Equations for “partitioning” and for transformation curves as a function of temperature are obtained and validated by measurements of plate martensite formation in Fe-Ni-C and Fe-C alloys.