Formation of a nanocrystalline structure during direct and reverse martensitic transformations

We studied different methods employing direct and reverse γ→α1→γ transformations for producing a fine fcc structure in austenitic steels alloyed with Ni, Cr or Ti. Finest nanodimensional austenitic crystals were obtained not by fast heating during the α1→γ transformation (or subsequent recrystallization) but by slow heating at a rate of 0.2–0.4 K min−1, when multiplication of γ-crystallite orientations occurred. The electron microscopy showed that 242=576 orientations of the γ-phase appeared in each initial austenitic grain during a single cycle of γ→α1→γ transformations. Crystallographic calculations of all possible misorientations of any adjacent γ-crystals having the Kurdyumov–Sachs relationship suggested that a limited number (approximately 10.5, 49.5, 60°, etc.) of unlike misorientation angles was formed. The nanocrystalline state and predominance of large-angle boundaries between fine γ-crystals account for a very strong strengthening of the steels. This may be used in practical applications.