A transmission electron microscopy study of strain-induced secondary twin and epsilon-martensitic transformation in Fe-15Mn-2Cr-0.6C-0.06N austenitic steel

Abstract

Strain-induced twin formation and gamma(f.c.c.)-austenite to epsilon(h.c.p.)-martensite phase-transformation are important processes for mechanical properties of high-Mn austenitic steel. One-directional primary twin and epsilon-martensite (T epsilon) are sequential and bi-sequential structures of stacking faults (SFs) formed with the passage of 1/6 <112> Shockley partial dislocations along every and every other {111} plane of the f.c.c. structure, respectively. As strain increased, secondary T epsilon, which do not satisfy the Shoji-Nishiyanma (S-N) orientation relationship, were developed by the intersection of two primary T epsilon-bands. The sequential process for the formation of secondary T epsilon-phase was studied by selective area diffraction patterns (SADPs) and high-resolution scanning transmission electron microscopy (STEM) analyses and compared with two atomistic models: (i) rigid-body rotation model and (ii) epsilon martensite {1 (1) over bar0(2)}(e) twinning-shear model. The rotation of the slip plane within a grain, referred to as the disclination process, is also developed by forming the secondary T epsilon-band at the intersection of two primary T epsilon-bands. The thickness change of four arms of primary T epsilon-bands contributes to the rotation of each subgrain of the gamma-matrix.