One-step model of the face-centred-cubic to body-centred-cubic martensitic transformation

A 'one-step' theory based on Pitsch distortion is proposed to explain the continuity between the orientations of the Kurdjumov-Sachs (KS), Nishiyama-Wassermann (NW) and Pitsch variants observed on the electron backscatter diffraction pole figures of martensitic alloys. The Pitsch distortion respects the hard-sphere packing of the iron atoms and implies the existence of a neutral line along the close-packed direction [110](gamma) = [111](alpha). Its principal strains are 0, -5.7 and 15.5%, well below the +12, +12 and -20% values of the Bain distortion. At the nucleation step, the distortion generates martensite that continuously deforms the austenitic matrix. Martensite continues to grow in Pitsch orientation inside the deformation field of the surrounding austenite to reach KS and NW orientations in reference to the bulk austenite. Some experimental results reported in the literature are revisited, such as the {225}(gamma) habit planes, the sometimes observed 'twins' at the midrib, the formation of butterfly martensite, and the effect of prior plastic deformation of austenite on the transformation temperature and on variant selection mechanisms.