Hidden pathway during fcc to bcc/bct transformations: Crystallographic origin of slip martensite in steels

As a classical structural phase transformation with a long history in physical metallurgy, the face-centered cubic (fcc) to body-centered cubic/tetragonal (bcc/bct) martensitic transformation in steels is one of the most studied crystal structure changes induced by either temperature or stress. However, it has yet to be realized that besides the well-known Bain path that leads to {112} twinning in the bct lattice, a hidden pathway could also be activated during the transformation, which leads to the formation of slip martensite and {110} twinning in the bct lattice. We show by symmetry and crystallographic analysis that activation of the hidden pathway is driven by internal or external stress coupled with short-range migration of interstitial carbon atoms. By considering both the hidden pathway and the Bain path, we construct a transformation pathway network for the fcc to bcc/bct transformation, which allows a systematic analysis of defects generation during transformation cycling. A new mechanism for the formation of slip martensite in steels is demonstrated through the analysis of symmetry breaking along both the hidden pathway and the Bain path, by which two distinctive types of martensites, slip and twinning martensites, are placed on an equal footing from a crystallographic point of view. The pathway network provides new insights into structural transformation mechanisms and the nature of transformation induced defects in steels.