The mechanisms of the fcc–bcc martensitic transformation revealed by pole figures

Face-centered cubic (fcc) to body-centered cubic (bcc) martensitic transformations occur in many materials, such as steels, FeNi meteorites or brass. The phenomenological theory has been the accepted theory for these transformations for more than half a century. However, it cannot explain the continuous singular features in the experimental electron backscatter diffraction or X-ray diffraction pole figures. Here we show that such patterns can be simulated by one discrete orientation relationship and two continuous rotations that correspond to a trace of the transformation mechanisms. A new theory of martensite transformation that is in full agreement with the experimental pole figures is proposed. In this theory, the fcc–bcc transformation results from a fcc–hexagonal close-packed (hcp) step followed by an hcp–bcc step. The advantages of this two-step theory over the phenomenological theory are discussed.