Influences of Thermal Martensites and Grain Orientations on Strain-induced Martensites in High Manganese TRIP/TWIP Steels

Strain-induced martensites in high manganese TRIP/TWIP steels were investigated in the presence of thermal martensites and under the influence of austenitic grain orientation by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). Before deformation, the morphology of alpha'-M depended mainly on the number of variants and growing period. Regardless of martensite morphologies and deformation, the Kurdjumov-Sachs (K-S) orientation relationships always maintained. The 6 alpha'-M variants formed from a plate of epsilon-M were of 3 pairs of twins with a common axis < 110 >(alpha') parallel to the normal of {111}(gamma) habit plane to minimize transformation strain. When alpha'-M could be formed only by deformation, it nucleated at the intersection of e-M variants and grew mainly in thick epsilon-M plates. Thick epsilon plates promoted significantly the alpha'-M and weakened the influence of grain orientations. During tension, the transformation in < 100 >-oriented grains was observed to be slower than that in < 111 >-oriented grains. Deformation twins promoted epsilon-M formation slightly and had no apparent effect on alpha'-M. Deformation increased the number of epsilon-M variants, but reduced that of alpha'-M variants.