Microstructural Study of the Phase Transformations Upon Cooling to Room Temperature of High Mn Steels

The phase transformations of high Mn steels during cooling have been characterized in this study. Widmanstatten plates occur in the austenite matrix upon cooling the steels from 1373 K (1100 A degrees C). The Widmanstatten plates are composed of not only the hexagonal close-packed epsilon-martensite but also the face-centered cubic (FCC) micro-twins. The formation mechanism of the Widmanstatten phases is probably various stacking faults induced from Shockley partial dislocations in the austenite. The epsilon-martensitic plates, along with the kappa-carbides, were observed in a Mn-Al steel at 873 K (600 A degrees C). As most of the FCC matrix has transformed to kappa-carbides, the partial dislocations neighboring epsilon-martensitic plates could not glide. The epsilon-martensite retained in the transformed matrix is the strongest evidence to support the above mechanism.