Formation mechanism of nanostructures in austenitic stainless steel during equal channel angular pressing

An ultra-low carbon austenitic stainless steel was successfully pressed from one to eight passes by equal channel angular pressing (ECAP) at room temperature. By using X-ray diffraction, optical microscopy and transmission electron microscopy, the microstructural evolution during ECAP was investigated to reveal the formation mechanism of strain-induced nanostructures. The refinement mechanism involved the formation of shear bands and deformation twins, followed by the fragmentation of twin lamellae, as well as successive martensite transformation from parent austenitic grains with sizes ranging from microns to nanometres through the processes gamma(fcc)->epsilon(hcp)->alpha'(bcc). After pressing for eight passes, two types of nanocrystalline grains were achieved: ( a) nanocrystalline austenite with a mean grain size of similar to 31nm and (b) strain-induced nanocrystalline alpha'-martensite with a size of similar to 74nm. The formation mechanisms are discussed in terms of microstructural subdivision via deformation twinning and martensite transformation.