On the formation and growth of grain boundary -carbides in austenitic high-Mn lightweight steels

The precipitation of grain boundary (GB) kappa-carbides critically influences the damage-tolerance ability of high-Mn high-Al lightweight steels, particularly in harsh environments (cryogenic and H environments). The formation and growth behavior of these carbides thus need to be understood. In this work, we use atom probe tomography and four-dimensional scanning transmission electron microscopy to study the formation mechanisms of GB kappa-carbides in a Fe-28Mn-8Al-1.3C (wt.%) steel that is aged at 550 degrees C, a temperature at which GB kappa-carbide formation is often believed to be delayed or avoided. We observe that the formation of GB kappa-carbides results from spinodal decomposition of grain interior (GI) kappa-carbides and their further interaction with GB planes rather than from heterogeneous GB nucleation, as has been formerly proposed. However, the subsequent growth of GB kappa-carbides shows different kinetics in comparison to GI kappa-carbides. The underlying reason and its implications for future microstructure design of such steels are discussed.