Nano-scale observation on the transformation behavior and mechanical stability of individual retained austenite in CMnSiAl TRIP steels

In the present study, the effects of microstructure, chemical composition on the transformation behavior and mechanical stability of individual retained austenite (RA) with the different sizes and morphology in TRIP steels were intensively investigated. In order to characterize the property of the individual RA, various analytical techniques including atom probe tomography (APT), step-wise straining EBSD and nano-indentation were applied. The blocky type RA as we categorized have many defects and lower carbon contents (~25%) compared to film type RA. And also, step-wise straining EBSD and nano-indentation results revealed that the mechanical stability of blocky type RA was lower than that of film type RA which means blocky type RA would be easily transformed into martensite. It is considered that many defects existing in blocky type RA could work as the nucleation site of martensite transformation. In contrast, high carbon contents and hard phases enclosing film type RA would increase the resistance to shear transformation resulting in the inhibition of transformation of RA to martensite.