Exceptional tensile properties under cryogenic temperature in heterogeneous laminates induced by non-uniform martensite transformation and strain delocalization

In the present study, the heterogeneous low C steel/304 austenite stainless steel (SS) laminates were observed to have exceptional tensile properties under cryogenic temperature, i.e., the yield strength and the uniform elongation were found to be significantly elevated simultaneously compared to these under room temperature. The underlying deformation mechanisms have been revealed by a novel tensile testing method coupled with in-situ digital image correlation imaging under cryogenic environment, and subsequent microstructure observations. Strain localization was found to initiate from low C steel, propagate across the interface and then towards 304 SS side. While the formed localized strain zone (LSZ) was observed to be delocalized at larger tensile strain due to the propagation of LSZ towards the un-deformed region along the gage length. Martensite transformation was found to be concentrated in the LSZ of 304 SS to regain strain hardening ability and reduce severity of strain concentration. Strain partitioning between 304 SS and low C steel was found to be more significant in the LSZ than that out of the LSZ. The non-uniform martensite transformation along the gage length should be the origin for the strain delocalization in the LSZ, resulting in large ductility in the laminates under cryogenic temperature.