Stress-state-dependent deformation and fracture behaviors in a cold-rolled 7Mn steel

Abstract

Tensile tests were conducted on a series of specially designed specimens to understand how the stress state affected the deformation and fracture characteristics of a recently developed 7Mn steel containing a high fraction of retained austenite (RA). The digital image correlation (DIC) method was utilized to measure the strain and displacements applied to the specimen shoulders. The results show that the fracture strain was sensitive to the stress state that changed from pure shear (PS) with a stress triaxiality of zero to plane strain with stress triaxiality of approximately 0.58. Microstructural observations indicate that the uniaxial tensile (UT) specimens exhibited more rapid transformation kinetics from austenite to martensite compared with those in-plane PS and notched tensile (NT) specimens. This was primarily related to the difference in the stability of RA resulting from the change in austenite grain morphologies with the imposed stress state. Finally, special attention was drawn to the characteristics of the microscopic deformation behaviors related to the austenite-to-martensite transformation through the detailed microstructural evolution mechanism diagrams under three typical stress states.