PNNL

Abstract

The martensite phase transformation dependence upon deformation modes and strain paths in a medium manganese (10 wt.%) TRIP steel stamped into a T-shape panel was quantified through combination of a 3D digital image correlation and synchrotron X-ray diffraction. The T-shape emulates a portion of a common anti-intrusion component. The stamping speed was kept intentionally slow (1 mm/s) so as to avoid excessive heat generation. The steel, which belongs to the third generation advanced high strength steel (3GAHSS) family, was chosen for two reasons: (1) it is two-phase, i.e. austenite and ferrite, with martensite resulting from deformation-induced phase transformation; (2) the 66 vol.% initial retained austenite volume fraction (RAVF) enabled a thorough examination of the martensite phase transformation at large deformation levels without exhaustion. Strain fields were coupled with measured RAVF values of small specimens extracted from specific locations on a formed T-shape panel. This enabled an exploration of the effects of linear, bilinear and non-linear strain paths as well as deformation modes such as tension, plane strain, biaxial tension and equibiaxial tension. Results suggest a significant martensite phase transformation dependence on deformation mode and strain path in the absence of fracture and when martensite phase transformation is unaffected by heat generated during forming. In general, the greatest decrease in RAVF occurs under biaxial tension, while the smallest decrease occurs under plane strain. Some discussion as to further application of the experimental methods detailed in this study to other 3GAHSS and the effects of fracture on martensite phase transformation is provided.