Effect of metastable retained austenite on dynamic mechanical properties of nanostructured bainitic steel under high-strain rate deformation

In this study, modifying the austenitizing temperature offers a strategically viable way towards creating blocky retained austenite (BRA) with varying sizes and morphologies in nanostructured bainitic steel, in strikingly contrast to the conventional method of adjusting the austempering temperature. Dynamic tensile and compressive evaluation at different strain rates of 1 x 103 s-1 and 5 x 103 s-1 were carried out to gain insights into the effect of BRA stability on the dynamic mechanical properties and its intrinsic relationship with high strain rates. Additionally, the phase transformations and the triggered transformation-induced plasticity (TRIP) effects of BRA at different high strain rates were analyzed. The results reveal that during dynamic deformation, most of metastable BRA underwent a two-stage martensitic transformation of gamma -> sM and sM ->alpha ' M, while the filmy retained austenite exhibits minimal phase transformation. BRA dominates the TRIP effect under the extremely short deformation time and adiabatic heating conditioning. Consequently, samples containing more and largersized BRA demonstrated superior dynamic mechanical properties, including strength and fracture strain. As the strain rate increases, the TRIP effect induced by less stable BRA contributes to more pronounced enhancement of properties. Besides the deterioration effect originating from the early transformed brittle martensite also gradually intensifies with the strain rate, which is manifested by the appearance of quasi-cleavage facets at the fracture comparable to the BRA size. Nevertheless, during dynamic deformation, the metastable BRA induced TRIP effect significantly enhances the mechanical properties, outweighing its deteriorating effect. Moreover, less stable BRA should be favored at higher the strain rate.