Mechanical Behavior of Multi-Phase Steels Comprising Retained Austenite

The retained austenite (RA) in advanced high-strength steel (AHSS) grades, such as dual-phase (DP) steels, plays an important role on their formability. Thanks to the transformation-induced plasticity (TRIP) effect that occurs during the mechanically induced transformation of RA into martensite, additional ductility is obtained. Martensite has a higher flow stress than austenite; hence, the transformation results in an apparent hardening, which is beneficial for the stability of deformation. The stability of RA at a given temperature strongly depends on its carbon content, which, in AHSS, is not uniform but distributed. The aim of this study is to build a model that predicts the transformation as well as TRIP in a DP steel grade with RA. A physics-based kinetic model is presented that captures the transformation of retained austenite based on the thermodynamic driving force of the applied stress. A direct analytical estimate of transformation plasticity is provided, which is consistent with the kinetic model. Transformation kinetics is incorporated in a self-consistent, mean-field homogenization-based constitutive model. Finally, an indication of the effect of transformation of retained austenite on formability is given.

Phase Transformation Kinetics of High Nb-TiAl Alloy during Continuous Heating

In this work, the phase transformation behavior of Ti-45Al-8.5Nb-(W, B, Y) alloy during continuous heating was investigated using dilatometer and optical microscopy. Results indicated that the phase transformation process of high Nb-TiAl alloy during continuous heating included two stages: ordered α2 → disorder α and tetragonal γ → hexagonal α. According to the microstructure analysis, the initial α2/γ lamellar structure transformed into the massive γ phase and α phase (retained as α2) during the heating process. The activation energy of α2 → α and γ → α was 989.65 kJ/mol and 995.30 kJ/mol, respectively. Moreover, the lower the heating rate was, the faster the phase transformation reached the equilibrium state.