On the strain rate sensitivity of aluminum-containing transformation-induced plasticity steels: Interplay between TRIP and TWIP effects

The primary objective of the study is to elucidate the effect of strain rate on the deformation behavior of Al-containing transformation-induced plasticity steels (TRIP) via combination of depth-sensing nanoindentation experiments and post-mortem analyses of deformed steels using transmission electron microscopy (TEM). The strain rate sensitivity decreased with increased Al-content. The activation volume of 2Al-steel was ~ half (6b3) of 6Al-steel (11b3), where b is the magnitude of the Burgers vector. The strain rate influenced the evolution of strain-induced martensite (TRIP effect), dislocation slip and deformation twinning (TWIP effect). The interplay between TRIP and TWIP effects as a function of strain rate is analyzed and discussed in terms of the three internal energies, namely γ→α (austenite→martensite) transformation Gibbs free energy, strain energy and stacking fault energy. These were impacted by the Al-content of the steels, which altered the austenite stability and propensity to deformation twinning. The study provides insights into the design of next generation of TRIP steels for the fabrication of automotive components.