High strength Fe–Mn–(Al, Si) TRIP/TWIP steels development — properties — application

Deformation twinning, martensitic phase transformation and mechanical properties of austenitic Fe-(15–30) wt.%Mn steels with additions of aluminium and silicon have been investigated. It is known that additions of aluminium increase the stacking fault energy γfcc and therefore strongly suppress the γ→ε transformation while silicon decrease γfcc and sustains the γ→ε transformation. The γ→ε phase transformation takes place in steels with γfcc⩽20 mJm2. For steels with higher stacking fault energy twinning is the main deformation mechanism. Tensile tests were carried out at different strain rates and temperatures. The formation of twins, α- and ε- martensite during plastic deformation was analysed by optical microscopy, X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The developed light weight high manganese TRIP (“transformation induced plasticity”) and TWIP (“twinning induced plasticity”) steels exhibit high flow stress (600–1100 MPa) and extremely large elongation (60–95%) even at extremely high strain rates of about 103 s−1. Recent trends in the automotive industry towards improved safety standards and a reduced weight as well as a more rational and cost effective manufacturing have led to great interest in these high strength and “super tough” steels.