Stacking-Fault Energy Measurements in Fe-Mn-Al-Si Austenitic TWIP Steels

The stacking-fault energy (SFE) is a composition- and temperature-dependent materials property that can greatly influence the deformation mechanisms of austenitic steels. A low SFE increases partial dislocation spacing and leads to reduced dislocation mobility via an impedance to cross slip. This reduction in dislocation mobility will induce the formation of twins during deformation. These substructures will create further barriers to dislocation glide causing high strain-hardening rates that inhibit local necking and result in total elongation-to-failure of up to 80 % [1]. The goal of this research is to determine the SFE experimentally for new Fe-Mn-Al-Si twinning-induced plasticity (TWIP) steels in order to gain a better understanding of the influence of this parameter on the relationship between structure and mechanical properties. Three Fe–XMn-3Al-3Si (X = 22, 25 and 28 wt%) alloys were induction melted, thermo-mechanically processed by hot and cold rolling, and recrystallized at 900oC for 30 min. After 1.5% tensile strain, electro-discharge machined 3-mm diameter disks were further annealed at 400°C, 650°C, and 700°C to induce equilibrium defect configurations. TEM samples, prepared by grinding the disks to 100 µm thickness and twin-jet electro-polishing in 30 % HNO3 and 70 % methanol at-30oC and 13.5 V, were analyzed using a Philips CM20T operating at 200 keV. Figure 1 shows th