Effects of strain rate on tensile ductility in Cu-added stable Fe–Cr–Ni-based austenitic stainless steels

Abstract

The mechanical properties of the austenitic stainless steels (ASSs) at high strain rates are receiving significant attention for interpreting the mechanical behavior of automotive members during the severe forming operations. Herein, the effects of Cu, as a stacking-fault energy (SFE)-increasing element, on the deformation mechanisms and consequent tensile ductility were investigated in the stable Fe–Cr–Ni-based ASSs by varying the tensile strain rate. At a slow strain rate (6.67 × 10−4 s−1), the elongation of the ASSs decreased considerably from 66 to 46% with increasing Cu content, whereas a fast strain rate (6.67 × 10−3 s−1) resulted into a decrease in the elongation from 47 to 42%. The decrease in the elongation with increasing strain rate was significantly smaller in the 4.0 wt.% Cu-containing ASS (4.0Cu steel, 4%) than that in the 0.8 wt.% Cu-containing ASS (0.8Cu steel, 19%). Two primary mechanisms of elongation-deterioration, i.e., (i) the increase in the SFE by the addition of Cu and (ii) the strain localization promoted by the fast strain rate, were competing. At a slow strain rate, the former mechanism was dominant, whereas the latter barely worked. At a fast strain rate, the former one was overridden by the latter mechanism because of the early occurrence of local instability even when the twin formation was activated, resulting in a small variation in the elongation of the three investigated steels.