Strain rate sensitivity and activation volume of AISI 321 stainless steel under dynamic impact loading: Grain size effect

Abstract

The rapid change in strain rate sensitivity (SRS) of metals at a critical true strain rate (ε̇Tc) is commonly attributed to change in rate-controlling deformation mechanism from thermally-activated process to viscous drag. In this study, metastable AISI 321 stainless steel with grain sizes of 0.24 μm (ultrafine [G1]), 3 μm (fine [G2]) and 37 μm (coarse [G3]) was deformed at high strain-rate. A change in SRS with increasing ε̇T is observed only in G2 and G3 and it occurs at ε̇Tc of ~5900 and 6800s−1, respectively; suggesting that grain size have an effect on the switch of rate-controlling deformation mechanisms. While the SRS of G1 is 0.101 and unchanged, those of G2 and G3 change from 0.094 to 0.326 and 0.091 to 0.634, respectively, once ε̇Tc is reached. Similarly, the estimated activation volume in G1 specimen is ~1.57b3, while those of G2 and G3 specimens change from ~2.95b3 to ~0.65b3 and from ~4.10b3 to ~0.45b3, respectively. Analysis of the deformed specimens using XRD, EBSD and TEM techniques revealed that the complexities in the activated deformation mechanisms in G2 and G3 specimens led to a change in ε̇Tc. Constant SRS in G1 specimen indicated no change in the operational deformation mechanisms.