Effect of strain rate on deformation mechanism for ultrafine-grained interstitial-free steel

Ultrafine-grained (UFG) interstitial-free steels with grain sizes of 0.39, 0.42, and 0.51 μm were used to ascertain effects of the strain rate (ε̇) on the primary deformation mechanism at room temperature. Tensile tests were performed to obtain the strain-rate sensitivity exponent of 0.2% proof stress. The value was evaluated as 0.02 at high strain rates but as −0.01 at low strain rates. The transition was observed at ε̇ of 10⁻³ s⁻¹ for each sample. Although the negative m value might result from strain aging, the influence of grain boundary sliding (GBS) increased remarkably at a low strain rate because it reached 76% of plastic strain and became about six times as much as that at a high strain rate. Therefore, it is claimed that the dominant deformation mechanism was changed by the strain rate from dislocation motion to GBS with decreasing strain rate across ε̇≈10⁻³ s⁻¹.