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 ( ε ̇ ) 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 ε ̇ 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 ε ̇ ≈10−3 s−1.