Deformation mechanism of SS400 steel at very high strain rates using activation volume

Constant strain rate tests were made on SS400 steel at high strain rates from about 2 × 103 to 2 × 104/s, where a dynamic flow stress depended strongly on the strain rate. In order to evaluate quantitatively the strain rate dependence of the dynamic flow stress, decrement strain rate tests were performed at the strain rates above 1 × 104/s. Also, to detect the effect of the strain on the strain rate dependence of the flow stress, the rate reduction tests were conducted at the constant strain rate of about 1 × 104/s at strains of 0.09, 0.16, 0.23 and 0.31, respectively. The flow stress drop caused by the rate reduction was assumed to be a response to an instantaneous strain rate. The results of measurement were treated on the basis of a theory of thermally activated process. The strain rate dependence of the dynamic flow stress which appeared strongly at high strain rates was due to the stress dependence of the activation energy E(τ) for the formation of kink pairs as the dislocation surmounted the Peierls potential barriers. The activation volume decreased with increasing the strain, which meant the strain rate dependence of the dynamic flow stress increased further as the strain increased. © 2015 The Society of Materials Science, Japan.