DISLOCATION DYNAMICS DURING RATE CHANGES

In crystals which slip by the thermal activation the transient function of the flow stress after a sudden strain rate change depends only weakly on the stress dependence of the activation volume or activation enthalpy. Detailed measurements of the transient function allow therefore a quantitative determination of the change in the preexponential factor and the density of mobile dislocations. Careful experiments on silver single crystals in stage II yield the following results: The ratio ρm2 ρm1 of the equilibrium density of mobile dislocations ρm1 before and ρm2 after a strain rate change, respectively, can be approximated by a power law ρm2 ρm1 = ( a ̇2 a ̇1)n(T). The exponent n(T) varies between 0.3 and 0.6 and increases with increasing temperature. There is also some influence of strain, crystal orientation and tempering. A comparison of stress-relaxation and strain-rate changes on testing machines of different stiffness shows that the apparent activation volume is not modified by changes in the preexponential factor only when obtained on hard machines by stress-relaxation. Measuring with soft machines one has to consider a correction factor. © 1978.