Microstructural study of the parameters governing coarsening and cyclic softening in fatigued ultrafine-grained copper

The cyclic deformation behaviour of ultrafine-grained (UFG) copper produced by equal-channel angular pressing was investigated. Special attention was paid to the parameters governing cyclic softening and cyclic grain coarsening. UFG copper shows significant cyclic softening for the tests performed at intermediate plastic strain amplitudes Deltaepsilon(pl)/2, that is in the range 2 x 10(-4) less than or equal to Deltaepsilon(pl)/2 less than or equal to 1.0 x 10(-3). Within this range, the cyclic softening as well as the intensity of grain coarsening increase with decreasing plastic strain amplitude. By contrast, under stress control, corresponding to a plastic strain amplitude range 2.4 x 10(-5) less than or equal to Deltaepsilon(pl)/2 less than or equal to 1.2 x 10(-4), cyclic softening as well as the intensity of grain coarsening decrease with decreasing plastic strain amplitude. Furthermore, cyclic softening and grain coarsening were also found to be enhanced by decreasing the deformation rate (and thus increasing the test time) and/or by increasing the temperature. These findings indicate that the responsible microstructural processes are thermally activated. Based on the experimental results, a dynamic 'recrystallization' mechanism is proposed and suggested to be responsible for the cyclic grain coarsening and to some extent also for the cyclic softening.