Strain rate sensitivity of binary Mg-Gd and Mg-Y solid solutions

The strain rate sensitivity (SRS) of binary Mg-Gd and Mg-Y alloys and the effect of solute on the rate-controlling mechanisms have been studied by analyzing rate changes during tension and compression tests at 78K and 298K. The steady-state SRS evaluated from the slope of Haasen plots at 78K increases with the solute concentration. The reverse behavior is observed at 298K, and the concentrated alloys exhibit the negative SRS. The activation work and the activation distance evaluated from strain rate jump tests at 78K discriminate regimes of plastic flow determined by solute-dislocation and dislocation-dislocation interactions. At the onset of plastic flow, dislocation-solute interactions control the plastic flow properties. This regime of low flow stress is represented by the activation work in the magnitude of 10(-2) eV and the activation distance in the range 10(-2)b -10(-1)b. The deformation by mechanical twinning occurring early in the compression conforms to these parameters and suggest no fundamental difference in the thermally activated glide of ordinary and twinning dislocations through the field of solute obstacles. The results suggest that by-passing of Gd and Y atoms by basal dislocations occurs by sequential activation of partial dislocations. At higher flow stress, the dislocation-dislocation interactions determine the work-hardening. The thermally activated motion of jogs produced during interactions of basal and prismatic dislocations is thought to be the rate-controlling process. The activation work and the activation distance signatures of this process are W* similar to 0.4 eV - 0.8 eV and d similar to 0.5b - 1b, depending on the solute content.