The effect of temperature on activation volume of ultrafine grained tantalum

The effect of temperature (from 77 to 473 K) on strain rate sensitivity, apparent and effective activation volume of ultrafine grained tantalum processed by equal-channel angular pressing (ECAP) is investigated in the present work. Microstructural characterization was performed using transmission electron microscopy (TEM) and electron-backscatter diffraction (EBSD). The strain rate sensitivity (SRS) was measured by strain rate jump tests at different temperatures. The SRS shows a monotonous increase with increasing the temperature. A small yield point phenomenon at 77 K during strain rate jump tests was observed because of the low mobility of screw dislocations. The Helmholtz free energy calculated according to the experimental data was estimated to be 1.14 eV. This suggests that the thermal activation process is controlled by dislocation slipping. However, the apparent and effective activation volumes show a power law dependence on the temperature, indicating a variation of rate controlling mechanism in the intermediate temperature range (from 373 to 473 K). It is concluded that the Peierls hill is much lower in the intermediate temperature thus the mechanism of intersection of dislocations becomes more difficult controlling the deformation instead.