Investigation on activation and strain-rate sensitivity in ultrafine-grained tantalum

The activation volume and strain-rate sensitivity (SRS) of a plastic deformation process and their relationship are investigated in the present work. Through theoretical modeling based on the movement of both isolated kinks and kink pairs in a dislocation line, a new form of theoretical relationship between the strain-rate sensitivity and activation volume was developed, which reduces to the conventional and frequently used form of the same relationship when SRS is approaching zero. The strain rate jump test during the necking stage of tensile testing process is validated thtough combined theoretical analytical approaches, for obtaining strain rate sensitivity of materials with short uniform tensile stage, such as UFG metals. To validate the theoretical approaches and modeling presented in this work, strain-rate jump tests and repeated stress relaxation experiments were conducted, to measure the SRS and activation volume of commercial pure ultrafine-grained tantalum prepared by equal channel angular pressing. The experimental relationship between the strain-rate sensitivity and the activation volume of these tantalum samples fits well with the new form of theoretical relationship, developed in this investigation. Experimental data from literature on nickel confirms the new form as well. The new form of theoretical relationship between strain-rate sensitivity and activation volume can be applied for both bcc and fcc metals as long as the major plastic deformation mechanism is dislocation gliding. (C) 2015 Published by Elsevier B.V.