Activation volume in fine grained metals from stress relaxation and nano-indentation

Fine grained copper was studied using the stress relaxation technique and creep testing in nano-indentation, to determine the activation volume involved in the micro-mechanism of the deformation. This material exhibits a near-perfect elasto-plastic deformation, featured by a steep work-hardening, after the elastic domain, followed by flow at a constant stress. Measurements of the activation volumes in the various domains reveal the role of the dislocations and the variation in the dislocation density in the deformation mechanism. This emphasizes the importance, in the determination of the activation volume, of the deformation domain investigated as well as the testing technique used and whether in both cases, the measurement is carried out in a transient domain or condition where variation in dislocation density occurs.