Deformation Mechanisms in Nanocrystalline Nickel at Low Temperatures

Abstract

It is the aim of the present paper to quantify and visualise the grain size induced transition of the deformation mechanism in metal polycrystals from the conventional dislocation–dislocation interaction at large grain sizes to (probably) dislocation–grain boundary interaction in the “nano” region. Since both types of interaction are thermally activated, thermal activation analysis can be used to discriminate between them. For this purpose dynamic tensile tests with stress relaxation tests were performed on pure pulsed electrodeposited nickel with 140 nm grain size at temperatures between 4 and 320 K. The results clearly indicate the transition temperature to be around 77 K. A rather unexpected result is the existence of a second transition of the deformation mechanism, which is only observable at very low temperatures namely from the homogeneous deformation mode governed by conventional dislocation–dislocation interaction towards localized deformation by “catastrophic shear”.