Work hardening and inherent plastic instability of nanocrystalline metals

We report an investigation of the mechanical behaviour of nanocrystalline (nc) Pd and Pd(90)Au(10) with mean grain size around 14 nm. All specimens were brittle in tension and ductile in compression. We analyze the parameters that control a material's potential for ductility in tension, namely strain hardening exponent, n, and strain rate sensitivity, m. The results reveal low values, n approximate to m approximate to 0.05. This constitutive behavior is known to promote plastic instability and, hence, rapid fracture in tension. Therefore, our results suggest that a tendency for localized deformation is an inherent feature of nc materials. It appears that the limited ductility of nc materials in tension is not related to microstructural flaws, but is a consequence of the concomitant low values of n and m. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim