Mapping the strain-rate and grain-size dependence of deformation behaviors in nanocrystalline face-centered-cubic Ni and Ni-based alloys

Tensile tests over a wide range of strain rates (1.04 x 10(-6) to 1.04 s(-1)) were performed on electro-deposited nanocrystalline (NC) Ni and Ni-Co alloys with different grain sizes to systematically investigate the coupling effects of strain rate and grain size on the mechanical behaviors. It was found that the grain size significantly affected the dependency of mechanical responses to the imposed strain rate. In particular, peculiar fluctuations of flow stress and elongation-to-failure with variation of strain rate were observed in the Ni-8.6 wt%Co alloy with the finest grain size. By carefully analyzing the changes in strain rate sensitivity exponent and apparent activation volume, such unique phenomenon was rationalized in terms of the synergy of grain size and strain rate on the transition of deformation mechanisms including thermal-activated and mechanical-driven grain boundary (GB) activities, interactions of dislocations with GBs and diffusional creep, which act alone or in concert to dominate the plasticity. Building on classifying the characteristics of plastic deformation on the nano-scale, a two dimensional deformation mechanism map was proposed to comprehensively elucidate the interactive effects of grain size and strain rate on the deformation mechanisms and the related mechanical behaviors in face-centered-cubic NC Ni and Ni-based alloys. (C) 2017 Elsevier B.V. All rights reserved.