Deformation and adiabatic heating of single crystalline and nanocrystalline Ni micropillars at high strain rates

Abstract

The deformation behavior of single crystal and nanocrystalline nickel were studied using in situ micropillar compression experiments from quasi-static to high strain rates up to 10^3 s-1. Deformation occurred by dislocation slip activity in single crystal nickel whereas extensive grain boundary sliding was observed in nanocrystalline nickel, with a shift towards more inhomogeneous, localized deformation above 1 s-1. The strain rate sensitivity exponent was found to change at higher strain rates for both single crystal and nanocrystalline nickel, while the overall strain rate sensitivity was observed to be of the same value for both. With increasing high strain rate micropillar compression tests being reported, the issue of adiabatic heating in micropillars becomes important. We report crystal plasticity based finite element modeling to estimate the adiabatic heating, spatially resolved within the pillar, at the highest tested strain rates. The simulations predicted a significant temperature rise of up to 200 K in nanocrystalline Ni at the grain boundaries, and 20 K in single crystalline Ni due to strain localization. Transmission Kikuchi Diffraction analysis of nanocrystalline nickel pillar post compression at 10^3 s-1 did not show any grain growth.