Dynamic deformation and failure of ultrafine-grained titanium

Dynamic deformation and shear localization of ultrafine-grained (similar to 120 nm) pure titanium are examined. The strain hardening can be considered as having two regimes: below and above a strain similar to 0.04; at this point there is a drastic decrease in the slope. The strain-rate sensitivity of ultrafine-grained titanium is found to be approximately the same as its coarse grained counterpart. Based on experimentally determined parameters, the Zerilli-Armstrong equation is modified to describe the mechanical response of the ultrafine-grained titanium over the strain rate range 10(-5) to 10(3) s(-1). Adiabatic shear banding is examined in a forced shear configuration where large strain is imposed in a narrow region. The microstructure inside the adiabatic shear band consists of a mixture of elongated grains and equiaxed nanograins (similar to 40 nm) that are significantly smaller than the initial grains (similar to 120 nm). The formation of equiaxed nanograins is modeled through a mechanism of rotational dynamic recrystallization. This further reduction in grain size from the one generated by ECAP is interpreted in terms of the Zener-Hollomon parameter for quasistatic and dynamic deformation. The adiabatic shear band eventually fractures by a combination of brittle and ductile failure. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.