Low-temperature plastic deformation and strain-hardening of nanocrystalline titanium

Abstract

Patterns of plastic deformation of nanocrystalline (NC) technical grade VT1-0 titanium, are studied in quasi-static tensile experiments, with the average size of the grain d, ranging from 35 nm to 2 μm, at the temperature interval 4.2 K < T < 395 K. The wide range of grain size, and grain size distribution, was made possible by cryomechanical grain fragmentation, which involves rolling at liquid nitrogen temperature, and subsequent annealing. At temperatures of T ≲ 30 K, smooth deformation curves become wavy, and as the temperature is continually lowered to Tjump ≈ 22 K, they become jagged. A correlation is found between the relative amplitude of the stress jump Δσ/σ and the rate of strain hardening θ = (∂σ/∂e)ė. A significant increase in plasticity is observed, especially noticeable at temperatures T ≲140 K, if a small fraction (≈15%) of submicron-sized grains is present. This is explained by a combination of dynamic grain growth under the influence of tensile stress, and nanotwinning activated in submicron grains. At cryogenic temperatures, abnormal grain growth favors nanotwinning during deformation. In nanometer-sized grains (d ≲ 50 nm), twinning is not observed.