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

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 mu m, at the temperature interval 4.2K < 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 less than or similar to 30 K, smooth deformation curves become wavy, and as the temperature is continually lowered to T-jump approximate to 22 K, they become jagged. A correlation is found between the relative amplitude of the stress jump Delta sigma/sigma and the rate of strain hardening theta - (partial derivative sigma/partial derivative e)((e) over dot). A significant increase in plasticity is observed, especially noticeable at temperatures T less than or similar to 140 K, if a small fraction (approximate to 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 less than or similar to 50 nm), twinning is not observed. (C) 2014 AIP Publishing LLC.