Recovery and recrystallization of cold-rolled polysynthetically twinned (PST) crystals of TiAl

Abstract

The recovery and recrystallization behavior of cold-rolled polysynthetically twinned (PST) crystals of TiAl with a nearly stoichiometric composition was investigated as a function of reduction in thickness and subsequent annealing time at 900 and 1000°C. The change in microhardness was found to occur in two stages regardless of the amount of reduction; the first stage associated with the decrease in dislocation density and the second stage related with the annealing-out of deformation twins introduced in TiAl lamallae during rolling. However, the recrystallization mode was found to vary depending on the amount of reduction in thickness. In specimens rolled to less than 20% reduction in thickness, the full recovery in hardness occurs preserving the lamellar structure. In contrast, in specimens rolled to more than 40%, a structure composed of equiaxed grains of TiAl is obtained after recrystallization. A mechanism is proposed to interpret the unusual recrystallization behavior of PST crystals of TiAl cold-rolled to less than 20%.