Superplastic behaviour and mechanical properties of two phase TiAl alloys

Abstract
High temperature plastic flow properties of two phase TiAl alloys containing 45 to 49 at.%Al have been investigated in thermo-mechanically grain refined materials in order to clarify the favourable microstructure and chemical composition for TiAl based superplastic materials. Grain sizes of thermomechanical treated materials and their grain size stability during subsequent high temperature deformation strongly depend on chemical composition. It was found that Ti-46at%Al offers the best [MATH] ratio which produces a fine and stable microstructure, whilst exhibiting superior superplasticity at temperatures exceedig 1100°C and a strain rate of around 1x10^-4s^-1 (with m-value of 0.44 at 1100°C and 0.64 at 1150°C) as well as preferred mechanical properties at temperature of up to 1000°C. This alloy was proposed as a baseline alloy for superplastic materials to be later modified by third elements effective for the formation of metallic phase. Correspondingly, effects of heat treatment on changes in microstructure and tensile properties have been studied in fine grain TiAl alloys in order to estimate the possibility of improving high temperature strength after superplastic forming. A new kind of microstructure consisting of coarse lamellar colonies and fine colony boundary grains of [MATH] and lamellar (to be refereed to as partially-transformed structure) was found to be obtained by heat-treatment of just above the [MATH]-transus. It was also found that the partially-transformed structures exhibit a better combination of room temperature ductility and high temperature strength than any other microstructure previously observed.