Superplastic behaviour and mechanical properties of two phase TiAl alloys

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

THE EFFECT OF GRAIN BOUNDARY PRECIPITATION ON THE SUPERPLASTICITY OF Al-Li ALLOYS

The conditions of thermomechanical treatment to obtain superplasticity at high strain rate,~ 10-3 s-1 in 8090 ( Al-Li-Cu-Mg-Zr) and 2090 (Al-Li-Cu-Zr) alloys were investigated and the mechanism of superplasticity is discussed from the metallurgical point of view. It is found that 2090 alloy has excellent superplasticity in an as hot-worked condition compared with 8090 one. In 2090 alloy, the elongation in superplastic deformation increases with decreasing the temperature of hot working to 673K or less, at which temperatures substructures are developed during hot working by acceleration of precipitation. These substructures result in the formation of fine grains, during superplastic deformation. Whereas 8090 alloy has less superplasticity in an as hot-worked condition. In this condition, grain growth is apt to occur during heating or superplastic deformation. To obtain excellent superplasticity in 8090 alloy, it is necessary to homogenize at 793K and cold-work at the reduction of 90% in addition to hot working at a low temperature. This process stabilizes the substructures and inhibits recrystallization. Fine grains form by dynamic recrystallization. After the formation of fine grains, it is important to inhibit the grain boundary precipitation and grain growth in order to improve superplasticity further