Superplastic behaviour of two extruded gamma TiAl(Mo,Si) materials

The superplastic properties of two intermetallic Ti-46.8Al-1.2(Mo,Si) and Ti-46Al-1.5(Mo,Si) (at.%) materials produced by arc melting and processed by hot extrusion in the temperature range between 1200 and 1250 °C were studied. The materials exhibited an equiaxic near γ microstructure with γ grains finer than 1 μm and some band like region of γ grains with a size ranging from 5 to 20 μm. The finer grained zone contained a volume fraction of about 12 vol% in the 46.8Al material and about 25 vol% in the 46Al material of finely dispersed α2-Ti3Al particles. Mechanical tests performed on both materials at strain rates ranging from 4.6×10−4 to 10−2 s−1 in the temperature range of 975-1050 °C showed strain rate sensitivity exponents of about 0.5 at most strain rates. A maximum elongation to failure of about 300% was obtained for the 46.8Al material while about 900% was recorded for the 46Al material at 1050 °C at a relatively high strain rate of 8×10−3 s−1. This difference is attributed to the larger volume fraction of α2-phase particles in the 46Al material that leads to a decrease of the number and size of band like regions of coarse γ grains. The microstructure in the fine-grained areas of both materials remains essentially constant during deformation. The mechanical behavior at high temperature of these superplastic materials can be explained by considering grain boundary sliding as the dominant deformation mechanism.