Microstructure Change and Mechanical Properties in Binary Ti-Al Containing Ti3Al

Grain morphology, phase stability and mechanical properties in binary Ti-Al alloys containing 43-52 mol% Al have been investigated. Isothermal forging was used to control the grain sizes of these alloys in the range of 5 to 350 mu m. Grain morphology and volume fraction of alpha(2) phase were observed by optical metallography and scanning electron microscopy. Compressive properties were evaluated at room temperature, 1070 K, and 1270 K in an argon atmosphere. Work hardening is significant at room temperature, but it hardly took place at 1070 K and 1270 K because of dynamical recrystallization. The grain morphologies were determined as functions of aluminum content and processing conditions. The transus curve of alpha and alpha+gamma shifted more to the aluminum-rich side than was the case in McCullough's phase diagram. Flow stress at room temperature depends strongly on the volume fraction of the alpha(2) phase and the grain size, whereas flow stress at 1070 K is insensitive to the alloy composition or the grain size, and flow stress at 1270 K depends mainly on the grain size. The alpha(2) phase in the alloys does not increase the proof stress at high temperatures. These observations indicate that improvement of both the proof stress at high temperature and the room temperature ductility should be achieved to obtain slightly Ti-rich TiAl base alloys.