抄録
A new type of microstructure has been developed by an optimum combination of macroalloying and thermomechanical processing in a high-purity gamma titanium-aluminide based ternary alloy: Ti-47at%Al-3at%Cr. The microstructure that is characterized by a microduplex structure of gamma and beta phases renders superplasticity to the alloy. A chromium enriched beta phase is formed by alloying of chromium as a beta stabilizing element by substitution of aluminum in the stoichiometric gamma composition. The beta phase of a body centered cubic structure is identified by crystallographic analyses with X-ray and electron diffraction and by compositional analyses with electron probe and energy dispersive X-ray spectroscopy. Thermomechanical processing induces precipitation of the beta phase along gamma grain boundaries. High temperature deformation behavior of the gamma plus beta microduplex alloy is studied in comparison with a fine grained binary stoichiometric gamma alloy. The microduplex alloy shows a total elogation of about 450% at an initial strain rate of 5.4×10–4 at 1473K, the strain rate sensitivity parameter being 0.57 far above 0.3 which is a criterion of superplasticity. Evolution of texture and microstructure is characterized for understanding of the mechanism operating in superplastic deformation. High temperature deformation structures of the microduplex alloy show no preferred orientation as an indication of random grain rotation. The beta phase is observed to spread along grain boundaries during deformation, contributing to promotion of the grain boundary sliding mechanism of superplasticity as well as to stabilization of the fine grained structure. Formation and coalescence of cavities at the interfaces of gamma and beta grains determine the limit of superplastic elongation.
