Diffusion bonding enhanced by phase transformation in micro-duplex titanium aluminides

Diffusion bonding of a gamma and beta micro-duplex titanium aluminide (Ti-45Al-5Cr) is investigated at temperatures between 1273 and 1473 K. It exhibits good contact at the bonding surface and a grain enriched by chromium across the interface is observed after bonding at 1473 K for 3.8 ks. under 10 MPa. It is expected that this grain formation is due to the beta phase contact at the interface and probably due to chromium diffusion through transformation from beta to α₂ phase. On the other hand, an equi-axed gamma binary alloy (Ti-50Al) exhibits good contact at the interface with increasing temperature and pressure while grain coarsening occurs. A cast and heat-treated gamma plus α₂ alloy (Ti-45Al-5Cr) fails to perform good diffusion contact but unbonding occurs. Grain coarsening is not observed in the gamma and beta micro-duplex alloy, suggesting that grain boundary phase plays a role to suppress grain growth. It is proposed that the mechanism for good diffusion bonding of the gamma and beta micro-duplex alloy is related to enhanced diffusion in chromium by transformation from beta to α₂, in addition to interdiffusion of constituent elements.