Diffusion Bonding Associated with Phase Transformation in (γ+β) Micro-duplex Titanium Aluminides

Abstract

Microstructure and bonding strength of diffusion-bonded y titanium aluminide alloys have been investigated focusing on phase transformation during diffusion bonding. High resolution scanning electron microscopy (SEM) observation revealed that lamellar grains are evolved near a jointed interface in (γ + β) micro-duplex alloys bonded at high temperatures, while not alloys bonded at low temperatures. This result is consistent with the proposed TTT diagram with the a lamellar nose. The transformation accompanied by the redistribution of Cr is evidenced by calculating Cr composition in each phase. Lamellar structure is also observed at a localized region in the (y + β) micro-duplex ternary alloy bonded at high temperatures. It is speculated that this region is exposed to a stress high enough to accelerate the transformation, thereby shifting the lamellar nose to shorter time or lower temperature in the TTT diagram. The phase transformation in the (y + β) micro-duplex ternary alloy is due to the low thermal stability of the β phase, enhancing atomic mass transport by Cr redistribution. Mechanical test revealed the high bonding strength in (y + β) micro-duplex alloys, in which fracture was characterized by rugged fractography across lamellar boundaries in fracture surface. Thin layer is produced at the jointed interface uniformly in the micro-duplex alloy and inhomogeneously in the other two samples.