The Relationship between Microstructure and Fracture Behavior of TiAl/Ti2AlNb SPDB Joint with High Temperature Titanium Alloy Interlayers

In this paper, spark plasma diffusion bonding technology was employed to join TiAl and Ti2AlNb with high temperature titanium alloy interlayer at 950 degrees C/10kN/60 min, then following furnace cooling at cooling rate up to 100 degrees C/min. After welding, the joint was aging heat-treated at 800 degrees C for 24 h. The microstructure and the elements diffusion of the TiAl/Ti2AlNb joint was analyzed by field emission scanning electron microscopy (FESEM) with EDS. Moreover, the tensile properties of the joint were tested at room temperature, 650 degrees C, and 750 degrees C. The results show that the spark plasma diffusion bonding formed a high quality TiAl/Ti2AlNb joint without microcracks or microvoids, while also effectively protecting the base metal. Significant differences in the microstructure of the joint appeared from TiAl side to Ti2AlNb side: TiAl BM (Base Metal) -> DP(Duplex) and NG (Near-Gamma) -> alpha(2)-phase matrix with needle-like alpha-phase -> bulk alpha(2)-phase -> needle-like alpha-phase -> metastable beta-phase -> Ti2AlNb BM. After heat treatment at 800 degrees C for 24 h, the microstructure of the TiAl side and the interlayer region did not change, but the density and size of the needle-like alpha-phase in region 3 increased slightly. The microstructure of Ti2AlNb near the weld changed obviously, and a large number of fine O phases are precipitated from the metastable beta phase matrix after heat treatment. Except for the Ti2AlN near-interface region, the effect of heat treatment on the microstructure of the joint is not significant. The microhardness of the joint is in the shape of a mountain peak. The maximum microhardness at the interface is above 500 HV, and it is significantly reduced to 400 HV after heat treatment. The fracture of the joint occurred at the interface at room temperature, 650 degrees C, and 750 degrees C. with the tensile strength 450 MPa, 540 MPa, and 471 Mpa, respectively, and mainly showing brittle fracture.