Enhancing thermal shock resistance of TiZrCuNi brazed joints via strategic niobium addition

This research conducted the brazing test of Ti-48Al-2Cr-2Nb (at.%) (Ti4822) and Ti-22Al-25Nb (at.%) (Ti₂AlNb) dissimilar intermetallic alloys using (TiZrCuNi)_1−xNb_x (wt.%) composite powder fillers. The microstructure, diffusion behavior of the alloying elements, volume fractions and distribution of the phases within the Ti4822/Ti₂AlNb joints were characterized using scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and electron backscatter diffraction (EBSD) methods. The microhardness of the containing phases was measured with the nanoindentation technique. The tensile properties of the joints were tested at room temperature and the fracture mechanism was analyzed from fractography. The experimental results indicated that the composite powder filler, which incorporates Nb particles into the TiZrCuNi powder using machine milling, enables reliable metallurgical bonding between Ti4822 and Ti₂AlNb. The brazed joints mainly consist with α₂-Ti₃Al, (Ti, Zr)(Ni, Cu)₃, and B2. The participation of the refractory Nb particles introduced more S/L interfaces into the joints, which allowed the molten filler to achieve isothermal solidification more quickly. The dissolved Nb elements also played a role in stabilizing the B2 phase and inhibiting the precipitation of (Ti, Zr)(Ni, Cu)₃ compounds. This both alleviates the residual thermal stresses typically present in conventional brazed joints of dissimilar alloys due to the thermal cycle and effectively improves the plastic deformation capacity of the joint. The Ti4822/Ti₂AlNb joint brazed with the filler containing 7.5 wt% Nb particles at 980 °C for 60 min exhibited the highest tensile strength of 410.85 ± 43.32 MPa, Additionally, this joint exhibits excellent thermal shock resistance, with no degradation in tensile strength after 50 times of rapid thermal cycle. Room-temperature tensile fracture occurred at the boundary between the α₂-Ti₃Al and B2 phases. The fracture surface exhibited both ductile dimples and cleavage surfaces, indicating a composite fracture mode.