Effect of Nb content on microstructure and mechanical properties in TiAl/Ti2AlNb alloy joint vacuum brazed with Ti50-xZr12.5Hf12.5Cu25Nbx filler metal

Abstract

TiAl/Ti2AlNb alloy composite components can effectively enhance the specific strength of aerospace vehicles. However, the presence of brittle phases and stress-mismatch issues within this composite structure limits its application. In this investigation, Ti50-xZr12.5Hf12.5Cu25Nbx filler metals with varying Nb contents were designed and prepared. The microstructural evolution of the brazed seam was investigated, revealing the strengthening and fracture mechanisms of the TiAl/Ti2AlNb brazed joint. The addition of an appropriate amount of Nb promotes the formation of the ductile β-(Ti, Nb) phase in the brazing joint. When the Nb content in the filler metal reached 4 wt%, Zone II contained the highest fraction of ductile β-(Ti, Nb) phase, which effectively alleviated stress concentration. Meanwhile, the brittle (Ti, Zr, Hf)2Cu phase became discontinuously distributed. With the increase of Nb content in the filler metal, grain refinement occurred across the seam. The proportion of high-angle grain boundaries (HAGBs) in Zone I increased to 94.4%, while low-angle grain boundaries (LAGBs) in Zones II and III rose to 39.2% and 20.2%, respectively. Correspondingly, substructure fractions in these Zones reached 40.7% and 86.6%. Zone II exhibits a maximum kernel average misorientation (KAM) angle of 4.95° relative to Zones I and III. Therefore, Zone II exhibits higher stress-strain values. When the Nb content in the filler metal is 4 wt%, the maximum shear strength of the joint reaches 157 MPa. Cracks initiated at the interface between (Ti, Zr, Hf)2Cu and β-(Ti, Nb) phases, and propagated along this boundary. Fractographic analysis revealed that the fracture occurred in a brittle manner.