Effect of Nb addition in novel amorphous/nanocrystalline Ti-Zr-Cu-Ni-Nb brazing fillers on improving microstructure and mechanical properties of TiAl alloy joints

Abstract

For high-strength brazing of TiAl alloys, novel amorphous/nanocrystalline Ti₄₅Zr₂₅Cu₁₅Ni₁₅, Ti₄₅Zr₂₅Cu₁₀Ni₁₅Nb₅, and Ti₄₅Zr₂₅Cu₁₀Ni₁₀Nb₁₀ (at%) alloys (denoted as Nb0, Nb5, and Nb10, respectively) were designed and synthesized in a melt-spun ribbon form as the brazing filler metals (BFMs). With the increase of Nb content in the Ti-Zr-Cu-Ni(-Nb) alloys, the glass-forming ability exhibited a decreasing trend, while continuous and flexible BFM ribbons with a thickness up to 40 μm could still be obtained since the amorphous phase serves as the matrix of the ribbons. The effect of Nb addition in Ti-Zr-Cu-Ni(-Nb) BFMs on the microstructure and mechanical properties of TiAl alloy joints was investigated. By using the Ti-Zr-Cu-Ni(-Nb) BFMs, the resultant joints brazed at 1223 K for 30 min exhibited the microstructure consisting of α2-Ti₃Al and (Ti, Zr)₂(Cu, Ni) as the matrix and different precipitated phases at the center of the braze zones. For the joints brazed with the Nb0 and Nb5 BFMs, the precipitated phases were α2-Ti₃Al + α-Ti with an island-like morphology. By contrast, in the joint brazed with the Nb10 BFM, the precipitated phases were the almost continuous B2 + O-Ti₂AlNb, and the fraction of the brittle (Ti, Zr)₂(Cu, Ni) phase was reduced. Due to the improved microstructure, by using the Nb10 amorphous/nanocrystalline BFM ribbon, high shear strength up to ∼236 MPa was achieved for the TiAl alloy joint brazed at the relatively-low brazing temperature. It is indicated that the addition of Nb in the Ti-based BFMs is effective in improving the microstructure and strength of the TiAl brazed joints.