Evolution of interfacial microstructure and mechanical properties of Nb521/GH99 brazed joints using Ti–35Ni filler alloy

The dissimilar brazing of Nb521 niobium alloy to GH99 superalloy was achieved successfully using Ti–35Ni brazing filler under vacuum. The effects of brazing temperature and holding time were systematically analyzed on the interfacial microstructure evolution and mechanical properties of joints. The joints brazed at 1120 °C for 10 min exhibited a typical interfacial structure composed of Nb521/β-(Nb, Ti) + TiNi/TiNi + Ti₂Ni/TiNi + TiNi₃/Cr-rich TiNi/Ti-rich (Ni, Cr)_ss/(Ni, Cr)_ss/GH99. The findings indicated that as the brazing temperature or holding time increased, the presence of brittle Ti₂Ni compounds decreased while the formation of TiNi₃ gradually increased and tended to coarsen. The shear strength of joints exhibited variations corresponding to changes in interfacial brittle compound, and reached the highest value of 121 MPa at 1120 °C for 10 min. In the context of shear testing, all joints displayed clear brittle fracture patterns, with fractures predominantly occurring at the brittle compounds, namely, Ti₂Ni and TiNi₃ phases.