Interfacial reaction of infrared brazed NiAl/Al/NiAl and Ni3Al/Al/Ni3Al joints

The early-stage microstructural evolution of NiAl/Al/NiAl and Ni3Al/Al/Ni3Al in temperature ranges between 800 and 1200°C for 2–290 s was studied by infrared brazing. Al3Ni phase is firstly formed in both Al-rich melt and the interface between nickel aluminide base material and Al-rich melt for specimens brazing at 800°C. The solid-state interdiffusion between Al3Ni and base material results in the formation of the interfacial Al3Ni2 phase, and further growth of Al3Ni2 phase at 800°C is impeded by the Al3Ni interlayer due to its stoichiometry. For specimens brazing at 1000°C, the reaction changes from hypereutectic into peritectic reaction. The Al-rich melt dissolves more Ni atoms with the increment of brazing temperature to 1000°C. The Al3Ni2 phase is now initially formed in both the Al-rich melt and joint interface. The growth of Al3Ni2 interlayer at 1000°C is much faster than that at 800°C. Transport of Al atoms in forming Al3Ni2 phase at 1000°C is greatly increased due to the contact between Al3Ni2 and liquid Al-rich melt. The Al3Ni shown in the joint is formed upon cooling cycle of the infrared brazing. The microstructural evolution in Ni3Al/Al/Ni3Al joint is similar to that in NiAl/Al/NiAl except for the formation of NiAl phase between Al3Ni2 and Ni3Al. It can also be attributed to the solid-state interdiffusion between Ni3Al and Al3Ni2 interlayer. However, the intermediate phase Ni5Al3, which is stable below 700°C, is not found in the experiment.