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/Ni3A l in temperature ranges between 800 and 1200 degreesC for 2-290 s was studi ed 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 f or specimens brazing at 800 degreesC. The solid-slate interdiffusion betwee n Al3Ni and base material results in the formation of the interfacial Al3Ni 2 phase, and further growth of Al3Ni2 phase at 800 degreesC is impeded by t he Al3Ni interlayer due to its stoichiometry. For specimens brazing at 1000 degreesC, the reaction changes from hypereutectic into peritectic reaction . The Al-rich melt dissolves more Ni atoms with the increment of brazing te mperature to 1000 degreesC. The Al3Ni2 phase is now initially formed in bot h the Al-rich melt and joint interface. The growth of Al3Ni2 interlayer at 1000 degreesC is much faster than that at 800 degreesC. Transport of Al ato ms in forming Al3Ni2 phase at 1000 degreesC is greatly increased due to the contact between Al3Ni2 and liquid Al-rich melt. The Al3Ni shown in the joi nt is formed upon cooling cycle of the infrared brazing. The microstructura l evolution in Ni3Al/Al/Ni3Al joint is similar to that in NiAl/Al/NiAl exce pt for the formation of NiAl phase between Al3Ni2 and Ni3Al. It can also be attributed to the solid-state interdiffusion between Ni3Al and Al3Ni2 inte rlayer. However, the intermediate phase Ni5Al3, which is stable below 700 d egreesC, is not found in the experiment. (C) 2001 Elsevier Science Ltd. All rights reserved.