DIFFUSION BRAZING OF NICKEL ALUMINIDES

NiAl is a promising candidate material for high-temperature applications. However, NiAl suffers from poor low-temperature ductility and toughness. Thus, in many potential applications, NiAl would have to be used locally, in combination with Ni-based alloys, rather than to form entire component. Hence, suitable technologies are required for NiAl to Ni-based alloy joining. In view of the poor low-temperature ductility and strong alumina-forming tendency of NiAl, diffusion brazing seems to be the most suitable technology for joining NiAl to itself and to Ni-based alloys. This paper examines the diffusion brazing oi NiAl to Ni using Ni-Si-B interlayers and draws comparisons with previous work by authors on NiAl/Ni-Si-B/NiAl and Ni/Ni-Si-B/Ni diffusion brazing. The progression of micro-structural development in the NiAl/Ni-Si-B/Ni joints is compared with that expected from standard models of the diffusion brazing process in which dissolution of the substrate material, isothermal solidification and solid-state homogenization occur sequentially. Evidence is presented that, while events on the NiAl side of the NiAl/Ni-Si-B/Ni joints are not incompatible with standard diffusion brazing process, major anomalies occur on the Ni side of the joint. These anomalies involve the formation of borides, seemingly at the brazing temperature, when that temperature for the ternary Ni-Si-B liquid is below the Ni-B binary eutectic temperature and extensive liquation of the Ni substrate when the brazing temperature is above the Ni-B binary eutectic temperature. Isothermal solidification of the NiAl/Ni-Si-B/Ni joints is dominated by epitaxial growth of the Ni substrate into the joint and is far more rapid than in the case of NiAl/Ni-Si-B/NiAl joining. The origins of these differences are discussed.