In-situ Joining of Nickel Monoaluminide to Iron by Reactive Sintering

Abstract

A cylindrical block of nickel monoaluminide, NiAl, is produced from a mixture of nickel and aluminum powders by sintering a powder compact under a pseudo-isostatic pressure, and is simultaneously joined to an iron block with the same shape. When the joining couple of the powder compact and the iron block is heated to approximately 900 K, a violent exothermic synthesis reaction, Ni + Al → NiAl, suddenly starts, and the temperature of the compact quickly rises owing to the heat of reaction and exceeds the melting point of NiAl, 1 911 K. Because the molten NiAl wets the contacting surface of the iron block, an iron-rich NiAl-base alloy and an iron-base ternary solid solution are produced on each side of the joining interface. No cracks or cavities are formed in the NiAl even in the vicinity of the joining interface. Hardness continuously changes across the joining interface from approximately 330 in NiAl to approximately 55 in iron. All of five specimens of a four-point bending test fractured in NiAl, the fracture surface being 1 to 2mm away from the joining interface.