Microstructure in NiAl/steel joint produced by reactive casting method

A NiAl block has been produced by pouring aluminium and nickel liquids onto a steel block placed in the bottom of a crucible. Due to an exothermic reaction between the elemental liquids, the temperature of the liquid mixture rises very quickly and exceeds 2800 K, which is extremely high compared to the pouring temperatures of 1023 K for aluminum and 1773 K for nickel. The liquid mixture melts the surface of the steel block and soon solidification of NiAl containing small amount of iron starts at the steel surface, and finally the NiAl and steel are strongly joined after solidification. The microstructure at the joint interface has a very fine dual-phase structure of rod-like beta-NiAl and gamma-ferrite. The morphology of the beta-NiAl gradually changes from rod to round grain, as the distance from the interface increases. Ibis gradual change in microstructure brings about a high joint strength exceeding 220 MPa.