On the microstructures of NiAl to titanium and NiAl to carbon steel joints obtained by microwave assisted SHS

Microwaves at 2.45 GHz can ignite the self-propagating high-temperature synthesis (SHS) in a 50 at% Ni and Al powder mixture to produce NiAl coating on Ti and carbon steel substrates. During microwave assisted SHS, the temperature of the compacted reacting powders, owing to the exothermic nature of the reaction, reaches the melting temperature of NiAl. Moreover, microwaves are able to convey energy to the system even during and after SHS occurred, a condition which can not be achieved by conventional heating methods, relying on heat transfer. In this study, microwave heating was used to extend the temporary presence of a molten intermetallic phase. This allowed the formation of reaction layers belonging to the Ni-Al-Fe and Ni- Al-Ti systems, at t he interface between the newly formed NiAl and the metallic substrates, achieving chemical bonding. This process involved different metallurgical phenomena such as melting, dissolution, diffusion and different solidification routes. The chemical effects, the phase transformations, the resulting interface microstructures were examined.