Ni–Al–Ti coatings obtained by microwave assisted SHS: Effect of annealing on microstructural and mechanical properties

Microwaves at 2.45 GHz have been used to ignite the self-propagating high-temperature synthesis (SHS) in a Ni and Al powder mixture to produce NiAl coatings on Ti. The temperature of the reacting powders, owing to the exothermic nature of the reaction and to the energy provided by the high frequency electromagnetic field, reaches the melting temperature of NiAl. The newly formed NiAl, in the liquid phase, reacts with the underlying Ti and rapidly forms at the interface a ternary eutectic belonging to the Ni–Al–Ti system. During cooling, the formed transient liquid eutectic layer at the Ti/NiAl interface originates an intermediate layer presenting an interlocking network, which constitutes an in-situ reinforcement phase. In this study it is shown that the thickness of the interface layer can be altered by annealing at temperature higher than the ternary eutectic temperature (904 °C), in a few minutes time. Experimental results demonstrated also that, due to its peculiar microstructure, the intermediate layer is tougher and harder than the parent NiAl coating.