Single step combustion synthesis of β-NiAl-coated γ-TiAl by microwave ignition and subsequent annealing

β-NiAl-coated γ-TiAl-based alloys were synthesised following a single combustion synthesis step in a mono mode microwave applicator operating at 2.45 GHz under a moderate pressure of 0.15 MPa, allowing the simultaneous synthesis and forming of the required shape. Two reactive cylindrical specimens composed respectively of substrate (Ti + Al, 50:50 at.%) and coating (Ni + Al, 50:50 at.%) powder mixtures were positioned in the region of maximum electric field strength of the microwave applicator. The high exothermicity of the reactions involved, together with the application of pressure, allowed synthesising in a self-propagating regime the new intermetallic phases and to promote a good adhesion between them, due to the formation of complex reaction interface belonging to the Ni–Al–Ti system. A subsequent isothermal annealing treatment at 1230 °C led to the obtainment of the desired homogeneous γ-TiAl structure into the substrate without affecting the single β-NiAl phase constituting the coating. The anomalous presence of annealing twins was observed and explained considering the different states in which combustion synthesis of aluminides in the Ni + Al, Ti + Al and Ni + Al + Ti systems occurs.