Effect of Ni, Si and Cr in the structural formation of diffusion aluminide coatings on commercial-purity titanium

To increase the lifetime and the ductility of the brittle diffusion layer Al3Ti, utilized as a protective coating against oxidation at high temperature, a trialuminide coating process was developed on commercial-purity Ti by hot-dipping aluminizing baths containing Ni, Si and Cr as added alloying elements and followed by two vacuum annealing treatments.The first was carried out at 700 °C for 48 h to transform the Al coating into Al3Ti, the second up to 870 °C for 170 h to complete the formation of aluminide layers, to verify the sufficient thermal stability of the diffusion coatings under the α–β Ti transition temperature and to complete the diffusion of the ternary Ni, Si and Cr added to the aluminizing baths. Compared to aluminide coatings composed by Al3Ti, AlTi and AlTi3 obtained by pure aluminium in the aluminizing bath after annealing at 870 °C, the as-coated aluminized layers obtained by the presence of Ni, Si and Cr in aluminizing baths, after annealing at 870 °C, shaped also as Si-rich and Ni-rich phases at the Al3Ti/AlTi interface. In the case of Si, as an added alloying element, the Si-rich phase may act on the Al inward diffusion and the outward Ti diffusion inside the coating forming a diffusion barrier and prolonging the lifetime of the protective aluminides. In the case of Ni and Cr addition, there is a possibility of transforming the brittle tetragonal DO22 structure to a cubic L12 Al3Ti structure with reduced hardness and increased cracking resistance of the transformed protective diffusion aluminide. This study showed that the non-structural Al3Ti coating layer, owing to its brittleness and hardness, can be transformed into a structural phase with lower hardness and brittleness up to 870 °C and that the formation of diffusion barriers, which depress the Al inward diffusion and the outward Ti diffusion, can reduce the decomposition of the unstable aluminides at high temperatures, prolonging their lifetime.