Microstructural analysis and growth mechanism of single-step aluminum–titanium diffusion coatings on a nickel-based substrate

Aluminum–titanium coatings have shown promising results for some aggressive high temperature environments in which protective oxides can hardly form. In this study, a series of aluminum–titanium diffusion coatings were developed on a nickel-based substrate via a single-step halide activated pack cementation process. Powder mixtures of aluminum, titanium, aluminum oxide and ammonium chloride were prepared with different mass ratios of aluminum to titanium. Coating experiments were carried out at 850, 900, 950, 1000 and 1050 °C for at least four hours. Coated specimens were characterized using conventional metallography, glow discharge optical emission spectroscopy and X-ray diffraction analysis. The results show that a zone of NiAl phase is formed in all the coatings right above the coating–substrate interface. Other zones towards the surface including τ2-Al2NiTi and τ1-(Al,Ni)3Ti in some of the specimens and τ4-AlNi2Ti and Ti2Ni in most of the others were found. The growth mechanism of the coatings was discussed based on the microstructural features.