Formation of nickel aluminide coating on γ-TiAl alloy

A nickel aluminide coating process was developed on γ-TiAl alloy by electroplating a Ni film followed by a high Al activity pack cementation carried out in a vacuum with a mixture of fine Al, NH4Cl, and Al2O3 powders at 1273 K for 18 ks. The coating has a duplex layer structure, an outer Ni2Al3 layer and an inner TiAl3/TiAl2/TiNiAl2 layer. The coated TiAl was oxidized in air for up to 3600 ks under thermal cycling between room temperature and 1173 K. A protective Al2O3 scale formed with little oxide exfoliation and the oxidation amount was 8 g/m2 after the 3600 ks oxidation. During oxidation at 1173 K the as-coated layer structure changed from an outer Ni2Al3 and inner TiAl3/TiAl2/TiNiAl2 layers to an outer β-NiAl and inner TiAl2 and TiNiAl2 layers with voids in the outer layer. The voids appear to be formed by the phase change from the Ni2Al3 to β-NiAl during oxidation. A part of the Ti–Al–Ni phase diagram at 1173 K was developed experimentally and it was shown that the solubility limits of Ti into both the Ni2Al3 and β-NiAl phases were less than 0.5at.%. The coating with more Al in the inner layer that in the outer layer would be formed by inward diffusion of Al, caused by a so-called up-hill diffusion phenomenon. The higher Al content in the inner layer was retained after 3600 ks oxidation.