Superior long-term oxidation resistance of Ni-Al coated TiAl alloys

A nickel aluminide coating, developed on γ-TiAl alloy by electroplating a Ni film followed by a high Al activity pack cementation, has a duplex layer structure with an outer δ-Ni2Al3 layer and an inner TiAl3/TiAl2/TiNiAl2 layer. The coated γ-TiAl was oxidized in air for up to 36,000 ks (10,000 h) under thermal cycling between room temperature and 1173 K. A protective Al2O3 scale formed with little oxide exfoliation and the average oxidation amount was 37 g/m2 after the 36,000 ks oxidation. During oxidation at 1173 K the outer δ-Ni2Al3 changed to β-NiAl with voids and then to TiNiAl2, and the inner TiAl3/TiAl2/TiNiAl2 layers to TiAl2 and TiNiAl2 layers and then to TiAl2 and τ3 layers. The voids in the outer layer were formed by the phase transformation from the δ-Ni2Al3 to β-NiAl during oxidation. It was found that after the 36,000 ks oxidation the higher Al contents in the inner layers were better retained than that in the outer layer.