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

Abstract 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-Ni 2 Al 3 layer and an inner TiAl 3 /TiAl 2 /TiNiAl 2 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 Al 2 O 3 scale formed with little oxide exfoliation and the average oxidation amount was 37 g/m 2 after the 36,000 ks oxidation. During oxidation at 1173 K the outer δ-Ni 2 Al 3 changed to β-NiAl with voids and then to TiNiAl 2 , and the inner TiAl 3 /TiAl 2 /TiNiAl 2 layers to TiAl 2 and TiNiAl 2 layers and then to TiAl 2 and τ 3 layers. The voids in the outer layer were formed by the phase transformation from the δ-Ni 2 Al 3 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.