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

A nickel alummide coating, developed on gamma-TiAl alloy by electroplating a Ni film followed by a high Al activity pack cementation, has a duplex layer structure with an outer delta-Ni2Al3 layer and an inner TiAl3/TiAl2/TiNiAl2 layer. The coated gamma-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/m(2) after the 36,000 ks oxidation. During oxidation at 1173 K the outer delta-Ni2Al3 changed to beta-NiAl with voids and then to TiNiAl2, and the inner TiAl3/TiAl2/TiNiAl2 layers to TiAl2 and TiNiAl2 layers and then to TiAl2 and tau(3) layers. The voids in the outer layer were formed by the phase transformation from the delta-Ni2Al3 to beta-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. (c) 2005 Elsevier Ltd. All rights reserved.