COATING OF Ni-ALUMINIDES ON TiAl INTERMETALLICS THROUGH UP-HILL DIFFUSION

摘要

A TiAl intermetallic was electrolytic-plated with 10μm Ni, followed by high Al activity pack cementation at 1273K for 5h to form a coated layer with an inner TiAl{sub}2/TiAl{sub}3/Al{sub}2TiNi layer and an outer Ni{sub}2Al{sub}3 layer. The coated layer grew due to inward, up-hill, diffusion of Al. The Ni-aluminized TiAl was oxidized in air for up to 1,000h under a thermal cycling condition between room temperature and 1173K. Mass gain per unit surface area was 8g/m{sup}2 after 1,000h and there was little exfoliation of the Al{sub}2O{sub}3 scale. After oxidation for up to 1,000h it was found that the coated layer maintains a duplex layer structure of an inner TiAl{sub}2/Al{sub}2TiNi layer and an outer Al-rich β-NiAl layer; the inner layer contains more Al than the outer layer. Both the Ni{sub}2Al{sub}3 and the Al rich β-NiAl contain less than 0.5at% Ti, in accordance with the experimentally determined phase diagram.