Brittle aluminides

A continuous compact alumina layer with a thickness of about 1 p,m was found on the surface of specimens coated with TiAls and oxidised at 900°C for 1000 cycles (Fig. 14.10(a)). The thermal barrier coating exhibited an excellent adhesion to the alumina layer, which was probably formed during the pre-oxidation heat treatment [10]. In the subsurface zone beneath the alumina layer, a large number of cracks were observed as a result of the brittleness of the TiAla diffusion layer [17] (Fig. 14.10(b)). Figure 14.11 shows the elemental profiles across the subsurface zone. The concentrations of Ti and A1 beneath the oxide scale did not correspond to the chemical composition of the aluminides TiAls or TiAl2. The amount of aluminium was... 

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