Melting, titanium aluminides

There are two important titanium aluminides Tig A1 which has a hexagonal structure with a density of 4.20 g/cm and a melting point of 1600°C and Ti A1 which has a tetragonal structure with a density of 3.91 g/cm and a melting point of 1445°C. As do all aluminides, they have excellent high temperature oxidation resistance owing to the formation of a thin alumina layer on the surface. They have potential applications in aerospace structures. 

The melting point of titanium is 1670°C, while that of aluminium is 660°C.142 In kelvins, these are 1943 K and 933 K, respectively. Thus, the temperature 625°C (898 K) amounts to 0.46 7melting of titanium and 0.96 melting of aluminium. Hence, at this temperature the aluminium atoms may be expected to be much more mobile in the crystal lattices of the titanium aluminides than the titanium atoms. This appears to be the case even with the Ti3Al intermetallic compound. The duplex structure of the Ti3Al layer in the Ti-TiAl diffusion couple (see Fig. 5.13 in Ref. 66) provides evidence that aluminium is the main diffusant. Otherwise, its microstructure would be homogeneous. This point will be explained in more detail in the next chapter devoted to the consideration of growth kinetics of the same compound layer in various reaction couples of a multiphase binary system. 

The much advanced nickel aluminides and titanium aluminides can be used only up to about 1000 °C because of their limited strength or oxidation resistance or both at higher temperatures, as has been stated before (Sauthoff, 1994). For applications significantly above 1000 °C other less-com--mon phases with higher melting temperatures have to be used. Such phases are available, and examples are shown in Fig. 34 (Sauthoff, 1992). In comparison to the nickel aluminides and titanium alu-... 

When microsecond pulse-heating became popular, the main interest was in liquid refractory metals such as Nb, Mo, Re, Ta, and W. As pyrometry advanced over time and technological interests changed, the focus shifted more and more towards lower-melting metals and alloys, such as noble metals [20] or titanium-aluminides [21,22]. 

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