Orthorhombic titanium aluminides

Of the titanium aluminide intermetallic phases, only TiAl2, TiAl3, and certain t phase compositions can form a protective AI2O3 scale over a wide range of temperatures in air. They will be treated first, followed by the y, oui, and orthorhombic phases. 

Figure 6-26. Schematic partial Ti-Al-Nb phase diagram (at.%) showing relevant phase equilibria for 2 and orthorhombic-based titanium aluminides (after 900 °C isotherm of Ward, 1992).

At higher niobium levels, the 02 phase evolves to a new ordered orthorhombic structure that is based on the composition Ti2AlNb (O phase), which has been observed in titanium aluminides with compositions finm Ti-25Al-12.5Nb to Ti-25A1-30Nb (Ref 1-4). The crystal structures of the o2 and ordered orthorhombic phases are compared in the accompanying figure, which shows the basal planes and atomic positions in the lanes above and below the plane of sheet. 

The titanium aluminide Ti2AlNb with an ordered orthorhombic crystal structure rather than the ordered hexagonal DOiq structure of Ti3Al was stronger and has higher fractvire toughness than... 

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