AbstractThe effect of silicon content on the microstructure and phases present in Ti–(20–23)Al–11Nb alloys has been studied in the temperature range 800–1270°C.Four phases,βo,α2, O, and a silicide, are formed. The parentβois ordered at 1270°C. At 1050°Cα2is formed which exhibits a higher silicon solubility than the parentβo. A peritectoid transformationβ0+silicide→α2is proposed. Assuming that niobium substitutes for titanium and silicon for aluminium, energy dispersive X-ray spectroscopic data suggest that theα2phase, unlike that in binary Ti–Al alloys, is highly stoichiometric and of the form (Ti+Nb)3(Al+Si). Similarly the silicide corresponds to binary Ti5Si3with the same site substitution as in theα2phase. The O phase is orthorhombic and similar in composition to theα2which it replaces: its formation is promoted by silicon.MST/3088