EFFECT OF SILICON ON MICROSTRUCTURE OF TI3AL ALLOYS CONTAINING NIOBIUM

The 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 degrees C. Four phases, beta(o), alpha(2), O, and a silicide , are formed The parent beta(o) is ordered at 1270 degrees C. At 1050 degrees C alpha(2) is formed which exhibits a higher silicon solubilit y than the parent beta(o). A peritectoid transformation beta(o) + sili cide --> alpha(2) is proposed. Assuming that niobium substitutes for t itanium and silicon for aluminium, energy dispersive X-ray spectroscop ic data suggest that the alpha(2) phase, unlike that in binary Ti-Al a lloys, is highly stoichiometric and of the form (Ti + Nb)(3)(Al + Si). Similarly the silicide corresponds to binary Ti5Si3 with the same sit e substitution as in the alpha(2) phase. The O phase is orthorhombic a nd sirnilar in composition to the alpha(2) which it replaces: its form ation is promoted by silicon. (C) 1996 The Institute of Materials.