Microstructure and phase relationships in Ti-Al-Si system

The phase equilibria between β and α titanium, Ti 3 Al (α 2 ), and Ti 5 Si 3 have been investigated in the temperature range 700-1200°C, for binary and ternary titanium alloys containing 14-22 at.-% aluminium and 1-3·5 at.-% silicon. These results have been used to construct sections of the ternary phase diagram. Silicon reduces the solubility of aluminium in both α and α 2 phases and increases the solubility of aluminium in the β phase. Extrapolation of the experimental data indicates the existence of eutectoid reactions given by β → Ti 5 Si 3 + α 2 and α → Ti 5 Si 3 + α 2 . The existence of a β + α 2 phase field at 1100°C suggests the existence of a β + α + α 2 phase field in the Ti-Al-Si system, and two possibilities arise to represent a peritectoid reaction involving these phases: β + α 2 → α and β + α → α 2 , depending on the alloy composition and on the effect of temperature on the solubility of silicon in the α phase. Finally, results indicate that the microstructure resulting from the allotropic decomposition of the β phase controls the morphology of α + α 2 and α 2 alloys.