Calculated ternary diagram of Ti–Al–Si system

Abstract

The phase equilibria between β (body centred cubic, bcc), α (hexagonal closed packed, hcp), Ti3 Al–α2 (hcp), and Ti5 Si3 (hcp) in the Ti–Al–Si system have been investigated in the temperature range 700–1200°C. Isothermal sections of the ternary phase diagram have been assessed employing thermodynamic software, which uses the compound energy model to describe the phases mathematically. Available experimental phase equilibria results on the Ti–Al–Si system were used to calculate missing binary and ternary interaction parameters and assess isothermal phase diagrams. Extrapolations in the resulting tie triangles indicate the existence of three eutectoid reactions in the Ti rich corner of the ternary diagram: β → α + Ti5 Si3 , α → α2 + Ti5 Si3 , and β → α2 + Ti5 Si3 . Additionally, extrapolations in the β + α2 + α tie triangle observed at 1100°C indicate that two possibilities arise to represent a peritectoid reaction involving α, β, and α2phases: β + α2 → α and β + α → α2 , depending on the alloy composition and the effect of temperature on the solubitlity of Si in the α phase.