Structural chemistry and phase relations in intermetallic systems Ti–{Pd,Pt}–Al

Abstract

Phase relations in the ternary systems Ti–{Pd,Pt}–Al have been experimentally established for the partial isothermal sections at 950°C in the Pd/Pt-poor region (<25 at.% Pd/Pt). The investigation is based on X-ray powder diffraction, metallography, SEM and EMPA techniques on about 45 alloys, which were prepared by various methods employing arc melting, levitation melting under argon or by powder reaction sintering in closed crucibles. Three ternary compounds were observed at 950°C in the Ti–Pd–Al system: τ 3-(Ti,Pd)(Ti,Pd,Al) 2 with Laves-MgZn 2-type, τ 2-(Ti,Al) 6(Ti,Pd,Al) 23+1 with a filled Th 6Mn 23+1-type and τ 1-(Ti,Pd,Al)(Ti,Pd,Al) 3 with AuCu 3-type. Due to the wide extension of the Laves phase field, there is no compatibility among γTiAl and τ 2-(Ti,Al) 6(Ti,Pd,Al) 23+1. The Ti–Pt–Al system at 950°C contains three ternary compounds: τ 3-(Ti,Al)(Ti,Pt,Al) 2 with Laves-MgZn 2-type, τ 2-(Ti,Al) 6(Ti,Pt,Al) 23+1 with the filled Th 6Mn 23+1-type and τ 1-(Ti,Pt,Al) with Cu-type. Compatibility exists for Al-rich γTiAl and τ 2-(Ti,Al) 6(Ti,Pt,Al) 23+1. The typical feature for both alloy systems studied is the three-phase equilibrium: α 2Ti 3Al+γTiAl+τ 3-(Ti,Pd/Al)(Ti,Pd/Pt,Al) 2. The solid solubility of palladium and platinum in the binary titanium aluminides, as observed from EMPA and X-ray data, is rather small and at 950°C accounts to about 2.5 at.% Pd and 2.0 at.% Pt. Two new oxide compounds Ti 3PdAl 2O x and Ti 3PtAl 2O x with a filled Ti 2Ni-type are observed in both quaternary systems.