Improving the microstructure and high temperature properties of the Ti–40Al–16Nb alloy by the addition of a minor Sc or La-rich Misch metal

This work investigated the effect of a slight increase in Nb content of approximately 1 at.% and the effect of adding a minor Sc or La-rich Misch metal (Mm) on the microstructure, phases and high temperature properties of Ti–40Al–15Nb alloy. Microstructural analysis and phase characterizations of the Ti–40Al–16Nb alloy with 0.39 wt.% Sc or La-rich Misch metal additions are summarized as follows. (i) The as-cast Ti–40Al–16Nb–0.39 wt.% M (M=Sc or Mm) alloys have an α2 matrix and contain α-Ti and B2 phase precipitates but few γ phase precipitates. The Sc2O3 or La2O3 oxide dispersoids, formed by internal oxidation, are observed in this work; but Ti3(Al,Sc) presented in the dual phases of γ-TiAl were not found. (ii) Adding Sc and La-rich Misch metal decreases the α-transus temperatures of the TiAl–Nb alloys, possibly stabilizing the α2 phase and inhibiting the B2 or γ phase formation. (iii) After homogenization, the σ phase and γ phase precipitates in the α2 phase matrix and the B2 phase are absent. The distributed morphology of the oxide dispersoids in the TiAl–Nb alloy with added La-rich Mm is more uniform and discontinuous than that in the TiAl–Nb alloy with added Sc, after the homogenization. The compressive peak flow stress increment measured from high temperature compression testing at 900 °C is around 450 MPa and the high temperature strength at temperatures above 900 °C is also enhanced. The significant improvement in high-temperature mechanical properties is attributed to the precipitation of fine oxide particles with high thermal stability in the alloys. Furthermore, adding either a minor Sc or Mm to the alloy increases the strength of adhesion between the oxide layers and the alloys and changes the morphology of the oxide particles formed in the TiAl alloy with a high niobium content.