Characterization of the invariant reaction involving the L, η, γ and σ phases in the Ti–Al–Nb system

Ti–Al–Nb alloys have shown promising properties for high-temperature applications. Although the phase diagram has been well researched, several controversies exist over the high-temperature phase equilibria. Among them, the invariant reaction involving the L, γ, σ, η phases, which has been classified as either class 1 (eutectic reaction) or class 2 (transition reaction), has not been well developed. To address this invariant reaction, two alloys that cross through the invariant plane during solidification were investigated. Differential thermal analysis in conjunction with detailed microstructural analysis of the as-cast as well as heat-treated samples determined the invariant reaction to be a ternary eutectic reaction. The results of this study indicate that a large energy barrier is associated with nucleation of the σ phase from the liquid. Additionally, extension of the γ phase field is shown to extend further into the Nb-rich corner of the ternary system at elevated temperatures.