Nucleation of the lamellar decomposition in a Ti–44Al–4Nb–4Zr alloy

Abstract The onset of the lamellar decomposition (α⇒α2+γ) in a titanium aluminide alloy containing Nb and Zr has been studied by transmission electron microscopy. Samples water-quenched from the solution-treatment temperature of 1350 °C show fault-like features resembling those reported previously as the precursors for the formation of the γ lamellae. High-resolution lattice images obtained from such features have revealed that the “faults” are actually embryonic γ lamellae, just a few atomic layers in thickness, which clearly exhibit the ordered L10 structure. This implies that the γ phase is formed directly, rather than via some intermediate disordered face-centred-cubic phase as suggested previously. Moreover, the character and configuration of the interfacial defects is consistent with this occurring in a diffusive-displacive manner with short-range fluxes across the risers of mobile perfect interfacial disconnections.