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

The onset of the lamellar decomposition (alpha double right arrow alpha(2) + gamma) 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 degreesC show fault-like features resembling those reported previously as the precursors for the formation of the gamma lamellae. High-resolution lattice images obtained from such features have revealed that the faults are actually embryonic gamma lamellae, just a few atomic layers in thickness, which clearly exhibit the ordered L1(0) structure. This implies that the gamma 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. (C) 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.