A high-resolution electron microscopy study of steps on lamellar γ-α2interfaces in a low-misfit TiAl-based alloy

Abstract

Abstract The defect character of steps on lamellar γ-α2 interfaces in a quinternary TiAl-based alloy for which the misfit between the α2 and γ phases is less than 0.1 % has been studied using high-resolution transmission electron microscopy. The interfaces consisted of atomically flat coherent terraces separated by interfacial steps across which equal even numbers of {111}γ and (0002)α 2 planes match. It has been found that the diversity of step heights is greater than that reported previously for lamellar TiAl-based alloys. Circuit mapping was used to identify the Burgers vectors of these steps from lattice images obtained at [10l]γ and [l10]γ zone axes. It was found that the Burgers vectors exhibited by the steps varied with both the height and the sense of the step. In each case these Burgers vectors were consistent with the steps being perfect interfacial disconnections as described by Pond's topological theory of interfacial defects. The defect character of the steps and the occurrence of certain combinations of steps of opposite sense was used to infer that the γ lamellae in this alloy grow by a diffusion-controlled step migration mechanism rather than by the glide of partial dislocations as proposed previously.