The defect nature of steps on lamellar γ-α2 interfaces in a quinternary TiAl-based alloy, where the misfit between the phases was less than 0.1%, was studied by using high-resolution transmission electron microscopy. The interfaces consisted of atomically flat coherent terraces which were separated by interfacial steps across which equal even numbers of {111}γ and (00•2)α2 planes matched. It was found that the diversity of step-heights was greater than that reported for lamellar TiAl-based alloys. Circuit mapping was used to identify the Burgers vectors of these steps, on the basis of lattice images which were obtained for [10¯1]γ and [¯110]γ zone axes. It was found that the Burgers vectors which were exhibited by the steps varied with respect to both the height and sense of the step. In each case, the Burgers vectors were consistent with the steps being perfect interfacial disconnections; as described by Pond’s topological theory of interfacial defects. The defect nature of the steps, and the occurrence of certain combinations of steps of opposite sense, were used to deduce that the γ lamellae grew via a diffusion-controlled step migration mechanism rather than via the glide of partial dislocations.

A High-Resolution Electron Microscopy Study of Steps on Lamellar γ-α2 Interfaces in a Low-Misfit TiAl-Based Alloy. P.Shang, T.T.Cheng, M.Aindow: Philosophical Magazine A, 1999, 79[10], 2553-75