High-resolution electron microscopy of steps on misfitting lamellar γ-α2 interfaces in a Ti-44at.% Al-8at.% Nb alloy

Abstract

The defect character of steps on lamellar γ-α2 interfaces in a ternary TiAl-based alloy has been studied using high-resolution transmission electron microscopy. Most of the steps are identical to those observed previously in a low-misfit quinternary alloy, for which equal even numbers of {111}γ and (0002)α2 planes match across the step riser, and the Burgers vectors depend on the height and sense of the step. Occasional steps were also observed with unequal numbers of {111}gamma and (0002)alpha2 planes across the step riser including a 3/4 step with b = 1\\3[111] and a 7/8 step with b = 1\\3[212]. In all cases the Burgers vectors were consistent with the steps being perfect interfacial disconnections as described by Pond's topological theory of interfacial defects. This implies 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.