High-resolution electron microscopy of steps on misfitting lamellar gamma-alpha(2) interfaces in a Ti-44 at.% Al-8 at.% Nb alloy

The defect character of steps on lamellar gamma-alpha(2) interfaces in a te rnary TiAl-based alloy has been studied using high-resolution transmission electron microscopy. Most of the steps are identical to those observed prev iously in a low-misfit quinternary alloy, for which equal even numbers of ( 111)(gamma) and (0002)(alpha 2) planes match across the step riser, and the Burgers vectors depend on the height and sense of the step. Occasional ste ps were also observed with unequal numbers of (111)(gamma) and (0002)(alpha 2) planes across the step riser including a 3/4 step with b = 1/3[(111) ov er bar] and a 7/8 step with b = 1/3[<(2)over bar 1(2)over bar>]. In all cas es the Burgers vectors were consistent with the steps being perfect interfa cial disconnections as described by Pond's topological theory of interfacia l defects. This implies that the gamma lamellae in this alloy grow by a dif fusion-controlled step migration mechanism rather than by the glide of part ial dislocations as proposed previously.