Shear boundaries in lamellar TiAl

Small-angle tilt boundaries and small-angle twist boundaries are familiar features of crystalline microstructures but small-angle shear boundaries are much less well known. Shear boundaries may be composed of cross-grids of either edge or screw dislocations. This paper reports some observations of shear boundaries on {111} planes in tetragonal TiAl which consist only of screw dislocations. In the electron microscope, shear boundaries look very similar to twist boundaries but they differ in that their two sets of dislocations have the same sign whereas twist boundaries have two sets with opposite signs. Asymmetric shear boundaries, composed of two sets of dislocations with different spacings, can be described as a superposition of a symmetric shear boundary and a twist boundary. An extreme case is a simple shear boundary which has just one set of screw dislocations.