Diffraction contrast from non-spherical distortions—in particular a cuboidal inclusion

Equations are derived for the displacement field due to a coherent cuboidal inclusion. The general treatment of the displacements due to a coherent precipitate of any shape or arrangement and for any set of elastic constants is indicated. Using two-beam dynamical equations and the column approximation it is shown that with transmission electron microscopy, a cuboidal inclusion can be distinguished from a spherical one by a ‘notch’ in the line of no contrast and/or an asymmetric image, when the operating reflection corresponds to a plane which is not a symmetry plane of the cube. Experimental observations on a Ni-Ti alloy confirm this result.