On interpreting weak beam images of microtwins

Abstract

The concept of a dynamically coupled set is of fundamental importance in carrying out theoretical calculations of image contrast of microtwins. Up to the present, considerations based on the dynamical theory have indicated that the diffracted beams taken into account in a particular calculation of image contrast must always reside in the same dynamically coupled set. The work presented in this paper indicates that for weak beam images this may not the case. The reason for this can be seen from Fig.1b in whichg, hand (i), (j) are reciprocal lattice vectors and branches of the dispersion surfaces in the matrix and microtwin respectively. It can be seen that if the condition (h − g) = Δγnfis obtained then the Bloch wave vectors in the matrix and twin,and, are parallel. Under these circumstanceswill make a coherent contribution to the intensity of diffracted beam g, even though h does not reside in the same dynamically coupled set.