Bloch waves and weak-beam imaging of crystals

The influence of the number of diffracted beams on weak-beam contrast simul ations of thickness contour lines and dislocation images is investigated. F or large deviation parameters s(g)-->thickness contour lines from two-beam simulations are similar to those from many-beam simulations. in many-beam s imulations of wedge-shaped bent samples extra thickness contour lines appea r at locations with ((g) over right arrow, 3 (g) over right arrow. These ex tra lines occur between the imaging condition ((g) over right arrow,-(g) ov er right arrow) and ((g) over right arrow, 3 (g) over right arrow). Therefo re, in the case of a more symmetrical imaging condition many-beam simulatio ns are mandatory. In bent samples the contributions of different Bloch wave s to weak-beam images change as a function of the imaging conditions ((g) o ver right arrow, x (g) over right arrow). Near ((g) over right arrow, 3.5 ( g) over right arrow) two Bloch waves dominate. In the case of x <3 two othe r Bloch waves with different wavelengths are most important for the image c ontrast. The 'classical' (g) over right arrow, 3 (g) over right arrow) weak -beam condition is not suitable to determine signs and magnitudes of Burger s vectors from terminating thickness contour lines. Higher deviation parame ters s(g)-->are necessary especially for dense dislocation arrangements.