Polycrystalline materials give contrast from their grain structure in scanning acoustic micrographs. Bulk wave propagation in anisotropic materials can be calculated from the Christoffel equation. The result can be visualized as a slowness surface of k/ω, which is the reciprocal of the phase velocity (and from which the group velocity can be determined). In the surface of an anisotropic material a pure Rayleigh wave can exist only in certain symmetry directions; in general, surface and pseudo‐surface waves dominate the reflectance function and hence the contrast in acoustic microscopy. These different modes can be measured by analysing V(z) measured with a cylindrical (line‐focus‐beam) lens. The V(z) response for a spherical (imaging) lens may be calculated from a complex mean reflectance function, and this enables the contrast to be understood from polycrystalline samples of aluminium, nickel, and copper. The contrast from grain boundaries requires separate treatment.
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