X-RAY-SCATTERING FROM SYSTEMS IN EARLY STAGES OF PRECIPITATION

Certain age-hardenable alloy systems can produce zones of large matrix distortion about precipitates having a size misfit with the surrounding matrix. These zones grow with ageing and give diffraction effects that are challenging to interpret. This paper describes such a model, based upon the random positioning of precipitates that produce severely distorted zones in the surrounding matrix. In an intermediate stage, matrix intensity is partitioned into Bragg peaks, static diffuse scattering and quasilines. The latter two forms of scattering are diffuse scattering that sharpens to widths not much broader than those of normal Bragg peaks. When all three forms of scattering are present, they are sufficiently localized about the reciprocal-lattice points to allow a column interpretation of the diffraction model much like that used in the study of cold-worked metals. The strain Fourier coefficients in this case are calculated from an elastic model. Theoretical results include Laue-like scattering from precipitates, which can also be influenced by displacement fields from other precipitates. In this case, the dominance of particle-size broadening reduces the role of strain broadening. Results may be expressed in terms of a few parameters that may be conveniently related to physical properties.