The formation and evolution of defect microstructures in irradiated materials is analysed in the framework of a dynamical model for the evolution of the two fundamental defects of irradiated microstructures, namely vacancy and interstitial clusters. The effects of irradiation on materials is described by dynamical equations for two mobile atomic size species (vacancies and interstitial atoms), and two basic immobile elements of the microstructure (vacancy and interstitial clusters). It is shown that uniform vacancy and interstitial loop distributions may become unstable during irradiation and that they will form large-scale spatially organized distributions, in a specific range of irradiation and material conditions. The selection and stability of the resulting microstructures are studied in the quasi-static approximation and in the weakly non-linear regime around the bifurcation point. It is shown that, after transients corresponding to three-dimensional BCC patterns, the final pattern should correspond to planar wall structures in agreement with experimental observations.
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