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Combining friction stir processing (FSP) with rapid cooling, samples with grain sizes of ∼100, 180, 300 and 500 nm have been produced in commercial 7075 Al by controlling the cooling rate. Microstructure characteristics of the processed materials were investigated. The nanocrystalline structures formed in the sample processed with the highest cooling rate consist of high-angle grain boundaries, and are free of dislocation cell structures. High temperature discontinuous dynamic recrystallization is the mechanism responsible for the nanocrystalline creation. Dislocations and recovery structures were observed in the large grains of samples with slower cooling rates. The developed grains are a result of the evolution of the initial nanocrystals formed around pin tool during the FSP.
Su et al. (Sun,) studied this question.