Relationship between Strength and Grain Size of Friction Stir Processed and Annealed High Purity Aluminum

A relationship between yield stress and grain size was examined in FSP-ed and annealed 5N-Al (99.9996% purity) in order to reveal the relationship on materials with equal chemical composition, because the large positive deviations of yield stress from the Hall–Petch relation obtained by plots with coarse grain size were reported in the fine grain sizes with SPD processed and we focused on amount of impurities during SPD processing about this phenomenon. The purity of FSP-ed samples on this study were 99.9988% Al (Fe +8at.ppm). Annealing this sample at various temperatures, the relationship between yield stress and grain sizes was obtained on materials with equal chemical composition. However, the yield stress of sample as FSP-ed is higher than that following Hall-Petch relation obtained by subsequently annealed samples plots. As a result, the positive deviation is occurred by factors other than the impurities.