In situ nuclear magnetic resonance investigation of deformation-generated vacancies in aluminum

Enhanced atomic diffusion under plastic deformation was observed in aluminum by means of an improved rotating-frame nuclear-spin-relaxation technique. The enhancement is caused by excess vacancies formed by the deformation process. 27Al NMR experiments were carried out as a function of temperature on polycrystalline, pure aluminum foils during deformation at constant strain rates. Evaluation of the data yields the actual concentration of the excess vacancies as a function of temperature, strain, and strain rate. The findings are in accord with a model which describes the formation mechanism of the vacancies through a fraction of the applied deformation power density (applied stress times strain rate) and the annihilation process by the diffusion of the vacancies to dislocations acting as vacancy sinks.