Producing nanograined microstructure in Mg–Al–Zn alloy by two-step friction stir processing

 Nanograined structures with an average grain size of ∼85 nm have been achieved in solution hardened AZ31 magnesium alloy by two-pass friction stir processing under rapid heat sink in which the second pass has a lower heat input. The mean hardness of the nanograined region reaches ∼1.5 GPa (or 150 Hv), about three times that of the matrix. The evolution of the nanograined structure is also investigated.