Excellent High‐Strain‐Rate Superplasticity of Fine‐Grained ZK60 Magnesium Alloy Produced by Submerged Friction Stir Processing

ZK60 magnesium alloy is produced via submerged friction stir processing (SFSP), and its microstructural characteristics and deformation behaviors at elevated temperature are investigated. Due to the severe plastic deformation with enhanced cooling rate, dynamic recrystallization occurs but grain growth is significantly retarded. Therefore, the as‐cast microstructure is greatly refined after SFSP, and an average grain size of 1.84 μm is attained. High‐strain‐rate superplasticity (HSRS) is observed at temperature of 573–723 K. A maximum elongation of 1205% is obtained at 1 × 10 −2 s −1 and 673 K. The excellent HSRS is attributed to the uniform fine‐grained structure and the relatively high fraction of high‐angle grain boundaries (71.5%), which facilitate grain boundary sliding. The comparatively high portion of dispersed fine particles/precipitates also enhances the microstructural stability of the SFSP specimens. Grain boundary sliding is the dominated deformation mechanism for superplasticity and the failure of the deformed samples is due to the coalescence of cavities.