Surface mechanical attrition treatment (SMAT) method has been widely acknowledged to obtain desired combination of strength and ductility in metals and alloys, and helpfully overcome the "trade-off" between the two mechanical properties. Within researches for more than 20-years, the favorable mechanical properties are related to the typical gradient structure (GS) in metals and alloys prepared by SMAT. In this overview, the principle and process parameters of SMAT are concisely presented. The strengthening mechanisms are interpreted by traditional dislocation-twin theory, hetero-deformation induced (HDI) theory and theory of shear band and strain delocalization. Besides, the strengthening mechanisms and methods to obtain excellent mechanical properties are discussed. Furthermore, both the latest progresses in design of GS materials prepared by SMAT and some other research interests are displayed.

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