High-order hierarchical nanotwins with superior strength and ductility

Microstructures dominate the mechanical properties of metals because of the interactions of dislocations and other defects. Hierarchical nanotwins (HNTs) in different orientations/orders are more effective in interacting with dislocations, which will enhance the mechanical properties of materials. However, the confirmation of high-order HNTs (>2) has not been formally established to fully explore the potential of mechanical property enhancement of HNT structured materials. In this work, HNTs up to five orders with sequential identification in pure silver (Ag) were fabricated by combining the techniques of surface mechanical attrition treatment and uniaxial pre-loading. The strength of this high-order HNT-structured Ag has been improved by over three times without sacrificing ductility. In addition, compared to other structures, the HNT structure can increase the limit of strength of the materials at high ductility. By performing in situ transmission electron microscopy, the HNT generation mechanism is revealed in real time, and the generation conditions of stable HNTs are established. Molecular dynamic simulations present the structure evolution details of HNTs at atomic scale, and the dislocation-based theory unveils the underlying mechanism. By using this generic approach, high-order HNT (≥5) structures can be achieved in crystalline materials.