Excellent strength-ductility synergy of Cu-Al alloy with a gradient nanograined-nanotwinned surface layer

Abstract

Gradient nanostructured (GNS) metallic materials have shown significant potential in achieving excellent mechanical properties. However, the underlying strengthening mechanisms resulting from plastically inhomogeneous deformation in GNS materials, particularly those involving nanotwinned structures, remain inadequately understood. In this study, a gradient nanograined-nanotwinned (GNNT) surface layer was generated on a Cu-Al alloy using the severe plastic deformation technique known as single-point diamond turning. Uniaxial tensile and localized micropillar compression tests were conducted to compare the mechanical properties of GNNT samples with conventional gradient nanograined (GNG) and uniform coarse-grained (CG) Cu/Cu-Al counterparts. The results revealed that the GNNT samples exhibit excellent strength-ductility synergy, with a yield strength of approximately 329 MPa, tensile strength of around 477 MPa, and uniform ductility of about 48 %, thereby demonstrating remarkable strain hardening capacity and mechanical stability. These characteristics are further supported by the observed strong back stress effect, well-preserved mobile dislocation density, and effective suppression of grain coarsening in the GNNT surface layer. In contrast to the evident mechanically induced grain coarsening through grain boundary (GB) migration in the GNG surface layer, grain coarsening through twin boundary (TB) migration in GNNT surface layers is effectively inhibited by Lomer-Cottrell (L-C) locks formed by TB-stacking fault and TB-TB intersections, resulting in significantly enhanced structural stability. Furthermore, the L-C locks also extensively contribute to the high density of mobile dislocations observed in the GNNT samples. These findings provide valuable insights into the optimization of GNS structures for achieving excellent strength-ductility synergy in metallic materials.