Gradient Nanograined Surface Layer Research Articles

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

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.

Friction and Wear Reduction in Copper with a Gradient Nano-grained Surface Layer.

A gradient nano-grained (GNG) surface layer is fabricated on a commercial-purity Cu sample, in which a significant reduction in the coefficient of friction and the wear loss is obtained compared to the coarse-grained and the nano-grained counterparts. A novel mild ploughing mechanism without subsurface damage has been identified in the GNG sample, giving rise to a much reduced wear rate. Sliding induced surface deformation brings about the unique inhomogeneous substructure in the GNG Cu: the topmost layer persists with nanograins without being oxidized, underneath which deformation is well accommodated by grain coarsening adjacent to the dynamic recrystallization layer. Both subsurface structural evolution and stress field model confirm that sliding-induced strain localization is suppressed, which is responsible for the superior friction and wear behaviors of the GNG Cu.

Tension-induced softening and hardening in gradient nanograined surface layer in copper

With surface mechanical grinding treatment, a gradient nanograined (GNG) surface layer is produced on a bulk coarse-grained (CG) pure Cu, where the grain size increases gradually from 20 nm (topmost surface) to micrometer scale. Microhardness measurements of the GNG/CG sample after tension revealed that tension induces softening for grains smaller than 165 nm and induces hardening above this size. This critical size agrees with the strain-induced saturation grain size of Cu subjected to severe plastic deformation.