Preparing bulk nanocrystalline Cu–Al alloys via rotary swaging

Although nanocrystalline (NC) metals and alloys have been studied for nearly 40 years, their preparation is limited to the laboratory as large-scale; low-cost commercial production remains a challenge. In this study, high-strength bulk NC Cu–Al alloys were prepared from coarse-grained Cu–Al alloy rods via rotary swaging. Rotary swaging is characterized by the low cost and infinite length of the processed samples; therefore, it can advance the industrial application of bulk NC alloys. Core–shell-structured Cu–Al alloy rods with a hard NC core (diameter of 2.2 mm) wrapped in a soft ultrafine-grained (UFG) shell with a thickness of 1.75 mm were prepared using a rotary swage. Tensile tests revealed that the hard NC Cu–Al alloy core exhibited an ultimate tensile strength of 1034 MPa, which surpassed current strength records. Microstructural characterization showed that the hard NC core was composed of NC fiber grains with widths of 45 nm and lengths of 190 nm. The edge of the rod contained numerous low-angle grain boundaries and shear bands, which provided it with a lower strength and higher elongation than those of the center. During swaging, strong (200) and (111) fiber textures perpendicular to the cross-section were produced during the early stages of deformation. In the latter deformation stages, the polar densities of the (200) and (111) textures weakened, and some complex textures were formed along with high-angle grain boundaries. The grain refinement mechanisms were dominated by multiple deformation twinning, stacking faults, and dislocation slips. Finite elemental analysis showed that triaxial compressive stress and a high strain rate were applied to grain refinement. In addition, the softer shell protects the harder core during deformation, preventing fracture. This study verified an effective preparation technique for bulk NC materials via rotary swaging because it is a simple process with low cost and broad industrial prospects.