Strong and thermally stable nanocrystalline Cu–Al alloy via Al segregation

Nanocrystalline (NC) metals and alloys are prone to mechanical and thermal instability under force and thermal fields due to their high Gibbs free energy, which limits their industrial applications. In this work, by employing rotary swaging (RS), bulk NC Cu–15 at.% Al alloys with both high strength and high thermal stability were prepared. Quasi-static tensile test results show that the yield strength is 1016 MPa. Moreover, the grain growth temperature was retarded up to 0.4 T_m, higher than the literature values. Microstructural characterizations revealed that after RS deformation, coarse-grained Cu–Al was refined into fibrous NC grains with a diameter of 45 nm and a length of 190 nm, and the contents of high-angle grain boundaries (GBs), low-angle GBs, and twin boundaries are 17%, 45%, and 38%, respectively. Moreover, there is a significant multiscale chemical fluctuation within the grains, at the GBs, and between the grains through extreme defect accumulation. The atomistic simulation suggests that the segregation behavior of Al solute is essentially driven by the atomic size and local stress state. Besides, Al segregation greatly reduces the grain boundary energy, which further improves the thermal stability of the material. The main strengthening mechanism is Hall–Petch strengthening and the strengthening brought by the chemical fluctuations. Our work provides ideas for designing strong and thermally stable bulk NC alloys.