Zr addition-dependent twin morphology evolution and strengthening response in nanostructured Al thin films

Abstract Manipulating the twin morphology to achieve the reinforcement of strength is a great challenge in Al with high stacking fault energy. In this work, the influence of Zr addition on the twin morphology and strengthening response of nanostructured Al films was symmetrically studied. The results showed that, for low Zr addition (≤ 4.0 at.%), the Zr atoms were homogeneously distributed within the matrix, while Zr segregation at grain boundaries was evident at higher Zr addition (> 4.0 at.%). Twins were substantially observed in all the films, and the twin morphology was highly dependent on the Zr addition. In the pure Al film, only twins with a single coherent twin boundary were detected. In comparison, nanotwins with coplanar twin boundaries (C-nanotwins) and 9R phase were predominant in the Al-Zr films with Zr addition ≤ 4.0 at.%. Further raising the Zr content, multiple nanotwins (M-nanotwins) coexisted with the C-nanotwins and 9R phase. In particular, a zero-strain twinning mechanism was applied to account for the C-nanotwins and 9R phase formation, and a zig-zag grain boundary feature induced by Zr segregation was responsible for the M-nanotwin formation. The hardness also exhibited a strong Zr dependence that increased monotonically with the Zr addition. The Al-13.4 at.% Zr film displayed a hardness of ∼4.3 GPa, about 11 times greater than the pure Al film. Strengthening mechanisms were quantitatively evaluated, and the highly-promoted hardness was mainly ascribed to the 9R phase and solid solution strengthening.