**4. Conclusions**

A hierarchical nanostructure of nanocrystalline Au–Cu alloys containing nanotwins was produced by electrodeposition from sulfite-based electrolyte. Microstructure investigation revealed that average grain sizes of the alloys were about 20 nm, and twin boundaries were observed in the nanograins. Due to the fine grain size, average spacings of the twins were all less than 10 nm, and this confirmed continuous strengthening was observed when the average twin spacing is thinned downed to sub 10 nm region. By making a hierarchical structure of twinned nanograins having the size in the HP region but close to the iHP region, a high yield strength of 1.5 GPa was obtained. In addition, the stress drop observed in the stress–strain curve was caused by evolution of the deformation twins, and the deformation twins were formed because of the reduced stacking fault energy in the Au–Cu alloys.

**Author Contributions:** Conceptualization, H.T., T.-F.M.C., and M.S.; data curation, H.T. and T.-F.M.C.; validation, H.T., C.-Y.C., H.I., and D.Y.; formal analysis, H.T. and T.N.; investigation, H.T. and Y.-W.C.; resources, M.S.; writing—original draft preparation, H.T.; writing—review and editing, T.-F.M.C.; visualization, H.T.; supervision, T.-F.M.C. and M.S.; project administration, K.M. (Katsuyuki Machida) and M.S.; funding acquisition, K.M. (Kazuya Masu) and M.S.

**Funding:** This work was supported by JST CREST grant number JPMJCR1433, Japan and the Grant-in-Aid for Scientific Research (S) (JSPS KAKENHI grant number 26220907).

**Acknowledgments:** The authors thank Suzukakedai Materials Analysis Division, Technical Department, Tokyo Institute of Technology, for TEM specimen preparation.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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