In this talk, I will present our recent atomic-resolution studies on Cu-Fe, Cu-Cr, and Cu-Zn nanocrystalline alloys using aberration-corrected high-resolution transmission electron microscopy and associated techniques (such as quantitative analysis).
1) The oxygen impurity effects in nanocrystalline were carefully explored. A comparative investigation of Cu-Fe nanocrystalline alloys generated from different initial materials with different oxygen contents reveals that oxygen atoms go either to GBs or dissolve into the matrix. The former effectively decreases the grain boundary mobility, facilitating grain refinement, while the latter leads to lattice expansion and interstitial strengthening.   
2) The study on 57 wt%Cu - 43 wt%Cr dual-phase alloys deformed by high pressure torsion (HPT) under 5 and 25 rotations have drawn the following conclusions. (i) an inverse grain-size effect on stacking fault (SF) and twinning was observed in Cu. (ii) During the early deformation stage, in a way, grain refinement in Cu was controlled by the interaction between Cr solutes (or clusters) that were forced to dissolve into the Cu matrix and extended dislocations motion. iii) Twins created during the highly strained state are associated with the grain boundary activities and the recovery procedure in the extremely fine grain size region. Moreover, the interaction between dislocations and twins plays a crucial role in forming the GBs of nanocrystalline materials.

Acknowledgments This work is financially supported by the Austrian Science Fund (FWF): No. P27034 - N20. Professor Reinhard Pippan is gratefully acknowledged for his helpful discussion.

Reference
1. Qinqin Shao et al. Journal of Alloys and compounds 832(2020)154994.
2. Yong Zhang et al., Materials Characterization, 157(2019)109886.
3. Jinming Guo et al, Acta Materialia, 166(2019)281.