In a face-centered cubic structure (FCC) metal or alloy, the twin boundary can effectively hinder the dislocation movement, so that the high-density nano-twin metal or alloy has both excellent strength and plasticity. For FCC copper, the growth/annealing twins are usually rectangular and contain abundant non-coherent twin boundaries (Incoherent Twin Boundaries,ITBs). The deformation twin usually has a spindle shape, and its interface is a step-shaped common lattice twin boundary (Serrated Coherent Twin Boundaries, SCTBs), which consists of a common lattice twin boundary and a twin order fault (Twinning Disconnections, TDs) step. The growth of deformation twins is through the movement of twin order faults, which consist of incomplete dislocations in one direction, usually with only one atomic layer height. Therefore, the structural difference of the twin interface leads to the obvious difference in the morphology of the growth/annealing twin and the deformation twin of FCC metal. A large number of studies have reported the formation mechanism of the twin interface, but the essence of the formation of the interface still needs experimental evidence.

Researcher Liu Yue, Professor Zeng Xiaoqin and Professor Wang Jian of University of Nebraska-Lincoln, State Key Laboratory of Metal Matrix Composites of Shanghai Jiaotong University, and others used in-situ HRTEM nano-compression technology to apply shear force in a specific direction on the common lattice twin boundary of FCC copper, and studied the process of interface migration of nano-twin copper during in-situ compression. The migration mechanism of twin interface under the action of specific shear force is analyzed, and the essential reason for the difference of growth/annealing twin and deformation twin morphology is explained from the interface structure and formation mechanism. The researchers found that the twin order fault induced by shear stress is different from the spontaneous formation mechanism of the ITB structure generated by the growth/annealing process. Under the action of shear force in a specific direction, the twin order fault can be generated, and the twin order fault can be expanded into the twin order fault of the two layers of atoms due to the interaction behavior, but the twin order fault of the two layers of atoms is more likely to decompose rather than continue to expand into the ITB structure. The relevant research results were published in the authoritative journal Scripta Materialia on the topic of "Migration kinetics of twinning disconnections in nanotwinned Cu: An in situ HRTEM deformation study. Li Quan, a doctoral student at Shanghai Jiaotong University, is the first author, Song Jian is the second author, and researcher Liu Yue and Professor Zeng Xiaoqin are the corresponding authors.

in-situ link: https://doi.org/10.1016/j.scriptamat. 2020.11.006

Fig.1 The microstructure of FCC copper twins studied in article

Fig.2 One-layer atomic twin order fault (Twinning Disconnections, TDs) Step Generation and Migration Process

Fig.3 Experimental Process

In the in-situ experiment, FCC copper is mounted on the loading end of the in-situ double-tilt sample rod of Zeyu Technology PicoFemto, and the three-dimensional nano manipulator opposite the sample can accurately apply pressure and shear force to the sample. It is worth mentioning that high-resolution video of the sample can be clearly observed even during in-situ loading.

Fig.4 The in-situ double tilting sample rod used in the experiment of Zeyou Technology PicoFemto