The new results of the team of Academician Tian Yongjun of Yanshan University are published in Nature again: room temperature structural transformation and migration of diamond interface

On January 4, 2024, Beijing time, Nature magazine published online the latest results of the team of Academician Tian Yongjun of the High Voltage Science Center of the State Key Laboratory of Metastable Materials Preparation Technology and Science at Yanshan University. Researchers systematically characterized multiple atomic-level configurations coexisting on Σ3{112}/<110>incoherent twin boundaries in nanotwinned diamond and recorded in situ the dislocation-mediated interface at room temperature. Structural transformation and structure-dependent interface migration behavior. Academician Tian Yongjun, Professor Xu Bo and researcher Hu Wentao of the High Voltage Science Center of Yanshan University are the co-corresponding authors, and postdoctoral fellow Tong Ke, Dr. Zhang Xiang and Dr. Li Zihe are the co-first authors.

In nanotwin materials with face-centered cubic structure or diamond structure, Σ3{112}/<110>incoherent twin boundary (ITB) is a common interface. In metals, this interface will migrate rapidly when the twin thickness is lower than a certain critical value, inducing the detwinization process, which is also the key reason for the softening of metal materials with nanotwin structures. Unlike metal materials, nanotwinned cubic boron nitride and nanotwinned diamond do not soften as the twins are further refined, and the hardness of the materials continues to increase. This shows that Σ3{112} ITB has high stability in covalent materials that is very different from that of metal systems. Exploring the structural origin of this high stability is of great significance for understanding the continuous hardening mechanism of nanotwinned diamond and developing high-performance nanotwinned structural materials.

Figure 1. Atomic-level configuration of Σ3{112}/<110>incoherent twin boundaries in nanotwinned diamond. a, Typical nanotwinned diamond grains, containing a large number of {111} coherent twin boundaries (CTB) and {112}ITB; b, Typical {112}ITB in nanotwinned diamond; c-h, at room temperature. Six coexisting {112} ITB atomic-level configurations; i, Structural search shows that the six configurations identified have lower interfacial energies.

To this end, the researchers used spherical aberration-corrected scanning transmission electron microscopy to systematically characterize the atomically resolved interface structure of nanotwinned diamond Σ3{112} ITB, and studied the interface structure with the help of the mechanical stress generated by the charging effect of electron irradiation. Transformation and migration processes and mechanisms. They found that due to the directionality and saturation of covalent bonds, as well as the existence of two hybrid states of sp2 and sp3, the Σ3{112} ITB in nanotwinned diamond exhibits a variety of configurations (two types-mirror symmetric and non-mirror symmetric). Mirror symmetry, three types each), and show different migration modes related to configuration: the mirror symmetry configuration can migrate quickly over long distances, and its structural and behavioral characteristics are similar to those observed in metallic materials; the non-mirror symmetry configuration Short-distance migration in shear-coupled mode. Under the action of stress, ITBs of different configurations can transform into each other through a dislocation-mediated mechanism, thereby affecting the migration mode of the interface; however, the transformation between asymmetric configuration and symmetric configuration involves the change of rigid body displacement on both sides of the interface, which is affected by Constrained by coherent twin boundaries, it has a higher transformation barrier. Therefore, Σ3{112} ITB in nanotwinned diamond mainly exists in an asymmetric configuration with low energy and low mobility, even when the twin thickness is as low as about 1 nm, which leads to the persistence of nanotwinned diamond. Hardening behavior.

Figure 2. Dislocation-mediated interface structural transformation. a-f, two typical examples of interface structure transformation; g, types and mechanisms of interface structure transformation.

Combining precession electron diffraction and molecular dynamics simulation results, the researchers further confirmed that the activation stress required for interface structural transformation is close to the critical shear stress required for the initiation of full dislocations in diamond, thus revealing the Σ3 in nanotwinned diamond. {112} The structural origin of the high stability of ITB. This work further explains the continuous hardening mechanism of nanotwinned diamond and also provides new ideas for studying its toughening and room temperature plasticity. In addition, it further enriches people's understanding of material interfaces and their behavior. It not only provides direct information on the room temperature structural transformation and migration of interfaces in covalent materials, but also lays a theoretical foundation for the design of new nanotwin materials and the implementation of strengthening and toughening strategies base.

This research work was supported by the National Natural Science Foundation, the National Key Research and Development Program, the Hebei Provincial Natural Science Foundation and other projects.

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The new results of the team of Academician Tian Yongjun of Yanshan University are published in Nature again: room temperature structural transformation and migration of diamond interface

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