With the support of the National Natural Science Foundation of China, the team led by Academician Tian Yongjun of the High Pressure Science Center, the State Key Laboratory of Metastable Materials Science and Technology, YSU cooperated with Professor Wang YanbinfromUniversity of Chicago in the United Statesand realizedin-situatomic-resolutionobservation of structuraltransitionand migrationofdiamond grain boundary at room temperature, revealing the structural characteristics of incoherent twin boundary (ITB) in diamond, the microscopic mechanism of atomic migration and interface stabilization. The research results were published onlineonNatureon January 3, 2024 (paper link:https://www.nature.com/articles/s41586-023-06908-6, Structural transition and migration of incoherent twin boundary in diamond).
Structural transitionof {112} incoherent twin boundary in nanotwinneddiamond.a:typical grain morphology in nanotwinned diamond; b-e:dislocation-mediated interface structure transition.
It has been proved that nano-twinning is an effective means and strategy to improve the strength and toughness of materials. Usually, when a large number of coherent twin boundaries are formed in face-centered cubic materials, Σ3{112} ITB will be formed between twins with different orientations. In metal materials, the ITB interface can migrate for a long distance, and the migration speed increases rapidly with the decrease of twin thickness at thenano scale, inducing the occurrence of de-twinning, which is also the key mechanism for the softening of nanotwinnedmetal materialswhen thethicknessis less thanthe critical twin thickness. However, nanotwinned cubic boron nitride and nanotwinned diamond continue to harden atthenanoscale, which is completely different from metal materials. This shows that the stability of Σ3{112} ITB in covalent materials is obviously higher than that in metal materials. Exploring the physical origin of this high stability is of great scientific significance for understanding the continuous hardening mechanism of nanotwinned diamond and developing high-performance nanotwinnedmaterials.
Therefore, the researchers systematically studied the interface structure of nanotwinned diamond Σ3{112} ITB by using spherical aberration corrected scanning transmission electron microscope, and recorded the dynamic process of structural transition and migration of ITB interface at room temperature by means of mechanical stressgeneratedbythecharging effectofelectron irradiation. It is found that Σ3{112} ITB in nanotwinned diamond presents six configurations, among which three are mirror symmetricand three are non-mirror symmetric.Mirror symmetric configuration canmigrate rapidly overlongdistances, which is similar to metal materials.However, the non-mirror symmetric configurationmigrates overshortdistancesin shear coupling mode.Under stress, although ITB with different configurations can transforminto each other throughadislocation-mediated mechanism, the potential barrierfor transformationbetween asymmetric and symmetric configurations is high.Σ3{112} ITB in nanotwinned diamond mainly exists inanasymmetric configuration with low energy and low mobility, even when the twin thickness is as low as about 1 nm,whichleadsto the continuous hardening behavior of nanotwinned diamond. Combiningthe results of precession electron diffraction and molecular dynamics simulation, the researchers found that the activation stress required for interface structuraltransition is close to the critical shear stress required forthe initiation offull dislocation in diamond, thus revealing the structural origin of high stability of Σ3{112} ITB in nanotwinned diamond.
This work not only reveals the relationship between grain boundary configuration, phase transition and migration andthe types ofchemical bondsin thematerial, but also deepens people's understanding of the continuous hardening behavior of nanotwinned diamond. This work is supported by the National Natural Science Foundation of China (U20A20238, 52288102, 52090020, 91963203), the National Key Research and Development Program of China (2018YFA0703400, 2018YFA0305904), Hebei Natural Science Foundation(E2022203109) and other projects. Postdoctoral fellow Tong Ke, Dr. Zhang Xiang and Dr. Li Zihe are co-first authors, and researcher Hu Wentao, Professor Xu Bo and Academician Tian Yongjun are co-correspondents.
