8:45 PM - *ST02.05.05
Nanoscale Dynamic Observations of Grain Boundary Fracture, Deformation, Migration and Twin Formation in Ceramics
University of Tokyo1,Japan Fine Ceramics Center2,Tohoku University3
Ceramics have been widely used for structural applications because of their superior mechanical properties. It has been known that the behavior of GB fracture, deformation and migration is strongly dependent on the GB characters such as misorientation angle between two adjacent crystals and GB plane, however, such effect has not been clarified yet. In this study, in order to clarify the atomistic mechanisms of GB fracture and deformation, bicrystal studies have been performed to find the relationship between the atomic structures and GB behavior of SrTiO3 and Al2O3 ceramics. Several kinds of bicrystals including GBs with specific geometrical configuration were fabricated, and some of them were doped by rare-earth elements to enhance the GB segregation.
It has been reported that single crystal of SrTiO3 can be plastically deformed even at R.T. by dislocation slip like metals. So far, many experimental investigations have been tried for understanding the dislocation-grain boundary interaction, but these experiments were mostly carried out statically, and the fundamental processes are still not well understood yet. In this study, the nanoindentation experiments were conducted for SrTiO3 crystals their bicrystals inside TEM. The SrTiO3 single crystals were indented with the sharp diamond tip and successfully observed the dislocation dynamics. In the case of the GBs, the interaction between the introduced lattice dislocations and the GBs were directly observed. The dislocation-GB interaction and its dependence on the GB characters will be discussed in detail.
GB fracture in Al2O3 is strongly dependent on the GB characters and the dopant segregated at GBs. In order to clarify the atomistic GB fracture mechanism and its dopant effect in Al2O3 ceramics, Al2O3 bicrystals including GBs with specific geometrical configuration were systematically fabricated, and some of them were doped by rare-earth elements. Then, the atomic structures and chemistry in thus fabricated GBs were characterized by atom-resolved STEM, and the dynamic behavior of GB fracture was observed by TEM nanoindentation experiment. The relationship between GB characters, segregated dopants and GB fracture behavior of Al2O3 will be discussed in detail.
It is known that Al2O3 shows twin formation at R.T. under high stress concentration. So far, many experimental investigations have been tried for understanding the twin formation mechanism, but these experiments were also carried out statically, and the fundamental atomistic processes are still not well understood yet. In this study, TEM in situ nanoindentation experiments were conducted also for Al2O3 single crystals and the bicrystals. Al2O3 single crystals were indented with the sharp diamond tip, and the twinning dynamics were successfully observed. In this case, the deformation twinning often occurs in the present experimental condition, and the mechanism can be explained by a shear process for each lattice layer, which is caused by twinning dislocations. It is suggested that the non-basal twinning systems, such as the rhombohedral twinning in Al2O3, can be completed by not only simple shear but also atomic shuffling. In this study, the dynamic behavior and atomic structures of the twinning dislocations were investigated for rhombohedral twinning in Al2O3, and the twin -GB interaction and its dependence on the GB characters will be discussed in detail.
1) S. Kondo, N. Shibata, T. Mitsuma, E. Tochigi, Y. Ikuhara, Appl. Phys.Lett., 100(18), 181906(2012)
2) S. Kondo, T. Mitsuma, N. Shibata, Y. Ikuhara, Sci. A dv., 2, e1501926(2016).
3) J.Wei, B. Feng, R. Ishikawa, T.Yokoi, K.Matsunaga, N.Shibata, Y. Ikuhara, Nature Materials, DOI：10.1038/s41563-020-00879-z (2021)