Akarsh Verma, Oliver K. Johnson, Gregory B. Thompson, Shigenobu Ogata and Eric R. Homer,“Solute influence in transitions from non-Arrhenius to stick-slip Arrhenius grain boundary migration”, Acta Materialia, 265 (2024) 119605-1-12.

Synthetic driving force based molecular dynamics simulations are used to evaluate the grain boundary velocities for an incoherent Σ3 [111] 60° {11 8 5} GB in elemental nickel and its copper-based alloys in the dilute limit. We examine the effects of temperature, solute content, and magnitude of the driving force on grain boundary velocity trends and their associated mechanisms. We observe that, for pure nickel and its copper alloys in the dilute limit at high driving forces, these special grain boundaries exhibit non-Arrhenius or anti-thermal migration behavior, where temperature and grain boundary velocity are inversely related. For lower driving forces, the increased copper content leads to stick-slip migration behavior and a likely transition from non-Arrhenius to Arrhenius temperature dependence. Interestingly, the ordered atomic motions are frustrated but unchanged by the solute content and stick-slip migration. While the results are generally consistent with the Cahn-Lücke-Stüwe (CLS) model, no solute drag is observed; rather, the solute effects are likely the result of solute pinning.

ページトップ

Rana Hossain, Hajime Kimizuka, and Shigenobu Ogata,“Asymmetry in core structure and mobility of basal dislocations in a Ti ₃ SiC ₂ MAX phase: An atomistic study with machine-learned force fields”, Physical Review Materials, 7-5, (2023) 053608-1-12.

MAX phases are a unique class of atomically layered ternary ceramics that deform plastically at room temperature owing to highly mobile basal dislocations (BDs). To understand and control the ductility of these materials, it is crucial to study the core structures and mobilities of BDs. In this study we developed a machine-learning-based spectral neighbor analysis potential (SNAP) to perform atomistic simulations of edge, screw, and mixed BDs in a Ti ₃ SiC ₂ MAX phase. The SNAP was trained on density functional theory (DFT)-calculated data. The SNAP calculations demonstrate that the BD core structure exhibits significant asymmetry that is dependent on the position of the weakly bonded Si layer relative to the Ti(4f)–Si slip plane. The dislocation core either splits into Shockley partials or remains compact, depending on whether compressive or tensile stresses act on the Si layer. This asymmetry in the BD core structure agrees well with DFT results. Differential displacement and Nye tensor distribution analyses reveal that undissociated BD cores are centered on Si layers and spread over adjacent parallel basal planes. Additionally, they are three orders of magnitude less mobile than partial BDs. The Peierls stresses of partial edge BDs (∼71 MPa) are closer to those of metals than those of ceramics, suggesting that edge BDs play a key role in the incipient plasticity of Ti ₃ SiC ₂ MAX phases at low temperatures. The findings of this study contribute to a deeper understanding of the complex behavior of BDs at the atomic level and provide theoretical support for elucidating the unique deformation modes of crystals with atomically layered structures owing to the motion of BDs.

ページトップ

Yan Li, Xufei Fang, Eita Tochigi, Yu Oshima, Sena Hoshino, Takazumi Tanaka, Hiroto Oguri, Shigenobu Ogata, Yuichi Ikuhara, Katsuyuki Matsunaga, Atsutomo Nakamura,“Shedding new light on the dislocation-mediated plasticity in wurtzite ZnO single crystals by photoindentation”, Journal of Materials Science & Technology, (2023) 206-216.

Dislocation-mediated plasticity in inorganic semiconductors and oxides has attracted increasing research interest because of the promising mechanical and functional properties tuned by dislocations. In this study, we investigated the effects of light illumination on the dislocation-mediated plasticity in hexagonal wurtzite ZnO, a representative third-generation semiconductor material. A (0001) 45^o off sample was specially designed to preferentially activate the basal slip on (0001) plane. Three types of nanoindentation tests were performed under four different light conditions (550 nm, 334 nm, 405 nm, and darkness), including low-load (60 μN) pop-in tests, high-load (500 μN) nanoindentation tests, and nanoindentation creep tests. The maximum shear stresses at pop-in were found to approximate the theoretical shear strength regardless of the light conditions. The activation volume at pop-ins was calculated to be larger in light than in darkness. Cross-sectional transmission electron microscope images taken from beneath the indentation imprints showed that all indentation-induced dislocations were located beneath the indentation imprint in a thin-plate shape along one basal slip plane. These indentation-induced dislocations could spread much deeper in darkness than in light, revealing the suppressive effect of light on dislocation behavior. An analytical model was adopted to estimate the elastoplastic stress field beneath the indenter. It was found that dislocation glide ceased at a higher stress level in light, indicating the increase in the Peierls barrier under light illumination. Furthermore, nanoindentation creep tests showed the suppression of both indentation depth and creep rate by light. Nanoindentation creep also yielded a larger activation volume in light than in darkness.