Grain rotation is a well-known phenomenon during plastic deformation and recrystallization of polycrystalline materials. Theoretical models suggest that atomic-scale processes of grain rotation commonly involve disconnection flow associated with grain boundary (GB) migration, but direct observations have been rare thus far. Here, we present direct evidences that grain rotation in polycrystals can proceed through the motion and annihilation of curved GB disconnections, using in situ nanomechanical testing and atomistic simulations. The dynamics of curved GB disconnections often accompanies an orientation change of grains in real polycrystals, by which the nanograin rotates gradually alongside GB migration and GB curving in three-dimensional (3D) space. This finding has implications for understanding GB-mediated processes in polycrystalline materials.
In this paper, the results of the first-stage research devoted to infrared thermographic detection of cracks in titanium alloys are presented. In application to the inspection of bottom-hole defects in 9.6 mm-thick Ti6Al4V titanium alloy samples, it has been found that a minimum detected defect should have diameter from one to two times greater than its depth. Images of phase are more noise-resistant and able to reveal deeper defects compared to images of amplitude. The test results obtained show that the Fourier analysis is a convenient data processing technique in active thermal NDT.
Grain boundary (GB) plasticity dominates the mechanical behaviours of nanocrystalline materials. Under mechanical loading, GB configuration and its local deformation geometry change dynamically with the deformation; the dynamic variation of GB deformability, however, remains largely elusive, especially regarding its relation with the frequently-observed GB-associated deformation twins in nanocrystalline materials. Attention here is focused on the GB dynamics in metallic nanocrystals, by means of well-designed in situ nanomechanical testing integrated with molecular dynamics simulations. GBs with low mobility are found to dynamically adjust their configurations and local deformation geometries via crystallographic twinning, which instantly changes the GB dynamics and enhances the GB mobility. This self-adjust twin-assisted GB dynamics is found common in a wide range of face-centred cubic nanocrystalline metals under different deformation conditions. These findings enrich our understanding of GB-mediated plasticity, especially the dynamic behaviour of GBs, and bear practical implication for developing high performance nanocrystalline materials through interface engineering.
Grain boundaries (GBs) serve not only as strong barriers to dislocation motion, but also as important carriers to accommodate plastic deformation in crystalline solids. During deformation, the inherent excess volume associated with loose atomic packing in GBs brings about a microscopic degree of freedom that can initiate GB plasticity, which is beyond the classic geometric description of GBs. However, identification of this atomistic process has long remained elusive due to its transient nature. Here, we use Au polycrystals to unveil a general and inherent route to initiating GB plasticity via a transient topological transition process triggered by the excess volume. This route underscores the general impact of a microscopic degree of freedom which is governed by a stress-triaxiality-based criterion. Our findings provide a missing perspective for developing a more comprehensive understanding of the role of GBs in plastic deformation.
Systematic study of interaction between graphene and hydroxyls is carried out by first-principles calculations. Although single hydroxyl adsorbed on graphene presents magnetic properties, hydroxyls prefer to adsorb on graphene in pairs without magnetic properties. The formation energy of hydroxyl pairs with graphene is coverage-dependent, and the most stable structure is half-covered by hydroxyl pairs along zigzag chains with alternative sp2 and sp3 hybridization between carbon atoms. The bandgap of this structure is 0.97 eV in GW approximation, close to the bandgap of Si, and this structure is stable at room temperature. It is possible to build graphene-based electronic circuits from graphene hydroxide without the need for cutting or etching.
Active thermal NDT is a promising technique for the detection of structural defects in solids. In this paper, the results of the first-stage research devoted to infrared thermographic detection of cracks in titanium alloys are presented. The test results obtained show that the Fourier analysis is a convenient data processing technique in active thermal NDT. Images of phase are more noise-resistant and able to reveal deeper defects compared to images of amplitude. In application to the inspection of bottom-hole defects in 9.6 mm-thick Ti6Al4V titanium alloy samples, it has been found that a minimum detected defect should have diameter from one to two times greater than its depth.
We report a computational discovery of novel grain boundary structures and multiple grain boundary phases in elemental bcc tungsten. While grain boundary structures created by the \gamma-surface method as a union of two perfect half crystals have been studied extensively, it is known that the method has limitations and does not always predict the correct ground states. Here, we use a newly developed computational tool, based on evolutionary algorithms, to perform a grand-canonical search of a high-angle symmetric tilt boundary in tungsten, and we find new ground states and multiple phases that cannot be described using the conventional structural unit model. We use MD simulations to demonstrate that the new structures can coexist at finite temperature in a closed system, confirming these are examples of different GB phases. The new ground state is confirmed by first-principles calculations.
The electrochemical selective oxidative transformation of lignin feedstocks into valuable oxygenated aromatics is essential to establish a sustainable biorefinery.
Abstract High‐performance MXene fibers are always of significant interest for flexible textile‐based devices. However, achieving high mechanical property and electrical conductivity remains challenging due to the uncontrolled loose microstructures of MXene (Ti 3 C 2 T x and Ti 3 CNT x ) nanosheets. Herein, high‐performance MXene fibers directly obtained through fluidics‐assisted thermal drawing are demonstrated. Tablet interlocks are formed at the interface layer between the outer cyclic olefin copolymer and inner MXene nanosheets due to the thermal drawing induced stresses, resulting in thousands of meters long macroscopic compact MXene fibers with ultra‐high tensile strength, toughness, and outstanding electrical conductivity. Further, large‐scale woven textiles constructed by these fibers offer exceptional electromagnetic interference shielding performance with excellent durability and stability. Such an effective and sustainable approach can be applied to produce functional fibers for applications in both daily life and aerospace.
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