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The interaction between nanotwin and dislocation pileup at twin boundary and interface cracks are of vital importance in studying the toughening mechanism of nanocrystalline materials. We have developed a theoretical model to discuss the effects of nanotwin and dislocation pileup on dislocation emission from an interfacial collinear crack tip in nanocrystalline bimaterials. The nanotwin as a stress source can be descripted by two disclination dipoles, and some dislocations produced by the grain boundary pile up at twin boundary. Using the complex potential method, the complex form of dislocation force and critical stress intensity factors (SIFs) corresponding to dislocation emission under remote plane loadings are deduced. Through numerical analysis, it is discussed that the effects of twin strength, twin orientation, twin size, twin position, dislocation pileup, crack length, and material constant on the critical SIF corresponding to dislocation emission in detail. The results show that the nanoscale twin and the dislocation pileup at the twin boundary will hinder dislocation emitted from the collinear crack tip and reduce toughness caused by dislocation emission. There is a critical nanotwin size making the critical SIF equal to zero and a best twin position minimizing the value of the critical SIF making dislocations emission easiest. The shear modulus of the two half plane have great influence on the critical SIF. [ABSTRACT FROM AUTHOR]

Molecular dynamics (MD) simulations under different mechanical and thermal constraints are carried out with a nanovoid embedded inside a single-crystal, face-centred-cubic copper. The dislocation emission angles measured from MD plots under 0.1 K, uniaxial-strain simulation are in line with the theoretical model. The dislocation density calculated from simulation is qualitatively consistent with the experimental measurement in terms of a saturation feature. The ‘relatively farthest-travelled’ atoms are employed to reflect the correlation between the dislocation structure and the void growth. At a smaller scale, the incomplete shear dislocation loops on the slip plane contribute to the local material transport. At a larger scale, the dislocation structures formed by those incomplete shear loops further facilitate the growth of nanovoid. Compared to the uniaxial-strain case, the void growth under the uniaxial-stress is very limited. The uniaxial-strain loading results in an octahedron void shape. The uniaxial-stress loading turns the nanovoid into a prolate ellipsoid along the loading direction. In the simulation, the largest specimen contains 12 million atoms and the lowest strain rate applied is 2 × 106s−1. Under all the different thermomechanical constraints concerned, the formation of incomplete shear dislocation loops are found capable of growing the void. [ABSTRACT FROM PUBLISHER]

Molecular dynamics (MD) simulation is performed to study the interaction between deformation twin and annealing twin in iron crystal. Two samples are simulated in this paper. The annealing twin in each sample is considered pre-existing automatically and the deformation twin is formed in the process of mode-I type loading and emitted from the crack tip. The distances between the twin boundary of the annealing twin and the crack tip in two samples are different. The applied stress is imposed to the top and the bottom of each sample. When the stress intensity is KI = 0·94 MPa m1/2, the partial dislocation is emitted from the crack tip of each sample, but the sample with larger annealing twin grain size will move a greater distance. As KI is increased to 2 MPa m1/2, the larger the annealing twin grain size is, the faster it will grow and the smaller the size of the deformation twin from the crack tip. That is to say, the size of the annealing twin will hinder the formation of the deformation twin and then affect the mechanical property of the material. [ABSTRACT FROM AUTHOR]

Theoretical model is suggested that describes the effects of the cooperative nanograin boundary sliding and stress-driven nanograin boundary migration (CNGBSM) process on the lattice dislocation emission from an elliptically blunt nanocrack tip in deformed nanocrystalline materials. Within the model, CNGBSM deformation near the tip of growing nanocrack carries plastic flow, produces two dipoles of disclination defects and creates high local stresses in nanocrystalline materials. By using the complex variable method, the complex form expression of dislocation force is derived, and critical stress intensity factors for the first lattice dislocation emission are obtained under mode I and mode II loading conditions, respectively. The combined effects of the geometric features and strengths of CNGBSM deformation, nanocrack blunting and length on critical SIFs for dislocation emission depend upon nanograin size and material parameters in a typical situation where nanocrack blunting and growth processes are controlled by dislocation emission from nanocrack tips. It is theoretically shown that the cooperative CNGBSM deformation and nanocrack blunting have great influence on dislocation emission from blunt nanocrack tip. [ABSTRACT FROM AUTHOR]