Qihong Fang

Edge dislocation interacting with an interfacial crack along a circular inhomogeneity

Abstract The elastic interaction of an edge dislocation, which is located either outside or inside a circular inhomogeneity, with an interfacial crack is dealt with. Using Riemann–Schwarz’s symmetry principle integrated with the analysis of singularity of the complex potentials, the closed form solutions for the elastic fields in the matrix and inhomogeneity regions are derived explicitly. The image force on the dislocation is then determined by using the Peach–Keohler formula. The influence of the crack geometry and material mismatch on the dislocation force is evaluated and discussed when the dislocation is located in the matrix. It is shown that the interfacial crack has significant effect on the equilibrium position of the edge dislocation near a circular interface. The results also reveal a strong dependency of the dislocation force on the mismatch of the shear moduli and Poisson’s ratios between the matrix and inhomogeneity.

Electro-elastic interaction between a piezoelectric screw dislocation and circular interfacial rigid lines

Abstract The electro-elastic interaction between a piezoelectric screw dislocation located either outside or inside inhomogeneity and circular interfacial rigid lines under anti-plane mechanical and in-plane electrical loads in linear piezoelectric materials is dealt with in the framework of linear elastic theory. Using Riemann–Schwarz’s symmetry principle integrated with the analysis of singularity of complex functions, the general solution of this problem is presented in this paper. For a special example, the closed form solutions for electro-elastic fields in matrix and inhomogeneity regions are derived explicitly when interface containing single rigid line. Applying perturbation technique, perturbation stress and electric displacement fields are obtained. The image force acting on piezoelectric screw dislocation is calculated by using the generalized Peach–Koehler formula. As a result, numerical analysis and discussion show that soft inhomogeneity can repel screw dislocation in piezoelectric material due to their intrinsic electro-mechanical coupling behavior and the influence of interfacial rigid line upon the image force is profound. When the radian of circular rigid line reaches extensive magnitude, the presence of interfacial rigid line can change the interaction mechanism.

Dipole of edge misfit dislocations and critical radius conditions for buried strained cylindrical inhomogeneity

A theoretical model is proposed for elastic stress relaxation of a buried strained cylindrical inhomogeneity, which assumes the edge misfit dislocation dipole formation in the soft matrix at some distance from the interface. The critical radius of the inhomogeneity for the formation of the edge misfit dislocation dipole is given and the influence of various parameters on the critical radius is evaluated. The result indicates that the critical radius decreases with increasing misfit strain and core radius of the misfit dislocation. It is also found that, compared to the edge misfit dislocation dipole formation in the interface, the critical radius of the inhomogeneity decreases when the location of an edge misfit dislocation dipole formation is in the soft matrix at some distance from the interface.

Influence of grain boundary sliding and grain size on dislocation emission from a crack tip

This article establishes a theoretical model to study the effect of grain boundary sliding deformation on the dislocation emission from the crack tip in nanocrystalline solids. Within our description, stress concentration near the crack initiates grain boundary sliding which results in the formation the dipole of disclinations at triple junctions of grain boundaries. The grain size-dependent criterion for the dislocation emission from a semi-infinite crack tip is derived. The influence of the grain size and the features of dipole of disclinations on the critical stress intensity factors for dislocation emission is evaluated. It is shown that grain boundary sliding deformation enhances the critical stress intensity factor for dislocation emission. The critical stress intensity factor will increase with the decrement of the grain size, which indicates that the emission of the dislocation becomes more difficult for smaller size of grain due to the effect of grain boundary sliding deformation.

Mechanical behaviors of AlCrFeCuNi high-entropy alloys under uniaxial tension via molecular dynamics simulation

Although a high-entropy alloy has exhibited promising mechanical properties, little attention has been given to the dynamics deformation mechanism during uniaxial tension, which limits its widespread and practical utility. According to the experiment, an atomic model AlCrFeCuNi HEA was built using a melting and quick quenching method. In this work, the mechanical behaviors of the AlCrFeCuNi high-entropy alloy under uniaxial tensile loading are studied using atomistic simulation to investigate the evolution of dislocation and stacking fault as well as deformation twinning. The results show that calculations for the elastic properties and stress–strain relations are in excellent agreement with recent experimental results. Above all, the AlCrFeCuNi1.4 HEA not only has high strength, but also exhibits good plasticity which is qualitatively consistent with the experiment. Similar to the mechanical properties of single-crystal metals, stress fluctuation during plastic deformation of the high-entropy alloy is always accompanied with the generation and motion of dislocation and stacking fault with the increase of strain. In addition, the dislocation–dislocation interaction, dislocation–solid solution interaction, deformation twinning and detwinning occur after the yield point. Furthermore, the dislocation gliding, dislocation pinning due to the severe lattice-distortion and solid solution, and twinning are still the main mechanisms of plastic deformation in the AlCrFeCuNi1.4 high-entropy alloy. This atomistic mechanism provides a fundamental understanding of plastic deformation in a high-entropy alloy.

Atomic-scale analysis of nanoindentation behavior of high-entropy alloy

Using molecular dynamics simulations, we study the elastic and plastic deformations of indentation in FeCrCuAlNi high-entropy alloy (HEA). The indentation tests are carried out using spherical rigid indenter to investigate the effects of high-entropy and severe lattice distortion in terms of shear strain, indentation force, surface morphology, defect structure, dislocation evolution and radial distribution function on the deformation processes. It can be found that when the indentation depth increases, the shear stress requires for the occurrence of the contact area between the indenter and the substrate increased, which is attributable to a higher probability to observe the dislocation evolution under a large indentation depth. The indentation test also shows that the equal element addition can significantly improve the mechanical properties of HEA compared with the conventional alloy. Based on the Hertzian fitting, the FeCrCuAlNi HEA has the Young’s modulus of 161GPa and hardness of 15.4GPa, respectively. These values are higher than that of traditional metal materials, due to the low stacking fault energy and the dense atomic arrangement in the slip plane of HEA. In the plastic region, the Fe element causes the more stable crystal structure, much stronger than the Cu element, presumably resulted from a variety of crystal structures for Fe in the multicomponent FeCrCuAlNi alloy. Further, this effective strategy is used to accelerate the discovery of excellent mechanical properties of HEAs.

A WEDGE DISCLINATION DIPOLE INTERACTING WITH A COATED CYLINDRICAL INHOMOGENEITY

A three-phase composite cylinder model is utilized to study the interaction of a wedge disclination dipole with a coated cylindrical inhomogeneity. The explicit expression of the force acting on the wedge disclination dipole is calculated. The motilities and the equilibrium positions of the disclination dipole near the coated inhomogeneity are discussed for various material combinations, relative thicknesses of the coating layer and the features of the disclination dipole. The results show that the material properties of the coating layer have a major part to play in alteringi the strengthening effect or toughening effect produced by the coated inhomogeneity.

Void formation of nanocrystalline materials at the triple junction of grain boundaries

A theoretical model of void formation at the triple junction of grain boundaries is described. Based on the combined effects of grain boundary diffusional creep and triple junction diffusional creep as well as dislocation climb, void formation time and void growth rate are derived. The results indicate that vacancy concentration increases with increasing creep strain rate and the angle of the Burgers vector to its dislocation line, and with decreasing grain size. It sharply declines at low creep strain rates, then the asymptotic behavior approaches a constant at high rates. It is also found that the dislocation density is noticeable for small grain sizes in nanocrystalline Cu, and the void growth rate decreases with creep strain rate and time, which are qualitatively consistent with the conclusions in previous work (Dongare et al 2010 J. Appl. Phys. 108 113518; Du et al 2010 Mater. Sci. Eng. A 527 4837).

Influence of laser nanostructured diamond tools on the cutting behavior of silicon by molecular dynamics simulation

In this study, a series of large-scale molecular dynamics simulations have been performed to study the nanometric cutting of single crystal silicon with a laser-fabricated nanostructured diamond tool. The material removal behavior of the workpiece using a structured diamond tool cutting is studied. The effects of groove direction, depth, width, factor, and shape on material deformation are carefully investigated by analyzing normal stresses, shear stress, von Mises stress, hydrostatic stress, phase transformation, cutting temperature, cutting force and friction coefficients. Simulation results show that a cutting tool groove orientation of 60° produces a smaller cutting force, less cutting heat, more beta-silicon phase, and less von Mises stress and hydrostatic stress. Moreover, tools with a smaller groove orientation, groove depth and groove width, and larger groove factor lead to more ductile cutting and an increased material removal rate. However, a cutting tool with a smaller groove width results in more heat during the nanoscale cutting process. In addition, the average temperature of the subsurface increases as groove factor increases, showing that a tool groove accelerates heat dissipation to the subsurface atoms. Furthermore, this V-shape groove cutting is shown to improve material removal ability in nanoscale cutting.

Theoretical analysis and finite element simulation of Poisson burr in cutting ductile metals

Abstract In metal machining, due to the material around the cutting edge subjected to high pressure, the subsurface generates a plastic deformation layer. And near the edge of finished surface the plastic deformation layer acts as a Poisson burr. This paper proposes an analytical model to predict the sizes of Poisson burr based on Flamant and Boussinseq equations in the plastic deformation problem. Besides the mechanical loading, the thermal effects are also taken into account in this model and assumed as moving heat sources during the cutting. The finite element simulation of aluminum 6061 alloy is utilized to verify the results of theoretical analysis. The results show that the sizes of Poisson burr has a great sensitivity to material properties and processing parameters. In addition, due to the serrated chips and brittle fracture of high strain rate, the Poisson burrs are not continuous along the longitudinal cutting direction. Through comparative study it is found that the theoretical model describes burr sizes with considerable accuracy. As a result, this paper may provide a better understanding the mechanism of Poisson burr formation and the theoretical basis of processing parameters optimization.