Evaluation of Mechanical Properties of Σ5(210)/[001] Tilt Grain Boundary with Self-Interstitial Atoms by Molecular Dynamics Simulation

Grain boundary (GB) can serve as an efficient sink for radiation-induced defects, and therefore nanocrystalline materials containing a large fraction of grain boundaries have been shown to have improved radiation resistance compared with their polycrystalline counterparts. However, the mechanical properties of grain boundaries containing radiation-induced defects such as interstitials and vacancies are not well understood. In this study, we carried out molecular dynamics simulations with embedded-atom method (EAM) potential to investigate the interaction of Σ5(210)/[001] symmetric tilt GB in Cu with various amounts of self-interstitial atoms. The mechanical properties of the grain boundary were evaluated using a bicrystal model by applying shear deformation and uniaxial tension. Simulation results showed that GB migration and GB sliding were observed under shear deformation depending on the number of interstitial atoms that segregated on the boundary plane. Under uniaxial tension, the grain boundary became a weak place after absorbing self-interstitial atoms where dislocations and cracks were prone to nucleate

Rotor profile design and numerical analysis of 2–3 type multiphase twin-screw pumps

Increasing demands for high-performance handling of fluids in oil and gas as well as other applications require improvements of efficiency and reliability of screw pumps. Rotor profile plays the key role in the performance of such machines. This paper analyses difference in performance of 2–3 lobe combination of twin-screw pumps with different rotor profiles. A-type profile formed of involute–cycloid curves and D-type formed of cycloid curves are typical representatives for 2–3 type screw pumps. The investigation is performed by use of a full 3-D computational fluid dynamics analysis based on a single-domain structured moving mesh obtained by novel grid generation procedure. The real-time mass flow rate, rotor torque, pressure distribution and velocity field were obtained from 3D computational fluid dynamics calculations. The performance curves were produced for variable rotation speeds and variable discharge pressures. The computational fluid dynamics model was validated by comparing the simulation results of the A-type pump with the experimental data. In order to get the performance characteristics of D-type profile, two rotors with D-type profile were designed. The first has the same displacement volume as A-type while the second has the same lead and rotor length as A-type but different displacement volume. The comparison of results obtained with two rotor profiles gave an insight on the advantages and disadvantages of each of them

Molecular dynamics simulation of fracture behaviour in nanocrystalline fcc structures

The study of material failure has always been very important for human beings. Previous studies were mostly conducted at the macroscopic and microscopic scale based on continuum mechanics. However, the effect of nanostructural features on fracture has not been fully understood. Therefore, it is necessary to investigate the fracture mechanics at the atomic scale. Computational modelling, particularly atomistic (or molecular) simulation is becoming an increasingly important technology with which to analyse fracture. In this thesis, molecular dynamics (MD) simulations were carried out to investigate the fracture behaviours in Face Centred Cubic (fcc) nanocrystals. Nanotwinned Copper (Cu) has an unusual combination of ultra-high yield strength and high ductility, and in fact a brittle-to-ductile transition was previously observed in nanotwinned Cu despite Cu being an intrinsically ductile metal. However, the atomic mechanisms responsible for brittle fracture and ductile fracture in nanotwinned Cu are still not clear, so in this thesis, MD simulations at different temperatures were carried out to investigate the fracture of a specimen of nanotwinned Cu with a single-edge-notched crack whose surface coincides with a twin boundary. Three temperature ranges were identified as being indicative of the distinct fracture regimes under tensile straining that are perpendicular to the twin boundary. Below 1.1 K, the crack propagated in a brittle fashion, but between 2 K and 30 K a dynamic brittle-to-ductile transition occurred, and above 40 K the crack propagated in a ductile mode. A detailed analysis has been carried out to understand the atomic fracture mechanism in each fracture regime

Optimal design of screw and flow field analysis for twin-screw pump

A new solving transcendental equation method combining graphics and analyzing approach—Matlab Anonymous Function Method (MAFM) was developed to solve the screw contact line equation. The influence of contact line on screw sealing property was revealed and the effect of involute meshing angle on the screw contact line properties of type A twin-screw pump was investigated. The tooth profiles, spiral surface equations and contact line equations of type A twin-screw pump were obtained based on the optimal involute meshing angle. The three dimensional model and flow field digital model with modified involute meshing angle for screw rotors was developed to analyze the pressure field, velocity field and other screw pump’s characteristics by applying finite volume method, and the volumetric efficiency of flow channel for twin-screw pump study has been gained. The study provides the theoretical basis for the further screw pump development and property evaluation

Effect of stress state on deformation and fracture of nanocrystalline copper: molecular dynamics simulation

Deformation in a microcomponent is often constrained by surrounding joined material making the component under mixed loading and multiple stress states. In this study, molecular dynamics (MD) simulation are conducted to probe the effect of stress states on the deformation and fracture of nanocrystalline Cu. Tensile strain is applied on a Cu single crystal, bicrystal and polycrystal respectively, under two different tension boundary conditions. Simulations are first conducted on the bicrystal and polycrystal models without lattice imperfection. The results reveal that, compared with the performance of simulation models under free boundary condition, the transverse stress caused by the constrained boundary condition leads to a much higher tensile stress and can severely limit the plastic deformation, which in return promotes cleavage fracture in the model. Simulations are then performed on Cu single crystal and polycrystal with an initial crack. Under constrained boundary condition, the crack tip propagates rapidly in the single crystal in a cleavage manner while the crack becomes blunting and extends along the grain boundaries in the polycrystal. Under free boundary condition, massive dislocation activities dominate the deformation mechanisms and the crack plays a little role in both single crystals and polycrystals

The shear response of copper bicrystals with Σ11 symmetric and asymmetric tilt grain boundaries by molecular dynamics simulation

Grain boundaries (GBs) are important microstructure features and can significantly affect the properties of nanocrystalline materials. Molecular dynamics simulation was carried out in this study to investigate the shear response and deformation mechanisms of symmetric and asymmetric Σ11 tilt GBs in copper bicrystals. Different deformation mechanisms were reported, depending on GB inclination angles and equilibrium GB structures, including GB migration coupled to shear deformation, GB sliding caused by local atomic shuffling, and dislocation nucleation from GB. The simulation showed that migrating Σ11(1 1 3) GB under shear can be regarded as sliding of GB dislocations and their combination along the boundary plane. A non-planar structure with dissociated intrinsic stacking faults was prevalent in Σ11 asymmetric GBs of Cu. This type of structure can significantly increase the ductility of bicrystal models under shear deformation. A grain boundary can be a source of dislocation and migrate itself at different stress levels. The intrinsic free volume involved in the grain boundary area was correlated with dislocation nucleation and GB sliding, while the dislocation nucleation mechanism can be different for a grain boundary due to its different equilibrium structures

A dual fracture transition mechanism in nanotwinned Ni

Molecular dynamic simulation was used to study the brittle versus ductile fracture behaviour in nanotwinned Ni at various temperatures. The simulation results show that three temperature regimes correspond to three different fracture behaviours: brittle, brittle-to-ductile transition and ductile. A dual fracture transition mechanism in nanotwinned Ni within a small temperature interval was observed: (1) local phase transformation and (2) ledge formation ahead of the crack tip induced a sharp fracture transition from brittle mode to ductile mode. Our simulation results reveal that the very rare double fracture transition mechanisms could be transformed quickly in nanotwinned Ni within a narrow temperature interval, suggesting a new interpretation of fracture and deformation of nanotwinned Face Centred Cubic metals

Molecular dynamics simulation on generalized stacking fault energies of FCC metals under preloading stress

Molecular dynamics (MD) simulations are performed to investigate the effects of stress on generalized stacking fault (GSF) energy of three fcc metals (Cu, Al, and Ni). The simulation model is deformed by uniaxial tension or compression in each of [111], [11-2], and [1-10] directions, respectively, before shifting the lattice to calculate the GSF curve. Simulation results show that the values of unstable stacking fault energy (γusf), stable stacking fault energy (γsf), and unstable twin fault energy (γutf) of the three elements can change with the preloaded tensile or compressive stress in different directions. The ratio of γsf/γusf, which is related to the energy barrier for full dislocation nucleation, and the ratio of γutf/γusf, which is related to the energy barrier for twinning formation are plotted each as a function of the preloading stress. The results of this study reveal that the stress state can change the energy barrier of defect nucleation in the crystal lattice, and thereby can play an important role in the deformation mechanism of nanocrystalline material

Evaluation of Mechanical Properties of Σ5(210)/[001] Tilt Grain Boundary with Self-Interstitial Atoms by Molecular Dynamics Simulation

Grain boundary (GB) can serve as an efficient sink for radiation-induced defects, and therefore nanocrystalline materials containing a large fraction of grain boundaries have been shown to have improved radiation resistance compared with their polycrystalline counterparts. However, the mechanical properties of grain boundaries containing radiation-induced defects such as interstitials and vacancies are not well understood. In this study, we carried out molecular dynamics simulations with embedded-atom method (EAM) potential to investigate the interaction of Σ5(210)/[001] symmetric tilt GB in Cu with various amounts of self-interstitial atoms. The mechanical properties of the grain boundary were evaluated using a bicrystal model by applying shear deformation and uniaxial tension. Simulation results showed that GB migration and GB sliding were observed under shear deformation depending on the number of interstitial atoms that segregated on the boundary plane. Under uniaxial tension, the grain boundary became a weak place after absorbing self-interstitial atoms where dislocations and cracks were prone to nucleate