查看更多>>摘要：Being able to seamlessly deal with complex three dimensional crack patterns like branching and merging, phase field models (PFMs) are promising in the computational modelling of fracture of solids. Regarding the damage field, if only its second order derivative is present in the governing equation we have a second order PFM and if its fourth order derivative is also present we have a fourth order PFM. Previous studies have demonstrated that fourth order PFMs, with its smoother damage profile, are better in convergence and able to model strong anisotropic fracture energies. However, previous fourth order PFMs were developed only for brittle fracture and do not embed the material strength directly in the formulation. This paper presents a fourth order phase field regularised cohesive zone model (fourth order PF-CZM) for both brittle fracture and quasi-brittle fracture. We present a semi-analytical approach to compute the coefficients of the rational degradation function used in the fourth order PF-CZM. The proposed model is tested with multiple benchmark problems for mode I and mixed-mode fracture, and the results demonstrate that the fourth order PF-CZM: (1) provides results independent of the length scale and (2) is more efficient than its second order counterpart. The proposed model was then applied to modelling strong anisotropic fracture energies. A detailed study on sawtooth like crack patterns, which is a signature of strongly anisotropic fracture, is carried out with focus on periodicity and length scale sensitivity.
查看更多>>摘要：Control of grain size during solidification is an important issue according to the desired usage properties. In this study, both microstructure characterizations and numerical simulations are performed for as-cast hypoeutectic Al-Si alloys to understand the mechanism of grain-size transition with initial Si contents. An improved threedimensional (3D) sharp-interface model is developed that couples cellular automaton (CA) approach with a deterministic mesh-anisotropy reduction (DMAR) algorithm. The improved sharp-interface model is able to accurately calculate interface curvature and reproduce reasonable dendrite morphology within a wide range of cooling rates. It is found that grain size first decreases with increasing the initial Si content to 3 wt% and then increases with further Si additions. The nucleation undercooling significantly increases with increasing the Si content from 3 wt% to 10 wt% due to a Si-poisoning effect. The grain-size transition is mainly determined by the variations in the nucleation undercooling. Analysis using the Interdependence model also supports that a wide nucleation-free-zone is formed during solidification of Al-10 wt% Si alloy induced by a large nucleation barrier and a decreased growth velocity.
查看更多>>摘要：While material design and mechanical properties have been extensively investigated in medium entropy alloy (MEA), resources are no available to evaluate more complex mechanical behaviors such as friction and wear. Here, the unique deformation mechanism of CoCrNi MEA during nanoscratching has been explored in depth at a qualitative level by molecular dynamics simulation. The characteristics of dislocation reaction and evolution during nanoscratching are emphasized and compared with pure nickel. We find that the typical defect structures are formed in multi-principal element alloys rather than in pure metals. The calculation of the shear stress component provides a reasonable explanation for the generation of defects in MEA. In particular, the focus is on the generation and evolution of the distorted prismatic dislocation loop in the compression region, as well as phase-transformations and nanotwins promoted in the tensile region. Moreover, we explain the work hardening and load drop events during nanoscratching by the analysis of dislocation evolution. The objective of this contribution is to provide a computational study incorporating defect theories to gain insight into the deformation mechanism of MEA under tribological loads, thereby facilitating the development of new MEAs with excellent tribological performance.
查看更多>>摘要：The deformation mode of some titanium (Ti) alloys differs from that of pure Ti due to alloying elements in the alpha-phase. Herein, we investigated all possible slip modes in pure Ti and the effects of Al and V solutes as typical additive elements on the dislocation motion in alpha-Ti alloys using density functional theory (DFT) calculations. The stacking fault (SF) energy calculations indicated that both Al and V solutes reduce the SF energy in the basal plane. In contrast, Al solute increases the SF energy in the prismatic plane, making the slip motion in different planes more comparable. DFT calculations were subsequently carried out to simulate dislocation core structures. The energy landscape of the transition between all possible dislocation core structures and the barriers for dislocation glide in various slip planes clarified the nature of dislocation motion in pure Ti; i) the energy of prismatic core is higher than the most stable pyramidal core, and thereby dislocations need to overcome the energy barrier of the cross-slip (22.8 meV/b) when they move in the prismatic plane, ii) the energy difference between the prismatic and basal cores is higher (127 meV/b), that indicates the basal slip does not activate, iii) however, the Peierls barrier for motion in the basal plane is not as high (16 meV/b) once the dislocation exists stably in the basal plane. Direct calculations for the dislocation core around solutes revealed that both Al and V solutes facilitate dislocation motion in the basal plane by reducing the energy difference between the prismatic and basal cores. Thus, the effect of solutes characterizes the difference in the deformation mode of pure Ti and alpha-Ti alloys.
查看更多>>摘要：With the rapid development of space technology, the requirements for the radiation resistance of solar cells are higher and higher. A clear understanding of the radiation damage mechanism of materials is very important for radiation hardening of space solar cells. Therefore, molecular dynamics (MD) method was utilized to simulate the irradiated defects evolution in mainly used materials (Si, Ge, and GaAs) for multi-junction solar cells. By simulating the cascade collision process took placed in different materials under different interatomic potential, it is determined that Tersoff potential was more suitable for radiation damage simulation of semiconductor materials. By comparing the number of Frenkel pair defects (FPs) produced in the three materials during irradiation, the radiation resistance of each material was obtained. The MD results were verified by comparing with empirical NRT model. The displacement evolution of off-site atoms and the final clusters formed by point defects, i.e. interstitials and vacancies were further quantitatively analyzed. The atomic image of irradiated defects provides new physical insights for displacement damage mechanism and gives better understanding for explaining the degradation of irradiated multi-junction solar cells.
查看更多>>摘要：In this paper, the dendrite-seaweed transition was studied by changing solidification conditions through a phasefiled method. Results show that with the increase of thermal gradient, the critical pulling velocity for the transition from seaweed morphology to dendrite morphology increases. And the splitting spacing of seaweeds decreases with increasing the thermal gradient. To quantitatively describe the seaweed-dendrite transition, here the fractal dimension was introduced. It is found that the fractal dimension increased with the pulling velocity in the dendritic regime and seaweed regime, while it decreased in the stage of transition from seaweed to dendrite. It indicates that the fractal dimension can be regarded as an effective tool to quantitatively describe the dendriteseaweed transition.
查看更多>>摘要：Corrosion behaviors, charge distribution, and oxide growth mechanisms of carbon steel have been studied by using reactive molecular dynamics simulations. The corrosion kinetics of carbon steel with a carbon content of 0.1% has been investigated in the 35%o salinity salt spray with different densities of 1.03, 0.52, and 0.10 g/ml at different temperatures, respectively. The results show that the corrosion is massive in normal density (1.03 g/ml) salt spray system and pitting in dilute density (0.52 g/ml and 0.10 g/ml) salt spray systems. Iron crystal lattice defects on the carbon steel surface will arouse O, H, and Cl particles in the salt spray to migrate inward, causing deep oxidation. The density of salt spray is found to take a dominant position in the carbon steel corrosion. Therefore, the influence of temperature on oxidation kinetics in normal density salt spray is much greater compared to dilute density. In addition, the "decarburization" phenomenon of carbon steel has been observed and verified that the carbon content in the decarburized layer continues to decrease as the oxidation deepens. In the long-term simulation of the 1.03 g/ml salt spray system, the oxide growth and corrosion behavior are found to occur mainly before 600 ps, and the corrosion rate at this time is determined by the chemical reaction rate. After 600 ps, the corrosion rate depends on the electron diffusion rate. Furthermore, the activation energy is calculated to estimate the oxidizing property of different salt spray systems by fitting the consumption rate of water molecules into Arrhenius equation. When the concentration of salt spray decreases, the activation barrier tends to increase and the oxidizing ability of salt spray becomes weaker.
查看更多>>摘要：We have studied the interaction of an individual dislocation and a pile-up of dislocations with {111} tilt grain boundary in iron by means of atomistic simulations. The {111} tilt grain boundary, under externally applied stress, can change orientation by forming steps of three plane high thanks to shuffling of two atoms per Coincident Site Lattice (CSL) unit cell. When an individual crystal dislocation interacts with the GB, there is no transmission of the dislocation. Instead, we observe the formation of the same steps as found under the application of external stress. Depending on the orientation of the glide plane of the dislocation, two situations may occur. (i) If the glide plane perpendicular to the GB, the GB transforms into a stepped segment and a {112} twin boundary. (ii) For the other glide planes, the dislocation is absorbed by the GB and form a facet along the glide plane. Up on the interaction with a pile-up of dislocations, the stress concentration accumulated in the interaction region enhances the same reaction process, i.e. in (i) there is a penetration of one grain into the other with the dislocation in the tip of the intrusion bounded by the symmetric (112) and asymmetric stepped segment respectively. In (ii), the second dislocation is absorbed increasing the length of the facet. Based on the obtained results, one can conclude that {111} GB acts as a strong obstacle for gliding dislocations, does not allow a direct dislocation transmission, which makes a contrast with other types of (110) GBs (e.g. (112) and (332)).
查看更多>>摘要：Although most of conventional carbon materials show super electricity-conducting performance, the excellent heat-conducting property always hinders their applications in thermoelectric area. The present work shows that such issue may be alleviated by employing the newly proposed carbon allotrope named pentadiamond. Comprehensive studies on mechanical property and thermal conductivity of pentadiamond via molecular dynamics (MD) simulations in this work reveal that pentadiamond possesses exceptional properties such as very light weight, extremely high specific Young's modulus and remarkably low thermal conductivity compared with other carbon allotrope structures (e.g. diamond, graphene and carbon nanotube, etc.). The mechanical property of pentadiamond is of great dependence on the crystal orientation under different loading conditions. The increasing temperature has a weakening effect on the mechanical property. Besides, the thermal conductivity of pentadiamond structure is dependent on the crystal orientation and the value along the crystal orientation [1 1 0] is slightly larger. The thermal conductivity decreases with increase in temperature. With the increase in the size of the pentadiamond, the thermal conductivity increases to a limit value which is determined as about 28 W/mK along the crystal orientation [1 1 0] via extrapolation method. Such findings can be of great help in the application of pentadiamond in thermoelectric field.
查看更多>>摘要：Unlike the conventional zinc-blende II-VI and III-V common-cation systems, which exhibit a general trend of decreasing band gap with increasing anion atomic number, the zinc-blende I-VII cuprous halides CuX (X = Cl, Br, and I) all have an approximately equal direct band gap. Here, using first-principles calculations, we demonstrate that this band gap anomaly in Cu halides is attributed to the unique energy level order of Cu 3d well above X p, making the valence band maximum (VBM) an antibonding state derived mostly from the Cu 3d orbital, thus, a relatively small variation of the band gap with respect to the anion atomic number.
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Elsevier