First-principles study on the mobility of screw dislocations in bcc iron

The fully two-dimensional Peierls barrier map of screw dislocations in body-centered cubic (bcc) iron has been calculated using the first-principles method to identify the migration path of a dislocation core. An efficient method to correct the effect of the finite size cell used in the first-principles method on the energy of a lattice defect was devised to determine the accurate barrier profile. We find that the migration path is close to a straight line that is confined in a {1 1 0} plane and the Peierls barrier profile is single humped. This result clarifies why the existing empirical potentials of bcc iron fail to predict the correct mobility path. A line tension model incorporating these first-principles calculation results is used to predict the kink activation energy to be 0.73 eV in agreement with experiment.     

A half-space Peierls–Nabarro model and the mobility of screw dislocations in a thin film

In this paper, a half-space Peierls–Nabarro (HSPN) model is proposed to re-examine the mobility of a screw dislocation along a thin film/substrate (half-space) interface. In this configuration, the screw dislocation is subjected to an image force due to the free surface, and we are concerned with the interaction between the dislocation and the free surface. Unlike the original Peierls–Nabarro (P–N) model, the HSPN model takes into account the effect of the image force, which leads to modifications on analytical expression of the Peierls barrier stress. The modified Peierls stress is a function of the thin film thickness, which allows us to accurately predict the mobility of a dislocation in the interface between the thin film and the substrate. Based on the proposed HSPN model, we have found that the Peierls stress of a surface screw dislocation may be about 5–15% less than that in bulk materials.     

Hydrogen effects on the character of dislocations in high-purity aluminum

The effect of hydrogen in solid solution on the edge/screw character of dislocations has been investigated by deforming samples of high-purity aluminum in a gaseous hydrogen environment in situ in a controlled environment transmission electron microscope. Solute hydrogen was found to stabilize edge segments of dislocations, which inhibited, and in some cases stopped dislocations from cross-slipping. Removal of hydrogen from the sample allowed the cross-slip process to continue. These observations provide an explanation for the macroscopic observation that hydrogen in solid solution can promote slip planarity.     

Interstitial loop strengthening upon deformation in aluminum via molecular dynamics simulations

The formation of prismatic interstitial loops during plastic deformation, as well as their interaction with dislocations, is systematically investigated in aluminum, using molecular dynamics simulations. First, direct dislocation interaction is responsible for producing various types of defects, typically vacancies and their clusters and interstitial loops. Secondly, small interstitial loops act as obstacles to dislocation gliding. When close to each other, a loop either decorates a dislocation with jogs or drags it elastically. The mobility of a dislocation decorated by a loop is significantly reduced. Depending on the relative position of the dislocation and the loop, the Peierls stress can increase several hundred times. The present work shows a complete picture of dislocation–loop interaction with atomic-scale details, which provides reliable information for parameterizing dislocation–debris interactions in constitutive models.     

Atomistic simulation of kink-pairs of screw dislocations in body-centred cubic iron

The nudged elastic band method is used to calculate the activation paths connecting jumps between the two degenerate states A and B of the screw dislocation core in body-centred cubic (bcc) iron. Kink-pairs are found to be involved in all such jumps. By comparing the activation energies, it is shown that the jump from state A to state B on the {101} plane is not the same as that from B to A, and whichever is easier depends on the direction of the applied stress. This asymmetry results in a characteristic zigzag pattern of slip, and forms the basis of an explanation for pencil glide at elevated temperatures. The activation energy of the rate-determining jump is found to decrease with increasing stress, but the predicted flow stress is about three times greater than the experimental value at all temperatures.     

Molecular dynamics simulation of fast dislocations in copper

Molecular dynamics simulations are used to describe the acceleration and motion of dissociated dislocations in copper to extreme velocities. Stationary dislocations are accelerated either by shear stress or by shear strain rates. Results indicate that dislocations can be accelerated into transonic and supersonic velocities. Stable dislocation motion is found in three distinguished regimes: (i) in a plateau of velocities close to 1.6 km s^?1 in the subsonic regime; (ii) in a narrow range of velocities around 2.6 km s^?1 in the first transonic regime; and (iii) in the second transonic regime with increasing velocities. For large velocities in the second transonic regime and in the supersonic regime, the shear stress is exceeded and dislocation motion is unstable as the crystal loses its mechanical stability. The stacking fault fluctuates around 35 Å in the subsonic regime, but subsequently declines with velocity to less than half the initial value in the second transonic regime. Both the decreasing stacking fault width and the plateau of velocities in the first transonic regime, indicating the existence of a radiation-free state, are in agreement with theoretical predictions.     

Ab initio investigation of the Peierls potential of screw dislocations in bcc Fe and W

The easy, hard and split core configurations of the 〈1 1 1〉 screw dislocation and the energy pathways between them are studied in body-centered cubic (bcc) Fe and W using different density functional theory (DFT) approaches. All approaches indicate that in Fe, the hard core has a low relative energy, close to or even below that of the saddle configuration for a straight path between two easy cores. This surprising result is not a direct consequence of magnetism in bcc Fe. Moreover, the path followed by the dislocation core in the (1 1 1) plane between easy cores, identified here using two different methods to locate the dislocation position, is almost straight, while the energy landscape between the hard core position and the saddle configuration for a straight path is found to be very flat. These results in Fe are in contrast with predictions from empirical potentials as well as DFT calculations in W, where the hard core has an energy about twice that of the maximum energy along the Peierls barrier, and where the dislocation trajectory between easy cores is curved. Also, the split core configuration is found to be unstable in DFT and of high energy in both Fe and W, in contrast with predictions from most empirical potentials.     

Preferential location of prilocaine and etidocaine in phospholipid bilayers: A molecular dynamics study

In this work, we report a 20-ns constant pressure molecular dynamics simulation of the uncharged form of two amino–amide local anesthetics (LA), etidocaine and prilocaine, present at 1:3 LA:lipid, molar ratio inside the membrane, in the hydrated liquid crystal bilayer phase of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC). Both LAs induced lateral expansion and a concomitant contraction in the bilayer thickness. A decrease in the acyl chain segment order parameter, ?S_CD, compared to neat bilayers, was also observed. Besides, both LA molecules got preferentially located in the hydrophobic acyl chains region, with a maximum probability at ～12 and ～10 Å from the center of the bilayer for prilocaine and etidocaine, respectively.     

Core structure and mobility of an edge dislocation in aluminum

The core structure of an edge dislocation in aluminum is studied by molecular dynamics simulation with the glue potential. The dislocation splits into two partials. The separation distance between the two partials is about 9 Å. The half width of the two partial dislocations is deduced to be 6.5 Å by fitting the Burgers vector density to an arctangent function, giving a half width of the whole dislocation of 12 Å. Dislocation mobility is studied by applying a shear stress on the crystal and observing the corresponding shift of the Burgers vector density. By considering the minor force acting on the dislocation, a Peierls stress in the order of 10⁻⁴ μ for the motion of the whole dislocation in aluminum is obtained.     

Modeling the mechanics and dynamics of arbitrary edge drills

This paper presents a new approach for modelling the cutting forces and chatter stability limits in drills with arbitrary lip geometry. The oblique cutting geometry at each point on the drill lip is modelled using parametric curve equations. The cutting force and process damping coefficients at different parts of the drill lip are identified empirically; the cutting force coefficients are identified from non-symmetric drilling tests, and the process damping coefficients are identified from chatter-free orthogonal turning tests. The presented approach provides a practical method for modelling the cutting forces and vibration stability without needing the detailed geometry of drill lips. The accuracy of presented model in predicting lateral and torsional-axial chatter stability limits is verified by conducting drilling tests using drills with various edge geometries.     

A size effect in grain boundary migration: A molecular dynamics study of bicrystal thin films

Molecular dynamics simulations of stress-driven grain boundary migration in bicrystal thin films demonstrate that the grain boundary mobility decreases as the films are made thinner. Examination of the surface morphology proves that this effect is not associated with grain boundary grooving. The simulation data demonstrate that the grain boundary mobility is a linear function of the inverse thickness. We present a simple model to explain this effect based upon the fundamental mechanism of grain boundary migration: the collective rearrangement of a large group of atoms. Decreasing system size implies that more of the boundary is near the surface. The presence of the free surface interferes with the collective rearrangement of the atoms during boundary motion and hence slows the migration. A simple heuristic analysis, based on this effect, is consistent with the observed functional dependence of boundary mobility on bicrystal thickness.     

Behavior of amorphous materials under hydrostatic pressures: A molecular dynamics simulation study

The atomic structural behavior of amorphous pure Ni under hydrostatic pressures has been investigated through a molecular dynamics simulation study based on a semi-empirical interatomic potential (MEAM). It was observed that the amorphous material crystallizes under hydrostatic compressive pressure but forms nanovoids under hydrostatic tensile pressure at room temperature. These results could be explained by the volume change effect on the nucleation energy barrier during crystallization. Consistent with this explanation, stress induced increase in the energy level (decrease of energy barrier) is proposed as the main reason for the mechanically driven nanocrystallization of amorphous materials.     

Microscopic study for the behavior of grain boundary using molecular dynamics

In this study, MD simulations have been performed to observe the behavior of a grain boundary in an a-Fe plate under 2-dimensional loading. In MD simulation, the acceleration of every molecule can be achieved from the potential energy and the force interacting between each molecule, and the integration of the motion equation by using the Verlet method gives the displacement of each molecule. Initially, four α-Fe rectangular plates which have different misorientation angles of grain boundary were modeled by using the Johnson potential and Morse potential. We compared the potential energy of the grain boundary system with that of the perfect structure model, which does not have a grain boundary inside. Also, we could obtain the width of the grain boundary by investigating the local potential energy distribution. The tensile loading for each grain boundary model at room temperature was applied and the behavior of the grain boundary was studied. From this study it was clarified that when the Johnson potential is used, the obvious fracture mechanism occurs along the grain boundary. With the Morse potential, the diffusion of the grain boundary appears instead of the grain boundary fracture.     

螺杆泵在液压机过滤冷却加热系统中的应用

大型液压机液压系统中的液压油必须保证一定的清洁度和温度,螺杆泵作为液压机过滤冷却加热系统的动力源,在整个系统中起着非常重要的作用.介绍了75 MN铝挤压机过滤冷却加热系统的组成及其工作原理,通过转速、结构形式和流量的要求确定了螺杆泵的型号,最后对螺杆泵在使用时的注意事项进行了总结.实践结果证明,螺杆泵的设计选型合适,该...