Atomistic study of pipe diffusion in Al–Mg alloys

Solute diffusion in an Al-rich binary Al–Mg alloy is studied by means of atomistic simulations. The activation energy for diffusion of Mg in the bulk is evaluated in the dilute solution limit for the nearest neighbor and the ring mechanisms. It is concluded that bulk diffusion at low and moderate temperatures must be assisted by vacancies. Further, diffusion of Mg along the core of edge, 60° and screw dislocations is studied. The activation energy for vacancy formation in the core and for vacancy-assisted Mg migration is evaluated for a large number of diffusion paths in the core region. It is observed that, similar to the bulk, Mg diffusion in absence of vacancies is energetically prohibitive. The paths of minimum activation energy are identified for vacancy-assisted diffusion, for all three types of dislocations. The lowest energy path is found in the core of the 60° dislocation, its activation energy being 60% of the activation energy in the bulk. Most diffusion paths have activation energies larger than 75% of the equivalent bulk quantity. This analysis is relevant for the discussion on the mechanism of dynamic strain aging in these alloys. The data presented here show that pipe diffusion, which is currently considered as the leading mechanism responsible for dynamic strain aging is too slow in absence of excess vacancies.     

Dislocation motion in thin Cu foils: a comparison between computer simulations and experiment

Discrete dislocation dynamics (DD) simulations in conjunction with stereo and in situ straining transmission electronic microscopy (TEM) were used to study dislocation motion in thin Cu foils. Stereo imaging prior to and following in situ tensile straining is utilized to describe the three-dimensional (3D) evolution of dislocation structures with incremental straining and observation by TEM. The initial 3D configuration is used as input for 3D discrete dislocation dynamics simulations, and the final 3D configuration serves to refine and validate the DD simulation, thereby providing a direct quantitative link between experiment and dislocation dynamics modeling. In the present experiment, we observed complex 3D structures of dislocations, with significant out-of-plane motion. Computer simulations incorporating the Friedel–Escaig cross-slip mechanism indicate that surface image forces are sufficiently strong to activate out-of-plane motion for screw dislocation segments near the surface. Cross-slip of screw segments and dislocation climb of edge components are shown to be necessary mechanisms for explaining the observed 3D dislocation motion.     

Temperature dependence of screw dislocation mobility on shuffle-set of silicon

Large-scale atomistic simulations are performed in order to observe local behaviors of screw dislocations located on the shuffle set of (111) in single crystal silicon, focusing on the propagation process of the screw dislocations. A quadrupolar arrangement of screw dislocations is utilized to impose the periodic boundary conditions along each of the three spatial directions. With the aid of molecular dynamics simulations, the dislocation mobility is investigated in terms of the critical resolved shear stress. Based on the results from the simulations, we discuss effects of the model size and temperature on the critical resolved shear stress. After choosing the proper model size to reduce undesirable interference between the dislocations, we further estimate the Peierls stress by fitting from a set of the critical resolved shear stresses at various temperatures. Meanwhile, we observe a double kink mechanism in the dislocation propagation which is the most energetically favorable dislocation movement in silicon. We investigate the formation and migration of kink pairs on an undissociated screw dislocation in silicon.     

Atomistic study of hydrogen distribution and diffusion around a {1 1 2}<1 1 1> edge dislocation in alpha iron

Despite extensive investigations, the distribution of hydrogen around a stress singularity field is still not well understood. In this study, we conducted molecular statics (MS) analyses of the hydrogen-trap energy around a {1 1 2}<1 1 1> edge dislocation in alpha iron. The distribution of hydrogen in crystals is generally assumed to be dominated by hydrostatic stress. However, the MS results indicate that the hydrogen-trap energy is sensitive to shear stress as well as hydrostatic stress, thus indicating that strong trap sites are distributed across a wide range on the slip plane around the dislocation core. We also performed molecular dynamics simulation of hydrogen diffusion, and revealed the anisotropic diffusion behaviour of hydrogen around the dislocation core.     

Spacing defects and disconnections in grain boundaries

The process of cutting of a grain boundary by a gliding or climbing dislocation is considered. Some planar dislocation arrays with long-range stress fields are also treated. Defects formed on the grain boundaries by these mechanisms include edge and screw disconnections, grain boundary dislocations, spacing defects and line forces. The cutting defects can also acquire kinks and jogs. The results have implications for emission of lattice dislocations from grain boundaries, trapping of dislocations at grain boundaries and grain boundary topography.     

Creep behaviour of Ti-6Al-2Sn-4Zr-2Mo: II. Mechanisms of deformation

Constant load creep tests are performed in Ti-6242(Si) alloy with a lath microstructure, at temperatures of 538 and 565 °C. A change in the stress exponent values from ˜1 at low stresses to between 5 and 7 at high stresses, is indicative of a change in creep mechanism. TEM analysis indicates that the deformation is dominated by a-type dislocations in the phase, with little evidence of dislocation activity in the β laths. At higher stress (310 MPa), the a-type dislocations are pinned frequently along their screw direction by tall jogs. A creep model is proposed based on the premise that movement of these jogged screw dislocations may control the creep rate. In contrast, at low stress (172 MPa), the a-type dislocations have long straight screw segments with no apparent pinning points. The near-edge segments are in climb configurations. The creep rates here are close to those predicted, based on Harper–Dorn creep, although the dislocation density is larger than that normally associated with this regime.     

Dislocation interaction with C in α-Fe: A comparison between atomic simulations and elasticity theory

The interaction of C atoms with a screw and an edge dislocation is modelled at an atomic scale using an empirical Fe–C interatomic potential based on the embedded atom method and molecular statics simulations. Results of atomic simulations are compared with predictions of elasticity theory. It is shown that a quantitative agreement can be obtained between both modelling techniques as long as anisotropic elastic calculations are performed and both the dilatation and the tetragonal distortion induced by the C interstitial are considered. Using isotropic elasticity allows the prediction of the main trends of the interaction, whereas considering only the interstitial dilatation will lead to a wrong interaction.     

Pressure Dependence of the Peierls Stress in Aluminum

The effect of pressure applied normal to the {111} slip plane on the Peierls stress in Al is studied via atomistic simulations. Edge, screw, 30°, and 60° straight dislocations are created using the Volterra displacement fields for isotropic elasticity. For each dislocation character angle, the Peierls stress is calculated based on the change in the internal energy, which is an invariant measure of the dislocation driving force. It is found that the Peierls stress for dislocations under zero pressure is in general agreement with previous results. For screw and 60° dislocations, the Peierls stress versus pressure relationship has maximum values associated with stacking fault widths that are multiples of the Peierls period. For the edge dislocation, the Peierls stress decreases with increasing pressure from tension to compression. Compared with the Mendelev potential, the Peierls stress calculated from the Mishin potential is more sensitive to changes in pressure.     

In situ observation of whiskers,pyramids and pits during the high-temperature oxidation of metals

A hot-stage, environmental scanning electron microscope has been used to observe the in situ development of oxide whiskers, pyramids, and pits in the oxidation of copper and nickel at elevated temperatures. The effects of oxidation temperature, metal deformation, and the presence of water vapor on these irregular oxidation features were studied. In each case, the feature results from the presence of a central screw dislocation which provides ledges for the extension of the oxide lattice, but the specific geometries are decided by factors such as surface diffusion along the dislocation core, the rate of the molecular dissociation step, and the balance of surface energy and dislocation line tension forces.     

Ti—56Al单晶的形变位错结构及反常屈服机制的研究

应用TEM对Ti-56Al单昌[103]有序畴区位错结构进行研究,并提出修正的钆轧－解钉模型以解释1／2＜110]普通位划不是长而直的纯螺位错及位错碎片不是沿纯螺方向排列等实验现象和反常屈服机制。认为,当TiAl合金中的Al含量较低时,1／2＜110]普通位划控制的反常屈服行为可用Viguier的钉扎－解钉模型解释;当TiAl合金名的Al含量较高时,1／2＜110]普通位划控制的反常屈服行为可用本文提出的修正的钉扎－解钉模型解释。     

Experimental study of dislocation mobility in a Ti–6Al–4V alloy

By in situ TEM deformation experiments, we studied in detail the deformation micromechanisms of a Ti–6Al–4V alloy at room temperature. All dislocations have an a-type Burgers vector and glide essentially in prismatic or basal planes. They are first emitted from α/β interfaces and take a preferential orientation along their screw direction. The motion of screw dislocations controls the strain rate. Our experiments allow the microscopic parameters of plasticity for this alloy to be determined for the first time. The results concerning the screw dislocation motion, its core structure and the influence of interfaces are then discussed in comparison with previously published results.     

Behavior of Vacancies and Interstitials at Semicoherent Interfaces

Using atomistic simulations on a model semicoherent interface, we show that the formation, migration, and clustering of vacancies and interstitials at semicoherent interfaces depend on the structure of the misfit dislocation network of the interface. Interfacial point defects trap at misfit dislocation intersections and migrate from one intersection to another along misfit dislocations by a multistage process. Interfacial point defect clusters are thermodynamically less stable than individual defects and the largest cluster size depends on the spacing between misfit dislocation intersections. Our results show that the behavior of interfacial point defects is intimately connected to interface structure.     

Atomic structure of dislocations in intermetallics with close packed structures: a comparative study

Dislocation cores influence very significantly plastic behavior of many materials and this is particularly important in intermetallic compounds. We first review the variety of possible core structures and related deformation phenomena and proceed then to show the most important features of dislocation cores that have been found in computer simulations for L1₂, L1₀, DO₁₉ and DO₂₂ intermetallic compounds. Unlike in elemental close-packed metals, cores of dislocations in these close-packed intermetallics are frequently non-planar and possess very high Peierls stresses. The first reason is, of course, the crystal structure. For example, both the L1₀ and DO₂₂ structures are tetragonal and thus the close packed directions in {111} planes are not all equivalent. However, crystallography alone is not capable of explaining the tendency for non-planar core configurations. The analysis presented here shows that, besides the crystallography, the most important parameter controlling the dislocation core structures in intermetallics is the ordering energy. This effect is mediated via its influence on the energy of the stacking fault-like planar defects that are present in the cores and play role in dislocation dissociations.     

Dynamic dislocation behaviour in the intermetallic compounds NiAl, TiAl and MoSi2

The dynamic behaviour of dislocations in NiAl, TiAl and MoSi₂ on ‘easy’ slip systems is studied by in situ straining experiments in a high-voltage electron microscope. At elevated temperatures, the dislocations are smoothly bent as in NiAl and TiAl or sometimes show superkinks as in MoSi₂, and they move in a viscous way. It is suggested that this dynamic behaviour as well as the flow stress anomaly are connected with the formation of atmospheres around the dislocations. A model is proposed assuming that the lowest energy configuration of a dislocation may require a certain number of antisite defects or other point defects in the dislocation core. This cloud of disordered structure may follow partly the moving dislocations to induce an additional friction, analogous to other diffusion controlled mechanisms. The view of atmospheres controlling the dislocation mobility in intermetallics at elevated temperatures is supported by measurements of the dependence of the strain rate sensitivity on the strain rate itself.     

Interactions between carbon solutes and dislocations in bcc iron

Carbon solute–dislocation interactions and solute atmospheres for both edge and screw dislocations in body-centered cubic (bcc) iron are computed from first principles using two approaches. First, the distortion tensor and elastic constants entering Eshelby’s model for the segregation of C atoms to a dislocation core in Fe are computed directly using an electronic-structure-based the total energy method. Second, the segregation energy is computed directly via first-principles methods. Comparison of the two methods suggests that the effects of chemistry and magnetism beyond those already reflected in the elastic constants do not make a major contribution to the segregation energy. The resulting predicted solute atmospheres are in good agreement with atom probe measurements.     

Atomic core structure of 90°(c)-bent screw threading dislocations in wurtzite GaN

The atomic and electronic structures of the c threading dislocations with an edge or screw character were compared using a tight binding formalism which takes into account charge transfer.The two dislocations do not exhibit dangling bonds.While the screw dislocation contains only constrained Ga-N bonds, the edge dislocation contains Ga-Ga and N-N wrong bonds.Both dislocations are found to induce shallow and deep gap states.     

Deformation-induced point defects in NiAl single crystals

NiAl single crystals, oriented for single slip, were deformed at room temperature to a strain of 2%, and were subsequently annealed in the temperature range of 673–873 K (T/T_m=0.35–0.45). In as-deformed samples, dislocation substructures consist of jogged edge and screw dislocations, and prismatic loops. Densities of vacancy- and interstitial-type loops are about equal. Annealing causes shrinkage and disappearance of the interstitial loops and significant growth of the vacancy loops. These observations suggest that excess vacancies are present after room temperature deformation. These non-equilibrium point defects may result from non-conservative motion of jogged screw dislocations.     

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.     

Dislocation dynamics simulations of plasticity in Fe laths at low temperature

Plastic deformation in 16MND5 steel made of Fe laths is investigated using three-dimensional dislocation dynamics (DD) simulations, adapted to treat the body-centred cubic crystalline structure, strained in the ductile to brittle transition temperature range. In that regime, the edge segment velocity is proportional to the local effective resolved shear stress, whereas the screw segments follow a thermal activation scheme. The adopted cross-slip rules are derived from atomistic simulations, implemented in the DD code using a kinetic Monte Carlo algorithm. Specific loading and boundary conditions are worked out, with a view to accounting for the bainitic microstructure of the steel and its specific deformation mode. In these conditions, the implemented cross-slip behaviour is shown to play an essential role in the development of specific dislocation arrangements forming at different temperatures, also observed in 16MND5 steel. The presented results also provide insights on dislocation-based deformation mechanisms possibly involved in damage initiation.     

Preparation and characterization of Mg alloy rods with gradient microstructure by torsion deformation

Extruded AZ31 Mg alloy rods were subject to free-end torsion deformation at room temperature. The microstructure features of the torsion deformed samples were characterized using electron backscatter diffraction technique. Mg rods with gradient microstructure can be fabricated by torsion deformation. Inhomogeneous distribution of microstructure along the radial direction of the twisted rods is attributed to the linearly increasing strain accumulation and strain rate from core to surface. With increasing equivalent strain, both the amount of {10-12} twins and dislocation density increase and the c-axes of texture tend to rotate towards torsion axis. Although both dislocation slips and {10-12} twinning can be activated during torsion, dislocation slips are considered as the dominated deformation mechanism and responsible for the change of macro-texture for present torsion deformation. {10-12} twins and dislocations in the twisted samples can generate refinement hardening and dislocation hardening, respectively, to increase the hardness value.