Observation of the critical thickness phenomenon in dislocation dynamics simulation of microbeam bending

Three-dimensional discrete dislocation dynamics simulations of thin aluminium beams in flexure give data in good agreement with critical thickness theory. The yield points (onset of significant plastic deformation) occur near the stress predicted by the theory. In the plastic regime, dislocation densities increase with plastic deformation within the bulk of the beam thickness, while the surface regions are relatively denuded of dislocations. The stress within the beam relaxes in the central part, then rises linearly to the free surfaces. This behaviour is quantitatively in agreement with critical thickness theory with a strain-thickness product for relaxation (plastic deformation) of 0.57 nm.     

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.     

An analysis of the size effect on void growth in single crystals using discrete dislocation dynamics

The effect of size on the mechanical behavior and the void growth rate in a voided single crystal was studied using two-dimensional discrete dislocation dynamics. The simulations were based on the methodology developed by Van der Giessen and Needleman [Van der Giessen E, Needleman A. Modell Simul Mater Sci Eng 1995;3:689], which was extended to non-convex domains through the use of finite elements with embedded discontinuities [Romero I, Segurado J, LLorca J. Modell Simul Mater Sci Eng 2008;16:035008]. Square crystals (in the range 0.5–2.5 μm) with an initial void volume fraction of 10% were deformed under plane strain conditions in uniaxial tension, uniaxial deformation and biaxial deformation. The results of the simulations show two size effects, one on the initial flow stress and strain-hardening rate of the voided crystal (“smaller is stronger”) and another on the void growth rate (“smaller is slower”). The magnitude of both size effects increased with triaxiality. The physical micromechanisms responsible for these size effects were elucidated from the simulation results.     

The effect of alloying elements on the dislocation climbing velocity in Ni: A first-principles study

By using density functional theory calculations in conjunction with the climbing images nudged elastic band method, the effects of alloying elements Re, W, Mo, Cr, Co and Ru on the velocity of dislocation climbing in gamma Ni were studied. The results shed a light on the mechanism of these elements suppressing the dislocation motion by connecting the stacking fault energy and the migration activation energy of vacancy with the dislocation climbing velocity. It is found that the elements can decrease the stacking fault energy of Ni and raise the migration activation energy of vacancy. The changes of these two energies result in the increase of the formation energy and the diffusion activation energy of the jog, thus the dislocation climbing is restricted. The results also reveal that the influences of alloying elements on dislocation climbing velocity depend on the characters of dislocations.     

Effect of grain disorientation on early fatigue crack propagation in face-centred-cubic polycrystals: A three-dimensional dislocation dynamics investigation

Three-dimensional dislocation dynamics simulations are used to study micro-crack interaction with the first micro-structural barrier, in face-centred-cubic bi-crystals loaded in high-cycle fatigue conditions. In the examined configuration, we assumed that micro-crack transmission occurs due to surface relief growth in the secondary grain ahead of the primary crack. This indirect transmission mechanism is shown to strongly depend on grain-1/grain-2 disorientation. For instance, small grain disorientation induces plastic strain localization ahead of the crack and therefore, faster transmission through the first barrier. Conversely, large grain-1/grain-2 disorientation induces plastic strain spreading similar to crack tip blunting, yielding slower indirect transmission. A semi-analytical micro-model is then proposed based on the present simulation results and complementary experimental observations, highlighting the original notion of first-barrier compliance. The model captures well-known experimental trends, including the effects of grain-size, grain disorientation and micro-crack retardation at the first barrier.     

Residual stress distribution in different depths of TiNi/Ti2Ni-based laser clad coating prepared at different environmental temperatures

This study aimed to effectively reduce the cracking susceptibility of the laser clad coating by enhancing the environmental temperature during laser cladding, and reveal the residual stress distribution in different depths of the coating. The TiNi/Ti₂Ni-based coatings were prepared on Ti6Al4V by laser cladding at different environmental temperatures of 25, 400, 600 and 800 °C. The changes in residual stress along the depth of the coatings were investigated in detail by the nanoindentation method. Results showed that the average residual stress of 2.90 GPa in the coating prepared at 25 °C was largest. With the increase in environmental temperature, the average residual stress was reduced to 1.34 GPa (400 °C), 0.70 GPa (600 °C) and 0 GPa (800 °C). For all the coatings, the residual stress was increased with increasing the distance from the coating surface. Enhancing the environmental temperature can effectively reduce the cracking susceptibility of the coatings.     

分子动力学模拟不同层错能单晶Ni及其合金拉伸变形行为

采用分子动力学模拟了不同尺寸模型的单晶Ni及Ni₅₇Cr₁₉Co₁₉Al₅合金[100]晶向拉伸变形过程,确定了具有稳定塑性流变应力的模型尺寸,进一步研究了在具有稳定塑性流变应力的相同模型下单晶Ni及其合金拉伸变形行为。结果表明,层错能较低的单晶Ni₅₇Cr₁₉Co₁₉Al₅合金在小尺寸模型拉伸变形时,容易形成多层孪晶结构或变形孪晶;模型的横截面边长大于30倍的晶格常数时,塑性流变阶段流变应力、相结构及位错密度随应变起伏趋于平稳。具有稳定流变应力的相同尺寸单晶Ni及其合金拉伸时,层错能越低,塑性变形时层错面的面积越大。Shockley不全位错在单晶Ni及其合金塑性变形过程中起主导作用,多层孪晶的形成伴随着位错耗尽,变形孪晶的形成与湮灭则主要由位错饥饿机制主导。     

Structural analysis for the stress variation of ta-C film with deposition energy: A molecular dynamics simulation

Molecular dynamics simulations are performed on the atomic origin of the evolution of residual stress in tetrahedral amorphous carbon (ta-C) film using the empirical Tersoff potential. The densities of and residual stresses in the amorphous films generated by molecular dynamics simulations were found to be in good agreement with the corresponding experimental results. A radial distribution function analysis shows that the peak at approximately 2.1 Å found in high-stress configurations, which is referred to as a satellite peak, is closely linked with the variation of the residual stress in ta-C film.     

涂覆WC-17Co涂层的Ni718合金在不同环境温度中的疲劳性能

利用超音速火焰喷涂技术在Ni718合金表面制备WC-17Co涂层.利用反复弯曲试验分析25,150,300℃条件下涂覆WC-17Co涂层Ni718合金疲劳性能,利用扫描电镜、X射线衍射仪分析涂层的断口形貌和相组成,并利用剥层法测量涂层中残余应力分布.结果表明,相同应变量条件下试样的疲劳寿命随着温度的升高而降低;循环载荷作用下裂纹由涂层表面产生,向基体方向扩展,最终形成整体断裂;室温至300℃温度范围内,涂层不会发生相变,但是随环境温度上升涂层中的残余压应力呈现下降趋势,这种趋势使得涂层中裂纹的扩展速度增加,最终导致疲劳寿命下降.     

中、高应变速率下AZ80镁合金高温变形力学行为研究

文章通过对AZ80镁合金在不同变形温度和应变速率下的压缩实验,研究了其高温变形力学行为。讨论了中、高应变速率变形条件下,变形过程中试样温升对流变应力的影响,并进行了修正。将修正后的流变应力应用于合金挤压变形过程的数值模拟。结果表明,模拟挤压变形力与实测值接近,说明修正后的流变应力能很好地反映材料的变形力学行为。     

CBGA不同尺寸焊点热循环载荷下应力应变的有限元模拟

采用ANSYS分析软件，将组件简化为二维模型的焊点应力应变有限元分析。通过研究三种BGA球（0．76mm、1．0mm、1．3mm）组件，利用有限元模拟CBGA（陶瓷球栅阵列）组件在热循环加载条件下其应力应变分布，并且计算了焊点的应力应变最大值，得出球径为0．76min的CBGA组件其可靠性最好，表明有限元方法可有效地用于研究微电子封装中各类焊点的可靠性。     

On the superposition of flow stress contributions at finite temperatures and in the athermal limit

The functional form of the equation describing the superposition of contributions to the flow stress due to various strengthening mechanisms is analyzed. Considering that the superposition can be written as a sum to which each mechanism contributes through function f, that the dependence of the critical resolved shear stress on the density of obstacles to dislocation motion is given by function g, and that f and g have the same functional form for all strengthening mechanisms considered, it is shown that these two function are necessarily power functions, and their exponents are related. Furthermore, requiring that the superposition law is valid both at finite temperatures and at 0 K leads to an equivalent expression for the strain rate sensitivity and imposes restrictions on the way in which contributions of various mechanisms to the flow stress are evaluated at finite temperatures.     

A verification of the thermal stress analysis,including the furan sand mold,used to predict the thermal stress in castings

The restraint exerted on a casting by a furan sand mold on the casting and the contraction of the casting during cooling was dynamically and simultaneously measured using a device that we developed. The measurements were compared during cooling with thermal stress analyses. The thermal stress analyses were based on the representative mechanical models for the furan sand mold, i.e., the elastic and elasto-plastic models used in previous studies. The comparison demonstrated that the elasto-plastic model simulates the restraint force more accurately than the elastic model. In the thermal stress analysis, it was important to describe the development of inelastic deformation and the fracture of the sand mold. However, the simulated restraint force was still twice as large as the measured force even in the elasto-plastic model. This error is most likely attributable to using the temperature-independent mechanical properties of the furan sand mold and the mechanical model of the casting alloy, which neglected the viscoplasticity at high temperature in the thermal stress analysis.     

Three dimension simulation analysis of the interpass stress and deformation during multipass welding

It has been widely studied about the final residual stress and deformation in muhipass welding of thick weldments. But there is a lack of a clear understanding of the interrelationship of interpass stress and deformation during multipass welding. In this study, a three dimension numerical model of a sixteen-pass double V-groove welded joint with 50 mm plate is developed to compute the stress field and deformation by using multiple CPU parallel processing technology. The following factors such as the non-linear of temperature, heat radiation, filling of material step by step and so on are considered. Distribution and evolution law of welding stress in the transverse and longitudinal section is analyzed in this paper, and the interpnss stresses are studied also. At the same time the evolution course of angular deformation amount is analyzed, and the experimental results show that the calculated resuhs accord with the measured results of angular deformation.     

Computational fluid dynamics analysis of working fluid flow and debris movement in wire EDMed kerf

In wire EDM, better exclusion of debris from the machined kerf is very important to obtain a stable machining performance. The purpose of this study is to investigate the fluid flow in the kerf and better jet flushing conditions of working fluid from the nozzles. The flow field and the debris motion in the kerf were analyzed by computational fluid dynamics (CFD) simulation, comparing with the observation by high-speed video camera. The influence of flow rate of working fluid from nozzles and the nozzle stand-off distance on flow field in the kerf and debris particle motion were discussed.