Precipitation and solute distribution in an interrupted-aged Al–Mg–Si–Cu alloy

Quantitative analysis of the precipitate species and solute distribution was carried out on Al–Mg–Si–Cu alloy 6061 aged to peak hardness using a conventional T6 heat treatment and the so-called T6I6 heat treatments. In this latter, a dwell period at reduced temperature (65°C) is introduced into the T6 ageing cycle (at 177°C or 150°C) which modifies the microstructure and results in the simultaneous improvement of both tensile properties and fracture toughness. Analysis of three-dimensional atom probe data reveals that the superior mechanical properties of the T6I6/177 temper are achieved by a combined effect of a greater consumption of solute atoms by precipitates, an increased number density of fine precipitates and the presence of greater fractions of the effective strengthening precipitates in the final microstructure. Three types of precipitates were found to be characteristic of the peak aged conditions: β′′ precipitates, Guinier–Preston zones and Mg–Si(–Cu) co-clusters. The composition of the strengthening precipitates was found to vary over a wide range for the different heat treatment schedules, corresponding to a variation in the number density of stable nuclei, without any accompanying change in their morphology. All precipitates were found to contain substantial quantities of aluminium. The results also indicate that the strengthening precipitates are preferentially formed from Si-rich nuclei that contain Cu atoms, as opposed to Cu-free nuclei.     

Si材料中60°位错的分子动力学研究

本文使用Stillinger-Weber势函数和周期性边界条件,通过在原子尺度上的分子动力学计算研究了60°位错的位错心能量和运动情况.首先提出了相对简单的建立位错偶极子的新方法.在此基础上,借助于最近得到的对周期性映像作用的评估理论,由不同大小的3维计算模型得到的位错心能量的平均值为0.43 eV,这一结果不同于先前文献中的报导.另一方面,为研究位错运动在较大温度和压力范围下的表现,提出了相应解决方法来避免位错心在高温模拟环境时测量的不精确性.模拟结果显示位错速度相对于温度的变化曲线表现为波动形式.而且,位错的速度随模拟温度的升高而降低,这一结果与声子拖拽模型相吻合.     

Dislocation dynamics simulations with climb: kinetics of dislocation loop coarsening controlled by bulk diffusion

Dislocation climb mobilities, assuming vacancy bulk diffusion, are derived and implemented in dislocation dynamics simulations to study the coarsening of vacancy prismatic loops in fcc metals. When loops cannot glide, comparison of the simulations with a coarsening model based on the line tension approximation shows good agreement. Dislocation dynamics simulations with both glide and climb are then performed. Allowing for glide of the loops along their prismatic cylinders leads to faster coarsening kinetics, as direct coalescence of the loops is now possible.     

有限温度下位错环的脱体现象

在FCC单晶铜中构造了滑移面为（111）,伯格矢量为b=［112］/6的圆形不完全位错环．采用分子动力学方法模拟了该位错环在0—350 K温度区间内的自收缩过程．模拟结果发现：零温度下,位错不能跨越Peierls-Nabarro势垒运动,迁移速度为0;50　K温度下,螺型和刃型位错具有基本相同的迁移速度;随温度增加,刃型位错具有较大迁移速度;温度较高时,位错核宽度进一步增加;小位错环周围的局部应力,引起4个脱体位错环;脱体位错环在原位错的应力作用下逐渐生长,原位错消失后,在自相关键词：单晶铜位错环分子动力学位错源     

A computational approach towards modelling dislocation transmission across phase boundaries

To study the nanoscopic interaction between edge dislocations and a phase boundary within a two-phase microstructure the effect of the phase contrast on the internal stress field due to the dislocations needs to be taken into account. For this purpose a 2D semi-discrete model is proposed in this paper. It consists of two distinct phases, each with its specific material properties, separated by a fully coherent and non-damaging phase boundary. Each phase is modelled as a continuum enriched with a Peierls–Nabarro (PN) dislocation region, confining dislocation motion to a discrete plane, the glide plane. In this paper, a single glide plane perpendicular to and continuous across the phase boundary is considered. Along the glide plane bulk induced shear tractions are balanced by glide plane shear tractions based on the classical PN model. The model's ability to capture dislocation obstruction at phase boundaries, dislocation pile-ups and dislocation transmission is studied. Results show that the phase contrast in material properties (e.g. elastic stiffness, glide plane properties) alone creates a barrier to the motion of dislocations from a soft to a hard phase. The proposed model accounts for the interplay between dislocations, external boundaries and phase boundary and thus represents a suitable tool for studying edge dislocation–phase boundary interaction in two-phase microstructures.     

基于点缺陷扩散理论与离散位错动力学耦合的位错攀移模型研究

位错的攀移运动对高温下晶体材料的塑性行为有重要影响,为了能够有效揭示攀移的物理本质及其对塑性行为的作用,本文基于点缺陷扩散理论,通过将体扩散和管扩散机理的共同作用与三维离散位错动力学耦合,建立了适用条件更广的位错攀移模型. 利用此模型我们模拟了单个及多个棱柱型位错环的收缩变形过程,发现影响位错攀移速率的决定因素不是传统理论认为的机械攀移力,而是位错周围(体扩散)及位错段上(管扩散)的空位浓度梯度. 该模型也能够完全重现棱柱型位错环群的粗化过程中不同位错环半径及晶体内平均空位浓度随时间变化的三个阶段.关键词：位错攀移点缺陷扩散理论位错动力学棱柱位错环     

Understanding acoustoplasticity through dislocation dynamics simulations

The acoustoplastic effect in metals is routinely utilised in industrial processes involving forming, machining and joining, but the underlying mechanism is still not well understood. There have been earlier suggestions that dislocation mobility is enhanced intrinsically by the applied ultrasound excitation, but in subsequent deliberations it is routinely assumed that the ultrasound merely adds extra stresses to the material without altering its dislocation density or intrinsic resistance to deformation. In this study, a dislocation dynamics simulation was carried out to investigate the interactions of dislocations under the combined influence of quasi-static and oscillatory stresses. Under such combined stress states, dislocation annihilation is found to be enhanced leading to larger strains at the same load history. The simulated strain evolution under different stress schemes also closely resembles certain previously obtained experimental observations. The discovery here goes far beyond the simple picture that the ultrasound effect is merely an added-stress one, since here, the intrinsic strain-hardening potency of the material is found to be reduced by the ultrasound, through its effect on enhancing dislocation annihilation.     

Non-singular dislocation continuum theories: strain gradient elasticity vs. Peierls–Nabarro model

Abstract

Non-singular dislocation continuum theories are studied. A comparison between Peierls–Nabarro dislocations and straight dislocations in strain gradient elasticity is given. The non-singular displacement fields, non-singular stresses, plastic distortions and dislocation core shapes are analysed and compared for the two models. The main conclusion of this study is that due to their characteristic properties, the non-singular displacement fields, non-singular stresses and dislocation core shape of screw and edge dislocations obtained in the framework of strain gradient elasticity are more realistic and physical than the corresponding fields of the Peierls–Nabarro model. Strain gradient elasticity of dislocations is a continuum dislocation theory including a weak non-locality within the dislocation core and predicting the size and shape of the dislocation core. The dislocation core is narrower in the strain gradient elasticity dislocation model than in the Peierls–Nabarro model and more evenly distributed in two dimensions. The present analysis shows that for the modelling of the dislocation core structure the non-singular dislocation fields of strain gradient elasticity are the suitable ones.     

位错密度的演化动力学方程

将位错的增殖、淹没与相互反应看作化学反应,位错在热激活作用下具有扩散性质。因此,位错系统是一个反应-扩散系统。依据自组织理论,给出了两个位错密度演化动力学方程,当系统发生结构失稳之后,第一个方程变成第二个方程;第一个方程含有对称性破缺,按Higgs机制将发射偶极子。     

基于多尺度分析的FCC金属应变率敏感性研究

 采用位错理论和分子动力学模拟研究了金属原子性质对其宏观应变率敏感性的影响。依据位错运动的Orowan关系,认为金属中位错速度对应力的依赖关系是此研究的关键,并分析提出研究金属原子性质与应变率敏感性关系的分析方法。构建了一个中等规模的二维分子动力学模型,应用此模型对单个位错在FCC金属中的运动进行模拟。综合位错理论分析和分子动力学模拟结果得出结论：影响金属应变率敏感性的原子性质是其原子量而不是其原子势。依据此结论分析得到的FCC金属应变率敏感性排序与试验结果相符。     

Softening and hardening of yield stress by hydrogen–solute interactions

Abstract

Hydrogen atoms have a wide variety of effects on the mechanical performance of metals, and the underlying mechanisms associated with effects on plastic flow and embrittlement remain to be discovered or validated. Here, the reduction in the plastic flow stress (softening) due to hydrogen atoms in solute-strengthened metals, previously proposed by Sofronis et al. is demonstrated at the atomistic level. Glide of an edge dislocation through a field of solutes in a nickel matrix, both in the absence of hydrogen and in the presence of H bound to the solutes, is modelled. The ‘solutes’ here are represented by vacancies, enabling use of accurate binary Ni–H interatomic potentials. Since vacancies have a misfit strain tensor in the Ni matrix and also bind hydrogen atoms, they are excellent surrogates for study of the general phenomenon. The binding of H to the solute (vacancy) reduces the misfit volume to nearly zero but also creates a non-zero tetragonal distortion. Solute strengthening theory is used to establish the connection between strength and solute/hydrogen concentration and misfit strain tensor. Simulations show that when a dislocation moves through a field of random vacancy ‘solutes’, the glide stress is reduced (softening) when H is bound to the solutes. Trends in the simulations are consistent with theory predictions. Trends of softening or hardening by H in metal alloys can thus be made by computing the misfit strain tensor for a desired solute in the chosen matrix with and without bound hydrogen atoms. Pursuing this, density functional theory calculations of the interaction of H with carbon and sulphur solutes in a Ni matrix are presented. These solutes/impurities do not bind with H and the complexes have larger misfit strains, indicative of H-induced strengthening rather than softening for these cases. Nonetheless, H/solute interactions are the only mechanism, to date, that shows nanoscale evidence of plastic softening due to hydrogen associated with the hydrogen-enhanced localised plasticity concept in fcc metals.     

基于连续介质位错理论的位错原子分布构造方法

基于连续介质位错理论提出一种新的位错原子分布构造方式,理论上可以构造出任意形状和任意Burgers矢量的位错结构.利用该方法,选用FCC单晶铜为模拟介质,构造Burgers矢量为b=[110]/2的刃型全位错和Burgers矢量为b=[112]/6圆环形不完全位错环,并使用分子动力学方法模拟全位错的扩展分解过程和不全位错环在自应力作用下的收缩过程,模拟结果与理论分析一致.该方法的优点在于可以方便地构造出其他传统方法难以构造的位错闭合结构——位错环,从而使位错环的细致研究成为可能.     

固氢金属化转变压力的理论计算

 利用简单金属的赝势理论方法计算了固氢的金属转变压力,并探讨了金属氢可能的晶体结构及力、热物性。计算结果表明,在绝对零度条件下,分子态固氢（HCP结构）向原子相金属氢（FCC结构）的转变压力p_t=465.95 GPa。