二维大取向角对晶界湮没过程的晶体相场模拟

【目的】针对大角正方相晶界的位错结构,揭示在外应变下的位错运动和位错反应的微观机理。【方法】采用双模晶体相场(PFC)模型,模拟大角度取向角位错湮没过程。【结果】晶界上的位错是由4个位错组成1个位错对。晶界湮没有如下主要过程:开始时位错沿晶界攀移,随后晶界发生位错分解并发射,位错运动方式由攀移转为滑移;滑移位错与其他位错在晶内相遇发生湮没,其余晶界位错进行攀移,再次出现晶界位错分解发射位错,位错滑移穿过晶界内部,到达对面晶界处发生湮没。在这个过程中部分位错滑移与其他位错相遇会形成新的位错,同样继续进行攀移、分解、滑移进而湮没的运动。【结论】PFC模型能较好地用于研究大角度正方相的位错在施加应力作用下的运动。     

晶界萌生裂纹扩展的晶体相场模拟

【目的】研究双晶样品在单轴拉伸应变作用下所发生的纳米微观尺度裂纹扩展运动,观察裂纹扩展特征,揭示位错对裂纹扩展的作用规律。【方法】应用晶体相场(PFC)方法模拟裂纹扩展演化及其位错运动。【结果】在位错没有发射之时,裂纹沿位错对柏氏矢量方向解理扩展,发射位错后裂尖沿顺时针旋转60°继续扩展。【结论】晶界滑移出的位错由于周围应力集中,萌生出微裂纹。在裂纹扩展中,裂纹与位错相互作用,可以有多种形式。     

共析钢经表面研磨处理后微观组织的演化

利用表面研磨处理(SMAT)在共析钢上制备出具有纳米晶体结构的表面层,并利用X射线衍射和透射电镜分析了纳米表面层的微观组织结构及其演变.结果表明:原始组织为层片状铁素体 渗碳体两相复合组织的共析钢经过SMAT后,铁素体晶粒由于位错产生、位错缠结形成位错胞,然后分割晶粒使铁素体细化至纳米尺度,而渗碳体在强烈塑性变形下,经历弯曲、断裂,最终分解成体心立方铁素体和底心正交的石墨.     

预变形方式对微合金钢中贝氏体热稳定性的影响

采用光学金相、硬度测量结合透射电镜观察,研究了经过弯曲或扭转变形的含铌微合金钢在等温受热时的组织稳定性问题. 研究发现:弯曲与扭转变形都可以大幅度提高微合金钢的硬度;在随后的550℃等温受热过程中,弯曲变形区的硬度迅速下降,同时伴随贝氏体向平衡组织的演变;而扭转区在等温过程中的硬度始终高于未变形区,同时钢中贝氏体组织基本得以保持. 扭转应变量越大,硬化效果越强,并且在随后的等温受热过程中能保持这种硬度上的优势. 弯曲与扭转变形均导致贝氏体板条内位错密度显著增加. 弯曲变形区的位错分布不均匀,其中的低位错密度区易于在随后的等温受热过程中演变为平衡组织多边形铁素体的形核核心;而扭转变形区内位错分布均匀,并且在随后的等温受热过程中位错分布不发生显著改变. 这些结果表明,不同的冷变形方式对微合金钢中贝氏体热稳定性的影响存在显著差异.     

晶体相场法模拟大角度晶界的变形过程

采用晶体相场法模拟大角度晶界在外加应力作用下的变形过程,研究外加应力方向对晶界结构及位错运动的影响.研究表明：大角度晶界在应力作用下通过改变晶界曲率和位错运动使晶界发生迁移;晶界处位错形核所需临界应变与体系所受应力方向有关,其中沿平行晶界方向比沿垂直晶界方向所需临界应变更大.研究结果揭示了晶界是位错的源和汇,不仅能够产生位错,而且能够吸收位错.     

映像力对高纯铝自由表面附近位错组态的影响

采用分析位错映像力的方法研究了纯铝表层区域直螺、直刃位错所承受的滑移应力,理论上计算出映像应力作用下直螺、刃位错临界滑移距离和纯铝表层低位错密度区尺寸.讨论了直刃位错临界攀移距离和温度的关系,指出了表面上相对稳定的位错组态.     

Dynamics of the silicon (111) surface phase transition

The manner in which phase transformations occur in solids determines important structural and physical properties of many materials. The main problem in characterizing the kinetic processes that occur during phase transformations is the difficulty of observing directly, in real time, the growth of one phase at the expense of another. Here we use low-energy electron microscopy to study the real-time kinetics of a phase transformation confined to the silicon (111) surface. We show that the transformation is governed by the rate at which material is exchanged between the first layer of the crystal and the surface. In bulk phase transformations, the dynamics are usually governed either by the rate of diffusion of material to the phase boundaries or by the structural rearrangement of atoms at the phase boundary. The kinetic process that we have identified here has no bulk analogue and leads to domain dynamics that are qualitatively different from those expected for bulk systems.     

压电螺型位错与含界面效应纳米夹杂干涉研究

研究在无穷远纵向剪切和平面内电场作用下压电智能材料中螺型位错与考虑界面应力纳米尺度夹杂(纤维)间的力电耦合交互作用.运用复势方法,求解了夹杂和基体中复势函数的解析解以及应力场和电位移场分量.利用广义Peach-Koehler公式,给出了作用在压电螺型位错上位错像力的解析解答.研究结果表明:当夹杂的半径缩减到纳米尺度时,界面效应对夹杂(纤维)附近位错运动和平衡位置的影响将变得非常显著.正界面效应将排斥基体中的位错;当存在正的界面效应时,软夹杂能排斥界面附近的压电螺型位错.     

Si晶体中螺位错滑移特性的分子动力学

在保持Si晶体模型完全周期性的边界条件下, 采用位错偶极子模型在其内部建立一对螺位错. 通过Parrinello-Rahman方法对模型施加剪应力, 并应用分子动力学计算位错运动速度及交滑移的发生与外加剪应力间的关系. 在此基础上进一步研究晶体内的空位缺陷对螺位错运动的影响. 结果表明, 在位错滑移面上的六边形环状空位聚集体可加速螺位错的运动, 并且螺位错能通过交滑移跨越该空位缺陷, 避免产生钉扎现象. 揭示了低温层中大量存在的空位缺陷是降低位错密度的原因.      

外应力作用下小角晶界的斜排位错运动研究

[目的]探讨纳米晶材料中小角度晶界位错在应力作用下的演化机理。[方法]通过分析位错受力,建立动力学方程,分析晶界斜排位错在受到外应力作用下的运动规律。[结果]晶界位错的3个部分结果不同,处在中心位错以上的位错(第1部分)在剪切应力不断增大的过程中,会经过滑移,波动,最后沿柏氏矢量方向滑移过程;中心位错(第2部分)会经过滑移,波动,再滑移过程;中心位错以下的位错(第3部分)会先逆柏氏矢量方向滑移,然后改变运动方向进行攀移,最后回到第5个位错的初始位置后沿柏氏矢量滑移。[结论]当晶界位错受到外应力作用时,晶界会马上瓦解,外应力是导致晶界瓦解的主要因素。对比水平位错的晶界,斜排位错组成的晶界瓦解不具有对称性,每个位错的运动规律都各有特点。     

晶体相场法模拟小角度晶界的位错结构及其演化

采用晶体相场模型模拟小角度晶界结构和在外加应力作用下晶界、位错的演化过程,从位错的运动形式、体系自由能与应变关系等角度,讨论晶界位错运动和湮没过程。结果表明:在稳定的小角度晶界中,位错间距与位向差成反比。位错和晶界区域具有较高的能量密度;外加应力会导致一个位错组脱离晶界而分解成两个位错组,且有诱发晶界湮没的趋势;晶界湮没过程可以出现4个阶段或2个阶段,且存在敏感位向差角,位向差稍高于5.85°时晶界湮没过程呈4个阶段,位向差稍低于5.85°时晶界湮没过程呈2个阶段;位向差越小,晶界湮没过程越简单快捷。     

Dislocation multi-junctions and strain hardening

At the microscopic scale, the strength of a crystal derives from the motion, multiplication and interaction of distinctive line defects called dislocations. First proposed theoretically in 1934 (refs 1-3) to explain low magnitudes of crystal strength observed experimentally, the existence of dislocations was confirmed two decades later. Much of the research in dislocation physics has since focused on dislocation interactions and their role in strain hardening, a common phenomenon in which continued deformation increases a crystal's strength. The existing theory relates strain hardening to pair-wise dislocation reactions in which two intersecting dislocations form junctions that tie the dislocations together. Here we report that interactions among three dislocations result in the formation of unusual elements of dislocation network topology, termed 'multi-junctions'. We first predict the existence of multi-junctions using dislocation dynamics and atomistic simulations and then confirm their existence by transmission electron microscopy experiments in single-crystal molybdenum. In large-scale dislocation dynamics simulations, multi-junctions present very strong, nearly indestructible, obstacles to dislocation motion and furnish new sources for dislocation multiplication, thereby playing an essential role in the evolution of dislocation microstructure and strength of deforming crystals. Simulation analyses conclude that multi-junctions are responsible for the strong orientation dependence of strain hardening in body-centred cubic crystals.     

Molecular dynamics simulation of microcrack healing in aluminium

The molecular dynamics method is used to simulate microcrack healing during heating or under compressive stress. A center microcrack in an Al crystal could be sealed under a critical compressive stress or by heating it over a critical temperature. During microcrack healing, dislocation and vacancy are generated and moved, and sometimes twin appears. The critical temperature necessary for microcrack healing depends upon the orientation of the crack plane. For example, the critical temperature of the crack along the ( 111 ) plane is the lowest. When there are pre-existing dislocations around the microcrack, the critical temperature necessary for microcrack healing will decrease. The energy condition for crack healing is б2π a ( 1-v2 ) / E + UT/ A ≥ 2 γ + γp, where б is the applied compressive stress, a the length of the crack, γ the Poisson' s ratio, E the Young' s modulus, UT heat energy driving crack healing, A the area of the crack, γ the surface energy, and γp the plastic deformation work. Pre-existing dislocations can reduce γp.     

Striped iron zoning of olivine induced by dislocation creep in deformed peridotites.

Deformation of solid materials affects not only their microstructures, but also their microchemistries. Although chemical unmixing of initially homogeneous multicomponent solids is known to occur during deformation by diffusion creep, there has been no report on their chemical zoning due to deformation by dislocation creep, in either natural samples or laboratory experiments. Here we report striped iron zoning of olivine ((Mg,Fe)2SiO4) in deformed peridotites, where the iron concentration increases at subgrain boundaries composed of edge dislocations. We infer that this zoning is probably formed by alignment of edge dislocations dragging a so-called Cottrell 'atmosphere' of solute atoms (iron in this case) into subgrain boundaries during deformation of the olivine by dislocation creep. We have found that the iron zoning does not develop in laboratory experiments of high strain rates where dislocations move too fast to drag the Cottrell atmosphere. This phenomenon might have important implications for the generation of deep-focus earthquakes, as transformation of olivine to high-pressure phases preferentially occurs in high-iron regions, and therefore along subgrain boundaries which would be preferentially aligned in plastically deformed mantle peridotites.     

一种基于面向对象模拟刃型位错运动图象及应力场的方法

位错是晶体中的一种线型缺陷，它在晶体中普遍存在，不仅影响着晶体的力学强度，而且对固体的许多物理性质均有重要影响.为了能形象地理解位错的基本性质，本文以刃型位错作为研究算例，通过编写Visual C++程序，成功地实现了对刃型位错原子动态图象及其应力场分量的计算机模拟，并详细阐述了程序设计原理和实现方法.本文的编程思想和通过定性及定量分析得出的结论对深入研究位错的特性提供了很好的参考思路.     

纳米铜单晶拉伸力学性能的分子动力学模拟

采用分子动力学模拟了绝对零度时三种不同边界条件下纳米铜单晶的拉伸力学性能。计算结果发现：不同边界约束对钢单晶的内在原子运动和整体力学行为有明显影响;纳米杆、纳米薄膜良好的延性主要来源于位错运动;铜单晶块体的破坏源于内部孔洞的发展,破坏时延性较差;此外,纳米杆、纳米薄膜存在较大的表面张力。     

高温应变作用下小角度晶界湮没过程的晶体相场模拟

【目的】采用晶体相场模型模拟应变作用下晶界的变形过程。【方法】分别模拟了对称倾侧双晶体系在远离熔点温度和接近熔点温度时,在外加应力作用下小角度晶界的湮没过程。【结果】研究表明,小角度晶界同一侧晶界处相邻位错组的柏氏矢量夹角为60°,并且当体系接近熔点温度时,晶界处位错组周围出现预熔区域。【结论】晶界处的位错组在应力作用下进行滑移运动,两种情况中晶界位错的湮没规律基本相同,但由于预熔情况的出现,使得位错运动的阻力降低,位错运动的速度较快,湮没时体系能量减小得更多。     

带锥台托盘KH2PO4晶体生长与位错分布

利用光学显微镜对采用锥台托盘及传统平托盘生长的晶体进行位错腐蚀观察，发现锥台托盘生长的晶体上半部分位错分布与平托盘生长的晶体相似，部分区域位错密度低于平托盘生长的晶体。对于锥台托盘生长的晶体下半部分，其柱面位错缺陷很少，位错集中在与锥台直接接触的下锥面。下锥面的存在阻止了柱面底部新位错的形成，为柱面生长提供了更好的生长条件。硬度分析表明使用锥台托盘一定程度上提高了晶体结晶完整性。     

Low cycle fatigue behavior of micro-grain casting K4169 superalloy at room temperature

The strain-controlled low cycle fatigue tests were carried out at 298 ?K for the newly developed micro-grain casting K4169 superalloy, and the cyclic stress response, deformation and fracture behaviors of the alloy were studied. The results show that the fatigue life of the alloy was increased by more than two times compared with other casting K4169 alloys. This can be partly attributed to grain refinement impeding crack initiation and propagation. When the strain amplitude was in plastic range (Δε_t/2 ?≥ ?0.5%), the alloy displayed a short period of initial cyclic stability followed by cyclic softening to fracture. By observing dislocation configurations, only a few dislocations moved around grain boundaries at initial cycles, leading to short cyclic stability. Afterwards, dislocations in the slip bands cut γ" phase continuously leading to cyclic softening. When the strain amplitude was in elastic range (Δε_t/2 ?≤ ?0.4%), the alloy exhibited elastic deformation and the peak stress kept stable. In addition, the crack initiation site changed from the MC carbides to the persistent slip bands on surface with increasing strain amplitudes.