晶体表面微观结构的反射电子显微术观察

本文用反射电子显微术观察了球状铂单晶表面的微观结构,分析了单晶形成中内部缺陷的变化。当球状液滴凝固时,晶体中存在位错、小角晶界等缺陷。在其后的退火过程中,晶粒内部的位错向亚晶界运动,降低了晶粒内部的位错密度,而亚晶界中的位错向晶体表面的运动,导致亚晶界的消失,形成单晶。 关键词：     

温度和应变速率耦合作用下纳米晶Ni压缩行为研究

本文利用低温力学测试系统研究了电化学沉积纳米晶Ni在不同温度和宽应变速率条件下的压缩行为. 借助应变速率敏感指数、激活体积、扫描电子显微镜及高分辨透射电子显微镜方法, 对纳米晶Ni的压缩塑性变形机理进行了表征. 研究表明, 在较低温度条件下, 纳米晶Ni的塑性变形主要是由晶界位错协调变形主导, 晶界本征位错引出后无阻碍的在晶粒内无位错区运动, 直至在相对晶界发生类似切割林位错行为. 并且, 在协调塑性变形时引出位错的残留位错能够增加应变相容性和减小应力集中; 在室温条件下, 纳米晶Ni的塑性变形机理主要是晶界-位错协调变形与晶粒滑移/旋转共同主导. 利用晶界位错协调变形机理和残留位错运动与温度及缺陷的相关性揭示了纳米晶Ni在不同温度、不同应变速率条件下力学压缩性能差异的内在原因.     

非对称倾侧亚晶界的晶体相场法研究

采用晶体相场法研究非对称倾侧亚晶界结构及其在应力作用下的微观运动机制.分别从温度、倾斜度角以及应力施加方向等方面对其结构及迁移过程进行分析和讨论.结果表明,非对称倾侧亚晶界由符号相同的一排刃型位错等距排列,部分出现由两个相互垂直排列的刃型位错构成的位错组;在应力作用下,非对称倾侧亚晶界迁移的微观机制包括位错的滑移和攀移、位错组分解、单个位错与位错组反应、单个位错分解以及位错湮灭;温度降低和倾斜度增大都会阻碍亚晶界的迁移过程;应力方向改变导致位错运动方向改变,从而改变晶界迁移形式.     

应变诱发四方相小角度对称倾侧晶界位错反应的晶体相场模拟

采用晶体相场法研究了外加应变作用下,不同取向差的四方相对称倾侧小角度晶界的位错运动与反应及反应过程中的位错组态,通过采用几何相位法对位错周围应变场进行了表征.结果表明,凝固弛豫达到稳态后,晶界两侧位错平行且方向相反,随晶界两侧晶粒取向差增大,位错数目增加,距离减小,且体系自由能增加.在外加应变作用下,晶界位错经历攀移、发射、反应湮灭等阶段,体系自由能呈现波动.当取向差增大时,位错运动方式由攀移向攀滑移转变,产生更多类型的位错组构型,并发生相应的位错与位错组之间的反应.对于不同构型的位错反应,正切应变驱动位错靠近,负切应变驱动位错湮灭.     

不同对称性下晶界结构演化及微观机理的晶体相场法研究

采用晶体相场法研究了单轴拉伸下三角相双晶变形过程及机理, 并重点分析了小角对称与非对称晶界和大角对称与非对称晶界在变形过程中的演化及微观机理, 变形过程中应力方向与初始晶界方向平行. 结果表明, 小角对称晶界由柏氏矢量夹角呈60°的两种刃型位错组成, 变形过程中不同类型的位错运动方向相反, 并各自与另一晶界上同一类型位错相互吸引以致部分位错发生湮没; 小角非对称晶界上的位错类型单一, 在应力作用下先沿水平方向攀移, 后各自分解成柏氏矢量约呈120°的两位错, 并通过位错运动和湮没最终形成理想单晶; 大角晶界在应力的作用下先保持水平状态而后锯齿化并发射位错, 伴随着位错运动和湮没, 最终大角非对称晶界发生分解, 而大角对称晶界则重新平直化, 表明大角对称晶界比大角非对称晶界更稳定, 这与实验和分子动力学模拟结果一致. 关键词： 晶体相场 双晶 晶界 对称性     

纳米晶体材料中小角度晶界湮没过程研究

本文针对纳米晶材料演化过程中的小角度晶界湮没,建立位错运动方程,计算模拟小角度晶界的晶格位错在外应力的作用下发生的运动,结果表明导致小角度晶界湮没的重要原因是切应力作用,破坏了晶界位错的受力平衡。这种湮没过程导致了高浓度的可整体移动的晶格位错形成。在纳米晶体材料中,这种被施加应力诱发的位错集体迁移,具有可塑的局部流动特性。     

晶体相场法研究晶粒缩小过程中的位错湮灭与晶界迁移

通过晶体相场法模拟了与基体三种不同取向圆形晶粒在缩小过程中晶界上的位错湮灭机制与晶界迁移机制.研究结果表明:当圆形晶粒和基体的取向差17°时,圆形晶粒和基体形成位错核心重叠的大角晶界,用位错模型难以解释该演化过程,但结果表明圆形晶粒半径的平方与演化时间成线性关系,该关系与弯曲晶界迁移理论相互印证;当取向差为4°时,圆形晶粒和基体形成由分离位错构成的小角晶界,在该晶粒缩小的过程中,位错以径向攀移为主且会发生晶粒转动以调整位错间距,随着位错间距的减小相互靠近的位错发生反应;当取向差为10°时,晶界既有位错核心重叠较小的部分也有由分离位错构成的部分,在晶粒缩小时晶界演化表现为位错径向攀移和切向运动,两种运动的耦合运动使得能相互反应的位错相互靠近并发生反应.     

纳米Ni在77 K温度下压缩行为的研究

利用低温力学测试系统研究了电化学沉积纳米Ni在77 K温度下的压缩行为. 室温下纳米Ni 的屈服强度为 2.0 GPa, 77 K温度下的屈服强度为3.0 GPa, 压缩变形量则由室温的10%左右下降到5%. 借助应变速率敏感指数、激活体积、扫描电子显微和高分辨透射电子显微分析, 对纳米Ni的塑性变形机制进行了表征. 研究表明, 在77 K温度下的塑性变形主要是由晶界-位错协调变形主导, 晶界本征位错弓出后无阻碍地在晶粒内无位错区运动, 直至在相对晶界发生类似切割林位错行为. 同时分析了弓出位错的残留位错部分在协调塑性变形时起到的增加应变相容性和减小应力集中的作用. 利用晶界-位错协调机制和残留位错运动与温度及缺陷的相关性揭示了纳米Ni室温和77 K温度压缩性能差异的内在原因. 关键词： 塑性变形 强度 位错     

温度对大角度晶界变形过程影响的晶体相场法研究

应用晶体相场法研究大角度晶界在外加应力作用下温度对位错运动的影响。研究表明,大角度晶界在应力作用下会发生形状变化;当变形达到临界应变时,晶界褶皱处产生位错并发射进入晶粒内部;温度较低时,晶界处位错形核所需的临界应变更大。在应力作用下大角度晶界通过改变曲率和位错运动产生迁移,温度较高时有利于晶界迁移。     

钼单晶体中亚晶界位错结构的研究

本文应用蚀象法对电子束浮区区熔法制得的原生态钼单晶体中的亚晶界位错结构进行了直接观测。对于实验结果进行了细致的分析,并与亚晶界的Frank公式的一些预期结果比较,全面地证实了理论预测。对(111)面上平行蚀线方向的测量表明,它们大体沿着1/2〈111〉刃型位错的滑移面及攀移面的交线,从而证实了它们是这种位错所组成的一组位错倾侧型晶界。通过对蚀斑三叉亚晶界的分析,检验了推广后的Read-Shockley公式,同时表明存在着两组位错的倾侧晶界。对于(111)面上观察到的15组蚀线网络进行了分析,结果表明其中5组是1/2〈111〉/〈100〉网络,9组是〈100〉/〈110〉网络。分析中,除去应用Carrington等所发展的极图分析法以外,我们还根据Frank公式所规定的网线间距的关系式,提出了进一步定量检验的分析方法。实践证明,当极图分析不能获得唯一的结果时,这种定量检验法可以有效地确定位错网络的Burgers矢量。此外,我们还观察到奇位错和亚晶界交互作用的事例,特别是奇位错在亚晶界上引起“台阶”以及夹杂物和亚晶界交互作用的迹象。不同类型的亚晶界交接以及非平衡态的亚晶界也是经常可以观察到的。以上结果表明,蚀象法对于定量地研究原生态晶体中的亚晶界位错结构是极其有效的,其能力并不亚于电子显微镜薄膜透射法。     

Strain mapping in nanocrystalline grains simulated by phase field crystal model

In recent years, the phase field crystal (PFC) model has been confirmed as a good candidate to describe grain boundary (GB) structures and their nearby atomic arrangement. To further understand the mechanical behaviours of nanocrystalline materials, strain fields near GBs need to be quantitatively characterized. Using the strain mapping technique of geometric phase approach (GPA), we have conducted strain mapping across the GBs in nanocrystalline grains simulated by the PFC model. The results demonstrate that the application of GPA in strain mapping of low and high angles GBs as well as polycrystalline grains simulated by the PFC model is very successful. The results also show that the strain field around the dislocation in a very low angle GB is quantitatively consistent with the anisotropic elastic theory of dislocations. Moreover, the difference between low angle GBs and high angle GBs is revealed by the strain analysis in terms of the strain contour shape and the structural GB width.     

Grain Boundary Phase Transitions and their Influence on Properties of Polycrystals

Grain boundary (GB) phase transitions can change drastically the properties of polycrystals. The GB wetting phase transition can occur in the two-phase area of the bulk phase diagram where the liquid (L) and solid (S) phases are in equlibrium. Above the temperature of the GB wetting phase transition a GB cannot exist in equlibrium contact with the liquid phase. The experimental data on GB wetting phase transitions in numerous systems are analysed. The GB wetting tie-line can continue in the one-phase area of the bulk phase diagram as a GB solidus line. This line represents the GB premelting or prewetting phase transitions. The GB properties change drastically when GB solidus line is crossed by a change in the temperature or concentration. The experimental data on GB segregation, energy, mobility and diffusivity obtained in various systems both in polycrystals and bicrystals are analysed. In case if two solid phases are in equilibrium, the GB “solid state wetting” can occur. In this case the layer of the solid phase 2 has to substitute GBs in the solid phase 1. Such GB phase transition occurs if the energy of two interphase boundaries is lower than the GB energy in the phase 1.     

应变效应对金属Cu表面熔化影响的分子动力学模拟

采用Mishin镶嵌原子势，通过分子动力学方法模拟了金属Cu 的(110)表面在不同应变下的熔 化行为，分析了表面熔化过程中系统结构组态和能量的变化以及固液界面迁移情况.金属Cu 的(110)表面在低于热力学熔点的温度下发生预熔化，准液体层的厚度随温度升高而增加.当 温度高于热力学熔点时，固液界面的移动速度与温度成正比，外推得到热力学熔点为1380K ，与实验结果1358K吻合良好.应变效应（包括拉伸和压缩）导致热力学熔点降低，并促进表 面预熔化进程.在相同温度条件下，准液体层的厚度随应变绝对值的增加而增大.应变效应导 致的固相自由能增加是金属Cu(110)表面热稳定性下降的主要因素，且表面应力和应变方向 的异同也会影响表面预熔化的进程. 关键词： 表面预熔化 热力学熔点 表面应力 分子动力学     

Nucleation and growth of helium bubble at(110) twist grain boundaries in tungsten studied by molecular dynamics

Migration of He atoms and growth of He bubbles in high angle twist grain boundaries(HAGBs) in tungsten(W) are investigated by atomic simulation method. The energy and free volume(FV) of grain boundary(GB) are affected by the density and structure of dislocation patterns in GB. The migration energy of the He atom between the neighboring trapping sites depends on free volume along the migration path at grain boundary. The region of grain boundary around the He bubble forms an ordered crystal structure when He bubble grows at certain grain boundaries. The He atoms aggregate on the grain boundary plane to form a plate-shape configuration. Furthermore, high grain boundary energy(GBE) results in a large volume of He bubble. Thus, the nucleation and growth of He bubbles in twist grain boundaries depend on the energy of grain boundary, the dislocation patterns and the free volume related migration path on the grain boundary plane.     

Atomistic simulation of the stacking fault energy and grain shape on strain hardening behaviours of FCC nanocrystalline metals

ABSTRACT

Ultra-fine grained copper with nanotwins is found to be both strong and ductile. It is expected that nanocrystalline metals with lamella grains will have strain hardening behaviour. The main unsolved issues on strain hardening behaviour of nanocrystalline metals include the effect of stacking fault energy, grain shape, temperature, strain rate, second phase particles, alloy elements, etc. Strain hardening makes strong nanocrystalline metals ductile. The stacking fault energy effects on the strain hardening behaviour are studied by molecular dynamics simulation to investigate the uniaxial tensile deformation of the layer-grained and equiaxed models for metallic materials at 300?K. The results show that the strain hardening is observed during the plastic deformation of the layer-grained models, while strain softening is found in the equiaxed models. The strain hardening index values of the layer-grained models decrease with the decrease of stacking fault energy, which is attributed to the distinct stacking fault width and dislocation density. Forest dislocations are observed in the layer-grained models due to the high dislocation density. The formation of sessile dislocations, such as Lomer–Cottrell dislocation locks and stair-rod dislocations, causes the strain hardening behaviour. The dislocation density in layer-grained models is higher than that in the equiaxed models. Grain morphology affects dislocation density by influencing the dislocation motion distance in grain interior.     

Grain boundary premelting of monolayer ices in 2D nano-channels

ABSTRACT

We employ molecular-dynamics (MD) simulations to study grain boundary (GB) premelting in ices confined in two-dimensional hydrophobic nano-channels. Premelting transitions are observed in symmetric tilt GBs in monolayer ices and involve the formation of a premelting band of liquid phase water with a width that grows logarithmically as the melting temperature is approached from below, consistent with the existing theory of GB premelting. The premelted GB is found rough for a broad range of temperature below the melting temperature, the two ice-premelt interfaces bounding the melted GB are engaged with long wave-length parallel coupled fluctuations. Based on current MD simulation study, one may expect GB premelting transitions exist over a wide range of low dimensional phases of confined ice and shows important consequences for crystal growth of low dimensional ices.     

Deformation study of bicrystalline and nano-polycrystalline structures using phase field crystal method

Deformation behaviors of bicrystalline and nano-polycrystalline structures of various tilt angles and inclination angles in two dimensions are investigated in detail using a two-mode phase field crystal model.The interaction between grain boundary(GB)and dislocation is also examined in bicrystals and nano-polycrystals that both contain asymmetric and symmetric tilt GBs,with energy analysis being carried out to analyze these processes.During deformation simulations,we assume the volume of each simulation cell at every time step is coincident with that of the initial state just before deformation.Our simulation results show that the behaviors of symmetric and asymmetric GBs in bicrystals and nano-polycrystals differ from each other depending on tilt angle and inclination angle.A new dislocation emission mechanism of interest is observed in bicrystals which contain low angle symmetric tilt GBs.Low angle GB has a higher mobility relative to high angle GB in both bicrystalline and nano-polycrystalline structures,as does asymmetric GB to symmetric GB.The generation,motion,pileup and annihilation of dislocations,grain rotation and grain coalescence are observed,which is consistent with the simulation results obtained by molecular dynamics.These simulation results can provide strong guidelines for experimentation.     

剪切应变下刃型位错的滑移机理的晶体相场模拟

针对刃型位错的滑移运动, 构建包含外力场与晶格原子密度耦合作用的体系自由能密度函数, 建立剪切应变作用体系的晶体相场模型. 模拟了双相双晶体系的位错攀移和滑移运动, 计算了位错滑移的Peierls势垒和滑移速度. 结果表明: 施加较大的剪切应变率作用, 体系能量变化为单调光滑曲线, 位错以恒定速度做连续运动, 具有刚性运动特征; 剪切应变率较小时, 体系能量变化出现周期波动特征, 位错运动是处于低速不连续运动状态, 运动出现周期“颠簸”式滑移运动, 具有黏滞运动特征; 位错启动运动, 存在临界的势垒. 位错启动攀移运动的Peierls势垒要比启动滑移Peierls势垒大几倍. 位错攀移和滑移运动特征与实验结果相符合.