晶体相场法模拟纳米晶材料反霍尔-佩奇效应的微观变形机理

采用晶体相场模型模拟获得了平均晶粒尺寸从11.61–31.32 nm的纳米晶组织, 研究了单向拉伸过程纳米晶组织的强化规律的微观变形机理. 模拟结果表明: 晶粒转动、晶界迁移等晶间变形行为是纳米晶材料的主要微观变形方式, 纳米晶尺寸减小, 有利于晶粒转动, 使屈服强度降低, 显示出反霍尔-佩奇效应.当纳米晶较小时, 变形量超过屈服点达到4%, 位错运动开启, 其对变形的直接贡献有限, 主要通过改变晶界结构而影响变形行为, 位错运动破坏三叉晶界, 引发晶界弯曲, 促进晶界迁移. 随纳米晶增大, 晶粒转动困难, 出现晶界锯齿化并发射位错的现象. 关键词： 晶体相场 纳米晶 反霍尔-佩奇效应 微观变形     

界面结构对Cu/Ni多层膜纳米压痕特性影响的分子动力学模拟

金属多层膜调制周期下降到纳米级时,其力学性质会发生显著改变. Cu-Ni晶格失配度约为2.7%,可以形成共格界面和半共格界面,实验中实现沿[111]方向生长的调制周期为几纳米且具有异孪晶界面结构的Cu/Ni多层膜,其力学性质发生显著改变.本文采用分子动力学方法对共格界面、共格孪晶界面、半共格界面、半共格孪晶界面等四种不同界面结构的Cu/Ni多层膜进行纳米压痕模拟,研究压痕过程中不同界面结构类型的形变演化规律以及位错与界面的相互作用,获取Cu/Ni多层膜不同界面结构对其力学性能的影响特征.计算结果表明,不同界面结构的样品在不同压痕深度时表现出的强化或软化作用机理不同,软化机制主要是由于形成了平行于界面的分位错以及孪晶界面的迁移,强化机制主要是由于界面对位错的限定作用以及失配位错网状结构与孪晶界面迁移时所形成的弓形位错之间的相互作用.     

纯铜[0 1 1]倾侧型非共格∑3晶界结构稳定性分子动力学模拟研究

有研究表明, 非共格∑₃晶界的行为在中低层错能面心立方金属晶界 特征分布演化中发挥着重要作用. 为了掌握不同界面匹配的非共格∑₃晶界的特性, 本文利用分子动力学(MD)模拟方法首先研究了纯铜的[0 1 1]倾侧型 非共格∑₃晶界在700–1100 K温度范围内和常压下的结构稳定性. MD模拟采用原子间相互作用长程经验多体势, 步长为5×10^-15 s. 模拟结果表明: 所研究的五个非共格∑₃晶界, 其结构稳定性存在很大差异, 其一般规律是, 与(1 1 1)/(1 1 1)共格孪晶界之间的夹角(Φ角)越小, 晶界匹配值越大, 则非共格∑₃晶界越稳定; 反之亦然. Φ角最小的 (2 5 5)/(2 1 1)非共格∑₃晶界较稳定, 在退火过程中几乎不发生变化. 随着Φ角的增大, 非共格∑₃晶界不再稳定, 这类晶界会通过Miller指数较高一侧晶体每三层原子面合并为一层原子面 (或Miller指数较低一侧晶体每一层原子面分解为三层原子面)的机理 转变为亚稳的“台阶”状晶界, 台阶面部分地处于精确的能量极低 的{111}/{111}共格孪晶界上; 当提高温度退火时, 这种“台阶”状晶界最终会全部转变成稳定平直的{111}/{111}共格孪晶界. 关键词： 纯铜 ∑₃晶界')" href="#">非共格∑₃晶界 分子动力学模拟     

高压导致纳米TiO2形变的电子显微研究

α-PbO₂相TiO₂高压相具有适宜的带隙能和可见光范围的光催化能力,是一种适用于可见光、高效且环保的光催化材料.本文使用金刚石对顶压砧对锐钛矿纳米球进行加压-卸压处理得到了α-PbO₂相TiO₂高压相.利用透射电子显微镜对比初始样品和卸压样品,观察结果表明晶粒发生了明显形变,高分辨图显示其晶粒中存在大量[100]方向层错和形变孪晶,其中亚微米级晶粒中形成了透镜形片层结构的形变孪晶带;纳米级晶粒中形成了扇形多重形变孪晶.研究表明高压下锐钛矿TiO₂可以发生明显的形变,其形变的微观机制与金属类似,主要为形变孪晶和层错滑移,形变孪晶的形成存在明显的尺寸效应.这些结果为TiO₂高压相变的尺寸效应研究提供了一个新的切入点,同时还为制备孪晶α-PbO₂相TiO₂高压相提供了方法.     

堆垛层错和温度对纳米多晶镁变形机理的影响

本文采用分子动力学模拟方法研究了在拉伸载荷下,堆垛层错和温度对纳米多晶镁力学性能的影响,在模拟中,采用嵌入原子势描述镁原子之间的相互作用．计算结果表明：在纳米晶粒中引入堆垛层错能明显增强纳米多晶镁的屈服应力,但堆垛层错对纳米多晶镁杨氏模量的影响很小;温度为300．0K时,孪晶在晶粒交界附近形成,孪晶随着拉伸应变的增加而逐渐生长．当拉伸应变达到0．087时,一种基面与X—Y面成大约35°角且内部包含堆垛层错的新晶粒成核并快速增长．也就是说,孪晶和新晶粒的形成和繁殖是含堆垛层错的纳米多晶镁在300．0K温度下的主要变形机理．模拟结果也显示,当温度为10．0K时,位错的成核和滑移是含堆垛层错的纳米多晶镁拉伸变形的主要形式．     

晶体相场方法模拟纳米晶生长过程

双模的PFC模型能够很好地模拟纳米晶的晶粒长大,可以在二维平面显示出不同晶面类型的晶粒长大过程、不同错配度和晶粒取向的晶界结构,还可以模拟出准共格晶界,观察到晶界处由于错配度不同引起的位错结构。这些模拟得到的结果,对实现人工控制纳米晶晶界结构,指导实验设计具有新型纳米晶界的结构材料具有重要意义。     

纳米多晶铁的冲击相变研究

利用分子动力学方法研究了不同晶粒度的纳米多晶铁在冲击压缩下的结构相变过程,模拟结果表明:纳米多晶铁的冲击结构相变(由体心立方(bcc)结构 α 相到六角密排(hcp)结构 ε 相)发生的临界冲击应力在15 GPa左右.纳米多晶铁在经过弹性压缩变形后,晶界导致的塑性变形开始发生,然后大多数相变从晶界成核并最终发展为大规模相变.不同变形过程在应力和粒子速度剖面上能得到清晰的体现,并通过微观原子结构分析分辨.冲击压缩后的微观结构以晶界原子和以fcc结构原子充当孪晶界的hcp原子为主.晶粒度明显影响晶界变形及相变 关键词： 冲击相变 纳米多晶铁 冲击波 分子动力学     

CVD金刚石薄膜孪晶形成的原子机理分析

采用X射线衍射技术、电子背散射衍射技术和扫描电镜分别观察了不同甲烷浓度条件下沉积的CVD自支撑金刚石薄膜的宏观织构、晶界分布和表面形貌. 研究了一阶孪晶在金刚石晶体{111}面生长的原子堆垛过程. 结果表明，由于一阶孪晶〈111〉60°的取向差关系以及{111}面的原子堆垛结构，使{111}面上容易借助碳原子的偏转沉积产生一阶孪晶. 低甲烷浓度时，碳原子倾向于在表面能较低的{111}面沉积，为孪晶的形成提供了便利，且高频率孪晶使薄膜织构强度减弱. 甲烷浓度升高使生长激活能较小的{001}面成为主要前沿生长面，因而只有〈001〉晶向平行薄膜法向的晶粒能够不断长大，因此孪晶形核概率明显减小. 另外，在薄膜中发现二阶孪晶，并对二阶孪晶的形成进行了分析. 关键词： 金刚石薄膜 孪晶 原子机理 取向差     

氧化锌纳米带中的面缺陷

面缺陷是纳米带中非常普遍和非常重要的一类缺陷.在有些情况下,面缺陷对于高表面能指数面的出现起着决定性的作用, 同时,它们可以诱导纳米带沿着特殊的方向生长.面缺陷可以是孪晶或双晶,层错和由杂质原子聚集在特定原子面所形成的间隙原子层.在本文中,利用透射电子显微术,我们将介绍氧化锌纳米带中被发现的几种面缺陷.我们确认了两种孪晶/双晶结构,它们的孪晶面分别是(0113)和(2112)面.基面层错有I1 和I2两种.在大尺寸的纳米带中,I1基面层错可以折叠到(2110)面形成棱面层错.当少量的In离子掺入氧化锌纳米带后,我们发现伴随着杂质In在基面的聚集,形成了两种倒反畴界.     

晶界对纳米多晶铝中冲击波阵面结构影响的分子动力学研究

用分子动力学方法研究了纳米多晶铝在冲击加载下的冲击波阵面结构及塑性变形机理.模拟研究结果表明:在弹性先驱波之后,是晶界间滑移和变形主导了前期的塑性变形机理;然后是不全位错在界面上成核和向晶粒内传播,然后在晶粒内形成堆垛层错、孪晶和全位错的过程主导了后期的塑性变形机理.冲击波阵面扫过之后留下的结构特征是堆垛层错和孪晶留在晶粒内,大部分全位错则湮灭于对面晶界.这个由两阶段塑性变形过程导致的时序性塑性波阵面结构是过去未见报道过的. 关键词： 晶界 塑性变形 冲击波阵面 分子动力学     

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.     

晶体相场方法研究二维六角相向正方相结构转变

应用双模PFC模型,计算二维PFC相图,模拟二维六角晶格向正方晶格的结构转变过程, 观察新相(二维正方相)的形核、长大特点,以及相结构转变的动力学特征. 结果表明:六角结构相向正方结构相的转变,正方相最易在六角相晶界处, 尤其是在三晶粒的交汇处首先生成正方相的晶核,之后是正方相逐渐通过吞噬六角相的边缘, 向六角相内部推进,并不断长大.对于结构转变生成的正方相晶粒,其晶粒取向几乎是随机的, 与原先六角相晶粒取向角没有明显的关系.正方相转变的面积分数随时间变化的动力学曲线 呈现典型的"S"形.由Avrami曲线可将相变曲线看成由两阶段组成. 计算模拟得到的Avrami曲线的第二阶段直线斜率K的范围在2.0和3.0之间, 与JMAK理论的指数n相符合. 关键词： 结构转变 晶体相场 相图 晶粒取向     

A phase-field model for elastically anisotropic polycrystalline binary solid solutions

A phase-field model for modeling the diffusional processes in an elastically anisotropic polycrystalline binary solid solution is described. The elastic interactions due to coherency elastic strain are incorporated by solving the mechanical equilibrium equation using an iterative-perturbation scheme taking into account elastic modulus inhomogeneity stemming from different grain orientations. We studied the precipitate interactions among precipitates across a grain boundary and grain boundary segregation–precipitate interactions. It was shown that the local pressure field from one coherent precipitate influences the shape of precipitates in other grains. The local pressure distribution due to primary coherent precipitates near the grain boundary leads to inhomogeneous solute distribution along the grain boundary, resulting in non-uniform distribution of secondary nuclei at the grain boundary.     

Experimental study of polarization bremsstrahlung from small-grained polycrystals

The spectrum of polarization Bremsstrahlung is measured in backscattering geometry for 7-MeV electrons and a polycrystalline Ni foil without texture. The average size of grains in the foil is 300 nm. The results show the possibility of using a new method in the diagnostics of the atomic structure of polycrystalline materials with nanosized grains. The method is based on measuring the coherent component of polarization Bremsstrahlung.     

Collective effects in grain boundary migration

In situ high-resolution transmission electron microscopy is used to study grain boundary structure and kinetics in bicrystalline Au films at elevated temperature. We report the first direct evidence for the existence of cooperative atomic motion in grain boundary migration. Certain nanoregions at grain boundaries, typically involving up to several hundred atoms, are found to switch back and forth between neighboring grains. Reversible structural fluctuations at temperatures near 0.5T(m) and above have been discovered in [110] and [001] tilt, as well as in general grain boundaries.     

Atomic structure of grain boundaries in YBa2Cu3O7-δ as observed by Z-contrast imaging

Determination of the atomic structure of grain boundaries is the key to fundamental understanding of the critical current density in polycrystalline superconductors. High-resolution images with incoherent characteristics, obtained using a high-angle annular detector on an atomic resolution scanning transmission electron microscope, are used to study the atomic arrangements of these technologically important boundaries. The incoherent Z-contrast images do not experience contrast reversals with defocus or sample thickness and display no Fresnel Fringe effects at boundaries. Observed rigid shifts of atomic columns at grain boundaries are independent of sample thickness and objective lens defocus. These characteristics allow unambiguous and intuitive interpretations of the generated images. We find the atomic structures at grain boundaries in YBa₂Cu₃O_7-δ are strongly influenced by the strong tendency of this compound to exist only as complete unit cells terminated at {001} and {100} planes. The weak-link behavior associated with high-angle grain boundaries may follow from this structure in which there is no clear connection between the {100} facets of adjacent grains. Symmetric grain boundaries where adjacent grains share a common boundary plane have also been observed in YBa₂Cu₃O_7-δ. In these boundaries partial structural coupling of the grains is maintained. There is evidence that these two boundary forms produce junctions with very different superconducting properties.     

Boundary between bcc and hcp lattices in a <Superscript>4</Superscript>He quantum crystal

The mechanism of transition between bcc and hcp phases, which is applicable to both classical and quantum crystals, has been considered at the microscopic level. The relative orientations of the lattices that provide coherent phase matching are specified. It is suggested that phase boundaries are coherent and atomic displacements do not exceed the unit-cell size. A new order parameter—relative displacement of atomic layers—is introduced. The phase boundary is described within the double-and triple-well potential models. The relationship of the lattice constant and barrier height with the parameters of these potentials has been found, as well as the shape and energy of the phase boundary. A mechanism leading to a decrease in the barrier height for the phase boundary in a quantum crystal is proposed. This mechanism makes it possible to explain the results of recent experimental investigations of the nucleation occurring during the bcc-hcp transition in solid ⁴He.     

Amorphous structures of iron obtained by quenching of the liquid state

The frozen-in structure of liquid iron was relaxed under periodic boundary conditions, where the potential of Pak and Doyama was used. The pair distribution functions of the final quenched structures resemble that of the amorphous structure, but depend on the details of the initial atomic arrangement.     

高温应变下的晶界湮没机理的晶体相场法研究

应用晶体相场方法研究高温应变下的预熔化晶界位错湮没机理. 结果表明, 原预熔化晶界上的位错在应变作用下发生分离运动, 形成新晶界, 即亚晶界. 该过程的实质是生成了亚晶粒; 亚晶界的迁移过程的本质是亚晶粒长大、吞噬旧晶粒的过程; 亚晶界之间的湮没是亚晶粒完全吞噬旧晶粒过程的结束, 体系转变成为单个晶粒结构. 根据原子密度序参数沿x和y方向的投影值随应变量的变化特征, 可以揭示出高温应变作用下, 预熔化亚晶界相遇湮没的本质是两对极性相反的偶极子位错对发生二次湮没, 该湮没的微观过程是通过位错连续二次滑移湮没而实现的, 其湮没的速率较低温时的湮没速率要小许多.