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本文首先说明晶体的弹性性质对其范性性质的重要作用,然后利用富氏变换方法推导出具有二重对称轴的晶体内由于集中作用力和位错所引起的应力分布。最后,利用本文中的公式讨论了Zn晶体的孪晶问题,指出孪晶的晶体可以使弹性能降低约35%左右。这就更进一步的证明了A.B.斯捷潘诺夫认为晶体的孪晶是由于弹性稳定性丧失的理论。 相似文献
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GaAs晶体的高质量生长对于制造高性能高频微波电子器件和发光器件具有重要意义.本文通过分子动力学方法对GaAs晶体沿[110]晶向的诱导结晶进行模拟,并采用最大标准团簇分析、双体分布函数和可视化等方法研究应变对生长过程和缺陷形成的影响.结果表明,不同应变条件下GaAs晶体的结晶过程发生显著变化.在初始阶段,施加一定拉应变和较大的压应变后,体系的晶体生长速率发生降低,且应变越大,结晶速率越低.此外,随着晶体的生长,体系形成以{111}小平面为边界的锯齿形界面,生长平面与{111}小平面之间的夹角影响固液界面的形态,进而影响孪晶的形成.施加拉应变越大,此夹角越小,形成孪晶缺陷越多,结构越不规则.同时,体系中极大部分的位错与孪晶存在伴生关系,应变的施加可以抑制或促进位错的形核,合适的应变甚至可以使晶体无位错生长.本文从原子尺度上研究GaAs的微观结构演化,可为晶体生长理论提供理论指导. 相似文献
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使用分子动力学方法,模拟研究了单晶Cu(001)薄膜在双向等轴拉伸应变下的塑性变形行为.当应变超过一定值时,样品通过产生位错、层错及孪晶而发生塑性变形.当应变相对较低时,不全位错首先在薄膜表面形核并在密排面上滑移,留下堆积层错;当应变增加时,位错在表面与内部同时成核生长,层错数量也随之增加.分析了相邻滑移面上的位错之间相互作用形成孪晶的微观过程.材料内部形成大量堆积层错及孪晶后,较大孪晶的密排面上的原子也会发生滑移,形成孪晶内部的层错结构以释放残余应力. 相似文献
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晶体中的孪晶,按其孪生对称元素一般分为反映和旋转孪晶两类.在LiNbO3晶体中,多数孪晶是反映孪晶. 本文所述的LiNbO3孪晶与常见的孪晶不同。它是从固-液同成分配方熔体中沿C轴方向提拉的.新晶是在原晶脊的晶面开始成核,沿着该晶面延伸发育,并与C轴斜交贯穿晶体(见图1). 这种少见孪晶的孪生对称元素是什么?它究竟属于哪类孪晶?我们采用X射线背射劳厄法和腐蚀法.对这些问题进行了比较详细的分析与讨论. 一、实 验 沿垂直拉晶方向将孪晶棒平行切割成三块晶片(图1中的α,β.γ), 每片都有原晶和新晶两部分。并分别用互和立表示.在各片I部分… 相似文献
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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. 相似文献
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In this study, molecular dynamics (MD) was used to simulate the rapid solidification process of Ni47Co53 and Ni48Co52 alloys at a cooling rate of 1012 K/s. The effects of HCP on the formation of twin boundaries and dislocations in two Ni–Co alloys are studied. It is found that the difference of HCP clusters is the main effect that producing discrepancies on microstructure of two alloys. The number of HCP clusters accounted for 9.23% in Ni47Co53 alloy. They are regularly arranged to form the number of single-layer twin boundaries, and each twin boundary ends in a dislocation. The FCC and HCP structures coexist in the same atomic layers, which is easy to create dislocations. The relatively standard FCC crystal and only 0.32% HCP clusters are formed in Ni48Co52 alloy at 300 K. That small amount of HCP clusters are dispersed on the surface, and cause the formation of dislocation in the border with FCC clusters. 相似文献
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Intergranular segregation of yttrium is investigated in a rhombohedral twin grain boundary of alumina. The deviation from the twin orientation is compensated by a periodic arrangement of intergranular dislocations. TEM tools (CTEM and HREM, EDXS, EFTEM and EELS/ELNES) have been used to characterize changes in chemical and electronic environments along this twin. Within the experimental limits, no Y is detected in the perfect twin parts. On the other hand, Y segregation occurs very locally in the dislocation cores on about four atomic planes perpendicularly to the GB, which in fact corresponds to the step height associated with the interfacial dislocations. By comparison with an undoped bicrystal, both the dislocation distribution and the Y segregation localization confirm the influence of Y on the dislocation mobility. Furthermore, in the Y-rich defects, high spatial resolution analysis of the energy loss near edge structures (ELNES) of the Al-L23 absorption edge brings some enlightenments about the Al3+ cation environment, provided the effects of local radiation damage are fully considered and under control. 相似文献
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ABSTRACT Void nucleation, growth and coalescence have been identified as the leading cause of ductile damage in metallic materials. To understand the underlying deformation and damage mechanisms, extensive theoretical, experimental and simulation efforts have been attempted on spherically voided metals. In this work, molecular dynamics simulations are performed to analyze the uniaxial straining deformation behaviours of both single-crystal and nanotwinned copper materials embedded with a preexisting spheroidal void. The coupling effects among twin boundary, spheroidal void aspect ratio and orientation on unidirectional elastoplastic behaviours are systematically examined. The dislocation-induced plastic deformation mechanism is also examined and compared with the one due to a perfectly spherical cavity. Simulation results show that elastic modulus increases with both spheroidal void aspect ratio and orientation. So do the yield stress, the first peak stress and the plasticity index. Another peak stress exists for most cases, except for a prolate void embedded in nanotwinned specimens. The slope between peak stresses decreases with both the spheroidal aspect ratio and orientation. The incorporation of a twin boundary results in lower elastic modulus, higher yield strength and smaller plasticity index. For an oblate void, the twin boundary gives rise to more severe strain softening behaviour. The dislocation extraction algorithm illustrates that the continuous nucleation, propagation and reaction of dislocations emanated from both the void front and twin boundary are responsible for the ductile damage of spheroidally voided crystals. The lower dislocation densities found in nanotwinned specimens indicate the desired suppression effects of twin boundary on dislocation activities. 相似文献
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《Physics letters. A》2020,384(22):126555
Molecular dynamics method is performed for analyzing the relationship of the twin boundary and grain boundary on the cyclic response of nanotwinned Cu. Results show that the strength difference among the grain boundary, the twin boundary and the variation of dislocation density are nearly 2-2.5 times. We predict twin boundary is only a factor that affects the stable response, however, the dislocation form and the time to reach stability is caused by the grain boundary. Furthermore, the phenomenon of cyclic hardening is found in all the nanotwinned Cu samples. 相似文献
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In situ straining in the transmission electron microscope and diffraction-contrast electron tomography have been applied to the investigation of dislocation/grain boundary and dislocation/twin boundary interactions in α-Ti. It was found that, similar to FCC materials, the transfer of dislocations across grain boundaries is governed primarily by the minimization of the magnitude of the Burgers vector of the residual grain boundary dislocation. That is, grain boundary strain energy density minimization determines the selection of the emitted slip system. 相似文献
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Wilto Kesternich † 《哲学杂志》2013,93(32):3449-3465
Segregation of silicon was induced by light-ion irradiation at elevated temperatures in Ni–8Si specimens. Its occurrence at external surfaces, helium-induced cavities, dislocation loops, coherent twin boundaries, grain boundaries, and precipitate-matrix interfaces has been investigated by transmission electron microscopy. Layers of ordered γ′ (Ni3Si) phase were formed at most of these point defect sinks. The behaviour of grain boundary precipitation was found to be exceptional in various respects. In particular, a high rate of precipitation distinguishes grain boundaries from all other kinds of point defect sinks investigated here. This phenomenon of rapid precipitation was found to be adjoined to precipitation-driven grain boundary migration and is attributed to a radiation-induced “discontinuous” precipitate reaction. Observations of helium bubble distributions created during α-particle irradiations at growing dislocation loops and at migrating grain boundaries are also briefly discussed. 相似文献
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对镍铬合金(20%Cr,1%Al,2.5%Ti)中层错边界处及共格孪晶界面上的不全位错进行了观察和分析,结果是:1.用g·bp=±2/3或±1/3作为不全位错是否显示衍衬是可行的,但不够严格。为此,应尽量选择{220}或{311}类型衍射成像,这时g·bp或者等于零,或者等于整数,比较容易确定不全位错的柏氏矢量。2.共格孪晶界面上有不全位错,大多数是全位错分解的产物,成对出现。3.平行滑移面上的层错在运动中可以相互重迭。重迭层错中内禀层错与外禀层错之间的不全位错,在g·bp=±2/3时无衍衬(在层错条纹的较强背景下是亮线),而在g·bp=±1/3时显示衍衬(暗线)。
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Grain boundary processes during plastic deformation of bicrystals were studied by TEM. Two methods were used. In situ straining in the electron microscope followed by post mortem examination and post mortem observation of specimens previously deformed by in situ synchrotron radiation X-ray topography. Two mechanisms governing slip propagation across a coherent twin boundary in a Fe-Si alloy bicrystal were identified. The first mechanism is a dissociation of a slip dislocation with the Burgers vector lying parallel to the boundary into three equal grain boundary dislocations. The second mechanism is a decomposition of a slip dislocation with Burgers vector inclined to the boundary into a dislocation mobile in the other grain and two screw grain boundary dislocations. 相似文献
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P. G. Partridge 《Czechoslovak Journal of Physics》1969,19(3):323-332
Slip band extrusions are formed in cadmium, magnesium and titanium, but not in zinc. The extrusions form preferentially in untwinned crystals. Filamentary growths occur at {10¯12} and {11¯21} twin interfaces during cyclic twinning.Possible dislocation interactions at these twin interfaces are described. The dislocation debris produced during cyclic strain in the slip bands and by cyclic twinning is shown to be similar and composed of a high density of dipole loops. It is therefore concluded that the occurrence and distribution of slip band extrusions in metals and the formation of twin boundary filamentary growths can be accounted for by a model based upon the glide of interstitial type dipole loops. Vacancy type loops will then cause crack nucleation in slip bands and deformation twin boundary regions.Twin boundary debris can also cause the observed fragmentation of twins by acting as a barrier to twin boundary movement.The author is grateful to Dr. A. G. Crocker, University of Surrey, for many discussions on the twinning mode in h.c.p. metals and to P. J. E. Forsyth for his interest and encouragement. The paper is published by permission of the Controller, H. M. Stationery Office. Crown copyright is reserved. 相似文献
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The microstructure features of carbides precipitated at twin boundary and Σ9 grain boundary (GB) in nickel-based Alloy 600 after aging at 715?°C for different time were investigated by high-resolution transmission electron microscopy. The carbides precipitated at incoherent twin boundary and Σ9 GB were identified as M23C6. The bar-like carbides grow parallel to each other and into each side of grain nearby incoherent twin boundary, while only growth into one side nearby Σ9 GB is observed. The carbides precipitated at incoherent twin and Σ9 GBs contain twins. The carbides have (1?1?1) matrix // (1?1?1) M23C6 orientation relationship with the nearby matrix. Based on the experimental results, the formation of twinning microstructure features in the M23C6 is discussed. 相似文献