压头晶体各向异性对纳米压痕变形机理的影响

为了研究压头晶体各向异性对纳米压痕变形机理的影响,用多尺度准连续介质(QC)法模拟了不同晶向的Ni压头与Ag薄膜的纳米压痕过程,对比不同晶向的压头在薄膜的弹-塑性转变点和最大载荷时薄膜中的原子滑移带,发现压头的晶向是决定薄膜开启原子滑移系的难易的关键因素。研究了压头在不同晶向下纳米压痕过程中Ag薄膜的变形机理,发现薄膜中原子滑移大都由压头拐角处触发。用RiceThomson位错模型计算得到压头表面正应力和切应力的分布图,借助应力分布图讨论了薄膜原子滑移的开启机理。     

一个特殊单滑移取向铜单晶疲劳位错结构研究

为了更细致地揭示面心立方金属单晶体的循环变形机制,利用扫描电镜电子通道衬度(SEM-ECC)技术观察研究了Schmid因子为0.5的[41841]单滑移取向铜单晶体的循环饱和位错结构.实验表明,在单滑移铜单晶体中,胞结构除了在高应变幅下的循环变形中出现外,还可能出现在循环应力-应变(CSS)曲线平台区的较低塑性应变幅下.驻留滑移带(PSBs)会随应变幅的增大而在试样表面聚集成内部含有位错胞的粗滑移带,带内的位错胞结构被认为是由于带内滑移阻力增大引起的应变集中所致形成的.此外,CSS曲线高应变幅区起始部分对应的循环饱和位错结构观察揭示出迷宫结构和胞结构是由PSBs逐渐演变而成的.     

轧制单层晶铜箔滑移启动和应变局部化的晶体塑性模拟

在晶粒尺度采用晶体塑性有限元模拟极薄带材轧制成形过程,对优化和改进材料模型以及探究极薄带材塑性变形机制具有重要作用.箔材轧制成形性能主要依赖材料的微观结构(晶界、滑移系、取向).采用退火态的单层晶铜箔为原料,进行箔轧实验和晶体塑性有限元模拟.建立反映晶粒形貌、晶界和取向各向异性的单层晶铜箔晶体塑性有限元模型,分析极薄带轧制成形中单/多滑移系启动状态和应变局部化现象.为准确构建晶体塑性有限元模拟的初始晶粒结构,消除微观组织亚表面的影响,采用垂直晶界即在厚度方向上建立只有一层晶粒的铜箔晶粒模型.结果表明:晶粒各向异性影响单层晶铜箔的轧制变形机制;晶界处的变形和滑移系运动状态完全不同于晶粒其他位置;单层晶轧制变形的滑移状态表现出明显的各向异性,出现局部滑移带和应变局部化,随轧制变形量的增大,滑移差异显著增大;晶界两侧局部区域存在滑移和变形的显著差异,这为亚晶和微观裂纹源的形核提供了有利的位置.     

同步辐射的基本知识 第五讲同步辐射中的成像术及其应用（二）

4．3衍衬成像术的应用 4．3．1缺陷应变矢量的测定 缺陷周围的应变场总是各向异性的,e^-i2π·R是因缺陷引人的附加相位因子。当衍射矢量与缺陷应变场的最大应变矢量平行时,衍射衬度最大,垂直时无衬度或消象。因此,可以通过改变衍射矢量的方法拍摄一系列衍射面的貌相图,     

单晶γ-TiAl合金微观滑移机制的研究

针对单晶γ-TiAl合金微观滑移机制方面研究的不足,建立了单晶γ-TiAl合金单轴拉伸时晶体滑移几何模型,根据几何模型中各夹角之间的几何关系和由Weiss晶带法则给出的滑移方向,计算出了单滑移系中各个滑移方向上的Schmid因子;通过对比计算结果发现在设定条件下晶体更易产生滑移的方向为(001)[01-1]和(11-1)[110];在由主滑移系和交滑移系组成的双滑移系同时开动时,计算出了临界外加拉伸应变为0.633;通过数值模拟验证了所给出的单晶γ-TiAl合金单轴拉伸时的微观滑移机制的正确性。     

晶体取向对铜单晶体循环硬化行为的影响

研究了不同取向铜单晶体的循环硬化行为.结果表明,晶体取向对双滑移和多滑移取向铜单晶体的循环硬化行为有强烈的影响.不同取向铜单晶体的初始循环硬化率θ0.2与晶体中的位错反应模式、强度以及交滑移机会密切相关.临界双滑移晶体在循环硬化初始阶段呈现出独特的应变突发行为.     

全片层TiAl合金临界屈服应力及影响因素的细观研究

基于PST晶体的微结构和γ相及α2相普通位错和孪晶滑移启动，结合细观力学方法，通过数值模拟两相中的各滑移系上的分切应力，得出PST晶体屈服应力和外加载荷与片层之间夹角θ的关系；详细讨论了γ相中孪晶与普通位错的临界切应力之差异对PST晶体屈服应力的影响。对单轴和双轴加载的情况分别进行了计算，得到的结论与已有的一些实验结果相吻合。     

用脉冲电流处理后疲劳铜单晶的位错行为

将[233]共面双滑移取向的铜单晶体在两种恒塑性应变幅下进行循环疲劳，形成密度不同的位错结构．用高密度脉冲电流对疲劳铜单晶体处理后，试祥中位错的结构由单纯的脉络结构转化成位错胞状结构．高密度脉冲电流处理引起的热压应力不但加强了主滑移系位错的运动，还使共面次滑移系开动，在主滑移系位错和共面次滑移系位错的共同作用下导致位错胞状结构的形成。     

合成金刚石晶体缺陷的同步辐射形貌研究

采用同步辐射白光貌相术研究合成金刚石单晶体中的晶体缺陷,观察到晶体中存在籽晶,籽晶周围存在着大量的位错线.位错线起源于籽晶表面,终止于晶体表面.计算了位错束的空间走向和位错密度.分析了晶体的生长阶段和影响晶体缺陷的主要因素,指出通过减少籽晶表面的缺陷,保持生长条件的稳定,能够有效地降低合成金刚石晶体中缺陷的密度,提高合成金刚石晶体的完整性.     

镁合金中〈c+a〉锥面滑移系开动的条件

〈c+a〉锥面滑移系的开动对充分发挥和改善镁合金的塑性变形能力具有十分重要的意义。评述了〈c+a〉位错滑移开动条件,如升高变形温度、加入合金元素Li、细化晶粒等,以及对镁合金塑性变形能力的影响,深入探讨了静水压力对〈c+a〉锥面滑移系开动的作用和进一步的研究方向。     

Microstructure and dislocation density evolutions in MgAlZn alloy processed by severe plastic deformation

Commercial MgAlZn alloy AZ31 was processed by hot extrusion and equal channel angular pressing (ECAP) known as EX-ECAP. Microstructure and defect structure evolution with strain due to ECAP were investigated by TEM, positron annihilation spectroscopy (PAS), and X-ray diffraction. Significant grain refinement was obtained by EX-ECAP. In the extruded condition relatively low density of dislocations was determined by PAS. Sharp increase of dislocation density occurred during the first two passes of ECAP, followed by the saturation and even a decline manifesting the dynamic recovery at higher strains. XRD line profile analysis confirmed the results of PAS with slightly higher values of dislocation densities in individual conditions. Detailed analysis of contrast factors allows to determine the type of dislocations and to draw conclusions about slip activation and its variations with strain. The influence of microstructure evolution on mechanical properties is discussed.     

Reactions of slip dislocations with twin boundary in Fe-Si bicrystals

Specimens for in situ TEM straining were prepared from Fe-5.5 at.%Si Σ 3 bicrystals with {112} grain boundary plane. They were strained under three different directions of the stress at the boundary with respect to the orientation of the grains. Transfer of slip across the boundary was analysed. In one case, the transfer of slip was realized by a transformation of the slip dislocation in one grain into the slip dislocation in the other grain. Low energy dislocation was created in the GB in accordance with general transfer criteria. In the second case, the incoming and outgoing slip systems were in direct contraction to the general transfer criteria. In the third case, oriented for common slip system in both grains, the trapped incoming slip dislocations dissociated into twinning dislocations which created twins on the other side of the boundary.     

The Dislocation Structure of a Single-Crystal γ + γ′ Two-Phase Alloy After Tensile Deformation

The dislocation structures of an industrial single-crystal γ + γ′ two-phase alloy DD3 after tensile deformation from room temperature to 1273K were studied by transmission electron microscopy. The strength of this alloy decreased with an increase in the temperature, and showed a strength peak at 1033K. At room temperature, the dislocations shearing the γ′ particles were found to be 1/3<112> partial dislocations on the dodecahedral slip system <112>{111}. Some dislocation pairs on the cubic <110>{100} system that blocked the glide of dislocations were found at a medium temperature of 873K. As a result, dislocation bands were formed. Shearing of γ′ particles by 1/3<112> partial dislocations on the dodecahedral slip system <112>{111} was also found at this temperature. At the peak temperature of 1033K, because of the strong interaction between dislocations on the {111} and {100} planes, the extent of dislocation bands with high dislocation densities was extensive. The 1/3<112> partial dislocations on the dodecahedral slip system <112>{111} also existed. When the temperature reached the high temperature of 1133K, the range of dislocation bands was limited. The γ′ particles were sheared by <110> dislocation pairs on the octagonal <110>{111} system and the cubic <110>{100} system. At 1273K, the regular hexagonal dislocation networks were formed in the γ matrix and at the γ/γ′ interface. The Burgers vectors of the network were found to be b₁ = 1/2[110], b₂ = 1/2[1–10], b₃ = [100], and the last one was formed by the reaction of b₁ + b₂ → b₃. Dislocations shearing the γ′ particles were found to be <110> dislocation pairs on the octagonal system <110>{111} and cubic slip system <110>{100} at 1273K.     

Plastic Deformation in Quench-and 650℃ Tempered Steel

The variations of the dislocation structuresin the quench and 650℃ tempered steel withincrease of elongations have been investigatedby using transmission electron microscopy. Inthe small elongation stage, the boundaries betweenferrite and carbide in this steel can releasedislocations. As the elongations increase, themoving dislocations in the ferrite slip ontothe carbides. Then, the interaction betweenmoving dislocations and dislocations releasedfrom this boundaries, and the interaction betweenthe dislocations moving to the carbides in everyslip plane occurs. Thereby, the dislocationtangles around the carbides can be formed.In the large elongation stage, the dislocationtangles with high dislocation density and thedeveloped dislocation cells are formed.     

Statistical analysis and recovery behaviors of dislocations in cold-rolled and annealed reactor pressure vessel steels

The statistical analysis and recovery behavior of dislocations in cold-rolled and annealed reactor pressure vessel steels with a thickness reduction of 50 % were investigated by using x-ray diffraction and electron backscattered diffraction analysis in this study. A new modified Kubin & Mortensen's method was proposed to evaluate the geometrically necessary dislocation density, and this method could effectively improve the accuracy of geometrically necessary dislocation evaluation by eliminating the effect of step size and measurement noises. Comparison analysis between modified Williamson-Hall and modified Kubin & Mortensen's methods was also performed in this study, which shows different recovery behavior of dislocations. The electron backscattered diffraction results show that geometrically necessary dislocation density remained unchangeable after annealing treatment below and at 550 °C, and it occurred to decrease above 550 °C due to the occurrence of recrystallization. However, dislocations started to migrate and annihilate at 340 °C according to x-ray diffraction results. A possible explanation is a difference in the effect of pairwise dislocation annihilation on statistically stored dislocations and geometrically necessary dislocations.     

Room temperature tensile behaviour of K640S Co-based superalloy

The room-temperature tensile properties of K640S cobalt-based superalloy were investigated by tensile tests and microstructure observation. The experimental results showed that the deformation mechanism of K640S alloy is dislocation slip; that the dislocation could be decomposed into continuous stacking faults with different orientations; and that with the increase of the number of dislocations, the dislocation tangle interacts with the decomposed stacking fault to increase the tensile strength of the alloy. As the stretching proceeded further, a plurality of slip systems were activated between different grains to coordinate the deformation, and the grains were gradually plastically deformed. The stress concentration occurred at the carbide interfaces, and microcracks were formed, causing mixed crystal fracture to the alloy.     

Geometrical effect on dislocation nucleation at crystal surface nanostructures

Crystal surface nanostructures such as steps and trenches in microelectronic layer structures have been considered as stress concentrators that may facilitate dislocation nucleation. Quantitative characterization of the critical condition of this atomic scale process is of considerable interest for the development of high performance electronic devices. This paper addresses this issue using a multiscale approach based on the variational boundary integral formulation of the Peierls–Nabarro dislocation model. By representing the profiles of embryonic dislocations as the relative displacements between the two adjacent atomic layers along the slip planes, the critical conditions for dislocation nucleation are obtained by solving the stress dependent activation energies required to activate embryonic dislocations from their stable to unstable saddle point configurations. The geometrical effect of surface nanostructures such as steps and trenches on dislocation nucleation is ascertained quantitatively. Our results show that the atomic scale surface nanostructures can reduce the critical stress for dislocation nucleation by nearly an order of magnitude and the trench configurations are more prone to dislocation nucleation than the step configurations. Nucleation of versatile dislocations in multiple slip systems at crystal surfaces may be attributed surface nanostructures of a variety of geometries.     

An edge dislocation of constant velocity near a static internal crack

An edge dislocation of constant velocity near a static internal crack was investigated. The dislocation slip and climb and dislocation source were considered. The crack surface was simulated with static continuous dislocations. After obtaining the distribution of static dislocations in the crack, we calculated the stress field in the entire space. Using the stress distribution, we then computed the stress intensity factors at both crack tips and the image force on the edge dislocation. Numerical results are provided to describe in detail the effect of velocity and crack length on toughness and image force.     

The dislocation structure of slip bands in deformed high entropy alloy nanopillars

Remarkable diversity is observed in dislocation interactions that are responsible for intermittent and sud-den crystal slips.While large crystal slips can be easily observed on the surface of deformed crystals,unraveling the underlying dislocation interaction mechanisms,however,has been a longstanding chal-lenge in the study of single-crystal plasticity.A recent study demonstrated that the sluggish dislocation dynamics in the high entropy alloy (HEA) of Al0.1CoCrFeNi enables the observation of slip bands for a direct link to dislocation avalanches in a nanopillar.Here,we further examined the dislocation structure of slip bands in the HEA nanopillars oriented for single slip.Experimental evidence was provided on the dislocation organization in a slip band based on groups of primary dislocations,secondary dislocations,and dislocation pileups.The results were compared with the previously proposed slip band models.The unique aspects of the HEA that enable such observations were also investigated through an examination of the dislocation microstructure and its response to applied forces in the HEA nanopillars.     

Study of residual elastic- and plastic-deformation in uniaxial tensile strained nickel-based Alloy 600 samples by polychromatic X-ray microdiffraction (PXM) and neutron diffraction methods

In order to assess the reliability of the relatively new polychromatic X-ray microdiffraction (PXM) method for measuring the magnitude and distribution of mechanical strains, PXM and the traditional technique—neutron diffraction measurements were made on the gauge section of an uniaxially 1% strained Alloy 600 tensile specimen and an unstressed sample of the same alloy. The average strain magnitudes for the grains analyzed by PXM were found to be similar with those measured from neutron diffraction within the large experimental uncertainty. Of particular interest was the behavior of dislocations in opposing grains across grain boundaries of differing orientations, which was studied by comparing the elongation and splitting of PXM spots. Similar dislocation densities, operating on the similar slip systems, were found on both sides of 60° boundaries, while considerable differences in the degree of elongation and splitting of diffraction spots occurred between grains with other misorientation angles.