Influence of reaction stresses induced by dislocation slips on the orientation evolution in bcc metals

A plastic deformation model for bcc metals is proposed in consideration of reaction stresses.The shear strains and the corresponding reaction stresses induced by the activation of dislocations are calculated in the model,which will influence the following dislocation activation.The rolling texture in bcc metals is simulated up tp 80% reduction,whille the ratio of critcal resolved shear stresses between the dislocations slippingon the {110}and{112}planes is chosen as 0.95.The corresponding calculation is also conducted with the activation of second dislocation,if the difference between the orientation factor of the two dislocations with maximal orientation factors is lower than 5%.It is shown that the simulated texture is closer to that of the 80% rolled interstitial free steels than other modeling.It is believed that the new model can give more attention to both of the strain and stress continuities during the plastic deformation of polycrystalline metals,and therefore approaches closer to the real deformation process in bcc metals.     

Effects of Strain Rate,Temperature and Grain Size on the Mechanical Properties and Microstructure Evolutions of Polycrystalline Nickel Nanowires: A Molecular Dynamics Simulation

Through molecular dynamics(MD)simulation,the dependencies of temperature,grain size and strain rate on the mechanical properties were studied.The simulation results demonstrated that the strain rate from 0.05 to 2 ns~(–1 )affected the Young’s modulus of nickel nanowires slightly,whereas the yield stress increased.The Young’s modulus decreased approximately linearly;however,the yield stress firstly increased and subsequently dropped as the temperature increased.The Young’s modulus and yield stress increased as the mean grain size increased from 2.66 to6.72 nm.Moreover,certain efforts have been made in the microstructure evolution with mechanical properties association under uniaxial tension.Certain phenomena such as the formation of twin structures,which were found in nanowires with larger grain size at higher strain rate and lower temperature,as well as the movement of grain boundaries and dislocation,were detected and discussed in detail.The results demonstrated that the plastic deformation was mainly accommodated by the motion of grain boundaries for smaller grain size.However,for larger grain size,the formations of stacking faults and twins were the main mechanisms of plastic deformation in the polycrystalline nickel nanowire.     

Insights into plastic deformation mechanisms of austenitic steels by coupling generalized stacking fault energy and semi-discrete variational Peierls-Nabarro model

The generalized stacking fault energy (GSFE) is a key parameter to determine the plastic deformation mechanisms of austenitic steels. However, the underlying physics why the GSFE can affect the plastic deformation behaviors remains unclear. In this paper, the plastic deformation mechanisms of austenitic steels with different carbon (C) additions were investigated by coupling the GSFE with the semi-discrete variational Peierls-Nabarro (P–N) model. The internal mechanisms behind the P–N stress and plastic deformation were explained at atomic scale. It is found that the positions and contents of C atoms affect the GSFE of austenite, and thus regulate plastic deformation behaviors of austenitic steels by influencing dislocation core structure. As exemplified that with 4 ​at.%C in austenite, the intrinsic stacking fault energy increases from −433 to −264 mJ/m², and the stacking fault width increases to 6.62b from 4.72b of FCC-Fe with b being the Burgers vector. This corresponds to the plastic deformation mechanism dominated by the ε martensitic transformation with the lattice changing from FCC to HCP. With increasing C contents to 8 ​at.%, the intrinsic stacking fault energy of austenite increases to −9.01 mJ/m², while the stacking fault width decreases to 6.03b. The plastic deformation tends to proceed via the mechanical twinning mode. The present investigation establishes a solid foundation for clarifying the plastic deformation mechanisms of austenitic steels from the perspective of the dislocation core structure.     

焦散线法测量断裂力学参数J积分的实验研究

根据焦散线法的基本原理，推出了该法测量线弹性断裂力学和弹塑性断裂力学参数Ｊ积分的计算公式，并列举了有机玻璃材料和幂硬化金属材料的Ｊ积分测量和计算实例。研究结果表明，焦散线法适用于处于平面应力状态下裂纹尖端塑性应力强度因子的测量，其实验设备简单，实验数据处理方便，便于工程应用。     

塑性流体在光滑圆管内的湍流流动及屈服应力对流动的影响

本文应用随机湍流模型对塑性流体在光滑圆管内的湍流流动规律进行了研究,得到了包括近壁区域在内的断面时均速度分布和平均速度、沿程阻力系数的计算公式。结果表明,塑性流体的屈服应力对湍流流动的影响是由比值τ0/τw(τo为屈服应力,τw为管壁处切应力)的大小来决定的,随着雷诺数Re的增大或管径减小,这种影响将很快衰减,而屈服应力对无因次速度剖面的分布影响很小。     

Intermittent dislocation flow in viscoplastic deformation

The viscoplastic deformation (creep) of crystalline materials under constant stress involves the motion of a large number of interacting dislocations. Analytical methods and sophisticated 'dislocation dynamics' simulations have proved very effective in the study of dislocation patterning, and have led to macroscopic constitutive laws of plastic deformation. Yet, a statistical analysis of the dynamics of an assembly of interacting dislocations has not hitherto been performed. Here we report acoustic emission measurements on stressed ice single crystals, the results of which indicate that dislocations move in a scale-free intermittent fashion. This result is confirmed by numerical simulations of a model of interacting dislocations that successfully reproduces the main features of the experiment. We find that dislocations generate a slowly evolving configuration landscape which coexists with rapid collective rearrangements. These rearrangements involve a comparatively small fraction of the dislocations and lead to an intermittent behaviour of the net plastic response. This basic dynamical picture appears to be a generic feature in the deformation of many other materials. Moreover, it should provide a framework for discussing fundamental aspects of plasticity that goes beyond standard mean-field approaches that see plastic deformation as a smooth laminar flow.     

CONFORM连续挤压金属变形行为的模拟实验研究

为了模拟研究CONFORM连续挤压过程中金属的变形行为,本文设计了一种专门的模拟实验装置,并利用莫尔云纹法研究了模拟实验时金属塑性变形的分布情况。研究结果表明,CONFORM连续挤压时的金属塑性流动不同于常规挤压时的情形,在塑性变形区内存在一个剧烈剪切变形带。剧烈剪切变形带的存在有助于减缓挤压塑性变形的不均匀性,尤其是对于金属颗粒挤压时,非常有利于增强颗粒之间的结合能力。     

低碳微合金双相钢拉伸断裂过程的动态观察

本文采用配备有拉伸装置的扫描电镜上连续地跟踪观察铁素体-马氏体双相组织在拉伸过程中微观断裂过程,并分析讨论了微裂纹的萌核与扩展条件及微观组织因素的影响。结果表明:双相钢的微观断裂机制是轴向应力和塑性座变的交互作用的结果。双相热处理工艺通过马氏体含碳量与体积率改变铁素体相的强度和变形行为以影响断裂机制。回火则由于碳化物在两相界面处析出与松驰应力集中而抑制显微裂纹的萌核与扩展。     

中深层位倾滑断裂带的固态流变

本文以江苏连云港地区变质基底的倾滑断裂带为例,通过透射电镜观察研究,对地壳中深层位中的自由位错、位错壁和亚颗粒等流变特点进行鉴定,并对位错密度、流动应力和流变速率等流变参数统计计算。其研究成果可资同浅层位、深层位的流变特征进行对比,并可作为研讨造山作用、变质作用和成矿作用的科学依据。     

柱壳轴向冲击屈曲实验研究及其应力波效应的分析

采用落锤及空气动力枪作为加载装置对铜、铝及钢质圆管进行了轴向冲击实验,得到了丰富的屈曲模态及冲击力时程曲线,并从应力波的角度定性地讨论了材料特性对圆柱壳出现塑性动力屈曲或渐进屈曲的影响,指出了高速撞击不是产生动力塑性屈曲的充要条件,对某些韧性材料撞击速度不很高时也能出现动力塑性屈曲.动力塑性屈曲和动力渐进屈曲形成条件应该是:前者仅仅是在一个持续的轴向塑性流动内能够产生,后者是塑性应变局部化的结果.     

Hydrogen effect on the mechanical behaviour and microstructural features of a Fe-Mn-C twinning induced plasticity steel

The influences of hydrogen on the mechanical properties and the fracture behaviour of Fe-22Mn-0.6C twinning induced plasticity steel have been investigated by slow strain rate tests and fractographic analysis.The steel showed high susceptibility to hydrogen embrittlement,which led to 62.9%and 74.2%reduction in engineering strain with 3.1 and 14.4 ppm diffusive hydrogen,respectively.The fracture surfaces revealed a transition from ductile to brittle dominated fracture modes with the rising hydrogen contents.The underlying deformation and fracture mechanisms were further exploited by examining the hydrogen effects on the dislocation substructure,stacking fault probability,and twinning behaviour in pre-strained slow strain rate test specimens and notched tensile specimens using coupled electron channelling contrast imaging and electron backscatter diffraction techniques.The results reveal that the addition of hydrogen promotes planar dislocation structures,earlier nucleation of stacking faults,and deformation twinning within those grains which have tensile axis orientations close to<111>//rolling direction and<112>//rolling direction.The developed twin lamellae result in strain localization and micro-voids at grain boundaries and eventually lead to grain boundary decohesion.     

Haemodynamic shear stress activates a K+ current in vascular endothelial cells

The endothelial lining of blood vessels is subjected to a wide range of haemodynamically-generated shear-stress forces throughout the vascular system. In vivo and in vitro, endothelial cells change their morphology and biochemistry in response to shear stress in a force- and time-dependent way, or when a critical threshold is exceeded. The initial stimulus-response coupling mechanisms have not been identified, however. Recently, Lansman et al. described stretch-activated ion channels in endothelial cells and suggested that they could be involved in the response to mechanical forces generated by blood flow. The channels were relatively nonselective and were opened by membrane stretching induced by suction. Here we report whole-cell patch-clamp recordings of single arterial endothelial cells exposed to controlled levels of laminar shear stress in capillary flow tubes. A K+ selective, shear-stress-activated ionic current (designated Ik.s) was identified which is unlike previously described stretch-activated currents. Ik.s varies in magnitude and duration as a function of shear stress (half-maximal effect at 0.70 dyn cm-2), desensitizes slowly and recovers rapidly and fully on cessation of flow. Ik.s activity represents the earliest and fastest stimulus-response coupling of haemodynamic forces to endothelial cells yet found. We suggest that localized flow-activated hyperpolarization of endothelium involving Ik.s may participate in the regulation of vascular tone.     

Diffusion of point defects in two-dimensional colloidal crystals

Uniform colloidal microspheres dispersed in a solvent will, under appropriate conditions, self-assemble into ordered crystalline structures. Using these colloidal crystals as a model system, a great variety of problems of interest to materials science, physical chemistry, and condensed-matter physics have been investigated during the past two decades. Recently, it has been demonstrated that point defects can be created in two-dimensional colloidal crystals by manipulating individual particles with optical tweezers. Direct imaging of these defects verified that their stable configurations have lower symmetry than the underlying triangular lattice, as predicted by numerical simulations for a number of two-dimensional systems. It was also observed that point defects can dissociate into pairs of well-separated dislocations, a topological excitation especially important in two dimensions. Here we use a similar experimental system to study the dynamics of mono- and di-vacancies in two-dimensional colloidal crystals. We see evidence that the excitation of point defects into dislocation pairs enhances the diffusion of di-vacancies. Moreover, the hopping of the defects does not follow a pure random walk, but exhibits surprising memory effects. We expect the results presented in this work to be relevant for explaining the dynamics of other two-dimensional systems.     

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.      

An experiment study of quartz-coesite transition at differential stress

In order to study quartz-coesite transition under the conditions of differential stress, experiments of quartzite deformation were carried out using a triaxial testing system with a Griggs type solid medium pressure vessel. Analyses on the plastically-deformed samples under optical microscope and Raman spectra show that fine-grained coesite was present in the region of samples adjacent to the pistons at temperatures of 950--I000~C, confining pressure of 1.3 GPa, differential stress of 1.5--1.67 GPa, and total strain of 75%--81%. It is evident that the transition pressure of quartz-coesite at differential stress and intensely-strained conditions is far lower than the pressure for coesite stability at isostatic pressure. In other words, the stress condition of coesite occurrence is not unique. The decrease in confining pressure for quartz-coesite transition under differential stress conditions is controlled by a combined effect of the maximum principal stress that provides a high stress environment, and differential stress that causes sample deformation. Coesite was produced in the plastically-deformed samples in this study, but it can occur in both semi-brittle and plastic deformation regimes as seen in previous studies. Phase transition in semi-brittle deformation regime is caused by local mechanical instability induced by shear deformation, and phase transition in plastic flow regime is due to strain instability induced by the presence of a high dislocation density within intensely-deformed quartz crystals.     

Subcellular localization of chitosan oligosaccharides in living cells

Chitosan oligosaccharides （COS） is derived from the chemical or enzymatic decomposition of chitosan. The exact mechanisms underlying many biological functions of COS have not been elucidated. Since subcellular localization is very important in determining biological activity in a number of molecules, we used fluorescein isothiocyanate-labeled chitosan oligosaccharides （FITC- COS） and investigated the localization of COS in living cells by laser scanning microscopy. We found that COS entered cells in a dose- and time-dependent manner, and we first showed that COS may enter cells by facilitated passive diffusion and was preferentially localized in the mitochondria. While in high concentration, it was also found in the cytoplasm and nucleus and was enriched in the nucleolus and karyotheca. The different subcellular localization of COS suggested that they played different effects. Determination of COS subcellular localization in cells is important to help us understand the potent mechanisms underlying its multiple functions.     

3D激光沉积增材制造TC4高应变率动态力学行为及本构关系研究

通过对3D激光沉积TC4在较宽温度（298~1 073 K）和应变率范围（0.001~5 000 s^-1）内的单轴压缩试验,系统研究了该材料的塑性流动行为,分析了材料的微观组织特性及其变形断裂微观机制.结果表明材料在压缩载荷下具有明显的应变率硬化和温度软化效应.在压缩加载条件下,材料的破坏模式为绝热剪切带的萌生和拓展,而初始缺陷成为诱导剪切带形成的主要原因.3D激光沉积TC4材料屈服强度与铸造TC4接近,略低于传统锻造TC4.文中基于位错动力学热激活理论建立了可以较好描述材料在不同温度不同应变率下的塑性流动行为物理概念的本构模型.     

Influence of beam current on microstructure of electron beam melted Ti-6Al-4V alloy

The defect microstructure of the samples manufactured from Ti-6Al-4V powder was studied using electron beam melting (EBM) in the beam current range of 17 - 13 mA. The hybrid digital complex combined positron lifetime spectroscopy and coincidence Doppler broadening spectroscopy was used to characterize the defect structure of the materials. The microstructure and defects were also analyzed by transmission electron microscopy. It has been established that the main type of the defects in the EBM manufactured samples is dislocations. According to the conducted measurements and calculations, the dislocation density in the EBM manufactured samples exceeds by two orders the similar value for the cast Ti-6Al-4Valloy. Formation of Ti-Ti-Al nanoscale clusters has been found in the EBM manufactured samples.     

Inferring nonlinear mantle rheology from the shape of the Hawaiian swell

The convective circulation generated within the Earth's mantle by buoyancy forces of thermal and compositional origin is intimately controlled by the rheology of the rocks that compose it. These can deform either by the diffusion of point defects (diffusion creep, with a linear relationship between strain rate and stress) or by the movement of intracrystalline dislocations (nonlinear dislocation creep). However, there is still no reliable map showing where in the mantle each of these mechanisms is dominant, and so it is important to identify regions where the operative mechanism can be inferred directly from surface geophysical observations. Here we identify a new observable quantity--the rate of downstream decay of the anomalous seafloor topography (swell) produced by a mantle plume--which depends only on the value of the exponent in the strain rate versus stress relationship that defines the difference between diffusion and dislocation creep. Comparison of the Hawaiian swell topography with the predictions of a simple fluid mechanical model shows that the swell shape is poorly explained by diffusion creep, and requires a dislocation creep rheology. The rheology predicted by the model is reasonably consistent with laboratory deformation data for both olivine and clinopyroxene, suggesting that the source of Hawaiian lavas could contain either or both of these components.     

异步轧制铜/铝双金属复合板变形行为的研究

采用异步轧制复合工艺制备了铜/铝双金属复合板,分析了轧制工艺参数对复合板变形行为的影响,结合轧制变形区金属受力状态探讨了复合过程中的金属变形及流动规律.结果表明:异步轧制变形区内界面摩擦剪切作用直接影响母材的受力状态,共同变形区内双金属间的搓轧作用对金属流动及结合效果影响最大.异步速比越大,硬质金属变形越大.总压下率增大时,组元金属压下率均呈正比关系增加,且软、硬两种金属的压下率差值越来越小.