基体对应力诱导的纳米晶W膜开裂行为的影响

采用磁控溅射方法同时在Si(100)和聚酰亚胺(PI)基体上沉积W膜,对比研究不同基体约束对纳米晶W膜微观结构及应力诱导的开裂行为的影响.结果发现,在两种基体上W膜的裂纹形态明显不同.在Si基体上W膜的裂纹呈楔形,而在PI基体上W膜的裂纹呈半圆柱形凸起于薄膜表面.这种裂纹形态的差异源于两种基体上W膜的变形机理不同.在刚性Si基体上,W膜的裂纹扩展是通过晶粒平面内的转动实现的,而在柔性PI基体上W膜裂纹扩展是通过排列晶粒在平面内、外的转动协调完成的.分析表明,两种截然不同的开裂行为与不同基体上薄膜内应力的变 关键词： W膜 残余应力 裂纹 晶粒     

Theory of bending of polycrystalline beams by creep at low stress

Attempts to extend diffusion creep theory from simple grain geometries to more complex polycrystalline structures generally make the assumptions that the vacancy creation (or annihilation) rate is constant over each grain face and that the volume of each grain is conserved. These assumptions do not permit grain rotation, a common feature of polycrystalline creep, nor is diffusion allowed to occur between individual grains. These two aspects are investigated theoretically in this paper, for the specific case of the grain boundary diffusion controlled bending of a polycrystalline beam consisting of a set of orthorhombic grains of dimensions X, Y and Z, with the Z dimension, parallel to the axis of bending, assumed large such that two-dimensional diffusion predominates. The grains are aligned with continuous boundaries across the beam height. For grains highly elongated along the beam length (X???Y), the derived rotation rate is identical to that for a bicrystal having the same height as the beam. For smaller X, diffusion in boundaries along the beam length make increasing contributions and the rotation rate increases. The novel prediction is made that the non-conservation of grain volume is an inevitable consequence of the grain boundary diffusion controlled deformation of this particular polycrystalline configuration.     

Electrodynamics and dispersion properties of a magnetoplasma containing elongated and rotating dust grains

The electrodynamics and dispersion properties of a magnetized dusty plasma containing elongated and rotating charged dust grains are examined. Starting from an appropriate Lagrangian for dust grains, a kinetic equation for the dust grain and the corresponding equations of motion are derived. Expressions for the dust charge and dust current densities are obtained with the finite size (the dipole moment) of elongated and rotating dust grains taken into account. These charge and current densities are combined with the Maxwell-Vlasov system of equations to derive dispersion relations for the electromagnetic and electrostatic waves in a dusty magnetoplasma. The dispersion relations are analyzed to demonstrate that the dust grain rotation introduces new classes of instabilities involving various low-frequency waves in a dusty magnetoplasma. Examples of various unstable low-frequency waves include the electron whistler, the dust whistler, dust cyclotron waves, AlfvÉn waves, electromagnetic ion-cyclotron waves, as well as lower-hybrid, electrostatic ion cyclotron, modified dust ion-acoustic waves, etc. Also found is a new type of unstable waves whose frequency is close to the dust grain rotation frequency. The present results should be useful in understanding the properties of low-frequency waves in cosmic and laboratory plasmas that are embedded in an external magnetic field and contain elongated and rotating charged dust grains.     

Microstructural evolution and grain subdivision mechanisms during severe plastic deformation of aluminum particles by ball milling

Electron backscattered diffraction technique was used to investigate the microstructure of aluminum particles deformed by high-energy ball milling. The lengths of different types of boundaries per area were calculated for different samples. The results show that the deformation mechanism and the rate of grain subdivision changed considerably as milling time increased. At the beginning of the milling, deformation banding subdivided grains and dynamic recovery formed a cellular structure of low angle boundaries. After further milling, particles were flattened; an increase in the aspect ratio of the original grains together with cold welding of the particles contributed to the formation of high angle grain boundaries (HAGBs). Lattice rotation progressively increased the misorientation of low and medium angle boundaries and transformed them to HAGBs, which resulted in formation of new small equiaxed grains by continuous dynamic recrystallization. This research shows subgrain rotation was the main mechanism for formation of new HAGBs.     

Analysis of radial segregation of granular mixtures in a rotating drum

This paper considers the segregation of a granular mixture in a rotating drum. Extending a recent kinematic model for grain transport on sandpile surfaces to the case of rotating drums, an analysis is presented for radial segregation in the rolling regime, where a thin layer is avalanching down while the rest of the material follows rigid body rotation. We argue that segregation is driven not just by differences in the angle of repose of the species, as has been assumed in earlier investigations, but also by differences in the size and surface properties of the grains. The cases of grains differing only in size (slightly or widely) and only in surface properties are considered, and the predictions are in qualitative agreement with observations. The model yields results inconsistent with the assumptions for more general cases, and we speculate on how this may be corrected. Received 4 June 1999 and Received in final form 28 September 1999     

Strain localisation in granular media

This paper discusses strain localisation in granular media by presenting experimental, full-field analysis of mechanical tests on sand, both at a continuum level, as well as at the grain scale. At the continuum level, the development of structures of localised strain can be studied. Even at this scale, the characteristic size of the phenomena observed is in the order of a few grains. In the second part of this paper, therefore, the development of shear bands within specimen of different sands is studied at the level of the individual grains, measuring grains kinematics with x-ray tomography. The link between grain angularity and grain rotation within shear bands is shown, allowing a grain-scale explanation of the difference in macroscopic residual stresses for materials with different grain shapes. Finally, rarely described precursors of localisation, emerging well before the stress peak are observed and commented.     

Anisotropic rheology during grain boundary diffusion creep and its relation to grain rotation,grain boundary sliding and superplasticity

The response of periodic microstructures to deformation can be analysed rigorously and this provides guidance in understanding more complex microstructures. When deforming by diffusion creep accompanied by sliding, irregular hexagons are shown to be anisotropic in their rheology. Analytic solutions are derived in which grain rotation is a key aspect of the deformation. If grain boundaries cannot support shear stress, the polycrystal viscosity is extremely anisotropic. There are two orthogonal directions of zero strength: sliding and rotation cooperate to allow strain parallel to these directions to be accomplished without any dissolution or plating. When a linear velocity/shear stress relationship is introduced for grain boundaries, the anisotropy is less extreme, but two weak directions still exist along which polycrystal strength is controlled only by the grain boundary “viscosity”. Irregular hexagons are characterised by four parameters. A particular subset of hexagons defined by two parameters, which includes regular hexagons as well as some elongate shapes, shows singular behaviour. Grain shapes that are close to that of the subset may exhibit large grain rotation rates and have no well-defined rheology unless there is a finite grain boundary viscosity. This new analysis explains why microstructures based on irregular but near equiaxed grains show high rotation rates during diffusion creep and it provides a framework for understanding strength anisotropy during diffusion creep.     

Lattice-diffusion-controlled rotation of crystals about a common interface

During the high-temperature deformation of polycrystalline materials, the interaction between neighbouring grains gives rise to grain shape changes, grain-boundary sliding and grain rotation. There is debate on whether sliding makes a direct contribution to strain or whether it merely accommodates the shape changes. In principle, it is possible to deduce any direct sliding contribution, by comparing the overall strain with grain strain. Such attempts are often confounded, however, by the existence of grain rotation. In a previous paper, by Burton, rotation occurring by interfacial diffusion was anaIysed. It may also occur by lattice diffusion and this is the subject of the present paper, where a numerical method is used to treat the rotation of a bicrystal configuration. The method is validated by adapting it to solve a related problem, that of lattice diffusion creep, and predictions are shown to agree with known analytical solutions. The rate of rotation is calculated as a function of bending moment, grain dimensions and grain aspect ratio. The steady-state vacancy concentration and diflusion fluxes within the bicrystal are determined. The fluxes at the free surfaces are shown to lead to apparent boundary ‘grooving’ and ‘mounding’ effects at the tensile and compressive ends of the interface. The method can be further adapted to solve the diffusion creep problem for a ‘bamboo’ structure and this has given important new results. It allows diffusion fluxes at the free surfaces to be calculated for the first time and the variation in the creep constant to be determined as a function of the grain aspect ratio. Reported measurements of enhanced grain-boundary grooving may be explained by these results.     

Effect of microstructure on ~3He migration in TiT_(1.9) films

Two kinds of films were prepared to study the effect of microstructure on helium migration in Ti tritides. Both films showed different release behaviors and helium bubble distributions. In the film consisting of columnar grains, a twolayered structure was observed. Inclusions with a strip feature were found at the grain boundary, and no helium bubbles were distributed in these inclusions. However, helium preferred to migrate to the boundaries of these inclusions. Bubble linkage as a ribbon-like feature developed parallel to the film surface in the film consisting of columnar grains. More cracks were developed at the grain boundaries of the film consisting of columnar grains, although the helium content in the film consisting of columnar grains was less than that in the film consisting of equiaxed grains. A surface region with a small number of bubbles, or "depleted zone", was observed near the surface. The cracks extending to the film surface were the pathways of the critical helium released from the film. The helium migration was strongly influenced by the grain microstructure.     

SnO2 nanograins Au-doped: A quantum mechanical evaluation of CO adsorption

In this study an analysis is made of the adsorption properties of nanocrystalline SnO₂ containing a metallic dopant. The analysis is based on semi-empirical Hartree–Fock and scattering theories and the structures considered are SnO₂ grains, with a rutile lattice, whose size and shape are comparable with the nanograins and nanowires produced in experiments. The grains contain rows of gold atoms located externally, on the grain surface, or in an endohedral position, in the grain interior, and the adsorbed system is generated by depositing CO molecules on the grain surface. The calculations illustrate the dependence of the binding energies and of the conductance on the grain size and on the location of the metallic additives in both the clean and in the CO-adsorbed grains. These results show that adsorption and current transport are determined by the intrinsic electronic structure of the adsorbing grains.     

Elevated temperature directional recrystallization of high-purity nickel

Electron backscattered diffraction has been used to characterise the three different kinds of boundaries that occur in grains that are generated by secondary recrystallization during directional annealing of high-purity nickel. Boundaries between columnar grains (CC boundaries) can be twin boundaries, low-angle boundaries or high-angle grain boundaries. The frequency of low-angle CC boundaries dropped from 25% to 0% while the frequency of the high-angle CC boundaries increased from 19% to 67% when the annealing temperature was increased from 1000°C to 1200°C. The misorientation angles of boundaries between columnar grains and small equiaxed grains ahead of them (CE boundaries) was random at 1200°C but had a 40° rotation relationship about ?111? at 1000°C. It was found out that the character of the CC boundaries is determined by relative mobility of the CE boundaries, which is determined by the processing temperature rather than the energy of the CC boundaries themselves. The character of the island grain boundaries sometimes found with columnar grains was not affected by the annealing temperature or the drawing velocity.     

Strain induced grain boundary premelting in bulk copper bicrystals

In bulk bicrystals strain induced grain boundary premelting (SIGBPM) occurs when heavy screw dislocation pileup can be held up to a certain high temperature, approximately 0.6T _M, where T _M is the melting point of bulk material in Kelvin. SIGBPM occurs at grain boundaries to which new twist component is added due to the rotation of both component crystals toward opposite direction about the axis perpendicular to the grain boundary plane. At the original grain boundary, grain boundary sliding takes place due to this relative rotation. In f.c.c. metals with relatively low stacking fault energies such as copper, nickel, brass(30Zn) and silver, dislocations dissociate into partials. Therefore high density tangled dislocations introduced during plastic deformation hardly loose. If these dislocations can be held to high temperatures, SIGBPM is promoted. Formation of static or dynamic recrystallized grains suppresses SIGBPM itself and the propagation of grain boundary cracks formed by SIGBPM.     

Surface behavior during abrasive grain action in the glass lapping process

In this study the surface behavior during its contact with the abrasive grain in the glass lapping process was studied using practical simulation which is the scratch test and the real contact between glass surfaces and α-alumina abrasive grains during lapping process. Formations and dimensions of the produced scratches were investigated to explain the grain action on the surface and the glass material removal rate. It has been found that humid environment created by the use of the slurry of loose abrasives causes more significant damages than the dry one. The use of slurry produces higher glass material removal rate in this environment and proves its utility in the lapping process. The shape of abrasive grains influences the nature of their action. Indeed, the worn grains produce scratches and chippings less than the sharp grains. During lapping, the number of scratches and theirs dimensions depend on the contact time and the abrasive grain size. It was concluded that the glass material removal rate during lapping depends on the cumulative actions of individual grains which produce scratches and chippings.     

The dust emission law in the wind erosion process on soil surface

The dust emission models to date cannot describe the relation between the transport rate of different sized grains and their grain size composition in soil surface, so Aeolian grain transport on a soil-like bed composed of fine sand and silt powder was measured in a wind tunnel. Six types of soil-like beds with different silt fractions have been tested in this experiment. The mass flux profiles of silt dust and sand grains are much different due to their different motion modes. Analysis of the vertical distribution of the powder and sand grains reveals that for a given soil bed, the ratio of the horizontal dust flux to the horizontal sand flux is directly proportional to their mass ratio in the bed. The dust flux is closely linked to the sand flux by the bombardment mechanism. For a given wind velocity and grain size of the bed, the slopes of the vertical mass flux profiles of sand grains larger than 100 μm are nearly equal in a log-linear plot and the ratio between the fraction of transport rate of each size group to the whole transport rate and the mass fraction of each size group in the bed is a constant only dependent on grain size. With this law, the transport rate of dust and different sized grains can be related with the grain size composition in the soil surface. Supported by the National Natural Science Foundation of China (Grant No. 50706031) and the Natural Science Foundation of Shanxi Province of China (Grant No. 2008021005)     

Non-conservation of grain volume during diffusion creep and its effect on denuded zones

It is reasoned in this paper that the traditional assumption of grain volume conservation during diffusion creep is correct only for special grain configurations. An analysis is presented that illustrates this, using a hypothetical arrangement of grains specifically chosen to be stable against both grain boundary sliding and grain rotation, so that the extent of grain volume non-conservation can be illustrated in the absence of these factors. The influence on the development of the ‘denuded’ zones that characterize diffusion creep in particle-containing materials is addressed. The analysis contributes to an explanation for the discrepancies between the creep strain estimated from zone sizes and the overall specimen strain, a discrepancy that has been used in the past as counter evidence for the diffusion creep mechanism. Suggestions are made for the improved modelling of diffusion creep in polycrystalline materials and duplex structures.     

Ion-beam-induced collective rotation of nanocrystals

Vapor-deposited nanocrystalline titanium layers have been irradiated at room temperature with 350-MeV-Au ions up to 4x10;{15} Au/cm;{2}. Bombardment-induced texture changes were determined at the BESSY synchrotron light source. During off-normal irradiation, the nanocrystals undergo grain alignment and rotation up to approximately 90 degrees at the highest ion fluence. At the same time, the whole layer exhibits shear flow very similar to that observed previously in amorphous materials. Below 1x10;{15} Au/cm;{2}, a reversal of the ion incidence angle leads to a back rotation of the grains. These effects are absent or immeasurably small in coarse-grained titanium but have also been found in nanocrystalline TiN and NiO. The observations can be modeled by assuming that grain boundaries behave during ion bombardment like amorphous matter or by assuming a generation of disclination dipoles moving along grain boundaries.     

The rapid prototyping of textured amorphous surfaces for the graphoepitaxial deposition of CdTe films using focused ion beam lithography

Cadmium telluride films deposited on amorphous substrates exhibit a grain structure characterized by [111]-oriented grains, but where the in-plane grain orientation is randomized due to the absence of epitaxy. Here, we explore the viability of promoting an in-plane grain alignment through graphoepitaxy. Fifteen different substrate surface textures were fabricated using focused ion beam lithography. This approach allows for the side-by-side deposition of surface textures where both the areal extent and depth of the surface features are varied in a systematic manner. CdTe films deposited overtop these textures show grain structures with dramatic variations, revealing that particular length scales have the most pronounced effect on the grain structure.     

Theory of diffusional rotation about the common boundary of a bicrystal

During the creep of polycrystals, individual grains may undergo shape changes, grain boundary sliding and grain rotation. Theoretical studies have focused on the first two of these processes but only recently has the theory of rotation received detailed attention. Diffusional rotation was analysed by Burton [Phil. Mag. A 82 51 (2002); Phil. Mag. 83 2715 (2003)], for a bicrystal with orthorhombic grains of dimensions X, Y and Z with the common boundary in the yz plane and with Z???X,Y. Rate equations were derived and the stress profile over the common boundary predicted, for cases where grain boundary and lattice diffusion predominate. In this paper, the analyses are extended using numerical methods, to the full two- and three-dimensional cases for boundary and lattice diffusion, respectively. For boundary diffusion, the results for Z/Y???1 reproduce those obtained by analytical means and this is regarded as a verification of the numerical method. When Z/Y?=?1, the rotation rates are shown to be about 30% faster, due to the additional diffusion contribution in the z direction. This contribution increases with decreasing values of Z/Y. The stress patterns at the rotating boundary are derived. For lattice diffusion, the stress pattern at the boundary, the shapes of the vacancy potential contours and the variation of the rotation rate with the ratios X/Y and Z/Y are presented.     

Water dynamics in ionic magnetic colloids studied by H nuclear magnetic resonance

In a previous nuclear magnetic resonance (NMR) study we observed that the NMR spectra of water in both surfacted and ionic ferrofluids are asymmetric and several orders of magnitude wider than the one of pure water. It has been proposed that this effect is produced by extremely strong magnetic field gradients in the intergrain volume and/or by surface interactions between the carrier liquid molecules and the grains surface. In the case of aqueous ionic ferrofluids the latter possibility should be interpreted as electric interactions between water (polar) molecules and the charges in the grain surface.

In this work we study a series of ionic and surfacted ferrofluids prepared at different magnetic grain concentrations and sizes, and with different surface charge densities. Our experiments clearly show that the sign and the density of the electric charge on the magnetic grains have no influence on NMR spectra. On the other hand, spectral widths increase with the magnetic grain concentration.     

薄膜中异常晶粒生长理论及能量各向异性分析

针对柱状晶薄膜，建立了异常晶粒生长理论模型.指出薄膜中的晶粒生长，除像传统的整体材料中的晶粒生长一样考虑晶界能外，还应当考虑表面能、界面能和应变能.对能量的各向异性进行了回顾性分析.从表面能的最小化考虑，面心立方和体心立方薄膜的择优取向或织构应分别为（111）和（110）；而从应变能的最小化考虑，面心立方和体心立方薄膜的择优取向或织构应分别为（110）和（100）. 关键词： 薄膜 异常晶粒生长 模型 织构