冶金硅定向凝固法制备太阳能级多晶硅及其微观组织与位错(英文)

以冶金硅为原料,探索采用具有高温度梯度的真空定向凝固技术制备低成本太阳能级多晶硅,并研究其在不同生长条件下的微观组织特征、晶界与晶粒大小、固液界面形貌以及位错结构。结果表明,当凝固速率低于60μm/s时,能获得具有高密度和良好取向的定向凝固多晶硅棒状试样,硅晶粒大小随凝固速率的增大而减小;在控制凝固过程,获得平的固液界面形貌是获得沿凝固方向排列柱状晶的关键;由于硅的小平面生长特性,微观组织中出现了位错生长台阶和孪晶结构;在晶粒中,位错分布呈现不均匀性,并且位错密度随凝固速率的增加而增加;在此基础上,讨论了多晶硅的生长行为以及位错形成机制。     

电磁搅拌对过共晶Al—Si合金初生Si分布的影响

系统研究了电磁搅拌下过共晶Al-Si合金初生Si的偏析规律,实验表明,虽然电磁搅拌引起过共晶Al-Si合金中初生Si的显著细化和圆整化,但常会使坯料表面出现初生Si偏析层,合金中Si含量越大,初生Si偏析层越厚,提高电磁摘抄功率、降低合金熔体冷却速率都会减小或消除初生Si偏析层;在电磁摘抄条件下,坯料表面产生初生Si偏析层的主要原因是固－液界面处的温度梯度过大和存在一层流附面层。     

Dislocation density simulations for bulk single crystal growth process

Computer simulations of the dislocation density during Czochralski (CZ) single crystal growth were performed for silicon crystals with 8-inch or 10-inch diameter using a finite element computer code developed by the authors. In the computer code, a dislocation kinetics model called the Haasen-Sumino model was used for the constitutive equation of a crystal at elevated temperatures. The computer code provides the dislocation density distributions and stress distributions during the CZ growth process. In the simulations, two values for the Young’s modulus for the silicon single crystal were used in order to examine the effect of the Young’s modulus on the dislocation density.     

基于分子动力学单晶硅的纳米压痕过程研究

基于分子动力学的基本理论,在微纳米尺度下建立了单晶硅的纳米压痕分子动力学模型。研究了在纳米压痕过程中单晶硅基体的变形机理、势能变化和温度变化。研究发现:在纳米压痕过程中基体上出现了位错、空位及滑移带,基体两侧有凸起现象。当压头撤离时,基体与压头间存在颈缩现象。在系统达到平衡时系统的势能出现不同,这是因为原子位错运动使得系统增加的势能小于压头原子所做的功。温度的变化与位错变形的程度相关,位错变形越剧烈系统温度升高的越快。     

Formation of a dislocation structure of aluminum single crystals under the action of pressure and gravitational field on the melt

The distribution of dislocation density has been investigated experimentally in aluminum single crystals with various concentrations of copper impurity (～0.0002, 0.005, and 0.05 wt %) that were obtained upon the crystallization of the melt under the action of a pressure and the gravitational-field component directed along the surface of the crystallization front. It has been found that a strong nonuniformity in the dislocation distribution arises in cross sections of the crystals in the direction of the field and this nonuniformity increases with increasing gravitational-field component and decreasing in the impurity concentration and rate of crystallization of the melt. It has been found that the degree of nonuniformity of the dislocation distribution in the crystals substantially depends on the magnitude of pressure at which the crystallization of the melt is realized and reaches a maximum at a certain pressure that is “optimum” for given conditions of the process of phase transformation.     

Strengthening mechanisms in nanostructured high-purity aluminium deformed to high strain and annealed

Samples of pure aluminium (99.99%) have been produced by accumulative roll-bonding to a large strain followed by a heat treatment, where a two-step annealing process has been used to produce samples with large variations in structural parameters such as boundary spacing, misorientation angle and dislocation density. These parameters have been quantified by a structural analysis applying transmission electron microscopy and electron backscatter diffraction, and the mechanical properties have been determined by tensile testing at room temperature. Strength–structure relationships have been analysed based on the operation of two strengthening mechanisms—grain boundary and dislocation strengthening—and good agreement with experiments has been found for the deformed sample. However, for samples where the density of dislocation sources has been reduced significantly by annealing, an additional strengthening mechanism, so-called dislocation source-limited hardening, may operate as a higher stress is required to activate alternative dislocation sources.     

Size effects and strength fluctuation in nanoscale plasticity

Stochastic, discontinuous flow is ubiquitous in the plastic deformation of small-volume metallic materials. We have identified a size-strengthening effect on the stress to initiate the jerky plastic yielding in nanoscale volumes of copper single crystals, subjected to nanoindentation in different orientations. Such a nanoscale size effect arises due to the stochastic nature of dislocation sources, in contrast to the microscale size effect often attributed to plastic strain gradients. The jerky response can result from the activation of either surface or bulk heterogeneous dislocation sources, as governed by the distribution and resistance of dislocation locks. Implications concerning the deformation mechanism in materials with flow defect-limited characteristics are discussed.     

Deformation behavior of Ni3Al single crystals during nanoindentation

The deformation behavior of (1 1 1), (1 1 0) and (0 0 1) oriented Ni₃Al single crystals was investigated by means of nanoindentation. Upon loading with the blunt and sharp indenter tips, the crystal deforms elastically in the initial stage followed by plastic deformation of which the onset is characterized by an obvious displacement burst (or pop-in) in the loading curve. This pop-in corresponds to homogeneous nucleation of dislocation loops under the indenter. When a sharp indenter tip was used, a major pop-in can be identified at a large load (7 mN for (1 1 1) crystal) in the plastic regime on loading. The major pop-in may be correlated with the special dislocation structure of the K–W locks in the Ni₃Al crystals that are formed during loading. The pop-in behavior during plastic deformation of Ni₃Al crystals is found to be closely related to the crystal orientation, pre-existing dislocation density in the sample surface, loading rate, and holding (at a constant sub-critical load) time as well. Pop-ins were also observed at critical loads in unloading processes.     

Investigation of Strain Rate Effect by Three-Dimensional Discrete Dislocation Dynamics for fcc Single Crystal During Compression ProcessEI北大核心CSCD

Microelectromechanical systems (MEMS) have become increasingly prevalent in engineering applications. In these MEMS, a lot of micro-components, such as thin films, nanowires, micro-beams and micropillars, are utilized. The characteristic geometrical size of those components is at the same scale as that of grain, the mechanical behavior of crystal materials exhibits significant size effect and discontinuous deformation. In addition, those MEMS are often subjected to high strain rate at work, such collision and impact loading. The coupling deformation characteristics of small scale crystals and high strain rate makes their mechanical behavior more complicated. Accordingly, investigation of the effect of the strain rate on crystal materials at micron scale is significant for both the academia and industry. In this work, a plastic deformation model of fcc crystal under axial compression was developed based on three-dimensional discrete dislocation dynamics (3D-DDD), which considered the influence of externally applied stress, interaction force between dislocation segments, dislocation line tension and image force from free surface on dislocation movement during the process of plastic deformation. It was applied to simulate the plastic deformation process of a Ni single crystal micropillar during compression under different loading strain rates. 3D-DDD and theoretical analysis are carried out to extensively investigate the effect of strain rate on flow stress and deformation mechanisms during plastic deformation process of crystal materials. The results show that the flow stress and the dislocation density increased with the loading strain rate. In the case of low strain rate, the flow stress was dominated by the activation stress of FreakRead (FR) source in plastic deformation. With the increase of strain rate, the contribution of activation stress of FR source to the flow stress decreases and the effective stress gradually dominated the flow stress. Under high strain rate loading, with the increase of the initial FR source, the dislocation density also increased at the same strain correspondingly, which makes it easier to meet the requirement of the loading strain rate, so the flow stress is smaller. In addition, under the low strain rate loading, a few activated FR sources can meet the requirement of the plastic deformation, a single slip deformation come up as a result. While, as the loading strain rate increases, more and more activated FR sources would be needed to coordinate the plastic deformation, the deformation mechanisms of the single crystal micropillar transformed from single slip to multiple slip.     

Features of forming structure of titanium pseudosingle crystal during β → α (bcc → hcp) polymorphic transformation

The structure of a titanium iodide single crystal obtained by zone melting has been studied by metallography, X-ray diffraction, and electron microscopy. It has been shown that the initial bcc titanium single crystal becomes a pseudosingle crystal upon cooling below the temperature of the β → α polymorphic transition. The pseudosingle crystal consists of macroscopic packets, i.e., crystals of lath morphology with a size of 0.1–0.5 cm² in different crystal sections. Each packet consists of α-phase laths of the same orientation, which are separated by dislocation boundaries. A total of six different types of packets in the pseudosingle crystal volume is realized in accordance with the Burgers orientation relationships. The structural heredity in the titanium pseudosingle crystal after the cycle of the α → β → α transformations is confirmed.     

High temperature creep property of silicon particles reinforced high Al Zn-based alloys

１　ＩＮＴＲＯＤＵＣＴＩＯＮＺｉｎｃｂａｓｅｄｃａｓｔａｌｌｏｙｓｃｏｎｔａｉｎｉｎｇａｈｉｇｈｃｏｎｃｅｎｔｒａｔｉｏｎｏｆａｌｕｍｉｎｕｍａｎｄｓｏｍｅｃｏｐｐｅｒｈａｖｅｂｅｅｎｆｏｕｎｄｔｏｂｅｅｃｏｎｏｍｉｃａｌａｎｄｅｆｆｅｃｔｉｖｅｆｏｒｓｕｂｓｔｉｔｕｔｅｓｆｏｒｂｒｏｎｚｅｉｎａｖａｒｉｅｔｙｏｆｇｅｎｅｒａｌｅｎｇｉｎｅｅｒｉｎｇｗｉｔｈｔｈｅｉｒｅｘｃｅｌｌｅｎｔｃｏｍｐｒｅｈｅｎｓｉｖｅｍｅｃｈａｎｉｃａｌｐｒｏｐｅｒｔｉｅｓａｎｄｗｅａｒｒｅｓｉｓｔａｎｃｅ…     

Some features of the formation and destruction of dislocation barriers in intermetallic compounds: III. Thermoactivated straightening of dislocations along a preferred direction in Ni<Subscript>3</Subscript>Al

Based on an analysis of the dislocation structure, the process of self-blocking, i.e., the transformation of glissile superdislocations into dislocation barriers without the effect of an external stress, has been investigated in the intermetallic compounds with an L1₂ superstructure. Using different regimes of heating without stress after preliminary deformation both below and above the temperature T _max corresponding to the peak in the yield-stress temperature dependence, a characteristic feature of self-blocking has been revealed in the intermetallic compounds on the basis of Ni₃Al, namely, the barriers were formed, but they were not destroyed. This feature was observed not only for single crystals of Ni₃(Al,Nb), but also for single crystals of the two-phase complex intermetallic alloy VKNA-4U, which contains 90% γ′ phase. By adjusting the temperature of heating and the duration of heating after preliminary low-temperature deformation, it was possible to observe the initial stages of the process of self-blocking of superdislocations for single crystals of Ni₃(Al,Nb). In experiments on the high-temperature deformation of intermetallic compounds, the process of extension (straightening) of dislocations along a preferred direction has been revealed and its thermoactivated nature has been proved. Based on the example of a well ordered alloy Ni₃Fe, the possibility of using the experiments with heating of preliminarily deformed intermetallic compounds for rapid analysis of the nature of the anomaly of the temperature dependence of yield stress has been demonstrated. In particular, for Ni₃Fe the observed anomaly is not connected with the transformations of superdislocations into barriers.     

Effect of grain size on creep properties of type 316LN stainless steel

The effect of grain size on creep properties of type 316LN stainless steel has been investigated at 600°C under different stresses. The initial strain at the beginning of creep tests decreased with the decrease of grain size. This was confirmed by the Hall-Petch relationship. The steady state creep rate decreased to a minimum value at the intermediate grain size (dm=80–130 μm) and then increased with the further increase of grain size. This result agreed with Garofalo's model stating that grain boundaries act simultaneously as both dislocation sources and barriers to dislocation movement. The rupture elongation at the intermediate grain size was minimal due to the cavity formed easily by carbide precipitates in the grain boundaries.     

一种太阳能高效多晶硅铸锭用坩埚底部引晶涂层的制备方法

介绍一种太阳能电池用高效多晶硅片生产过程中,铸锭用坩埚底部引晶涂层的制备方法。此方法是在坩埚底部分别制备两个涂层:第一层是硅粉和无机陶瓷胶的混合物,第二层是氮化硅粉和无机硅溶胶、去离子水的混合物。此方法的创新点是将硅粉替代传统的石英砂作为引晶晶核来制作引晶层。结果表明,与传统石英砂引晶层相比,此方法产生的多晶硅锭晶体均匀、位错少,制成的太阳能电池转换效率高(平均达到17.8%)。     

降低冷轧取向硅钢残余应力和位错密度的磁-热耦合工艺

为了得到一种高效去除冷轧取向硅钢残余应力的工艺,制定了低温、低强度脉冲磁场的短时磁-热耦合处理试验方案,分别利用X射线残余应力仪和XRD测定了不同工艺处理前后取向硅钢的残余应力和位错密度。结果表明,采用低温、低强度脉冲磁场的短时磁-热耦合处理可以有效降低冷轧取向硅钢的残余应力和位错密度,当采用处理时间为3 min、处理温度为400 ℃、峰值电流为180 A的磁-热耦合工艺时,可取得最佳处理效果,残余应力降幅为55.5%,比单纯只施加低温热场或低强度脉冲磁场的处理效果优异。宏观残余应力的降低与位错密度具有紧密的联系,两者变化规律基本一致。短时、低温、低脉冲磁场强度磁-热耦合处理去除残余应力的微观机制是脉冲磁场和温度场耦合作用下进一步提高材料内部位错运动,实现了局部回复,达到位错密度和残余应力减小的目的。     

Plasticity of bcc micropillars controlled by competition between dislocation multiplication and depletion

Recent micropillar experiments have shown strong size effects at small pillar diameters. This “smaller is stronger” phenomenon is widely believed to involve dislocation motion, which can be studied using dislocation dynamics (DD) simulations. In the present paper, we use a three-dimensional DD model to study the collective dislocation behavior in body-centered cubic micropillars under compression. Following the molecular dynamics (MD) simulations of Weinberger and Cai, we consider a surface-controlled cross-slip process, involving image forces and non-planar core structures, that leads to multiplication without the presence of artificial dislocation sources or pinning points. The simulations exhibit size effects and effects of initial dislocation density and strain rate on strength, which appear to be in good agreement with recent experimental results and with a simple dislocation kinetics model described here. In addition, at the high strain rates considered, plasticity is governed mainly by the kinetics of dislocation motion, not their elastic interactions.     

Atomistically informed crystal plasticity model for body-centered cubic iron

The glide of screw dislocations with non-planar dislocation cores dominates the plastic deformation behavior in body-centered cubic iron. This yields a strong strain rate and temperature dependence of the flow stress, the breakdown of Schmid’s law and a dependence of dislocation mobility on stress components that do not contribute to the mechanical driving force of dislocation glide. We developed a constitutive plasticity model that takes all these effects into account. The model is based on the crystal plasticity approach and parameterized by performing molecular statics calculations using a semi-empirical potential. The atomistic studies yield quantitative relations between local stress tensor components and the mobility of dislocations. Together with experimental stress–strain curves obtained for two different orientations of iron single crystals taken from the literature, the constitutive law is completely parameterized. The model is validated by comparing numerical single crystal tension tests for a third orientation to the equivalent experimental data from the literature. We also provide results for the temperature and strain rate dependence of the new atomistically informed constitutive model.     

A discrete dislocation analysis of the Bauschinger effect in microcrystals

The Bauschinger effect is theoretically investigated in micron-sized crystals using mechanism-based discrete dislocation plasticity. Use is made of constitutive rules that are formulated at the fundamental dislocation level to represent dislocation interactions at short-range, including dislocation multiplication and escape at free surfaces, dislocation intersections, and subsequent dynamic source and obstacle creation and destruction. Various overall responses emerge as a natural outcome to the collective behavior of discrete dislocations, depending on specimen size and initial dislocation source density. At low source density, where the behavior is multiplication controlled, tension/compression asymmetry is often realized and the scatter increases with decreasing specimen size. At sufficiently high source density, hardening mechanisms dominate the behavior and a strong Bauschinger effect is predicted in crystals with heights in the sub-micron range. In this case, the Bauschinger effect is qualitatively correlated with a new, evolving structural measure, which is solely expressible in terms of kinematic quantities.     

Dislocation annihilation in plastic deformation: I. Multiscale irreversible thermodynamics

Irreversible thermodynamics is employed as a framework to describe plastic deformation in pure metals and alloys. Expressions to describe saturation stress in single crystals and nanocrystals are employed over wide ranges of temperature, strain rate and grain size. The importance of the roles played by vacancy self-diffusion in dislocation climb and in plasticity is shown. Equations to describe the stress–strain response of single crystals and ultrafine-grained metals are derived. The activation energy for dislocation annihilation plays a central role in the mechanical response of the systems. Succinct formulations for predicting hot deformation behaviour and relaxation of industrial alloys are presented; the influence of composition in the activation energy for dislocation annihilation is shown. All formulations describing stress–strain relationships can be reduced to Kocks–Mecking classical formulation, but incorporating grain size and compositional effects. The importance of the recovery term in such formulation is established, as well as the need to obtain it employing more fundamental approaches.     

Macrosegregation During Re-melting and Holding of Directionally Solidified Al-7 wt.% Si Alloy in Microgravity

As-cast aluminum-7 wt.% ailicon alloy sample rods were re-melted and directionally solidified on Earth which resulted in uniform dendritically aligned arrays. These arrays were then partially back-melted through an imposed, and constant, temperature gradient in the microgravity environment aboard the International Space Station. The mushy zones that developed in the seed crystals were held for different periods prior to initiating directional solidification. Upon return, examination of the initial mushy-zone regions exhibited significant macrosegregation in terms of a solute-depleted zone that increased as a function of the holding time. The silicon (solute) content in these regions was measured on prepared longitudinal sections by electron microprobe analysis as well as by determining the fraction eutectic on several transverse sections. The silicon content was found to increase up the temperature gradient resulting in significant silicon concentration immediately ahead of the mushy-zone tips. The measured macrosegregation agrees well with calculations from a mathematical model developed to simulate the re-melting and holding process. The results, due to processing in a microgravity environment where buoyancy and thermosolutal convection are minimized, serve as benchmark solidification data.