Crystal plasticity modeling of irradiation growth in Zircaloy-2

Abstract

A physically based reaction-diffusion model is implemented in the visco-plastic self-consistent (VPSC) crystal plasticity framework to simulate irradiation growth in hcp Zr and its alloys. The reaction-diffusion model accounts for the defects produced by the cascade of displaced atoms, their diffusion to lattice sinks and the contribution to crystallographic strain at the level of single crystals. The VPSC framework accounts for intergranular interactions and irradiation creep, and calculates the strain in the polycrystalline ensemble. A novel scheme is proposed to model the simultaneous evolution of both, number density and radius, of irradiation-induced dislocation loops directly from experimental data of dislocation density evolution during irradiation. This framework is used to predict the irradiation growth behaviour of cold-worked Zircaloy-2 and trends compared to available experimental data. The role of internal stresses in inducing irradiation creep is discussed. Effects of grain size, texture and external stress on the coupled irradiation growth and creep behaviour are also studied and compared with available experimental data.     

Crack growth on the basal plane in single crystal zinc: experiments and computations

Crack propagation on the basal planes in zinc was examined by means of in situ fracture testing of pre-cracked single crystals, with specific attention paid to the fracture mechanism. During quasistatic loading, crack propagation occurred in short bursts of dynamic crack extension followed by periods of arrests, the latter accompanied by plastic deformation and blunting of the crack-tip. In situ observations confirmed nucleation and propagation of microcracks on parallel basal planes and plastic deformation and failure of the linking ligaments. Pre-existing twins in the crack path serve as potent crack arrestors. The crystallographic orientation of the crack growth direction on the basal plane was found to influence both the fracture load as well as the deformation at the crack-tip, producing fracture surfaces of noticeably different appearances. Finite element analysis incorporating crystal plasticity was used to identify dominant slip systems and the stress distribution around the crack-tip in plane stress and plane strain. The computational results are helpful in rationalizing the experimental observations including the mechanism of crack propagation, the orientation dependence of crack-tip plasticity and the fracture surface morphology.     

超长定向含铁多壁碳纳米管阵列的制备

采用无模板化学气相沉积法,以二茂铁为催化剂,二甲苯为碳源,利用单温炉加热装置制备了定向碳纳米管阵列。运用扫描电子显微镜、透射电子显微镜、拉曼光谱和X射线衍射仪等对定向碳纳米管阵列的形貌、成分和物相进行细致的分析和表征。结果表明:制得的碳纳米管阵列具有良好的定向性和多壁管状结构,并且石墨化程度高;碳纳米管中除碳元素外,管中包含有少量以纳米颗粒和纳米线形式存在的铁及其化合物,主要成分是铁和碳化铁。结合碳纳米管的制备和透射电子显微镜分析表征结果,认为超长碳纳米管阵列的生长模式为底部生长方式,即经历催化剂分解、催化、成核、长大、中毒、凝聚成粒和连接成线的循环过程,正是由于碳源和催化剂的连续供应促成了碳纳米管阵列的快速定向生长。     

自组生长的硅纳米管的稳定性研究

文章作者的研究小组在世界上首次合成自组生长的硅纳米管（SiNTs）后,对它的稳定性研究又获得进展.采用5wt%的HF酸对自组生长的硅纳米管的稳定性进行了研究,研究表明HF酸可以去除硅纳米管的氧化物外层,只剩下晶体硅纳米管,说明所得到的硅纳米管是一种稳定结构,因而使其应用研究开发成为可能.研究表明,硅纳米管的稳定性与其生长形成过程密切相关。     

生长温度对碳纳米管阴极场发射性能的影响

碳纳米管(Carbon Nanotubes,CNTs)场发射平面显示器(Field Emission Display,FED)与其他显示器比较显示了其独特优点,被认为是未来理想的平面显示器之一。碳纳米管阴极作为器件的核心部分,其性能的好坏直接影响显示器的性能。针对30~60英寸(76.2~152.4cm)大屏幕显示器所用的厚膜工艺,即采用丝网印刷法制备了碳纳米管阴极阵列,研究了化学气相沉积法在不同温度下生长的CNTs的场发射电流-电压特性,找到了适合FED用碳纳米管的最佳生长温度。结果表明生长温度越高(750℃),CNTs场发射性能越好。并用荧光粉阳极测试这些CNTs的场发射发光显示效果,验证了上述结论。     

面心立方晶体外延膜沉积生长中失配位错的结构与形成过程

用分子动力学方法对5%负失配条件下面心立方晶体铝薄膜的原子沉积外延生长进行了三维模拟.铝原子间的相互作用采用嵌入原子法(EAM)多体势计算.模拟结果再现了失配位错的形成现象.分析表明，失配位错在形成之初即呈现为Shockley扩展位错，即由两个伯格斯矢量为〈211〉/6的部分位错和其间的堆垛层错组成，两个部分位错的间距、即层错宽度为1.8 nm,与理论计算结果一致；外延晶体薄膜沉积生长中，位错对会发生滑移，但其间距保持稳定.进一步观察发现，该扩展位错产生于一种类似于“局部熔融-重结晶”的表层局部无序紊乱- 关键词： 失配位错 外延生长 薄膜 分子动力学 铝     

The steady state growth rate of a recrystallization front in regions of heterogeneous dislocation density

The paper investigates whether a change from a homogeneous to an inhomogeneous dislocation distribution, assumed to be caused by a slight additional deformation, can lead to an increase of the recrystallization temperature of a deformed metal. In this case, the higher temperature would indicate a more stable deformation structure despite the increase of stored energy. The recrystallization temperature is related to the growth rate. Hence, the steady state velocity of a recrystallization front moving either parallel or vertically to the stripes of a simplified two-dimensional heterogeneous dislocation distribution of parallel sections of higher and lower dislocation densities is calculated. The results show that if a front growths through the high and low density sections in series an overall slower rate despite higher mean dislocation density is, indeed, possible. However, growing in the parallel arrangement always leads to a higher growth rate compared with the homogeneous case of slightly less stored energy. Since in a real structure the faster growth is likely to succeed, the recrystallization temperature observed will be lowered with additional deformation in accordance with experimental experience.     

An efficient lasing action from pyrromethene 556 dye-doped organically modified silicates

Organically modified solid-state silicates (ORMOSILS) doped with a new laser dye 1,3,5,7,8-pentamethylpyrromethene-2,6-disulfonate-BF₂ complex (pyrromethene 556) have been synthesized by a sol-gel method and the compositional effects on pore characteristics, fluorescence and lasing properties have been investigated. It is found that the use of dimethylsulfoxide and γ-glycidoxypropyltrimethoxysilane could greatly change the structure properties of sol-gel derived ORMOSILS cage, and thus the fluorescence and lasing properties of the materials could improve significantly. A successful laser oscillation from this dye-doped ORMOSILS sample has been achieved upon pumping with a Q-switched frequency-doubled Nd:YAG laser at 532 nm. A slope efficiency of 54% with a useful lifetime greater than 10,000 shots has been demonstrated at a pump repetition rate of 1 Hz and a pump intensity of 1 J/cm² by using the new ORMOSILS cage on our newly designed laser system. Our results have shown that it is possible to obtain a high-efficiency with a long-lifetime for a compact new laser device by low cost dye-doped solid-state ORMOSILS.     

Optical spectroscopy of Yb/Er codoped NaY(WO4)2 crystal

Erbium and ytterbium codoped double tungstates NaY(WO₄)₂ crystals were prepared by using Czochralski (CZ) pulling method. The absorption spectra in the region 290-2000 nm have been recorded at room temperature. The Judd-Ofelt theory was applied to the measured values of absorption line strengths to evaluate the spontaneous emission probabilities and stimulated emission cross sections of Er³⁺ ions in NaY(WO₄)₂ crystals. Intensive green and red lights were measured when the sample were pumped by a 974 nm laser diode (LD), especially, the intensities of green upconversion luminescence are very strong. The mechanism of energy transfer from Yb³⁺ to Er³⁺ ions was analyzed. Energy transfer and nonradiative relaxation played an important role in the upconversion process. Photoexcited luminescence experiments are also fulfilled to help analyzing the transit processes of the energy levels.     

The Effect of Boundary Conditions on Gas-phase Synthesised Silver Nanoparticles

We have prepared spherical non-agglomerated silver nanoparticles by an evaporation–condensation–dilution/cooling technique. Silver was evaporated from a crucible in a tubular flow reactor. A porous tube diluter was used to quench the carrier gas at the outlet of the reactor to enhance the formation of small particles and to suppress agglomeration and other particle growth mechanisms. The number size distribution of the prepared particles was measured with a differential mobility analyser–condensation nucleus counter combination and the size and the shape of the particles were analysed with transmission electron microscope. The system was modelled using a sectional aerosol dynamics computer code to estimate the importance of different aerosol processes. In all conditions the particles obtained were non-agglomerated and spherical. The mean particle diameter varied from 4 to 10-nm depending on boundary conditions. From the modelling studies it can be concluded that the nucleation rate is the most important parameter controlling the final particle size.