Dislocation and defect density-based micromechanical modeling of the mechanical behavior of fcc metals under neutron irradiation

A rate-independent dislocation and defect density-based evolution model is presented that captures the pre- and post-yield material behavior of fcc metals subjected to different doses of neutron radiation. Unlike previously developed phenomenological models, this model is capable of capturing the salient features of irradiation-induced hardening, including increase in yield stress followed by yield drop and non-zero stress offset from the unirradiated stress–strain curve. The key contribution is a model for the critical resolved slip resistance that depends on both dislocation and defect densities, which are governed by evolution equations based on physical observations. The result is an orientation-dependent non-homogeneous deformation model, which accounts for defect annihilation on active slip planes. Results for both single and polycrystalline simulations of OFHC copper are presented and are observed to be in reasonably good agreement with experimental data. Extension of the model to other fcc metals is straightforward and is currently being developed for bcc metals.     

Crystal plasticity-based constitutive modelling of irradiated bcc structures

A constitutive crystal plasticity model is proposed and developed for the inelastic deformation of irradiated bcc ferritic/martensitic steels. Defects found in these irradiated materials are used as substructure variables in the model. Insights from lower length- and time-scale simulations are used to frame the kinematic and substructure evolution relations of the governing deformation mechanisms. Models for evolution of mobile and immobile dislocations, as well as interstitial loops (formed due to irradiation), are developed. A rate theory-based approach is used to model the evolution of point defects generated during irradiation. The model is used to simulate the quasi-static tensile and creep response of a martensitic steel over a range of loading histories.     

Dislocation model for an incoherent nonthin twin

A condition for the equilibrium of a nonthin twin is obtained from a dislocation model using the approximation of a continuous distribution of twinning dislocations at twin boundaries. The model of a nonthin twin is shown to be applicable to the elastic and inelastic stages of twinning. The theory of a thin twin is found to be a particular case of the developed dislocation model of a nonthin twin.     

Explicit incorporation of cross-slip in a dislocation density-based crystal plasticity model

We present a crystal plasticity model that incorporates cross-slip of screw dislocations explicitly based on dislocation densities. The residence plane of screw dislocations is determined based on a probability function defined by activation energy and activation volume of cross-slip. This enables the redistribution of screw-dislocations and dislocation density patterning due to the effect of stacking fault energy. The formulation is employed for explaining the cross-slip phenomenon in aluminium during uniaxial tensile deformation of ?100? single crystal and a single slip orientation of single crystal, and compare the results with experimental observations. The effect of cross-slip on the stress–strain evolution is seen using this explicit treatment of cross-slip.     

Dislocation model for the behavior of fractal dimension of the microstructure of a strained solid

A single-parameter model describing the behavior of fractal dimension of the structure of a deformed solid in a wide range of strains in terms of nonequilibrium thermodynamics has been proposed based on the analysis of dislocation kinetics. Comparison with the literature and own experimental data has been presented, which gives evidence in favor of the proposed model.     

A constitutive model of the human vocal fold cover for fundamental frequency regulation

The elastic as well as time-dependent mechanical response of the vocal fold cover (epithelium and superficial layer of the lamina propria) under tension is one key variable in regulating the fundamental frequency. This study examines the hyperelastic and time-dependent tensile deformation behavior of a group of human vocal fold cover specimens (six male and five female). The primary goal is to formulate a constitutive model that could describe empirical trends in speaking fundamental frequency with reasonable confidence. The constitutive model for the tissue mechanical behavior consists of a hyperelastic equilibrium network in parallel with an inelastic, time-dependent network and is combined with the ideal string model for phonation. Results showed that hyperelastic and time-dependent parameters of the constitutive model can be related to observed age-related and gender-related differences in speaking fundamental frequency. The implications of these findings on fundamental frequency regulation are described. Limitations of the current constitutive model are discussed.     

A nonlinear magneto-thermo-elastic coupled hysteretic constitutive model for magnetostrictive alloys

This paper presents a general hysteretic constitutive law of nonlinear magneto-thermo-elastic coupling for magnetostrictive alloys. The model considered here is thermodynamically motivated and based on the Gibbs free energy function. A nonlinear part of the elastic strain arising from magnetic domain rotation induced by the pre-stress is taken into account. Furthermore, the movement of the domain walls is incorporated to describe hysteresis based on Jiles–Atherton's model. Then a set of closed and analytical expressions of the constitutive law for the magnetostrictive rods and films are obtained, and the parameters appearing in the model can be determined by those measurable experiments in mechanics and physics. Comparing this model with other existing models in this field, the quantitative results show that the relationships obtained here are more effective to describe the effects of the pre-stress or in-plane residual stress and ambient temperature on the magnetization or the magnetostriction hysteresis loops.     

A model for the dissolution behaviour of a graphite overlayer

A model based on a two-step first-order reaction mechanism is proposed to explain the dissolution behaviour of a graphite overlayer into the substrate. The two steps are (a) the decomposition of the graphite layer on and (b) the dissolution of the individual atoms into the substrate. The model was used to successfully explain some results from the literature.     

Ising-like model for protein mechanical unfolding

The mechanical unfolding of proteins is studied by extending the Wako-Sait?-Mu?oz-Eaton model. This model is generalized by including an external force, and its thermodynamics turns out to be exactly solvable. We consider two molecules, the 27th immunoglobulin domain of titin and protein PIN1. We determine equilibrium force-extension curves for the titin and study the mechanical unfolding of this molecule, finding good agreement with experiments. By using an extended form of the Jarzynski equality, we compute the free energy landscape of the PIN1 as a function of the molecule length.     

Quantum mechanical behaviour of deuterated methyl groups

We discuss the temperature dependence of deuterium NMR spectra and spin-lattice relaxation rates 1/T ₁ of deuterated methyl groups. We restrict ourselves to the temperature range where it is sufficient to consider only the two lowest librational levels of the CD₃ group. Specifically we derive explicit expressions for 1/T ₁ and the width _NMR of the so-called tunneling resonances in the high-field NMR spectrum in the limit where the tunneling frequency ₀ is large compared to the quadrupole coupling constant of a methyl group. We demonstrate that the disappearance of the tunneling resonances at elevated temperatures follows the scenario of the NMR of nuclei which exchange classically between two inequivalent sites. We further show that the NMR linewidths and relaxation rates are strongly related to the widths of the quasielastic and inelastic lines in incoherent neutron spectra. It turns out that for NMR spectra the classical limit is usually reached at lower temperatures than for neutron spectra. All calculations are based on the projection operator formalism.     

Low-strain fatigue in AISI 316L steel surface grains: a three-dimensional discrete dislocation dynamics modelling of the early cycles I. Dislocation microstructures and mechanical behaviour

The early stages of the formation of dislocation microstructures in low-strain fatigue are analysed, using three-dimensional discrete dislocation dynamics modelling. Simulations under various conditions of loading amplitude and grain size have been performed. Both the dislocation microstructures and the associated mechanical behaviour are accurately reproduced in single-slip as well as in double-slip loading conditions. The microstructures thus obtained are analysed quantitatively, in terms of number of slip bands per grain, band thickness and band spacing. The simulations show the crucial role of cross-slip both for the initial spreading of strain inside the grain and for the subsequent strain localization in the form of slip bands. A complete and detailed scheme for the persistent slip band formation is proposed, from the observation of the numerical dislocation arrangements.     

一种计算金属剪切模量的本构模型：以Al为例

用两步法构建了一个与温度和压力相关的适用于金属材料的剪切模量本构模型,其中的第一步任务是求得沿0 K等温线上剪切模量随压力的变化规律,即求得G₁=G₁（P,0 K）的函数式.第二步是从0 K等温线上某一给定P的G值出发,求出沿等压线上剪切模量随温度T变化的规律,从而最终求得剪切模量本构模型G=G（P,T）的具体表达式.在这两个阶段的研究中都利用了超声波测量和第一性原理计算方法的研究结果.用铝为模型材料,对本模型的合理性进行了检验.结果表明,G的模型预测数据与试验测量及理论计算数据相比较,无论G的演化是沿冲击压缩轨迹、等熵压缩轨迹、等温压缩轨迹还是等压线轨迹,都能达到令人满意的程度,故可认为本模型具有良好的普适性和合理性. 关键词： 铝 本构模型 剪切模量 冲击波压缩     

Investigation of agglomerated Cu seed on Cu oxidation after chemical mechanical planarization

After chemical mechanical planarization (CMP), the reason which caused the formation of Cu-oxide defects at the interface between Cu deposit and TaN barrier layer has been studied. The experimental results of atomic force microscopy, secondary ion mass spectroscopy, X-ray diffraction demonstrated that the agglomeration phenomenon was found on Cu seed in the thickness of only 10 nm, thus resulting in the electrodeposited Cu film with more abundant C impurities at Cu/TaN interface and lower (1 1 1)/(2 0 0) ratio compared to the thick one (30 nm). Therefore it caused the Cu deposit with poor corrosion resistance and then the Cu-oxide defects were easily formed after CMP. As a result, the correlation between Cu-oxide defects at the Cu/TaN interface and the agglomeration on Cu seed layer was proposed herein.     

Anomalous mechanical characteristics of Au/Cu nanocomposite processed by Cu electroplating

An Au/Cu nanocomposite is produced by electroplating Cu on a nanoporous Au, and its mechanical characteristics are investigated by hardness tests. The Au/Cu nanocomposite showed a lower hardness and a lower elastic modulus than the nanoporous Au. Furthermore, annealing caused the nanocomposite to harden twice. Large lattice strains in the Au lattice for the nanocomposite were observed by high-resolution transmission electron microscopy. Also, first-principle calculations showed that lattice strains induce the decreased elastic modulus. Therefore, both the inverse mixing behaviour and the hardening via annealing are suggested to be related to the large lattice strains.     

Grain boundary influence on the mechanical behaviour of oriented bicrystals

It is shown through several experiments centred on dislocation transmission through a GB that relating macroscopic mechanical properties of a bicrystalline specimen to the atomic structure of the GB or to local dislocation reactions is not straightforward. Not only the long and short range stresses and the plastic properties of the two grains must be taken into consideration, but also the kinetics of events has to be taken into account to explain the final result.     

Size effect on the mechanical behaviour of polycrystalline copper microbeams

To reduce costs and to remain competitive in the worldwide electronics industry, semiconductor manufacturers continually miniaturize devices. Today, the interconnect lines linking electronic components have diameters of the order of 100?nm or smaller. At the nanometre scale, strong size effects modify the mechanical properties of materials. To examine such effects, freestanding microbeams with geometrical and microstructural properties similar to those of interconnect lines have been designed. The yield stress dependence of the microbeams on their microstructure, shape and dimensions was investigated. As predicted by the Hall–Petch law, an increase in the yield stress with a decrease in the grain size was observed. In addition, a decrease in the cross-section of the microbeams at a fixed grain size led to a decrease in the yield stress. Hence, the yield domain of interconnect lines was observed to be controlled by two competitive size effects. This result imposes some restrictions on the design of electronic devices.     

A macro-mechanical constitutive model of shape memory alloys

It is of practical interest to establish a precise constitutive model which includes the equations describing the phase transformation behaviors and thermo-mechanical processes of shape memory alloy (SMA). The microscopic mechanism of super elasticity and shape memory effect of SMA is explained based on the concept of shape memory factor defined by the author of this paper. The conventional super elasticity and shape memory effect of SMA are further unified as shape memory effect. Shape memory factor is redefined in order to make clear its physical meaning. A new shape memory evolution equation is developed to predict the phase transformation behaviors of SMA based on the differential relationship between martensitic volume fraction and phase transformation free energy and the results of DSC test. It overcomes the limitations that the previous shape memory evolution equations or phase transformation equations fail to express the influences of the phase transformation peak temperatures on the phase transformation behaviors and the transformation from twinned martensite to detwinned martensite occurring in SMA. A new macro-mechanical constitutive equation is established to predict the thermo-mechanical processes realizing the shape memory effect of SMA from the expression of Gibbs free energy. It is expanded from one-dimension to three-dimension with assuming SMA as isotropic material. All material constants in the new constitutive equation can be determined from macroscopic experiments, which makes it more easily used in practical applications. Supported by the National Natural Science Foundation of China (Grant No. 95505010), the National High Technology Research and Development Program of China (Grant No. 2006AA03Z109), the China Postdoctoral Science Foundation (Grant No. 20080430933), the Open Foundation of Institute of Engineering Mechanics of National Seism Bureau of China (Grant No. 2007B02), and the Harbin Talent Foundation of Scientific and Technical Innovation (Grant No. RC2009QN-017046)