Molecular-dynamics simulation of the high-pressure properties of rubidium

An embedded-atom potential for rubidium has been calculated with the parameters chosen with the use of the results of the static tests at a temperature of 300 K and pressures up to 45 GPa, as well as the results of the shock tests at pressures up to 39 GPa. The molecular-dynamics simulation has been performed for temperatures of 300–10 000 K and pressures up to ∼94 GPa. The potential determined from the shock-test data does not provide complete agreement with the static data for 300 K. The pressure, energy, and specific heats C _V and C _p have been calculated for the compression up to 20% of the normal pressure and for temperatures up to 10 000 K. The derivative (∂p/∂T) _V is positive for all of the molar-volume and temperature values except for a compression ratio of 30%. Compression up to a factor of 2.5 or more is accompanied by the partial amorphization of the models, which is enhanced with heating. The calculations of the temperature along the Hugoniot curve under the assumption that the Grüneisen parameter and adiabatic compression modulus are independent of the temperature provide an incorrect molar-volume dependence of the pressure at 0 K.     

Simple empirical equation of state for gaseous nitrogen in the high-pressure range

A two-parameter equation of state is presented for dense gases, which describes the available experimental data for nitrogen to a high accuracy over a wide temperature range.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 40, No. 4, pp. 732–735, April, 1981.     

Superplastic flow in a nanostructured aluminum alloy produced using high-pressure torsion

Samples of a spray-cast Al-7034 alloy were processed by high-pressure torsion (HPT) at temperatures of 293 or 473 K using an imposed pressure of 4 GPa and torsional straining through five revolutions. Processing by HPT produced significant grain refinement with grain sizes of 60 and 85 nm at the edges of the disks for the two processing temperatures. In tensile testing at room temperature, the alloy processed by HPT exhibited higher strength and lower ductility than the unprocessed material. Good superplastic properties were achieved in tensile testing at elevated temperatures with a maximum elongation of 750% for the sample processed at 473 K and tested in tension at 703 K under an initial strain rate of 1.0 × 10⁻² s⁻¹. The measured superplastic elongations are lower than in samples prepared by equal-channel angular pressing because of the use of very thin disks in the HPT processing.     

Effect of strain reversals on the processing of high-purity aluminum by high-pressure torsion

High-purity aluminum was processed by high-pressure torsion (HPT) under conventional monotonic (m-HPT) and cyclic (c-HPT) conditions where strain reversals are introduced in c-HPT during processing. Measurements show higher values of the Vickers microhardness in the center regions of all disks but these values are higher when processing by c-HPT by comparison with m-HPT for the same total number of turns. Slightly smaller grain sizes are observed in the c-HPT samples. It is shown that all of the microhardness values correlate with the estimated values of the equivalent strain and the results are consistent with earlier data reported under c-HPT conditions when it is recognized that the variation of hardness with equivalent strain is dependent upon the level of recovery within the material.     

吸油烟机气动噪声的数值模拟

基于混合CAA(Computational Aeroacoustics)方法,对某型号吸油烟机的气动噪声进行了数值模拟研究.首先,利用CFD软件Fluent对吸油烟机的非稳态流场进行计算,得到吸油烟机非稳态流场的压力和速度分布.然后,利用Lighthill声类比方法和Curle方程,采用声学软件Actran计算得到吸油烟机的声场.最后,分析了吸油烟机内声源频域结果和声场的声压级云图,得到了吸油烟机噪声传播指向特性.研究结果表明,吸油烟机噪声主要为中低频噪声,且以离散噪声为主.在叶片通过频率下,吸油烟机噪声具有明显的指向性特征,噪声主要由吸油烟机内离心风机主次进风口向外部传播.     

圆柱体工件高压气淬过程中的数值模拟

利用导热微分方程和单值性条件推导了轴对称短圆柱体温度分布的通用数学微分方程,可计算工件上任意点的温度和任意时间的温度分布;采用伽辽金(Galerkin)加权余量法推导了二维轴对称圆柱体瞬态温度场的有限元计算的积分方程;采用大型有限元计算软件ANSYS对5Cr1MoV轴对称实心圆柱体工件淬火过程的温度变化及热应力/应变分布状况进行了模拟计算.     

Microstructural evolution and the mechanical properties of an aluminum alloy processed by high-pressure torsion

Processing by high-pressure torsion (HPT) was performed on disks of an Al-7075 alloy at room temperature. The alloy was initially annealed at 753 K and then processed by HPT under a pressure of 6.0 GPa up to a maximum of ten turns. Measurements of the Vickers microhardness showed lower values at the centers of the disks after small numbers of turns but higher numbers of turns led to a reasonable hardness homogeneity across each disk. After five turns, the grain size at the edge of the disk was ~250 nm. It is demonstrated that results from mechanical testing are consistent with the hardness and microstructural data.     

高压均质物料流场的数值分析

 利用计算流体动力学商业软件对均质阀内部流场进行可视化研究.采用CFD建模并模拟求解所得到的结果与先前研究人员采用半经验半理论的研究方法所得结果较为吻合.基于研究结果,提出一种计算压降与间隙的基本数学模型,并采用遗传算法和非线性回归算法对数值结果进行拟合,验证了模型的合理性.该研究结果能够给出均质阀体内流场中各个参数变化的详细信息,同时展示了实验手段所不能观察到的流动现象,对进行均质阀结构设计以及对流场的深入分析具有一定的指导作用.     

Computer simulation of martensitic textures

We consider a Ginzburg-Landau model free energy F(ε, e₁, e₂) for a (2D) martensitic transition, that provides a unified understanding of varied twin/tweed textures. Here F is a triple well potential in the rectangular strain (ε) order parameter and quadratic e₁², e₂² in the compressional and shear strains, respectively. Random compositional fluctuations η(r) (e.g. in an alloy) are gradient-coupled to ε, ˜ − ∑_rε(r)[(Δ_x² − Δ_y²)η(r)] in a “local-stress” model. We find that the compatibility condition (linking tensor components ε(r) and e₁(r), e₂(r)), together with local variations such as interfaces or η(r) fluctuations, can drive the formation of global elastic textures, through long-range and anisotropic effective ε-ε interactions. We have carried out extensive relaxational computer simulations using the time-dependent Ginzburg-Landau (TDGL) equation that supports our analytic work and shows the spontaneous formation of parallel twins, and chequer-board tweed. The observed microstructure in NiAl and Fe_xPd_{1 − x} alloys can be explained on the basis of our analysis and simulations.     

Computer simulation of serrated yielding

A model for serrated yielding based on the negative resistance characteristics of materials is discussed. An analog computer based on this model is described. The simulated curves show oscillations which are regular and uniform in amplitude. To simulate more realistic tensile test curves, a refined model which includes the effects of fluctuations in dislocation density and velocity is described. Some simulated curves using this refined model are presented. These results are close to observed tensile test curves.     

Computer simulation of iceberg instability

The possibility of an iceberg rolling to an orientation of deeper draft and colliding with the sea-bed has important implications for the offshore oil and gas industry on the Grand Banks of Newfoundland. In this paper that possibility is given substance. An interactive computer program was used to study a class of icebergs that increase their draft by as much as 50% on being perturbed from a position of near unstable equilibrium.     

Computer simulation of tensile testing

A brief overview of the molecular dynamics method, with emphasis on the work of Parrinello and Rahman, is presented. Molecular dynamics is a method for studying classical statistical mechanics of well-defined systems through a numerical solution of Newton’s equations. A set ofN particles with coordinatesr _i (i=1, …,N) and confined in a cell are allowed to interact through a potential. The bulk is usually simulated by periodically repeating the cell in space. Newton’s equations are then solved numerically and the statistical averages of dynamical properties are calculated as temporal averages over the trajectory of the phase space. This method has already been used to simulate a liquid. Now, based on a Lagrangian formulation, it is possible to study systems under the most general conditions of externally applied stress. Unlike the earlier calculations, in this procedure, shape and size are governed by equations of motion obtained from the Lagrangian. Thus it allows us to study structural transformations which may be brought about by an interplay of external and internal stresses. By applying this technique to a single crystal of Ni, Parrinello and Rahman observe that the stress-strain relations obtained confirm with reported results. Under compressive loading it is found that Ni shows a bifurcation in its stress-strain relation and the system changes from cubic to hexagonal close packing. Such a transformation could perhaps be observed under extreme conditions of shock. Finally the scope of computer simulation is highlighted and the limitations of employing such a method are pointed out. only summary of the talk     

Computer simulation of pressure sintering

The densification process during pressure sintering has been analyzed using finite element analysis. This analysis uses an iterative solution algorithm. With this the densification process in complex geometries with complex boundary conditions can be analyzed and this technique is particularly suited for tackling material nonlinearity. Evolution of dense structures with gradual closure of pores is described for two typical geometries.     

Computer simulation of liquid zinc

An embedded atom model potential for zinc has been developed, which makes it possible to calculate liquid zinc properties both under normal pressure and in strongly compressed states using the molecular dynamics method. In order to calculate the potential, the data on density, energy, and compressibility of liquid zinc and the data on shock compression of zinc were used. Pair contribution to the potential and the embedding potential are represented by analytical functions. Liquid zinc properties are calculated at temperatures up to 1500 K. The values of energy, bulk compression modulus, and self-diffusion coefficient, as well as pair correlation functions at T < 1000 K, agree well with the experiment. The electron contribution to the thermal capacity at those temperatures is not high. Zinc models are constructed for densities up to 15.86 g/cm³ and pressures up to 773 GPa. Zinc models melt in the case of shock compression at compression rates of V/V ₀ < 0.7 and temperatures above 1900 K. A significant contribution of electron excitation energy to the zinc energy is observed at temperatures above 20000 K. The estimated average surplus thermal capacity of electrons at 30000–50000 K is ∼12 J/mole K. Discrepancies between the molecular dynamic calculation and the Gruneisen model at low temperatures are relatively low; however, they rise as temperature increases. A series of zinc nanocluster models with magic sizes of 55 and 147 atoms is constructed. The clusters have an amorphous structure with slightly lower energy than that of icosahedral or cuboctahedral configuration, after cooling from 600 to 10 K. The surface energy of zinc at T = 0 calculated based on the dependence of energy of clusters on size is 1.3 J/m².     

Computer simulation of bias recording

The technique of AC bias recording is explained by a newly developed theoretical approach. It is demonstrated that the model can explain the physical background of the linearization of the remanence curve and be used to determine the dependence of the maximum output level (MOL) on various technical and media parameters. The merits and drawbacks of changes in the different media data are evaluated quantitatively by computer simulation. All theoretical results are compared with corresponding experimental data and thus verified     

Computer simulation of ultrasonics in a solid

High-speed, large-memory computers have made feasible accurate computer simulations of ultrasound in solids. The basic equations and models used for the simulations are presented, and some 400 results are outlined. The results are shown to be quite accurate by comparison with experiments. Current and possible future applications of the technique to NDT are discussed.     

Computer simulation of interfacial packing in concrete

Computer simulation of particle packing against an aggregate surface was undertaken to show the effects of four variables on interfacial porosity profiles. The variables in order of significance and their assumed physical meaning are: sticking probability (tendency of cement particles to flocculate), amplitude of particle motion (energy of mixing), travel distance of particle to surface (thickness of water film surrounding aggregate), and original particle density (roughly related to water/cement ratio). In all cases, simulations demonstrated that interface porosity decreased from nearly 100% directly at the interface to that of the bulk paste at two to three particle diameters. Flocculation (sticking probability) was found to be the single most-significant variable. Highly flocculated systems produced very porous interfaces. When flocculation was reduced, packing became more efficient. It was also found that energy of mixing (amplitude of motion), was not an entirely independent variable. The simulation showed that, if the tendency to flocculate was high, gentle mixing (low amplitude of motion) was found to result in better packing and a less porous interfacial zone. If, on the other hand, flocculation was low, then vigorous mixing (high amplitude of motion) promoted better packing near the interface. The thickness of the water film surrounding the aggregate (travel distance) was found to have only a minor effect on the outcome of simulations, while original packing density (w/c) resulted in no significant differences at all.