Effect of the Strain Rate and Microstructure on Damage Growth in Aluminum

Materials used in soldier protective structures, such as armor, vehicles and civil infrastructures, are being improved for performance in extreme dynamic environments. Nanocrystalline metals show significant promise in the design of these structures with superior strengths attributed to the dislocation-based and grain-boundary-based processes as compared to their polycrystalline counterparts. An optimization of these materials, however, requires a fundamental understanding of damage evolution at the atomic level. Accordingly, atomistic molecular dynamics simulations are performed using an embedded-atom method (EAM) potential on three nano-crystalline aluminum atom systems, one a Voronoi-based nano-crystalline system with an average grain size of 10 nm, and the other two single crystals. These simulations are performed under the condition of uniaxial expansion at several strain rates ranging from 10⁶s^-1 to 10¹⁰s^-1. Results for the effective stress are discussed with the aim of establishing the role of the strain rate and microstructure on the evolution of the plastic strain and void volume fraction and the eventual loss of stress carrying capability of the atom systems.     

On the nucleation and growth of voids at high strain-rates

The nucleation and growth of voids at high strain-rate is studied in copper as a model face centered cubic (fcc) material using large scale molecular dynamics (MD) methods. After a brief introduction to dynamic fracture, results are presented for the homogeneous nucleation of voids in single crystal copper and the heterogeneous nucleation in nanoscale polycrystalline copper. The simulations suggest void growth occurs through anisotropic dislocation nucleation and emission in agreement with experiment and the observed anisotropy of the tensile flow stress in fcc crystals. A phenomenological model for the transition from intergranular to transgranular fracture at high strain-rate is presented. This revised version was published online in July 2006 with corrections to the Cover Date.     

Molecular Dynamic Simulation of Lattice Distortion Region Produced by Rounded Grain Boundary in Nanocrystalline Materials

The distortion structure in nanocrystalline NiAl is studied using molecular dynamics simulation.The rounded grain boundaries in these nanograins are a direct source for the observed lattice distortion.The change of grain size affects directly the volume fraction of the distorted lattice in the nanograin.     

金属Cu液态结构的纳米晶粒模型

为研究液态Cu的短程有序结构,建立了液态微观结构的纳米晶粒模型,从晶体X射线衍射学的角度出发,通过对固态Cu衍射峰的宽化计算出了液态X射线衍射强度曲线.计算出的强度曲线与实验获得的液态Cu的X射线衍射强度曲线具有较好的一致性,这证明纳米晶粒模型的正确性,同时也证明了Cu的液态短程有序结构与其固态晶体结构存在着密切相关性,即Cu的液态短程序结构为fcc.     

Densification Behavior of Nanocrystalline Mg2Si Compact in Hot-pressing

Densification behavior of nanocrystalline Mg2Si （n-Mg2Si） with grain size about 30-50 nm was investigated by hot-pressing at 400℃. The results indicated that the densification process of n-Mg2Si exhibited three linear segments： p〈0.3 GPa, 0.3 GPa〈p〈1.2 GPa, and p〉1.2 GPa determined by Heckel formula, among which the third fast increasing segment in high pressure range p〉1.2 GPa has seldom been reported in conventional coarse-grained polycrystalline materials. Nevertheless, in the whole pressure range （0.125-1.500 GPa） investigated the densification behavior of n-Mg2Si can be well described by a Kawakita formula p/C=（1/a）p＋ 1/（ab） with constant α=0.452 being in good agreement with the initial porosity of the compact.     

Molecular dynamics simulations of spallation in metals and alloys

We report on the results of large-scale molecular dynamics simulations of the mechanical behavior of two-dimensional metallic systems. The specific impact phenomenon studied is that in which a flyer of mass M moving with x-velocity v impacts a target of mass 2M moving with x-velocity −v/2. Simulations of such a spallation experiment have been performed for a generic metal, modelled with an embedded atom potential and also for a Cu-Ni alloy system, modelled with truncated Lennard-Jones potentials.

Our simulations indicate cold-welding upon impact, and shock wave generation, followed by rebound from the boundaries. The alloy was less ductile than the generic metal and consequently the system came apart due to the cooperative effect of the reflected shock waves.     

Molecular Dynamics Study on Interfacial Energy and Atomic Structure of Ag/Ni and Cu/Ni Heterophase System

The results of molecular dynamics calculations on the interfacial energies and atomic structures of Ag/Ni and Cu/Ni interfaces are presented. Calculation on Ag/Ni interfaces with low-index planes shows that those containing the (111) plane have the lowest energies, which is in agreement with the experiments. Comparing surface energy with interfacial energy, it is found the order of the interfacial energies of Ag/Ni and Cu/Ni containing the planes fall in the same order as solid-vapor surface energies of Ag, Cu and Ni. In this MD simulation, the relaxed atomic structure and dislocation network of (110)Ag||(110)Ni interface are coincident to HREM observations.     

Thermo-activated Dislocation Emission at the Cu/Nb Interface Revealed by Molecular Dynamics Simulations

Nanolayered Cu-Nb composites offer a series of enhanced properties for their use in extreme conditions, e.g. high field magnets and high irradiation resistance. However, the stability of the Cu/Nb heterogeneous interface needs confirmation under various conditions. In the present work, molecular dynamics simulations were carried out to investigate the interfacial behavior under various temperatures with initial stress at the interface. It is found that the interface becomes unstable at simulation temperatures higher than 600 K, resulting in the emission of dislocations and loops within one or more slip systems. The emission process is found to be thermally-activated, i.e., the higher temperature, the shorter annealing time needed. The present study is believed to assist the experimental synthesis of the Cu-Nb multilayer nanocomposites for multiple applications.     

纳米多晶铝压缩塑性力学行为分子动力学研究

目的 研究纳米多晶铝在不同温度与应变速率下的力学响应与塑性变形行为以及不同变形条件下的塑性力学行为。方法 通过ATOMSK软件构建了晶粒取向随机的纳米多晶铝模型,利用LAMMPS软件在300～700 K温度以及1×10⁹、5×10⁹、1×10¹⁰、1×10¹¹ s^-1应变速率下完成了纳米多晶铝的压缩模拟,借助后处理OVITO软件对模拟结果进行了分析。结果 随温度的升高,晶界原子所占比例增大,纳米多晶铝的弹性模量逐渐下降,在压缩过程中总位错密度随温度的升高而增大。随着应变速率的增大,材料硬化速率增加,纳米多晶铝表现出更高的屈服强度。当应变速率较低时,位错大量存在于小晶粒之中,且中央大晶粒相较于初始位置旋转了20°。当应变速率达到1×10¹¹ s^-1时,材料的硬化速率极大提高,且在晶粒内部出现了孪晶。在塑性变形过程中,1/6<112>(不全位错)的数量最多,在位错运动中占主导地位。结论 温度升高导致材料弹性模量降低,这主要是由...     

Al纳米晶在Fe表面熔化行为的分子动力学模拟

本文应用分子动力学(MD)方法,采用嵌入势模型,在Al纳米晶熔点以上、Fe熔点以下的温度范围内,对Al原子在Fe(001)、(110)和(111)面上的扩散现象进行了系统研究。结果发现,在模拟时间内Al原子的扩散主要发生在Al和Fe直接接触的第一原子层上,且大部分Al原子沿着Fe表面即x-y平面扩散,仅有极少数Al原子沿着z方向向下扩散。Al原子在不同Fe表面上的主要扩散通道并不相同:在Fe(001)面上Al原子沿[110]和[1-10]的扩散几率大致相同;在Fe(110)面上Al原子主要在Fe表面沿[001]方向扩散;在Fe(111)面上Al原子沿[1-10]、[1-01]和[01-1]的扩散几率大致相同。     

Dynamic Recrystallization and Precipitation Behavior of Mn-Cu-V Weathering Steel

The hot deformation behavior of a Mn-Cu-V weathering steel was investigated at temperatures ranging from 850 to 1050℃ and strain rates ranging from 0.01 to 5 s-1 using MMS-300 thermal-mechanical simulator. The activation energy for dynamic recrystallization and stress exponent were calculated to be 551 kJ/mol and 7.73, respectively. The accurate values of critical strain were determined by the relationship between work hardening rate and flow stress (θ-σ) curves. The hyperbolic sine constitutive equation was employed to describe the relationship between the peak stress and Zener-Hollomon parameter during hot deformation. The interaction between dynamic recrystallization and dynamic precipitation of V(C,N) at a low strain rate was analyzed. The results showed that precipitation particles size of weathering steel increased with increasing strain at deformation temperature 950℃ and strain rate 0.1 s-1. The calculation results of the recrystallization driving force and pinning force showed that dynamic precipitation could retard the progress of dynamic recrystallization but not prevent it while the pinning forces is less than driving force. On the contrary, dynamic precipitation can effectively prevent the progress of dynamic recrystallization.     

分子结构对高分子材料动态力学性能影响的研究进展

从材料的阻尼性能的表征出发,系统总结了高分子材料阻尼机理研究状况。主要以LA和TA分析法为例,从分子设计角度,总结阐述了近年来有关分子结构对高分子材料动态力学性能影响的研究成果和存在的问题,并对未来此方面的研究进行了简单的预测。     

Modelling of boron nitride: Atomic scale simulations on thin film growth

Molecular-dynamics simulations on ion-beam deposition of boron nitride are presented. A realistic Tersoff-like potential energy functional for boron nitride, which was specially fitted to ab initio-data, has been used. The impact of energetic boron and nitrogen atoms on a c-BN target is simulated with energies ranging from 10 to 600 eV. The structural analysis of the grown films shows that a loose, dominantly sp²-bonded structure arises at high ion flux. In no case the formation of a sp³-bonded phase is observed, but the obtained films partially reveal textured basal planes as found in experiment. Two different growth regimes are identified for ion energies above and below 100 eV.     

Molecular dynamics study of thermal properties of noble metals

Molecular dynamics simulations have been applied to investigate thermal properties of Ag and Au. Semi-empirical potentials, based on the embedded atom method (EAM) have been employed to calculate lattice parameter, energy per atom, mean square displacements and radial distribution function for the two metals. Thermal properties like specific heat, thermal coefficient of linear expansion and melting temperature are deduced from the calculated parameters. Results are found to compare well with the experimental results.     

Ultra-elastic and inelastic impact of Cu nanoparticles

The degree of elasticity for the impact of a particle with a rigid wall is normally characterized with the restitution parameter, R. We examine such impact behavior of Cu nanoparticles with molecular dynamics simulations, for different particle sizes (1-15 nm in radius) and impact velocities (25-200 m s^− 1). The impact can be ultra-elastic (R > 1) or inelastic (R < 1). Ultra-elastic or inelastic impact may occur for the smallest nanoparticles soly due to fluctuations, and the impact is inelastic but can be highly elastic (R ∼ 0.9-1) for larger sizes. R decreases with increasing size and impact velocity in general. Impact-induced structure transitions (e.g., dislocations) can be reversible and induce irreversible heating regardless of their reversibility. Such heating along with remnant plasticity is the key mechanism for impact inelasticity. Inelastic impact may occur with little remnant plasticity.     

Self-Irradiation Cascade Simulations in Plutonium Metal: Model Behavior at High Energy

The Modified Embedded Atom Method model for Pu metal is revised so that it more accurately captures the behavior of the Ziegler-Biersack-Littmark model of ion-ion interactions. Two revision are tested with somewhat different stiffnesses in the 2-1000 eV range. The revised models show higher damage levels at 20 KeV than an earlier model, suggesting that the behavior of the models above 100 eV is dominating damage production, at least in the earlier stages of the cascade. Work was performed at Los Alamos National Laboratory under the auspices of the US Department of Energy, under contract DE-AC52-06NA25396.     

Quantifying the Microstructures of Pure Cu Subjected to Dynamic Plastic Deformation at Cryogenic Temperature

A pure Cu (99.995 wt%) has been subjected to dynamic plastic deformation at cryogenic temperature to a strain of 2.1. Three types of microstructures that are related to dislocation slip, twinning and shear banding have been quantitatively characterized by transmission electron microscopy (TEM) assisted by convergent beam electron di?raction (CBED) analysis. Microstructures originated from dislocation slip inside or outside the shear bands are characterized by low angle boundaries (<15°) that are spaced in the nanometer scale, whereas most deformation twins are deviated from the perfect Σ3 coincidence (60°/<111>) up to the maximum angle of 9°. The quantitative structural characteristics are compared with those in conventionally deformed Cu at low strain rates, and allowed a quantitative analysis of the flow stress-structural parameter relationship.     

Failure analysis of a high-temperature thermocouple

A solid oxide fuel cell unit operates at high temperatures of 700 to 800°C. The operation of the unit is controlled by a set of instrumentation including sensors, thermocouples, and voltage leads. Two S-thermocouples were inserted in the after-burner where temperature is constantly at 1000°C. During operation of one prototype unit and after 1000 h, the thermocouples started to give erratic readings and led to complete shutdown of the unit. Analysis of the thermocouples revealed a series of events may have occurred due to stress induced by the design, materials and operating conditions, contamination during manufacturing, or a reducing environment caused by sheathing. The specification from S-type to B-type may remove the risk of reoccurrence. A design change in platinum burner thermocouples to ensure an oxygen-rich environment for the platinum was also recommended.     

Tensile Properties of Electrodeposited Nanocrystalline FCC Metals

Bulk nanocrystalline Ni and Ni-15wt%Fe alloy were fabricated via electrodeposition techniques. The nominal grain size of nickel samples was varied from 15 to 200 nm by employing different deposition parameters. The grain size was further reduced to 9 nm by alloying nickel with iron. The tensile properties were evaluated at room temperature using dog-bone shaped samples. The results of this study confirm that strength and strain hardening rate increase with decreasing grain size. The fracture behavior was found to depend on the grain size, presence of large and small defects, and the stress state. The tensile elongation and reduction in area varied significantly among the samples and did not correlate with the fracture behavior. Three categories of behavior were identified. In Type I the samples showed completely ductile fracture but very low tensile elongation. In Type II the samples showed a relatively brittle behavior but impressive tensile elongation. In Type III the samples showed ductile behavior with reasonable tensile elongation. In this article, the tensile elongation and the fracture mode of nanocrystalline face centered cubic (FCC) metals are discussed in terms of deformation behavior and presence of defects.