双边扩散枝晶生长的定量相场模型

本文针对非对称双边扩散条件下的二元合金枝晶生长,建立了一个包含溶质截流项的定量相场模型．本模型耦合了非线性热力学函数并采用化学势相等的界面条件．通过对相场方程进行二阶的薄界面渐进分析,并结合溶质拖拽模型,推导出相场迁移率和溶质截流项．随后将模型简化为二元稀溶液合金等温枝晶生长的相场模型以对其进行理论验证．通过在各种相场界面厚度条件下进行数值模拟,测试了本模型的数值收敛性．用所建立的模型模拟了Fe-0．15m01％C合金的等温枝晶生长,将相场模拟结果和经典Gibbs—Thomson关系,线性可解性理论以及改进的Lipton—Glicksman—Kurz（LGK）解析模型进行比较,取得了良好的符合．模拟结果表明本模型能有效地消除延拓的界面厚度所导致的界面异常效应,具有良好的定量模拟能力．而且,本模型能够定量地描述从单边扩散到对称扩散的各种固相扩散迁移率条件下的枝晶生长．     

二元合金微观偏析的相场法数值模拟

使用耦合溶质场的相场模拟研究了Ni-Cu二元合金枝晶生长过程中固相溶质扩散系数D_s 对枝晶形貌和微观偏析等的影响.计算结果表明，随着D_s的减少，固液界 面前沿溶质扩散层越薄，枝晶生长越有利，枝晶尖端生长的速度越大，侧向分支越发达；D< sub>s越小，固相中溶质成分的波动越强烈，随着D_s的增大，固相中溶质 成分的波动相应减小；溶质微观偏析程度随D_s的增大而减小. 关键词： 相场方法 微观偏析 固相扩散系数 数值模拟     

等温凝固多晶粒生长相场法模拟

采用Kim模型，利用耦合溶质场的相场模型对Al-2-mole-Cu合金等温凝固过程中多晶粒相互影响下枝晶的生长过程进行数值模拟，为了提高计算效率，采用差分去实现宏微观场之间的耦合.研究了不同过冷度对多晶粒枝晶形貌和溶质分布的影响，结果表明：成分过冷对枝晶生长速度和溶质分配有着重大的影响，溶质元素Cu在固液界面前沿重新分布，结果导致实际热过冷减小，进而影响枝晶的生长和溶质向外层的扩散，致使相互接触的枝晶产生萎缩而其余没有受到抑制的枝晶生长方向产生择优现象. 关键词： 相场法 模拟 择优生长 等温凝固 二元合金     

A phase-field model for simulating various spherulite morphologies of semi-crystalline polymers

A modified phase-field model is proposed for simulating the isothermal crystallization of polymer melts. The model consists of a second-order phase-field equation and a heat conduction equation. It obtains its model parameters from the real material parameters and is easy to use with tolerable computational cost. Due to the use of a new free energy functional form, the model can reproduce various single crystal morphologies of polymer melts under quiescent conditions, including dendritic, lamellar branching, ring-banded, breakup of ring-banded, faceted hexagonal, and spherulitic structures. Simulation results of isotactic polystyrene crystals demonstrate that the present phase-field model has the ability to give qualitative predictions of polymer crystallization under isothermal and quiescent conditions.     

A new algorithm for anisotropic solutions

We establish a new algorithm that generates a new solution to the Einstein field equations, with an anisotropic matter distribution, from a seed isotropic solution. The new solution is expressed in terms of integrals of an isotropic gravitational potential; and the integration can be completed exactly for particular isotropic seed metrics. A good feature of our approach is that the anisotropic solutions necessarily have an isotropic limit. We find two examples of anisotropic solutions which generalise the isothermal sphere and the Schwarzschild interior sphere. Both examples are expressed in closed form involving elementary functions only.     

Fast solute diffusion in solid and supersaturated solid solutions

In this paper, we review the experimental data concerning fast diffusion of solute in Pb matrix and Pb (solute) solid solutions. Using both calorimetric and radiotracer techniques the temperature dependence of solute diffusion coefficients in Pb (solute) supersaturated solid solutions is determined. On the basis of experimental results it is shown that these coefficients can only be understood by assuming different defect states for solutes in solid and supersaturated solid solutions.     

多晶薄膜屈服强度的一个模型

从位错运动的应力功和应变能关系导出了附着在基体上并有钝化层薄膜的屈服强度公式.该式表明多晶薄膜的屈服强度由两个影响因子（晶粒取向和位错类型）和三个强化因子（钝化层强化，基体强化和晶粒强化）确定.和已报道的实验结果基本一致表明了该模型的合理性. 关键词： 多晶薄膜 屈服强度     

On definitions and assumptions in the dislocation theory for solid solutions

A recent paper criticized the standard treatment of Cottrell atmospheres, relevant to solid-solution hardening. We show that the treatment in current texts is correct within the standard assumptions of dislocation theory. Nonlinear treatments of the atmosphere are discussed. We also show that no current theory of such atmospheres includes complete nonlinear screening of the dislocation strain field.     

A numerical algorithm for the solution of a phase-field model of polycrystalline materials

We describe an algorithm for the numerical solution of a phase-field model (PFM) of microstructure evolution in polycrystalline materials. The PFM system of equations includes a local order parameter, a quaternion representation of local orientation and a species composition parameter. The algorithm is based on the implicit integration of a semidiscretization of the PFM system using a backward difference formula (BDF) temporal discretization combined with a Newton–Krylov algorithm to solve the nonlinear system at each time step. The BDF algorithm is combined with a coordinate-projection method to maintain quaternion unit length, which is related to an important solution invariant. A key element of the Newton–Krylov algorithm is the selection of a preconditioner to accelerate the convergence of the Generalized Minimum Residual algorithm used to solve the Jacobian linear system in each Newton step. Results are presented for the application of the algorithm to 2D and 3D examples.     

体心立方固溶体合金中的"团簇+连接原子"结构模型

为体现固溶体合金中的溶质原子产生的化学短程序,文章提出了配位数为14的团簇在体心立方(bcc)点阵中的堆垛模式,并建立了基于bcc结构的"团簇+连接原子"结构模型,用团簇成分式[团簇](连接原子)_x表述. 此模型中,与基体组元具有相对大的负混合焓的溶质原子占据团簇心部,其他原子作为连接或者替代团簇壳层基体原子. 1 ∶1结构模型[团簇](连接原子)₁由于最大程度地保证了团簇与连接原子的近邻,构成了连接原子最有效的合金化方式. 在两个实用bcc固溶体合金体系中, 1 ∶1模型指导设计了低V含量的储氢合金 V₁和低弹性模量高强度的 Nb₁合金. 关键词： 体心立方固溶体成分设计 "团簇+连接原子"结构模型 Ti-Cr-V合金 Ti-Zr-Mo-Nb合金     

BST solid solutions, temperature evolution of the ferroelectric transitions

Solid solutions Ba_1 − xSr_xTiO₃ (BST) are of high technological importance, particularly in microwave domain. Barium titanate has “naturally” three transitions, between four stable ferroelectric phases: (C) cubic, (T) tetragonal, (O) orthorhombic, (R) rhombohedral. Jaffe et al. [B. Jaffe, W.R. Cook, H. Jaffe, Piezoelectric Ceramics, Academic Press, 1971] has given the dependence of the transition temperatures up to 30% of Sr content. We have extrapolated these temperatures and we have found that some phases might disappear at higher Sr concentrations. A family of solid solutions with x = 25, 50, 75, 90% was prepared by standard solid-state reaction and sintered at 1230 and 1260 °C, respectively. The permitivities and the dielectric losses were measured with a self-acting bridge (1 kHz), on a large temperature range (±200 °C). The composition x = 25% shows three peak values of permittivity as expected, while the composition x = 50%, only two peak values, corresponding to phase transitions cubic-tetragonal-rhombohedral, phase orthorhombic being excluded. Compositions with x ≥ 63%, Sr shows only one peak value corresponding to a genuine transition cubic-rhombohedral. The cubic transition to several lower phases shows almost a linear decrease of the Curie point with the increase of Sr fraction. For Sr concentration x ≥ 80%, the Curie point appears to fall more rapidly than linear. To our best knowledge, there is for the first time, this effect is reported.     

A lattice-gas hamiltonian for micellar binary solutions

An explicit Ising-spin lattice Hamiltonian is proposed as a model for studying the phase diagrams of micellar binary solutions on the micellar length scale. Incorporating many essential features, it can be used to check the validity of a previously suggested scenario for nonuniversality at the consolute point.     

固溶态染料激光物质的研究

本文从不同改进的掺Rh6G聚甲基丙烯酸甲脂(PMMA)固溶态物质获得了有效的激光.热稳定性得到改善以及工作寿命得到延长.研究了它们的激射和光谱特性.     

面心立方固溶体合金的团簇加连接原子几何模型及典型工业合金成分解析

工业合金牌号的成分选择体现了固溶体合金的化学短程有序结构,满足由最近邻两层原子组成的"团簇加连接原子"模型,例如对于置换型面心立方固溶体Cu-Zn,其合金牌号成分可以表述为[Zn-Cu_(12)]Zn_(1-6)和[Zn-Cu_(12)](Cu,Zn)_6,其中方括号内为第一近邻配位多面体团簇.基于此,本文赋予团簇式以具体原子结构的含义,对置换型面心立方固溶体结构中团簇的可能存在形式进行了穷尽,得出团簇式所对应的所有"团簇加连接原子"结构单元模型,给出团簇和连接原子之间的比例和空间排列的所有可能,并对Cu-Zn和Cu-Ni合金常用牌号对应的团簇式给出了三维结构模型,进一步验证了前期关于合金团簇式解析的正确性.用这些模型中原子化学序描述合金的成分,赋予团簇式以具体的原子结构意义,为进一步开发新的合金提供了理论依据.     

Modelling of short-range ordering kinetics in dilute multicomponent substitutional solid solutions

ABSTRACT

Short-range ordering as the formation of couples and pairs between solutes is affected by their formation energies. This is not reflected in the standard regular solution model. Here, we present a new thermodynamic model which accounts for the dependence of the molar Gibbs energy on the concentrations of couples and pairs and their formation energies. The model treats kinetics of couples and pairs formation controlled by diffusion. This new model uses tracer diffusion coefficients of solutes and bond formation energies, which can be taken from ab initio calculations. Insofar, the current concept bridges the gap between ab initio methods and non-equilibrium thermodynamics. The reliability of the model is checked by comparison with kinetic Monte Carlo simulations. The model is applied to an Al-Mg-Si-Cu system. Finally, the configurational entropy for a binary system evaluated with the current model is compared with Bethe’s approximation, which allows estimating of applicability limits of the current model.     

Compositional dependence of serrated flow in nickel binary solid solutions during high-temperature microindentation

At 450?°C, pure nickel and nickel binary solid solutions, containing Ru, Rh, W, Re, Ir and Pt, have a smooth load–depth curves. In contrast, the alloying addition of Nb, Mo, Pd and Ta results in the occurrence of serrations in their load–depth profiles. Analyses show the load increment of the quasi-elastic segments scales linearly with the load at their starting point. The slope of such linear relationships was normalized by the solute concentration and then plotted against the diffusion rate of transition metal solutes within the nickel lattice and along dislocation cores. Linear relationships were observed in both cases. Results suggest that the solute diffusion along dislocation cores coupled with the atomic size misfit are mainly responsible for such serrations.     

Effects of internal stresses and intermediate phases on the coarsening of coherent precipitates: A phase-field study

Phase stability, topology and size evolution of precipitates are important factors in determining the mechanical properties of crystalline materials. In this article, the Cahn-Hilliard type of phase-field model was coupled to elasticity equations within a mixed-order Galerkin finite element framework to study the coarsening morphology of coherent precipitates. The effects of capillarity, particle size and fraction, compositional strain, and inhomogeneous elasticity on the kinetics and kinematics of coherent precipitates in a binary dual phase crystal admitting a third intermediate stable/meta-stable phase were investigated. The results demonstrated the ability of the model to simulate coarsening under the concomitant action of Ostwald ripening and mismatch elastic strain mechanisms. Using a phenomenological coarsening power law, coarsening rates were determined to depend on precipitate size and volume fraction, compositional strain, and strain mismatch between precipitates and the matrix. Results also showed that the necking incubation time between two neighboring precipitates depends inversely on the precipitate’s initial sizes; however, under fixed volume fraction of precipitates, any increase in the initial sizes of the precipitates mitigates the coarsening. Meanwhile, the compositional strain and the growth of the intermediate stable/meta-stable phase leads to substantial enhancements of precipitate coarsening.