Influence of Interfacial Stress on Microstructural Evolution in NiAl Alloys

A phase-field model for the phase transition between austenite and martensite and twinning between two martensitic variants is presented from our previous theory [1] with the main focus on the influence of interfacial stress that is consistent with the sharp interface limit. Each variant-variant transformation can be represented by only one order parameter. Thus, it allows us to get the analytical solution of interface energy and width. Coupled phase-field and elasticity equations are solved for cubic-to-tetragonal phase transformation in NiAl shape memory alloy. The effects of interfacial stress are studied for martensite-martensite interfaces in detail, which was absent in [1]. Additionally, stress and temperature-induced growth of the martensitic phase inside austenite and twining are simulated. Some of the nontrivial experimentally observed microstructures reproduced in the simulations [1] are analyzed in detail. It includes tip splitting and bending, and twins crossing. This theory can be extended for electric, reconstructive, and magnetic phase transformations.

     

The lamellar-disorder interface: one-dimensional modulated profiles

We study interfacial behavior of a lamellar (stripe) phase coexisting with a disordered phase. Systematic analytical expansions are obtained for the interfacial profile in the vicinity of a tricritical point. They are characterized by a wide interfacial region involving a large number of lamellae. Our analytical results apply to systems with one dimensional symmetry in true thermodynamical equilibrium and are of relevance to metastable interfaces between lamellar and disordered phases in two and three dimensions. In addition, good agreement is found with numerical minimization schemes of the full free energy functional having the same one dimensional symmetry. The interfacial energy for the lamellar to disordered transition is obtained in accord with mean field scaling laws of tricritical points. Received: 28 March 1997 / Revised: 6 February 1998 / Accepted: 16 February 1998     

Bulk viscosity of hot dense Quark matter in the PNJL model

Starting from the Kubo formula and the QCD low energy theorem, we study the the bulk viscosity of hot dense quark matter in the PNJL model from the equation of state. We show that the bulk viscosity has a sharp peak near the chiral phase transition, and that the ratio of bulk viscosity over entropy rises dramatically in the vicinity of the phase transition. These results agree with those from the lattice and other model calculations. In addition, we show that the increase of chemical potential raises the bulk viscosity.     

Solid/liquid interfacial free energies in binary systems

The interfacial segregation and the free energy of segregation for solid/liquid interfaces between binary solutions are computed for the (111) boundary of face-centered-cubic crystals. A lattice-liquid interfacial model and pair-bonded regular solution model are employed in the treatment with an accommodation for liquid interfacial entropy. It is concluded that the zone of compositional transition across the interface is generally a few atomic layers in width and is moderately narrower for ideal solutions. The free energy of the segregated interface depends primarily upon the solid composition and the heats of fusion of the component atoms, the composition difference of the solutions, and the difference of the heats of mixing of the solutions. Master plots are presented for predicting the segregation and interfacial free energies in general binary systems.     

Computation of multiphase systems with phase field models

Phase field models offer a systematic physical approach for investigating complex multiphase systems behaviors such as near-critical interfacial phenomena, phase separation under shear, and microstructure evolution during solidification. However, because interfaces are replaced by thin transition regions (diffuse interfaces), phase field simulations require resolution of very thin layers to capture the physics of the problems studied. This demands robust numerical methods that can efficiently achieve high resolution and accuracy, especially in three dimensions. We present here an accurate and efficient numerical method to solve the coupled Cahn–Hilliard/Navier–Stokes system, known as Model H, that constitutes a phase field model for density-matched binary fluids with variable mobility and viscosity. The numerical method is a time-split scheme that combines a novel semi-implicit discretization for the convective Cahn–Hilliard equation with an innovative application of high-resolution schemes employed for direct numerical simulations of turbulence. This new semi-implicit discretization is simple but effective since it removes the stability constraint due to the nonlinearity of the Cahn–Hilliard equation at the same cost as that of an explicit scheme. It is derived from a discretization used for diffusive problems that we further enhance to efficiently solve flow problems with variable mobility and viscosity. Moreover, we solve the Navier–Stokes equations with a robust time-discretization of the projection method that guarantees better stability properties than those for Crank–Nicolson-based projection methods. For channel geometries, the method uses a spectral discretization in the streamwise and spanwise directions and a combination of spectral and high order compact finite difference discretizations in the wall normal direction. The capabilities of the method are demonstrated with several examples including phase separation with, and without, shear in two and three dimensions. The method effectively resolves interfacial layers of as few as three mesh points. The numerical examples show agreement with analytical solutions and scaling laws, where available, and the 3D simulations, in the presence of shear, reveal rich and complex structures, including strings.     

计入固液界面作用的润滑热力学模型与分析

摩擦与润滑过程是典型的能量耗散过程, 在机理上与非平衡热力学中的熵增、耗散结构等理论颇有相似之处. 通过热力学分析可以对一些典型的摩擦磨损过程做出合理的机理揭示与推测. 本文利用热力学理论对典型的润滑过程进行了建模分析. 采用分离压模型表征和计入了微尺度下的固液界面作用, 揭示分析了润滑热力学模型与润滑状态Stribeck曲线的联系. 从分析计算结果来看, 润滑Stribeck曲线的摩擦系数最低点与系统热力学上的熵增率最低点具有相当好的对应关系, 而润滑状态从弹流润滑向薄膜润滑的转变过程, 可以用耗散结构理论加以机理解释. 文中的热力学模型和方法能够有效地体现出润滑过程中多物理要素跨尺度非线性耦合的作用, 对实际工程与实验有着重要的指导作用.     

Thermal Effects of Interfacial Dynamics

Dynamical Ginzburg-Landau theory is applied to the study of thermal effects of motion of interfaces that appear after different phase transitions. These effects stem from the existence of the surface internal energy, entropy and temperature gradients in the interfacial transition region. Evolution equations for the interfacial motion are derived. For the experimental verification of the thermal effects the expression is derived for the amplitude of temperature waves during continuous ordering.     

Force-field dependence on the interfacial structure of oil–water interfaces

We investigate the performance of different force-fields for alkanes, united (TraPPE) and all atom (OPLS-AA) models, and water (SPC/E and TIP4P-2005), in the prediction of the interfacial structure of alkane (n-octane, and n-dodecane)–water interfaces. We report an extensive comparison of the interfacial thermodynamic properties as well as the interfacial structure (translational and orientational). We use the recently introduced intrinsic sampling method, which removes the averaging effect of the interfacial capillary waves and provides a clear view of the interface structure. The alkane interfacial structure is sensitive to the environment, i.e. alkane–vapour or alkane–water interfaces, showing a stronger structure when it is in contact with the water phase. We find that this structure is fairly independent of the level of detail, full or united atom, employed to describe the alkane phase. The water surface properties show a small dependence on the water model. The dipole moment of the SPC/E model shows asymmetric fluctuations, with a tendency to point both towards the alkane and water phases. On the other hand the dipole moment of the TIP4P-2005 model shows a tendency to point towards the water phase only. Analysis of the intrinsic electrostatic field indicates that the surface water potential is confined to an interfacial region of about 8 Å. Overall we find that the intrinsic structure of alkane–water interfaces is a robust interfacial property, which is independent of the details of the force-field employed. Hence, it should provide a good reference to interpret experimental data.     

Morphological Aspects of In-Situ Compatiblized Binary Polymer Blends With Variable Viscosity Ratios of Components

The in-situ compatiblized binary polymer blend polypropylene(PP)/polystyrene(PS)/ anhydrous aluminum chloride(AlCl₃) was selected as a model system of a reactive polymer blend to investigate the effect of viscosity ratio of components at a constant shear rate on the phase morphological behavior in in-situ compatibilized systems. The results showed that the well-known interfacial compatibilization effect was related to variations of viscosity ratios of components in the reactive PP/PS blends with different contents of AlCl₃ catalyst. The phase morphology evolution of the in-situ compatiblized reactive blend was determined by both the interfacial compatibilization and the variation of the viscosity ratio of components under the fixed mixing conditions, which showed characteristics obviously different from and much more complex than those in binary polymer blends generally compatiblized by added compatiblizers. The results implied that the variation of the viscosity ratio of components should be checked carefully and taken into account if necessary, when the phase morphology of binary polymer blends is investigated, especially in complex in-situ compatiblized reactive polymer blends.     

Nematic liquid crystals at rough and fluctuating interfaces

Nematic liquid crystals at rough and fluctuating interfaces are analyzed within the Frank elastic theory and the Landau–de Gennes theory. We study specifically interfaces that locally favor planar anchoring. In the first part we reconsider the phenomenon of Berreman anchoring on fixed rough surfaces, and derive new simple expressions for the corresponding azimuthal anchoring energy. Surprisingly, we find that for strongly aligning surfaces, it depends only on the geometrical surface anisotropy and the bulk elastic constants, and not on the precise values of the chemical surface parameters. In the second part, we calculate the capillary waves at nematic-isotropic interfaces. If one neglects elastic interactions, the capillary wave spectrum is characterized by an anisotropic interfacial tension. With elastic interactions, the interfacial tension, i.e., the coefficient of the leading q² term of the capillary wave spectrum, becomes isotropic. However, the elastic interactions introduce a strongly anisotropic cubic q³ term. The amplitudes of capillary waves are largest in the direction perpendicular to the director. These results are in agreement with previous molecular dynamics simulations.     

The influence of graded interfaces in the electronic spectrum of nanometer silicon dots

The effects imposed to the electron energy levels of Si/SiO₂ quantum dots by the presence of smooth graded interfaces and interfacial carriers trap is studied. For small diameter quantum dots, while the existence of graded interfaces strongly blue shifts the carrier energy states (up to a few hundred meV), the effect of the interfacial carriers trap is to red shift the energies, but to a lesser extend (under 50 meV). In addition, slight changes in the distance of the carriers trap in relation to the center of the dot does not alter significantly the energy spectrum.     

Rheological Characterization of in situ Compatibilized Binary Polymer Blends With Variable Steady Shear Viscosities of Components

Zero shear viscosities of binary polymer blends, η_0,bc, of polypropylene (PP)/polystyrene (PS), in situ compatibilized by anhydrous aluminum chloride (AlCl₃) catalyst, were obtained by fitting their shear rate sweep curves according to the modified Carreau model. The results showed that the dependence of η_0,bc on AlCl₃ content was complicated and obviously influenced by viscosity variations of the components as well as the interfacial compatibilization effect of the in situ formed PP-g-PS copolymer. For further investigation, η_0,bc was divided into three parts: contribution of the viscosity of components, contribution of phase geometry, and contribution of the interfacial compatibilization effect. The results showed that when the apparent value of the third part was experimentally determined, the significant influence of viscosity variations of the components had to be considered, while the influence of phase morphology geometry resulting from viscosity variations of the components could be ignored experimentally and reasonably within the whole experimental range of AlCl₃ content. The contribution of the interfacial compatibilization effect to η_0,bc could be used as the rheological parameter to characterize the interfacial character and could be used to interpret the variations of η_0,bc of the in situ compatibilized polymer blend successfully. In addition, η_0,bc is more sensitive to the shear viscosity variations of the components than the phase structure geometry evolution of the reactive blends.     

Free energy of semiflexible polymers and structure of interfaces

The free energy of semiflexible polymers is calculated as a functional of the compositional scalar order parameter and the orientational order parameter of second-rank tensor S_ij on the basis of a microscopic model of wormlike chains with variable segment lengths. We use a density functional theory and a gradient expansion to evaluate the entropic part of the free energy, which is given in a power series of .The interaction term of the free energy is derived with a random phase approximation. For the rigid rod limit, the nematic-isotropic transition point is given by , N and w being the degree of polymerization and the anisotropic interaction parameter, respectively, and the degree of ordering at the transition point is 0.33448. We also find that the contour length of polymer chains becomes larger in a nematic phase than in an isotropic phase. Interface profiles are obtained numerically for some typical cases. In the neighborhood of isotropic-isotropic interfaces, polymer chains tend to align parallel to the interface on the polymer-rich side and perpendicular on the poor side. When an isotropic region and a nematic region coexist, orientational order parallel to the interface is preferred in the nematic region. Received: 28 May 1998 / Revised: 12 August 1998 / Accepted: 8 September 1998     

Theory of the Ising model on a Cayley tree

The Ising model on a Cayley tree is known to exhibit a phase transition of continuous order. In this paper we present a complete and quantitative analysis of the leading singular term in the free energy which is associated with this phase transition. We have been able to solve this problem by considering the distribution of zeros of the partition function. The most interesting new feature in our results is a contribution to the free energy which performs singular oscillations as the magnetic field approaches zero.     

Internal friction related to viscous motion of phase interfaces during thermoelastic martensitic transformation

Two internal friction peaks were observed in CuAlNiMnTi polycrystalline shape memory alloy by a partial phase transition method in non-isothermal measurements. The low-temperature internal friction peak arising from the soft mode effect caused by the viscous movement of atoms along phase interfaces was studied in the present study in terms of internal friction fundamentals. An internal friction model related to the peak was established, which was shown to be in good agreement with experimental results.     

Nd0.7Sr0.3MnO3陶瓷中界面“陷阱态”相关电阻转变行为

通过固相烧结和高能球磨后热处理两种方法分别得到不具晶(相)界和具有明显晶(相)界的两种Nd_0.7Sr_0.3MnO₃陶瓷样品, 并用两线法和四线法分别对这两种样品的电极-块体接触界面和晶(相)界界面的I-V和电脉冲诱导电阻转变效应(EPIR)进行研究. 结果发现, 在两线法测试下, 电极-块体界面具有回滞的非线性I-V特征, 并能产生稳定的EPIR效应, EPIR的稳定性随温度的升高逐渐减弱并消失; 而对具有明显晶(相)界的陶瓷样品, 四线法测试结果表明, 虽然其I-V行为也具有非线性和回滞性特点, 但不能产生EPIR 效应. 这些奇特的界面输运行为与界面中的各种缺陷充当“陷阱”并实现对载流子的捕捉和释放过程密切相关. 而大量的晶(相)界界面及其复杂的连接方式导致较大的漏导则是晶(相)界不能出现EPIR效应的主要原因.     

Critical Quantum Metrology in the Non-Linear Quantum Rabi Model

The quantum Rabi model (QRM) with linear coupling between light mode and qubit exhibits the analog of a second-order phase transition for vanishing mode frequency which allows for criticality-enhanced quantum metrology in a few-body system. We show that the QRM including a nonlinear coupling term exhibits much higher measurement precisions due to its first-order-like phase transition at finite frequency, avoiding the detrimental slowing-down effect close to the critical point of the linear QRM. When a bias term is added to the Hamiltonian, the system can be used as a fluxmeter or magnetometer if implemented in circuit QED platforms.     

Irradiation-induced amorphization processes in nanocrystalline solids

A theoretical model is suggested which describes irradiation-induced amorphization in nanocrystalline solids, using the rate theory approach. In the framework of the model, interfaces (grain boundaries) cause the two basic effects on irradiation-induced damage and amorphization processes in nanocrystalline solids where the volume fraction of the interfacial phase is extremely large. First, amorphization is enhanced in nanocrystalline solids, because high-density ensembles of interfaces essentially contribute to the total energy of the crystalline state and thereby provide a shift in the energetics of amorphization. Second, interfaces serve as effective sinks of irradiation-produced point defects and thereby hamper amorphization driven by defect accumulation. The competition between these effects is described by kinetic equations for densities of point defects in nanoscale grains in nanocrystalline solids under irradiation treatment. This competition is shown to be responsible for the specific behavior of irradiated nanocrystalline solids, which is different from that of their coarse-grained counterparts. The suggested model accounts for the experimental data reported in the literature.PACS 61.46.+w; 61.72.Cc; 61.80.Az