Phase field model for strong interfacial energy anisotropy of HCP materials

Based on the Karma model and the Eggleston regularization technique of the strong interfacial energy anisotropy, a phase-field model was established for HCP materials. An explicit finite difference numerical method was used to solve phase field model and simulate the dendrite growth behaviors of HCP materials. Results indicate that the dendrite morphology presents obvious six-fold symmetry, and discontinuity in the variation of interface orientation occurs, resulting in a fact that the corners were formed at the tips of the main stem and side branches. When the interfacial energy anisotropy strength is lower than the critical value（1/35）, the steady-state tip velocity of dendrite increases with anisotropy as expected. As the anisotropy strength crosses the critical value, the steady-state tip velocity drops down by about 0.89%. With further increase in anisotropy strength, the steady-state tip velocity increases and reaches the maximum value at anisotropy strength of 0.04, then decreases.

Simulation of facet dendrite growth with strong interfacial energy anisotropy by phase field method

Numerical simulations based on a new regularized phase-field model were presented, to simulate the solidification of hexagonal close-packed materials with strong interfacial energy anisotropies. Results show that the crystal grows into facet dendrites,displaying six-fold symmetry. The size of initial crystals has an effect on the branching-off of the principal branch tip along the<100> direction, which is eliminated by setting the b/a(a and b are the semi-major and semi-minor sizes in the initial elliptical crystals, respectively) value to be less than or equal to 1. With an increase in the undercooling value, the equilibrium morphology of the crystal changes from a star-like shape to facet dendrites without side branches. The steady-state tip velocity increases exponentially when the dimensionless undercooling is below the critical value. With a further increase in the undercooling value, the equilibrium morphology of the crystal grows into a developed side-branch structure, and the steady-state tip velocity of the facet dendrites increases linearly. The facet dendrite growth has controlled diffusion and kinetics.

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.

The effect of interfacial energy anisotropy on planar interface instability in a succinonitrile alloy under a small temperature gradient

The morphological stability of a planar interface with different crystallographic orientations is studied under a small positive temperature gradient using a transparent model alloy of succinonitrile. Novel experimental apparatus is constructed to provide a temperature gradient of about 0.37 K/mm. Under this small temperature gradient, the planar interface instability depends largely on the crystallographic orientation. It is shown experimentally that the effect of interfacial energy anisotropy on planar interface stability cannot be neglected even in a small temperature gradient system. Higher interfacial energy anisotropy leads the planar interface to become more unstable, which is different from the stabilizing effect of the interfacial energy on the planar interface. The experimental results are in agreement with previous theoretical calculations and phase field simulations.

Controlling upconversion through interfacial energy transfer(IET):Fundamentals and applications

Photon upconversion has received substantial attention owing to its great promise in broad applications from bioimaging to other frontier fields like display,upconversion laser,information security and anticounterfeiting.A smart control and manipulation of the upconversion luminescence has always been a key topic,however,to date the most efficient mechanism for upconversion nanoparticles remains the energy transfer upconversion and recently reported energy migration mediated upconversion.Recently,we found that the interfacial energy transfer(IET)is also an efficient approach for enabling and tuning photon upconversion of lanthanide ions.Moreover,it can be used for the mechanistic understanding of the interionic interactions such as energy transfer and energy migration on the nanoscale.In this review,the recent advances of the research on the IET are summarized,the principles for designing IET process and typical examples are discussed together with its applications in both mechanistic research and frontier information security.The challenges and perspectives for future research are also commented.

THEORETICAL CALCULATION OF SPECIFIC INTERFACIALENERGY OF SEMICOHERENT INTERFACE BETWEEN MICROALLOY CARBONITRIDES AND AUSTENITE

According to the misfitting dislocation theory,a method of theoretical calculation was devel- oped for the specific energy of the semicoherent interface between microalloy carbonitrides and austenite matrix.The calculating formulae were derived and the results were satisfactorily applied on the research works.

Atomistic calculations of surface and interfacial energies of Mg_(17)Al_(12)-Mg system

It is well known that precipitation hardening in magnesium(Mg)alloys is far less effective than in aluminum alloys.Thus,it is important to understand the surface and interfacial structure and energetics between precipitates and matrix.In upscale modeling of magnesium alloys,these energy data are of great significance.In this work,we calculated the surface and interfacial energies of Mg_(17)Al_(12)-Mg system by carefully selecting the surface or interface termination,using atomistic simulations.The results show that,the higher fraction of Mg atoms on the surface,the lower the surface energy of Mg_(17)Al_(12).The interfacial energy of Mg/Mg_(17)Al_(12)was calculated in which the Burgers orientation relationship(OR)was satisfied.It was found that the(011)P|(0002)Mg interface has the lowest interfacial energy(248 mJ/m 2).Because the Burgers OR breaks when{10¯12}twin occurs,which reorients the matrix,the interfacial energy for Mg_(17)Al_(12)and a{10¯12}twin was also calculated.The results show that after twinning,the lowest interfacial energy increases by 244 mJ/m^(2),and the interface becomes highly incoherent due to the change in orientation relationship between Mg_(17)Al_(12)and the matrix.

Helical Configurations of Elastic Rods in the Presence of Interfacial Traction

The Kirchhoff thin elastic rod models are always the important basis to explore the configuration mecha- nism of the flexible structures in both the macroscopic and microscopic scale. As a continuum model of DNA, a thin elastic rod subjected to interfacial interactions is used to investigate the helical equilibrium configuration of DNA in salt solution. In this paper, the Kirchhoff＇s equations in the presence of interracial traction and the free energy density functions of different configurations are studied. The transition formula of the free energy between B-DNA and Z- DNA is obtained, and the results show that the free energy of the transition is mainly determined by the salt concentra- tion, which agrees well with the experimental data.

Penny-shaped interfacial crack between dissimilar magnetoelectroelastic layers

A penny-shaped interfacial crack between dissimilar magnetoelectroelastic layers subjected to magnetoelectromechanical loads is investigated,where the magnetoelectrically impermeable crack surface condition is adopted. By using Hankel transform technique,the mixed boundary value problem is firstly reduced to a system of singular integral equations,which are further reduced to a system of algebraic equations. The field intensity factors and energy release rate are finally derived. Numerical results elucidate the eects of crack configuration,electric and/or magnetic loads,and material parameters of the magnetoelectroelastic layers on crack propagation and growth. This work should be useful for the design of magnetoelectroelastic composite structures.

Microstructure evolution of hot pressed AZ91D alloy chips reheated to semi-solid state

AZ91D magnesium alloy chips, which were directly collected on the spot of machining process, were recycled to prepare billet via hot pressing for semi-solid processing. The semi-solid microstructure evolution of the billet during reheating was investigated. The results indicate that there are three stages during reheating to semi-solid state: the dissolution of Mg17Al12 and diffusion of Al into α-Mg matrix, the melting of the region with high content of solute and formation of isolated solid particles, and spheroidization and growth of solid particles. Meanwhile, a number of entrapped liquid droplets form within solid particles. In addition, the number and size of entrapped liquid droplets rely on the holding time in the semi-solid temperature range. With increasing isothermal holding time, the solid fraction remains unchanged when the solid-liquid system reaches the dynamic equilibrium at last, while the solid particles become more globular and the average size of solid particles increases owing to the decreasing of interfacial energy and the effect of interfacial tension.

Kinetics of reactive wetting of graphite by liquid Al and Cu-Si alloys

In order to reveal the physical essence of the spreading process of reactive wetting,a sort of model of energy to explain the driving force and wetting mechanism was presented.The reactive wetting of molten A1 and Cu Si on graphite was studied by a modified sessile drop method under a vacuum,in which the contact angles were measured by ADSA software.The thermodynamic and kinetic processes of the typical reactive wetting were focused on,the thermodynamic equations of energy relations were derived,the interfacial energy of graphite and solid-liquid interfacial energy versus time at the triple line were calculated,and the dynamics model of interface energy is established.The presented dynamics model is verified by means of experimental results,and it is shown that solid liquid interfacial energy decreases with time in exponential relationship.It provides a new method for reference to explain the process from the angle of energy.

RAFTING PREDICTION CRITERION FOR NICKEL-BASE SINGLE CRYSTALS UNDER MULTIAXIAL STRESSES AND CRYSTALLOGRAPHIC ORIENTATION DEPENDENCE OF CREEP BEHAVIOR

Based on the relief of interfacial energy density by dislocation generation at the gamma - gamma ' interfaces, a rafting prediction criterion has been developed for nickel-base single crystals under multiaxial stresses. The diagrams of rafting have been presented, and confirmed by experimental results. The rafting process have been analyzed quantitatively by the relief of interfacial energy. The criterion has been applied to study the creep behavior. The example of creep life analysis shows that the criterion can be correlated greatly to the crystallographic orientation dependence of creep behavior. (Author abstract) 11 Refs.

THEORETICAL PREDICTION OF THE KINETICS CURVES OFPEARLITIC TRANSFORMATION IN HYPO-PROEUTECTOID STRUCTURAL STEELS

Supposing carbon contents of ferrite phases in pearlite precipitating from austenite in multicomponent steel at temperature T and in Fe-C ystem at T' are the same the pearlite formation temperature diference, can be calculated from the FeX phase diagrams and the equilibrium temperature Al. Using Tp and Fe-C binary thermodynamic model, the driving forces for phase transformation from austenite to pearlite in multicomponent steels have been successfully calculated. Through the combination of simplified Zener and Hillert's model for pearlite growth with Johnson-Mehl equation, using data from known TTT diagrams, the interfacial energy parameter and activation energy for pearlite formation can be determined and expressed as functions of chemical composition in steels by regression analysis. The calculated starting curves of pearlitic transformation in some commercial steels agree well with the experimental data.

Simulations of Coarsening Behavior for M_(23)C_6 Carbides in AISI H13 Steel

Based on the local equilibrium assumption, coarsening behavior of M23C6 carbide at 700℃ in H13 steel was simulated by DICTRA software. The results from the calculations were compared with transmission electron microscopy （TEM） observations. The results show the interracial energy for M23C6 in H13 steel at 700℃ is thus probably 0.7J·m^-2, which fits the experiments well. The influence of composition and temperature on the coarsening rate was also investigated by simulations. Simulations show a decrease in the coarsening rate when V/Mo ratio is increased, while the coarsening rate increases with increasing temperature.

An improved model for predicting hydrate phase equilibrium in marine sediment environment

Based on the models of hydrate phase equilibrium in bulk water and porous media,an improved model was proposed to predict the methane hydrate equilibrium in marine sediment environment.In the suggested model,mechanical equilibrium of force between the interfaces in hydrate-liquid-vapor system was considered.When electrolyte was present in pore water,interfacial energy between hydrate and liquid was corrected by an equation that is expressed as the function of temperature and electrolyte concentration.The activity of water is calculated based on the Pitzer model and the interfacial energy between liquid and gas is solved using the Li method.The prediction results show good agreement with the experimental data.By comparison with other models,it is proved that this model can improve the accuracy for predicting hydrate phase equilibrium in marine sediment environment.

High interfacial-energy heterostructure facilitates large-sized lithium nucleation and rapid Li+desolvation process

High interfacial energy Li^(0)-electrolyte interface contributes to larger Li^(0) nucleation embryos and a more stable interface,so the interfacial energy is essential for highly reversible Li^(0) deposition/stripping.Herein,a high interfacial-energy artificial solid electrolyte interphase(SEI)with rich LiF embedded in lithiated poly-2-acrylamido-2-methylpropane sulfonic acid(PAMPS-Li)network is designed to realize favorable Li^(0) nucleation and rapid desolvation of Li+simultaneously.The Li-F bonds in LiF(001)exhibit stronger ion-dipole interactions with Li atoms,offering higher interfacial energies.When the growth surface energy and total interfacial energy of Li^(0) are balanced,the high interfacial energy SEI with abundant LiF can promote the formation of larger Li^(0) nucleation embryos.In addition,the PAMPS-Li with immobilized anions presents weaker interaction with Li^(0) and possesses higher polymer-Li interfacial energy,and its amide and sulfonic acid groups exhibit higher binding energies with Li^(+).Therefore,PAMPS-Li can easily promote the Li+to escape from the solvent sheath and weaken the desolvation energy barrier.The highly reversible Li^(0) deposition behavior with restricted side reactions is achieved based on the synergistic modification of high interfacial energy SEI with heterostructure.Most importantly,lifespan of multi-layered Li^(0) pouch cell(330 Wh kg-1)with a low N/P ratio(1.67)is over 100 cycles,verifying its potential practical application.

EVOLUTION OF VOIDS IN DUCTILE POROUS MATERIALS AT HIGH STRAIN RATE

In the present work, a dynamic damage model in ductile materials under the application of dynamic general stresses loading is presented. The evolution equation of ductile voids has the closed form, in which work-hardening, the change of surface energy of voids, rate-dependent, inertial effects are taken into account. The expressions of critical stresses for the growth and compaction of voids are directly obtained from the evolution equations of voids. Numerical analysis of the model indicates that the growth of voids is sensitive to the strain rates. The voids grow quickly as the increase of strain rates. It is also shown that the influence of the inertial effects on the void growth is great at high loading rates. It appears to resist the growth of voids. In addition, a dynamic collapse model of ductile voids is also proposed, which can be applied to study the problems of compaction in powder and other materials.

Effect of interface anisotropy on tilted growth of eutectics:A phase field study

Interfacial energy anisotropy plays an important role in tilted growth of eutectics. However, previous studies mainly focused on the solid-solid interface energy anisotropy, and whether the solid-liquid interface energy anisotropy can significantly affect the tilted growth of eutectics still remains unclear. In this study, a multi-phase field model is employed to investigate both the effect of solid-liquid interfacial energy anisotropy and the effect of solid-solid interfacial energy anisotropy on tilted growth of eutectics. The findings reveal that both the solid-liquid interfacial energy anisotropy and the solid-solid interfacial energy anisotropy can induce the tilted growth of eutectics. The results also demonstrate that when the rotation angle is within a range of 30°-60°, the growth of tilted eutectics is governed jointly by the solid-solid interfacial energy anisotropy and the solid-liquid interfacial energy anisotropy;otherwise, it is mainly controlled by the solid-solid interfacial energy anisotropy. Further analysis shows that the unequal pinning angle at triple point caused by the adjustment of the force balance results in different solute-diffusion rates on both sides of triple point. This will further induce an asymmetrical concentration distribution along the pulling direction near the solid-liquid interface and the tilted growth of eutectics. Our findings not only shed light on the formation mechanism of tilted eutectics but also provide theoretical guidance for controlling the microstructure evolution during eutectic solidification.

A novel computational model for isotropic interfacial energies in multicomponent alloys and its coupling with phase-field model with finite interface dissipation

In this work,a novel computational model for the description of the temperature-and composition-dependent isotropic interfacial energy in multicomponent alloys was first developed in the framework of the CALculation of PHAse Diagram(CALPHAD)approach and implemented in a home-made code.By linking to the open-source code for interfacial energy calculation in alloys,OpenIEC,the databases for isotropicγ/liquid andγ/γ’interfacial energies in Ni-Al,Ni-Cr,Al-Cr,and Ni-Al-Cr systems were then efficiently established.After that,a direct coupling strategy between the current CALPHAD interfacial en-ergy database and the phase-field model with finite interface dissipation was proposed and applied to three-dimensional(3-D)phase-field simulations of the primaryγdendritic growth in both Ni-Al and Ni-Al-Cr alloys during isothermal solidification.The effect of the interfacial energy on the morphology,tip growth rate,and partitioning coefficients in primaryγdendrites of binary Ni-Al and ternary Ni-Al-Cr alloys was investigated by comprehensively comparing the phase-filed simulation results using the composition-/temperature-dependent interfacial energies with those using the constant value.It is an-ticipated that the presently developed CALPHAD model for interfacial energy is of general validity for different multicom ponent alloys and should be integrated with the phase-field model for quantitative simulation of their microstructure evolution.

Estimation of Nucleation Thermodynamical Parameters of La2Sr2xCuO4 (LSCO) and YBa2Cu2O7–δ (YBCO) Crystallizing from High Temperature Solution

The growth kinetics of LSCO and YBCO single crystals from high temperature solution of LSCO-CuO solute-solvent and YBCO-CuO solute-solvent systems has been investigated. Based on regular solution model and classical nucleation theory, the thermodynamical data investigated for the systems are used to determine the nucleation parameters: interfacial free energy, metastable zone-width, volume free energy, critical energy barrier for nucleation and radius of critical nucleus for LSCO and YBCO which leads to the understanding of the nucleation phenomena of LSCO and YBCO.