晶体取向对定向凝固枝晶生长的影响

采用类金属透明模型合金丁二腈-1.0 wt%乙醇(SCN-1.0 wt% Eth)合金, 考察了晶体取向对定向凝固过程中晶粒的平界面失稳孕育时间、枝晶形态演化以及枝晶一次间距的影响. 结果表明, 随着枝晶择优生长方向与温度梯度方向夹角的增大, 晶粒的平界面失稳孕育时间增加, 界面的稳定性增强; 对于不同晶体取向的枝晶形态演化, 枝晶择优生长方向与温度梯度方向夹角越大, 枝晶二次臂不对称生长越严重, 同时, 具有生长优势的枝晶二次臂对相邻枝晶的生长的抑制越强烈; 至于不同晶体取向的枝晶一次间距, 随着枝晶择优生长方向与温度梯度方向夹角的增大, 枝晶一次间距增大. 关键词： 定向凝固 平界面失稳 枝晶间距 晶体取向     

Phase field investigation on the initial planarinstability with surface tension anisotropy during directional solidification of binary alloys

Phase field investigation reveals that the stability of the planar interface is related to the anisotropic intensity of surface tension and the misorientation of preferred crystallographic orientation with respect to the heat flow direction. The large anisotropic intensity may compete to determine the stability of the planar interface. The destabilizing effect or the stabilizing effect depends on the misorientation. Moreover, the interface morphology of initial instability is also affected by the surface tension anisotropy.     

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.     

Interfacial Free Energy at the Metallic Crystal–Melt Interface

The technique for estimating the interfacial free energy of transition-metal nanocrystals and its anisotropy at the interface with their melts has been developed. The expression for the coordinate of the Gibbs’ interface, which takes into account the size dependence, has been derived. The interfacial free energy of crystal faces at the interface with the related melts of monomorphic 4d and 5d metals decreases nonlinearly with a decrease in the nanocrystal size and, at a certain size, disappears. At the nanocrystal radius of more than 10 nm, the interfacial free energy of the faces approaches that for a macrocrystal. The temperature dependence of the interfacial free energy at the crystal–melt interface is almost linear. The technique developed is shown to be in agreement with the known experimental data for mono- and polycrystals and applicable for estimating the orientational, temperature, and size dependences of the interfacial free energy at the interfaces of nano-, micro-, and macrocrystals with their melts.     

晶体取向对定向凝固平界面失稳行为的影响

采用丁二腈-丙酮透明模型合金研究了不同晶体取向的晶粒在定向凝固条件下的平界面失稳过程.实验选择了三个界面失稳后具有不同生长形态的典型晶粒作为研究对象, 分别为择优生长枝晶、倾斜枝晶和海藻晶.结果表明可发展为择优生长枝晶的晶粒的平界面失稳孕育时间和初始扰动波长最小,海藻晶次之, 倾斜枝晶最大,这与以往的解析结果和相场模拟结果一致. 同时,实验观察发现可发展为择优生长枝晶和倾斜枝晶的晶粒的界面非稳态演化过程与海藻晶显著不同,这表明平界面失稳的非稳态演化过程与晶体取向相关.     

界面能各向异性对定向凝固枝晶生长的影响

采用元胞自动机 (cellular automaton, CA) 模型研究了界面能各向异性对二维定向凝固枝晶生长的影响. 模拟结果显示当晶体的择优生长方向与热流方向一致时, 随着界面能各向异性强度的增大, 凝固组织形态由弱界面能各向异性时的海藻晶转变为强界面能各向异性时的树枝晶. 同时, 界面能各向异性强度会影响稳态枝晶尖端状态的选择, 界面能各向异性越强, 定向凝固稳态枝晶尖端半径越小, 尖端界面前沿的液相浓度和过冷度越小. 稳态枝晶生长的尖端状态选择参数与界面能各向异性强度也存在标度律的指数关系, 而枝晶一次间距则受界面能各向异性强度影响较弱. 当晶体的择优生长方向与热流方向呈-40°夹角时随着界面能各向异性强度的增大, 凝固组织形态由海藻晶逐渐转变为退化枝晶, 后又逐渐转变为倾斜枝晶. 关键词： 元胞自动机 枝晶 界面能各向异性     

Decaying turbulence: What happens when the correlation length varies spatially in two adjacent zones

We have imagined a numerical experiment to explore the onset of turbulent intermittency associated with a spatial perturbation of the correlation length. We place two isotropic regions, with different integral scales, inside a volume where the turbulent kinetic energy is initially uniform and leave them to interact and evolve in time. The different length scales produce different decay rates in the two regions. Since the smaller-scale region decays faster, a transient turbulent energy gradient is generated at the interface between the two regions. The transient is characterized by three phases in which the kinetic energy gradient across the interface grows, peaks and then slowly decays. The transient lifetime is almost proportional to the initial ratio of the correlation lengths. The direct numerical simulations also show that the interface width grows in time. The velocity moments inside this interaction zone are seen to depart from their initial isotropic values and, with a certain lag, the anisotropy is seen to spread to small scales. The longitudinal derivative moments also become anisotropic after a few eddy turnover times. This anisotropic behaviour is different from that observed in sheared homogeneous turbulent flows, where high transverse derivative moments are generated, but longitudinal moments almost maintain the isotropic turbulence values. Apart from the behaviour of the energy gradient transients, the results also show the timescaling of the interface diffusion width, and data on the anisotropy of the large and small scales, observed through one-point statistics determined inside the intermittency sublayer, which is associated with the interaction zone.     

Anisotropy of the solid–liquid interface properties of the Ni–Zr B33 phase from molecular dynamics simulation

Solid–liquid interface (SLI) properties of the Ni–Zr B33 phase were determined from molecular dynamics simulations. In order to perform these measurements, a new semi-empirical potential for Ni–Zr alloy was developed that well reproduces the material properties required to model SLIs in the Ni_50.0Zr_50.0 alloy. In particular, the developed potential is shown to provide that the solid phase emerging from the liquid Ni_50.0Zr_50.0 alloy is B33 (apart from a small fraction of point defects), in agreement with the experimental phase diagram. The SLI properties obtained using the developed potential exhibit an extraordinary degree of anisotropy. It is observed that anisotropies in both the interfacial free energy and mobility are an order of magnitude larger than those measured to date in any other metallic compound. Moreover, the [0 1 0] interface is shown to play a significant role in the observed anisotropy. Our data suggest that the [0 1 0] interface simultaneously corresponds to the lowest mobility, the lowest free energy and the highest stiffness of all inclinations in B33 Ni–Zr. This finding can be understood by taking into account a rather complicated crystal structure in this crystallographic direction.     

Complex angular dependence of exchange bias on (001) epitaxial NiO-Co bilayers

We have sputter-deposited NiO-Co bilayers on MgO(001) substrates. NiO and Co grow epitaxially on MgO and reproduce its fcc structure. The high quality of our samples, in terms of flatness and crystallographic coherence of the interface, allows the observation of an additional fourfold magnetic anisotropy term by standard magnetometry. This term is induced by interfacial interaction assigned to the same origin as exchange bias. Additional measurements of exchange bias azimuthal dependence versus the crystallographic axes of the film plane reveal unusual behaviors with several sign changes related to this fourfold anisotropy.     

过冷熔体中枝晶生长的相场法数值模拟

利用相场法模拟了过冷纯金属熔体中的枝晶生长过程,研究了各向导性、界面动力学、热扩散和界面能对枝晶生长的影响.结果表明,热噪声可以促发侧向分支的形成,但不影响枝晶尖端的稳态行为;随着各向异性的增加,枝晶尖端生长速度增加,尖端半径减小;当界面动力学系数减小及在界面动力学系数小于1的条件下热扩散系数减小时,枝晶尖端生长速度随之减小,而尖端半径相应增大;界面能趋于增大枝晶尺度并保持界面在扰动下的稳定,界面能越大,形成侧向分支的趋势越小 关键词： 过冷 枝晶生长 相场法 数值模拟     

Nonlinear dynamics of an interface between shear bands

We study numerically the nonlinear dynamics of a shear banding interface in two-dimensional planar shear flow, within the nonlocal Johnson-Segalman model. Consistent with a recent linear stability analysis, we find that an initially flat interface is unstable with respect to small undulations for a sufficiently small ratio of the interfacial width l to cell length L(x). The instability saturates in finite amplitude interfacial fluctuations. For decreasing l/L(x) these undergo a nonequilibrium transition from simple traveling interfacial waves with constant average wall stress, to periodically rippling waves with a periodic stress response. When multiple shear bands are present we find erratic interfacial dynamics and a stress response suggesting low dimensional chaos.     

Absolute stability of the solidification interface in a laser resolidified Zn--2wt.%Cu hypoperitectic alloy

This paper reports on laser surface remelting experiments performed on a Zn--2wt.%}Cu hypoperitectic alloy by employing a 5kW CW CO₂ laser at scanning velocities between 6 and 1207mm/s. The growth velocities of the microstructures in the laser molten pool were accurately measured. The planar interface structure caused by the high velocity absolute stability was achieved at a growth velocity of 210~mm/s. An implicit expression of the critical solidification velocity for the cellular--planar transition was carried out by nonlinear stability analyses of the planar interface. The results showed a better agreement with the measured critical velocity than that predicted by M--S theory. Cell-free structures were observed throughout the whole molten pool at a scanning velocity of 652~mm/s and the calculated minimum temperature gradient in this molten pool was very close to the critical temperature gradient for high gradient absolute stability (HGAS) of the \eta phase. This indicates that HGAS was successfully achieved in the present experiments.     

A study of the effective magnetic anisotropy and the corresponding spin configurations in two dimensional ferromagnetic/antiferromagnetic system

The spin configurations of two dimensional ferromagnetic/antiferromagnetic system were investigated using model calculations and Monte-Carlo simulation methods. The lowest energy state was obtained under various coupling conditions to investigate the role of interfacial interaction on anisotropy. We found that the total ferromagnetic layer anisotropy is contributed not only from its own crystalline anisotropy but also from the antiferromagnetic layer spin flop effect. The overall ferromagnetic layer effective anisotropy is calculated as a function of the exchange energy of antiferromagnetic layer and the interfacial interaction energy. If the effective anisotropy from the spin flop effect is comparable with the crystalline anisotropy, the asymmetric spin configuration is generated. In this configuration, the magnetization direction of the ferromagnetic layer is neither perpendicular nor parallel to the antiferromagnetic spin direction. Temperature effect on the perpendicular-to-collinear coupling transition was also investigated using Monte-Carlo simulation, and the relationship between the effective anisotropy and the temperature was obtained.     

The effect of anisotropic surface tension on the morphological stability of planar interface during directional solidification

This paper considers the effect of the anisotropic surface tension on the morphological stability of the planar interface during directional solidification. When the expression exhibiting the four-fold symmetry is included, the modified absolute stability criterion is obtained by employing the multi-variable expansion method. The linear stability analysis reveals that for the given temperature gradient, as the anisotropic surface tension parameter increases, the stability zone tends to decrease.     

A cellular automaton model for the solidification of a pure substance

An improved cellular automaton model for describing the evolution of solidification morphology of a pure substance is presented. The mesh-induced anisotropy was investigated for the traditional capture rules such as Von Neumann’s and Moore’s capture rules. A random zigzag capture rule was developed, which can greatly reduce the mesh-induced anisotropy. The method for the calculation of solid/liquid (SL) interface curvature was also improved by using a bilinear interpolation method, which increased the accuracy of the calculation of the SL interface curvature. Based on these improvements, the effect of interfacial energy anisotropy on the dendritic growth behavior was analyzed.     

Grain boundary transitions in binary alloys

A thermodynamic diffuse interface analysis predicts that grain boundary transitions in solute absorption are coupled to localized structural order-disorder transitions. An example calculation of a planar grain boundary using a symmetric binary alloy shows that first-order boundary transitions can be predicted as a function of the crystallographic grain boundary misorientation and empirical gradient coefficients. The predictions are compared to published experimental observations.     

Experimental Measurement of Anisotropy in Crystal-Melt Interfacial Energy

Experimental methods for the determination of anisotropy in crystal-melt interfacial energy are explored. Equilibrium shape measurements of liquid droplets entrained in single-phase solid are utilized and results are reported for an Al-Cu and an Al-Si alloy. The grain boundary groove method for determination of interfacial energy anisotropy is also examined. A complete coupled-groove solution is presented for a general tilt-boundary groove. The effects of various physical parameters, including interface energy anisotropy, on the groove shape are discussed.     

Growth and equilibrium morphology of metal precipitates formed inside an oxide matrix

Metal precipitation occuring during internal reduction of various mixed oxides, such as magnesia or alumina, doped with more noble transition metal oxides, is investigated for different doping contents, reaction temperature and driving forces. The precipitate morphology developing inside the solid oxide matrix and the corresponding orientation relationships between the metal and the oxide phase are analysed by transmission electron microscopy. The evolution of precipitate shape across the reduction scale is interpreted in terms of ageing of the metal inclusions; the various morphologies identified are ascribed to different stages of nucleation, diffusional growth and final equilibration.Precipitate morphology is analysed in terms of diffusion-controlled growth of small metal particles in their local and long range diffusion field. The influence of interfacial energy and related anisotropy is experimentally demonstrated for selected examples. The general role played by these parameters during growth and final equilibration is also discussed. The implications of the elastic strain fields building up during growth of the metal inside the oxide matrix are analysed in view of our experimental results. Shape changes taking place during the growth process are considered to be controlled by local matter transport properties. The influence of the reaction mechanism at an atomic scale and the relative fit of the crystallographic lattices are briefly summarised.