Comparison of dendrite and dispersive structure in rapidly solidified Cu-Co immiscible alloy with different heat flow modes

Rapid solidification of Cu-Co immiscible alloy was investigated by glass-fluxing, spray casting and melt-spinning techniques. Both the transition from dendrite to dispersive structure and corresponding scale evolution were revealed and further elucidated in terms of the heat flow mode, nucleation and growth processes under different solidification conditions. With the increase of undercooling, columnar dendrite is replaced by dispersive structure due to the immiscible effect. In contrast, equiaxed dendrite forms in spray cast alloy due to multiple nucleation events and becomes thinner for the case of higher cooling rate. Ascribed to the enhanced non-equilibrium effect and insufficient period for collision and coagulation processes between separated droplets, fine globular dispersion appears upon the diameter of spray casting reaching 4 mm. As for the melt-spun ribbon with the highest cooling rate, a single-phase solid solution microstructure with refined grain of cellular morphology can be obtained, which is attributed to the suppression of liquid phase separation by instant solidification.

Modeling and Simulation of the Microstructure Evolution during a Cooling of Immiscible Alloys in the Miscibility Gap

The microstructure development during a cooling period of alloys being immiscible in the liquid state such as Al-Pb or AI-Bi has gained renewed scientific and technical interest during the last decades. Experiments have been performed to investigate the phase transformation kinetics in the liquid miscibility gap and numerical models have been developed to simulate and analyze the solidification process. The recently developed computational modeling techniques can, to some extent, be applied to describe the decomposition, the spatial phase separation and the microstructure evolution during a cooling period of an immiscible alloy through the miscibility gap. This article overviews the researches in this field.

Calculation of Activity Coefficient from Immiscible Binary Alloy Phase Diagram by Means of Modified Sub-regular Solution Model

The modified sub regular solution model was used for a calculation of the activity coefficient of immiscible binary alloy systems. The parameters needed for the calculation are the interaction parameters, λ 1 and λ 2, which are represented as a linear function of temperature, T . The molar excess Gibbs free energy, G m E, can be written in the form G m E= x A x B[( λ 11 + λ 12 T )+( λ 21 + λ 22 T ) x B ] The calculation is carried out numerically for three immiscible binary alloy systems, Al Pb, Cu Tl and In V. The agreement between the calculated and experimentally determined values of activity coefficient is excellent.

Mechanical Alloying of Ag-Cu Immiscible Alloy System

Mechanical alloying has been performed in Ag-Cu immiscible alloy system with five different compositions. X-ray diffraction analysis was carried out to determine the structural characterization of the milled powders. Lattice constants of the milled powders were determined and the solubility for Ag in Cu was calculated. The results demonstrated that MA indeed produced a face center cubic (f.c.c.). Cu-based Cu-Ag solid solution and the solid solubility has been extended to x(Ag)=30% for Ag in Cu when the grain size of Cu-based Cu-Ag solid solution is about 10 nm after MA. There is a three-phases co-existence during the process of MA in this alloy system which agrees well with other experimental and theoretical results. Based on the experimental results a formation model was proposed in this paper to understand the formation of Ag-Cu solid solution during MA.

Effect of high magnetic field on solidification microstructure evolution of a Cu-Fe immiscible alloy

The liquid phase separation behavior and the evolution of the solidification microstructure of a binary Cu_(50)Fe_(50) alloy were investigated under the conditions of without and with a 10 T magnetic field,with different undercooling during the solidification process.Results show that the combined effect of Stokes motion and Marangoni convection leads to the formation of the core-shell structure under the condition without the magnetic field.In addition,specific gravity segregation is reinforced by increasing the undercooling,resulting in Fe-rich phase drifts towards the sample edge.In the 10 T magnetic field,the Fe-rich phase is elongated in the parallel direction of the magnetic field under the action of demagnetization energy due to the difference of static magnetic energy and surface energy.In the vertical direction,through the action of Lorentz force,the convection in the melt is inhibited and Fe-rich phase becomes more dispersed.Meanwhile,the diffusion of the two phases and the coagulation of the Fe-rich phases are also restrained under the magnetic field,therefore,the phase volume fraction of the Fe-rich phase decreases at the same undercooling in the 10 T magnetic field.The magnetic field inhibits the segregation behavior in the vertical direction of the magnetic field,and at the same time,improves the gravitational segregation to a certain extent,which has a very important impact on microstructure regulation.

Model for calculation of microstructural development in rapidly directionally solidified immiscible alloys

A model has been developed for the calculation of the microstructural evolution in a rapidly directionally solidified immiscible alloy. Numerical solutions have been performed for Al Pb immiscible alloys. The results demonstrate that at a higher solidification velocity a constitutional supercooling region appears in front of the solid/liquid interface and the liquid liquid decomposition takes place in this region. A higher solidification velocity leads to a higher nucleation rate and, therefore, a higher number density of the minority phase droplets. As a result, the average radius of droplets in the melt at the solid/liquid interface decreases with the solidification velocity.

Simulation of nucleation and coalescence of second phase droplets during earth-based solidification process of immiscible alloys

The solidified microstructure of immiscible alloys strongly depends on the nucleation, diffusional growth, especially the coalescence of the second phase droplets in the miscibility gap. A numerical model based on the discrete multi-particle approach was developed to simulate the nucleation and coalescence mode of the second phase droplets during the earth-based processing of immiscible alloys (in this case, the effect of gravity cannot be neglected). The cooling rate is the major factor influencing the coalescence mode. Under the super-rapid or rapid solidification condition (>104K/s), Brownian collision is the dominant coalescence mode. Marangoni collision becomes the dominant mode under the sub-rapid solidification condition (>102K/s). In the conventional slow cooling scope(101K/s), Stokes collision becomes the dominant coalescence mode, correspondingly, leading to a serious phase segregation.

Microstructure evolution of immiscible alloys during rapid cooling through miscibility gap

A numerical model was developed to describe the coarsening of the second phase droplets under the common action of nucleation, diffusional growth and Brownian collision between minority phase droplets during rapidly cooling a hypermonotectic alloy through its miscibility gap. The simulated results show that Brownian motion is an important factor influencing the coarsening process. A faster cooling rate leads the supersaturation of the matrix liquid and the nucleation rate to grow up to a higher level, but leads to a smaller droplet radius and a higher number density. This model is used to predict the microstructural evolution of melt spun Al 30%In ribbon. The model reflects the real physical processes well and is expected to be applicable to other immiscible alloys or other preparing processes.

Microstructural Features of Spray-Deposited Immiscible Bearing Alloys Al-43Sn and Al-7Sn-1.3Cu-1.3Ni

Two commercial grade aluminum based immiscible bearing alloys were spray-deposited using convergent-divergent type of nozzle. The processing parameters for spray-deposition were adjusted in such a way that most of the droplets arrived on the deposition substrate in either liquid or semi-liquid state. The microstructural features of spray-formed and as-cast alloys are compared. In spray-formed alloys equiaxed grains were observed. The cell boundaries and intercellular regions were observed to be decorated with sub-micron sized particles whereas in normal casting the second phase was observed to be segregated along grain boundaries. The morphology and distribution of second phase were observed to have similarity with those in over-spread and atomized powders produced under similar processing conditions. The microstructural features observed with variation in spray conditions are discussed in detail.

Numerical simulation of the Liquid-liquid phase separation and microstructure evolution of Al-In immiscible alloys during cooling

A numerical model has been developed to describe the microstructural evolution of Al In immiscible alloys through the miscibility gap. The model considers the common action of nucleation, diffusible growth, Brownian collision and motion collision between the second phase droplets. The simulation results are dynamically visualized and show that the volume fraction, distribution and size of the second phase droplets satisfactorily agree with the experimental results. So the model can be used to predict the microstructural evolution of Al In immiscible alloys during the cooling process.

Formation Mechanism of Microstructure of Melt Spun Al-In Immiscible Alloys

Immiscible alloys are attractive for their valuable physical and mechanical properties. In this paper, Al-ln immiscible alloy is prepared by melt spinning process and its morphological evolution is studied at various indium contents. The results show that the morphologies of the matrix phase depend on the indium content. Different morphologies lead to different distribution of the second phase particles. Due to a particular solidification mechanism of immiscible alloys, even under the melt spinning rapid solidification condition, it is still impossible to produce homogeneous Al-ln hypomonotectic alloy ribbons. But for Al-ln hypermonotectic alloys, there is almost no segregation of the second phase throughout the cross section of the ribbons.

Effect of Processing Parameters on Microstructures of Melt-Spun Al-In Immiscible Alloys

Melt spinning rapid solidification technique was employed to fabricate homogeneous Al-ln immiscible alloys and their final microstructures and morphologies were investigated. There existed a transition of columnar Al grain-equiaxed grain for the thicker ribbon, but only columnar Al grain for the thinner ribbon throughout the thickness. In the columnar grain field, most of the fine In particles was distributed within the cells, but a minority of bigger In particles or short rods was perpendicularly distributed at the grain boundaries. In the equiaxed grain field, the fine In particles were located in Al grains and coarser particles were situated at the boundaries. The average particle size increased with increasing distance from the chilled surface throughout the ribbon. At the same wheel speed (same cooling rate), the average particle size increased with raising In content. At the same composition condition, the average particle size decreased with increasing wheel speed and/or decreasing ejection temperature.

Liquid phase separating mechanism and preparation techniques of immiscible alloys

Immiscible alloys have attracted growing interest for their valuable physical and mechanical properties. However, their production is difficult because of metallurgical problems in which there is a serious tendency for gravity separation in the region of the miscibility gap. So far the study on the liquid separation mechanism is still one of the important projects in the spatial materials science and the spatial fluid science. The studied results about the liquid phase separating mechanism of immiscible alloys are presented, at the same time the preparation techniques of homogeneous immiscible alloys are summarized, and the existing problems and the related researching areas in the future are also pointed out.

Modeling of Coalescence and Separation of Liquid Droplets During Solidification of Immiscible Alloys

Directional solidification methods are being used f or in-situ production of metallic immiscible composites. A quantitative understa nding of the dynamic behavior and growth kinetics of the nucleated second phase during solidification is necessary to produce homogeneous dispersion in solidifi ed composites. This paper presents a mathematical model for describing the grow th of nucleated dispersed phase in the two-liquid phase region ahead of the sol idification front and the entrapment of these droplets by the moving solid-liqu id interface in vertical unidirectional solidification systems. The model has t wo components. A macro-heat transfer model for describing the temperature prof iles and the rate of advance of the solidification front. The dynamic behavior and coalescence and growth of nucleated droplets in the two-liquid phase region under the influence of effective gravity and thermocapillary forces were repres ented through the solution the droplet momentum and mass conservation equations in particle space. These two components of the models were coupled through a sp ecial algorithm for tracking the particle location and size with respect to movi ng solidification front in the solidification time scale. The model is used to study the particle size distribution in unidirectional solidified Zn-Bi hypermo notectic alloys at reduced gravity conditions. It has been found that the parti cle size and distribution in the solidified alloy depends on solidification rate and the ratio of effective gravity to thermocapillary forces. It was also foun d that uniform dispersion could only be obtained in a very narrow range of effec tive gravity values near zero gravity. The model predictions were compared agai nst experimental measurements obtained at different effective gravity conditions in a novel unidirectional solidification apparatus that uses electromagnetic fo rces to modulate gravitational forces. The model was found to reasonably predic t the experimentally measured particle size and distribution over the entire ran ge of effective gravity investigated as well as gravity conditions for settling and flotation of the second phase during solidification. The practical signific ance of these findings will be discussed.

Modeling of the Microstructure Evolution in Continuously Cast Al-pb Alloys

A numerical model is presented describing the microstructure evolution of an immiscible alloy under the continuous casting conditions. Calculations are carried out to investigate the microstructure evolution in a vertical strip cast sample of Al+5wt pct Pb alloy. The numerical results show that there exists a peak value for the supersaturation in front of the solid祃iquid interface, and the minority phase droplets are nucleated in a region around this peak. Under strip casting conditions the Marangoni migration dominates the motion of droplets. This leads to an accumulation of the minority phase droplets in front of the solid祃iquid interface.

Influence of forging and heat treatment on wear properties of Al-Si and Al-Pb bearing alloys in oil lubricated conditions

The tribological behaviours of aluminium-based bearing alloys with different compositions, forged and heat treated materials, were investigated in oil lubricated conditions. Tested materials were Al-8.5Si-3.5Cu and Al-15Pb-3.7Cu-1.5Si-1.1Fe. The effects of hardness, heat treatment and forging on wear behaviours of the tested materials were investigated. In forging process, 10%-20%strains were applied. Heat treatment （T6） was performed to the materials. The wear tests of all specimens were performed with a pin-on-disc wear test machine. Forging process increased hardness value of the tested materials. A forging strain of 10%-20%has no significant effect on mass loss.

Al-Pb合金的快速定向凝固及其模拟

对Al-Pb合金开展了快速定向凝固实验,并根据实际凝固条件模拟计算了合金的组织演变过程.结果表明,当凝固速度不变时,Pb相粒子尺寸及其分布范围随Pb含量的增加而增大;当合金成分不变时,Pb相粒子尺寸及其分布范围随凝固速度的增加而减小.模拟结果与实验数据吻合较好,展现了快速定向凝固条件下Al-Pb合金的凝固组织形成过程.

Al-Pb互不溶体系机械合金化过程中固溶度的计算

计算了 8种fcc金属 (Ag ,Al,Au ,Cu ,Ni,Pb ,Pd和Pt)和Al Pb互不溶体系的嵌入原子势 (EAM) ,并计算了用EAM模型计算的结构稳定性。计算结果和实验结果吻合较好 ,而且拟合得到的fcc模型在结构上是稳定的。运用拟合得到的数据计算了Pb在Al中的溶解热 ,结果与abinitio计算结果相近。根据机械合金化扩展固溶度的理论 ,计算了Al Pb互不溶体系机械合金化后的固溶度 ,约为 0 .19% (摩尔分数 )。

增强型Al-Pb层状复合电极材料制备与性能研究

以Pb-Al层状复合电极材料与传统Pb合金电极为研究对象,通过在Al与Pb合金之间引入过渡金属Sn制备出Al-Sn-Pb层状复合电极材料,与Pb合金电极对比研究了物理性能及电化学行为.测试电阻分布、重量、电极极化曲线、耐蚀性、槽电压等分析表明:与传统Pb合金电极相比,其电阻减少24%,重量减轻37.6%,电极极化电位降低2.3%,腐蚀损耗降低90.8%,槽电压降低200mV。因此ASP层状复合电极材料是一种重量轻、导电好、耐腐蚀的电极材料,有着重要的开发应用前景。

Al-Pb合金快速定向凝固的数值模拟

在考虑凝固界面前沿第二相液滴形核、长大以及迁移综合作用的基础上 ,提出了描述偏晶合金在快速定向凝固条件下微观组织形成过程的数学模型 ,并对Al-Pb轴承合金在垂直Bridgeman定向凝固条件下的凝固组织进化过程进行了计算分析 .结果表明 :在大的凝固速度条件下 ,凝固界面前沿存在成分过冷区 ,液 -液相分解在此区域内进行 ;在恒定的温度梯度条件下 ,凝固速度越快 ,第二相液滴的形核速率越大 ,液滴的数量密度越高 ,平均半径越小 ;凝固界面前沿液滴的平均半径 (R)与凝固速度 (v)之间存在如下指数关系 :R(z =0 ) =C2 v-0 .3 9± 0 .0