MICROSTRUCTURAL FEATURE OF Al-In MONOTECTIC ALLOY PROCESSED UNDER MICROGRAVITY

The microstructural feature of Al-In monotectic alloy processed under microgravity has been investigated It was found that in the sample cooled in space,there are a lot of In particles in A! dendrite,but no particle has been observed in the sample cooled on the ground.Moreover, the In particles distribute with regularity and the A1 dendrite has an obvious boundary layer marked by a string of particles.Thus,the distinction between them may reflect the character- istics of the solid/liquid interface during solidification under different gravity conditions.

Coarsening Manner and Microstructure Evolution of Al-In Hypermonotectic Alloy during Rapidly Cooling Process

A numerical model reflecting the real physical processes well has been developed to predict the coarsening manner of the second phase droplets and the microstructural evolution under the common action of nucleation, diffusional growth, colliding coagulation during rapid cooling Al-ln hypermonotectic alloys. The model reflects the real physical processes well and is also applicable to other immiscible alloys.

Microstructure evolution of Cu-Pb monotectic alloys during directional solidification

Planar, cellular and dendrite morphologies were observed at different concentrations in the directional solidification of Cu-Pb monotectic alloys. In Cu-Pb hypomonotectic alloys, the directional solidification microstructure changes from columnar dendrite to the irregular rod composite structure with increasing lead content and growth rate. In Cu-Pb hypermonotectic alloys, the structure changes from the band structure and elongated droplets to irregular rod composite structure with increasing growth rate. The range of composition of forming the rod composite structure around the monotectic points increases with the increasing growth rate. The transient morphology of Cu-Pb alloys in the directional solidification was obtained. The solid/liquid interface of Cu-Pb alloys presents planar and the second liquid droplets are pushed by growing front under the high temperature gradient. With increasing growth rate or decreasing temperature gradient the planar interface becomes unstable and the cellular structures with L2 phase at the cell boundaries are developed.

Liquid-liquid phase separation in highly undercooled Ni-Pb hypermonotectic alloys

Liquid-liquid phase separation in the undercooled Ni-20%Pb(mole fraction, the same below if not mentioned) hypermonotectic melts was investigated by the observation of the water-quenched structure and DTA analysis. The results indicate that the number of spherical cells in the water-quenched microstructure increases with dropping temperature, and the cells gather and grow up obviously. The spherical cell origins from L1 phase separated from homogeneous melt, and is the product of monotectic reaction. Both results of the water-quenched structures and DTA analysis prove that liquid phase separation still occurs in the highly undercooled Ni-Pb hypermonotectic alloy melts, and liquid phase separation in the immiscible gap can not be fully inhibited by high undercooling and rapid solidification.

Non-equilibrium solidification of undercooled Ni-31.44%Pb monotectic alloy melts

By using the method of molten glass denucleating combined with superheating cycling, solidification behavior of the bulk undercooled Ni 31.44% Pb monotectic alloy melts was systematically investigated. The results indicated that the undercooled monotectic alloy solidifies in form of dendrite essentially during the stage of rapid solidification and after recalescence, the residual melts between the dendrites solidify in the equilibrium mode. Within the achieved undercooling range, the solidification structures are classified into three categories. When the undercooling is less than 50?K, the structures are composed of coarse dendrites and interdendritic lead phase. With the undercooling increasing into the range of 70 ～ 232?K, the dendrite clusters are refined and fine lead particles separate out from the supersaturated primary dendrite arms because of solute trapping. When the undercooling exceeds 242?K, the granular grains form and fine lead particles homogeneously distribute in the whole sample. Based on the observation of the solidification structures and the calculated results with BCT model, it is found that the granulation mechanism of the granular grains is owing to the primary dendrite disintegration and recrystallization.

Tribological property and wear mechanism of undercooled Ni-Pb monotectic alloys

The tribological properties of Ni-31.44%Pb monotectic alloys were measured by using a SRV reciprocating tribo-tester. The effects of load, sliding speed and melt undercooling on wear rate of the sample were investigated. The worn surface of Ni-31.44%Pb was examined using scanning electron microscope （SEM） and X-ray photoelectron spectroscope （XPS）. The results show that the wear properties of the samples undercooled by 80 K and 310 K are obviously superior, which is attributed to more efficient transfer of Pb from the bulk material to the worn surface. The lubricating film is identified as a mixture of Ni2O3 and PbO by XPS analysis. At the same load and sliding speed, the predominant wear mechanisms can be identified as oxidative wear for the lower and larger undercooling, and plastic deformation and fracture for the medium undercooling.

Some Physicochemical and Thermal Studies on Organic Analog of a Nonmetal-Nonmetal Monotectic Alloy;2-Cyanoacetamide–4-chloronitrobenzene System

The phase equilibrium data on organic analog of the nonmetal-nonmetal system, involving 2-cyanoacetamide (CA)―4-chloronitrobenzene (CNB), show the formation of a monotectic (0.10 mole fraction of CNB) and a eutectic (0.98 mole fraction of CNB) with a large miscibility gap starting from 0.10 mole fraction of CNB of monotectic (M) and ending at 0.92 mole fraction of CNB of monotectic horizontal (Mh);the upper consolute temperature Tc being 63?C above the monotectic horizontal at 118?C and eutectic temperature is at 85?C. The values of enthalpy of fusion of the pure components, the eutectic and the monotectic were determined by the differential scanning calorimetry (Mettler DSC-4000 system). Using these data, the size of the critical radius, interfacial energy, excess thermodynamic functions, entropy of fusion, and enthalpy of mixing were calcu-lated. The solid-liquid interfacial energy data confirm the applicability of the Cahn wetting condition. While growth data obey the Hillig-Turnbull equation, the microstructural investigations give typical characteristic features of the eutectic and the monotectic of the system.

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.

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.

铜基偏晶合金的研究现状

铜基偏晶合金(Cu-Co、Cu-Fe、Cu-Cr等)具有优异的电、磁、力学等特性,受到国内外材料科学领域学者的广泛关注。在制备过程中,铜基偏晶合金由于发生液相分离行为并形成严重的偏析组织现象,被称为不混溶合金。过去的研究主要集中于寻求使铜基偏晶合金组织均匀的制备方法,然而近期研究表明,偏析组织异质结构的强度-韧性协同作用有利于提高材料的韧性,这为铜基偏晶合金的进一步发展提供了新思路。本文综述了目前国内外对铜基偏晶合金的液相分离和偏析组织形成机理的理论研究成果(包括液相分离组织结构演变、液相分离热力学以及第二相液滴形核生长动力学研究),归纳了目前主流的改善偏析组织方法(包括微合金化法、快速凝固法和激光成形法),阐述了铜基偏晶合金因液相分离特性产生的双相复合微观组织结构的强度-韧性协同作用,最后对铜基偏晶合金未来的研究发展方向进行了展望。

高温度梯度下Al-In偏晶合金定向凝固组织的演化规律

应用高温度梯度的方法,研究了在定向凝固条件下温度梯度与凝固速率的比值G／R对Al-17.5％In(质量分数)合金 凝固组织的影响.结果表明,偏晶合金的定向凝固与共晶合金定向凝固的生长规律类似.在高温度梯度下,仅在很低的生长速度时 才能形成二相有序排列的共生.在偏晶合金定向凝固进入稳态生长以后,在各自的相凝固前沿富集了另一相的溶质,由于两相的层 间距不大,长大过程中的横向扩散占主导地位.随着生长速度的增大或温度梯度的降低,Al-17.5％In合金定向凝固组织从纤维 结构到周期性或规则排列的串状结构再到弥散分布结构转变.这种转变与固-液界面形状的转变有密切关系.

化学成分对定向凝固Al-In偏晶合金显微组织的影响

在高温度梯度下对不同化学成分的Al-In偏晶合金进行了定向凝固,分析了其定向凝固组织形成机制。Al-In亚偏晶合金定向凝固中,S1相首先析出,造成溶质在生长界面前沿的富集,在凝固界面的推移过程中不断长大,在晶界处形成粗化的第二相。Al-In偏晶合金在较低的定向凝固生长速度下,凝固界面呈现胞状生长模式,形成一个较长的被周围富集Al的S1相所包围的L2相纤维。Al-In过偏晶合金定向凝固过程中,凝固通过液相不混溶区时,两个液相将迅速发生分离,分离后的两个液相由于显著的密度差进行沉积和凝并,形成弥散组织。随着抽拉速度的增大,第二相在基体中分布更加弥散和均匀。

添加第三组元调控Al-Bi偏晶合金凝固组织研究进展

Al-Bi偏晶合金熔体冷却过程中由均一的液相经液-液相转变为两互不混溶的液相,在常规重力铸造条件下极易形成宏观偏析严重的凝固组织,极大限制其工业应用,因此有必要对Al-Bi偏晶合金凝固组织进行有效调控(细化和均匀化)。近年来,研究者对如何改善Al-Bi等偏晶合金偏析组织开展了大量研究,如快速凝固、施加外场和添加第三组元等,其中添加第三组元最有可能实现其低成本/规模化制备。基于此,本文主要综述了Al-Bi偏晶合金添加第三组元(合金元素、Al-(Ti)-(B)体系、陶瓷颗粒)的相关研究进展,其中添加含陶瓷颗粒的第三组元能够有效细化和均匀化富Bi相,获得弥散型Al-Bi基偏晶合金,该研究对于实现其大规模工业应用具有重大意义。

Rapid solidification and dendrite growth of ternary Fe-Sn-Ge and Cu-Pb-Ge monotectic alloys

The phase separation and dendrite growth characteristics of ternary Fe-43.9%Sn- 10%Ge and Cu-35.5%Pb-5%Ge monotectic alloys were studied systematically by the glass fluxing method under substantial undercooling conditions. The maximum undercoolings obtained in this work are 245 and 257 K, respectively, for these two alloys. All of the solidified samples exhibit serious macrosegregation, indicating that the homogenous alloy melt is separated into two liquid phases prior to rapid solidification. The solidification structures consist of four phases including α-Fe, (Sn), FeSn and FeSn2 in Fe-43.9%Sn-10%Ge ternary alloy, whereas only (Cu) and (Pb) solid solution phases in Cu-35.5%Pb-5%Ge alloy under different undercool- ings. In the process of rapid monotectic solidification, α-Fe and (Cu) phases grow in a dendritic mode, and the transition "dendrite→monotectic cell" happens when alloy undercoolings become sufficiently large. The dendrite growth velocities of α-Fe and (Cu) phases are found to increase with undercooling according to an exponential relation.

Microstructure Evolution of a Ternary Monotectic Alloy During Directional Solidification

A model was developed to describe the microstracture evolution in a directionally solidified ternary monotectic alloy.The directional solidification experiments were carried out on Al-3Pb-lSn（wt%） alloys by using a Bridgman apparatus.The microstracture evolution in the directionally solidified sample was calculated.The numerical results agree well with the experimental ones.It is demonstrated that the nucleation of the minority phase droplets occur at two different positions.One corresponds to the liquid-liquid decomposition,which occurs in front of the solidification interface.The other is at the liquid/solid interface.The nucleation rate of the minority phase droplets at the liquid/solid interface is significantly higher than at the position in front of the solidification interface.The characteristic of the nucleation process leads to a bimodal size distribution of the minority particles in the directionally solidified sample.

The Effect of AC Electro-magnetic Body Force on the Solidified Microstructure of Zn-10%Bi Hyper-Monotectic Alloy

Monotetic alloy have been researched for hundred years.Recent years,researchers have prepared homogeneous monotectic alloy by electro-magnetic body force(EMBF).But the mechanism of separation and refinement of the second phase in liquid matrix by EMBF is not very clear.This paper investigated the effect of different AC EMBF on the microstructure and solidification process of Zn-10%Bi hyper-monotectic alloy by quenching the samples in water under high static magnetic field.It is shown that,the superimposing of AC EMBF can influence the moving speed of the second phase droplet and damp the gravity segregation in solidifying Zn-10%Bi alloy.When the AC EMBF reaches 5×10~5N/m^3 in different magnetic flux densities from 2 to 10 Tesla,the homogenuous distribution of bismuth particles could be achieved,A physical model is established to illustrate the mechanism of separation and refinement of the second Bi droplets at the stage of liquid-liquid separation by AC EMBF.The results show that the second droplets can be divided into two or three smaller droplets by Lorenz force caused by induced current at the former and later location of bismuth droplet.When the bismuth droplet moves at a proper speed which is adjusted by EMBF,it will be broken down by Lorenz forces.

Solidification of Hyper-Monotectic Zinc-Bismuth Alloy in a Strong Magnetic Field

In order to obtain homogenous Zn-Bi hyper-monotectic alloys and investigate what mechanism the magnetic field functions,we carried out the solidification experiments of Zn-4wt%Bi,5wt%Bi,6wt%Bi,7wt%Bi, 15wt%Bi and 30wt%Bi alloys under a 18T static magnetic field which was set up in LNCMI.Water quenching was also chosen to further damp the segregation caused by Stokes convection and Marangoni movement.The results indicated that when the content of Bismuth is 5-7wt%,the superimposition of 18Tesla magnetic field can damp the segregation remarkably,and the size of the second phase particles also is decreased.Furthermore,to Zn-4wt%Bi alloy solidified in 18T magnetic field,no spherical bismuth particles are found even magnifying 1000 times by microscope, which hints that the 18T magnetic field may change its solidifying character.To Zn-15wt%Bi and Zn-30wt%Bi alloy, due to their strong segregation trend,the 18T magnetic field still cannot damp the Stokes settlement thoroughly even by quenching way,however,no layered bismuth and zinc appears when compared to 0T,large Bismuth block are formed in the lower half part of the sample.

Phase field simulation of monotectic transformation for liquid Ni-Cu-Pb alloys

Based on the subregular solution model, the liquid phase separation of ternary (NixCu100-x)50Pb50 monotectic alloys is simulated by the phase field method. It is found that if the surface segregation potential is not incorporated, the dynamic morphologies of alloy melt show a transition from disperse microstructure into bicontinuous microstructure with the increase of fluidity parameter. When the surface segregation potential is coupled, Pb-rich phase migrates preferentially to the surface of the liquid alloy, and the Ni-rich phase depends on the Pb-rich phase to nucleate. With the extension of the phase separation time, the surface layer is formed through coagulation and growth, and its thickness gradually increases. The Ni-rich phase migrates to the central part, and finally a two-layer core-shell microstructure is produced. The concentration in the surface layer fluctuates more conspicuously than that inside the bulk phase, which subsequently transfers from the surface to the interior by a wave. The fluid field near the liquid-liquid interface is strong at the beginning of phase separation, and reduces later on. The surface segregation is essential to the formation of the surface layer, concentration profile variation, fluid field distribution and phase separation morphology.

In-Situ Analysis of Bulk Solidifying the Hyper-Monotectic Alloy Under a Compound Electric-Magnetic Field by Physical Simulation

A physical simulation was carried out to investigate the realistic experiment of bulk solidifying the Zn-Bi hyper-monotectic alloy under various compound electric-magnetic fields(CEMF).For this experiment,two crucial parameters determinate the cast microstructure,the one is electric-magnetic force(EMF)and the other is the frequency of AC current.Results show that the minor phase could be mixed in the other phase from the initial layered structure when the EMF above a specific value under fixed frequency,and the average diameter of minor phase droplet decreases with increasing EMF.The evolution of the liquid phases structure is reasonable agree with the realistic experiment of Zn-Bi hyper-monotectic alloy,which suggests that the mechanism revealed by the physical simulation could represent the one in the realistic experiment.

偏晶合金液液相分离机制和均质偏晶合金的制备方法

本文介绍了偏晶合金液液分离机制的研究现状及偏晶合金形核特点,在此基础上,分析了影响偏晶合金液液相分离的诸多因素和机制,并系统介绍了制备均质偏晶合金的各种方法。指出:影响偏晶合金液液相分离的诸因素的作用过程及均质偏晶合金的制备和应用,将是该领域今后研究的重点。