Liquid immiscible alloys

The microstructure formation, during casting, of alloys being immiscible in the liquid state such as copperlead or aluminium-lead has gained renewed scientific and technical interest during the last fifteen years. Especially, a new experimental tool, research under reduced gravity conditions, was able to cast new, unexpected results and theories into the discussion on the nature of the complex process of microstructure evolution in such alloys. Prior to the first experiments performed at reduced levels of gravity acceleration, it was generally agreed that the process of phase separation during cooling through the miscibility gap is dictated solely by gravity-induced effects such as natural convection and sedimentation. Fundamental and applied research in space and in earth laboratories could show that there are other mechanisms operating concurrently and under suitable conditions with equal strength. In addition applied research was able to utilize the often unexpected results from space experimentation to develop new casting processes which allow one to produce microstructures on earth suitable for bearings in automotive applications. Therefore this article describes the extensive progress that has been made during the last decade and also the fundamentals of immiscibles. In addition it will be shown that the combination of classical laboratory research, research under reduced gravity conditions and a newly developed computational modelling technique seems to be just becoming available to solve the problems of decomposition, spatial phase separation and microstructure evolution during cooling of an alloy through the liquid miscibility gap.     

Microstructure Evolution in a Rapidly Solidified Cu85Fe15 Alloy Undercooled into the Metastable Miscibility Gap

1.IntroductionThe Cu-Fe alloy system is well known as a peritecticsystem[1].It also exhibits a metastable miscibility gap inthe undercooled liquid state,as shown in Fig.1.Whena single-phase liquid is cooled into the miscibility gap,itseparates into two liquids:one is Cu-rich(L1),and theother is Fe-rich(L2).Although many researches on theCu-Fe system have been carried out[2~6],most of themfocused on the thermodynamic aspect.The effect of thecooling rate and undercooling on the liquid phase …     

Processing of immiscible metallic alloys by rheomixing process

Abstract

Mixing immiscible alloys has been a long standing challenge to both materials scientists and processing engineers. Despite great efforts made worldwide, including extensive space experiments, no casting techniques so far can produce the desirable fine and uniform dispersed microstructure. Based on extensive experience in mixing immiscible organic liquids offered by the polymer processing community, the authors have successfully developed a rheomixing process for mixing immiscible alloys. The rheomixing process utilises first the intensive shear stress-strain field offered by a twin screw extruder to create a fine homogeneous liquid dispersion within the miscibility gap and then the viscous force offered by a semisolid slurry at a temperature below the monotectic temperature is used to counterbalance the gravitational force and the Marangoni effect. A laboratory scale rheomixer has been designed and constructed to realise this two step mixing strategy. The Ga–Pb and Zn–Pb systems were selected to demonstrate the principles of rheomixing. The experimental results showed that the rheomixing process developed is capable of creating a fine and uniform microstructure from immiscible alloys. This paper describes the rheomixing process in detail and the preliminary experimental results on rheomixing in the Ga–Pb and Zn–Pb systems are discussed.     

Microstructure of Zn–Pb immiscible alloys obtained by a rheomixing process

Abstract

A rheomixing process has been developed for processing immiscible metallic liquids. In this paper, a binary Zn–Pb system is used to demonstrate the rheomixing process. During the rheomixing process, liquid Zn and Pb are premixed in the miscibility gap using a propeller mixer to achieve coarse dispersion of Pb liquid droplets in Zn liquid matrix. The coarse mixture is then transferred into a twin-screw rheomixer, where it is continuously cooled down to a temperature below the monotectic temperature to form a semi-solid slurry under the intensive shear mixing action of the twin-screw rheomixer. The semi-solid slurry is finally extruded through a cylindrical extrusion die to form a continuous bar. The microstructure of immiscible alloys produced by this method is characterised by a fine dispersion of Pb particles distributed uniformly throughout a Zn matrix. The effects of rheomixing temperature and alloy composition on the resultant microstructure have been investigated. It has been found that the average size of Pb particles increases linearly with increasing rheomixing temperature and with increasing Pb concentration in the Zn–Pb alloys.     

Microstructure and phase selection in supercooled copper alloys exhibiting metastable liquid miscibility gaps

The influence of both bulk supercooling and cooling rate on the microstructure and phase selection during solidification of Cu–Co, Cu–Co–Fe, and Cu–Nb alloys exhibiting metastable liquid miscibility gaps were investigated using scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. Containerless electromagnetic levitation was used to achieve large bulk supercoolings in the specimens. Supercooling of these alloys below a certain temperature resulted in metastable separation of the melt into two liquids, a Cu-lean (Co, Co?+?Fe, or Nb enriched) melt (L1) and a Cu-rich melt (L2). Usually, the microstructure of the phase-separated alloys consisted of spherulites corresponding to one of the phase-separated liquids embedded in a matrix corresponding to the other. The microstructure and phase selection are found to depend on factors such as: alloy composition, supercooling level, whether the material was dropped before or after recalescence, and the cooling rate during solidification. The following results were observed: (1) solidification of metastable ε-Cu with enhanced Co (or Co?+?Fe, or Nb) solubility; (2) partitionless solidification of the L1 and L2 liquids; (3) spinodal decomposition of the supercooled liquid, and (4) secondary melt separation. The results are discussed and related to current solidification theories regarding solidification paths for the different conditions examined. The miscibility gap boundaries for the different alloys were determined and compared with those reported in the literature.     

Study of the containerless undercooling of Ti-Ce immiscible alloys

Ti-Ce immiscible alloys of compositions across the miscibility gap were containerlessly processed in both a low-gravity and a unit-gravity environment. Although undercooling of the single-phase liquid into the miscibility gap could not be observed, undercooling did occur across the miscibility gap for the separated liquid Ti-rich phase. The low gravity, quiescent environment favored higher undercooling over the unit-gravity samples. Every undercooled sample had massive separation of the liquid phases. Metallurgical analysis of samples undercooled in unit-gravity showed signs of vigorous convective stirring and shearing of the L₁ Ti-liquid by the applied levitation electromagnetic field. In low-gravity processed samples, the L₁ liquid formed a near-concentric sphere within a Ce shell with some residual smaller spherical particles dispersed throughout the Ce. This configuration is predicted from wetting theory and from Marangoni separation. Plots of both the melting and solidification temperatures indicate that the monotectic temperature is 1831 ± 12°K rather than the 1723°K as reported in the literature. From chemical and diffraction analysis, the solubility of Ce in the Ti-rich phase was found to be extended; also, some cerium oxide precipitates formed but no perceptible dissolved oxygen within the Ce or Ti phases was found which indicates that the higher monotectic temperature reported here is probably not an oxygen effect.     

Effect of in situ phases on microstructure and properties in Al–Bi immiscible alloy

Al–Bi immiscible alloy is of particular interest as potential self-lubricating wear materials with a homogeneous distribution of minority phase. However, it is difficult to obtain a homogeneous microstructure by conventional casting methods due to liquid phase separation of Al–Bi immiscible alloy. We have developed a new strategy to restrain liquid phase separation and improve the properties of Al–Bi immiscible alloy by in situ phases. The in situ AlB₂ phase acts as heterogeneous nucleation site to accelerate the nucleation and slow down the velocity of the Bi-rich droplet, resulting in a significant size reduction and a homogeneous microstructure of Al–Bi immiscible alloy. The self-lubricating wear resistance of Al–Bi immiscible alloy can be further enhanced by in situ Al₂Cuphase.     

On the metastable miscibility gap in liquid Cu–Cr alloys

Electromagnetically levitated Cu–Cr alloy melts containing 5–70 at.% Cr were splat-quenched onto a chill substrate. The microstructure of the solidified alloys was investigated by scanning electron microscopy. The alloys containing 5–60 at.% Cr showed a droplet-shaped microstructure consisting of Cr-rich spheroids or and dendrites in a Cu-rich matrix, whereas those containing 65 and 70 at.% Cr showed a banded microstructure consisting of alternative Cu-rich and Cr-rich bands. Both types of microstructure presented evidence for metastable phase separation in Cu–Cr alloy compositions, thus verifying the existence of a broad miscibility gap in the undercooled liquid. However, the results suggested that the miscibility gap has a Cr-rich critical composition and a skewed geometry.     

Microstructure formation and electrical resistivity behavior of rapidly solidified Cu-Fe-Zr immiscible alloys

The immiscible Cu-Fe alloy was characterized by a metastable miscibility gap. With the addition element Zr, the miscibility gap can be extended into the Cu-Fe-Zr ternary system. The effect of the atomic ratio of Cu to Fe and Zr content on the behavior of liquid-liquid phase separation was studied. The results show that liquid-liquid phase separation into Cu-rich and Fe-rich liquids took place in the as-quenched Cu-Fe-Zr alloy. A glassy structure with nanoscale phase separation was obtained in the as-quenched(Cu_(0.5) Fe_(0.5))40Zr_(60) alloy sample, exhibiting a homogeneous distribution of glassy Cu-rich nanoparticles in glassy Fe-rich matrix. The microstructural evolution and the competitive mechanism of phase formation in the rapidly solidified Cu-Fe-Zr system were discussed in detail. Moreover, the electrical property of the as-quenched Cu-Fe-Zr alloy samples was examined. It displays an abnormal change of electrical resistivity upon temperature in the nanoscale-phase-separation metallic glass. The crystallization behavior of such metallic glass has been discussed.     

Dynamics of solidification and development of morphology of Cu‐base alloys with a metastable miscibility gap in the range of the undercooled melt

The Cu‐Co system shows a metastable miscibility gap in the range of the undercooled melt. In this work the method of electromagnetic levitation (EML) and drop tube experiments have been used to examine the metastable state of Cu‐Co alloys. The experiments show that both methods allow deep undercooling of the melt into the range of the miscibility gap. Due to the deep undercooling the velocity of the solidification front is very high and the actual microstructure is frozen in. The process of demixing can be observed and the binodal has been determined with high precision. The microstructure of samples processed in the electromagnetic levitation shows an influence of the electromagnetic stirring due to the induction of electric currents into the melt. Drop tube experiments, which lead to a rapid solidification under reduced gravity conditions, in contrary result in a homogeneous distribution of spherical particles of the minority phase. For this reason space experiments under microgravity conditions in the TEMPUS facility are under consideration. In these experiments the stirring effect would be greatly reduced compared to the EML.     

Computer Modeling of the Evolution of Dendrite Microstructure in Binary Alloys During Non-isothermal Solidification

Two-dimensional simulations of the evolution of dendrite microstructure during isothermal and non-isothermal solidifications of a Ni-0.41Cu binary alloy are carried out using the phase-field method. The governing evolution equation for the phase field variable, the solute mole fraction and the temperature are formulated and numerically solved using an explicit finite difference scheme. To make the computations tractable, parallel computing is employed. The results obtained show that under lower cooling rates, the solidification process is controlled by partitioning of the solute between the solid and the liquid at the solid/liquid interface. At high cooling rates, on the other hand, solute trapping takes place and solidification is controlled by the heat extraction rate. An increase in the cooling rate is also found to have a pronounced effect on the dendrite microstructure causing it to change from poorly developed dendrites consisting of only primary stalks, via fully developed dendrites containing secondary and tertiary arms to the diamond-shaped grains with cellular surfaces. These findings are in excellent agreement with experimental observations.     

Surface tension, phase separation and solidification of undercooled Cu-Co

The system Cu-Co has a metastable miscibility gap in the under-cooled liquid phase which can be accessed by electromagnetic lévitation. Unsupported two-phase liquid drops display a variety of physical phenomena, including wetting, phase separation and solidification, which can be studied on this model system. This paper reports theoretical and experimental results which have been obtained within the CuCool project, funded by ESA, DLR and CNES through the MAP programme.     

Undercooling in Co–Cu alloys and its effect on solidification structure

Undercooling behaviour and solidification morphology change of various Co–Cu alloys were examined. Each alloy was melted in an alumina crucible under an argon atmosphere by high-frequency induction, and the cyclic heating and cooling was repeated several times in the temperature range between 1300 and 1850 K. The temperature change during the experiment was analysed under the Newtonian cooling assumption. The temperature curve showed that the undercooling in a first few cycles was negligibly small but it increased remarkably. The alloy was undercooled below the metastable liquid miscibility gap after the next several cycles. In these samples, liquid separation was observed. The homogeneously mixed spherical grains of copper-enriched phase were observed in cobalt-enriched matrix for the samples solidified immediately after the liquid separation. The two melts became coarser after the separation by mutual coalescence. In the case of the slow start of the solidification after the separation, they formed a clear interface between the upper cobalt-enriched layer and the lower copper-enriched layer located in the lower part according to the density difference between the two melts. It depended on the cooling rate after liquid separation. The very fine duplex structure can be obtained by the rapid cooling of the melt at the initial stage of the separation.     

Surface Tension,Phase Separation,and Solidification of Undercooled Cobalt–Copper Alloys

By using electromagnetic levitation, liquid Cu–Co alloys can be undercooled below their liquidus temperature into the metastable miscibility gap, leading to a phase separation into a cobalt‐rich L₁ phase and a cobalt‐poor L₂ phase. This paper reports on experimental and theoretical investigations into the properties of this system, including equilibrium shape, surface, and interfacial tension, phase separation, as well as solidification behavior. Solidification experiments were performed in microgravity in order to minimize the effect of convection on the resulting microstructure.     

An effective approach for bonding of TZM and Nb-Zr system:Microstructure evolution,mechanical properties,and bonding mechanism

A novel method of liquid metallic film(LMF)bonding was developed to join titanium zirconium molyb-denum alloy(TZM)and Nb-Zr alloy with a Ni interlayer.Using this method,a Ni-Zr liquid phase was formed by the eutectic reaction and then squeezed out from the gap due to a transient pressure,leaving an LMF.It not only achieved a reliable metallurgical bonding but also served as a transition layer between TZM and Nb-Zr alloy to reduce the mismatch between them thus further improving its performance.The bonding mechanism of the TZM and Nb-Zr system was discussed based on theoretical calculation and high-resolution microscopy analysis.The advantages of this method were established by comparing the microstructure and mechanical properties of LMF bonded joints with that of traditional contact-reaction brazing and direct diffusion bonding.Additionally,the feasibility of the LMF bonding method was also demonstrated by the reliable joining of other high-temperature and immiscible systems.     

Undercooling and demixing of copper-based alloys

Since the beginning of materials science research under microgravity conditions immiscible alloys have been an interesting subject. New possibilities to investigate such systems are offered by containerless processing techniques. Of particular interest is the ternary system Cu-Fe-Co, and its limiting binaries, Cu-Co and Cu-Fe. They all show a metastable miscibility gap in the regime of the undercooled melt. Within the ESA-MAP project “CoolCop”, different aspects of this alloy have been investigated; results obtained so far are reported here.     

聚丙烯酸酯与聚碳酸酯共混体系相容性及酯交换反应研究

利用示差扫描量热计和傅立叶变换红外光谱仪，研究了双酚Ａ型聚碳酸酯与一系列聚丙烯酸烷基酯的相容性及热处理过程中的酯交换反应，实验证明了ＰＣ与聚丙烯酸酯是不相容的。在高温状态下，共混体系中存在酯交换反应，可以导致该共混体系均相比。     

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

Anumerical 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-In hypermonotectic alloys.The model reflects the real physical processes well and is also applicable to other immiscible alloys.     

On the mathematical theory of the microstructure evolution during the final stage of liquid phase sintering

A new mathematical theory of the microstructure evolution during the final isothermal stage of liquid phase sintering has been developed. The basis of this theory has been formed by materials science and continuum mechanics. The microstructure evolution has been described by the system of non-linear differential equations, which contain more than ten parameters. Finite difference analysis has been performed for a heavy alloy cylindrical specimen. The results of the calculations have been compared with experimental data.