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.

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.

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.

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.

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.

Mass transport in a highly immiscible alloy on extended shear deformation

Forced mixing to a single-phase or supersaturated solid solution(SSS)and its prerequisite microstructure evolution in immiscible systems has been a focus of research for fundamental science and practical applications.Controlling the formation of SSS by shear deformation could enable a material design beyond conventional equilibrium microstructure in immiscible systems.Here,a highly immiscible Cu-50 at.%Cr binary alloy(mixing enthalpy of∼20 kJ mol^(−1))was employed to investigate the microstructure evolution and localized tendencies of SSS during severe shear deformation.Our results demonstrate the dislocation mediated microstructural refinement process in each phase of the binary alloy and the mechanisms associated with localized solute supersaturation as a function of shear strain.Pronounced grain refinement in the softer Cu phase occurs owing to the strain localization driving the preferential dynamic recrystallization.The grain refinement of the Cr phase,however,is enabled by the progressive evolution of grain lamination,splitting,and fragmentation as a function of shear strain.The solute supersaturation is found to be strongly dependent on the local environments that affect the dislocation activity,including the level of microstructure refinement,the interfacial orientation relationship,the mechanical incompatibility,and the localized preferential phase oxidation.Ab initio simulations confirm that it is more favorable to oxidize Cr than Cu at incoherent Cu/Cr interfaces,limiting the mass transport on an incoherent boundary.Our results unveil the mechanism underpinning the non-equilibrium mass transport in immiscible systems upon severe deformation that can be applied to produce immiscible alloys with superior mechanical properties.

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(Cu0.5Fe0.5)40Zr60 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.

Solidification of Immiscible Alloys： A Review

Immiscible alloys gained a considerable interest in last decades due to their valuable properties and potential applications. Many experimental and theoretical researches were carded out worldwide to investigate the solidification of immiscible alloys under the normal gravity and microgravity condition. The objective of this article is to review the research work in this field during the last few decades.

Selective laser melting of bulk immiscible alloy with enhanced strength:Heterogeneous microstructure and deformation mechanisms

To overcome the dimension limits of immiscible alloys produced by traditional techniques and enhance their mechanical properties,bulk Cu-Fe-based immiscible alloy with abundant nanotwins and stacking faults was successfully produced by selective laser melting(SLM).The SLM-produced bulk immiscible alloy displays a heterogeneous microstructure characterized by micro-scaledγ-Fe particles dispersed in fineε-Cu matrix with a high fraction(~92%)of high-angle grain boundaries.Interestingly,abundant nanotwins and stacking faults are generated in the interior of nano-scaledγ-Fe particles embedded withinε-Cu matrix.The heterogeneous interface of soft domains(ε-Cu)and hard domains(γ-Fe)not only induces the geometrically necessary dislocations(GNDs)but also affects the dislocation propagation during plastic deformation.Therefore,the bimodal heterogeneous interface,and the resistance of nanotwins and stacking faults to the propagation of partial dislocation make the bulk immiscible alloy exhibit an enhanced strength of~590 MPa and a good ductility of~8.9%.

Microstructure Formation in Al-Bi-Co Immiscible Alloys Directionally Solidified with Different Melt Superheat Temperatures

The directional solidification has been carried out for the AI-413i-2,5Co （wt pct） alloys with different melt superheat temperatures. The microstructure characterization and the quantitative metallographic analysis have been performed. The results indicated that the Bi-rich sphere size and cellular spacing decrease with increasing melt superheat temperature. The interaction between the advancing solidification interface and the Bi-rich spheres with different sizes was analyzed. The effect of the melt superheat treatment on microstructure evolution was discussed for the immiscible alloys. The microstructure development in ternary Al-Bi-Co alloys directionally solidified with different melt superheat temperatures was clarified.

Imaging of Structure Evolution in Solidifying Al–Bi Immiscible Alloys by Synchrotron Radiography

Using synchrotron X-ray imaging technique,the segregation evolution in solidifying Al-10 wt% Bi immiscible alloys was investigated at different cooling rates.Irrespective of the cooling rate,most of the Bi solute appeared at the upper part of the sample after solidification.The reason for this Bi enrichment phenomenon is different for different cooling rates.Besides Marangoni motion,positive segregation,which has rarely been noticed before,can also make Bi solute transfer to the hot top zone.It is also found that,bubbles（or pores） appear in solidifying Al-10 wt% Bi alloys,and the number of bubbles（or pores） increases with the increase of the cooling rate,while the size of the bubbles（or pores） decreases.

Observation of Bi Coarsening and Dissolution Behaviors in Melting Al–Bi Immiscible Alloy

Using synchrotron radiography, a melting process of the AI-10 wt% Bi immiscible alloy was fully observed, and Bi solute coarsening and dissolution behaviors as well as their effect on the microstructural evolution were also investi- gated. We found that two different steps including coarsening and dissolution processes can be used to describe the structure evolution in melting AI-10 wt% Bi immiscible alloy. And diffusion coalescence controlled the coalescence of Bi droplets at the early stage of the melting AI-10 wt% Bi immiscible alloy, while the coagulation dttring movement did at later stage. Also, the larger the droplet, the larger the distance of the steady-state concentration distribution around the droplet.

Ultrafine-grained Nb-Cu immiscible alloy implants for hard tissue repair:Fabrication,characterization,and in vitro and in vivo evaluation

The biocompatible metallic implants with strong osteointegration often lack the ability of anti-infection.The biocompatible niobium(Nb)containing the antibacterial copper(Cu),the obtained Nb-Cu alloy,could be a potential candidate to solve this issue.To test this hypothesis,ultrafine-grained Nb-Cu immiscible alloys were fabricated via mechanical alloying and spark plasma sintering.The aim of this study was to investigate the microstructure,mechanical properties,magnetic susceptibility,corrosion behavior,ion release,and the bactericidal activity,biocompatibility and osteogenic potential of the Nb-Cu alloys in vitro and their osteogenesis and osteointegration ability in vivo with a comparison with pure Nb.The rat cranial defect model and the bone screws insertion in rabbit femoral bone were used to evaluate the osteogenesis and osteointegration ability,respectively.The results showed that after the addition of 3 wt.%of Cu,the compressive strength was significantly improved from 1.57 GPa to 2.21 GPa and the magnetic susceptibility slightly decreased.The Nb-3 wt.%Cu(Nb-3Cu)alloy exhibited higher corrosion resistance than pure Nb in Hank’s solution and strong bactericidal activity against both E.coli and S.aureus.In vitro,the Nb-3Cu alloy showed comparable biocompatibility with pure Nb.The addition of 3 wt.%Cu also significantly enhanced the expression of osteogenesis-related genes(RUNX2,ALP,COLA1 and OCN)of pre-osteoblasts.In vivo,the Nb-3Cu alloy promoted bone regeneration at the defect sites and showed enhanced osteointegration after 12 weeks of implantation.Such a good combination of high mechanical strength and corrosion resistance,strong antibacterial activity and improved osteogenesis and osseointegration ability enables the present Nb-3Cu alloy a promising candidate for heavy load-bearing hard tissue repair.

Solidification of Immiscible Alloys Under the Effect of Electric and Magnetic Fields

Immiscible alloys have aroused considerable interests in the last decades on account of their special physical andmechanical properties and potential applications. A considerable number of researches have been implemented toinvestigate the solidification behaviors of immiscible alloys in electric and magnetic fields. It has been indicated that themagnetic field and electric current can remarkably affect the solidification process and microstructures of immisciblealloys. The solidification techniques under the effects of electric and magnetic fields have great potentials for the fabri-cation of immiscible alloys. This paper reviews the research work in this field in recent years.

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.

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.

Effect of Brownian Coagulation on the Liquid-liquid Decomposition in Gas-atomized Alloy Drops

Modeling and simulation have been carried out for Al-Pb alloys to investigate the Brownian coagulation effect on the microstructure development in a gas-atomized drop during the liquid-liquid decomposition. The results indicate that Brownian coagulation has a weak effect on the nucleation and a relatively strong effect on coarsening the minority phase droplets. The influence of Brownian coagulation on the liquid-liquid decomposition decreases with the increase in the diameter （or the decrease in the cooling rate） of the atomized drop.

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.