Minority Phase Droplets Research Articles

The Influences of Micro-Alloying Element Sn and Magnetic Field on the Microstructure Evolution of Al–Bi Immiscible Alloys

An investigation was conducted through directional solidification experiments to explore the impact of micro-alloying element Sn and a magnetic field on the solidification behavior of immiscible Al–Bi alloys, as well as the combined effect of Sn and the magnetic field. Experimental results show that the size distribution of the dispersed particles in the low-speed solidified Al–3.4 wt.%Bi alloy presents two peaks, while it only shows one peak when solidified at a relatively high speed. The addition of Sn not only can enhance the nucleation rate and the number density of the Bi-rich droplets in the sample, but also decrease the Marangoni migration velocity and the axial resultant velocity of minority phase droplets in front of the solidification interface. Thereby it promotes the formation of Al–Bi alloys with a well-dispersed microstructure. A static magnetic field with the strength of 0.2 T increases the number density of the dispersed particles and decreases the average size and the size distribution width of the dispersed particles. Under the effect of Sn addition and static magnetic field, the average radius of the dispersed particles R and the solidification velocity V0 satisfy R∝V0−1/3 when the alloy was solidified at a relatively low velocity, R and V0 satisfy R∝V0−1/2 when the alloy is solidified at a high velocity.

Microstructure evolution of immiscible alloy solidified under the effect of composite electric and magnetic fields

Solidification experiments were performed with Lead–Aluminum immiscible alloy under the effect of composite electric and magnetic fields (CEMFs). The results demonstrate that CEMFs not only decrease the size of minority phase particles (MPPs) but also promote a more uniform distribution of the MPPs. A theoretical model was built to describe the microstructure evolution during cooling the immiscible alloy. The solidification process of Pb–0.4 wt.%Al alloy under the effect of the CEMFs was simulated. The numerical results are well consistent with the experimental data. These results demonstrate that CEMFs affect the solidification process through changing melt convection and the nucleation behavior of minority phase droplets (MPDs). On one hand, the CEMFs can inhibit the convection and lead to the homogeneous distribution of MPPs along the radial direction of the sample. On the other hand, the CEMFs can increase the nucleation driving force for the MPDs, which decreases the average radius of MPDs and boosts the formation of dispersed solidification structures. This research indicates that the application of CEMFs is a promising strategy for controlling the microstructure of immiscible alloys.

Microstructure evolution in immiscible alloys during rapid directional solidification

Abstract A model has been developed by taking into account the common action of the nucleation, the diffusional growth, the collisions and coagulations of the minority phase droplets and the spatial phase segregation to describe the microstructure evolution in a rapidly directionally solidified immiscible alloy. Numerical solutions have been performed to investigate the microstructure evolution in a directionally solidified hypermonotectic Al –Pb alloy. It has been shown that an undercooled region appears in front of the solid/liquid interface and the liquid –liquid decomposition takes place there. If the solidification velocity is high enough, a steady state of solidification can be established and a fine dispersion of minority phase particles in a hypermonotectic alloy can be obtained.

Effect of microgravity on the solidification of aluminum\u2013bismuth\u2013tin immiscible alloys

Directional solidification experiment was carried out with Al-Bi-Sn immiscible alloy under microgravity environment onboard the Tiangong 2 space laboratory of China. Sample with a well-dispersed microstructure was obtained by properly designing the experimental scheme, the matrix shows equiaxed morphology, and there is no visible gas cavity or pinhole in the sample. In contrast, the reference samples solidified on earth show phase-segregated structure and contain some gas cavities or pinholes. The grain morphology of the terrestrial sample depends on the solidification direction, it is equiaxed when the sample ampoule was withdrawn against the gravity direction, while it is columnar when the sample ampoule was withdrawn along the gravity direction. The solidification process and affecting mechanisms of microgravity on the microstructure formation are discussed. The results indicate that the microgravity conditions can effectively diminish the convective flow of the melt and the Stokes motions of the minority phase droplets and gas bubbles, which are helpful for suppressing the occurrence of macro-segregation and preventing the formation of porosity. The results also demonstrate that the microgravity conditions favor the detachment between the melt and the wall of crucible, thus increasing the nucleation undercooling of α-Al nuclei and promoting the formation of equiaxed grain.

Solidification of Al-Bi-Sn immiscible alloy under microgravity conditions of space

Al-Bi-Sn alloy was solidified directionally in space. The sample shows a well-dispersed microstructure. A model was developed to describe the microstructure formation during solidification. Calculations demonstrate that the Stokes settlement of the Minority Phase Droplets (MPDs) and the convection of the melt are against the establishment of the steady state of solidification and macro-segregation occurs soon after the beginning of the solidification on ground. The diminished melt convection and the Stokes motions of the MPDs under microgravity conditions promote the establishment of the steady state of solidification and the formation of a well-dispersed microstructure.

Tsallis Entropy Index q and the Complexity Measure of Seismicity in Natural Time under Time Reversal before the M9 Tohoku Earthquake in 2011.

The observed earthquake scaling laws indicate the existence of phenomena closely associated with the proximity of the system to a critical point. Taking this view that earthquakes are critical phenomena (dynamic phase transitions), here we investigate whether in this case the Lifshitz–Slyozov–Wagner (LSW) theory for phase transitions showing that the characteristic size of the minority phase droplets grows with time as is applicable. To achieve this goal, we analyzed the Japanese seismic data in a new time domain termed natural time and find that an LSW behavior is actually obeyed by a precursory change of seismicity and in particular by the fluctuations of the entropy change of seismicity under time reversal before the Tohoku earthquake of magnitude 9.0 that occurred on 11 March 2011 in Japan. Furthermore, the Tsallis entropic index q is found to exhibit a precursory increase.

Continuous Solidification of Immiscible Alloys and Microstructure Control

Immiscible alloys have aroused considerable interest in last few decades due to their excellent physical and mechanical characteristics as well as potential industrial applications. Up to date, plenty of researches have been carried out to investigate the solidification of immiscible alloys on the ground or in space and great progress has been made. It is demonstrated that the continuous solidification technique have great future in the manufacturing of immiscible alloys, it also indicates that the addition of surface active micro-alloying or inoculants for the nucleation of the minority phase droplets and proper application of external fields, e.g., static magnetic field, electric current, microgravity field, etc. may promote the formation of immiscible alloys with an expected microstructure. The objective of this article is to review the research work in this field.

Solidification of Pb–Al Alloys Under the Influence of Electric Current Pulses

Continuous solidification experiments are carried out with Pb–Al alloys under the influence of the electric current pulses (ECPs). The results demonstrate that the ECPs mainly affect the microstructure formation through changing the energy barrier for the nucleation of the minority phase droplets (MPDs) and minority phase particles (MPPs) during cooling Pb–Al alloys in the liquid–liquid and liquid–solid phase transformation temperature ranges in advance of the solidification of the matrix liquid. For Pb–Al alloys with Al-rich droplets/particles as the minority phase, the ECPs lower the energy barriers for the nucleation of the MPDs/MPPs and cause a significant increase in the nucleation rate of the MPDs/MPPs and, thus, promote the formation of Pb–Al alloys with a well-dispersed or even nanoparticles dispersed microstructure. The ECPs parameters show an important influence on the microstructure formation of Pb–Al alloys. The refinement extent of the MPDs/MPPs increases with the increase in the peak current density. For a given peak current density, the refinement extent of the MPDs/MPPs increases with the increases in the pulse frequency and pulse width first, and then level off and become asymptotic.

Microstructure evolution during the liquid-liquid phase transformation of Al-Bi alloys under the effect of TiC particles

Solidification experiments were carried out for Al-Bi alloys with the addition of TiC. It is demonstrated that the average size of the Bi-rich minority phase particles (MPPs) shows a tendency of keeping constant, increasing, decreasing and then keeping constant again with the increase of the additive amount of TiC. The MPPs can be refined effectively with the addition of an appropriate amount of TiC. A model was developed to describe the kinetic behavior of the TiC particles in the melt and the microstructure evolution during the liquid-liquid (L-L) phase transformation of the Al-Bi alloys with the addition of TiC. The numerical results show that TiC particles may dissolve and coarsen during the heating period, and precipitate out during the cooling period of the melt. There may be two kinds of TiC particles: the one remained during holding temperature and the one precipitated during a cooling of the melt. Both of them can work as the inoculants for the nucleation of the minority phase droplets. What determines the refining efficiency is the resultant number density of the TiC particles in the melt cooled to the binodal line temperature. This research demonstrates that inoculation is an effective method for controlling the L-L phase transformation. The addition of the efficient inoculant may promote the formation of a monotectic alloy with a well dispersed microstructure.

Influence of a Direct Current on the Solidification of Immiscible Alloys

Continuous solidification experiments were carried out with immiscible alloys under the effect of a direct current. The experimental results demonstrate that a direct current shows a significant effect on the migration of minority phase droplets (MPDs) in continuously solidified immiscible alloys. It can promote the formation of a well dispersed microstructure or a core/shell microstructure. A model describing the kinetics of the microstructure evolution in a continuously solidified immiscible alloy was developed. The microstructure formation in the alloys was calculated. The numerical results are in favorable agreement with the experimental ones. They demonstrate that a direct current may affect the microstructure development through changing the spatial motions of MPDs. The alloys show a well dispersed microstructure when they are solidified with such a direct current density that the direct current causing motion of the MPDs is almost equivalent to the radial component of the Marangoni migration velocity of the MPDs. The alloys show a core/shell microstructure when they are solidified with such a direct current density that the direct current causing motion of the MPDs dominates the migration of the MPDs along the radial direction of the sample. A wire or rod with well dispersed microstructure or a core/shell microstructure can be prepared by solidifying immiscible alloys under the effect of a direct current properly chosen.

Stable phase separation and heterogeneity away from the coexistence curve

Phase separation, i.e., the coexistence of two different phases, is observed in many systems away from the coexistence curve of a first-order transition, leading to a stable heterogeneous phase or region. Examples include various quantum ferromagnets, heavy-fermion systems, rare-earth nickelates, and others. These observations seem to violate basic notions of equilibrium thermodynamics, which state that phase separation can occur only on the coexistence curve. We show theoretically that quenched disorder allows for phase separation away from the coexistence curve even in equilibrium due to the existence of stable minority-phase droplets within the majority phase. Our scenario also answers a related question: How can a first-order transition remain sharp in the presence of quenched disorder without violating the rigorous lower bound $\nu \geq 2/d$ for the correlation-length exponent? We discuss this scenario in the context of experimental results for a large variety of systems.

Influence of electric current pulses on the solidification of Cu-Bi-Sn immiscible alloys.

Continuous solidification experiments were carried out with Cu-Bi-Sn alloys under the effects of Electric Current Pulses (ECPs). A model describing the microstructure evolution was developed. The formation of the microstructure in the continuously solidified alloys was calculated. The calculations demonstrated that ECPs mainly affect the solidification process through changing the energy barrier for the nucleation of the minority phase droplets (MPDs). When the matrix liquid has a lower electric conductivity compared to the MPD, the ECPs lead to a decrease in the energy barrier for the nucleation of the MPDs which then promote the formation of a finely dispersed microstructure. When the matrix liquid has a higher electric conductivity compared to the MPD, the ECPs cause an increase in the energy barrier for the nucleation and lead to the formation of a phase segregated microstructure.

Microstructure Evolution of a Ternary Monotectic Alloy During Directional Solidification

A model was developed to describe the microstructure evolution in a directionally solidified ternary monotectic alloy. The directional solidification experiments were carried out on Al–3Pb–1Sn (wt%) alloys by using a Bridgman apparatus. The microstructure 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.

Solidification of Immiscible Alloys and Convective Effect<sup></sup>

A model describing the microstructure formation in a directionally solidified immiscible alloy under the convective effect is presented. The microstructure evolution in a directionally solidified Al-Pb alloy is investigated. It is demonstrated that convective flows have great effects on the solidification of immiscible alloys. A convective flow against the solidification direction causes an increase in the nucleation rate while a convective flow along the solidification direction causes a decrease in the nucleation rate. The convective flows lead to a more uneven distribution of the minority phase droplets in the melt. It causes an increase in the size of the largest minority phase droplets and is against the obtaining of the immiscible alloys with a well dispersed microstructure.

EFFECT OF Sn ON THE SOLIDIFICATION PROCESS AND MICROSTRUCTURE OF Al-Pb MONOTECTIC ALLOYS

When a homogeneous, single-phase liquid of monotectic alloy is cooled into the miscibility gap, the components are no longer miscible and two liquid phases develop. Generally, the liquid-liquid decomposition causes the formation of the microstructure with serious phase segregation. Many efforts have been made to use the demixing phenomenon for the production of well dispersed composite materials. It has been demonstrated that the rapid directional solidification technique is an effective method to prevent the formation of the phase segregated microstructure in immiscible alloys. Directional solidification experiments were carried out to study the influence of the addition of Sn on the solidification process of Al-Pb alloys. The experimental results show that the addition of a small amount of Sn causes a decrease in the interface energy between the matrix and the minority phase liquids and, thus, an increase in the nucleation rate of the minority phase droplets during the liquid-liquid phase transformation or a decrease in the average size of the minority phase particles. With the increase of the Sn content, both the volume fraction of the minority phase droplets and the temperature range of the liquid-liquid phase zone and the liquid-liquid-solid tertiary phase zone of the phase diagram increase. These are favorable for the coarsening of the minority phase droplets. The addition of Sn leads to the formation of a dendrite solid/liquid interface. This may promote the formation of a well dispersed microstructure and shows great effect on the distribution of the minority phase particles.

Solidification Behavior of Immiscible Alloys under the Effect of a Direct Current

Directional solidification experiments were carried out with Al–Pb alloys under the effect of a direct current (DC). The experimental results show that the DC causes a migration of the minority phase droplets (MPDs) from the middle part to the surface region of the sample. Samples with either a finely dispersed microstructure or a shell/core structure were obtained by solidifying the alloy under the effect of the properly selected DC densities. A model was developed to describe the microstructure evolution in an immiscible alloy directionally solidified under the effect of the DC. The microstructure formation in the Al–Pb alloys was calculated. The numerical results are in favorable agreement with the experimental ones. They demonstrate that the DC affects the microstructure formation mainly through changing the spatial motions of the MPDs and the temperature field of the melt in front of the solid/liquid interface. The formation mechanisms of the finely dispersed microstructure as well as the shell/core structure were sufficiently clarified.

Rapid solidification behavior of Cu–Co–Fe alloy

Abstract

Influence of the static high magnetic field on the liquid–liquid phase separation during solidifying the hyper-monotectic alloys

Magnetic in-situ quenching refers to fixing and quenching the sample at a static high magnetic field (SHMF) up to 18 T; it has been achieved by a specially designed facility. Zn-7wt%Bi and Zn-10wt%Bi hyper-monotectic melts were quenched under different magnetic flux densities to investigate the influence of SHMF on the liquid–liquid phase separation process in solidifying hyper-monotectic alloys. Because this separation is mainly caused by the growth of minority phase droplets (Bi droplets in the present study), and such growth is attributed to the diffusion of Bi element and the coalescence between the droplets, the influence of SHMF on the growth of Bi droplets was analyzed. Results show that the imposed SHMF prevented the formation of layered structure in the Zn-10wt%Bi alloy and refined the Bi particles in the Zn-7wt%Bi alloy, which indicates that the SHMF retarded the liquid–liquid phase separation during solidifying the hyper-monotectic alloys. Indeed, the two motions of droplets in determining the coalescence, Marangoni migration and Stocks sedimentation, were slowed down by the applied SHMF. Analytical estimations of the magnitude of such damping effect have been made and show that the 18 T SHMF could reduce the speed of Stokes sedimentation and Marangoni migration of the minority phase droplets by about 95.5 % and 62.4 %, respectively.

Effect Mechanism of a Direct Current on the Solidification of Immiscible Alloys

Continuous solidification experiments are carried out with Al-Pb alloys under the effect of a direct current. The microstructure evolution in the samples is calculated. The numerical results obtained indicate that a direct current has great effects on the solidification of immiscible alloys. It mainly affects the microstructure evolution during a liquid-liquid decomposition by changing the spatial motions of the minority phase droplets and the temperature field in front of the solidification interface. A sample with either a well dispersed or a core/shell microstructure can be obtained by solidifying Al-Pb alloys under the effect of properly selected direct currents.

Microstructure Formation in a Gas-Atomized Drop of Al-Pb-Sn Immiscible Alloy

Al-Pb-Sn alloy is rapidly solidified by using the high-pressure gas-atomization technique. A model describing the microstructure evolution in an atomized drop is developed. This model takes into account the concurrent actions of the nucleation, diffusional growth, spatial motions of the minority-phase droplets, and the solidification of the matrix liquid. The microstructure formation in the gas-atomized drop is simulated by coupling the thermodynamic and kinetic calculations. The numerical results show a favorable agreement with the experimental results. They demonstrate that under the rapid cooling conditions of gas atomization, the spatial phase separation due to the Marangoni migration of the minority-phase droplets mainly affects the microstructure formation in a small region close to the atomized drop surface. All the minority-phase droplets are nucleated during the liquid-liquid phase decomposition period. For an Al-(5-9) wt pct Pb-3 wt pct Sn alloy, the average radius and number density of the minority-phase particles depend exponentially on the powder diameter. The microstructure formation process is discussed in detail.