Monotectic Solidification Research Articles

Multiple ultrasounds assisted phase separation and monotectic solidification of liquid ternary Al81.5Cu14.7Bi3.8 immiscible alloy

Multiple power ultrasounds were employed to investigate the phase transition process of ternary Al81.5Cu14.7Bi3.8 immiscible alloy by various exerting modes. As the ultrasonic sources increased, the liquid phase separation pattern transformed from (Bi)-rich layered macrosegregation into the uniform distribution of secondary (Bi) droplets. Meanwhile, the primary (Al) phase evolved from coarse dendrites into plenty of small spherical grains and also tended to be uniformly dispersed. The subsequently formed ternary (Al) + (Al2Cu) + (Bi) monotectic structure, featured by the alternative (Al) and (Al2Cu) lamellar structure with fine (Bi) grains distributed, was coarsened first and then refined. Numerical simulations showed that the transient cavitation and the acoustic streaming strength were significantly enhanced by increasing ultrasonic beams, with the fourfold ultrasounds producing the most prominent effects on the phase separation process. The intensive and enlarged cavitation areas greatly accelerated the nucleation of both the secondary liquid phase and primary solid phase, which refined the growing (Bi) droplets and (Al) dendrites. The strength and morphology of acoustic streaming were the key factors in offsetting Stokes motion and carrying the growing grains to various regions, resulting in a uniform microstructure. Furthermore, increasing ultrasonic sources improved the friction and wear properties of the solidified alloy, which indicated that the Al81.5Cu14.7Bi3.8 immiscible alloy may become an excellent wear-resistant material owing to the uniform monotectic structure fabricated by the fourfold ultrasounds.

Phase separation and subsequent solidification of peritectic Fe–Cu–Ge alloys subjected to substantial undercooling processing

Liquid ternary Fe50-x/2Cu50-x/2Gex (x = 5 and 15) alloys were highly undercooled by glass fluxing technique, and the achieved maximum undercoolings are 300 K (0.18TL) and 268 K (0.16TL), respectively. At small undercoolings, these two alloys solidify in a normal pathway of peritectic solidification. If the alloy undercooling continues to increase beyond a certain value, metastable liquid phase separation takes place. The critical undercooling for Fe47.5Cu47.5Ge5 alloy is 70 K and that for Fe42.5Cu42.5Ge15 alloy is 110 K. At larger undercoolings, the homogeneous liquid alloy separates into an Fe-rich zone and a Cu-rich zone. In such a case, the undercooled alloy displays a monotectic solidification structure. The rapid solidification of the Fe-rich zone is the keystone of alloy solidification, and the growth velocity of the primary phase rises with increasing alloy undercooling according to a power law relation. Within the experimental undercooling ranges, the primary γFe phase of Fe47.5Cu47.5Ge5 alloy attains a maximum growth velocity of 19 m/s, while the growth velocity of the primary Fe0.84Ge0.16 phase reaches 11 m/s in undercooled Fe42.5Cu42.5Ge15 alloy. Moreover, energy dispersive spectroscopy (EDS) analysis shows that the Fe-rich and Cu-rich liquid phases have a similar affinity with solute Ge.

Liquid phase separation and monotectic structure evolution of ternary Al62.6Sn28.5Cu8.9 immiscible alloy within ultrasonic field

Power ultrasound was introduced to investigate its effects on the liquid phase separation and monotectic solidification process of ternary Al62.6Sn28.5Cu8.9 immiscible alloy. As ultrasound power increases, the macrosegregation induced by liquid demixing is strikingly reduced. Meanwhile, a large number of spherical ternary (Al+Sn+θ(Al2Cu)) monotectic cells are formed within ultrasonic field. Cavitation effect, which produces strong shockwaves and facilities the radial symmetry of temperature, concentration, and flow fields in front of liquid–solid interface accounts for these microstructure variations.

A comparative study of dendritic growth within undercooled liquid pure Fe and Fe50Cu50 alloy

The dendritic growth characteristics within undercooled liquid pure Fe and binary Fe50Cu50 alloy were investigated by glass fluxing and electromagnetic levitation methods. The growth velocity of pure Fe dendrite increases with undercooling, which reaches 69ms−1 at the maximum undercooling of 280K (0.15Tm). The microstructure appears as well-defined dendrites under slight undercooling, while it is characterized by a network of grain boundaries without discernible dendrites under substantial undercooling. A glass fluxing method was used to study the dendritic growth within the undercooled liquid Fe50Cu50 alloy. The maximum undercooling achieved was 261K (0.15TL), and the corresponding growth velocity was 15ms−1. A critical undercooling for metastable liquid phase separation of 7K was determined. At larger undercoolings, the sample separates into an Fe-rich zone and a Cu-rich zone. The microstructure of the Fe-rich zone displays well-branched dendrites in the condition of slight undercooling, whereas it transforms into monotectic solidification structures in the highly undercooled regime. Slender dendrites and spheres are characteristics of the structural morphology of the Cu-rich zone at all of the undercoolings. Energy-dispersive spectroscopy analysis shows that the average compositions of phase-separated zones have a large deviation from the original alloy composition, and conspicuous solute trapping has taken place.

The Effect of Shear Flow on the Formation of <I>L</I><SUB>2</SUB> Droplet During Directional Monotectic Solidification

The Effect of Shear Flow on the Formation of <I>L</I><SUB>2</SUB> Droplet During Directional Monotectic Solidification

Thermoelectric performance of the ordered In4Se3–In composite constructed by monotectic solidification

In4Se3 has recently been found to be a layered compound which shows promising thermoelectric (TE) properties. In the present study, a textured In4Se3-based material grown by the zone melting technique is found to be a composite with elemental indium embedded in the In4Se3 matrix. The TE properties of this composite are evaluated and a maximum figure of merit ZT of 0.9 is achieved in the direction perpendicular to the growth direction. Such a result should provide a useful guidance for composition control towards further ZT enhancement in this material.

Solidification in syntectic and monotectic systems

We present theoretical studies of syntectic and monotectic solidification scenarios. Steady-state solidification along the liquid-liquid interface in a syntectic system is considered by means of a boundary-integral technique developed previously. We study the case of small asymmetry of the pattern and extract from the results the scaling relations in terms of the undercooling and the asymmetry parameter. We also investigate monotectic solidification using the phase-field method. We present two kinds of two-phase fingers, with the solid phase being either the exterior phase or the interior phase, and the pattern corresponding to the growth along the solid-liquid interface. We finally analyze the asymptotic shape of these new morphologies far behind their tip.

Spinodal decomposition and droplets entrapment in monotectic solidification

In this article, we present two models to simulate solidification morphologies in monotectic alloys. With the first model, we investigate the morphological evolution under the influence of spinodal decomposition. The model requires that a gradient energy contribution for the concentration field should be incorporated, in order to stabilize phase separation when the liquid concentration is inside the region of miscibility gap. The free energy of the system in this model is derived from direct interpolation of the bulk energy densities. This, however, results in simulation regions in nanometer scale due to contributions from the chemical free energy of the system to the total surface excess. With the second model, our purpose is to develop a phase-field model to simulate scales that are larger than nanometer, where the departures from equilibrium are very small resulting in phase concentrations outside the spinodal region. In view of this, we exclude the concentration gradient contribution to the grand chemical potential functional, and develop a model based on [M. Plapp, Phys. Rev. E 84, 031601 (2011); A. Choudhury and B. Nestler, Phys. Rev. E 85, 021602 (2012)]. The advantage is that the free energy excess across the interface at equilibrium disappears, and hence it is easier to derive the required surface energies with higher interface widths. Due to this benefit, we employ the method to simulate the dynamic entrapment process in the monotectic reaction and study the influence of liquid(1) - liquid(2) surface energy and undercooling on the entrapment process.

Regular Structure Formation of Hypermonotectic Al-In Alloys

Since the liquid / liquid separation occurs in hypermonotectic alloys and the liquid / liquid interface agitates mass transfer around solidifying front, it is rather difficult to achieve the aligned-rod structure. The high magnetic filed such as 10T achieved the aligned-rod structure in the Al-10at%In alloys. The in-situ observations of the monotectic solidification in the Al-10at%In alloys were performed using synchrotron radiation X-ray. Coarse and fine In rods coexisted during the unidirectional solidification without magnetic field. The local melt flow induced by the In-rich liquid / Al-rich liquid interface enhanced the mass transfer and consequently the coarse rods could continuously grow. The suppression of the local melt flow of which scale was several 10 m by the high static magnetic field resulted in the aligned-rod structure.

The real-time, high-resolution x-ray video microscopy of solidification in aluminum alloys

The directional solidification of thin alloy sheets in a Bridgman furnace has been studied by x-radiography using high-brilliance synchrotron x-radiation in combination with a low-noise, fast-readout camera. Spatial resolutions down to 1.5 μm and a temporal resolution of about 0.15 s have permitted real-time video microscopy of microstructural evolution during columnar and equiaxed dendrite growth and eutectic and monotectic growth. The technique has also allowed for direct observations of important solidification phenomena such as dendrite fragmentation and porosity formation, primarily in aluminium alloys. As a result, insights have been gained into mechanisms of dendrite fragmentation, criteria for dendrite tip kinetics and interface stability during transient growth, and microstructure formation mechanisms during monotectic solidification. The results are expected to be important for validation of dendrite growth models. This paper presents a review of the technique as well as examples of images obtained during solidification of aluminum alloys.

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 undercoolings. 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.

Fabrication of Porous Aluminium and Copper Media by Using Monotectic Solidification under a Magnetic Field

Unidirectional solidification of Al-In, Cu-Pb and Cu-Pb-Al monotectic alloys was performed under static magnetic fields up to 10T for formation of the regularly aligned-rod structure. mThe imposition of static magnetic fields exceeding 4T suppressed movement of the In liquid droplets at the Al solidifying front and enhanced the engulfment of the In droplets into the front. As a result, the eutectic-like structure was obtained in the Al-10 and 15at%In hypermonotectic alloys. The micro X-ray tomography indicated that the continuous In rods with diameters of 10-20 µm were aligned parallel to each other. In the case of the Cu-Pb and the Cu-Pb-Al alloys, the imposition of static magnetic fields also enhanced the formation of the aligned Pb rods. The electrochemical etching by using a 10% HNO3 solution successfully removed the minor phase, and the porous aluminum and copper with deep pores were fabricated.

On a Morphological Transition in Monotectic Solidification

Al-Bi and Al-In of exact monotectic composition were directionally solidified with solidification velocities ranging from 0.3 µm/sec to 5 µm/sec and temperature gradients from 2 K/mm to 6.5 K/mm. Three distinct microstuctures could be observed: fibers, strings of pearls or irregular arrays of second phase droplets. The transitions from fibers to strings of pearls is discussed theoretically and compared with the experimental results. The origin of this transition seems to be a Nichols and Mullins instability of the fibers during cooling from the monotectic to the eutectic temperature caused by surface perturbations driving volume diffusion of solute in the liquid fibers of the second phase.

Laser cladding of quasi-crystal-forming Al-Cu-Fe-Bi on an Al-Si alloy substrate

We report here the results of an investigation aimed at producing coatings containing phases closely related to the quasi-crystalline phase with dispersions of soft Bi particles using an Al-Cu-Fe-Bi elemental powder mixture on Al-10.5 at. pct Si substrates. A two-step process of cladding followed by remelting is used to fine-tune the alloying, phase distribution, and microstructure. A powder mix of Al64Cu22.3Fe11.7Bi2 has been used to form the clads. The basic reason for choosing Bi lies in the fact that it is immiscible with each of the constituent elements. Therefore, it is expected that Bi will solidify in the form of dispersoids during the rapid solidification. A detailed microstructural analysis has been carried out by using the backscattered imaging mode in a scanning electron microscope (SEM) and transmission electron microscope (TEM). The microstructural features are described in terms of layers of different phases. Contrary to our expectation, the quasi-crystalline phase could not form on the Al-Si substrate. The bottom of the clad and remelted layers shows the regrowth of aluminum. The formation of phases such as blocky hexagonal Al-Fe-Si and a ternary eutectic (Al + CuAl2 + Si) have been found in this layer. The middle layer shows the formation of long plate-shaped Al13Fe4 along with hexagonal Al-Fe-Si phase growing at the periphery of the former. The formation of metastable Al-Al6Fe eutectic has also been found in this layer. The top layer, in the case of the as-clad track, shows the presence of plate-shaped Al13Fe4 along with a 1/1 cubic rational approximant of a quasi-crystal. The top layer of the remelted track shows the presence of a significant amount of a 1/1 cubic rational approximant. In addition, the as-clad and remelted microstructures show a fine-scale dispersion of Bi particles of different sizes formed during monotectic solidification. The remelting is found to have a strong effect on the size and distribution of Bi particles. The dry-sliding wear properties of the samples show the improvement of wear properties for Bi-containing clads. The best tribological properties are observed in the as-clad state, and remelting deteriorates the wear properties. The low coefficient of friction of the as-clad and remelted track is due to the presence of approximant phases. There is evidence of severe subsurface deformation during the wear process leading to cracking of hard phases and a change in the size and shape of soft Bi particles. Using these observations, we have rationalized possible wear mechanisms in the Bi-containing surface-alloyed layers.

Rapid monotectic solidification during free fall in a drop tube

Droplets of Ni-31.4%Pb monotectic alloy with different sizes are rapidly solidified during free fall in a drop tube. The theoretical calculations indicate that the undercooling was achieved before solidification exponentially depends on droplet diameter. The maximum undercooling of 241 K (0.15Tm) is obtained in the experiments. With the increase of undercooling, the volume fraction of monotectic cells increases, and the L2(Pb) grains are refined. Calculations of the nucleation rates of L2(Pb) and α-Ni phases indicate that L2(Pb) phase acts as the leading nucleation phase during the monotectic transformation.

Rapid monotectic solidification during free fall in a drop tube

Rapid monotectic solidification during free fall in a drop tube

Fabrication of porous aluminum with deep pores by using Al–In monotectic solidification and electrochemical etching

This paper presents fabrication of porous Al with deep pores. Unidirectional solidification under a magnetic field of 10 T achieved the uniformly aligned structure of the In rods (10–20 μm in diameter) in the Al matrix for the Al–10at%In alloys. The micro X-ray computer tomography showed the continuous In rods in the matrix. The electrochemical etching successfully removed the In rods by using a 10% HNO 3 solution. Consequently, the porous Al with deep pores was fabricated.

The effect of marangoni convection on fibrous monotectic growth

Directional solidification of monotectic alloys can lead, under certain conditions of growth velocity and temperature gradient in the melt, to composite microstructures with a rodlike appearance. For a theoretical description, most often the Jackson and Hunt model of rod eutectic growth is used. A relation between the mean rod distance R and the solidification velocity v, similar to eutectics, is generally observed in experiments. A comparison between theory and experiments always led to discrepancies not yet fully resolved. In the approach presented here, we propose an additional mode of mass transport ahead of the monotectic solidification front, namely, Marangoni convection. The main difference between monotectic and eutectic solidification seems to be the liquid phase state of the (rod) L2 phase growing simultaneously within a nearly perfectly pure solid matrix. We assume that the thermocapillary effect causes convection at the interface of the fibrous liquid phase. This Marangoni convection induces a flow field in the matrix and has an influence on the solute transport if the fluid flow Peclet number is large enough. First, an analytical model is described using a suitably modified convective diffusion equation for the concentration field; and, second, a numerical model is presented. The main result of both models is a new functional relation between fiber spacing and solidification velocity being explicitly dependent on the temperature gradient ahead of the solidification front.

Monotectic composite growth with fluid flow

It is a well-known fluid-mechanical phenomenon that thermocapillary forces induce surface convection on a fluid–fluid interface. This so-called Marangoni convection depends on the variation of the surface energy along the interface. In our present work we focus our attention on the evolution of a fibrous monotectic microstructure with liquid L 2 fibers. We will show, that the Marangoni convection has a strong influence on the transport of solute in front of the solidification front, despite the flow induced by density differences. The resulting flow field affects the constitutional undercooling and therefore the mean undercooling of a monotectic solidification front. In a previous paper we discussed qualitatively the influence of fluid flow on the microstructure evolution of composite monotetic growth (C. Stöcker, L. Ratke, J. Crystal Growth 203 (1999) 582). We introduced an analytical model that takes the density differences of the phases and the surface convection on the L 1–L 2 surface into consideration. With this extended Jackson and Hunt theory for composite monotectic growth we derived a characteristic equation for the inter-rod distance depending on solidification velocity and temperature gradient. In this paper we develop a more accurate model. We solve numerically the diffusion equation coupled with the Navier–Stokes equation in the L 1 phase to find the minimal undercooling for a given velocity and temperature gradient. We derive a Jackson and Hunt diagram and show that the fluid flow leads to a strong dependence of the inter-rod distance on the temperature gradient opposite to eutectic solidification.

Analysis of monotectic growth: infinite diffusion in the L2 phase

The Jackson-Hunt model of eutectic solidification is applied to monotectic solidification in which a liquid (L1) transforms into rods of a different liquid (L2) in a solid matrix. Limiting cases of no diffusion and infinite diffusion (complete mixing) in the L2 phase are considered. An adaptive refinement and multigrid algorithm (MGGHAT) is used to obtain numerical solutions for the concentration field in the L1 phase; this allows consideration of a general phase diagram. Density differences between the three phases, which cause fluid flow, are treated approximately. Specific calculations are carried out for aluminum-indium alloys. Infinite diffusion in the L2 phase has only a small effect on the relationship between interface undercooling and rod spacing.