Correlated process of phase separation and microstructure evolution of ternary Co-Cu-Pb alloy
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- Yan, N., Wang, W.L., Luo, S.B. et al. Appl. Phys. A (2013) 113: 763. doi:10.1007/s00339-013-7586-6
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The phase separation and rapid solidification of liquid ternary Co45Cu42Pb13 immiscible alloy have been investigated under both bulk undercooling and containerless processing conditions. The undercooled bulk alloy is solidified as a vertical two-layer structure, whereas the containerlessly solidified alloy droplet is characterized by core-shell structures. The dendritic growth velocity of primary α(Co) phase shows a power-law relation to undercooling and achieves a maximum of 1.52 m/s at the undercooling of 112 K. The Pb content is always enriched in Cu-rich zone and depleted in Co-rich zone. Numerical analyses indicate that the Stokes motion, solutal Marangoni convection, thermal Marangoni convection, and interfacial energy play the main roles in the correlated process of macrosegregation evolution and microstructure formation.
As one of the most important phase transformation phenomena, phase separation is commonly observed in various condensed matter systems such as polymers, oxides, and metallic alloys [1–3]. Especially for metallic alloys, the phase separation process of immiscible alloy melts has attracted extensive research both theoretically and experimentally in relevant fields for several decades [4, 5]. Such alloys provide an effective approach to the development of novel advanced composite materials applied to self-lubricating bearings and high electrical conductivity devices . The properties of these alloys are determined, to a great extent, by their solidification mechanism and final microstructure. A uniformly dispersive structure is expected to form under microgravity condition or during rapid solidification due to the suppression of gravity-dependent phenomena such as buoyancy-driven convection. However, the removal of the effect of gravity alone cannot insure a dispersive structure for immiscible alloys, but induce the formation of various morphologies . Many gravity independent factors, such as thermocapillary convection, the minimization of interfacial energy between immiscible liquids  and the preferential wetting of container wall by one of the liquid phases  may also cause migration, coalescence, and massive segregation of immiscible liquids.
The dendritic growth within undercooled immiscible alloy melts has also drawn more and more attention due to its potential technological applications [9, 10]. When bulk undercooling becomes large, the dendrite growth velocity is usually enhanced and the solidification kinetics is far from equilibrium condition. The related nonequilibrium effects have been studied by many researchers for simple alloys . Similarly, the phase separation of binary immiscible alloys has been extensively investigated . In contrast, the research on ternary immiscible alloys has been mainly focused on some simple systems . Further investigations remain to be done on the phase separation and dendrite growth mechanisms of more complicated ternary immiscible alloys. The Co-Cu based alloy, which is characteristic of metastable phase separation has been widely investigated owing to their excellent giant magnetoresistance (GMR) properties . In order to control the phase separation and solidification microstructure of the alloy, different third elements have been introduced to binary Co-Cu alloy systems . The objective of this work is to investigate the macrosegregation formation and microstructure evolution mechanisms of ternary Co-Cu-Pb immiscible alloy under both bulk undercooling and containerless processing conditions. Special attention is also paid to the dendritic growth of primary α(Co) phase within undercooled ternary Co45Cu42Pb13 immiscible alloy.
2 Experimental methods
The bulk samples were prepared from the high purity elements Co (99.95 %), Cu (99.999 %), and Pb (99.999 %). Co and Cu were primarily melted in an arc-melting furnace and then melted together with Pb by induction heating under the protection of B2O3 fluxing agent in argon atmosphere. Each sample had the mass of 1 g and was contained in an 8 mm internal diameter × 10 mm outer diameter × 12 mm length alumina crucible together with a suitable amount of 70 % B2O3 + 20 % Na2B4O7 + 10 % CaF2 fluxing agent. The vacuum chamber was evacuated to 2×10−4 Pa and then backfilled with argon gas until 105 Pa. The sample was melted and superheated to 200∼300 K above its liquidus temperature. After being maintained at the overheating state for 3∼5 min, the sample was cooled naturally by switching off the induction heating power. Its heating and cooling curves were recorded by an infrared pyrometer, while the dendrite growth velocity during rapid solidification was measured with an infrared photodiode device.
As an alternative approach to achieve rapid solidification, the bulk alloy melt was atomized into a series of tiny droplets with diameters from 100 to 800 μm. These alloy droplets fell freely inside a 3 m drop tube  and were solidified in a containerless state. After the experiments, the solidified samples were sectioned, polished, and then etched with a solution of 5 g FeCl3 + 10 ml HCl + 50 ml H2O for about 8 s. The phase constitution and solute distribution of the solidified alloy were examined with Rigaku D/max 2500 X-ray diffractometer and Oxford INCA Energy 300 energy dispersive spectrometer, respectively. Their microstructural morphologies were analyzed with a Zeiss Axiovert 200MAT optical microscope and FEI Sirion 200 scanning electron microscope.
3 Results and discussion
3.1 Macrosegregation formation and dendritic growth under bulk undercooled condition
3.2 Phase separation and microstructure evolution during free fall
To have a clear understanding of the solute redistribution characteristics in rapidly solidified Co45Cu42Pb13 alloy droplets, EDS analysis was used. The average compositions of Co-rich and Cu-rich zones are demonstrated in Figs. 1(b) and (c). It is shown that the content of solute elements in liquid phases decreases with the reduction of droplet diameter. When the droplet diameter is 800 μm, the Co and Pb solute contents in Cu-rich zones are 9.2 and 14.9 at%, respectively. With the decrease of droplet diameter, the solubilities of Co and Pb in Cu-rich zones reduce to 5.8 and 14.1 at% in the droplet with a diameter of 200 μm. Also, in the Co-rich zone, the Cu and Pb solute contents display a declining variation of 36.2–27.2 at% Cu and 2.5–1.5 at% Pb when the droplet diameter changes from 800 to 200 μm. The Pb element content is always rich in the Cu-rich zone and poor in Co-rich zone. As is clearly seen in Fig. 1(a), in the droplet with diameter of 800 μm, the alloy melt separates into a Co-rich zone with the composition Co61.3Cu36.2Pb2.5 (designated as point C1) and a Cu-rich zone with the composition Co9.2Cu75.9Pb14.9 (marked as point C2). Similarly, in an alloy droplet of 200 μm diameter, the alloy melt is separated into a Co-rich zone with the composition Co71.3Cu27.2Pb1.5 (designated as point D1) and a Cu-rich zone with the composition Co5.8Cu80.1Pb14.1 (marked as point D2). It should also be noticed that the solubility of Pb in Cu-rich zone is almost five times larger than that in Co-rich zone, indicating that Pb element has a stronger affinity with the Cu liquid and a repulsive behavior with the Co liquid.
3.3 Dynamic characteristics of phase separation process
Physical parameters for evaluating the interfacial energies of immiscible liquids
ρ0/103 kg m−3
In calculation, the (Co), (Cu), and (Pb) phases are taken as f.c.c. crystal structures. Figures 5(b) shows the calculated results of interfacial energies between pseudobinary liquid phases of Co-Cu, Cu-Pb, and Co-Pb systems in respective miscibility gaps [17, 18]. The interfacial energy is zero at each critical temperatures and increases with the decrease of temperature. The interfacial energy between the Co-Pb system is much larger than that of Cu-Pb and Co-Cu systems. This means that the Pb-rich liquid has a weak wettability with the Co-rich liquid if the Co-, Cu-, and Pb-rich liquids can coexist in the ternary system at a certain temperature. Thus, Pb element tends to exist in the Cu-rich liquid and is always isolated from (Co) phase by a thin layer of (Cu) phase.
Physical parameters of two different alloy droplets
Thermal conductivity of Co-rich liquid phase, k1 (W m−1 K−1)
Thermal conductivity of Cu-rich liquid phase, k2 (W m−1 K−1)
Viscosity of Co-rich liquid phase, η1 (Pa s−1)
Viscosity of Cu-rich liquid phase, η2 (Pa s−1)
Density of Co-rich liquid phase, ρ1 (kg m−3)
Density of Cu-rich liquid phase, ρ2 (kg m−3)
The calculated results for the above two types of Marangoni migrations of Cu-rich globules are demonstrated in Fig. 6(b). The Cu-rich globule with a radius of 20 μm displays a thermal Marangoni migration velocity of 0.52 mm s−1 within the alloy droplet of 800 μm diameter. In contrast, it migrates at a solutal Marangoni migration velocity of 16.8 mm s−1, which is about thirty times as large as the thermal Marangoni migration velocity. Since a smaller alloy droplet with 200 μm diameter maintains a larger temperature and concentration gradients , the corresponding thermal and solutal Marangoni convections propel a Cu-rich globule of 20 μm radius to migrate at velocities as high as 0.86 and 40.3 mm s−1, respectively. It is noteworthy that the solutal Marangoni migration velocity is much larger than thermal Marangoni migration velocity within the alloy droplet during phase separation. And the above two Marangoni migration velocities of Cu-rich globules are about three to five orders of magnitude larger than the Stokes motion velocities. Solutal Marangoni migration can completely dominate the phase separation process inside freely falling alloy droplets. Under such conditions, the Cu-rich liquid globules mostly accumulate at the droplet surface, forming a Cu-rich shell layer surrounding the Co-rich core. Therefore, Marangoni convection contributes significantly to the phase separation and structure evolution of ternary Co45Cu42Pb13 immiscible alloy under free fall condition.
The solidification microstructure is always composed of α(Co), (Cu) and (Pb) solid solution phases. The undercooling of bulk alloy varies from 30 to 112 K at the cooling rate of 11∼18 K/s. In contrast, the cooling rate of alloy droplets attains 1.5×103 to 4.3×104 K/s in drop tube.
The macrosegregation pattern of the undercooled bulk alloy solidified under glass fluxing condition shows a vertical two-layer structure, which is composed of a top Co-rich zone and a bottom Cu-rich zone. As a comparison, the containerlessly solidified alloy droplets in drop tube are mainly characterized by two- or three-layer core-shell structures, where the Co-rich zone is located at droplet center and the Cu-rich shell surrounds the Co-rich core.
The measured dendritic growth velocity of primary α(Co) phase increases with the enhancement of undercooling by a power relation, which achieves a maximum value of 1.52 m/s at the undercooling of 112 K.
The Pb solute has a stronger affinity with Cu-rich liquid and a repulsive behavior with Co-rich liquid, which is mainly caused by the high interfacial energy within Co-Pb system.
Stokes motion is the major dynamic mechanism of Cu-rich globules during the phase separation of undercooled bulk alloy. However, it is significantly suppressed and the solutal Marangoni convection becomes the dominant role in the macrosegregation formation process during the containerless rapid solidification inside drop tube.
The authors are grateful to Dr. W.J. Xie, Dr. H.P. Wang, Mr. J. Chang, and Mr. S.S. Xu for their help with the experiments and helpful discussion. This work was financially supported by National Natural Science Foundation of China under Grant Nos. 50971105 and 51101123, and the Fundamental Research Fund of Northwestern Polytechnical University under Grant No. JC20110278.