The surface tension and specific heat of stable and metastable liquid Ni70.2Si29.8 eutectic alloy were measured by electromagnetic levitation oscillating drop method and drop calorimetry. The surface tension depends on tempera- ture linearly within the experimental undercooling regime of 0―182 K (0.12 TE). Its value is 1.693 N·m?1 at the eutectic temperature of 1488 K, and the temperature coefficient is ?4.23×10?4 N·m?1·K?1. For the specific heat measurement, the maximum undercooling is up to 253 K (0.17 TE). The specific heat is determined as a polynomial function of tem- perature in the experimental temperature regime. On the basis of the measured data of surface tension and specific heat, the temperature-dependent density, excess volume and sound speed of liquid Ni70.2Si29.8 alloy are predicted theoreti- cally.
Bulk samples and small droplets of liquid Fe-10%Sb alloys are undercooled up to 429 K (0.24TL) and 568 K (0.32TL), respectively, with glass fluxing and free fall techniques. The high undercooling does not change the phase constitution, and only the αFe solid solution is found in the rapidly solidified alloy. The experimental results show that when the undercooling is below 296 K, the growth velocity of αFe dendrite rises exponentially with the increase of undercooling and reaches a maximum value 1.38 m/s. Subsequently, the growth velocity begins to decrease if undercooling further increases. The αFe phase grows into coarse dendrites under small undercooling conditions, whereas it becomes vermicular dendrites in highly undercooled melts. The solute trapping is closely related to the dendrite growth velocity and cooling rate rather than undercooling. Although the solute trapping can be remarkably suppressed by the rapid dendrite growth, the segregationless solidification is not observed in the present experiments due to the large solidification temperature range.
The rapid solidification of Al-30%Cu-18%Ag ternary alloy is investigated by using the free fall method. Its solidified microstructure is composed of θ(Al2Cu), α(Al) and ξ(Ag2Al) phases. The liquidus temperature and solidus temperature are determined as 778 and 827 K, respectively. The alloy melt undercooled amounts up to ΔTMax=171 K (0.20TL). Its microstructural evolution is investigated based on the theoretical analysis of undercooling behavior and nucleation mechanics. It is found that the undercooling increases with the decrease of the diameter of the alloy droplet. When ΔT<78 K, the primary θ (Al2Cu) phase of the alloy grows into coarse dendrite. When 78 K≤ΔT≤171 K, its refined θ (Al2Cu) phase grows alternatively with α(Al) phase. Once ΔT≥171 K, its microstructure is characterized by the anomalous (θ+α+ξ) ternary eutectic.
The metastable liquid phase separation occurs in the ternary Cu50Fe37.5Co12.5 peritectic alloy droplets during free fall. The separated alloy melt rapidly solidifies and evolves core-shell microstructure composed of L1(Cu) and L2(Fe,Co) phases. Based on the determination of the phase transition temperature, the core-shell microstructure evolution, the interfacial energy, the temperature gradient and the Marangoni migration are analyzed. The interfacial energy of the separated liquid phase increases with the decrease of the temperature. The temperature gradient changes from large to small along the radius direction from inside to outside in the alloy droplet. The Marangoni force (FM) acting on the micro-droplet of L2(Fe,Co) phase increases with the increase of the size of the L2(Fe,Co) phase, and decreases with the increase of undercooling. Driven by FM, the micro-droplet of L2(Fe,Co) phase migrates from outside to inside in the alloy droplet, collides and coagulates each other during migration, and then forms different types of core-shell microstructures.
Rapid solidification of bulk Ag42.4Cu21.6Sb36 ternary eutectic alloy is accomplished by glass fluxing method,during which the maximum undercooling attains 114 K (0.16 TE). Under high undercooling conditions,the ternary eutectic consists ofε (Ag3Sb),(Sb)and θ(Cu2Sb)phases,instead of (Ag),(Sb)and θphases as predicted by the phase diagram.In the sample of small undercooling,the alloy microstructure is characterized by the mixture of primary θ(Cu2Sb),(ε+θ) and (ε+Sb) pseudobinary eutectics,and regular (ε+θ+Sb) ternary eutectic.With the increase of undercooling, θ (Cu2Sb) primary phase and pseudobinary eutectics disappear gradually,and ternary eutectic transfers from regular to anomalous structure.When undercooling exceeds 102 K,anomalous (ε+θ+Sb) ternary eutectic is the unique microstructure.Competitive nucleation and growth of these three eutectic phases is the main cause for the formation of complex growth morphologies.Based on the current experiments and theoretical calculations,it can be concluded that the intermetallic compound phaseθ(Cu2Sb) is the leading nucleating phase.
Rapid growth behavior of ■ phase has been investigated in the undercooling experiments of Cu-14%Ge, Cu-15%Ge, Cu-18.5%Ge and Cu-22%Ge alloys. Alloys of the four compositions obtain the maximum undercoolings of 202 K(0.17TL), 245 K(0.20TL), 223 K(0.20TL) and 176 K(0.17TL), respectively. As the content of Ge increases, the microstructural transition of "α (Cu) dendrite + ■ peritectic phase →■ peritectic phase →■ dendrite + (ε +■ ) eutectic" takes place in the alloy at small undercooling, while the microstructural transition of "fragmented α (Cu) dendrite + ■ peritectic phase →■peritectic phase →■ dendrite + ε phase" happens in the alloy at large undercooling. EDS analysis of the Ge content in ■ peritectic phase indicates that undercooling enlarges the solid solubility of α dendrite, which leads to a decrease in the Ge content in ■ phase as undercooling increases. In the Cu-18.5%Ge alloy composed of ■ peritectic phase, the Ge content in ■ phase increases when undercooling increases, which is due to the restraint of the Ge enrichment on the grain boundaries by high undercooling effect.
The specific heat and related thermophysical properties of liquid Fe77.5Cu13Mo9.5 monotectic alloy were investigated by an electromagnetic levitation drop calo-rimeter over a wide temperature range from 1482 to 1818 K.A maximum under-cooling of 221 K(0.13 Tm)was achieved and the specific heat was determined as 44.71 J·mol1·K1.The excess specific heat,enthalpy change,entropy change and Gibbs free energy difference of this alloy were calculated on the basis of experimental results.It was found that the calculated results by traditional esti-mating methods can only describe the solidification process under low under-cooling conditions.Only the experimental results can reflect the reality under high undercooling conditions.Meanwhile,the thermal diffusivity,thermal conductivity,and sound speed were derived from the present experimental results.Furthermore,the solidified microstructural morphology was examined,which consists of(Fe)and(Cu)phases.The calculated interface energy was applied to exploring the correlation between competitive nucleation and solidification microstructure within monotectic alloy.
The solidification characteristics of three types of Pb-Sb-Sn ternary alloys with different primary phases were studied under substantial undercooling conditions. The experimental results show that primary (Pb) and SbSn phases grow in the dendritic mode, whereas primary (Sb) phase exhibits faceted growth in the form of polygonal blocks and long strips. (Pb) solid solution phase displays strong affinity with SbSn intermetallic compound so that they produce various morphologies of pseudobinary eutectics, but it can only grow in the divorced eutectic mode together with (Sb) phase. Although (Sb) solid solution phase and SbSn intermetallic com- pound may grow cooperatively within ternary eutectic microstructures, they sel- dom form pseudobinary eutectics independently. The (Pb)+(Sb)+SbSn ternary eutectic structure usually shows lamellar morphology, but appears as anomalous eutectic when its volume fraction becomes small. EDS analyses reveal that all of the three primary (Pb), (Sb) and SbSn phases exhibit conspicuous solute trapping effect during rapid solidification, which results in the remarkable extension of sol- ute solubility.
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 undercool- ings. 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.
The liquid to solid transformation of ternary Ag42.4Cu21.6Sb36 eutectic alloy was ac- complished in an ultrasonic field with a frequency of 35 kHz, and the growth mechanism of this ternary eutectic was examined. Theoretical calculations predict that the sound intensity in the liquid phase at the solidification interface increases gradually as the interface moves up from the sample bottom to its top. The growth mode of (ε + θ + Sb) ternary eutectic exhibits a transition of "divorced eutectic— mixture of anomalous and regular structures—regular eutectic" along the sample axis due to the inhomogeneity of sound field distribution. In the top zone with the highest sound intensity, the cavitation effect promotes the three eutectic phases to nucleate independently, while the acoustic streaming efficiently suppresses the coupled growth of eutectic phases. In the meantime, the ultrasonic field accelerates the solute transportation at the solid-liquid interface, which reduces the solute solubility of eutectic phases.
