Wei Bingbo

High undercooling and rapid solidification of Ni32.5% Sn eutectic alloy

Abstract Ni32.5% Sn eutectic alloy has been undercooled up to 397 K (0.283 T E ) by levitation melting and superheating within 45 special inorganic glasses. Its rapid solidification behaviour is studied by high speed infrared pyrometry and cinematography. It is found that interface heterogeneous nucleation may take place in preference to homogeneous nucleation, even though undercooling exceeds 0.2 T E . The transient solidification velocity during recalescence attains 784 mm/s at an undercooling of 302 K. If undercooling is below 100 K, an abundance of feathery lamellar eutectic grains form, and β-Ni 3 Sn serves as a better nucleant for α-Ni phase than vice versa. As undercooling increases, both β-Ni 3 Sn and α-Ni phases can nucleate primarily and independently, and a microstructure morphology transition from regular lamellar eutectic into irregular anomalous eutectic occurs. It is concluded that anomalous eutectic is the product of rapid solidification whereas lamellar eutectic forms under slow solidification conditions. An undercooling of 376 K leads to an extension of Sn solid solubility in α-Ni phase from the equilibrium limit of 19.0% to a maximum of 25.8%.

Unidirectional dendritic solidification under longitudinal resonant vibration

Abstract Longitudinal steady-state sinusoidal vibration is introduced into the unidirectional dendritic solidification process of Al−3% Mg alloy. The first (470 Hz), second (1050 Hz), and third (1736 Hz) order resonant frequencies of solidification system is utilized to produce strong vibrational response out of less exciting energy. Solidification front temperature is raised by vibration under constant solidification conditions, whereas liquid temperature gradient ahead of solidification front and solidification rate are both reduced. Dendrite fragmentation is the most conspicuous variation of structural morphology, which is characterized by primary stalk disintegration and secondary arm detachment. The first order resonance results in a periodically fractured bamboo-like dendritic structure and most severe solute segregation. Contrary to its grain refinement effect, vibration expedites the coarsening of resultant structures. The alloy mechanical properties are markedly deteriorated under the first order resonance. But they are appreciably improved if the second and third order resonant vibrations are applied.

Rapid Growth of Ice Dendrite in Acoustically Levitated and Highly Undercooled Water

Water drops with diameters ranging from 2.5 to 4 mm are highly undercooled by up to 24 K with the acoustic levitation technique. Compared to the case of water contained in a tube, acoustic levitation has efficiently avoided the heterogeneous nucleation from container walls and consequently increased the undercooling level. However, the cavitation effect induced by ultrasound may prematurely catalyse nucleation, which hinders the further achievement of bulk undercooling. The growth velocity of ice dendrite determined experimentally in highly undercooled water is characteristic of rapid dendritic growth, which reaches 0.17 m/s at the undercooling of 24 K. The Lipton-Kurz-Trivedi dendritic growth model is used to predict the kinetic characteristics of rapid growth of ice dendrite under high undercooling conditions, which shows good agreement with the experimental results.

Formation of ζ phase in Cu-Ge peritectic alloys

Rapid growth behavior of ζ 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 + ζ peritectic phase → ζ peritectic phase → ζ dendrite + (ɛ+ζ) eutectic” takes place in the alloy at small undercooling, while the microstructural transition of “fragmented α(Cu) dendrite + ζ peritectic phase → ζ peritectic phase → ζ dendrite + ɛ phase” happens in the alloy at large undercooling. EDS analysis of the Ge content in ζ peritectic phase indicates that undercooling enlarges the solid solubility of α dendrite, which leads to a decrease in the Ge content in ζ phase as undercooling increases. In the Cu-18.5%Ge alloy composed of ζ peritectic phase, the Ge content in ζ phase increases when undercooling increases, which is due to the restraint of the Ge enrichment on the grain boundaries by high undercooling effect.

The liquid phase separation of Bi-Ga hypermonotectic alloy under acoustic levitation condition

Containerless treatment of Bi-58.5at%Ga hypermonotectic alloy is successfully performed with acoustic levitation technique. Under acoustic levitation condition, the second phase (Ga) distributes almost homogeneously in solidification sample, opposite to macrosegregation in solidification sample under conventional condition. Stokes motion of the second liquid droplet (Ga) is significantly restrained under acoustic levitation condition. The analyses indicate that the melt vibration in the gravity direction forced by acoustic field can induce steady flow around the second liquid droplet, which influences droplet shape during its moving upward and consequently restrains Stokes motion velocity of the second liquid droplet.

Resonance Shift of Single-Axis Acoustic Levitation

The resonance shift due to the presence and movement of a rigid spherical sample in a single-axis acoustic levitator is studied with the boundary element method on the basis of a two-cylinder model of the levitator. The introduction of a sample into the sound pressure nodes, where it is usually levitated, reduces the resonant interval Hn (n is the mode number) between the reflector and emitter. The larger the sample radius, the greater the resonance shift. When the sample moves along the symmetric axis, the resonance interval Hn varies in an approximately periodical manner, which reaches the minima near the pressure nodes and the maxima near the pressure antinodes. This suggests a resonance interval oscillation around its minimum if the stably levitated sample is slightly perturbed. The dependence of the resonance shift on the sample radius R and position h for the single-axis acoustic levitator is compared with Leungs theory for a closed rectangular chamber, which shows a good agreement.

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.

Thermophysical properties of Ni-5%Sn alloy melt

The surface tension and specific heat of Ni-5%Sn alloy melt were measured by the oscillating drop method and the drop calorimetric method using electromagnetic levitation, respectively. The temperature coefficient of surface tension is 6.43×10−4 N·m−1K−1 within the temperature regime of 1464–1931 K. The enthalpy change was measured in the temperature range from 1461 to 1986 K, and the average specific heat was obtained as 43.03 J·mol−1K−1. Some other thermophysical properties, such as viscosity, solute diffusion coefficient, density, thermal diffusivity and thermal conductivity of this alloy melt, were derived based on the experimentally measured surface tension and specific heat. Using these thermophysical parameters, the relation between solute trapping and undercooling in rapidly solidified α-Ni was calculated, and the theoretical prediction shows a good agreement with experimental data.

A molecular dynamics study on surface properties of supercooled water

Molecular dynamics simulations were performed to study the surface properties of water in a temperature range from 228 to 293 K by using the extended simple point charge (SPC/E) and four-site TIP4P potentials. The calculated surface tension increases with the decrease of temperature, and moreover the slopes of the surface tension-temperature curves show a weak rise below 273 K, whereas no obvious anomalies appear near 228 K, which accords with the previous experiments. Compared with the measured values, the SPC/E potential shows a good agreement, and the TIP4P potential underestimates the surface tension. The main reason for that may be the reasonable description of the surface structure of supercooled water for the SPC/E. When simulating the orientational distributions of water molecules near the surface, the SPC/E potential produces higher ordering and larger surface potentials than the TIP4P potential.

Surface tension and specific heat of liquid Ni70.2Si29.8 alloy

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 temperature 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 temperature 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 theoretically.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 temperature 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 temperature 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 theoretically.