B. Wei

Parametric study of single-axis acoustic levitation

Remarkable enhancement of the single-axis acoustic levitation force is achieved by properly curving the surface and enlarging the section of the reflector so as to levitate high density material like tungsten (ρs=18.92u200ag/cm3). A two-cylinder model incorporating the boundary element method simulations is presented for systematic study of the relationship between levitation capabilities and geometric parameters. The model proves to be successful in predicting resonant modes and explaining deviation of the levitated samples near the reflector and driver. The dependence of levitation force on resonant mode, reflector section radius Rb and curvature radius R is revealed and summarized, which agrees with the experiment in principle and suggests that a reflector with large Rb and small R (when Rb/λ⩾0.982) working under mode 1 assures better levitation capabilities.

Acoustic method for levitation of small living animals

Ultrasonic levitation of some small living animals such as ant, ladybug, and young fish has been achieved with a single-axis acoustic levitator. The vitality of ant and ladybug is not evidently influenced during the acoustic levitation, whereas that of the young fish is reduced because of the inadequacy of water supply. Numerical analysis shows that the sound pressures on the ladybug’s surface almost reach the incident pressure amplitude p0 due to sound scattering. It is estimated that 99.98% of the acoustic energy is reflected away from the ladybug. The acoustic radiation pressure pa on the ladybug’s surface is only 1%–3% of p0, which plays a compression role on the central region and a suction role on the peripheral region.Ultrasonic levitation of some small living animals such as ant, ladybug, and young fish has been achieved with a single-axis acoustic levitator. The vitality of ant and ladybug is not evidently influenced during the acoustic levitation, whereas that of the young fish is reduced because of the inadequacy of water supply. Numerical analysis shows that the sound pressures on the ladybug’s surface almost reach the incident pressure amplitude p0 due to sound scattering. It is estimated that 99.98% of the acoustic energy is reflected away from the ladybug. The acoustic radiation pressure pa on the ladybug’s surface is only 1%–3% of p0, which plays a compression role on the central region and a suction role on the peripheral region.

Liquid-liquid phase separation in undercooled Co-Cu alloys

Abstract The liquid–liquid phase separation in undercooled Co–Cu alloy melts has been investigated by differential thermal analysis in combination with glass fluxing technique over a composition range of 16.0–87.2 at.% Cu. The DTA signals, obtained during isochronous cooling, indicate that this separation process is exothermic and proceeds till the rapid solidification of Co-rich liquid phase occurs. The metastable miscibility gap that was determined directly and reproducibly from the onset temperatures of this process is slightly shifted to the Cu-rich side and roughly symmetrical about a Cu concentration of 53 at.%. The measured critical temperature of phase separation is 1547±1.5 K and is about 108 K below the corresponding liquidus temperature. In the present measurements the separated Co-rich liquid always solidified prior to the Cu-rich phase, which crystallized near the peritectic temperature. Lower surface tension and better wetting properties of the Cu-rich liquid phase with glass flux are responsible for the Co-rich phase to be always encased by the Cu-rich phase. In addition, thermodynamic calculations have been accomplished leading to a binodal line, which is in sufficient agreement with the experimental results.

Rapid solidification of Cu84Co16 alloy undercooled into the metastable miscibility gap under different conditions

Abstract The Cu 84 Co 16 alloy melt processed by differential thermal analysis, electromagnetic levitation and drop tube experiences a liquid phase separation when it is undercooled into the metastable miscibility gap. The phase separation and coagulation processes are mainly controlled by the degree of undercooling, cooling rate and convection level in the containerless states. Disperse structures have been formed in droplets solidified during free fall in the drop tube.

Microstructure evolution and solidification kinetics of undercooled Co–Ge eutectic alloys

Abstract The Co-29.7% Ge eutectic and Co-33% Ge hypereutectic alloys processed in drop tube attain large undercoolings and show “lamellar eutectic to anomalous eutectic” and “dendritic to equiaxed” morphology transitions respectively. The eutectic coupled zone is calculated on the basis of current eutectic and dendritic growth theories, which takes the shape of peanut and leans toward Co-rich side.

Specific heat and thermodynamic properties of undercooled liquid cobalt

Co is undercooled to 227 K (0.128 Tm) and its specific heat has been determined as 40.6 Jmol−1u200aK−1 by electromagnetic levitation drop calorimeter, whose good accuracy is verified by the measured specific heat data of pure nickel. On the basis of measured specific heat data, the thermodynamic properties, such as thermodynamic driving force and relative nucleation rate, are calculated. Moreover, the theoretical models of Turnbull and Dubey–Ramachandrarao are also used to calculate the thermodynamic properties for a comparison. It is found that both the thermodynamic driving force and relative nucleation rate have large deviation compared with the calculated results based on experimental data. This indicates that, since the deviation of specific heat value has a drastic influence on thermodynamic driving force and nucleation rate for the undercooled liquid metals, it is necessary to measure the specific heat quantitatively by experiments.

Containerless rapid solidification of highly undercooled Co-Si eutectic alloys

Abstract Droplets of Co–62.1%Si and Co–43.5%Si eutectic alloys with different sizes are rapidly solidified during containerless processing in drop tube. The microstructures of Co–62.1%Si eutectic alloy are mainly characterized by lamellar eutectic plus anomalous eutectic of CoSi 2 and Si phases, whereas those of Co–43.5%Si eutectic alloy are mainly characterized by lamellar eutectic plus anomalous eutectic of CoSi and CoSi 2 phases. For both alloys, the experimental results show that the microstructural evolution depends mainly on undercooling. In the case of Co–62.1%Si eutectic alloy, the smaller the droplet diameter, the larger the volume fraction of anomalous eutectic. When its diameter is very small, the droplet exhibits anomalous eutectic morphology entirely. As for Co–43.5%Si eutectic alloy, with the decrease of droplet size, the microstructural transition proceeds from lamellar eutectic to anomalous eutectic, even to dendrites. The nucleation rates of each phase have been calculated. The TMK eutectic growth and LKT/BCT dendritic growth theories are applied to analyze the rapid solidification process and investigate the microstructural transition mechanisms. The coupled zone around Co–43.5%Si eutectic alloy has also been calculated on the basis of TMK and LKT/BCT models, which covers a composition ranging from 40.8 to 43.8%Si. And calculated coupled zone of Co–62.1%Si covers a composition ranging from 60.2 to 81.6%Si.

Rapid eutectic growth under containerless condition

Rapid eutectic growth in a highly undercooled liquid was accomplished by containerlessly processing Co–Mo eutectic alloy in a drop tube. The containerless state during free fall produces substantial undercoolings up to 391 K (0.24TE) in falling droplets before crystallization. The eutectic growth mechanism is found to transform from lamellar eutectic to anomalous eutectic if the droplet undercooling exceeds a critical value of about 56 K, which coincides well with the lower boundary of the calculated eutectic coupled zone. Although the reduced gravity level of 10−2–10−3u2009g has little influence on this eutectic growth mechanism transition, it frequently results in the formation of spherical anomalous eutectic grains owing to the symmetrical temperature field and concentration field surrounding solid/liquid interface. Both theoretical analyses and experimental observations indicate that the independent nucleation and cooperative branched growth of two eutectic phases are responsible for the eutectic growth m...

Thermophysical properties of undercooled liquid cobalt

The surface tension of undercooled liquid cobalt has been measured by the oscillating-drop technique combined with electromagnetic levitation. The accuracy of the method was verified by measurements of the surface tension of liquid nickel. The liquid cobalt was undercooled by up to 231K (0.13T m), and its surface tension determined to be σCo =1875 0.348(T-T m)mNm-1. From this result, the viscosity, self-diffusion coefficient, density and thermal diffusivity of undercooled liquid cobalt were derived. Using these thermophysical parameters, the growth velocity of cobalt dendrite is calculated and shown to agree well with experimental results. Furthermore, the Marangoni number and the Rayleigh number are calculated; these increase slowly with increasing degree of undercooling.

Rapid solidification behaviour of Ag-Cu-Ge ternary eutectic alloy

Abstract Ag 38.5 Cu 33.4 Ge 28.1 ternary eutectic alloy was undercooled and solidified by glass fluxing method and drop tube technique, respectively, and its growth mode and phase selection in these two processes were investigated. By using B 2 O 3 as a denucleating agent, this ternary eutectic alloy is undercooled by a maximum of 185 K (0.228 T m ). It is found that with the increase of undercooling, a transition from the cooperative growth of three eutectic phases, namely the semiconducting phase (Ge), the solid solution phase (Ag), and the intermetallic compound phase η (Cu 5 Ge 2 ), to the preferential growth of (Ge) phase followed by that of the (Ag) and η phases occurs. The fact that the latter two phases easily form lamellar structure indicates that these two phases have a good affinity. In addition, as undercooling increases, the macrosegregation of semiconducting phase (Ge) becomes weak and the solubilities of three eutectic phases are all extended. Under containerless processing conditions in drop tube, the solidification behaviour is significantly influenced by droplet size which can usually determine the actual magnitude of undercooling. The cooling rate and undercooling level in this process are estimated and their effect on growth kinetics and structural morphology of semiconducting phase (Ge) is discussed.