L. Hu

Thermodynamic properties and solidification kinetics of intermetallic Ni7Zr2 alloy investigated by electrostatic levitation technique and theoretical calculations

The thermodynamic properties, including the density, volume expansion coefficient, ratio of specific heat to emissivity of intermetallic Ni7Zr2 alloy, have been measured using the non-contact electrostatic levitation technique. These properties vary linearly with temperature at solid and liquid states, even down to the obtained maximum undercooling of 317u2009K. The enthalpy, glass transition, diffusion coefficient, shear viscosity, and surface tension were obtained by using molecular dynamics simulations. Ni7Zr2 has a relatively poor glass forming ability, and the glass transition temperature is determined as 1026u2009K. The inter-diffusivity of Ni7Zr2 alloy fitted by Vogel–Fulcher–Tammann law yields a fragility parameter of 8.49, which indicates the fragile nature of this alloy. Due to the competition of increased thermodynamic driving force and decreased atomic diffusion, the dendrite growth velocity of Ni7Zr2 compound exhibits double-exponential relationship to the undercooling. The maximum growth velocity is...

Dendrite growth within supercooled liquid tungsten and tungsten-tantalum isomorphous alloys

The dendrite growth in both supercooled liquid pure W and binary W-Ta isomorphous alloys has been observed and measured by an electrostatic levitation technique. The liquid W and W-x%Ta (xu2009=u200925, 50, 75) alloys were substantially supercooled by up to 733u2009K (0.2u2009Tm) and 773u2009K (0.23TL), respectively. The measured density and the ratio of specific heat to emissivity displayed a linearly increasing tendency versus supercooling. The thermal dendrites in supercooled liquid tungsten achieved a maximum growing velocity of 41.3u2009m·s−1, and the concurrent recalescence process exhibited Johnson-Mehl-Avrami type kinetics. Liquid W-Ta alloys showed stronger supercoolability but a lower maximum dendrite growth velocity of only 35.2u2009m·s−1. The dendritic growth kinetics was always characterized by a power function relation to liquid supercooling. The microstructure of equiaxed grains transforms to the well-developed dendrites with the increase of supercooling. The grain refinement effect resulting from dendrite fragmentati...

Density Measurement and Atomic Structure Simulation of Metastable Liquid Ti-Ni Alloys

The temperature dependence of the densities of liquid Ti-Ni alloys was investigated by the electrostatic levitation technique and molecular dynamics simulation. The average cooling rate by natural radiation decreases with a reduction in Ti content and reaches its minimum at Ti55Ni45 alloy. The Ti-Ni alloy system exhibits a negative excess volume and it becomes smaller with the increase in undercooling. This indicates that the interactions among atoms are enhanced with the decrease in temperature. The pair correlation functions and static structure factors are obtained from the molecular dynamics results. It is found that the packing of the Ni atoms does not occur through replacement of the Ti atoms with the addition of Ni atoms. In addition, the clusters are abundant in liquid Ti-Ni alloys, and a tetragonal bipyramid atomic configuration of may exist. It is found that the Ni-Ni bonds transform to Ti-Ni bonds with the increase in Ni content.

Dynamics of water drop detachment from a superhydrophobic surface induced by an ultrasonic field

We present the dynamics of sessile water drops during their detachment from a superhydrophobic surface induced by ultrasound. The superhydrophobic surface not only serves as a reflector of the ultrasound emitted from the source but also reduces the adhesive force between the drop and the solid surface. The drop is subject to an acoustic radiation force in the ultrasonic field due to the nonlinear effect of the latter. By shifting the reflector upward to approach the first resonance distance, the sessile drop is first elongated in the vertical direction, with its contact line and contact angle decreasing, and finally detaches from the superhydrophobic surface when the acoustic radiation force overcomes the sum of the gravitational and adhesive forces. The acoustic radiation pressure and acoustic radiation force are calculated by solving the acoustic field with the finite element method. The results indicate that the distribution of acoustic radiation pressure provides the upward force to make the drop detach. After its detachment from the reflector, the drop undergoes vertical vibration accompanied by shape oscillations. Oscillations of a water drop that is pinned on the reflector are also demonstrated.We present the dynamics of sessile water drops during their detachment from a superhydrophobic surface induced by ultrasound. The superhydrophobic surface not only serves as a reflector of the ultrasound emitted from the source but also reduces the adhesive force between the drop and the solid surface. The drop is subject to an acoustic radiation force in the ultrasonic field due to the nonlinear effect of the latter. By shifting the reflector upward to approach the first resonance distance, the sessile drop is first elongated in the vertical direction, with its contact line and contact angle decreasing, and finally detaches from the superhydrophobic surface when the acoustic radiation force overcomes the sum of the gravitational and adhesive forces. The acoustic radiation pressure and acoustic radiation force are calculated by solving the acoustic field with the finite element method. The results indicate that the distribution of acoustic radiation pressure provides the upward force to make the drop deta...

Local atomic structure correlating to phase selection in undercooled liquid Ni-Zr peritectic alloy

The local atomic structure and its correlation with phase selection during rapid solidification of undercooled liquid Ni5Zr peritectic alloy has been investigated by combining the electrostatic levitation technique and first-principles molecular dynamics simulations. The calculated density agrees well with the present experimental measurements. The partial pair distribution functions indicate that liquid Ni5Zr alloy exhibits a remarkable chemical short range order, which leads to a preferred association of Ni and Zr atoms. Atomic three-dimensional structure analyses reveal that a high fraction of atom pairs locates in perfect or distorted icosahedral environments (>36.9%) although the number of the fully developed icosahedra or distorted icosahedra is rather rare (<2.5%), implying that this liquid contains a lot of fragmented clusters with perfect or distorted pentagonal faces. Moreover, we demonstrate that the short range orders of the undercooled liquid differ topologically from those in the ideal Ni5Zr crystal but are similar to those in the ideal Ni7Zr2 crystal in the temperature range from the liquidus temperature of 1612u2009K to 1400u2009K, which indicates that the nucleation of primary phase Ni7Zr2 from the undercooled liquid is structurally favored due to the low free energy barrier. However, the nucleation of primary Ni7Zr2 phase is presumed to be replaced by the direct formation of the peritectic Ni5Zr phase if a sufficient undercooling is achieved, which is ascertained by the solidified microstructures and X-ray diffraction patterns. These results shed light on the relationship between the local atomic structure and phase selection during the rapid solidification of the undercooled peritectic alloys.

Dendrite growth kinetics of βZr phase within highly undercooled liquid Zr−Si hypoeutectic alloys under electrostatic levitation condition

The liquid Zr100-xSix (xu2009=u20091, 3, and 5) alloys were substantially undercooled by up to 392u2009K (0.19TL), 423u2009K (0.21TL), and 451u2009K (0.23TL), respectively, under the electrostatic levitation condition. The measured dendrite growth velocity of the primary βZr phase increased with the enhancement of liquid undercooling. Although the undercooling ability showed an increasing tendency with the increase in the Si content, dendrite growth was obviously depressed due to the dominance of solute diffusion controlled growth. Their critical undercoolings for kinetics transition from solute diffusion to thermal diffusion controlled growth were determined. It demonstrates a remarkable refinement of dendrites and an increasing trend of Si solubility in αZr dendrites with increasing undercooling.