H.P. Wang

Measurement and calculation of surface tension for undercooled liquid nickel and its alloy

The surface tensions of metastable undercooled liquid nickel and its alloy are experimentally measured and theoretically calculated by electromagnetic levitation oscillating droplet method and molecular dynamics method, respectively. The experimental undercoolings for liquid Ni and Ni90.1Si9.9 alloy are 201 and 206 K, whereas the calculated undercoolings are up to 426 and 323 K. The measured surface tension displays the same undercooling dependence as the molecular dynamics calculation. The surface tension increases linearly with the increase in undercooling and no break occurs at the melting temperature. It is found that the correlation of surface tension with temperature predicted by molecular dynamics calculation agrees with the experimental results for both pure Ni and its alloy.

Remarkable solute trapping within rapidly growing dendrites

Solute microsegregation always takes place during dendritic crystal growth. Although this may be reduced with the increase of crystal growth velocity, the realization of segregationless dendritic growth is quite difficult. Here the authors present the results of remarkable solute trapping within the rapidly growing dendrites of highly undercooled liquid Ni–5wt%Si alloy. The dendrites grow at a velocity of 15m∕s at the maximum experimental undercooling of 304K. Such a high growth velocity results in the pronounced solute trapping and almost segregationless solidification. Furthermore, a model is proposed to describe the correlation between dendritic growth velocity and undercooling. It agrees well with the experimental results in the whole undercooling regime and provides a reasonable prediction for the dendritic growth trend under extremely great undercooling conditions.

Thermophysical properties of a highly superheated and undercooled Ni–Si alloy melt

The surface tension of superheated and undercooled liquid Ni–5 wt % Si alloy was measured by an electromagnetic oscillating drop method over a wide temperature range from 1417 to 1994 K. The maximum undercooling of 206 K (0.13 TL) was achieved. The surface tension of liquid Ni–5 wt % Si alloy is 1.697 N m−1 at the liquidus temperature 1623 K, and its temperature coefficient is −3.97×10−4 N m−1 K−1. On the basis of the experimental data of surface tension, the other thermophysical properties such as the viscosity, the solute diffusion coefficient, and the density of liquid Ni–5 wt % Si alloy were also derived.

Surface tension of superheated and undercooled liquid Co–Si alloy

The surface tension of superheated and undercooled liquid Co 25wt.% Si alloy was measured by an electromagnetic oscillating drop method. The experimental temperature regime was from 1384 to 2339K and a maximum undercooling of 223K (0.14TL) was achieved. The surface tension of liquid Co 25wt.% Si alloy is 1.604Nm−1 at the liquidus temperature of 1607K, and its temperature coefficient is −4.0×10−4Nm−1K−1. On the basis of previous research results of pure Co and Si, an expression is developed to predict the surface tension of binary Co–Si alloy system. The other thermophysical properties, such as the viscosity, the solute diffusion coefficient, and the density of liquid Co 25wt.% Si alloy are also derived by the relevant theoretical models.

Theoretical prediction and experimental evidence for thermodynamic properties of metastable liquid Fe–Cu–Mo ternary alloys

The prediction of thermodynamic properties for metastable undercooled liquid is important in the research on liquid structure and phase transition. Here we report the theoretical prediction of specific heat for metastable undercooled liquid Fe–Cu–Mo ternary alloys with a molecular dynamics method. Furthermore, experimental measurements were also performed by electromagnetic levitation drop calorimeter to confirm the predicted results. For liquid Fe78Cu15Mo7 and Fe71.5Cu3.5Mo25 alloys, the calculated specific heat values are 37.5 and 36.3 J mol−1 K−1, which agree well with the experimental results of 40.0 and 38.3 J mol−1 K−1, respectively. The computed undercooling range of about 700 K is sufficiently broader than the experimental regime of 223 K.The prediction of thermodynamic properties for metastable undercooled liquid is important in the research on liquid structure and phase transition. Here we report the theoretical prediction of specific heat for metastable undercooled liquid Fe–Cu–Mo ternary alloys with a molecular dynamics method. Furthermore, experimental measurements were also performed by electromagnetic levitation drop calorimeter to confirm the predicted results. For liquid Fe78Cu15Mo7 and Fe71.5Cu3.5Mo25 alloys, the calculated specific heat values are 37.5 and 36.3 J mol−1 K−1, which agree well with the experimental results of 40.0 and 38.3 J mol−1 K−1, respectively. The computed undercooling range of about 700 K is sufficiently broader than the experimental regime of 223 K.

Rapid monotectic solidification during free fall in a drop tube

Droplets of Ni-31.4%Pb monotectic alloy with different sizes are rapidly solidified during free fall in a drop tube. The theoretical calculations indicate that the undercooling was achieved before solidification exponentially depends on droplet diameter. The maximum undercooling of 241 K (0.15Tm) is obtained in the experiments. With the increase of undercooling, the volume fraction of monotectic cells increases, and the L2(Pb) grains are refined. Calculations of the nucleation rates of L2(Pb) and α-Ni phases indicate that L2(Pb) phase acts as the leading nucleation phase during the monotectic transformation.

Rapid dendritic growth within an undercooled Ni–Cu–Fe–Sn–Ge quinary alloy

A liquid quinary alloy with composition Ni–5%Cu–5%Fe–5%Sn–5%Ge has been prepared from a containerless state by undercooling. Dendritic growth of α-Ni phase took place with a velocity of 28 m s−1 at the maximum degree of undercooling, which was as high as 405 K (0.24T L). All of the four solute elements Cu, Fe, Sn and Ge exhibited a significant solute trapping effect during the rapid dendrite growth. Segregation-less solidification is consequently realized when the degree of undercooling is sufficiently large. The lattice constant of α-Ni solid solution phase is found to increase with the amount of multicomponent solute trapping.

Surface tension of substantially undercooled liquid Ti–Al alloy

It is usually difficult to undercool Ti–Al alloys on account of their high reactivity in the liquid state. This results in a serious scarcity of information on their thermophysical properties in the metastable state. Here, we report on the surface tension of a liquid Ti–Al alloy under high undercooling condition. By using the electromagnetic levitation technique, a maximum undercooling of 324 K (0.19 T L) was achieved for liquid Ti-51 at.% Al alloy. The surface tension of this alloy, which was determined over a broad temperature range 1429–2040 K, increases linearly with the enhancement of undercooling. The experimental value of the surface tension at the liquidus temperature of 1753 K is 1.094 N m−1 and its temperature coefficient is −1.422 × 10−4 N m−1 K−1. The viscosity, solute diffusion coefficient and Marangoni number of this liquid Ti–Al alloy are also derived from the measured surface tension.

Thermophysical properties of substantially undercooled liquid Ti-Al-Nb ternary alloy measured by electromagnetic levitation

The thermophysical properties of undercooled liquid alloys at high temperature are usually difficult to measure by experiment. Here, we report the specific heat of liquid Ti45Al45Nb10 ternary alloy in the undercooled state. By using electromagnetic levitation technique, a maximum undercooling of 287 K (0.15 T L) is achieved for this alloy. Its specific heat is determined to be 32.72 ± 2.51 J mol−1 K−1 over a broad temperature range of 1578–2010 K.

Surface tension measurement of undercooled liquid Ni-based multicomponent alloys

The surface tensions of liquid ternary Ni–5%Cu–5%Fe, quaternary Ni–5%Cu–5%Fe–5%Sn and quinary Ni–5%Cu–5%Fe–5%Sn–5%Ge alloys were determined as a function of temperature by the electromagnetic levitation oscillating drop method. The maximum undercoolings obtained in the experiments are 272 (0.15T L), 349 (0.21T L) and 363 K (0.22T L), respectively. For all the three alloys, the surface tension decreases linearly with the rise of temperature. The surface tension values are 1.799, 1.546 and 1.357 N/m at their liquidus temperatures of 1719, 1644 and 1641 K. Their temperature coefficients are −4.972 × 10–4, −5.057 × 10−4 and −5.385 × 10−4 N/m/K. It is revealed that Sn and Ge are much more efficient than Cu and Fe in reducing the surface tension of Ni-based alloys. The addition of Sn can significantly enlarge the maximum undercooling at the same experimental condition. The viscosity of the three undercooled liquid alloys was also derived from the surface tension data.