Thermophysical Properties And Core-Shell Solidification Microstructure Of Highly Undercooled Alloy Melts

Both the thermophysical properties and rapid solidification of highly undercooled metallic melts, which are important research subjects in the field of materials science in space, have aroused great research interest. The present thesis carried out detailed investigations on the measurement of surface tensions and specific heats of undercooled alloys. Meanwhile, the monotectic solidification of Ni-Pb binary alloy systems is accomplished by drop tube technique.An oscillating drop method combined with electromagnetic levitation was employed to measure the surface tensions of liquid Ni-5%Si and Co-25%Si alloys. The maximum undercoolings of the two alloys obtained in the experiment were 206K (0.13TL) and 223K (0.14 TL) , respectively. A linear relationship between surface tension and temperature was found. Within the experimental undercooling regime, the surface tensions of the two alloys are Ni-5%/si=l.697-3.97 x 10-4(T-TL) N/m and co-25%si=1.604-4 x 10-4(T-TL) N/m. Based on the experimental data, some other thermophysical properties, such as the viscosity, the solute diffusion coefficient, and the density of liquid Ni-5%Si and Co-25%Si alloys are derived. Meanwhile, the corresponding apparent activation energy for viscous flow and the apparent activation energy for diffusion are also obtained.Using the electromagnetic levitation combined with drop calorimeter method, the specific heat of liquid Ni-5%Si alloy is determined as 39.54+1.87 J/mol/K within the undercooling range. The maximum undercooling achieved in the experiments is 218 K. The calculated excessive specific heat by some well-known theoretical models indicates that they are not in agreement with the experimental data. The relationship of thermal diffusivity and thermal conductivity changing with temperature is also calculated from the measured specific heat and surface tension.For Ni-Pb hypermonotectic alloys, rapid solidification was accomplished in a 3m drop tube. Core-shell microstructures are obtained in Ni-40%Pb, Ni-50%Pb and Ni-58%Pb hypermonotectic alloys. For Ni-40%Pb and Ni-50%Pb alloys, Pb-rich phase always occupies the center part, whereas both Ni-rich phase and Pb-rich phase can be the core for Ni-58%Pb alloy. Meanwhile, It is found that Ni separated form Pb-rich phase grows in a dendritic way. The nucleation rates of La (Pb) phase of the above three alloys are calculated as functions of undercooling, which indicated that homogenous nucleation with no undercooling can occur to Ni-58%Pb alloy, andhomogenous nucleation with only small undercooling can take place in the other two alloys. With the increase of undercooling, the interfacial energy also increases. If the composition is smaller than 58%, the more solute Pb the alloy contains, the smaller the interfacial energy. The temperature field inside droplets has been calculated, and it is inferred that the temperature gradient in the center part is smaller than that near the surface. For tiny liquid nucleus of L2 (Pb), the larger the diameter, the higher the Marangoni convection velocity. In terms of the calculation, there exists enough time to separate the L2 phase from alloy melt for the three Ni-Pb monotectic alloys. Once the droplet size of the second phase is tiny enough, its migration is driven by Brownian movement. When the size exceeds 1 x 10-8m, Marangoni convection is the dominating factor for the migration.