Research On The Solid/Liquid Interfacial Energy And Nucleation Undercooling Of Metals

The solid/liquid interfacial energy is important paremeter for solidification. It is impossible to comprehend the essences of nucleation and growth of crystals without a clear knowledge of the solid-liquid interface energy.This dissertation carried out investigations on the solid/liquid interface energies and nucleation undercoolinges of Ni-21.4at%Si eutectic alloy, Al-Si eutectic alloy and bismuth. The main results and contributions of the thesis are as follows:Adopting molten glass denucleating and cycle superheating technology, the Ni-21.4at%Si eutectic alloy melts were highly undercooled. The factors affecting undercooling were investigated, including the chemical composition of glass, the temperature, the holding time, and the number of cycle overheating and so on. It is found that the chemical composition of glass is the most important factors affecting the undercooling. By denucleating Ni-21.4at%Si eutectic alloy in glass, the maximum undercooling obtained was 365K, which is in agreement with the undercooling predicted from the solid/liquid interface energy model reported. The microstructure of Ni-21.4at%Si eutectic alloy depends on the undercooling. When the undercooling is less than 143K, the primary phase is Ni3Si, the secondary phase is a(Ni), the remainder is irregular eutectic. When the undercooling is in the region from 143 to 299K, the microstructure is the mixture of irregular and the complicated regular eutectic. When the undercooling is in the region from 299 to 365K, the microstructure is a(Ni) and irregular eutectic, and the leading phase is a(Ni).The solidification structure of undercooled bismuth depends on the nucleation uddercooling. When the undercooling is lower than 49 K, the solidification structure is composed of layer grains with pronounced edges and faces. A mixture of elongated grains and refined equiaxial grains was obtained in undercooling region from 49 to 95 K. When the undercooling is higher than 95 K, the structure is refined equiaxial grains with several smooth spherical bulges on the surface of each grain. The critical undercoolings for bismuth from lateral to intermediate growth and that from intermediate to continuous growth were determined to be 49 and 95 K, respectively. The microstructures inside the sample grains obtained in undercooling regions are not the same, but they all show the feature of anisotropic growth.The solid-liquid interface energy and the homogenous-nucleation undercooling of bismuth can be predicted from the critical growth-transition undercoolings. The predicted results for bismuth are in good agreement with the experimental results.Measurement of the solid/liquid interface energy between solid Al and Al-Si liquid solution has been made by grain boundary groove shape. After the sample was hold for time 11 days, the equilibrated grain boundary groove shapes for the Al solid solution in Al-Si liquid solutions were successfully obtained. From the observed grain boundary groove shapes, the Gibbs-Thomson coefficient for solid Al in Al-Si liquid solutions has been determined to be 5.7x10-5K/cm. The solid/liquid interface energy between solid Al and Al-Si liquid solution has been obtained to be 0.45J/m2 from the Gibbs-Thomson equation.