Font Size: a A A

The Research On Super Undercooling Of Eutectic Alloys And Crystallization Of Bulk Amorphous Alloys

Posted on:2005-06-01Degree:DoctorType:Dissertation
Country:ChinaCandidate:J PuFull Text:PDF
GTID:1101360152968811Subject:Materials science
Abstract/Summary:PDF Full Text Request
It is well known that rapid solidification of melts may refine their microstructure, andthus improve material properties. Some new materials which are difficult to synthesizeusing conventional ways can be reproduced by rapid solidification. High undercooling ofmelts can result in rapid solidification of melts at low cooling rates, thereby providing apossible way to extensive research on rapid solidification kinetics. Therefore, highundercooling technique has been widely used in fundamental research of rapidsolidification. In this study, the glass fluxing method is applied to study the undercoolingbehavior of simple eutectic alloys system (binary and ternary alloy). Growth of binary eutectic alloy involves the competitive nucleation and simultaneousgrowth of two solid phases from one liquid phase. The procedure is affected byundercooling, heterogeneous nucleation of melt and environmental conditions. Firstly,Ni81P19 eutectic alloys are adopted as a model to investigate the transformation ofsolidification structure with respect to undercooling. At low undercooling, α-Ni and Ni3Pnucleate simultaueously and form a rod eutectic microstructure. At medium undercooling,solidification structure starts to transform from regular (rod) to anomalous eutectics. Weproposed a remelting model of the α-Ni eutectic phase with dendritic characteristic toexplain the formation of the anomalous eutectics. By increasing the undercooling of themelt, the volume fraction of the anomalous eutectic will gradually increase. At highundercooling, the anomalous eutectic of Ni-P transforms to cellular eutectic, which isconsistent with interfacial stability of undercooled melts. Further more, nucleation of twophases in Ni-P alloy is analyzed to calculate eutectic coupled zone in Ni-P alloys according tonucleation theory. The solidification structures of different composition of Ni-P alloys areobserved to describe the shape and location of eutectic coupled zone in Ni-P alloys. At the sametime, the influence of undercooling of the melt on microstructure of primary phase α-Ni andNi3P is investigated. Recently, grain refinement of undercooled metal and alloys has been studied extensively.Experience shows that when a eutectic alloy melt is undercooled below its liquidus, metastableliquid phase separation takes place. Basically, undercooled melt separates into two differentregions from one homogeneous liquid. There are two kinds of decomposition mechanismsin a phase separation reaction, which are nucleation and growth and spinodaldecomposition. In the latter, the microstructure can be described as net-like. Phaseseparation morphology can be "frozen" by rapid quenching. Cahn predicted that thewavelength λ of phase separation decreases monotonically with undercooling below thecritical undercooling of miscibility gap. It agrees with our experimental results for Pd82Si18alloys. With increased undercooling, connected networks in the melt are broken up byinterfacial tension into liquid droplets. The increase of undercooling will diminish the IIIdimensions of liquid droplets in the nanometer range. In the study, we prepared bulkPd82Si18 nanomaterials with the largest diameter of 2 mm showing many droplets rangingfrom 30~50nm in diameter. Bulk Pd82Si18 amorphous alloy was prepared by water quenching the molten alloy withflux medium in a quartz tube. According to nucleation theory, metal melt can beundercooled below its Tg and form bulk amorphous alloys at a slow cooling rate bycontrolling the nucleation process. Critical cooling rate of Pd82Si18 amorphous alloy may beobtained by Temperature-Time- Transformation (TTT) diagrams which is calculated basedon typical nucleation and growth theory and homogeneous nucleation condition. Somefactors that influence the critical cooling rate are discussed. Theoretic calculation andexperimental datum demonstrate that there are strong correlation between critical coolingra...
Keywords/Search Tags:Crystallization
PDF Full Text Request
Related items