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Title: Studies of solute distribution at solid-liquid interfaces
Author: Sharp, R. M.
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 1971
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An attempt has been made in this thesis to put on a sound basis, the relation between interface morphology and distribution of solute in alloys solidifying in a cellular or dendritic manner. Specimens have been grown under precisely known and regulated conditions and quenching by rapid withdrawal into water has ensured that the solute distribution existing in the liquid has been retained in the solid. Electron microprobe analysis coupled with optical microscopy studies have then been used to give information concerning the effect of growth variables on the nature of the interface. The results of this work fall into two distinct parts. First, data are presented which relate to the tips of the cells or dendrites solidified under conditions involving convective flow or transport by diffusion only, and with both heavy and light solutes. Second, extensive annealing of the initial structure behind the tips has been investigated to establish the mechanism involved in post-solidification ripening. It has been found that over the range of experimental conditions used, the amount of lateral segregation produced by cells and dendrites increases with increasing growth velocity and concentration and with a decrease in the value of the imposed temperature gradient. The results suggest that the solute segregation adjusts until the average gradient of liquidus temperature normal to the interface is equal to the imposed temperature gradient. At the same time as lateral segregation increases, there is a gradual change in morphology from simple cells to cells with branches and finally to crystallographic dendrites which have well-developed side arms. Increasing growth rate was found to cause a decrease in the size of branched cells and dendrites, However, the size of simple cells appears to be independent of growth velocity and alloy concentration but is linked to the magnitude of the concentration gradient at the tips. In specimens in which there is convection in the melt, a boundary layer exists, inside which there is no stirring. Outside this layer the liquid is fully mixed. The thickness of the layer was found to depend on alloy concentration, and temperature gradient, both variables which affect the magnitude of the density inversion. Annealing of the cellular structure behind the growth front was found to occur by removal of solute-rich liquid from the cell walls to the cell corners. When the interface plane is (100) or (111) this takes place randomly, but if it is (110), then cell walls disappear preferentially in one direction. The reason for this has been discussed and the conclusion reached that the mechanism causing changes in cell structure is the reduction in surface area of the solute-rich liquid films. Annealing in the branched cell and dendrite specimens takes place by the disappearance of branches or secondary arms behind the interface, the rate of disappearance depending on concentration and being essentially different for the two morphologies involved. The results presented give the spacing of arms as a function of time allowed for annealing.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available