The kinetics of growth competition during rapid solidification of ternary steel alloys
Author(s)Kertz, Joan Elizabeth, 1975-
Massachusetts Institute of Technology. Dept. of Materials Science and Engineering.
Merton C. Flemings.
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The rapid solidification growth behavior of ternary stainless steel alloys was examined using electromagnetic levitation and high-speed digital imaging, and the growth behavior of the stable fee austenite (gamma) phase into the pre-existing metastable bee ferrite [delta] solid/liquid dendrite array was successfully modeled. Alloys with equal thermal driving force for growth during secondary recalescence but different solute concentrations were selected and compared. The growth velocity of stable -y into the metastable [delta]/l semi-solid (Vss ) was confirmed to be independent of undercooling and was determined to be a function of solute concentration. Because the build-up of solute ahead of the dendrite acts to slown down the growing tip, growth velocity decreases as solute concentation increases. It has been shown previously that at a given undercooling, the fee phase grows faster into the bee solid/liquid array when compared to growth into liquid alone. A growth model involving heat extraction was developed that successfully characterizes this behavior. In the model, heat is extracted from the austenite dendrite tip to the pre-exisitng ferrite solid, which acts as a heat sink. The latent heat of fusion rejected at the tip to the liquid is reduced. The solid fee is able to grow into the bee semi-solid at a faster rate because the barrier to growth is curtailed. The model predicts that the amount of heat extracted to the solid, H s, is a linear function that decreases to zero as thermal driving force increases. After heat extraction ceases. the growth velocity of austenite into semi-solid is expected to behave analogous to the growth rate into liquid. It was determined that the difference between the growth velocity into semi-solid and growth into liquid, [Delta]V, actually depends on the product of Hs and Vss , or the rate of heat extraction, J. As thermal driving force increases, both [Delta]V and J increase to the a local maximum before decreasing to zero at the same rate. The model values of Hs can be used in conjunction with existing dendrite growth theory to predict Vss. The resulting velocity for alloys with equal solute concentration is a linear function of thermal driving force, with the slope depending on the empirically determined kinetic coefficient. Experimental values and model predictions for both Hs and Vss agree well.
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2001.Includes bibliographical references (leaves 99-102).
DepartmentMassachusetts Institute of Technology. Dept. of Materials Science and Engineering.
Massachusetts Institute of Technology
Materials Science and Engineering.