The Grain Structure of Castings

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9521875 Hellawell This two-year renewal provides for completion of research on the development of equiaxed grains in metal castings. The initial source for the grain solidification is thought to be dendrite fragments that have become detached within the earlier mushy region of the casting. The fragments are transported out into the bulk liquid by thermo-solutal convection. Surviving fragments grow, sink, and eventually block columnar grains. The steps in modeling the process have been identified and analyzed in some detail, and it is recognized that the conditions that promote dendrite arm detachment are associated with transient growth conditions. These conditions mirror those in a casting, where columnar dendritic growth front is decelerating continuously and the temperature gradient is falling simultaneously. This critical step in the process is examined analytically and experimentally using transparent analogues and selected metal alloys. %%% This topic addresses a major gap in understanding the development of the microstructure of alloy castings. This complex process should be clarified by the experimental and theoretical work proposed. ***