Grain Size in Cast Gold Alloys

¹ New Rochelle, New York

Fine grain dental golds are superior to coarse grains in mechanical properties and in uniformity of such properties. The traditional method of grain refining dental golds by inoculation with "seed" crystallization nuclei has two disadvantages: (1) The dispersoid tends to segregate in the melt, and hence production lots cut from one ingot and from ingot to ingot vary in properties and in polishability. (2) There is no control over the distribution of the dispersoid on remelting in the dental casting operation.

By selecting addition agents that have high melting points and very low solubility, it is nevertheless still possible to obtain grain refining without introducing a second phase dispersoid.

An explanation for the grain refining by this method is that homogeneous rather than heterogeneous nucleation has occurred. The increase in nuclei on homogeneous nucleation is apparently due to the lowering of the surface energy of the solidification nucleus, which in turn lowers the critical radius of the nucleus and thereby converts an increased fraction of the embryos into solidification nuclei. It would appear that grain refining attributed to heterogeneous nucleation by seeding may be overexaggerated. On examining the data for molybdenum and aluminum grain refining, it is found that all or at least a significant part of grain refining occurs with addition compositions below that at which a seed nucleus can occur. Researchers⁶ in solidification theory apparently have not heretofore noted the homogeneous nucleation possibilities described in this report.

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