1995 Volume 35 Issue 6 Pages 589-595
An investigation on the static undercooling behavior of Ni and Ni-Co alloys using an alumina crucible under both air and Ar atmosphere was carried out. The degree of undercooling in both Ni and Ni-Co alloys decreased with increasing oxygen content. Impurities of spinels NiAl2O4 and CoAl2O4 were detected in the low undercooled specimens by XMA and X-ray diffraction methods. Both spinels have low value of planar disregistry which means they have a good potency to be heterogeneous uncleation sites. These inclusions are created as results of reactions between an alumina crucible, soluble oxygen, and molten metal. Furthermore, an equation which gave the fraction solid was derived considering the effect of the crucible. The equation is a function of undercooling, cooling rate, thermal properties of crucible and melt, and time. This equation gives a relationship between undercooling and solidification time by substituting '1' in place of the fraction solid. The calculated solidification time is in good agreement with the experimental data. In low undercooling, the maximum recalescence temperature was almost constant and a refining of casting structure by oxygen was achieved.
