INVESTIGATION OF FORMATION MECHANISM FOR EQUIAXED GRAINS USING FE-NI ALLOYS

The formation mechanism of equiaxed grains during solidification has been investigated in Fe-Ni alloys, since the diffusion coefficient of nickel in the solid iron is small and this system has the peritectic reaction as the Fe-C system does. Alloys were melted in the vacuum induction furnace and the melt was cast in the ingot mold made of MgO. In this ingot mold, fine thermocouples covered by thin quartz tubes were set to measure the temperature change. The ingot was cut and metallographically investigated. The equiaxed ratio was then measured as a function of superheat and nickel content. When superheat was less than 50 K globular grains were found independent of composition. In the center of the globular grains, a higher concentration region was found in the alloys with high nickel contents in which the γ-phase was solidified as a primary phase. The globular grains found in this study are formed by the nucleation in the slightly undercooled alloy melt. The mathematical model for nucleation has been also established to estimate the value of the undercooling and concentration difference. The result of this model agrees well with the result of temperature measurement.