Grain refinement of undercooled single-phase Fe₇₀Co₃₀ alloys

The grain refinement mechanism of undercooled single-phase Fe₇₀Co₃₀ alloy was investigated. With increasing the initial melt undercooling (ΔT), the first grain refinement occurs within 22 K<ΔT<87 K, and the second one if ΔT>183 K. Applying a dendrite break-up model (i.e. on the basis of morphological instability) in combination with the BCT model calculation, a theoretical description for the observed grain refinements is given. Both grain refinements are concluded to be caused by dendrite remelting. Good agreement between theoretical expectation and experimental result was found for the second grain refinement. As for the first grain refinement, only the initiating undercooling (i.e. ΔT=22 K) can be predicted using the above model, whereas large deviation remains for the ending undercooling (i.e. ΔT=87 K). This can be ascribed to the fact that the effect of solutal diffusion was deduced to be negligible for ΔT>25 K in BCT model calculation.