EFFECTS OF MN AND CO ADDITION ON MORPHOLOGY OF UNIDIRECTIONALLY SOLIDIFIED FESI2 EUTECTIC ALLOYS

Solidification structure was examined, which was obtained by the unidi rectional solidification of various (FeSi2)(100-x)(M)(x), (M = Mn or C o, x = 1-3at.%) alloys. The transition of the solidification structure from irregular eutectic to well-aligned rod-like eutectic alloys occu rred at a critical growth rate depending on alloys. Its value was 25 m u m s(-1) for the binary Fe-Si alloy and decreased by the small additi on of Mn or Co. The effect of these elements on the transition was exp lained by the constitutional undercooling criterion in the ternary eut ectic alloys. The rod spacing (lambda) of the eutectic structure varie d with the growth rate (R) and the value of lambda(2)R was constant fo r a given alloy depending on the third element. The values of lambda(2 )R for the ternary alloys were larger than those for the binary alloy. The minimum undercooling assumption proposed by Jackson and Hunt can be applied in this case.