A substantial undercooling up to 250 K was produced in the IN718 superalloy melt by employing the method of molten salt denucleating, and the microstructure evolution with undercooling was investigated. Within the achieved undercooling, 0–250 K, the solidification microstructure of IN718 undergoes two grain refinements: the first grain refinement occurs in a lower range of undercooling, which results from the ripening and remelting of the primary dendrite, and at a larger range of undercooling, grain refinement attributes to solidification shrinkage stress and lattice distortion energy originating from the rapid solidification process. A ‘lamellar eutectic anomalous eutectic’ transition was observed when undercooling exceeds a critical value of ∼250 K. When undercooling is small, owing to niobium enrichment in interdendrite, the remaining liquid solidifies as eutectic ( γ+Laves phase); whereas, if the undercooling achieves 250 K, the interdendrite transforms from eutectic ( γ+Laves phase) to Laves phase, which results from the formation of divorced eutectic arising from the huge variance of the growth velocities of γ and Laves phases.

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This paper reports our investigation on the corrosion resistance and corrosion behaviour of Fe-6?5 wt-%Si atomised powders. Results showed that corrosion resistance was strongly related to powder sizes. When powder sizes were ,38 mm, they showed excellent corrosion resistance compared with the larger ones. Analysis suggested that most of the corrosion started from corrosion pits when the powder sizes were .53 mm. Weaker corrosion resistance can be explained by severe Si content heterogeneity and more grain boundaries in the larger sizes atomised powders.