The nickel-based superalloy CMSX-4 is used commercially to cast single crystal components. During casting, a pre-fabricated seed is used to nucleate the desired orientation. During casting the top portion of the seed is partially melted and this mushy zone is the origin of defect stray grains. A number of seeds cast at Rolls-Royce CRDF that exhibit extensive defect grain formation in the melt-back region have been analysed using optical and electron microscopy techniques. These defect grains

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... e found to nucleate exclusively at the casting surface where the alloy contacts the mould. Nucleation of the defect grains is a transient behaviour, it occurs only during the preliminary stages of withdrawal. After this time, no further defect grain nucleation was observed. Due to growth propensity effects, non-axially oriented seeds exhibit more prolific defect grains towards one side of the casting.