Investigating the Effect of Diameter-to-Height Ratio of the Starter Block on the Microstructure and Crystallographic Orientation of CMSX-4 Single Crystal
DOI:
https://doi.org/10.24425/afe.2026.158023Abstract
The present study investigates the effect of starter-block diameter-to-height (D/H) ratio on grain selection, crystallographic orientation, solidification microstructure, and γ′ precipitate morphology in directionally solidified CMSX-4 single-crystal castings. Starter blocks with D/H ratios of 0.3, 0.5, and 0.7 were produced using a Bridgman-type directional solidification process at a withdrawal rate of 9 mm/min. Laue X-ray diffraction, optical and scanning electron microscopy, image analysis, and ProCAST numerical simulations were employed to evaluate the crystallographic and microstructural characteristics of the castings. All investigated castings exhibited single-crystal growth in the examined cross-sections. The measured misorientation angles from the preferred crystallographic orientation were 6.3°, 21.7°, and 3.3° for D/H ratios of 0.3, 0.5, and 0.7, respectively, indicating a non-monotonic dependence on starter-block geometry. The relatively large misorientation at D/H = 0.5 was associated with altered local thermal conditions, increased sensitivity to lateral heat extraction, and possible destabilization of the solid-liquid interface. Stray grains were observed in the D/H = 0.3 and 0.5 castings but were not detected in the D/H = 0.7 casting. The simulations indicated that the solidification front advanced into the colder furnace region beyond the heat baffle, where the local thermal gradient decreased substantially and could promote wall-associated stray-grain nucleation. Nevertheless, the survival of these grains was also influenced by dendrite alignment and competitive growth. The measured primary and secondary dendrite arm spacings showed substantial overlap, and no statistically conclusive relationship with the starter-block D/H ratio was identified. Grain-count analysis demonstrated a progressive reduction in grain density with increasing distance from the chill plate. Both D/H ratio and distance had statistically significant main effects, whereas their interaction was not significant. Image analysis of γ′-related regions in the primary dendrite cores showed comparable area fractions and broadly similar shape descriptors for D/H ratios of 0.3 and 0.5, while the lower feature count at D/H = 0.5 suggested a relatively coarser or more interconnected morphology. Overall, the results show that starter-block geometry primarily affects local thermal conditions and competitive grain selection, thereby influencing crystallographic quality and stray-grain formation, whereas its effect on average dendrite spacing and local γ′ morphology was comparatively limited within the investigated conditions.
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