In order to better understand the geometrical effect on freckle formation in superalloys, samples with uniform increases and decreases in their cross sections were directionally solidified in a Bridgman furnace. In comparison to our conventional knowledge, some new features of freckle appearance have been observed. Freckles could occur at sloped surfaces where the freckle pattern is no longer roughly parallel to the direction of gravity but has the same slope as the surface. At significantly angled surfaces, the freckles appear as discrete flakes, having the shape of tree roots, instead of the long and narrow chains which are usually observed. The component portions having inward sloping surfaces are very freckle prone while those with outward sloping surface are mostly freckle free, although the completely opposite freckling tendency is indicated by the simulated solidification condition. Therefore, as an independent factor the geometrical feature of the components can more effectively affect the freckle formation than the local thermal conditions.
展开▼
