Influence of Crystal Orientation on Freckle Formation in Single Crystal Heavy-Plate Castings

The effect of the crystal orientation on freckle formation has been investigated in single crystal Ni-base superalloy heavy-plate castings. Single crystal superalloy heavy-plate castings grown along the <001> , <011> and <111> crystallographic orientations were prepared by the bottom seeding technique and Bridgman method. Optical microscopy (OM) and scanning electron microscopy (SEM) were employed to observe the microstructure, and electron backscatter diffraction (EBSD) was used to characterize the crystallographic orientation of the castings. The morphology of the mushy zone during directional solidification was simulated by ProCAST finite element software. The experimental results show that the space between primary dendrites at the (010) crystal plane of <011> oriented plate casting and the (100) crystal plane of <111> oriented plate casting is wider than that at the same corresponding crystal plane of <001> oriented plate casting. The occurrence of freckles depends not only on orientation but also on dendrite morphology. Compared with orientation, the freckle is more sensitive to dendrite morphology and the space between primary dendrites of the single crystal plates. The freckle formation tendency of the <001> orientation casting was the weakest among the three crystal orientation castings, and the reason for this tendency was discussed.

Graphical Abstract