Microstructural investigation of downward directionally solidified single crystal superalloys

En savoir plus sur le livre

This dissertation examines the downward directional solidification process (DWDS) for casting single crystal Ni-based superalloy components. The study demonstrates that DWDS significantly enhances the thermal gradient and minimizes variations in this value along the component's height and across different casting sizes. These improvements lead to faster solidification rates, refined cast microstructures, better segregation of alloying elements, reduced microporosity, and increased microstructural homogeneity. The DWDS process effectively eliminates freckle-type defects by analyzing segregation and density profiles in the mushy zone. As withdrawal rates increase, the samples show progressively refined microstructures and reduced segregation of alloying elements. The relationship between primary dendrite arm spacings and withdrawal rates aligns with the theoretical model of Bouchard and Kirkaldy, with a dendrite-calibrating factor of 13.5. Furthermore, the solidification sequence of the eutectic pool begins with a fine (γ+γ′) structure in the center of interdendritic regions or near γ phases, evolving into a petal-like coarse (γ+γ′) structure, followed by the precipitation of γ′ particles from the γ phase between the coarse petal-like γ′ phases.