Abstract

In the present work, two solidification experiments of a single crystal superalloy were conducted in opposite directions. During conventional upward directional solidification, severe freckle defects and upward accumulation of eutectic structure occurred in the castings. This is attributed to the segregation of alloying elements and density reversal in the mushy zone liquid, leading to an unstable state and then an upward convection of the liquid. While the solidification direction was changed to downward, the lighter residual liquid is located at the upper end of the mushy zone, forming an absolutely stable state of liquid. In this case, the thermal solutal convection, as well as the caused freckle defects and eutectic accumulation could be completely avoided.

Keywords:Single crystal superalloy; Solidification direction; Freckle; Eutectic accumulation