Directional solidification is a process in which an alloy melt is solidified by cooling from the bottom of the mold. In the resulting casting, the grain boundaries transverse to the solidification direction are virtually eliminated. The material thus obtained shows increased creep rupture strain and improved thermal fatigue behavior, two important characteristics for the modern gas turbine blades. It is, however, not uncommon to find long-range differences in the alloy composition between different parts of the castings. These are known as macrosegregations, and they are serious defects which adversely affect the physical and mechanical properties and workability. Even more deleterious are localized segregates, commonly referred to as freckles or channel segregations, which are solute-rich, columnal regions parallel to the direction of cooling in DS-processes. The objective of this research is to determine the mechanisms of fluid mechanical origin by which freckles are formed in directionally solidified alloys. Experiments and numerical investigations will be carried out simultaneously to enhance the beneficial interactive effect.
