  Freckles, one of the common defects in blades used in heavy duty gas
turbines, hugely deteriorates blades mechanical properties and liability under
service conditions. Thermal-solutal convection theory is a widely adopted
formation mechanism but few solid experimental evidences were reported. Here
for the first time we systematically studied the microstructure of 117 grains
in freckle chains from four different Nickel-based superalloys of either single
crystal or directionally solidified alloys. The relationship between the
internal stress and the misorientation throughout the freckle chains is studied
by means of state-of-the-art electron microscopy. All results give new
experimental proof to the theory of thermal-solutal convection, which is
further supported by the fact that borides at the boundary are randomly
orientated to alloys. Our results enrich the methodology of freckle study,
providing a new sight of the formation mechanism of casting defects.
