Quantitative study of the microstructure evolution along the thickness direction in the nickel-based single crystal superalloy DD6 at 1323 K thermal exposure

The wall thickness of turbine blade is becoming thinner because of the need for cooling, which leads to the thickness debit effect. The microstructure evolution along the thickness is deemed one of the major factors. The present work quantitatively investigates the microstructure evolution along the thickness of a second generation nickel-based single crystal superalloys DD6 under pure thermal condition. Experiments are performed on specimens with thickness of 0.3 mm, 0.6 mm and 1.5 mm. Microstructure is analyzed using scanning electron microscopy (SEM) in combination with quantitative image analysis. The interplay of oxidation and substrate causes apparently different microstructures along the thickness which are classified into three layers except the base material layer. The growth of each layer is barely influenced by the specimen thickness and their evolutions with time are fitted by both logarithmic and power equations. The microstructure along the thickness is not obvious different in the base material layer by naked eyes. Quantitative analysis of the average size and probability distribution function of the gamma' particle area and circularity reveals the variation of microstructure along the thickness. The average size of the gamma' particle area increases with the increase of the distance from the thickness symmetry axis. The average value of circularity at the position with the same distance from the thickness symmetry axis is larger in the thin specimen than that in the thick specimen, which represents more severe rafting in the thin specimen. A diffusional mechanism is used to interpret the observed microstructure changes at the base material layer. The new findings are important for mechanical performance analysis of thin-walled structure in turbine blade applications.