Abstract
The core and the interdendritic regions of an as-cast nickel based single crystal turbine blade were observed by electron microscopy to understand the microstructural development during an investment casting process. The dendrite core region shows an irregular morphology of gamma prime in gamma due to a relatively short casting time, which prevented the development of gamma prime expected in a solution heat-treated microstructure. By comparison, the interdendritic region comprises three different regions composed of: several elongated gamma prime particles, relatively tiny and irregular gamma prime, and gamma prime with relatively regular morphology. The chemical analysis of these phases showed that, regardless of the analysis point in the core or the interdendritic region, almost the same compositions were acquired in the regular type of gamma and gamma prime phases. This result suggests that if the gamma prime forms in the gamma matrix, the composition of gamma prime is almost uniform regardless of the region and prevailing general chemical composition. In contrast, the composition of the elongated gamma prime in the interdendritic region was slightly different depending on the analysis point even within the same elongated particle.
Highlights
Ni-based single crystal superalloys have been designed to maximise the combination of significantly high mechanical strength and surface stability at high operating temperatures, allowing severe mechanical stressing to nearly 80% of the melting point, and are ideal for turbine blade applications [1,2,3,4]
An image acquired at a higher magnification of 25,000× reveals a number of particles (γ0 phase) of dark contrast and this γ0 phase was not uniformly precipitated in the core because the turbine blade was only cast without further processes such as solution and aging heat treatments, which form the more familiar microstructure composed of uniformly precipitated γ’ and thin γ channels
The core region showed a relatively uniform microstructure, whereas the interdendritic region showed a range of microstructures depending on the area observed
Summary
Ni-based single crystal superalloys have been designed to maximise the combination of significantly high mechanical strength and surface stability at high operating temperatures, allowing severe mechanical stressing to nearly 80% of the melting point, and are ideal for turbine blade applications [1,2,3,4]. The casting process forms dendrites, containing the primary phase of γ and fine γ0 precipitates, with a specific direction of {001}, followed by eutectic γ and γ0 in the interdendritic region This mixed microstructure is undesirable in the highest performing turbine components, due to an inhomogeneous chemical distribution, which requires homogenisation during the subsequent heat treatment [10]. Even though the electron beam is targeted to bombard just the thin γ phase, it is inevitable that the analysis includes a number of X-ray photons emitted from the surrounding bulky γ0 in the chemical composition analysis using EPMA Another analysis technique for the compositions of γ and γ0 is the use of TEM due to their fine sizes. TEM sampling method using a FIB lift-out technique was used to make two lamellae on one TEM grid, and the microstructure and the compositions of γ and γ0 in the core and the interdendritic region were investigated by electron microscopy
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