The approaches to thin film preparation and TEM observations on slurry Si-modified aluminide coatings

Transmission electron microscopy (TEM) can be used as a precision characterization tool to identify very small precipitates in diffusion aluminide coatings. However, in order to successfully prepare the appropriate samples for TEM observation, often non-traditional thin film preparation techniques need to be employed. In this work, two sample preparation methods of twin jet electro-polishing and ion milling were experienced to characterize fine precipitates (<1 mu m), in Si-alummide coatings applied on Ni-base superalloy In-738LC by slurry technique. These precipitates are concentrated throughout the topcoat zone. It was found that the preparation of thin film exactly from the outer zone of the coating is only possible using ion milling process. The ion-milled specimens were utilized to observe by JEOL high resolution TEM operating at an accelerating voltage of 300 W Electron diffraction patterns, bright field and EDS were used to identify the precipitate phases as well as the coating matrix. The results showed that the fine precipitates are typically chromium silicides in nature, mostly as Cr3Si and CrSi, distributed in the beta-NiAl matrix phase.