Application of Analytical Electron Microscopy and Tomographic Techniques for Metrology and 3D Imaging of Microstructural Elements in Allvac^® 718Plus™

The development of innovative materials for aeronautics requires use of modern research methods to characterize their structure on the level from micro- to nanoscale. Allvac^® 718Plus™ (718Plus) is a relatively new nickel-based superalloy that has high strength, is corrosion resistant and has improved higher temperature performance compared to the 718, while retaining the excellent processing characteristics of this alloy. The aim of this study was the application of analytical electron microscopy and tomographic techniques to perform the qualitative and quantitative characterization of structural elements in the wrought and cast 718Plus. Determination of crystallographic structure of selected phases was performed using electron diffraction method supported by JEMS software. 3D imaging of microstructural elements and chemical compositions of selected phases were performed using STEM-EDX tomography technique using a probe Cs corrected Titan³ G2 60-300 with ChemiSTEM™ system. Tomographic reconstruction was performed using Simultaneous Iterative Reconstruction Technique (SIRT) method on a tilt series, which allowed visualizing the three-dimensional distribution of selected elements (Al, Cr) in the analysed volume. The shape and distribution of γ′ particles in the reconstructed volume, as well as complex particle composition of η-phase were analyzed using STEM-EDX and FIB-SEM tomography techniques. 3D imaging of precipitates in the interdendritic region (Laves and η phases) was performed using FIB-SEM tomography technique. The 3D visualization of reconstructed space was performed using ImageJ and Avizo Fire software. The study showed that advanced microscopic techniques and test methods in conjunction with tomography technique permits to obtain complementary information about the microstructure of 718Plus superalloy.