Imaging and characterization of fine γ′ precipitates in a commercial nickel-base superalloy

Empirical datasets of volume fractions and size distributions of small γ′ precipitates from “real” multi-component engineering nickel-based superalloys are vital to calibrate and validate the computer models which predict high sensitivities of mechanical properties to size and volume fraction of these fine γ precipitates, and in order to accelerate microstructure and alloy development. Consequently, we investigated a number of imaging techniques available in a Tecnai F-20 FEG/TEM and selected the technique which best enabled rapid and extensive acquisition of these datasets using the engineering alloy, René 88`DT. The EFTEM technique was found to be the most appropriate method for imaging fine γ′ precipitates while further investigation showed that the Cr-M-edge, in comparison with other ionization-edges provided the best images based largely on contrast-to-noise ratio. Imaging of the Cr-M-edge elemental maps were further improved by investigating the effects of microscope parameters, imaging filter parameters and analysis of the experimental electron energy loss spectra obtained from this alloy. In addition, a novel technique to determine the volume fraction of the fine γ′ precipitates without the need to determine the absolute thickness of the TEM foil is proposed.