Macroscopic physical properties are correlated to the microstructure in nickel base superalloys. In these materials mechanical properties are strongly influenced by the presence of sub-micrometer sized γ’-phase precipitates. In this presentation the detailed phase compositions of single crystalline superalloy PWA1483 (nominal composition Ni60Cr14Co9Al8Ti5Ta1W1Mo1) is elucidated. PWA1483 forms globular to cubical, 0.2 to 0.6 µm sized γ’-phases surrounded by 0.1 to 0.3 µm broad γ-matrix channels. The chemical compositions of the highly alloyed solid solutions determine the crystal structure and the γ and γ’-phase fractions formed during heat treatment. The two phases γ (Cr,Co-rich) and γ’ (Al,Ti-rich) are described in the literature as Ni (space group: Fm3m) and as Ni3Al (space group: Pm3m) crystal structure, however, chemical and structural details are frequently not reported.

In transition metals X-ray absorption and overlapping X-ray lines are challenges for a quantitative analysis. SEM-EDX and STEM-EDX were applied for a quantitative EDX analysis. The nominal composition of PWA1483 was measured and confirmed by SEM-EDX with an accuracy of up to 1.5 at.% using multiple scan areas of 0.25 mm². STEM-EDX spectra were acquired on the same sample on a Jeol 2200FS (EDX-detector: Jeol Centurio SSD) at Siemens and a Fei Tecnai F20 (EDX-detector: EDAX Apollo XLTW SSD) at the USTEM. Only FIB-prepared lamellae were used mounted on a Cu grid.

Tabulated k-factors of the standardless Cliff-Lorimer method were found to be inaccurate up to 20%. Quantitative analysis of the nominal composition yielded modified k-factors using STEM-EDX mappings acquired over a 5 µm² wide area of the TEM lamellas. After recalibration of the k-factors the chemical composition of the individual phases were determined yielding Ni54Cr24Co11Al5Ti2Ta1W2Mo1 for the γ-matrix and Ni68Cr3Co5Al12Ti8Ta2W1Mo1 for the γ’-phases. Both TEM systems at USTEM and at Siemens yielded the chemical composition for the two phases in accordance, which proves the significance of EDX analysis for such complex materials. The chemical composition of the individual phases together with the nominal composition yielded the volume fraction of the γ’-phase and was calculated as of 45 ±5%. This result was proved to be more reliable than phase fractions yielded from SEM secondary electron grayscale images.

The presented analysis of the γ and γ’-phases in PWA1483 by STEM-EDX provides a unique and quick method for analyzing the chemical composition of the individual phases with high accuracy. From this data the phase fractions can be determined if the nominal composition of the superalloy is known.