Quantitative measurement of gamma prime precipitates in two industrial nickel-based superalloys using extraction and high resolution SEM imaging

• Improvement of a gamma prime selective extraction method.
• Determination of crystallographic structures and chemical composition of precipitates.
• Determination of size and morphology via high-resolution SEM.
• Nanometric precipitates are clearly observed.
• Ratio Ti/(Al + Ti + Nb) and Nb/(Al + Ti + Nb) are very similar in the grade and in the precipitates.

A technique for the microstructural study of superalloys, based on the use of matrix dissolution to collect the very fine precipitates formed in superalloys during heat treatments, has been considerably improved. This technique was applied to two different superalloys, characterized by different titanium, aluminum and niobium contents. The technique was implemented in order to accurately determine experimental data including the crystallographic structure, chemical composition of gamma prime precipitates and the chemical composition of the matrix after precipitation. Following a special treatment, it was possible to analyze the size and morphology of the precipitates using high-resolution SEM, for a large range of dimensions down to 10 nm. From the chemical compositions determined using ICP-OES, gamma prime molar fractions were determined for both studied superalloys. Finally, the Nb/(Ti + Al + Nb) and Ti/(Ti + Al + Nb) ratios were found to be very similar in the grade and in the precipitates.