Microstructural properties of superalloys investigated by nanoindentations in an atomic force microscope

Abstract

The microstructure of nickel-base superalloys with differently shaped γ′ precipitates determines their excellent high-temperature behaviour. A reliable quantitative determination of volume fraction and particle size distribution (PSD) of these precipitates is difficult, since the size of the precipitates is often below 100 nm. With an atomic force microscope (AFM), sections through specimens are analysed with a resolution in the nanometre range, which allows a quantitative determination of the γ′ volume content and PSD for different superalloys. Thus, determined volume fractions for the γ′ phase in the superalloys CMSX-6 and Waspaloy amount to 56% and 27%, respectively. A combination of an AFM with a nanoindentation system also allows the measurement of local mechanical properties such as hardness and elasticity. These quantities are determined for the first time directly on the superalloys CMSX-6 and Waspaloy for the γ′ and matrix phases, separately. The γ′ phase in both alloys shows a significantly higher but different hardness than the matrix phase, whereas the moduli of elasticity are similar. A depth dependence of the hardness was found for very small indentations.