Elasticity mapping of precipitates in nickel-base superalloys using atomic force acoustic microscopy

Abstract

The paper reports mapping of elastic properties of various precipitates such as γ′, δ, and carbides in nickel-base superalloys. Atomic force acoustic microscopy is used to measure the elastic properties with a lateral resolution of better than 100 nm in polycrystalline alloy 625 and directionally solidified alloy CM247A. A novel approach of simultaneous acquisition of two contact resonances and using the isotropic indentation modulus of matrix in every scan line as reference is used to circumvent the effect of change in tip condition while mapping elastic properties. Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy is performed to identify the precipitates and their compositions. Carbides exhibited highest modulus followed by δ precipitates, γ′ and γ matrix, respectively. It is demonstrated that due to the close-packed orientation relationships of precipitates with the matrix, the modulus of a precipitate measured by the methodology described in this study is not affected by the orientation of individual grain in which the measurement is made.