Imaging and characterization of γ′ and γ nanoparticles in Inconel 718 by EDX elemental mapping and FIB-SEM tomography

Microstructural characterization of Inconel 718 superalloy after three different heat treatment variants was performed by electron microscopy and electron tomography techniques, taking advantage of recent development in quantitative electron microscopy. Distribution maps of the chemical elements, collected by ChemiSTEM (TM) EDX system, offer a clear contrast between gamma', gamma '', and the gamma matrix. It was found that the gamma' phase contains mainly Ni, Al, and Ti, while the gamma '' phase contains Ni, Nb, and Ti. Thus application of the Al and Nb STEM-EDX elemental maps enables identification and size measurements of gamma' and gamma '' nanoparticles. 3D morphology of gamma' and gamma '' precipitates was examined by electron microscopy and FIB-SEM tomography. Employed methods revealed that in all three heat treatment variants the gamma' particles are almost spheroidal while the gamma '' precipitates are mainly elongated-disc shaped. However, the precipitate sizes differed for each variant contributing to differences in the yield strength. Tomographic images were used for estimation of the volume fraction of the both strengthening phases. (C) 2014 Elsevier Inc. All rights reserved.