Characterization of nanoscale precipitates in superalloy 718 using high resolution SEM imaging

Abstract

At present, obtaining accurate volume fraction and size measurements of γ″, γ′, and δ precipitates in Superalloy 718 has been challenging due to their size, crystal structures, low volume fractions, and similar chemistries. These measurements are necessary to validate precipitation models that in turn enhance selective laser melting fabrication. Superalloy 718 is a promising candidate for selective laser melting fabrication due to a combination of excellent mechanical properties and workability. A new technique, combining high resolution distortion corrected SEM imaging and with high resolution x-ray energy dispersive spectroscopy, has been developed to accurately and independently measure the size, volume fraction, and number densities of these three precipitates. A specimen of selective laser melted superalloy 718 was obtained and underwent a conventional heat treatment for superalloy 718 where it was solutionized at 1010 °C for one hour, gas quenched and followed by a two-step age (718 °C/11 h → 621 °C/6 h) in order to produce a microstructure with all three precipitate types present. These results were further validated using x-ray diffraction and phase extraction methods.