Microstructure evolution in the nickel base superalloy Allvac®718Plus™

Nickel base superalloys experience significant changes of the microstructure during processing as well as in service. The mechanical properties are predominantly determined by microstructure parameters like grain size, γ’-precipitate diameter and volume fraction, etc. Thus, knowledge about the microstructure evolution during processing is a necessary prerequisite to employ novel alloys as Allvac®718Plus™ in service. In the present study, microstructure evolution during heat treatment in particular has been investigated. Electron microscopy and especially transmission electron microscopy (TEM) have been used to characterise the microstructure changes in Allvac®718Plus™. The two major phases were found to be γ’- and δ-phase. By analysing selected area diffraction patterns from a large number of particles, it could be shown that nearly no γ’-phase is present in Allvac®718Plus™. Emphasis was put on the development of γ’- and δ- phase during typical heat treatment steps performed during processing. Here particle growth, the volume fractions of the phases as well as their crystallographic relationship and chemical composition were monitored using imaging, diffraction and energy dispersive spectroscopy in TEM. The interfaces between the δ- and surrounding γ/ γ’-phase were investigated using high resolution TEM. They are flat and parallel to the {111} plane of γ/ γ’-phase and the (010) plane of δ-phase. (AU) © 2010 by The Materials, Metals, & Materials Society. All rights reserved.