Microstructure Evolution in the Nickel Base Superalloy Allvac 718Plus®

Abstract

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.

Correlation Between Local Chemical Composition and Formation of Different Types of Ordered Phases in the Polycrystalline Nickel‐Base Superalloy A718Plus

“…This confirmed findings of Xie et al [23] and Viskari et al, [4] who reported on the presence of a hexagonal Ni 3 Al 0.5 Nb 0.5 phase in 718Plus. This is in contrast to many other former studies [8,21,24] that described the grain boundary phase as δ phase like in alloy 718. This explained also, why the chemical composition of the grainboundary phase in 718Plus had often been found to be different from Ni 3 Nb with significant concentrations of Al and Ti.…”

“…This explains, why a γ 00 phase was detected in this study in contrast to many former publications. [15][16][17][18][19][20][21] The γ 0 / γ 00 compounds are similar to the compact morphologies in alloy 718 that were characterized by Cozar and Pineau [7] and further investigated by He et al [27] The alloy IN706, which is also hardened by γ 0 precipitates, shows the same compounds of γ 0 and γ 00 phases. [29] Analogous to the precipitates of alloy 718, the γ 00 discs nucleate on the {001} γ/γ 0 interface planes because of the given crystal orientation relationship of (001) γ 00 ∥ {001} γ 0 , [100] γ 00 ∥ 〈001〉 γ 0 between both phases and the smaller lattice misfit between γ 00 and γ 0 compared with γ 00 and γ.…”

“…This demonstrates that the phase fractions of η and δ can differ from grain boundary precipitate to grain boundary precipitate and that the local chemical composition at the given heat treatment step determines which phases form predominantly. This explains, why some studies found only or mainly the δ phase in 718Plus, [11,16,21] whereas others detected only or mainly the η phase. [15,22]…”

“…Other studies, such as the bright-field TEM imaging and diffraction analysis by Pickering et al, [15] could not confirm the presence of γ 00 phase in 718Plus. [15][16][17][18][19][20][21] To this day, it is not entirely clear why some conditions promote the formation of γ 00 phase in 718Plus. It is hypothesized that an envelope of γ 00 DOI: 10.1002/adem.202100558 Allvac 718Plus is a derivative of alloy 718 for applications at higher temperatures in rotating and static components of gas turbines.…”

Influence of Isothermal Holding on the Microstructure and Mechanical Properties of Electron Beam Welded Dissimilar Inconel 718/ATI 718Plus® Joint

“…The principal difference in 718Plus chemistry is lower Ti-to-Al ratio as well as lower amount of Fe, which is replaced by the addition of Co and W. These modifications led to replacement of metastable c¢¢ main strengthening phase in IN718 with more stable c¢ phase in 718Plus that caused a increase in thermal stability up to 704°C. Other constituents of 718Plus microstructure are c matrix and precipitates of hexagonal g-Ni 3 Ti, g*-Ni 6 AlNb or Ni 6 (Al, Ti)Nb phases distributed mainly at the grain boundaries (Ref [6][7][8][9].…”