Microstructure investigation of Allvac 718Plus superalloy after heat treatment with temperature gradient

Abstract

ATI Allvac 718Plus® superalloy (718Plus alloy) exhibits high strength and good corrosion resistance in high temperatures. The 718Plus alloy was designed to increase the maximal temperature of application without strong increase of production costs. It was achieved by improving the microstructure stability up to 700 °C and was possible due to the changes in the main strengthening mechanism of the 718Plus alloy. The 718Plus alloy typical chemical composition is as follows: Ni‐18Cr‐10Fe‐9Co‐5.1(Nb+Ta)‐2.7Mo‐1W‐0.7Ti‐1.5Al‐0.03C (wt%). Those elements create multiple phases and proper characterisation of those phases is the key for understanding properties of the 718Plus alloy. The 718Plus alloy microstructure consists of a γ matrix (Ni‐base solid solution) with ordered face centred cubic γ'‐Ni 3 (Al,Ti) type phase, some orthorhombic δ‐Ni 3 Nb and hexagonal η‐Ni 3 Ti, η*‐Ni 6 AlNb or Ni 6 (Al,Ti)Nb particles precipitated mainly on the grain boundaries [1,2]. The aim of this study was to investigate the microstructural changes, with a special focus on the evolution of secondary phases, during the multiple‐step heat treatment conditions. Establishing such understanding will allow to define an optimal heat treatment route for the best alloy performance and stability, and can provide major savings in production costs. Several microscopy techniques were used for microstructure investigations, mainly SEM, TEM/HRTEM and STEM‐EDX spectrometry. Phase identification was performed by XRD, EDX and electron diffraction (SAED, nD) supported by JEMS. The samples were prepared by conventional jet electropolishing and by FIB techniques. The research was conducted utilizing Merlin G20 TWIN (SEM) and a probe Cs corrected Titan 3 Cubed G2 60‐300 with a ChemiSTEM system. It provided a possibility of very detailed analyses mainly focused on identification, morphology and chemical composition of the phases strengthened the 718Plus alloy. STEM imaging using HAADF contrast and EDX mapping were used for characterization of the particles' nanostructure down to the atomic level. The as‐received 718Plus alloy microstructure (Figs 1, 2) consists of spherical γ'‐Ni 3 (Al,Ti) phase particles and various plate‐like precipitates, some of them with a very complex structure. Observations in TEM dark‐field revealed stripes of additional phase inside some of the plate‐like precipitates (Fig. 3), which were not observed in the bright‐field images. Fig. 4 shows HRSTEM‐HAADF image of plate‐like precipitate at atomic level, as seen along [110] axis. Identification by electron diffraction and EDX showed that it is a hexagonal η‐Ni 3 Ti phase enriched in Nb (possibly η*‐Ni 6 AlNb or Ni 6 (Al,Ti)Nb phase), however some reflections indicated a presence of a different phase. It seems to be orthorhombic δ‐Ni 3 Nb phase (“white stripes”), which has similar chemical composition (Ni, Nb) but different crystal structure. Unambiguous identification of phases forming complex plate‐like particles is in progress.