Transmission Electron Microscopy (TEM) Observations of Phase-Separations of Gamma-Prime Precipitates in Ni-Al-Fe and Ni-Si-Fe Ternary Alloys

Abstract

In Ni-13.0at%Si-3.1at%Fe alloy, when γ/γ’ two-phase microstructure formed at 1123 K is isothermally heated at 923 K which is lower than the temperature where the initial γ/γ’ microstructure forms, the phase-separation of γ/γ’ precipitate phase occurs and γ particles newly appear in each cuboidal γ’ precipitate. While in Ni-10.2at%Al-10.8at%Fe alloy, when γ/γ’ two-phase microstructure formed at 1023 K is isothermally heated at 1123 K which is higher than the temperature where the initial γ/γ’ microstructure forms, the phase-separation of γ’ precipitate phase takes place and γ particles newly appear in each cuboidal γ’ precipitate. Such appearance of new γ particles in γ’ precipitates can be explained by the difference in the volume fraction of γ phase that should exist in the γ/γ’ two-phase system depending on the heating temperature.

Mapping the evolution of hierarchical microstructures in a Ni-based superalloy

“…This behaviour is not only fascinating from the scientific point of view but also technologically important because of the associated potential to strengthen alloys. Other alloy systems [7][8][9] have also been reported to exhibit similar phaseseparation phenomena. In so-called 'inverse' alloys, g 0 can serve as a matrix for g precipitates [10][11][12][13] that act in a similar way as g particles inside g 0 precipitates 6 .…”

Modeling the precipitation processes and the formation of hierarchical microstructures in a single crystal high entropy superalloy

“…Our interest in this HESA is spotted by the formation of a hierarchical microstructure comprising secondary  spherical particles formed inside primary ′ cuboid precipitates and its elevated temperature tensile yield strength which overcomes that of advanced superalloys such as the CMSX-4 benchmark. Hierarchical microstructures are not specific to HESA, they were also observed in several aged Ni-based superalloys such as IN-738 [8], Mar-M200, IN-100 [9], and CMSX-4 [10], as well as in model ' precipitation hardened ternary alloy such as Ni-Al-Ti [11,12], Ni-Si-Fe [13,14], Ni-Al-Fe [13], and Ni-Al-Si [15]. Their formation was first explained by H. F. Merrick [8] and Oblak [9] as the precipitation of  within ' particles during aging.…”