Effects of a Tantalum Addition on the Morphological and Compositional Evolution of a Model Ni-Al-Cr Superalloy

Abstract

The effects of a 2.0 at.% addition of Ta to a model Ni-Al-Cr superalloy aged at 1073 K are assessed using scanning electron microscopy and atom-probe tomography. The addition of Ta results in appreciable strengthening, and the morphology is found to evolve from a bimodal distribution of spheroidal precipitates, to cuboidal precipitates aligned along the elastically soft <001>-type directions. Tantalum is observed to partition preferentially to the γ'-precipitate phase and decreases the mobility of Ni in the γ-matrix sufficiently to cause an accumulation of Ni on the γ-matrix side of the γ'-precipitate/γ-matrix heterophase interface.

The effects of refractory elements on Ni-excesses and Ni-depletions at γ(f.c.c.)/γ′(L12) interfaces in model Ni-based superalloys: Atom-probe tomographic experiments and first-principles calculations

“…The morphological evolution of the γ'-precipitates in the base Ni-Al-Cr alloy [30], and alloys F (with Ta) [30] and H (with W) [32,33] follows the trend observed for most Ni-based superalloys [59,60]; that is, a sphere-to-cuboid transition in the later stage of coarsening. The transition for alloys F [30] and H [32,33] occurs at a smaller R value than the base Ni-Al-Cr alloy because of their increased lattice parameter misfit, due to the Ta or W additions. In contrast, the morphology of the γ'precipitates in alloys G (with Re) [5,31] and E (with Ru) [34] is spheroidal through 256 h of…”

“…Here, we review γ'-morphology, γ'mean radii ( R ), γ'-number density (N v ), edge-to-edge γ'-inter-precipitate distance ( ee   ), and γ'volume fraction  in the four quaternary model alloys E to H, containing only one refractory elemental addition, R, at a time, based on our prior research [5,[30][31][32][33][34]. The morphological evolution of the γ'-precipitates in the base Ni-Al-Cr alloy [30], and alloys F (with Ta) [30] and H (with W) [32,33] follows the trend observed for most Ni-based superalloys [59,60]; that is, a sphere-to-cuboid transition in the later stage of coarsening. The transition for alloys F [30] and H [32,33] occurs at a smaller R value than the base Ni-Al-Cr alloy because of their increased lattice parameter misfit, due to the Ta or W additions.…”

“…Unlike Ni retention-excesses in ternary Ni-Al-Cr alloys [25,26], which emerge during the early stages of phase separation and disappear at later stages, an accumulation of Ni is observed to develop on the γ-matrix side of the (f.c.c. )/γ'(L1 2 ) interface during aging times 0-256 h in Ni-10.0Al-8.5Cr-2.0Ta at.% [32,33]. But in just one Ni-Al-Cr-Ta alloy has a Ni interfacial excess been reported, which is lacking an in-depth explanation.…”

“…Additionally, we have implemented different techniques to study systematically the roles of small additions (2 at.%) of refractory elements on the γ/γ' decompositions when a model ternary Ni-10.0Al-8.5Cr at.% superalloy is aged at 1073 K (800 o C) for seven aging times varying from 0 to 256 h [5,[29][30][31][32][33][34][35]. More specifically, we reported previously on four quaternary alloys Ni-10.0Al-8.5Cr-2.0Re [5,31], Ni-10.0Al-8.5Cr-2.0W [29,30], Ni-10.0Al-8.5Cr-2.0Ta [32,33] and Ni-10.0Al-8.5Cr-2.0Ru at.% [34], one quinary alloy Ni-10.0Al-8.5Cr-1.0Re-1.0Ru…”

A correlative four-dimensional study of phase-separation at the subnanoscale to nanoscale of a Ni Al alloy

“…A solutionized specimen ( ) was analyzed by 3-D APT to establish whether or not γ'(L12)-precipitates were present in the initial quenched-in state, and none were detected: thus, . Additional details of the procedures are given elsewhere [18,23,30].…”