Abstract
ATI 718Plus® is a nickel-based superalloy developed to replace Inconel 718 in aero engines for static and rotating applications. Here, the long-term stability of the alloy was studied and it was found that topologically close-packed (TCP) phases can form at the γ-η interface or, less frequently, at grain boundaries. Conventional and scanning transmission electron microscopy techniques were applied to elucidate the crystal structure and composition of these TCP precipitates. The precipitates were found to be tetragonal sigma phase and hexagonal C14 Laves phase, both being enriched in Cr, Co, Fe and Mo though sigma has a higher Cr and lower Nb content. The precipitates were observed to be heavily faulted along multiple planes. In addition, the disorientations between the TCP phases and neighbouring η/γ were determined using scanning precession electron diffraction and evaluated in axis-angle space. This work therefore provides a series of compositional and crystallographic insights that may be used to guide future alloy design.
Highlights
The nickel-based superalloy ATI 718Plus® has been developed with a view to replace the current workhorse alloy in the gas turbine industry, Inconel 718
We report the observation of topologically close-packed (TCP) phases in 718Plus subjected to high temperature annealing and characterise TCP precipitates both chemically and crystallographically using transmission electron microscopy and diffraction
The following conclusions can be drawn from this work: 1. TCP phases were found in annealed conditions of the commercial nickel-based superalloy 718Plus
Summary
The nickel-based superalloy ATI 718Plus® (hereafter 718Plus) has been developed with a view to replace the current workhorse alloy in the gas turbine industry, Inconel 718 (hereafter 718). This new alloy offers an operating temperature increase of about 37 K whilst maintaining workability and weldability [1]. (Ni3Al/Ti, L12, cubic, Pm3m), whereas the main strengthening phase in 718Plus is g0 [2] some have argued that g00 may still occur [3] This is key to performance because g0 is stable to higher temperatures than g00, which readily transforms to d phase above 650 C [4,5]. The other major difference is that where the dominant grain boundary precipitates in alloy 718 are needle-like d phase (Ni3Nb, D0a, orthorhombic, Pmmn) the dominant precipitates in 718Plus are h phase (Ni3Nb0.5(Al/Ti)0.5, D024, hexagonal, P63/mmc)
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