The wrought γ′-strengthened superalloy Nimonic 263 develops a tensile ductility minimum near 800 °C that Chung, Huh and Jung (Materials Characterization 140 (2018) 9-14) traced to transgranular decohesion of the glide plane: as temperature rises, cutting of the L1₂-ordered γ′ precipitates — whose flow stress increases anomalously with temperature — gives way to Orowan by-pass, dislocations pile up at γ/γ′ interfaces, and strain localizes until the slip plane separates. Because this behavior is inherent to the ordered γ′ phase, the authors argued it cannot be removed by composition alone and posed, as an open scientific and engineering question, which thermo-mechanical processing routes, forging parameters, or post-deformation heat treatments can suppress the strain localization and glide-plane decohesion. No processing route reported between 2018 and 2026 has been shown to suppress the glide-plane-decohesion ductility minimum of Nimonic 263 or of any wrought superalloy sharing this mechanism, even though recent work has suppressed grain-boundary-mediated intermediate-temperature embrittlement in other alloy systems by grain boundary engineering, phase engineering, duplex aging, or heterogeneous grain structures and single-crystal studies confirm an intrinsic γ/γ′ contribution. The problem remains open: identify processing routes — with the parameters, the resulting microstructure, and the room-temperature-to-900 °C tensile and fracture-mode experiments that would establish their effect — that remove or substantially attenuate this ductility minimum.

Background

Nickel-based superalloys are the standard structural materials for the hottest components of gas turbines and of advanced ultra-supercritical (A-USC) coal-fired power plants, which operate at steam temperatures of roughly 700-760 °C under high pressure. Nimonic 263 is a wrought (hot-rolled/forged) nickel-cobalt-chromium-molybdenum superalloy used in such applications; it is strengthened by a dense dispersion of γ′ precipitates — coherent particles of the ordered intermetallic Ni₃(Al,Ti) with the L1₂ crystal structure — embedded in the disordered face-centred-cubic γ matrix, delivered by a standard treatment of solution annealing at 1150 °C followed by ageing at 800 °C for 8 h.

Many precipitation-strengthened nickel alloys lose tensile ductility in an intermediate temperature window of roughly 600-850 °C: elongation passes through a minimum even though strength is largely retained, and it recovers at higher temperatures. The phenomenon is called intermediate temperature brittleness (ITB, also intermediate temperature embrittlement, ITE) or the ductility minimum. A 2012 review by Zheng, Schmitz, Meng, Chellali and Schlesiger established its generality across Ni and Ni-based superalloys and judged the mechanism unresolved, favouring nonequilibrium segregation of impurities to grain boundaries; other work attributed intermediate-temperature intergranular failure to oxygen-assisted grain-boundary attack (Németh et al., 2017) or to grain-boundary carbides.

Chung, Huh and Jung (2018) reported a different, intragranular mechanism for Nimonic 263. In tensile tests from room temperature to 900 °C, elongation rose slightly to 500 °C, fell to a minimum of about 19% at 800 °C, and recovered at 900 °C. Transmission electron microscopy showed the deformation mechanism changing with temperature: at 500-700 °C dislocations travel in pairs that shear the ordered γ′ precipitates (the ordered lattice must be cut in paired unit dislocations to preserve ordering), while at 900 °C they circumvent the particles by Orowan by-pass, looping around them. The key ingredient is the anomalous temperature dependence of the L1₂ ordered phase: the strength of γ′ rises as temperature increases over this range, whereas the γ matrix softens, so cutting becomes progressively harder relative to by-pass. Near 800 °C the two resistances become comparable and mobile dislocations are trapped at the γ/γ′ interfaces, producing intense dislocation pile-ups in slip bands; the concentrated strain cannot be accommodated and the crystal separates along the strained glide plane — decohesion of the glide plane (DGP) — giving a mixed transgranular/intergranular fracture at the minimum. Because grain-boundary cracks were observed at all test temperatures (not only at the minimum) and γ′ retained its ordered structure up to 900 °C, the authors excluded grain-boundary embrittlement and γ′ instability as causes and concluded that the ductility minimum is an inherent property of the γ′-strengthened system. On this basis they posed the question this problem records: since composition alone cannot remove an effect intrinsic to the ordered phase, which thermo-mechanical processing routes, forging parameters, or post-deformation heat treatments can suppress the strain localization and glide-plane decohesion at intermediate temperatures?

Subsequent literature has both confirmed and complicated this picture. Single-crystal superalloys, which contain no grain boundaries, still show intermediate-temperature ductility loss (Huo et al., Intermetallics 2024; Zhang et al., Int. J. Mech. Sci. 2026, combining in-situ electron microscopy with crystal-plasticity modelling), so the intrinsic γ/γ′ contribution the source paper identified is real. At the same time, in other alloys and heat-treatment states the minimum is grain-boundary-mediated: M23C6 grain-boundary carbide precipitation and sulfur segregation in solution-treated Nimonic 105 (Peng et al., 2025), and oxidation-accelerated grain-boundary fracture in GH4151 (Zhou et al., Acta Materialia 2025), where the single-crystal version of the alloy is fully ductile. A 2020-2026 wave of studies demonstrates that these grain-boundary forms can be suppressed by microstructure engineering: duplex ageing — a two-step ageing heat treatment — stabilizes grain boundaries and eliminates intermediate-temperature embrittlement in an L1₂-strengthened high-entropy alloy (Yang et al., 2020); hierarchical fiber-like structures built by selective laser melting — a laser-based powder-bed additive-manufacturing process — overcome it in another high-entropy alloy (Xu et al., 2023); grain boundary engineering — deliberately altering grain-boundary population and character by thermo-mechanical treatment, here including ductile or bridging phases deposited at the boundaries — evades ITB in a precipitation-strengthened CoNiCr alloy, raising elongation at 800 °C from about 1% to about 10% (Zhang et al., Materials Futures 2025), and in (NiCoCr)94Al3Ti3 (Cai et al., 2026); phase engineering overcomes it in L1₂-strengthened NiCoCr-based high-entropy alloys (Zhang et al., 2025); and a fine-grain, bimodal-γ′ grain-boundary-engineered treatment suppresses grain-boundary brittleness in GH4151 from room temperature to 1000 °C while enabling superplastic forming (Zhou et al., Acta Materialia 2026). However, all of these mitigations act on intergranular fracture. None addresses the transgranular glide-plane decohesion that produces Nimonic 263's ductility minimum, and newly designed easy-to-deform wrought superalloys still exhibit ITB-type elongation minima at 750-850 °C (Hao et al., Materials Futures 2026). Which processing route — if any — can suppress the DGP mechanism in Nimonic 263 therefore remains an open scientific and engineering question.

Problem Statement

For Nimonic 263 — the γ′-strengthened wrought superalloy whose intermediate-temperature ductility minimum arises from strain localization and decohesion of the glide plane in the standard solution-treated and aged condition (1150 °C solution, water quenched, 800 °C for 8 h; elongation minimum of about 19% at 800 °C) — identify specific thermo-mechanical processing routes that suppress this embrittlement: hot-working and forging schedules (temperature, strain, strain rate, and cooling path), post-deformation or otherwise modified heat treatments, and the microstructural states they produce (grain size and boundary character, crystallographic texture, γ′ precipitate size and volume-fraction distribution), that suppress or substantially attenuate the ductility minimum and the glide-plane-decohesion fracture mode in the intermediate-temperature range of roughly 600-850 °C. Experimental validation of a candidate route means specifying the processing parameters, characterizing the resulting microstructure, and specifying the tensile tests from room temperature to 900 °C, together with the microstructural and fractographic analyses, that would demonstrate that the minimum is raised or removed relative to the standard-treated baseline at the same temperatures without loss of yield or ultimate tensile strength, and that strain localization and glide-plane decohesion are genuinely reduced.

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