Creep & Stress-Rupture Strength of Nimonic 90 at 650–870°C: Design Data Overview

Nimonic 90 (UNS N07090 / W.Nr. 2.4632 / China ref. GH90 / GH4090) is a γ′-precipitation hardened Ni–Cr–Co–Ti–Al superalloy​ containing nominally Ni–19.5Cr–18Co–2.6Ti–1.5Al​ with (Al+Ti)total ≈ 3.8–4.8 wt%. It is specifically engineered for 650–870°C (1200–1600°F) creep-resistant service​ in aircraft engine turbine blades, nozzle guide vanes (NGVs), high-temperature springs, and fasteners.

Its creep strength originates from a relatively high volume fraction of coherent γ′ phase [Ni₃(Al,Ti)], approximately 18–22% after standard double-ageing, which provides Orowan bypass resistance to dislocation motion. This article consolidates typical stress-rupture / creep data for design reference, compares Nimonic 90 with related alloys, and defines practical temperature / stress envelopes for preliminary sizing.

1. Heat Treatment & Metallurgical Condition for Reported Data

Unless otherwise stated, all creep / rupture values below refer to the standard production heat treatment:

• Solution Anneal (SA):​ 1080–1150°C (typically 1120–1140°C) × soak per section → oil or water quench​ (must be rapid enough to retain γ′ solutes in solution).

• Double-Stage Aging (Precipitation Hardening):

  • 1st stage: 700–750°C × 16 h / AC or FC

  • 2nd stage: 650°C × 16 h / AC

• Result: Bimodal γ′ (primary ~ 0.1–0.3 µm + secondary ~ 20–50 nm), optimized stress-rupture + tensile.

⚠️ Over-aging (> 950–1000°C long hold) or insufficient quench → reduced γ′ fraction / coarse η (Ni₃Ti hex) → lower creep strength. Data below assume proper mill HT.

2. Typical Stress-Rupture (Time-to-Rupture) Values — Isothermal, Air

Values are typical averages from producer data & BS HR 1 qualification lots​ — for preliminary estimation only, not ASME Section II-D design allowables.

→ 100 h / 1000 h master curves (approx.):

• 750°C: 275 MPa ≈ 100 h; 240 MPa ≈ 1000 h

• 815°C: 180 MPa ≈ 100 h; 150 MPa ≈ 1000 h

• 870°C: 120 MPa ≈ 100 h; 100 MPa ≈ 1000 h

3. Minimum Creep Rate & Secondary Creep Reference

Typical secondary (steady-state) creep rate (˙ε_s) for aged Nimonic 90:

Design codes (e.g. aerospace engine structural integrity) often specify allowable secondary creep rate ≤ 1×10⁻⁷ – 1×10⁻⁶ s⁻¹​ depending on strain accumulation budget over mission life.

4. Effect of γ′ Volume Fraction & Aging on Creep

• γ′ (~Ni₃(Al,Ti)):​ After double-age, ~ 18–22 vol.% (higher than Nimonic 263 ≈ 8–12%, far above solid-solution alloys).

  • Primary γ′ (coarse, from 1st stage 700–750°C) → stability at service T.

  • Secondary γ′ (fine, from 2nd stage 650°C) → pin dislocations at early creep.

• γ′ Solvus:​ ≈ 985–1010°C → alloy usable to ~ 870°C design; above ~ 925–950°C long-term γ′ coarsens → creep accelerates.

• η Phase (Ni₃Ti hex):​ Forms on extreme over-age (> 1000°C or 800°C × > 5000 h extreme) → depletes γ′, lowers strength. Standard double-age minimizes η in as-supplied condition.

5. Comparison With Related Alloys (Creep Context, 750–870°C)

→ Nimonic 90 provides the best balance of creep strength + oxidation resistance + established pedigree for 650–870°C rotating / stressed parts​ among the Nimonic family. For > 900°C sustained centrifugal load, consider Waspaloy, Udimet 700/720 or René 80 (cast).

6. Oxidation / Environmental Limit Context (Creep + Oxidation Coupled)

• Oxidation Limit (static air):​ Cr₂O₃ scale + minor Al₂O₃ sub-layer → continuous useful to ≈ 1040°C (1900°F) for oxidation alone; coated if > 900°C gas path (MCrAlY / aluminide).

• Creep is usually the governing design factor at 650–870°C metal temperature; oxidation is acceptable if coating/maintenance schedule addresses scale.

• Hot Corrosion (Type I / II):​ Similar to other Cr≈20% Ni-base; sea-salt environment may require coating for long campaigns.

7. High-Temperature Spring / Fastener Application Note

• Nimonic 90 is extensively used for valve springs, lock rings, turbine casing tie-bolts​ in the 400–600°C design range.

• Stress Relaxation Test (typical):​

  • Pre-load to 60–70% Rp0.2 @ RT → heat to 600°C for 100–1000 h → retained load > 80–85% of initial (aged condition).

  • Superior to carbon/alloy steel springs; comparable to Inconel X-750 but usable to higher T (X-750 limited ~ 650°C max oxid.+ relax).

• Wire is usually supplied cold-drawn + aged (or annealed + age by customer)​ to achieve UTS 1300–1600 MPa (spring grade).

8. Recommended Design Envelopes (Preliminary)

9. Summary Creep Data Takeaways

• Nimonic 90 (UNS N07090 / W.Nr. 2.4632), properly solution-treated + double-aged, delivers:

  • ~ 240 MPa / 1000 h @ 750°C

  • ~ 100 MPa / 1000 h @ 870°C

  • Secondary creep rates in 10⁻⁷–10⁻⁶ s⁻¹ range under design stress.

• Creep strength derives from high γ′ volume (18–22%) and Co-enhanced γ′ solvus (~ 985–1010°C).

• Suitable for turbine blades, NGVs, high-temp springs/fasteners in 650–870°C creep regime; not a substitute for Nimonic 263 in welded sheet-metal (SAC risk) nor for disc alloys > 950°C sustained centrifugal load.

• Always verify final design allowables against qualified material test data per purchaser's engineering specification (often more conservative than typical producer data shown here).