Waspaloy (UNS N07001 / AMS 5708)

Technical Whitepaper for Aerospace Engine Components

Issued by: Shanghai Hangbo Alloy Group — Technical Whitepaper Series Document: HB-TWP-WASP-003 | Revision: 1.0 | Date: August 2026 Applicable Specifications: AMS 5708 (bar & forgings), AMS 5704/5706/5707, AMS 5544 (sheet, strip, plate), AMS 5828 (welding wire), ASTM B637, UNS N07001, MMPDS Chapter 6.3.8

Abstract. Waspaloy is a wrought, age-hardenable nickel-base superalloy developed by Pratt & Whitney in the early 1950s for gas-turbine rotating hardware. Its precipitation-hardened microstructure, built on a γ′ (Ni₃(Al,Ti)) dispersion and a carefully controlled carbide/boride grain-boundary network, delivers a room-temperature tensile strength near 190 ksi (1310 MPa) that degrades slowly with temperature: the alloy still exceeds 165 ksi (1137 MPa) ultimate at 1200 °F (649 °C) and maintains useful creep-rupture strength through 1600–1700 °F (870–927 °C), outperforming Alloy 718 above roughly 1200–1300 °F. This whitepaper details the AMS 5708 chemistry, room/elevated-temperature mechanical property database, physical properties, oxidation and corrosion performance, fabrication and heat-treatment practice, engine component applications with case studies, and a procurement checklist for aerospace buyers.

1. Introduction: History & Market

Waspaloy was developed in the early 1950s by Pratt & Whitney Aircraft (East Hartford, Connecticut) to meet the emerging demand for turbine-disk and rotating-component alloys that could sustain high stresses at 1200–1400 °F (650–760 °C) — conditions beyond the capability of the austenitic stainless steels and early Ni–Cr alloys then in service. The alloy's name derives from the famous "Wasp" engine family, whose radial and early jet powerplants defined Pratt & Whitney's identity; the trademark is held by United Technologies Corporation. Compositionally, Waspaloy refined the precipitation-hardening concept of the earlier Nimonic series by pairing a γ′-forming addition of aluminum plus titanium (total ~4.5%) with solid-solution strengthens (Cr, Co, Mo) and micro-alloy additions of boron and zirconium for grain-boundary control — a combination that proved exceptionally stable and forgiving in service.

For more than seven decades Waspaloy has been a workhorse of the aero-engine industry. It is specified for high- and low-pressure turbine disks, compressor spools and disks, shafts, spacers, seal rings, casings, and fasteners in commercial and military turbofans, turboprops (including the Pratt & Whitney Canada PT6A family), and auxiliary power units. It also appears in rocket turbopump hardware (including components of the Space Shuttle Main Engine / RS-25 lineage) and in industrial gas turbines. Today, Waspaloy continues to be manufactured in quantity under AMS 5708 and related specs, while newer alloys such as Alloy 718, René 88DT, RR1000, and HAYNES 282 progressively displace it in new designs — 282, in particular, offers better fabricability and creep strength in the 1200–1600 °F band. Nevertheless, the installed engine fleet, MRO (maintenance, repair, and overhaul) demand, and legacy design approval keep Waspaloy one of the highest-volume superalloys in production, with a fully populated property database in MMPDS (6.3.8) that simplifies design approval.

Shanghai Hangbo Alloy Group supplies Waspaloy billet, bar, plate, sheet, strip, wire, and forgings to AMS 5708/5544, including triple-melt (VIM + ESR + VAR) material with full traceability and NADCAP-compliant heat-treatment documentation.

2. Metallurgy: Precipitation Hardening & Microstructural Control

Waspaloy derives its strength from three coordinated mechanisms:

1. γ′ precipitation hardening. Solution treatment (1825–1875 °F per AMS 5708; 1950–2000 °F per Haynes practice) dissolves the γ′ phase, which then re-precipitates as coherent, ordered Ni₃(Al,Ti) particles (volume fraction roughly 20–25%) during the staged aging treatment. The fine γ′ dispersion pins dislocation motion at both room and elevated temperature — the source of the alloy's high yield strength and resistance to creep.
2. Solid-solution strengthening. Chromium (18–21%), cobalt (12–15%), and molybdenum (3.5–5.0%) raise the matrix's high-temperature strength, lower stacking-fault energy, and retard diffusion-controlled softening.
3. Grain-boundary engineering. Boron (0.003–0.010%) and zirconium (0.02–0.12%) segregate to grain boundaries, stabilizing the boundary carbide (M₂₃C₆, M₆C) and boride precipitates that suppress grain-boundary sliding and cavitation during creep. Grain size is a controlled variable: fine grains (ASTM 5–7) maximize tensile and low-cycle-fatigue strength for disks; coarser grains improve creep and crack-propagation resistance where dwell time dominates.

Processing is therefore as important as chemistry. Aerospace-grade Waspaloy is triple-melted (VIM + ESR + VAR) to minimize non-metallic inclusion content and centerline segregation in large billets, then forged with thermomechanical control (often isothermal or near-isothermal dies for disks) to develop the required grain structure before the final solution and aging cycles.

3. Chemical Analysis (AMS 5708)

Table 1 — Chemical Requirements, AMS 5708 (UNS N07001), wt%

Note: The Ti/Al ratio (nominally ~2:1) is deliberate — titanium-rich γ′ minimizes coarsening rate, while the residual γ′ solvus temperature (~1850–1950 °F) dictates the solution-treatment window. Carbon, boron, and zirconium are functional micro-alloy elements, not impurities: carbon feeds grain-boundary carbides, and B + Zr must be held within specification because both strengthen boundaries but excess boron degrades weldability.

4. Mechanical Properties

4.1 Room-Temperature Properties (Age-Hardened)

Table 2 — Typical RT Tensile, Waspaloy Bar, Age-Hardened (1825 °F/2 h AC + 1550 °F/4 h AC + 1400 °F/16 h AC)

Minimum AMS 5708 requirements for forgings and bar (typical): UTS 170–175 ksi, YS 115 ksi (0.2%), elongation 15%, with RT hardness in the range of 34–44 HRC depending on section. The age-hardened alloy's ductility (20–25%) is high for a 190-ksi material, reflecting the clean, inclusion-controlled melt practice.

4.2 Elevated-Temperature Properties (>1200 °F Focus)

The defining attribute of Waspaloy is the slow decline of strength with temperature. Table 3 lists typical tensile data from 1000 to 2000 °F.

Table 3 — Typical Elevated-Temperature Tensile, Age-Hardened Waspaloy

At 1200 °F the alloy still carries 165 ksi (1137 MPa) UTS — approximately 87% of its room-temperature value — and its strength is superior to Alloy 718 above 1200–1300 °F, which makes Waspaloy the conservative choice for long-life turbine disks that see sustained 1200–1400 °F rim temperatures. Note also the rising elongation with temperature (32% at 1400 °F, 48% at 1600 °F): Waspaloy is not embrittled by short-time elevated-temperature exposure, an important consideration for thermal-stress management in cyclic service.

4.3 Creep-Rupture Properties

Table 4 — Creep & Stress-Rupture Strength, Age-Hardened Sheet (approximate initial stress to produce specified creep in 100 h / 1000 h; R = rupture)

Two practical readings: (a) at 1200 °F the 1000-h rupture stress of 80 ksi (552 MPa) is among the best of the wrought age-hardenable superalloys, and (b) the gap between 1%-creep and rupture stresses (about 12–15%) is narrow, so life prediction is controlled by creep-deformation tolerance rather than sudden fracture.

4.4 Thermal Stability After Long Exposure

Waspaloy is metallurgically stable: after 8000 h at 1200 °F, RT yield strength increases slightly (130 → 138 ksi) as γ′ coarsens and boundary carbides develop, with negligible loss of ductility (21.8% elongation). Even after 8000 h at 1400 °F, RT UTS remains ~178 ksi. The alloy's response to prolonged service is thus predictable and non-degenerate — a key requirement for on-wing time and disk life-cycle management.

5. Physical Properties

Table 5 — Typical Physical Properties

The modulus drop with temperature (~30% from RT to 1800 °F) is normal for nickel superalloys and is fully captured in the MMPDS design curves. Thermal conductivity rises with temperature (12.6 → 24.5 W/m·K), which helps disks shed heat in cooled-turbine designs.

6. Oxidation & Corrosion Performance

6.1 Oxidation Resistance

Waspaloy forms a protective, adherent Cr₂O₃ scale in oxidizing service. In flowing-air cyclic oxidation testing (1008 h, 168-h cycles), the alloy shows:

Table 6 — Cyclic Oxidation in Flowing Air (1008 h)

In high-velocity burner-rig testing (30-min cycles, 1000 h), average metal affected is ~4.3 mil (109 µm) at 1600 °F and ~13.6 mil (345 µm) at 1800 °F — good, though slightly behind the newer HAYNES 282 and R-41 in the same tests. The practical reading: continuous service to ~1600 °F (870 °C) is well supported; short-time excursions to 1800 °F (982 °C) are acceptable with design allowance for scale loss. This aligns with the alloy's stated capability "up to about 1800 °F (980 °C)" for strength and its combustion-environment rating of ~1600 °F.

6.2 Hot Corrosion & Aqueous Behavior

Waspaloy exhibits good resistance to hot gas-path corrosion (sulfidation and low-temperature hot corrosion) in aviation-fuel combustion environments, supported by its Cr + Co + Mo balance. It is not, however, immune: under high-sulfur fuel with sea-salt ingestion, coating (e.g., aluminide or overlay MCrAlY) is applied to blades and vanes where hot corrosion dominates. In aqueous service, Waspaloy is rarely specified — like most γ′-strengthened superalloys it has moderate resistance to oxidizing acids but is susceptible to pitting in chloride-bearing solutions; use it only where elevated-temperature strength, not aqueous corrosion, is the design driver. For wet chloride service, consider Alloy C-276 or Alloy 625.

7. Fabrication Guide

7.1 Hot & Cold Forming

Waspaloy is hot-worked at 1900 °F (1040 °C) or above — standard practice is 1950–2150 °F with controlled finish temperature to develop the required grain size. Disk forgings use multiple upset/draw sequences with intermediate reheats; large sections require careful thermal-mechanical control to avoid adiabatic heating and grain growth. In the annealed condition the alloy is cold-formable (sheet components, seal rings), with 50%+ elongation available in the soft condition.

7.2 Machining

Waspaloy is at the difficult end of the superalloy machining spectrum — tough, work-hardening, with low thermal conductivity that keeps heat at the tool edge:

• Use carbide (C-2/C-3) or ceramic/whisker-reinforced inserts for turning; run rigid setups with continuous, light cuts and large coolant volumes.
• Typical turning speeds: 30–60 SFPM (carbide) on bar stock; slow, constant feeds prevent work-hardening of the surface.
• Drilling and tapping require specialized cobalt/HSS-E or carbide tooling, frequent pecking, and abundant water-soluble coolant.
• Because machined surface layers work-harden, final dimensions on fatigue-critical hardware are achieved by grinding or low-stress machining, followed by stress relief before the final aging cycle if machining precedes heat treatment.

7.3 Welding & Strain-Age Cracking

Waspaloy is weldable but carries a strain-age cracking (SAC) risk under high restraint, because the γ′ precipitation that strengthens the weldment causes slight volumetric contraction during post-weld aging. Guidelines:

• Minimize restraint with generous joint design and pre-heat as required; use matching filler (AMS 5828, Waspaloy composition) for weld-repair of Waspaloy base metal.
• Post-weld heat treatment is mandatory to develop properties: solution anneal followed by the three-step age (Section 7.4). Heat to the solution temperature as fast as the furnace permits to limit SAC on heating.
• For attaching Waspaloy to other alloys, consult the welding guidance (e.g., Haynes Welding SmartGuide); gas tungsten-arc with matching filler is the standard repair process in engine MRO.
• Welding in the overaged condition is sometimes used in repair schemes to reduce cracking tendency, followed by full re-solution and age.

7.4 Heat Treatment (Precipitation Hardening)

The standard AMS 5708 sequence and the Haynes-recommended optimum sequence are given below; both are widely accepted:

Table 7 — Heat-Treatment Cycles for Waspaloy

The aging response is the precipitation of fine γ′ (and grain-boundary M₂₃C₆ + borides), raising YS from ~64 ksi (annealed) to ~130 ksi. The lower AMS solution temperature retains a finer grain for tensile/LCF-critical disks; the higher Haynes window optimizes creep and rupture strength for blades and spacers. Every heat-treated lot must carry a furnace chart and a hardness/tensile verification — heat treatment is the single most audited step in superalloy manufacture (NADCAP AMS 2750 pyrometry compliance expected).

8. Industry Applications & Case Studies

Waspaloy's 70-year service record spans virtually every gas-turbine hot-section architecture. Principal applications: HP/LP turbine disks, compressor spools and disks, turbine and compressor shafts, spacers, seal rings, knife-edge seals, casings, bolts and fasteners, and (in sheet form) combustion hardware and ducting; in rockets, turbopump shafts, disks, and housings.

8.1 Commercial Turbofan Turbine Disks

Case Study — Long-Life HP Turbine Disk. A mature widebody turbofan (a Pratt & Whitney PW/JT9D-class design) operates its high-pressure turbine disk at rim temperatures of ~1200–1300 °F (649–704 °C). Waspaloy forgings, triple-melted and fine-grained, deliver the required combination of 1200 °F yield (~114 ksi), low-cycle-fatigue resistance, and 1000-h rupture strength (~80 ksi at 1200 °F). Fleet experience shows the disk's lifting to be governed by crack-initiation LCF with stable, non-degenerate properties after tens of thousands of cycles — the thermal-stability data of Section 4.4 being the basis for extended time-on-wing programs. MRO shops rework Waspaloy disks by controlled grind/blend of service indications and shot-peening, with weld repair reserved for non-rotating hardware.

8.2 Turboprop & Regional Hot Section

Case Study — PT6A Compressor Turbine. The Pratt & Whitney Canada PT6A family — one of the most produced turbine engines in history — uses Waspaloy for its compressor turbine disk and hot-section static hardware. The alloy's forgiving forgeability, its tolerance of the moderate firing temperatures of the small-engine cycle, and its well-documented repair history make it the cost-effective choice in a market segment where disk replacement must be predictable at 3000–6000 h intervals. This case illustrates the broader truth that Waspaloy is chosen not only for peak capability but for provenability — 60 years of field data, MMPDS curves, and established repair recipes.

8.3 Rocket Turbopump Hardware

Case Study — RS-25/SSME Lineage. In the Space Shuttle Main Engine and its RS-25 successor, the high-pressure fuel turbopump operates with liquid-hydrogen inlet temperatures near −253 °C at the inlet and combustion-gas temperatures above 980 °C in the turbine. Waspaloy turbine components bridge this extreme thermal gradient: the alloy retains cryogenic-capable toughness while carrying hot-gas stresses at the turbine end — a combination few alloys match. Its use in rocket turbopump disks and shafts leverages the same properties that serve gas turbines, and component life is validated by hot-fire testing and rotating-pool qualification.

9. Procurement Checklist for Engineers

• <input type="checkbox" disabled> Specification: AMS 5708 (bar/forgings), AMS 5544 (sheet/strip/plate), AMS 5704/5706/5707, ASTM B637; confirm which AMS revision and supplement (e.g., AMS 5708B) applies to the program.
• <input type="checkbox" disabled> Melt practice: require VIM + ESR + VAR (triple melt) for rotating hardware; VIM + VAR minimum; verify on the certificate.
• <input type="checkbox" disabled> Chemistry certification per heat against Table 1 — pay special attention to Ti, Al, B, Zr, and the C range; verify S and P for weldability.
• <input type="checkbox" disabled> Grain size (typically ASTM 5–7 for disks) and ultrasonic quality class per AMS 2631 (e.g., Class A) for rotating parts.
• <input type="checkbox" disabled> Mechanical verification: RT tensile per AMS 5708 minimums; where design-critical, request elevated-temperature (1200 °F) tensile and stress-rupture sampling per AMS 5708/AMS 2259.
• <input type="checkbox" disabled> Heat-treatment documentation: furnace charts, pyrometry (AMS 2750) compliance, and age-cycle verification; NADCAP-accredited heat treater.
• <input type="checkbox" disabled> NDT: ultrasonic, fluorescent-penetrant (FPI per AMS 2647), and, for large disks, eddy-current or phased-array as drawing requires.
• <input type="checkbox" disabled> Traceability: heat-lot-to-part traceability, COC per EN 10204 3.2 (witnessed) for critical parts, and country-of-origin documentation.
• <input type="checkbox" disabled> Forging quality: forging map/simulation evidence, die-lubricant control (no glass residue), and macro-etch verification of grain flow where specified.
• <input type="checkbox" disabled> Supplier experience: AS9100/AS9110 quality system, aerospace forging and MRO pedigree, and documented history of AMS 5708 supply.

10. FAQ — Waspaloy

1. What temperature can Waspaloy serve? Continuous service to ~1600 °F (870 °C) with good margins; strength data are published through 1800 °F (982 °C); creep capability falls off steeply above 1600 °F.
2. Why is Waspaloy better than Alloy 718 above 1200 °F? Above ~1200–1300 °F, 718's metastable γ″ phase overages rapidly; Waspaloy's stable γ′ and boundary carbides hold strength and creep resistance longer.
3. Is Waspaloy weldable? Yes, with care — matching filler and mandatory post-weld solution + age; heavy-restraint joints risk strain-age cracking.
4. What does triple melting buy? VIM removes gases/inclusions, ESR removes macro-segregation and oxide stringers, VAR removes residual porosity — cleanliness that translates directly to LCF life.
5. What is the difference between AMS 5708 and AMS 5544? 5708 covers bar/forgings (rotating hardware); 5544 covers sheet, strip, and plate (static hardware, ducting).
6. Does long exposure at 1400 °F degrade Waspaloy? No — after 8000 h at 1400 °F, RT UTS is still ~178 ksi; properties are stable and predictable.
7. Can Waspaloy be cold formed? Yes, in the solution-annealed condition (50%+ elongation); strain-rate-sensitive, so form slowly.
8. What is the γ′ solvus and why does it matter? The γ′ dissolution temperature is ~1850–1950 °F; solution treatment must exceed it (or control it precisely for grain size) to dissolve hardening phase fully.
9. Is Waspaloy used for blades? Mostly disks/shafts/static hardware; blades in extreme service are cast (e.g., Mar-M-246, René 80) — Waspaloy's forged, fine-grain structure suits rotating disks better than airfoils.
10. What are typical hardness values? 34–44 HRC age-hardened depending on section; ~93 HRBW sheet in the annealed state.
11. How is Waspaloy repaired in MRO? Blend/grind of indications, shot peening, and for non-rotating hardware, TIG repair with AMS 5828 filler followed by full re-solution and age.
12. Why boron and zirconium? They strengthen grain boundaries against creep cavitation — without them, rupture ductility and long-life creep at 1200–1400 °F would be unacceptably low.

11. References

1. SAE AMS 5708, Nickel Alloy, Corrosion and Heat-Resistant, Bars, Forgings, and Rings (Waspaloy, UNS N07001).
2. SAE AMS 5544, Nickel Alloy, Corrosion and Heat-Resistant, Sheet, Strip, and Plate (Waspaloy).
3. SAE AMS 5828, Nickel Alloy, Corrosion and Heat-Resistant, Welding Wire (Waspaloy).
4. Haynes International, HAYNES® Waspaloy Product Data (Principal Features, Tensile, Creep-Rupture, Oxidation, Physical Properties, 2024).
5. MMPDS-01, Metallic Materials Properties Development and Standardization, Chapter 6.3.8 (Waspaloy), FAA.
6. ASM International, ASM Specialty Handbook: Heat-Resistant Materials and ASM Handbook Vol. 2.
7. ASTM B637, Standard Specification for Precipitation-Hardening Nickel Alloy Bars, Forgings, and Forging Stock for High-Temperature Service.
8. Donachie, M. J., Superalloys: A Technical Guide, 2nd ed., ASM International, 2002.

Disclaimer: Data presented are typical values from public manufacturer data sheets and standards; they are indicative only and must be verified against actual mill certifications and application testing. © 2026 Shanghai Hangbo Alloy Group. Waspaloy is a registered trademark of United Technologies Corporation; HAYNES and 282 are trademarks of Haynes International, Inc.; all trademarks belong to their respective owners.