Nimonic 115: High-Creep Superalloy & Turbine Blade Applications up to 1010°C

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Overview

Nimonic 115 (UNS N07115, W.Nr. 2.4636, also designated GH4115 under China's GB/T 14992 standard) is a nickel-chromium-cobalt precipitation-hardenable superalloy developed by Special Metals Corporation (formerly Henry Wiggin & Company) as a high-performance evolution of the widely used Nimonic 105. With approximately 20% higher aluminum and titanium content than its predecessor, Nimonic 115 achieves a gamma-prime (γ') volume fraction of approximately 55% after full heat treatment — among the highest for any wrought nickel superalloy. This exceptionally high strengthening-phase content is the foundation of the alloy's class-leading creep-rupture performance at temperatures up to 1010°C (1850°F).

Developed during the era of rapidly advancing gas turbine technology, Nimonic 115 was specifically engineered for the hottest stages of aircraft engines where turbine blades must withstand simultaneous extremes of temperature, centrifugal stress, and oxidizing combustion gases. The alloy remains a benchmark material for cooled and uncooled turbine blade designs in both military and commercial aero-engine applications. Its Chinese equivalent GH4115 is widely produced by domestic superalloy mills and used in domestically developed gas turbine engines.

The alloy's microstructural design centers on maximizing the volume fraction of coherent gamma-prime precipitates (Ni₃(Al,Ti)) while maintaining sufficient chromium content (14–16%) for hot-corrosion resistance. Molybdenum provides additional solid-solution strengthening of the gamma matrix, while cobalt raises the gamma-prime solvus temperature, extending the effective strengthening range to higher temperatures. The carefully controlled carbon content produces grain boundary carbides (primarily M₂₃C₆) that inhibit grain boundary sliding during creep.

At Hangbo Alloy Group, Nimonic 115 is supplied in extruded bar, forged bar, ring forging, and investment casting forms per UNS N07115 and GH4115 specifications. Due to its high gamma-prime content, the alloy is not typically available in sheet or plate form, and its fabrication is primarily through forging, extrusion, and investment casting processes rather than cold forming or welding.

Chemical Composition

The chemistry of Nimonic 115 represents the upper limit of gamma-prime-forming element additions achievable in a wrought nickel alloy while retaining sufficient hot-workability for commercial production. The Al+Ti total of 8–10% is approximately 20% higher than Nimonic 105, yielding a gamma-prime solvus temperature approaching 1160°C and a volume fraction of approximately 55% at service temperatures. Cobalt is present at ~14% to increase the gamma-prime solvus, while chromium at 14–16% provides the necessary hot-corrosion resistance for turbine blade environments.

Element	Nominal %	Range %
Nickel (Ni)	55.0	Balance
Chromium (Cr)	15.0	14 – 16
Cobalt (Co)	14.0	13 – 15.5
Molybdenum (Mo)	4.0	3 – 5
Aluminum (Al)	5.0	4.5 – 5.5
Titanium (Ti)	4.0	3.5 – 4.5
Al + Ti Total	9.0	8 – 10
Carbon (C)	0.10	0.05 – 0.15
Iron (Fe)	—	max 1.0
Manganese (Mn)	—	max 0.5
Silicon (Si)	—	max 0.5
Copper (Cu)	—	max 0.2
Boron (B)	—	max 0.02
Zirconium (Zr)	—	max 0.04

Physical Properties

Nimonic 115 has a face-centered cubic (FCC) austenitic matrix with approximately 55% gamma-prime phase distributed as coherent precipitates after full heat treatment. The relatively low density of 7.85 g/cm³ is advantageous for rotating turbine components, where centrifugal stress is directly proportional to density. The modulus of elasticity is comparable to other precipitation-hardened nickel alloys at room temperature but declines more gradually at elevated temperatures due to the high gamma-prime solvus temperature.

Property	Value	Unit
Density	7.85	g/cm³
Melting Range	1280 – 1320	°C
Modulus of Elasticity (21°C)	220	GPa
Modulus of Elasticity (800°C)	160	GPa
Mean CTE (21–500°C)	12.5	μm/m·°C
Mean CTE (21–900°C)	14.8	μm/m·°C
Thermal Conductivity (21°C)	10.5	W/m·K
Thermal Conductivity (800°C)	23.0	W/m·K
Gamma-Prime Solvus	~1160	°C
Gamma-Prime Vol. Fraction	~55	%

Mechanical Properties

Nimonic 115 achieves its full strength only after the complete multi-stage precipitation hardening heat treatment. The alloy is not supplied or used in the solution-annealed condition for service applications; all mechanical property specifications reference the fully heat-treated condition. The extremely high gamma-prime volume fraction produces room-temperature strength levels that approach those of powder metallurgy superalloys.

Room Temperature Properties (Fully Heat Treated):

Property	Typical Value
Tensile Strength	1100 – 1300 MPa (160 – 189 ksi)
Yield Strength (0.2% offset)	750 – 900 MPa (109 – 131 ksi)
Elongation in 50 mm	15 – 25%
Reduction of Area	18 – 28%
Hardness	350 – 400 HB

Elevated Temperature Tensile Properties:

Temperature (°C)	Tensile Strength (MPa)	Yield Strength (MPa)	Elongation (%)
21 (Room)	1200	820	20
650	1050	750	15
800	850	600	12
900	550	400	10
1000	280	220	15

A notable characteristic of Nimonic 115 is that ductility actually increases at temperatures above 900°C as the gamma-prime phase begins to partially dissolve, improving stress relaxation capability which is beneficial for thermal-mechanical fatigue resistance in turbine blade applications.

Heat Treatment

The heat treatment of Nimonic 115 is a sophisticated multi-stage process that must be executed with precise temperature control to achieve the intended gamma-prime particle size distribution. The process is more complex than that of lower-gamma-prime alloys and is a significant contributor to the alloy's cost.

• Solution Annealing: 1180–1190°C (2160–2180°F) for 2–4 hours depending on section size, followed by air cool or controlled-rate cooling. This temperature is just above the gamma-prime solvus (~1160°C), dissolving all primary gamma-prime and most carbides into solid solution. Temperature uniformity is critical — overheating can cause incipient melting at grain boundaries.
• Primary (High-Temperature) Aging: Approximately 1050°C (1920°F) for 4–6 hours, air cool. This step precipitates relatively coarse gamma-prime particles at grain boundaries and in the interdendritic regions, providing grain boundary strengthening and creep resistance.
• Secondary (Low-Temperature) Aging: 750–850°C (1380–1560°F) for 16–24 hours, air cool. This final step precipitates a fine dispersion of secondary gamma-prime within the grains, maximizing room-temperature and intermediate-temperature strength.

The two-stage aging process produces a bimodal gamma-prime distribution that is optimal for both creep resistance (coarse precipitates at grain boundaries) and tensile strength (fine precipitates within grains). The total heat treatment cycle time can exceed 30 hours, reflecting the precision required for turbine-blade-grade material.

Creep and Stress-Rupture Properties

Creep resistance is the defining performance characteristic of Nimonic 115 and the primary reason for its development. The approximately 55% gamma-prime volume fraction provides exceptional resistance to dislocation climb and glide at temperatures where most wrought superalloys have lost effective strengthening. This makes Nimonic 115 one of the few wrought alloys suitable for the hottest turbine blade stages.

1000-Hour Stress-Rupture Strength:

Temperature (°C)	1000-h Rupture Strength (MPa)	Comparison: Nimonic 105 (MPa)
750	420	350
850	200	160
950	70	50
1000	35	20

The creep advantage of Nimonic 115 over Nimonic 105 becomes more pronounced at higher temperatures, with approximately 75% higher rupture life at 1000°C. This behavior is a direct consequence of the higher gamma-prime solvus temperature, which maintains particle stability and strengthening effectiveness at temperatures where lower-Al+Ti alloys experience significant gamma-prime coarsening and dissolution.

Metallurgical Design — The 55% Gamma-Prime Architecture

The microstructure of Nimonic 115 after full heat treatment is dominated by its exceptionally high gamma-prime volume fraction. Understanding this microstructure is essential for appreciating both the alloy's capabilities and its processing challenges.

• Bimodal Gamma-Prime Distribution: The two-stage aging process creates coarse primary gamma-prime (~200–500 nm) at grain boundaries and fine secondary gamma-prime (~20–50 nm) within grains. The coarse precipitates pin grain boundaries against sliding during creep, while the fine precipitates provide dislocation cutting and Orowan looping resistance within grains.
• Grain Boundary Carbides: M₂₃C₆ chromium-rich carbides precipitate preferentially at grain boundaries during the primary aging step, forming a discontinuous chain morphology that inhibits grain boundary migration and sliding without causing embrittlement.
• Gamma-Prime Solvus Temperature: At approximately 1160°C, the solvus is remarkably high for a wrought alloy, giving a wide processing window between solution annealing and incipient melting, and ensuring gamma-prime stability at service temperatures up to 1010°C.
• Topologically Close-Packed (TCP) Phase Control: The chromium and molybdenum levels are carefully balanced to avoid the formation of sigma or mu TCP phases during long-term service exposure, which would detract from creep ductility.

Fabrication and Processing

Nimonic 115 is challenging to process compared to lower-gamma-prime alloys, and its fabrication is typically limited to forging, extrusion, and investment casting. The high gamma-prime content gives the alloy a narrow hot-working window and high flow stress, requiring specialized equipment and expertise.

• Hot Working: Forging and extrusion must be performed within a narrow temperature window of approximately 1120–1180°C. Below 1120°C, gamma-prime precipitation during cooling rapidly increases flow stress; above 1180°C, incipient melting risk increases. Proper preheat and rapid transfer from furnace to press are essential.
• Investment Casting: Nimonic 115 can be vacuum investment cast for turbine blade production. The casting must be carefully gated to ensure directional solidification and minimize porosity. Hot isostatic pressing (HIP) may be applied to close internal porosity.
• Machining: In the fully heat-treated condition, Nimonic 115 is very abrasive and generates high cutting temperatures. Carbide or ceramic tooling, rigid setups, abundant coolant, and conservative feeds/speeds are required. Machining in the solution-annealed condition (before aging) is generally preferred.
• Welding: Nimonic 115 is generally NOT recommended for welding. The very high gamma-prime content makes the heat-affected zone extremely susceptible to strain-age cracking, a phenomenon where welding-induced residual stresses combine with gamma-prime precipitation strains during post-weld cooling to cause intergranular cracking. For applications requiring joining, brazing or mechanical fastening methods are preferred.

Typical Applications

Nimonic 115 was developed specifically for the most demanding rotating and static hot-section components in gas turbine engines, and its application portfolio reflects this highly specialized origin.

• Aircraft Gas Turbine Blades: Both cooled and uncooled blade designs for the first and second turbine stages of high-thrust military and commercial engines, where metal temperatures can approach 1000°C and centrifugal stresses exceed 200 MPa.
• Nozzle Guide Vanes (NGVs): Stationary airfoils immediately downstream of the combustor that direct hot gases onto the first-stage turbine blades, experiencing the highest gas temperatures in the engine.
• Turbine Discs and Seals: Forged disc components in the hottest stages, where creep resistance under rim loading is the design-limiting criterion.
• Industrial Gas Turbine Hot Section: Components in large-frame industrial gas turbines used for power generation, particularly in the first-stage turbine section.
• Aerospace Structural Components: High-temperature fasteners, locating pins, and structural brackets in the hottest zones of aero-engine casings.

Comparison with Similar Alloys

Property	Nimonic 115	Nimonic 105	Nimonic 90	Waspaloy	Inconel 713C
Max Service Temp	1010°C	980°C	920°C	870°C	980°C
γ' Vol. Fraction	~55%	~45%	~20%	~25%	~65%
Density (g/cm³)	7.85	8.01	8.18	8.22	7.91
Form	Wrought/Cast	Wrought	Wrought	Wrought	Cast Only
Weldability	Poor	Poor	Fair	Fair	Poor
RT Tensile (MPa)	1100–1300	1100–1250	1200–1400	1275–1410	850–1035
Cost Factor	Premium	High	High	High	Medium-High

Nimonic 115 occupies a unique niche among wrought nickel superalloys — it provides gamma-prime volume fraction and temperature capability approaching that of cast alloys like Inconel 713C (~65% γ'), while retaining the forged microstructure benefits of a wrought product. The trade-offs are high cost, challenging processing, and limited product forms compared to lower-gamma-prime wrought alloys.

Frequently Asked Questions