Superalloys used for aerospace fastener require a combination of mechanical strength, notch toughness, elevated temperature capability and corrosion/oxidation resistance.

Typical superalloy requirements

Mechanical strength requirements might include tensile, shear, fatigue, creep and/or stress rupture strengths, depending on the application demands. Strength requirements have increased as new developments in engine design and other technologies have arisen.

The continual need for greater thrust output and better fuel efficiency has resulted in faster-spinning, hotter-running gas turbine engines. This, in turn, has created the need for alloys that can withstand higher stresses and temperatures, a trend that is expected to continue.

Another critical material property is the ability to resist corrosion at ambient and elevated temperatures, including general corrosion, crevice corrosion, stress corrosion, oxidation and sulfidation. For joints exposed to potentially corrosive environments, the fastener should be galvanically compatible with the joint. That is, the materials used should be relatively close on the galvanic series. If this is not practical, platings or coatings can be applied to the “noble” corrosion resistant fasteners to promote galvanic compatibility with structures assembled from more “active” materials, such as aluminum alloys.

It is important to understand that the properties of a superalloy are not merely the result of its composition. Material processing and fastener manufacturing play roles in achieving the desired combination of properties. Cold working, thermal treatments and the sequence of critical manufacturing operations significantly enhance performance.

Marine, space and medical applications are just a few of the environments that challenge the superalloys . Other applications are...

• Space Shuttle Orbiter & Solid Rocket Motors

• Aircraft Structure & Landing Gears

• Aircraft Gas Turbine Engines

• Gas Turbine Engines for Power Generation

• High-Performance Automotive Engines

• Marine—Ships, Submarines, Naval Aircraft

• Petrochemical Equipment

• Chemical Processing Equipment

• Medical X-Ray Imaging Equipment

• Cryogenic Uses

MP159 is an excellent superalloy invented by sps. The details of this alloy are as follows.

MP159 Basics

MP159 was developed in response to the need for a fastener alloy that could perform under the higher operating temperatures developed by turbine engines and other high performance motors. MP159 has advantages similar to MP35N, but can perform in applications up to 1100°F, and is suitable for short term use at even higher temperatures. This capability makes MP159 alloy fasteners desirable for use in jet engines, rocket motors and other applications.

MP159 alloy's unique attributes led to its use for high strength landing gear bolts on commercial aircraft and to secure the Space Shuttle aboard its Boeing 747 ferry plane, as well as applications in the solid rocket boosters.

MP159 also exhibits excellent forgeability, superior to that of MP35N, which allows the manufacture of tension rated, 260 ksi tensile strength bolts above 0.75 inch diameter.

Nominal Composition

Cobalt 36%

Nickel 25%

Chromium 19%

Iron 9%

Molybdenum 7%

Titanium 3%

Columbium 0.6%

Aluminum 0.2%

Metallurgical Strengthening Mechanisms 

Like MP35N, this alloy undergoes the MULTIPHASE reaction, but also benefits from a second strengthening mechanism. Elements have also been added to the chemical composition to cause the precipitation of the gamma prime (γ ') phase during age hardening. The gamma prime phase, which forms in many conventional nickel-base superalloys, is responsible for hightemperature strength.

MP159 Properties Profile

• High strength coupled with excellent ductility and toughness

• Higher operating temperature than MP35N: 1100°F

• High creep strength at 1100°F

• Highly resistant to stress relaxation at operating temperatures; clamping load is maintained

• Capable of being forged and manufactured into complex configurations

• Corrosion resistance equivalent to that of MP35N

• Excellent fatigue resistance.

MP159 Corrosion Resistance

Extensive laboratory testing of MP159 bolts has verified the alloy's excellent resistance to crevice corrosion, stress corrosion cracking and hydrogen embrittlement:

Resistance to crevice corrosion was demonstrated in a 10% ferric chloride solution test, reported in the MP35N section of this literature.  

Stress corrosion cracking resistance was evaluated by loading MP159 alloy bolts to 75% of the minimum ultimate tensile strength and alternately immersing the loaded bolts in a 3.5% NaCl solution for 10 minutes and air drying for 50 minutes in accordance with MIL-STD-1312, test no. 9 procedures. After 5000 hours of testing, the bolts were sound, free of cracks and any visible corrosion. 

Salt swab tests were performed on MP159 alloy bolts loaded to 75% of the minimum ultimate tensile strength in aluminum alloy cylinders. A salt solution was applied to the bolts every 100 hours. These conditions promote the cathodic generation of hydrogen on the bolt surface. After 1000 hours of this severe test, no bolt failures occurred, indicating exceptional resistance of MP159 to galvanically induced hydrogen embrittlement.

MP159 Fastener Applications

• Gas turbine engines

• Airframes

• Shuttle propulsion system

• Aircraft landing gear

• Marine equipment

• Chemical processing apparatus

• Petrochemical refineries

• Pulp and paper processing plants

• Power generation equipment

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