Superalloys

• Superalloys are heat resistant alloys of nickel, cobalt, and iron– nickel that frequently operate at temperatures exceeding 1000 °F.
• They are required to exhibit combinations of high strength, good fatigue and creep resistance, good corrosion resistance, and the ability to operate at elevated temperatures for extended periods of time (i.e., metallurgical stability)
• Their combination of elevated temperature strength and resistance to surface degradation is unmatched by other metallic materials.
• In general, the nickel-based alloys are used for the highest temperature applications, followed by the cobalt-based alloys and then the iron–nickel alloys
• Nickel has an FCC crystalline structure, a density of 0.322 lb/in3 (8900 kg/m3 ) and a melting point of 2650 oF (1728 oC)
• Iron has a BCC structure at room temperature
• Cobalt has a HCP structure at room temperature
• Iron- and cobalt-based superalloys are so highly alloyed (large % of solid solution elements such as iron, chromium, cobalt, molybdenum, tungsten, titanium and aluminium) that they have an austenitic FCC structure at room temperature.
• The compositions of commercial superalloys are complex (some contain as many as a dozen alloying elements)
• Nickel based superalloys are the most complex of the superalloys and are used in the hottest parts of aircraft engines, constituting over 50% of the engine weight
• They are either solid solution hardened for lower temperature use or precipitation hardened for higher temperature use
• Cobalt based superalloys have high melting points and high temperature capability at moderate stress levels, excellent hot salt corrosion resistance and better weldability than the nickel based alloys.
• Cannot compete with the Ni based alloys at high temperatures and high stress levels.
• Cobalt based superalloys are much simpler than the nickel based alloys. ◼ Cast cobalt alloys contain about 50–60% cobalt, 20–30% chromium, 5–10% tungsten and 0.1–1.0% carbon.
• Wrought alloys contain about 40% cobalt and high nickel contents ∼20% for increased workability.
• No precipitates that result in a large strength increase have been found for the cobalt based alloys ➔ they must rely on a combination of solid solution and carbide strengthening, which limits their use in many applications.
• The cobalt alloys display good stress rupture properties at temperatures higher than 1830 F but cannot compete with the nickel based alloys for highly stressed parts, so are used for low stress long lived static parts
• The iron–nickel based alloys have high strengths below 1200 F (922 C) and are more easily processed and welded than the nickel based alloys.
• The most common precipitate is γ′ as in the alloy A-286 (contains 26% Ni)
• In alloys containing niobium, such as Inconel 718, the γ′′ precipitate Ni3Nb is the predominate strengthener
• The most commercially important superalloy, Inconel 718, is listed as an iron– nickel based alloy even though it contains more nickel than iron.


  This classification fits with the traditional classification for this alloy, although many newer works list it as a nickel based alloy.
• Solution heat treating is conducted at 1925–2250 F (1325-1505 C) for 10–30 min, followed by rapid cooling.
• Typical annealing heat treatments are conducted at minimum temperatures of 1750–2050 F (1228-1395 C) for 5–20 min followed by rapid cooling through the range of 1200–1600 F (922-1145 C).
• An intermetallic compound is a type of metallic alloy that forms an ordered solid-state compound between two or more metallic elements
• Unique combination of properties ◼ Examples: Nickel aluminide Iron aluminide Titanium aluminide
• (Intermetallic compounds) Low density, good high temperature strength, less corrosion but brittle.


  0.1 % Boron and 6-9 % Cr added to reduce embrittlement and to increase ductility
• Applications of intermetallic compounds: Jet engine, pistons, furnace parts, magnetic applications (Fe3Si)
• stoichiometric intermetallic compounds have a fixed composition.
• Examples of stoichiometric intermetallic compounds. 1. Au2Pb, AlSb, MgNi2
• nonstoichiometic intermetallic compounds have a range of compositions and are sometimes called intermediate solid solutions.
• Examples of nonstoichiometic intermetallic compounds. 1. CuAl2, TiAl3, Mg2Al3
• Refractory metals are a class of metals that are extraordinarily resistant to heat and wear. ◼ The most common (niobium, molybdenum, tantalum, tungsten, and rhenium).
• Refractory metals all share some properties, including a melting point above 2000 °C and high hardness at room temperature. ◼ They are chemically inert and have a relatively high density. ◼ Their high melting points make powder metallurgy the method of choice for fabricating components from these metals. ◼ Partly due to the high melting point, refractory metals are stable against creep deformation to very high temperatures.
• Thermal barrier coatings (TBCs) are advanced materials systems usually applied to metallic surfaces operating at elevated temperatures, such as gas turbine or aero-engine parts, as a form of exhaust heat management.
• These 100 μm to 2 mm thick coatings of thermally insulating materials serve to insulate components from large and prolonged heat loads and can sustain an appreciable temperature difference between the load-bearing alloys and the coating surface.
• In doing so, these coatings can allow for higher operating temperatures while limiting the thermal exposure of structural components, extending part life by reducing oxidation and thermal fatigue.
• Superalloy engine blades are often coated to prevent environmental degradation, and more recently, to provide thermal barriers which allow even higher operating temperatures.
• Superalloy coatings are divided into two main categories: – diffusion coatings are coatings that diffuse into the surface and react with alloying elements to form the protective coating, – overlay coatings that are deposited on the surface but only react with the substrate to the extent that an adherent bond is formed.


  While the diffusion and overlay coatings are applied to provide environmental resistance to oxidation and hot corrosion, ceramic thermal barrier coatings (TBC) are applied to allow the turbine blades to operate at even higher temperatures.
• TBC must be sufficiently thick, have a low thermal conductivity, have a high resistance to thermal shock, and contain a certain percentage of voids to provide more thermal insulation
• TBCs for aerospace applications are frequently comprised of an yttria-stabilized zirconia ceramic layer ranging in thickness from 100 μm to 2 mm.