Super Alloys

SUPER ALLOYS

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General Introduction to Super Alloys A super alloy, or high-performance alloy, is an alloy that exhibits several key characteristics: • Excellent mechanical strength, • Resistance to thermal creep deformation, • Good surface stability, • And resistance to corrosion or oxidation. The crystal structure is typically face-centered cubic austenitic. Examples of such alloys are Hastelloy, Inconel, Waspaloy ,Rene alloys, Haynes alloys, Incoloy, MP98T, TMS alloys. . 26/12/2016 2 .

General Introduction to Super Alloys • Super Alloy development has relied heavily on both chemical and process innovations. • Super Alloys develop high temperature strength through solid solution strengthening. • An important strengthening mechanism is precipitation strengthening which forms secondary phase precipitates such as gamma prime and carbides. • Oxidation or corrosion resistance is provided by elements such as aluminum and chromium • The primary application for such alloys is in turbine engines, both aerospace and marine

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Nickel Based Super Alloys • Gamma (γ): This phase composes the matrix of Nibased super alloy. • It is a solid solution fcc austenitic phase of the alloying elements. • Alloying elements found in most commercial Ni-based alloys are, C, Cr, Mo, W, Nb, Fe, Ti, Al, V, and Ta. • During the formation of these materials, as the Ni-alloys are cooled from the melt, carbides begin to precipitate, at even lower temperatures γ'phase precipitates

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Nickel Based Super Alloys Carbide Phases: Carbide formation is usually considered deleterious although in Ni-based super alloys they are used to stabilize the structure of the material against deformation at high temperatures.

Carbides form at the grain boundaries inhibiting grain boundary motion Crystal structure for γ" (Ni3Nb) (Body Centered Tetragonal)

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Cobalt Based Super Alloys Gamma (γ): Similar to Ni-based super alloys, this is the phase of the super alloy’s matrix. While not used commercially to the extent of Nibased super alloys, alloying elements found in research Co-based alloys are C, Cr, W, Ni, Ti, Al, and Ta. Chromium is also used in Cobalt based super alloys (occasionally up to 20 wt.%) as it provides oxidation and corrosion resistance, critical for material use in gas turbines.

Gamma Prime (γ'): Just as in Ni-based super alloys, this phase constitutes the precipitate used to strengthen the alloy. In this case, it is usually close packed with a L12structure of Co3Ti Co3T, though both W and Al have been found to integrate into these cuboidal precipitates quite well. 26/12/2016

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Cobalt Based Super Alloys The elements Ta, and Ti integrate into the γ’ phase and are quite effective at stabilizing it at high temperatures. This stabilization is quite important as the lack of stability is one of the key factors that makes Co-based super alloys weaker than their Ni-base cousins at elevated temperatures. Carbide Phases: As is common with carbide formation, its appearance in Co-based super alloys does provide precipitation hardening, but does decrease lowtemperature ductility. 26/12/2016

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Iron Based Super Alloys Iron-based super alloys are characterized by high temperature as well as room-temperature strength and resistance to creep, oxidation, corrosion, and wear The iron-based grades, which are less expensive than cobalt or nickel-based grades, are of three types: alloys that can be strengthened by a martensitic type of transformation, alloys that are austenitic and are strengthened by a sequence of hot and cold working (usually, forging at 2,000 to 2,100°F followed by finishing at 1,200 to 1,600°F), and austenitic alloys strengthened by precipitation hardening. 26/12/2016

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Magnesium Alloys

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Manufacturing Processes associated with Super Alloys Spray Forming/CastingThe typical spray forming/casting process begins with vacuum-induction melting. The molten metal is then delivered to a tundish and metered from the tundish through a controlled orifice or nozzle. This metered alloy stream is then "atomized" into very fine droplets by highpurity argon gas impingement. When producing billets, the spray is then deposited onto a preheated carbon steel mandrel, which rotates under the spray. Spray-formed rings can also be produced by spraying onto a rotating mandrel. The spray formed/cast material may then be HIPed and/or subjected to deformation processing

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Manufacturing Processes associated with Super Alloys Deformation processing of wrought super alloys is performed to alter the cross-section of the material as well as to impart the appropriate microstructural refinement. Typical processes include forging, hot rolling and ring rolling. Forging can be done in closed dies, where the metal is constrained or in open dies, where it is not. Hot rolling to produce sheet and bar products must be done at very high temperatures with many reheats in between rolling passes. Ring rolling is a forging process used to produce a hollowcentered round forging 26/12/2016

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Heat Treatment and Surface Treatment of Super Alloys The term "heat treatment" when applied to super alloys may mean many different processes, including stress relievable, in process or full annealing, solution treating, precipitation hardening or diffusion of coatings. In-process annealing may be used after welding to relieve stress or in between severe forming operations. Full annealing is used to obtain a fully recrystallized, soft and ductile structure. Solution treating is done to dissolve second phases so that the additional solute is available for precipitation hardening. Precipitation hardening (also called age hardening) is used to bring out strengthening phases and to control carbides and the topologically close packed phases. Applications of coatings also involve exposures to elevated temperatures. 26/12/2016

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Surface Treatment of Super Alloys Since super alloys are prone to hot corrosion and oxidation at temperatures below which these alloys are designed to operate, protective coatings of three basic types have been developed: Aluminide (diffusion) coatings, overlay coatings and thermal barrier coatings. The aluminide coatings (usually CoAl or NiAl) are formed by a "pack diffusion" method which is a form of chemical vapor deposition. The coatings are protective because of the alumina which forms on the coating but they degrade as that oxide spalls. Overlay coatings, called MCrAlY, where M can be iron, nickel, cobalt or a combination, are two phase coatings, i.e. an aluminide in a ductile matrix.

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Surface Treatment of Super Alloys These coatings are more ductile than the aluminide coatings. Ceramic thermal barrier coatings protect by providing insulation to lower the temperature of the super alloy component by 150°C or more. A thermal barrier coating system is comprised of a ceramic layer (top coat) over a metallic layer (bond coat).

Each coating type has its advantages and limitations. 26/12/2016

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