A heat-resistant alloy with a large amount of the L1₂ ordered phase γ' (Ni₃Al) coherently precipitated in an FCC Ni matrix. Used for turbine blades, it retains strength up to temperatures close to its melting point.
Japanese name: Ni基超合金
Nickel-based superalloy appears in 7 practice questions across 3 topics, including High-temperature materials and creep, Computational materials science, Materials processing and joining.
A nickel-based superalloy is an alloy in which the L1₂ ordered phase γ' (Ni₃(Al,Ti)) precipitates as cuboids in the matrix γ, an FCC Ni solid solution. Because γ and γ' have nearly the same lattice parameter and coherent interfaces, γ' resists coarsening even at high temperature, and in alloys for single-crystal blades it occupies 60 to 70% of the volume. When a dislocation in γ enters γ', it disturbs the ordered arrangement and creates an antiphase boundary, so dislocations must travel in pairs and meet a large resistance. In addition, Cr, Mo, W, Re and other elements provide solid-solution strengthening of the matrix.
In turbine blades pulled for long times at high temperature, grain boundaries perpendicular to the stress become the starting points of creep damage. Blades with columnar grains, made by solidifying in one direction from the bottom of the mold so that the boundaries are aligned with the stress, were therefore produced, followed by single-crystal blades grown by selecting just one crystal. A single crystal can do with fewer grain-boundary-strengthening elements, so solution treatment can be done at high temperature to make γ' uniform and fine. The blade length is aligned with the low-modulus 〈001〉 orientation to reduce thermal stress.
On the surface of a blade exposed to hot gas, Al and Cr in the alloy oxidize to form protective Al₂O₃ or Cr₂O₃ scales. Adding many strengthening elements may force Al or Cr to be reduced, so coatings rich in Al (aluminide coatings or MCrAlY coatings) are applied to the surface to make up the oxidation resistance. A zirconia thermal barrier coating is then layered on top to lower the metal temperature, and air flows through internal cooling holes to cool the blade, so that it can be used in gas at temperatures that can exceed the melting point of the alloy.
Answer: B. It is Ni₃Al with the L1₂ ordered structure (an ordered phase based on FCC), with nearly the same lattice parameter as the matrix γ, and precipitates finely with coherent interfaces
The γ' phase is an L1₂ ordered phase of composition Ni₃(Al,Ti), in which Al occupies the corners and Ni the face centers of the FCC lattice. Its lattice parameter differs little from that of the matrix γ (an FCC Ni solid solution) (small misfit), so fine cuboidal particles precipitate with coherent interfaces, reaching 60 to 70% by volume in alloys for single-crystal blades. When a dislocation in γ enters γ', it disturbs the ordered arrangement and creates an antiphase boundary, so dislocations must travel in pairs, which gives a large resistance. γ' is stable up to high temperatures and also shows an anomalous temperature dependence in which the yield stress rises with temperature up to about 700 to 800 °C. Calling it BCC mistakes the crystal structure, and calling it a grain boundary carbide confuses it with another phase; a film-like carbide is actually a cause of brittleness.
Answer: C. Because removing grain boundaries perpendicular to the stress eliminates the sites where creep damage starts, and a single crystal can do without grain-boundary-strengthening elements, so it can be solution treated at high temperature to make γ' uniform and fine
Turbine blades are pulled along their length by centrifugal force, and grain boundaries perpendicular to the stress become the starting points of creep voids and cracks. Directional solidification produces columnar grains aligned with the stress, removing the perpendicular boundaries. In single-crystal blades, grown so that only one crystal is selected, there are no grain boundaries at all. When C, B, Zr and other grain-boundary strengtheners are no longer needed, the temperature at which the alloy begins to melt rises, so solution treatment can be done at high temperature to dissolve γ' fully and then reprecipitate it uniformly and finely. The statement about adding more strengtheners reverses this relationship. Single crystals still contain dislocations, and creep proceeds by their motion within the crystal.
They came to be called by this name as alloys that keep high strength and oxidation resistance even at high temperatures at which the strength of ordinary steels and similar alloys drops sharply. Besides Ni-based superalloys, there are also Co-based and Fe-Ni-based superalloys.
The Ni₃Al of γ' shows an anomalous temperature dependence in which the yield stress rises with temperature, being strongest at about 700 to 800 °C. This is because heat promotes cross slip that converts dislocations into a configuration that moves with difficulty. At still higher temperatures, however, γ' coarsens or dissolves and the strength falls.
Re slows diffusion and raises creep strength, but too much makes brittle TCP phases such as the σ phase likely to precipitate as needles or plates, lowering strength and ductility. Re is also expensive and dense, so the amount added is set by balancing these factors.
The refractory mold withstands high temperatures, so the method is also used for precision parts in Ni-based superalloys, such as jet-engine turbine blades, and directionally solidified and single-crystal blades are made this way as well.
Which casting method makes a pattern of the same shape as t…Al₂O₃ grows even more slowly and is stable to high temperatures, so nickel-based superalloys, bond coats of thermal barrier coatings and Fe-Cr-Al heating elements are made to form Al₂O₃.
Which statement about the protective scales on heat-resista…The creep strength of nickel-based superalloys is sensitive to temperature, so this temperature difference strongly affects the life and the allowable gas temperature.
A heat flux of 1.0 MW/m² passes uniformly through the thick…Creep Dislocation Thermal barrier coating Columnar grains Solid-solution strengthening CALPHAD method Gibbs energy Investment casting Thermal stress Deformation mechanism map