High-temperature materials and creep
What is the main purpose of adding a few percent of heavy elements such as rhenium (Re) to single-crystal nickel-based superalloys?
- ATo dissolve in the γ matrix and slow diffusion, giving solid-solution strengthening and slower γ' coarsening, and so raise creep strength at high temperature
- BTo form on the surface a dense film of rhenium oxide that hardly volatilizes, raising the oxidation resistance at high temperatures in place of the Al₂O₃ film
- CTo lower the alloy's density, reducing the centrifugal stress on the fast-rotating blade and so extending the creep rupture life at the same temperature
Show answer and explanation
Answer: A. To dissolve in the γ matrix and slow diffusion, giving solid-solution strengthening and slower γ' coarsening, and so raise creep strength at high temperature
Re is an element that diffuses very slowly in the alloy and partitions mainly to the γ matrix, where it dissolves. Besides solid-solution strengthening of γ, it slows the coarsening of γ' and rafting, in which γ' joins into plates under stress at high temperature, greatly raising creep strength. Generations of single-crystal alloys are sometimes distinguished by their Re content, such as second-generation alloys with about 3% Re and third-generation alloys with about 6%. However, too much Re makes brittle TCP phases such as the σ phase likely to precipitate. Re is a heavy element, so it raises rather than lowers the density, and its oxide is volatile and does not form a protective film. Oxidation resistance is provided mainly by the Al₂O₃ film formed by Al.
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