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High-temperature materials and creep

Which statement about the γ' phase that provides the high-temperature strength of nickel-based superalloys is correct?

  1. AIt is a BCC (B2-type) ordered phase with a large lattice mismatch and incoherent interfaces with the matrix γ, so it precipitates as coarse particles at grain boundaries and in the grains that strongly pin dislocations
  2. BIt 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
  3. CIt is a carbide such as M₂₃C₆ that precipitates as a continuous film on grain boundaries; the more of it there is, the better grain boundary sliding is stopped and the higher both ductility and creep strength become
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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.

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More High-temperature materials and creep questions

  1. Why are directionally solidified columnar-grained or single-crystal nickel-based superalloys used for jet engine turbine blades?
  2. What is the main purpose of adding a few percent of heavy elements such as rhenium (Re) to single-crystal nickel-based superalloys?
  3. When an alloy was oxidized in air at high temperature, it followed the parabolic law x² = k_p t, in which diffusion through the dense oxide scale controls the rate, and the scale was 2.0 µm thick after 100 h. Which is closest to the scale thickness after 900 h of oxidation at the same temperature?
  4. The Pilling–Bedworth ratio of a metal's oxide scale (PBR, the ratio of the volume of oxide to the volume of metal used to form it) is calculated as PBR = (M_ox ρ_M)/(n M_M ρ_ox) (M is the molar mass, ρ is the density, and n is the number of metal atoms in one formula unit of the oxide). For magnesium (24.31 g/mol, 1.74 g/cm³) and MgO (40.30 g/mol, 3.58 g/cm³, n = 1), which combination of PBR and scale behavior is correct?
  5. Which statement about the protective scales on heat-resistant alloys used in air at high temperatures above 1000 °C is correct?