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High-temperature materials and creep

Which statement about the protective scales on heat-resistant alloys used in air at high temperatures above 1000 °C is correct?

  1. AA Cr₂O₃ scale does not volatilize at any temperature, so above 1000 °C it is always more protective than an Al₂O₃ scale
  2. BA Cr₂O₃ scale tends to be lost as volatile CrO₃ above about 1000 °C, so at higher temperatures alloys that form an Al₂O₃ scale are used
  3. CThe faster and thicker a scale grows, the more protective it is, so choose alloys forming oxides with a large parabolic rate constant
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Answer: B. A Cr₂O₃ scale tends to be lost as volatile CrO₃ above about 1000 °C, so at higher temperatures alloys that form an Al₂O₃ scale are used

A protective scale at high temperature must be dense, have slow diffusion through it (a small parabolic rate constant) and adhere well to the substrate; the typical examples are Cr₂O₃, Al₂O₃ and SiO₂. Cr₂O₃ is a good protective scale up to about 1000 °C, but at higher temperatures in oxidizing atmospheres it forms gaseous CrO₃ and volatilizes, thinning the scale. 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. Adding small amounts of Y or Hf improves the adhesion of the scale (the reactive element effect). A scale that grows fast and thick means the metal is consumed fast, which is the opposite of being protective.

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More High-temperature materials and creep questions

  1. Which statement about the structure of a thermal barrier coating (TBC) applied to gas turbine blades is correct?
  2. A heat flux of 1.0 MW/m² passes uniformly through the thickness of the top coat of a thermal barrier coating (thickness 250 µm, thermal conductivity 1.2 W/(m·K)). Treating it as steady one-dimensional heat conduction, which is closest to the temperature difference between the two faces of the top coat?
  3. Which statement about the three stages of a creep curve (strain versus time) measured at constant load and temperature is correct?
  4. The steady-state creep rate of a heat-resistant alloy follows the power law ε̇ = Aσⁿexp(−Q/RT) with a stress exponent of n = 5. If the stress is raised by 20% at the same temperature, by roughly what factor does the steady-state creep rate increase?
  5. An alloy has a steady-state creep rate that follows ε̇ = Aσⁿexp(−Q/RT) with an activation energy of Q = 300 kJ/mol. If the temperature is raised from 1000 K to 1050 K at the same stress, by roughly what factor does the steady-state creep rate increase? Take R = 8.314 J/(mol·K).