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High-temperature materials and creep

Which statement about the deformation mechanism map of Ashby and coworkers (with stress normalized by the shear modulus, σ/G, on the vertical axis and homologous temperature T/Tm on the horizontal axis) is correct? Tm is the melting point in absolute temperature.

  1. AFor a fixed grain size, it shows the field of the fastest mechanism at each stress and temperature; diffusion creep lies at low stress and high temperature, and power-law creep at higher stresses
  2. BThe field of each mechanism is set only by the kind of material, so the boundary between diffusion creep and power-law creep does not move at all whether the grains are refined or coarsened
  3. CThe field of dislocation glide widens toward lower stresses and higher temperatures, and the diffusion creep field appears only at high stresses close to the ideal strength and at low temperatures
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Answer: A. For a fixed grain size, it shows the field of the fastest mechanism at each stress and temperature; diffusion creep lies at low stress and high temperature, and power-law creep at higher stresses

A deformation mechanism map uses the rate equation of each mechanism to find which one deforms the material fastest at a given stress and temperature, and shades its field. Because several mechanisms act in parallel, the fastest one is taken to be dominant. Dislocation glide dominates at high stresses close to the ideal strength, power-law creep involving dislocation climb at intermediate stresses and high temperatures, and diffusion creep at low stresses and high temperatures. Since the rate of diffusion creep is inversely proportional to the square or cube of the grain size, refining the grains enlarges the diffusion creep field, so the boundaries do move. Contours of constant strain rate are also drawn, so the mechanism and the deformation rate under service conditions can be read off. Placing dislocation glide on the low-stress side gets the layout wrong.

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  1. At room temperature, refining the grains raises strength (the Hall–Petch relation), but components loaded for long times at high temperature often use coarse grains or single crystals. Which is the correct reason?
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