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Joined 2026-09-21 Message
Incoming links: Vacancy diffusion
Diffusion creep 2026-10-20
Permanent shape change driven by stress-biased atomic or vacancy diffusion. Different chemical potentials at differently stressed grain surfaces drive material transfer, requiring grain-shape accommodation. Bulk and boundary transport give different grain-size dependencies.
Past exam of the materials science course of the University of Cambridge 2016 Paper 1 7 a Solution 2026-10-20
Material creep is time-dependent permanent deformation under sustained stress. Its rate is often appreciable at a large homologous temperature , because diffusion and obstacle-escape processes become rapid. In an activated regime a useful form iswith additional elastic, geometric and temperature prefactors absorbed into for the chosen approximation. The activation energy belongs to the rate-controlling process: bulk diffusion, grain-boundary diffusion and dislocation motion need not have the same value.
- A: power-law dislocation material creep. Dislocations glide and climb around obstacles; vacancy transport enables climb and recovery. The rate is strongly stress-dependent, typically with several times greater than one. It dominates the higher-stress, hot field below the glide-strength boundary.
- B: Nabarro-Herring creep. Stress-directed vacancy diffusion through the grain interiors transfers material between differently stressed boundaries, extending grains along tension. In the simple equiaxed-grain model . It occupies the high-temperature, low-stress field.
- C: Coble creep. The analogous transport runs predominantly along grain boundaries. The characteristic scaling is , where is an effective boundary thickness. The lower activation cost of boundary diffusion makes it competitive at lower temperature than bulk-diffusion material creep.
Boundary sliding can accommodate grain-shape changes during diffusional material creep; it is not a reason to identify either diffusion field as dislocation glide. The supplied map's A/B/C placement, including the boundary between B and C, determines these assignments. Its numerical contours apply to the material state used to construct that map, not to arbitrary grain sizes.