Steady-state creep occurs via atomic diffusion, either as a mass transport mechanism in itself (“diffusion creep”) or as a rate determining step in dislocation motion (“dislocation creep”). In diffusion creep, transport occurs on the scale of the grain, so coarser grains lead to slower creep rates. Like diffusion, the creep rate has an Arrhenius dependence on temperature. It also has a power-law dependence on stress. The value of the stress exponent indicates which mechanism of creep is acting. For diffusion creep, the exponent is around unity, while for dislocation creep it is usually higher (~3-8).
You should now be familiar with a particular experimental set-up to measure creep-rate as a function of stress and temperature. You should also understand how to use experimental data to calculate the stress exponent and the activation energy of creep.
This TLP has largely focused on the creep of solder, since this can be observed on a short timescale at quite low temperatures. It is important to realise that most metals do not experience creep at room temperature, but that it can be a problem at high temperatures even if the material is operating under stresses much lower than its yield stress.
Materials can be engineered to resist creep. Removing grain boundaries increases
the material’s resistance to diffusional creep. An array of finely spaced
precipitates can inhibit the motion of dislocations. These methods are used
in the manufacture of nickel-based superalloys for use in aero-engine turbine
blades.
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