Superplastic extension of the aluminium–zinc eutectoid results primarily from grain-boundary sliding and grain rotation. The strain rate (έ), flow stress (σ), grain size (L), and temperature (T) are related empirically: ɛ ˙ = K σ 2 L 2 exp [ − U k T ] where K is a constant, K is Boltzmann's constant, and U correlates with the activation energy for grain-boundary diffusion.The following proposed mechanism is quantitatively in agreement with our observations on superplasticity:

Certain grains that obstruct the easy relative motion of groups of grains by grain-boundary sliding yield under the resulting stress concentration;

under superplastic conditions, dislocations traverse such yielded grains and pile up at grain boundaries until their back stress prevents the grain-boundary sliding;

the high stress at the head of the pile-up causes accelerated diffusion and dislocations rapidly escape by climb into and along grain boundaries. The replacement of these dislocations makes possible further boundary sliding by the obstructed group of grains.

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