Superplasticity in the aluminium-zinc eutectoid—an early model revisited

Superplastic extension of the aluminium-zinc eutectoid results primarily from grain-boundary sliding and grain rotation. The strain rate (i), flow stress (σ), grain size (L) and temperature (T) are related empirically, i = Kσ2/L2 exp[−U/KT]. The following mechanism is in agreement with our observations: (i) Certain grains that obstruct the easy relative motion of groups of grains by grain boundary sliding yield under the resulting stress concentration; (ii) dislocations traverse such yielded grains and pile up at grain boundaries; (iii) the stress at the head of the pile-up causes accelerated diffusion and dislocations escape by climb into and along grain boundaries. The replacement of these dislocations makes possible further boundary sliding.