Superplastic extension of the aluminium-zinc eutectoid results·primarily from grain-boundary sliding and grain rotation. The strain rate (e:), flow stress (0), grain size (L), and temperature (T) are related empirically:.where K is a constant, k is Boltzmann's constant, and U correlates with the activation energy for grainboundary diffusion.The following proposed mechanism is quantitatively in agreement with our observations on superplasticity:(i) Certain grains that obstruct the easy relative motion of groups of grains by grain-boundary sliding yield under the resulting stress concentration; (ii) under superplastic conditions, dislocations traverse such yielded grains and pile up at grain boundaries until their back stress prevents the grain-boundary sliding; (iii) 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.The superplastic behaviour of many alloys1 is of considerable interest because of the extremely large deformations that occur at quite high rates of strain, but neither the mode nor the mechanism of superplastic flow has been established. Superplasticity is most evident in eutectic or eutectoid alloys. However, the marked superplasticity of the quenched aluminium-zinc eutectoid suggests that the structural metastability of such alloys is not the cause of superplasticity; this alloy decomposes spontaneously at room temperature 2 ,3 and separation of the phases would normally occur before testing. The high strain-rate-sensitivity of the flow stress during superplastic deformation led to the proposal 4 that the flow occurred by a combination of Herring-Nabarro creep and dislocation motion. At high temperatures and low strain rates the Herring-Nabarro n1echanism would be dominant and m would approach unity in the equation cr = CXgm, where cr is the stress, g the strain rate, and cx is a constant. However, the maximum observed values of m, at temperatures just below the eutectic or eutectoid temperatures, consistently approach 0.5. 5 -7 In addition, calculations 6 ,8 show that the maximum possible strain rate by pure HerringPaper No. MS 103. Manuscript Nabarro creep (nl = 1),either through bulk or grain-boundary diffusion, is far below those experimentally recorded.The observation 9 that grains of lead-tin and lead-bismuth alloys remained equiaxed during extensions of ,...., 2000% led to the conclusion that grain-boundary sliding was the important mode of deformation. This view is supported by recent metallographic investigations of superplasticity.5,10 Despite this evidence, Chaudhari ll has,recent1y proposed that deformation proceeds solely by dislocation movement within the grains; his mechanism does not explain how the grains can remain equiaxed, and his proposal consequently appears unreasonable.The present work attempts to correlate the microstructure, as observed by electron microscopy: with th...
