Abstract
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: <disp-formula> <mml:math> <mml:mrow> <mml:mover> <mml:mi>ɛ</mml:mi> <mml:mi>˙</mml:mi> </mml:mover> <mml:mo>=</mml:mo> <mml:mfrac> <mml:mrow> <mml:mi>K</mml:mi> <mml:msup> <mml:mi>σ</mml:mi> <mml:mn>2</mml:mn> </mml:msup> </mml:mrow> <mml:mrow> <mml:msup> <mml:mi>L</mml:mi> <mml:mn>2</mml:mn> </mml:msup> </mml:mrow> </mml:mfrac> <mml:mtext> exp</mml:mtext> <mml:mo>[</mml:mo> <mml:mfrac> <mml:mrow> <mml:mo>−</mml:mo> <mml:mi>U</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>k</mml:mi> <mml:mi>T</mml:mi> </mml:mrow> </mml:mfrac> <mml:mo>]</mml:mo> </mml:mrow> </mml:math> </disp-formula> 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: <list list-type="roman-lower"> <list-item>
Certain grains that obstruct the easy relative motion of groups of grains by grain-boundary sliding yield under the resulting stress concentration;</list-item> <list-item>
under superplastic conditions, dislocations traverse such yielded grains and pile up at grain boundaries until their back stress prevents the grain-boundary sliding;</list-item> <list-item>
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.</list-item> </list>
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