Abstract
The influence of the grain size on the plastic deformation of the Al-Zn-Mg-Cu alloy was investigated in a broad temperature range. The strengthening effect of grain boundaries was observed at temperatures below 473 K and the softening effect at higher temperatures. The increasing ability of grain boundaries to act as lattice dislocation sinks with increasing temperature is probably the reason for this transition. Superplastic properties were observed at 773 K where grain boundary sliding is the most important deformation mechanism. The existence of a grain-size-dependent threshold stress can explain the increase of the apparent grain size exponent p at low stresses.

Key words
Al-Zn-Mg-Cu alloy, plastic deformation, a grain-size-dependent threshold stress

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