Abstract
Acoustic emission was monitored and analysed during tensile tests at room temperature of extruded magnesium alloy AZ31 (Mg-2.9wt.%Al-0.98wt.%Zn-0.29%Mn). Hydrostatically and indirectly extruded rods were water-cooled or air-cooled, respectively, and exhibit different microstructures, e.g. a variation of the grain size, as well as different mechanical properties. The validity of the Hall-Petch relation was proven. The results are explained in terms of dislocations glide and deformation twinning during plastic deformation.

Key words
acoustic emission, extrusion, tensile testing, Hall-Petch relation, AZ31

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