Abstract
The effects of ageing treatment on phase transformations in Mg–Al alloy have been assessed. The techniques of scanning and transmission electron microscopy, microhardness and differential scanning calorimetric tests were used to characterize the materials obtained after application of artificial ageing. An ageing treatment at 175 °C causes the β-Mg17Al12 precipitation to become evenly distributed along the grain boundary and within the grain together with precipitation of the Al6Mn and the Τ-Mg32(Al,Zn)49 phases. The transformation processes are associated with increased hardness values. The hardness value increases with the ageing time and attains maximum value after 10 h of ageing time. The presence of the β-Mg17Al12 phase acts as an effective barrier to dislocation motion, thus improving the mechanical properties of the alloy. Analysing the DSC data it is found a shift of peak temperatures to higher temperatures with increasing heating rates, which suggests that the solid state reactions are thermally activated and kinetically controlled. The fraction and the rate of transformation, the transformation function and the kinetic parameters such as activation energy and frequency factor for the alloy in artificial ageing conditions were determined.
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