Effect of overageing temperature on the superplastic behaviour in magnesium alloys

An especially fine-grained structure d < 10 μm is necessary for superplastic forming. Conventional magnesium alloys in cast condition show only a very coarse-grained structure with an average grain size of d > 100 μm. Yet through overageing and successive warm extrusion moulding a sufficient number of recrystallization nuclei are formed in the magnesium alloys so that during the final recrystallization treatment a very fine microstructure results. It has been shown that the optimal temperature for overageing lies in the range from 300 to 350 °C for the magnesium alloys AE42, AZ91, ZRE1 and QE22. Using this temperature range we obtained an average grain size of 6.2 μm in the AE42 alloy and 6.4 μm in the AZ91 alloy. In the ZRE1 magnesium alloy a grain size of 1.2 μm and in the QE22 a grain size of 0.7 μm were obtained. The heat-treated magnesium alloys QE22 and ZRE1 investigated here elongate up to 750% and have an m value of 0.75 under a constant strain rate of 3 × 10−4 s−1 and a deformation temperature of 420 °C. Under the same conditions AZ91 und AE42 elongate up to 260% and have m-values of 0.45.