Microstructure of magnesium alloys in dependence of manufacture and heat treatment

Microstructural investigations are carried out an the Magnesium alloys AZ91 and AE42. The alloy AZ91 was provided both in the form of a gravity castin g and a pressure die casting, and the AE42 alloy as a pressure cast plate. In each case the as-cast condition and the T4- and T6- heat treatment condi tions were taken into account. Static bending tests, impact tests and hardn ess measurements were carried out in order to characterise the mechanical p roperties of the alloys. From the results of the investigations it is clear that in the case of the gravity cast GK-AZS1 alloy failure takes place by crack initiation and growth in the brittle beta - phase Mg17Al12, whilst in the case of the pressure cast alloy GD-AZ91 micro - shrinkage cavities and gas pores cause the cracking initiation under loading. The pressure cast alloy GD-AE42 proved to be considerably easier to deform. Crack propagation is caused by the plastic deformation of the matrix combi ned with precipitation at the grain boundaries.