Creep behavior of Deep Cryogenic Treated AZ91 Magnesium alloy

The Deep Cryogenic Treatment (DCT) is known to have beneficial effects on mechanical behavior/properties of some alloys. It was also reported to be beneficial for the creep strength of Mg alloys. The present study is aimed at checking the effect of DCT on the high temperature behaviour of the Mg alloy AZ91 by means of comparative creep tests at 100 and 200°C carried out on high pressure die-cast specimens. In addition, the role played by initial microstructure was investigated by comparative creep tests on annealed and solution treated specimens. The mechanical tests were joined to optical and scanning electron microscopy observations on the creep specimens prior and after their high temperature exposure and to Differential Scanning Calorimetry analyses to investigate the thermal stability and the microstructural evolution of the alloy. The investigations results demonstrated that the effect of initial microstructure is by far more important for the creep properties of the Az91 alloy than the cryogenic treatment. DSC analyses revealed to be a useful tool to get information on the microstructural stability in view of the development of microstructure-based creep models.