Deep cryogenic treatment of HPDC AZ91 magnesium alloys prior to aging and its influence on alloy microstructure and mechanical properties

Studies of deep cryogenic treatment have mainly focused on ferrous alloys, despite the advantages of this form of treatment when applied to other non-ferrous alloys. This study examines the microstructural behavior of a high pressure die-casting (HPDC) AZ91 magnesium alloy, submitted to T6 and T6 with deep cryogenic treatment (DCT) prior to aging. Differences between continuous and discontinuous β-Mg17Al12 phase precipitation are analyzed by means of TEM, SEM and optical microscopy to explain the mechanical behavior following each type of heat treatment. The mechanical properties were improved by both treatments though yield strength was higher following T6 treatment and elongation was greater following T6 with deep cryogenic treatment. The conclusion is that continuous precipitation is promoted by cryogenic treatment, resulting in an improvement of elongation by 20%. This factor is important in view of the limitations of poor plasticity and its consequences for HPDC magnesium alloy applications.