Monotonic and Fatigue Behavior of Magnesium Extrusion Alloy AM30: An International Benchmark Test in the “Magnesium Front End Research and Development Project” 2010-01-0407

Magnesium alloys are the lightest structural metal and recently attention has been focused on using them for structural automotive components. Fatigue and durability studies are essential in the design of these load-bearing components.

In 2006, a large multinational research effort, Magnesium Front End Research & Development (MFERD), was launched involving researchers from Canada, China and the US. The MFERD project is intended to investigate the applicability of Mg alloys as lightweight materials for automotive body structures. The participating institutions in fatigue and durability studies were the University of Waterloo and Ryerson University from Canada, Institute of Metal Research (IMR) from China, and Mississippi State University, Westmorland, General Motors Corporation, Ford Motor Company and Chrysler Group LLC from the United States. This paper presents the results of benchmark coupon testing that were obtained for monotonic and cyclic conditions on extruded AM30 alloy samples. Tests were performed independently in Canada, China, and the US. In general, the results reported by different institutions were in good agreement.

Microstructure analyses revealed strong material texture with a unique orientation of extension twinning with respect to the initial basal plane. The cyclic deformation, therefore, was seen to be dominated by twinning and detwinning. The unusual asymmetric hysteresis of AM30 observed for fully reversed cyclic loading is attributed to twinning under compression in the extrusion direction, detwinning upon unloading from compression and dislocation slip in tension.

The monotonic tests were performed under different strain rates and at room temperature or 125°C. Cyclic tests were performed under strain controlled conditions. Two strain amplitudes were considered, 0.3% and 0.6% and all fatigue tests were performed under standard laboratory conditions.

Raising the temperature from standard laboratory conditions to 125°C had a significant effect under monotonic loading: both the yield and tensile strength dropped by about 25%, while ductility increased by 300%. Under fatigue loading at room temperature, extruded AM30 exhibits asymmetrical cyclic behavior at a strain amplitude of 0.6%, whereas the cyclic behavior at 0.3% was symmetric. The material showed significant plastic strain recovery, cyclic hardening, and a clear endurance limit.