Cyclic deformation behavior of rare-earth containing magnesium alloys

Abstract

Lightweighting in ground vehicles is considered as one of the most effective strategies to improve fuel economy and reduce anthropogenic environment-damaging, climate-changing and costly emissions. Magnesium (Mg) alloy, as a strategic ultra-lightweight metallic material, has recently drawn a considerable interest in the transportation industry to reduce the weight of vehicles due to their high strength-to-weight ratio, dimensional stability, and good machinability and recyclability. However, the hexagonal close-packed crystal (HCP) structure of Mg alloys gives only limited slip systems and develops sharp crystallographic textures associated with strong mechanical anisotropy and tension-compression yield asymmetry. For the vehicle components subjected to dynamic loading, such asymmetry could exert an unfavorable influence on the material performance. These problems could be tackled through texture modification via addition of rare-earth (RE) elements. These RE-Mg alloys possess relatively weak initial textures, which lead to improved ductility and strength, and a reduction of the tension-compression asymmetry present in the conventional wrought Mg alloys. Despite the fact that the addition of RE elements sheds some light on the alterations in the mechanical anisotropy and the tension-compression yield asymmetry, the potential advantage of such RE-Mg alloys as structural components under cyclic loading condition has not been well appreciated. Thus, the main objective of this dissertation was to explore the cyclic deformation behavior of RE-Mg alloys under varying strain amplitudes and strain ratios, and correlate the behavior to the microstructural change and crystallographic texture weakening in the RE-Mg alloys in different states (extruded and heat-treated). Unlike the RE-free Mg alloys, these alloys exhibited essentially cyclic stabilization and fairly symmetrical hysteresis loops due to the weaker texture and reduced twinning-detwinning activities. While these alloys had a lower cyclic strain hardening exponent than the RE-free extruded Mg alloys, it had a longer fatigue life which can also be described by the Coffin-Manson law and Basquin’s equation. Fatigue crack was observed to initiate from the specimen surface with some cleavage-like facets near the initiation site. Crack propagation was basically characterized by fatigue striations in conjunction with secondary cracks. A detailed analysis for understanding the obstructive role of the precipitate to twinning has been also presented.