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.
Highlights
Background and motivationThe increasing climate extremes such as severe droughts, worrisome water risks, superstorms and destructive floods under global warming, which are known to be largely irreversible on timescales of many centuries [1], have today been recognized to be a consequence of the anthropogenic greenhouse gas emissions [2-10]
RE-containing particles were present in the GW103K alloy in all the three conditions
Since {10 1 2} extension twinning is a key deformation mechanism in Mg alloys [25-28,248], the combination of twinning and crystallographic textures in wrought Mg alloys is mainly responsible for the tension-compression yield asymmetry which is commonly observed in most RE-free wrought Mg alloys [16,25-28,38-40]
Summary
Background and motivationThe increasing climate extremes such as severe droughts, worrisome water risks, superstorms and destructive floods under global warming, which are known to be largely irreversible on timescales of many centuries [1], have today been recognized to be a consequence of the anthropogenic greenhouse gas emissions [2-10]. Advanced high-strength steels, aluminum alloys, magnesium (Mg) alloys, and polymers are being used to reduce vehicle weight and the subsequent emissions [16,17], but substantial reductions could be achieved further by more applications of Mg alloys which have been considered as a strategic ultra-lightweight material in the automotive and aerospace sectors [11,12,18-23]. It is well known that a metal subjected to repeated or fluctuating stresses will fail at a stress level much lower than that required to cause fracture on a single application of load [33]. Such a failure occurring under conditions of dynamic loading is called fatigue failure. Knowledge on the cyclic deformation and fatigue behavior of Mg alloys is of vital importance for the design and durability evaluation of structural engineering components
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
