Dislocation-mediated plasticity in magnesium alloys

Abstract

The pursuit of lightweight design and material innovation is crucial for enhancing sustainability and addressing fuel consumption and emissions across various industries. Magnesium (Mg), the lightest structural metal on earth, stands out for its low density, high specific stiffness, and excellent castability and workability. Nonetheless, its insufficient room-temperature mechanical properties, which have long been believed to be associated with limited dislocation activities, hamper the practical use of Mg and its alloys in engineering components. Tailoring the mechanical properties of Mg alloys, as demanded by both academics and industries, requires a comprehensive understanding of the intrinsic dislocation mechanisms that accommodate the macroscopic plastic flow. This thesis aims to provide a new scientific understanding of the dislocation-driven plasticity mechanisms of Mg alloys, achieved predominantly by analyzing their static and dynamic tensile deformation responses at various strain rates via the use of dislocation theory. This hybrid methodology—combining systematic mechanical testing, multiple-length-scale microstructural characterizations, and comprehensive dislocation modelling and calculations—help to effectively reveal some abnormal strain rate sensitivities and associated intriguing dislocation behaviors under the impact of chemical compositions, thereby offering new insights into the dislocation mechanisms of Mg alloys. Moreover, these scientific findings can usefully guide designs of Mg with improved ductility. In the first part, fundamental works have been focused on the plasticity mechanisms of different Mg alloys. For the dilute Mg alloy, the dislocation-mediated plasticity mechanisms were investigated, revealing a rate-dependent transition of dislocation activities from easy-glide <a> dislocations at low strain rates to glissile <c + a> dislocations at high strain rates. Meanwhile, it was found that such <c + a> dislocations, generated at high strain rates, do not necessarily lead to enhanced ductility, contrary to the common belief. For the Mg alloy incorporating beneficial rare-earth free additives, disparities in strain rate sensitivity regarding both yielding and postyield strain hardening behaviours were studied between the solid-solution and the aged treatment Mg alloy. The distinct roles of solid solutes and precipitates on the rate-controlling plasticity mechanisms and on the strain rate sensitivity were revealed, showing the fundamental effects of different heat treatments on both the thermally activated dislocation behaviors at yielding and on the dislocation multiplication. In the second part, a simple, inexpensive, and robust strategy for developing ductile Mg bulk pieces was proposed using the idea of dislocation engineering. It was found that instead of traditionally adding complex alloying elements, stable <c + a> dislocation sources can be induced by applying a directional warm deformation process, which not only provides the activation energies necessary for <c + a> dislocations but also prevents their recovery. Based on experiments, the transition path for the induced <c + a> dislocation loops was revealed, with cross-slip from the basal plane onto the pyramidal plane. Besides, simultaneous enhancement of strength and ductility has been achieved in a Mg alloy through engineering the dislocation. Additional <c + a> dislocations were found in the pre-warm deformed Mg alloy in place of twinning, accommodating the plastic incompatibility in the Mg alloy.