Deformation behavior in lightweight alloys: effects of rare-earth microalloying and carbon nanotube reinforcement

One of the most important strategies for improving fuel efficiency and reducing anthropogenic emissions is vehicle lightweighting by the use of lightweight materials such as Mg and Al alloys in the automotive industry. The structural application of these alloys inevitably requires the mechanical properties and their continuous performance improvement to meet the increasingly stringent safety and durability requirements. An effective method to enhance the deformation resistance is to alloy with rare-earth (RE) elements for Mg alloys or develop composites with the addition of reinforcement for Al alloys. The objective of this dissertation was to identify the effects of RE element and carbon nanotube (CNT) reinforcement on the deformation behavior, focusing mainly on the deformation mechanisms. The deformation behavior of a RE-free extruded AZ31 Mg alloy was first studied. It was observed that the propagation of distinct twin variants led to the confinement of the spaces constrained by the fine twin lamellas. Various double twinning structures acknowledged through atomistic simulations were experimentally observed via progressive electron backscatter diffraction (EBSD) analyses during stepwise compression. The vanishing of primary {1121} embryonic twins via the nucleation and growth of either single or multiple {1012} secondary extension twins was detected, and two new ladder-like and branching-like twin-twin interaction phenomena were observed. Then a low-RE containing Mg alloy was exploited via texture and cyclic deformation studies. The addition of 0.2 wt.% Nd in ZEK100-O Mg alloy led to a weaker basal texture in comparison with AZ31 Mg alloy. Fatigue life of ZEK100 alloy was longer than that of AZ31 alloy, due to a good combination of strength with ductility. Asymmetry of hysteresis loops was improved because of texture weakening and grain refinement, however anelastic behavior largely remained arising from the presence of twinning and detwinning. The last investigation involved deformation behavior of CNT reinforced Al composites where the addition of 2.0 wt.% CNT in a 2024Al alloy led to considerable grain refinement. Deformation resistance of the composite was effectively enhanced due to CNT load transfer, Hall-Petch strengthening, thermal mismatch and Orowan looping. In a nutshell, this work constitutes a valuable benchmark for understanding the factors affecting the performance of two lightweight alloys in the automotive and aerospace applications.