Correlation between Microstructure and Properties of Magnesium-Lithium-Aluminium Alloys

Abstract

Magnesium-lithium (Mg-Li) alloys are ultra-lightweight engineering alloys (density, ρ = 1.3-1.65 g/cm3) that have attracted considerable scientific and commercial interest due to their moderate specific strengths and good formability. Aluminium (Al) is the most effective strengthening element in Mg-Li alloys, such that Mg-Li-Al alloys have been widely investigated, including the recent discovery of a body-centred cubic (BCC) Mg-11Li-3Al alloy (wt.%, LA113) that exhibited high specific strength, good ductility and excellent corrosion resistance. The aims of this thesis are to investigate the effect of composition and microstructure of Mg-Li-Al alloys on their mechanical properties and oxidation behaviour, with specific focus on the benchmark LA113 alloy. The structure/mechanical property investigations include standard thermal and mechanical processing of selected alloys through to studies of localised structural and compositional effects using diffusion couples in conjunction with site-specific microanalysis techniques. In addition to the more standard structural analysis techniques, atom probe tomography, high-energy synchrotron X-ray diffraction and high-resolution transmission electron microscopy were used for investigating these structure/property relationships. The mechanism/s of oxidation of LA113 on exposure to atmospheric air was investigated, with particular focus on the effect of changes in composition and microstructure on oxidation kinetics and film morphology. Specific analysis techniques used include secondary ion mass spectrometry and X-ray photoelectron spectroscopy. The response of Mg-Li-Al alloys to solution treatment and water quenching and subsequent natural ageing were investigated on bulk alloys in conjunction with diffusion couple experiments. Substantial strengthening of LA113 almost immediately after quenching was associated with the formation of a uniform distribution of nano-sized, rod-shaped, semi-coherent D03-Mg3Al precipitates. Subsequent softening on natural ageing was associated with the gradual coarsening and loss in coherency of these Mg3Al precipitates. Based on detailed structural and theoretical investigations, a new methodology was proposed for producing structurally stable, high specific strength Mg-Li-Al base alloys. A Mg-15Li-6Al alloy (wt.%, LA156) was designed that quench strengthened to a hardness of ~ 128 HV and naturally aged to a stable peak hardness of ~ 148 HV. Such a high hardness and exceptionally low density (~ 1.32 g/cm3) equates to an estimated specific strength of over 300 KN∙m/Kg, which makes LA156 one of the highest specific-strength structural alloys developed so far. The surface oxidation of LA113 in the BCC β condition occurred via electrochemical reactions with the assistance of water vapour adsorption onto the alloy surface in atmospheric air; i.e., cathodic reduction of oxidants (water and dissolved oxygen) and anodic oxidation of metals (Li and Mg). Development of the oxide layer was initiated by the growth of MgO and Mg(OH)2. Subsequent formation of Li2O and LiOH was the result of rapid diffusion of Li to the free surface. Li2CO3 predominantly formed an outermost layer through the chemical reaction of LiOH with carbon dioxide in the air. After exposure to air for over 70 h, further oxidation became minimal. Such a complex, multilayered oxidation film prevented the underlying alloy from further oxidation. The diffusion couple experiments highlighted the marked effect of microstructure on the oxidation behaviour of Mg-Li-Al alloys. For HCP α + BCC β two-phase microstructures, oxidation is accelerated due to localized galvanic corrosion via attack of the more electrochemically active β phase. The significant effect of microstructure on oxidation has significant implications for commercially producing corrosion resistant Mg-Li-base alloys.