Phase transformations and mechanical properties of BCC Mg-Li-Al alloys

Abstract

BCC Mg-Li-Al alloys are a type of ultralight magnesium alloys. In this thesis, the strengthening mechanisms of a series of BCC Mg-Li-Al alloys were investigated using a powerful new cryogenic preparation method for atom probe tomography (APT) in combination with standard and advanced ex situ and in situ structural analysis techniques. To fundamentally support these structural studies, first principles, phase-field and physical-based modelling were carried out. A combination of modelling and experimental techniques generated conclusive morphological, chemical, crystallographic and thermodynamic evidence that the rapid and substantial strengthening in a model ternary alloy designed herein, Mg-14Li-7Al (LA147), was due to rapid spinodal decomposition. A leaner ternary alloy, Mg-11Li-3Al (LA113), was also investigated in detail, where it was shown that quench strengthening is attributed to the precipitation of coherent, nano-sized, rod-shaped Mg3Al (θ) phase which was considered to originate from the prior spinodal. The principal difference between LA147 and LA113 is that the latter generates a larger lattice mismatch between the spinodal (Al-rich zones) and BCC matrix and results in a higher rate of transformation of the Al-rich zones to the θ phase. While the θ phase in BCC Mg-Li-Al alloys is widely regarded to be a coherent but metastable phase that plays a major role in strengthening, such a microstructure has not been studied in any considerable detail. Herein, the structural and crystallographic features of θ phase was characterized. Such results were used as a platform for proposing an alloy design strategy for creating new types of ultra-high-specific strength BCC Mg-Li-Al alloys with improved thermal stability during natural and artificial ageing. The stability the metastable θ phase in the foregoing Mg-Li-Al alloys was found to be very temperature dependent, whereby temperatures above ~100 °C were sufficient to transform θ into a stable AlLi phase to cause softening. This transformation was systematically studied and correlated with changes in mechanical properties. It was found that the generation of a nano-sized HCP α Mg phase between the transforming θ phase and BCC matrix was the main factor causing a loss in strength in these alloys