Microstructural evolution and mechanical properties in age-softening resistant Mg-Li-Al-Y alloys

Abstract

Of the engineering alloys, magnesium-lithium (Mg-Li) alloys are the lightest structural metallic materials (density, ρ=1.3-1.6 g/cm3) that have attracted substantial scientific research due to their relatively high specific strength and good formability. However, their low strength and poor thermal stability and corrosion resistance largely limit their industrial applications, which can be addressed by microalloying and designing proper heat treatments. The aims of this thesis are to investigate the effects of alloy composition, solid solution temperature and ageing treatments on the evolution of the microstructure and properties (hardness and corrosion) of a series of thermomechanically processed body centred cubic (BCC) Mg-Li-Al-Y alloys (designated A1 to A7).

It was found that the hardness of as-quenched A2 alloy increased with the increasing solution treatment temperature, with the highest hardness ~157 HV achieved at 380-400°C, this represents a hardness increase of more than 100% over its as-cast state. This substantial strengthening is closely associated with the formation of a uniform distribution of Al-rich nanostructures in the BCC matrix. Moreover, the as-quenched A2 and A7 alloys optimized via hot forging and rolling showed excellent softening resistance during ageing, with a hardness above ~ 133 HV after over 60 days, and still above ~100 HV after over 3 years of natural ageing, or with a hardness above ~110 HV after 30 h of ageing at 70 °C and ~ 95 HV after more than 3 years of natural ageing. The mechanism behind this excellent thermal stability was explained by the formation of uniformly distributed Al-rich nanostructure on water quenching that generated a significant strengthening effect and retarded the nucleation of plate-shaped nanoparticles from the supersaturated matrix, thus reducing age softening. Also, the absence of soft AlLi precipitation back to the matrix and the impeded decomposition of the strengthening MgLi2Al θ phase into the AlLi phase also helped to slow the rate of age softening. Furthermore, the HR-WQ A7 alloy was the most corrosion resistant alloy during the various ageing procedures (1.37–5.84 µA/cm2), which was consistently lower than both high purity Mg (7.56 µA/cm2) and LA113 (6.20 µA/cm2) (Xu et al., Nature Materials, vol. 14, p. 1229, 2015). The corrosion resistance of the alloy decreased during ageing, which was argued to be a result of the precipitation of new phases and coarsening of the nanostructure.