Effect of thermal processing on structure and properties of a magnesium-lithium alloy

Ultra-lightweight alloys with high strength, ductility and corrosion resistance are desirable for applications in the automotive, aerospace, biomedical, sporting and electronic goods sectors. Of these materials, magnesium (Mg) is the lightest (~ 1.74 g/cm3) of the structural metals, significantly lighter than steel, titanium and aluminium alloys. A new class of Mg alloys containing lithium (Li) were first developed by the United States National Aeronautics and Space Administration (NASA) in the 1960s with ultra-low densities (1.33-1.65 g/cm3) that are arguably the most lightweight of the engineering alloys. These alloys can also contain single- and multiple-phase microstructures depending on alloying additions and processing. Binary MgLi alloys have a Li content dependent microstructure: Type I - hexagonal close-packed (HCP) Mg structure (termed α phase) (0 to 5 wt.% Li); Type II - mixed α+β structure where β phase is body-centered cubic (BCC) (5 to 10.3 wt.% Li); and Type III - fully BCC β phase structure (> 10.3 wt.% Li). A new type of MgLi alloy was developed recently that contains (wt.%) Mg-10.95Li-3.29Al-0.59Y. In its near-equilibrium state (hot extruded and air cooled), this alloy has the α+β structure (Type II) and a small volume fraction of intermetallic compounds, such as Al2Y. In this state, the alloy is soft and ductile, but highly susceptible to corrosion. However, this alloy was also capable of generating a unique property profile after further thermal and mechanical processing, i.e. high strength, appreciable ductility and good corrosion resistance (Xu et al., Nature Materials, vol. 14, p. 1229, 2015). Such a property profile was argued to be a result of the formation of both an unusual Li-rich solute nanostructure within the single-phase BCC Mg-rich β matrix and a uniform lithium-carbonate film on the alloy surface. While this MgLi alloy demonstrated an exceptional structural and mechanical (+ corrosion) response to heat treatment, many processing parameters are still not well understood ...