Orbital-free density functional theory characterization of the β′- M g2 A l3 Samson phase

Nearly all intermetallic compounds in the Mg-Al phase diagram have been characterized using Kohn-Sham density functional theory (KSDFT), providing insight into their properties. Two compounds with a common chemical formula of Mg2Al3, known as the β and β′ Samson phases, remain challenging for KSDFT to interrogate due to their large unit cells containing partially occupied atomic sites and many hundreds of atoms. The much less expensive orbital-free DFT (OFDFT) is as accurate as KSDFT for Al-Mg alloys, provided one employs a nonlocal kinetic energy density functional. Here, we use nonlocal OFDFT to evaluate properties of the relatively simpler β′-Mg2Al3 phase with a unit cell of 879 atoms, as a first step toward full DFT characterization of these Samson phases. We employ the virtual crystal approximation to treat the potentials associated with the partially occupied atomic sites in the β′-Mg2Al3 unit cell. OFDFT lattice and elastic constants, with pair distribution functions (PDFs) of β′-Mg2Al3 computed via OFDFT molecular-dynamics simulations, are consistent with available experimental data; the PDF is a prediction, as it has not yet been measured. The predicted OFDFT formation energy of β′-Mg2Al3 (derived from the phonon density of states) increases with temperature, consistent with the experimental observation that this phase becomes increasingly unstable at higher temperatures. Using two common metrics of ductility (Pugh's ratio and brittleness), OFDFT-derived ideal tensile and shear strains/stresses of β′-Mg2Al3 and Mg17Al12 (precipitates present at Al/Mg interfaces) reveal that β′-Mg2Al3 should be more ductile than Mg17Al12. The superior ductility of β′-Mg2Al3 suggests that welding of Al to Mg should be done at temperatures where this alloy remains stable and under Al-rich conditions to favor its formation.