Surface and Interface Energies of Complex Crystal Structure Aluminum Magnesium Alloys

Alloying Al with Mg can improve its structural properties but also can lead to the formation of grain-boundary precipitates of β-Mg₂Al₃ that lead to failure by intergranular fracture and corrosion. Simulating the properties of the β phase is difficult because it has a complex structure with more than 1000 atoms per unit cell. We approximate the experimental β structure by the β′ structure, which has about 300 atoms per unit cell, and we compute the fracture behavior of the material from density functional theory calculations of relevant surface and interface energies. We report also on experimental measurements of the orientation and fracture properties of the α-Al(Mg)–β-Mg₂Al₃ interface and compare them with the atomistic simulations. We have computed the surface energy of face-centered cubic α-Al with up to 10 at. pct Mg, as well as the decohesion energy of β′-Mg₂Al₃ and the interfacial decohesion energy between β′-Mg₂Al₃ and pure α-Al with geometry similar to that observed experimentally. We find that the β′-Mg₂Al₃ decohesion energy is nearly isotropic and is lower than the pure Al surface energy and the α-Al–β′-Mg₂Al₃ interface decohesion energy. This result is consistent with the experimental observations of fracture within the β phase rather than at the α-Al(Mg)–β-Mg₂Al₃ interface or within the α-Al(Mg) phase.