Abstract.Magnesium has multiple dislocation and twinning systems with starkly different properties, which make its plastic deformation strongly anisotropic and highly complex. Existing empirical interatomic potentials fail to capture the full scope of these properties, making current molecular statics and dynamics simulation results of limited quantitative and predictive use. Here, based on the work by Kim et al, a new modified embedded-atom method potential for magnesium is introduced and rigorously validated against existing ab initio, continuum theory and experimental results. The new potential satisfactorily reproduces all the necessary mechanical properties for plastic deformation, including the various generalized stacking fault energy surfaces, dislocations core structures, Peierls stresses, surface energies, and basal plane cohesive strength. The capability of this potential to accurately describe all the important slip systems and fracture behavior makes it valuable for future realistic atomistic studies of general magnesium deformation and failure problems.

We demonstrate experimentally that a part-per-million addition of Sn solutes in Al-Mg-Si alloys can inhibit natural aging and enhance artificial aging. The mechanism controlling the aging is argued to be vacancy diffusion, with solutes trapping vacancies at low temperature and releasing them at elevated temperature, which is supported by a thermodynamic model and first-principles computations of Sn-vacancy binding. This "diffusion on demand" solves the long-standing problem of detrimental natural aging in Al-Mg-Si alloys, which is of great scientific and industrial importance. Moreover, the mechanism of controlled buffering and release of excess vacancies is generally applicable to modulate diffusion in other metallic systems.

The ion mass spectrometer on Cassini detected enhanced ion flux near Saturn's main rings that is consistent with the presence of atomic and molecular oxygen ions in the thermal plasma. The ring “atmosphere” and “ionosphere” are likely produced by UV photosputtering of the icy rings and subsequent photoionization of O2. The identification of the O+ and O2+ ions is made using time‐of‐flight analysis and densities and temperatures are derived from the ion counting data. The ion temperatures over the main rings are a minimum near synchronous orbit and increase with radial distance from Saturn as expected from ion pick up in Saturn's magnetic field. The O2+ temperatures provide an estimate of the neutral O2 temperature over the main rings. The ion mass spectrometer also detected significant O2+ outside of the main rings, near the F ring. It is concluded that between the F and G rings, the heavy ion population most likely consists of an admixture of O2+ and water group ions O+, OH+, and H2O+.