Self-organized Sr leads to solid state twinning in nano-scaled eutectic Si phase

A new mechanism for twin nucleation in the eutectic Al-Si alloy with trace Sr impurities is proposed.Observations made by sub-angstrom resolution scanning transmission electron microscopy andX-ray probing proved the presence of <110> Sr columns located preferentially at twin boundaries.Density functional theory simulations indicate that Sr atoms bind in the Si lattice only along the<110> direction, with preferential positions at first and second nearest neighbors for interstitialand substitutional Sr, respectively. Density functional theory total energy calculations confirm thattwin nucleation at Sr columns is energetically favorable. Hence, twins may nucleate in Si precipitatesafter solidification, which provides a different perspective to the currently accepted mechanism whichsuggests twin formation during precipitate growth.