  In this work, we explore the role of atomistic-scale energetics on
liquid-metal embrittlement of Al due to Ga. Ab initio and molecular mechanics
were employed to probe the binding energies of vacancies and segregation
energies of Ga for <100>, <110> and <111> STGBs in Al. We found that the GB
local arrangements and resulting structural units have a significant influence
on the magnitude of vacancy binding energies. For example, the mean vacancy
binding energy for <100>, <110>, and <111> STGBs at 1st layer was found to be
-0.63 eV, -0.26 eV, and -0.60 eV. However, some GBs exhibited vacancy binding
energies closer to bulk values, indicating interfaces with zero sink strength,
i.e., these GBs may not provide effective pathways for vacancy diffusion. The
results from the present work showed that the GB structure and the associated
free volume also play significant roles in Ga segregation and the subsequent
embrittlement of Al. The Ga mean segregation energy for <100>, <110> and <111>
STGBs at 1st layer was found to be -0.23 eV, -0.12 eV and -0.24 eV,
respectively, suggesting a stronger correlation between the GB structural unit,
its free volume, and segregation behavior. Furthermore, as the GB free volume
increased, the difference in segregation energies between the 1st layer and the
0th layer increased. Thus, the GB character and free volume provide an
important key to understanding the degree of anisotropy in various systems. The
overall characteristic Ga absorption length scale was found to be about ~10, 8,
and 12 layers for <100>, <110>, and <111> STGBs, respectively. Also, a few GBs
of different tilt axes with relatively high segregation energies (between 0 and
-0.1 eV) at the boundary were also found. This finding provides a new atomistic
perspective to the GB engineering of materials with smart GB networks to
mitigate or control LME and more general embrittlement phenomena in alloys.

Materials Science