Direct Evidence of Nanometric Invasionlike Grain Boundary Penetration in theAl/GaSystem

We report the first in situ results of deformation during grain boundary penetration in the $\mathrm{Al}/\mathrm{Ga}$ system, obtained with a novel, nondestructive hard x-ray synchrotron projection microscopy technique. Focusing the beam to a state-of-the-art spot size of $90\times 90{\mathrm{nm}}^2$, we demonstrate that penetration is accompanied by continuous relative separation of the Al grains of the same final amplitude as the final Ga layer thickness in the absence of external stress. The formation of nanometric intergranular liquid layers is originated by a crack propagation process and inherently implies the presence of weak stress levels.