A possible mechanism for shock-induced failure in Al involved atomic vacancies diffusing through the crystal lattice and agglomerating to form voids, which continued to grow and ultimately resulted in ductile fracture. Use was made of orbital-free density functional theory, a linear-scaling first-principles quantum mechanics method, to study vacancy formation, diffusion and aggregation in Al under shock-loading conditions of compression and tension. Vacancy formation and migration energies were calculated and it was found that, while nearest-neighbour vacancy pairs were unstable, next-nearest neighbour vacancy pairs were stable. As the number of nearby vacancies increased, it was predicted that vacancy clusters preferentially grew via next-nearest neighbour vacancies. The energetics were found to be greatly affected by expansion and compression; leading to insights as to how vacancies behaved under shock conditions.

Energetics and Kinetics of Vacancy Diffusion and Aggregation in Shocked Aluminium via Orbital-Free Density Functional Theory. G.Ho, M.T.Ong, K.J.Caspersen, E.A.Carter: Physical Chemistry Chemical Physics, 2007, 9[36], 4951-66