Flow stresses and activation volumes for highly deformed nanoposts

Recent interest in high-strength nanocrystalline structures has prompted a call for understanding scale effects in deformation mechanisms. One aspect, producing nanostructures by severe plastic deformation, promoted the present examination of nanoposts undergoing large strain compression. On e-beam lithography produced nanoposts of both aluminum and permalloy with radii ranging from 50 to 150 nm, single and repeat compression tests produced measurements of flow stress and apparent activation volume. Flow stresses, continuously measured with a nanoindenter allowed theoretical assessment of the deformation mechanism. It was concluded that the high stresses (1-3 GPa) and small activation volumes (1-10b3) where b is the modulus of the Burgers vector, were consistent with dislocation nucleation. A traditional model is shown to give good first order accountability for these two face-centered cubic metals.