Strengthening of NiAl nanofilms by introducing internal stresses

Molecular dynamics simulations of notched NiAl nanofilms tension are carried out. The stress-strain curves are calculated for a dislocation-free nanofilm and for nanofilms with periodic arrays of prismatic dislocations introduced in order to create tensile internal stress in the inner part of the nanofilms and compressive stress near their surfaces. It is demonstrated that under uniaxial tensile load the nanofilms with the dislocation loops can show higher strength, strain to failure, and energy to failure compared to the dislocation-free nanofilm. Larger strength of the nanofilms with dislocations is naturally explained by the compressive internal stress at the surfaces which detains crack initiation at the notch under tensile loading. Increase in the strain and energy to failure is due to the particular mechanism of elastic strain observed for the nanofilm. There exists a domain of strain where homogeneous deformation of the nanofilm is thermodynamically unstable. As a result, domains with larger elastic strain appear and elastic deformation of the nanofilm occurs at practically constant stress by growth of the domains with larger elastic strain in expense of the domains with smaller elastic strain. We believe that this mechanism of non-homogeneous elastic deformation is due to competing interaction of atoms of different sorts and thus, it cannot be realized in pure metals but can happen in ordered alloys and intermetallic compounds. Our results demonstrate that strengthening by introducing internal stresses, widely used for macroscopic structures, can also be applied for nanomaterials such as nanofilms and nanowires.