[SY-C5] Nucleation of dislocation in ultra-hard ceramic nanoparticles modelled by molecular dynamics and nudged elastic band simulations
Micro- and nano-sized structures have attracted substantial interest due to their special mechanical behavior: they generally show an increased yield strength compared with the bulk material as well as an improved ductility. Among them, nanoparticles (NPs) which are generally used for their shape-dependent functional properties also appear as perfect candidates for submicronic plasticity investigations due to their broad range of sizes and their variety of shapes. While metallic and silicon NPs have been widely investigated since almost two decades, less is known about the strength of ceramic NPs maybe due to the brittleness of their bulk counterparts.
In the light of recent but preliminary in situ TEM observations, we investigate here the mechanical behavior of <100>-oriented MgO nanocubes. First, incipient plasticity mechanisms are investigated using molecular dynamics simulations (MD) of virtual nanocompression tests at constant strain rate. Results show that the plastic deformation of MgO nanocubes starts with the nucleation from the surface of perfect ½<110>{110} dislocations under ultra-high stresses, one order of magnitude larger than what is generally observed in fcc metals. However, as MD simulations can only be carried out at strain rates several orders of magnitude larger than experimental ones,we used the nudged-elastic-band method to calculate the activation energy for the nucleation of the dislocation as a function of stress, particle size and dislocation nucleation site. With the help of the transition state theory, the nucleation stress can then be estimated as a function of temperature and experimental strain rate. We find that due to the large stiffness of the activation energy vs.stress curve, the strain rate has a much smaller influence on the nucleation stress than what is found for metallic nanocrystals and therefore, that the nucleation stress estimated using MD is close to that corresponding to experimental strain rate.
In the light of recent but preliminary in situ TEM observations, we investigate here the mechanical behavior of <100>-oriented MgO nanocubes. First, incipient plasticity mechanisms are investigated using molecular dynamics simulations (MD) of virtual nanocompression tests at constant strain rate. Results show that the plastic deformation of MgO nanocubes starts with the nucleation from the surface of perfect ½<110>{110} dislocations under ultra-high stresses, one order of magnitude larger than what is generally observed in fcc metals. However, as MD simulations can only be carried out at strain rates several orders of magnitude larger than experimental ones,we used the nudged-elastic-band method to calculate the activation energy for the nucleation of the dislocation as a function of stress, particle size and dislocation nucleation site. With the help of the transition state theory, the nucleation stress can then be estimated as a function of temperature and experimental strain rate. We find that due to the large stiffness of the activation energy vs.stress curve, the strain rate has a much smaller influence on the nucleation stress than what is found for metallic nanocrystals and therefore, that the nucleation stress estimated using MD is close to that corresponding to experimental strain rate.