Abstract
The extremely elevated strength of nanoceramics under compression arises from the necessity to nucleate highly energetic dislocations from the surface, in samples that are too small to contain pre-existing defects. Here, we investigate the site dependence of surface dislocation nucleation in MgO nanocubes using a combination of molecular dynamics simulations, nudged-elastic-band method calculations and rate theory predictions. Using an original simulation setup, we obtain a complete mapping of the potential dislocation nucleation sites on the surface of the nanoparticle and find that, already at intermediate temperature, not only nanoparticle corners are favorable nucleation sites, but also the edges and even regions on the side surfaces, while other locations are intrinsically unfavorable. Results are discussed in the context of recent in situ TEM experiments, sheding new lights on the deformation mechanisms happening during ceramic nanopowder compaction and sintering processes.
Highlights
It has long been recognized that materials are stronger when smaller[1,2]
Reducing the size of single-crystal pillars or wires induces strengthening due to two size-dependent contributions: dislocation line tension and dislocation nucleation, both originally observed in metals using transmission or scanning electron microscopy (TEM and SEM), molecular dynamics (MD) and dislocation dynamics (DD) simulations[8,9,10,11,12,13]
We focus on dislocation nucleation at the surface of magnesium oxide (MgO) nanoparticles
Summary
It has long been recognized that materials are stronger when smaller[1,2]. The reason is a size-dependent competition between deformation and fracture that has attracted an intense scientific attention over the past 20 years. In the case of sharp nano-objects, the dislocation nucleation process often initiates from surfaces as commonly observed in atomic-scale simulations When coupled to the transition state theory (TST), this approach predicts the yield strength of nano-objects on a wide range of temperatures and strain rates that fill the usual gap between classical MD and experimental conditions of deformation[13,24,25,26] These studies are generally restricted to metal nanowires in which only one or a few pre-selected nucleation sites are considered. The MgO nanocubes yielded in the GPa range by the nucleation from the surface of perfect dislocations in the slip. B Surface dislocation nucleation in a MgO nanocube observed during in situ TEM nanocompression test at room temperature
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