A. R. Beaber

Deconfinement leads to changes in the nanoscale plasticity of silicon

Silicon crystals have an important role in the electronics industry, and silicon nanoparticles have applications in areas such as nanoelectromechanical systems, photonics and biotechnology. However, the elastic-plastic transition observed in silicon is not fully understood; in particular, it is not known if the plasticity of silicon is determined by dislocations or by transformations between phases. Here, based on compression experiments and molecular dynamics simulations, we show that the mechanical properties of bulk silicon and silicon nanoparticles are significantly different. We find that bulk silicon exists in a state of relative constraint, with its plasticity dominated by phase transformations, whereas silicon nanoparticles are less constrained and display dislocation-driven plasticity. This transition, which we call deconfinement, can also explain the absence of phase transformations in deformed silicon nanowedges. Furthermore, the phenomenon is in agreement with effects observed in shape-memory alloy nanopillars, and provides insight into the origin of incipient plasticity.

Smaller is tougher

“Smaller is stronger” is now a tenet generally consistent with the predominance of evidence. An equally accepted tenet is that fracture toughness almost always decreases with increasing yield strength. Can “smaller is tougher” then be consistent with these two tenets? It is taught in undergraduate engineering courses that one design parameter that allows for both increased strength and fracture toughness is reduced grain size. The present study on the very brittle semiconductor silicon proves this exception to the rule and demonstrates that smaller can be both stronger and tougher. Three nanostructures are considered theoretically and experimentally: thin films, nanospheres, and nanopillars. Using a simple work per unit fracture area approach, it is shown at small scale that toughness is inversely proportional to the square root of size. This is supported by experimental evidence from in situ electron microscopy nanoindentation at length scales of less than a micron. It is further suggested that dislocation shielding can explain both strength and toughness increases at the small scales.

Alloys: Strength from modelling

A new multiscale computational method that is capable of predicting solute strengthening of alloys without adjustable parameters may lead to the development of new engineering materials.

Connectivity between plasticity and brittle fracture: An overview from nanoindentation studies

Nanoindentation and scanning probe microscopy techniques applied to deforming thin films, nanospheres, nanowires, and nanocrystalline structures have uncovered new mechanical property phenomena dependent on size scale. This overview addresses several segments — those associated with measurement by nanoindentation, as well as the resulting properties of elasticity and plasticity, and fracture toughness of semi-brittle crystals. Specific to volumes in indentation or compression, it is shown that both pressure and scale effects can become dominant for both elasticity and plasticity at sizes less than 100nm. A strong inverse relationship between yield strength and activation volume for both single crystal and nanocrystalline structures is reviewed. Equally strong is a relationship between fracture toughness, the number of shielding dislocations accommodating indentation prior to fracture, and the basic mechanical and physical properties of semi-brittle solids. The link between fracture toughness and plasticity in these semi-brittle materials is shown to be the activation volume for dislocation nucleation in these strong solids.