Microstructure and size effects on metal plasticity at limited length scale
(NSF CAREER award)

The advent of nanotechnology has given rise to new fundamental questions on the strength and failure mechanisms of metals in nanowire (NW) form. However, meaningful answers can only be obtained if the influences of microstructure and sample size on the plasticity of NWs are fully characterized. To this regard, the specific aims of this project are:

•  to use atomistic simulation to characterize how size, surface morphology and grain boundaries influence dislocation emission and plasticity in FCC metal NWs.

•  to fundamentally understand the plasticity of crystalline NWs subjected to nanoindentation, as a simple means to provide direct insights into size effects from both simulation and experimental approaches.

•  to characterize the synergistic effects of twinning and surface faceting on mechanical properties of realistic metallic nanowires made from self-assembly of nanoparticles or electrochemical method.

Size-dependent yielding behavior and plastic flow. Using large-scale molecular dynamics simulations, we have elucidated the rate-controlling mechanism of yielding in twinned gold NWs under uniform deformation, and proposed a predictive model for the size-dependence of plastic flow of these materials based on site-specific surface dislocation emission and dislocation image effects in the vicinity of coherent twin boundaries. As a result, it was revealed that the addition of nanoscale twins to crystalline NWs can act to either increase or decrease their resistance to slip in tension, depending on both sample diameter and number of twins per unit length. We have also shown that a sharp transition from quasi-brittle behavior to significant strain-hardening and ultrahigh flow stresses exists in periodically twinned gold NWs when balancing NW diameter and twin boundary spacing at the nanoscale.

Surface morphology effects. Experimentally observed metallic NWs usually yield far before the ideal strength, because different types of surface defects act to decrease the stress required for dislocation nucleation and irreversible deformation. However, we have predicted near-ideal strength and low strain rate sensitivities in pure Au NWs with complex surface morphologies and {111} surface facets, similar to realistic NWs. Surface faceting in Au NWs gives rise to a novel yielding mechanism associated with the nucleation and propagation of full dislocations along {001}<110> slip systems, instead of the common {111}<112> partial slip observed in face-centered cubic metals. It was thus found that special defects can be utilized to approach the ideal strength of gold in NWs by microstructural design.

Stacking fault energy effects. We have also examined the differences in plastic behavior of periodically-twinned metal NWs in Au, Ag, Al, Cu, Pb and Ni. The simulations revealed a fundamental transition of plasticity in twinned metal NWs from sharp yielding and strain-softening to significant strain-hardening as the stacking fault energy of the metal decreases. This effect was shown to result from the relative change, as a function of the unstable stacking fault energy, between the stress required to nucleate new dislocations from the free surface and that for the resistance of twin boundaries to the glide of partial dislocations.

Size effects on NW hardness. Another major finding from our simulations is also that the hardness of single-crystalline Ni NWs decreases as the sample diameter decreases; causing important softening effects in smaller NWs durin g spherical indentation. The interactions of prismatic loops and dislocations, which are emitted beneath the contact ing tip, with free sample boundaries we re shown to be the main factor for the size dependence of hardness in Ni NWs during indentation.

 

 
© 2005-present, F. Sansoz - The University of Vermont - Burlington, VT, 05405