Prestraining and Annealing of Gold Micropillars: Strengthening and Weakening Turned Upside Down

Friday, January 29, 2010 - 11:00 p.m. to Saturday, January 30, 2010 - 11:55 p.m.
ChEMS Seminar

Featuring William D. Nix, Ph.D.
Professor, Department of Materials Science and Engineering
Stanford University

Location
:  PCB 1200
Free and open to the public

Abstract:
When soft metals are plastically deformed, they get stronger, mainly because the dislocation density increases dramatically. Correspondingly, annealing of strain-hardened metals commonly leads to softening in part because the annealing causes the dislocation density to decrease. Recent experiments with gold micropillars have shown that metals behave in a very different way at the sub-micrometer scale.  At that scale plastic deformation leads to softening and annealing leads to hardening, just the opposite of what occurs in bulk metals.  These results suggest that plasticity at the sub-micrometer and nanometer scale is controlled not by the elastic interactions of dislocations, as in bulk metals, but by the operation of dislocation sources.  In addition, compressive deformation experiments on dislocation-free gold microcrystals reveal ideal strengths and whisker-like behavior, also indicating source-controlled plasticity.  Recent dislocation dynamics modeling of these effects is reviewed in an effort to determine the nature of dislocation sources in micropillars.  The modeling shows that dislocations readily escape from the micropillars during plastic flow, leading to a nearly dislocation-free state in which plastic flow might be controlled by the nucleation of dislocations, most likely at the free surfaces.  The nature of dislocation sources in micropillars is discussed in light of these simulations.  The unusual effects of prestraining/annealing in microcrystals can be described using a classical dislocation dynamics model wherein dislocations as carriers of plasticity are emphasized relative to their elastic interactions. The model is inspired by Johnston and Gilman’s dislocation dynamics approach to plasticity and celebrates the 50th anniversary of that classic work.

About the Speaker:
William D. Nix, Ph.D., received his Ph.D. degree in 1963 from Stanford University.  He has been engaged in the study of mechanical properties of materials for more than 40 years. His early work was on high temperature creep and fracture of metals, focusing on techniques for measuring internal back stresses in deforming metals and featuring the modeling of diffusional deformation and cavity growth processes. His students and he also studied high temperature dispersion strengthening mechanisms and described the effects of threshold stresses on these creep processes. Since the mid-1980's they have focused most of their attention on the mechanical properties of thin film materials used in microprocessors and related devices. They have developed many of the techniques that are now used to study of thin film mechanical properties, including nanoindentation, substrate curvature methods, bulge testing methods and the mechanical testing of micromachined (MEMS) structures. Their current work deals with the mechanical properties of nanostructures and with strain gradients and size effects on the mechanical properties of crystalline materials.