Dislocation Starvation and Nanoscale Deformation Behavior of Crystals

ChEMS Seminar

Featuring Julia R.Greer, Ph.D.
Assistant Professor, Materials Science
California Institute of Technology
Host:  Professor Martha Mecartney               

Location: ICS 174
Refreshments to follow

While “super-sizing” seems to be the driving force of our food industry, the direction of materials research has been quite the opposite: the dimensions of most technological devices are getting ever smaller.  The functionality of these devices directly depends on their structural integrity and mechanical stability, driving the necessity to understand and to predict mechanical properties of materials at reduced dimensions. Yield and fracture strengths at the nanoscale, however, deviate from classical mechanics laws and therefore can no longer be inferred from the bulk response or from the literature.  The few existing experimental techniques for assessing mechanical properties at the nanoscale are insufficient, not easily accessible, and generally limited to thin films.  Greer has developed innovative experimental approaches to assess strengths of nanoscale specimens.  She found that single crystal gold nanopillars ranging in diameter from 100 nm to several microns (made by Focused Ion beam (FIB)) reached strengths of 800 MPa, a value ~50 times higher than that of bulk gold.  To fully appreciate the significance of this finding, one should recognize that it has been known for nearly a century that crystalline materials were typically made stronger by introducing defects into them, i.e. by work-hardening (also known as strain-hardening).  These defects are called dislocations, and work-hardening is a result of their interactions with each other as they multiply and require application of higher stresses to accommodate further deformation.  Greer’s work demonstrates for the first time that, contrary to conventional strain-hardening, plastic deformation in single crystals at nanometer scale might occur via Hardening by Dislocation Starvation, a fundamentally opposite strengthening mechanism based on elimination rather than multiplication of defects during plastic deformation. 

About the Speaker:
Julia R. Greer received her B.S. degree in chemical engineering with a minor in advanced music performance from Massachusetts Institute of Technology in 1997.   After graduation from MIT, Julia worked as an intern at Intel while pursuing an M.S. in materials science at StanfordUniversity, where she built novel X-ray diffraction equipment in order to determine stress states in Cu and Al interconnect lines on Si substrates.  Subsequently, Greer received a Ph.D. in materials science and engineering from Stanford University (2005), studying size effects in plasticity of metals at the nano-scale with Professor William D. Nix.   Prior to starting her appointment as an assistant professor of materials science and engineering at California Institute of Technology in June of 2007, Greer was a post-doctoral fellow at Palo Alto Research Center, investigating flexible electronics.  Greer is also a concert pianist, with a recent performance of Brahms Concerto No. 2 with the Redwood Symphony.