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Board Members

Nai-Chang Yeh
Co-Director

Keith Schwab
Co-Director

Harry A. Atwater, Jr.
Julia R. Greer
James R. Heath
Nathan S. Lewis
Oskar J. Painter
Michael L. Roukes
Axel Scherer
Kerry J. Vahala

 

image Julia R. Greer
Professor of Materials Science and Mechanics

B.S., Massachusetts Institute of Technology, 1997; M.S., Stanford University, 2000; Ph.D., Stanford University, 2005.

phone: 626.395.4127
mail code: 309-81

Research Group |

A key focus in Professor J.R.Greer's research group is the development of innovative experimental approaches to assess strengths of specimens whose dimensions have been reduced to nanoscale not only vertically but also laterally. We have developed unique fabrication techniques involving the use of Focussed Ion Beam (FIB) to "carve out" single crystal nanopillars ranging in diameter from 100 nm to several microns. Their strengths in uniaxial compression are subsequently measured in the Nanoindenter with a flat punch tip to remove the strain gradient effect from the observed mechanical response. These small pillars were found to reach 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 can be made stronger by introducing defects into them, i.e. by work-hardening (also known as strain-hardening). This concept has been fully utilized in the manufacturing of steels, super-strong alloys, and other building materials. 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. Julia’s work demonstrated for the first time that contrary to the 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. In this mechanism, the mobile dislocations have a higher probability of annihilating at a nearby free surface than being pinned by other dislocations. When the starvation conditions are met, plasticity is accommodated by the nucleation of new dislocations rather than by motion and interactions of existing dislocations, as is the case for bulk crystals.

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