| SIZE MATTERS: Nano-scale Mechanical Properties of Crystals |
| Julia R. Greer, Caltech 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. These advances in nanotechnology have a tremendous impact on parts of the economy as diverse as information, energy, health, agriculture, security, and transportation. Some of the examples include data storage at densities greater than one terabit per square inch, high-efficiency solid-state engines, single-cell diagnostics of complex diseases (e.g. cancer), and the development of ultra-light yet super-strong materials for vehicles, with the component sizes comprising these technological devices reduced to the sub-micron scale. 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, for example, have been found to deviate from classical mechanics laws and therefore can no longer be inferred from the bulk response or from the literature. Unfortunately, the few existing experimental techniques for assessing mechanical properties at that scale are insufficient, not easily accessible, and are generally limited to thin films. In order to design reliable devices, a fundamental understanding of mechanical properties as a function of feature size is desperately needed; with the key remaining question whether materials really are stronger when the instrumental artifacts are removed, and if so then why and how. A key focus in Professor J.R.Greer's research 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. One such approach involves the fabrication of single crystal nanopillars ranging in diameter from 100 nm to several microns by using Focused Ion beam (FIB), as shown in Figure 1. Their strengths in uniaxial compression are subsequently measured by modifying a standard nanoindentation setup 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, as shown in Figure 2. 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. |