Abstract
Since its development in the mid-1980's, nanoindentation and other related mechanical testing techniques have led to the discovery of a variety of unique small-scale deformation phenomena in materials. Among these are the indentation size effect, in which hardness at small indenter penetration depths increases; indentation pop-in, in which sudden displacement excursions are caused by homogenous nucleation of dislocations at stresses approaching the theoretical strength; and micro-pillar compression testing, in which the nanoindenter is used as a small-scale compression testing apparatus to explore deformation phenomena in samples small enough to probe single dislocation events. One common theme in these observations is that "smaller is stronger", and a great deal of research over the past decade has focused on the physical origin of these effects. In this presentation, a simple stochastic model based on the behavior of specimens containing a small number of dislocations is presented that explains many of the observations. The model is tested by comparing its predictions to recent experimental observations of the yield strengths of sub-micron diameter Mo alloy fibers tested in tension and compression as well as nanoindentation pop-in data for pure single crystals. The model correctly predicts not only the size dependence of strength, but also how the scatter in the strength should vary with specimen size.
About the speaker
Prof George M. Pharr received his PhD in Materials Science and Engineering from Stanford University in 1979. After one year of postdoctoral study at the University of Cambridge, he moved to Rice University and became faculty member of Mechanical Engineering and Materials Science. He joined the University of Tennessee at Knoxville (UT) in 1998, and is currently Chancellor's Professor and McKamey Professor of Engineering. He also holds a joint faculty position in the Materials Science and Technology Division at the Oak Ridge National Laboratory (ORNL), and is the Director of the UT/ORNL Joint Institute for Advanced Materials.
Prof Pharr’s research focuses on mechanisms of plasticity and fracture in solids, especially at small scales. His other research areas include nanoindentation and nanomechanical testing, thin film and small-scale mechanical behavior and finite element modeling of indentation contact. He is an associate editor of the Journal of the American Ceramic Society and principal editor of the Journal of Materials Research. He has authored or co-authored more than 200 scientific publications, including 4 book chapters.
Prof Pharr received numerous awards including the ASM International’s Bradley Stoughton Award, the Humboldt Senior Scientist Award, and the Materials Research Society's inaugural Innovation in Materials Characterization Award, etc. He is a Member of the US National Academy of Engineering, and a Fellow of ASM International and the Materials Research Society.
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