Speaker: George M. Pharr ( Member of National Academy of Engineering, Chancellor's Professor, McKamey Professor of Engineering,and UT-ORNL Joint Faculty)

Title: Stochastic Processes and the Strength of Metals Containing a Limited Number of Dislocations

Time: 9:00–11:00am, Sep.12, 2014 (Friday)

Venue: Room 205, School of Materials Science and Engineering (MSE)

 [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.

[Introduction]

George M. Pharr received his BS in Mechanical Engineering at Rice University in 1975 and Ph.D. in Materials Science and Engineering from Stanford in 1979. After one year of postdoctoral study at the Engineering Department of the University of Cambridge, England, he returned to Rice in 1980 as a faculty member in the Department of Mechanical Engineering and Materials Science. During a sabbatical leave in 1987-88, he worked with the Ceramic Sciences Group at the IBM Thomas J. Watson Research Center in Yorktown Heights, NY. He moved to his current joint position at UT/ORNL in 1998 and was head of the UT Materials Science and Engineering Department during the period 2006-11.

He received ASM International’s Bradley Stoughton Award for Young Teachers of Metallurgy in 1985 and was elected a Fellow of ASM International in 1995. His honors also include the Amoco Award for Superior Teaching at Rice University in 1995 and the Innovation in Materials Characterization Award of the Materials Research Society in 2010. In 2012, He was elected a Fellow of Materials Research Society. In the spring of 2014 he was elected Member of National Academy of Engineering.

He has been an Associate Editor of the Journal of the American Ceramic Society since 1990 and served as Volume Organizer for the MRS Bulletin in 2000. He has chaired or co-chaired many large professional society meetings including the 1995 Spring Meeting of the Materials Research Society in San Francisco and the 2000 Gordon Research Conference on Thin Film Mechanical Behavior, which he co-founded in 1998. He is an author or co-author of more than 220 scientific publications, including 4 book chapters, and a Thomson ISI "Highly Cited Researcher in Materials Science". His research focuses on nanoindentation and small-scale mechanical behavior of materials.

About Prof.George:  http://web.utk.edu/~mse/faculty/pharr/


 Professor George M. Pharr