The Pursuit of the Small: From Grain-Boundary Cavities to Nanocrystalline Metals
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The Pursuit of the
Small: From GrainBoundary Cavities to Nanocrystalline Metals Julia R. Weertman
Abstract The following article is based on the Von Hippel Award presentation given by Julia Weertman of Northwestern University on December 3, 2003, at the Materials Research Society Fall Meeting in Boston. Weertman received the award for “her lifelong exceptional contributions to understanding the basic deformation processes and failure mechanisms in a wide class of materials, from nanocrystalline metals to high-temperature structural alloys, and for her inspiring role as an educator in materials science.” It has been said that “the best things come in small packages,” and that is certainly in Weertman’s mind in this presentation. She has spent much of her career “in pursuit of the small.” In this article, she first looks back at her experiences studying grain-boundary cavities and life in the spaces between grains. She then fast-forwards to modern work on nanocrystalline mechanical behavior, confirming that such nanocrystalline materials are indeed strong, but also brittle. Some of her experiences in studying these phenomena are also described. Keywords: grain-boundary cavitation, mechanical behavior, nanocrystalline metals.
Introduction First, I want to thank the Materials Research Society for this great award. When I look at the past recipients, I am daunted by being in their company. But unlike certain movie stars, I have no intention of giving this award back! Again, I’m very grateful for this honor. A good deal of my materials science career has been involved in looking at phenomena associated with small entities: first, grain-boundary cavities; later, defects and structures in nanocrystalline metals. I am going to talk first about the grainboundary cavity work, which took place some time ago, then move on to the nanocrystalline mechanical behavior and characterization work, which has stretched over about three academic units of time. (I count an academic unit of time as the “lifetime” of a PhD graduate student, about
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five years. So, I have been working on this latter subject for about 15 years.)
husband, was developing a theory of grain-boundary cavitation at the time and suggested that I study that phenomenon. At the time I started working on grainboundary cavitation in the early 1970s, it was a hot area of research. The phenomenon is an important damage mechanism in materials subjected to stresses at elevated temperatures, and it presents interesting scientific problems. An important paper by Raj and Ashby1 that had just been published calculated the high localized stresses that arise at serrations in grain boundaries and the effect of these stresses in driving cavity growth by diffusion. Under creep conditions, the high stresses rapidly smooth out. We decided to examine cavity growth under fatigue loading.2 At a brisk, fully reversed cycling of 17 Hz, the initial stresses have little chance to relax. Instead, they tend to produce voids at the predicted position near the tips of the grain-boundary s
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