Session 1A: Special Lecture

The Zen of grain boundaries

Abstract: Grain boundaries are interfaces between two adjacent crystals; interphase boundaries have crystals on both sides of different chemistry or crystal structure. Grain boundaries are ubiquitous in nature, ranging from conventional metals and ceramics in daily use to minerals and rocks in mountains and tectonic plates. They play an important role in many different areas, such as deformation, failure, sintering, manufacturing, avalanches, and possibly dust coagulation and related processes in outer space. There is a resurgence in recent interest in grain boundaries because their volume fraction is substantial in nanocrystalline materials

A common theme underlying this presentation will be dual role played by grain boundaries under many different conditions, so that they have a yin-yang characteristics of positive and negative effects. As an example, consider the role of grain boundaries in superplasticity which involves extremely large elongations to failure in polycrystalline materials at high temperatures. On the one hand, they play a positive role in enabling superplasticity, by facilitating grain boundary sliding that is the major strain contributing process. On the other hand, they may hinder superplasticity as grain boundary sliding leads to stress concentrations at junctions where multiple boundaries meet, and this can lead to premature failure by the nucleation, growth and interlinkage of cavities. Many such similar examples will be discussed in the presentation, including strength in nanocrystals.