February 28, 2007
InterSchool Lab, 7th Floor CEPSR
Hosted by: Center for Integrated Science & Engineering
Speaker: Gregory S. Rohrer, Carnegie Mellon University
It is widely recognized that the types of grain boundaries in a material and the manner in which they are connected affect a wide range of properties and, ultimately, a material's performance and lifetime. Understanding causal structure/property relationships relies on accurate descriptions of the grain boundary network, which is structurally complex. To distinguish one grain boundary from another, it is necessary to characterize five independent parameters. Furthermore, the different types of grain boundaries are connected in non-random configurations. To capture this complexity, we have developed techniques to measure the five-dimensional grain boundary character distribution (the relative areas of different boundary types, distinguished by lattice misorientation and grain boundary plane orientation). Based on observations in a range of metals and ceramics (Al, grain boundary engineered Ni, Cu, and ƒÑ-brass, Fe-1%Si, WC, MgO, SrTiO3, TiO2, MgAl2O4, and Al2O3), we are beginning to understand how the grain boundary character distribution evolves with time and is influenced by impurities and processing conditions. One general observation that will be described in this talk is that grains within polycrystals have preferred habit planes that correspond to the same low energy, low index planes that dominate the external growth forms and equilibrium shapes of isolated crystals of the same phase. A second topic will be the probable existence of a steady state grain boundary character distribution that is correlated to grain boundary energies and is established in the early stages of growth. A theory for the development of steady state, characteristic grain boundary character distributions will be described.