CAREER: Grain Growth and Topological Evolution of Polycrystals

Rensselaer Polytechnic Institute, Troy NY

Investigators

Abstract

TECHNICAL SUMMARY: Recent studies have proven that for nonzero boundary mobility, grain growth is a persistent feature of the microstructural stability of materials. This is due entirely to the measurable geometry and topology of the grains in a polycrystal. On average, no grain with an isotropic surface energy can have zero mean curvature along a grain boundary and simultaneously satisfy Plateau's rules. This inevitably leads to grain growth and subsequent environmental instability in polycrystalline materials. Available grain growth theories can be used to compute the existence of a steady state, self similar, grain size distribution. This has been confirmed experimentally and theoretically through serial sectioning and computer simulation for aggregates between ten and one-hundred grains. However, further inspection of the published data reveals that the distribution of the number of faces per grain and the number of edges per face are relatively limited as compared to the number of possible ways of constructing grains. This implies that grains in a polycrystal sample a relatively small number of shapes out of all permissible shapes, leading the PI to ask the following questions: 1) There is a self similar distribution of grain sizes. Is there also a self similar distribution of grain topologies? 2) Is the narrow distribution that which biases someone's intuition when they observe experimental microstructures and classify them as "likely" or "unlikely"? 3) Do the formation conditions or the materials processing conditions establish an initial topological distribution that can be identified? 4) Are there some distributions that are more resistant to grain growth than others? 5) Is the topological distribution universal, or, do material properties such as crystalline anisotropy affect the outcome? Answers to these questions will provide a more complete theory of grain growth that, in turn, will improve our ability to engineer and predict materials properties in service conditions. NON-TECHNICAL SUMMARY: In general, metallic materials are polycrystalline and macroscopic materials properties are strongly dependent on the grain size. Environmental factors, such as high temperature exposure, can vary the grain size of materials and in some cases lead to deleterious properties and premature failure of engineered components. Therefore, it is important to understand the factors that lead to or inhibit grain growth and to control those factors to improve the materials properties critical to engineering performance. The PI will systematically investigate the topology of grains in both simulation and experiment and will synthesize these data sets into a foundation for understanding the kinetics and thermodynamics of grain growth in metallic materials. This research program advances the understanding of grain growth by addressing aspects of grain topology that have received little attention and will help to rationalize discrepancies between published theoretical predictions, experiment, and simulation data. In this program high-school students, undergraduates, and graduate students will participate in activities related to material interfaces, morphology of polycrystals, and kinetics of grain growth. Hands-on application of laboratory techniques and student-led research projects related to this research program will complement graduate and undergraduate courses in kinetics and electron microscopy. The author will continue the successful implementation of the Capital District Materials Camp for high-school students and teachers in both high- and middle-school. The results and large data sets generated as part of this research program will be published in journal articles, conference presentations, and made available for analysis by other researchers.