Harnessing Abnormal Grain Growth for the Production of Single Crystals

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Non-technical summary:Most engineering materials are commonly found in polycrystalline form, in which a solid consists of many tiny crystals with different orientations. In some cases, superior properties can be achieved when the material is made in a single crystal form, as in the case for jet-engine turbine blades and solar cells. However, single crystals are expensive and time-consuming to produce. Recently researchers have developed a method for the production of single crystals by exploiting abnormal grain growth induced by cyclic heat treatment, in which a metal alloy is heated and cooled repeatedly. During abnormal grain growth, a few grains preferentially grow by engulfing the neighboring grains. The goal of this project is to discover why and how this process occurs. The project integrates emergent research in structural characterization and simulations of grain growth, capitalizing on our ability to watch the evolution of the grain network in real-time, and leveraging high performance computing resources to simulate microstructural evolution. Developing this fundamental understanding of abnormal grain growth could lead to a paradigm shift in the manufacture of single-crystalline materials, and therefore it promotes global competitiveness in manufacturing and technology and national prosperity. The project also promotes the development of a highly trained future workforce; two graduate students are trained in state-of-the-art techniques in experiments, modeling, simulations, and data analysis. Outreach activities are carried out through the Females Excelling More in the Math, Engineering, and the Sciences program and Washtenaw Elementary Science Olympiad and include a virtual reality demonstration that allows students to walk through an evolving microstructure during heat treatment. Technical summary:A cyclic heat treatment that induces abnormal grain growth holds promise for solid-state processing of single crystals and otherwise large-grained materials. However, its full potential has not been realized due to the poor understanding of the mechanisms underlying the process. The objective of this project is to advance the science governing the growth of the abnormal grains during cyclic thermal treatment. The following fundamental questions are addressed: What is the mechanism by which abnormal grain growth is initiated during a cyclic heat treatment? Which grains are most likely to become abnormal and what are their microstructural signatures? How does the abnormal grain grow into new microstructural neighborhoods? To answer these questions, emergent research in structural characterization, phase-field and phase-field-crystal modeling, and graph theory methods are synergistically integrated. High-resolution synchrotron-based X-ray diffraction microscopy is utilized to visualize and quantify the evolution of the grain and subgrain network in real-time and also enable microstructural evolution simulations based on the measured space-, time-, and orientation-resolved datasets as initial conditions. The resulting high-dimensional data is then distilled into a network model that succinctly describes the microstructure and the local driving forces for grain boundary motion. Scientific understanding of abnormal grain growth upon thermal cycling will ultimately inform the process design of single-crystal fabrication.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.