Strength Design Maps for Nanoscale Metallic Multilayer Thin Films

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Technical: Nanoscale multilayer thin films are extraordinary systems with which to study structure-mechanical property relations. First, these systems display remarkable strength, far surpassing the strength of single-phase systems with comparable grain size. Second, the methods of synthesis allow for unprecedented control of composition and microstructure, so that they have become a test bed to study correlations between structure, chemistry, interfaces, and physical properties such as strength. For metallic systems, that strength hinges on the ability to confine slip to small volumes. Currently, there exists no systematic means by which to predict yield strength in such systems, nor does a corresponding approach for systematic validation exist. Consequently, the multilayer thin film community lacks a broad design strategy to optimize yield strength in applications involving MEMS, hard coatings, refractive optical elements, band gaps for semiconductors, and magneto-elastic and magneto-optical films. The overall aim of the proposed program is to develop and validate yield strength design maps for A/B metallic multilayer thin films. These maps will include as input the individual phase properties, bilayer period, volume fractions, epitaxial relationships, and direction/sign of external loading. The approach is to advance current predictive methods through a combined program of advanced 3D dislocation-based simulation techniques, dislocation theory, and novel experimental verification methods. These include verification of internal stress maps via x-ray diffraction measurements; verification of interfacial dislocation content maps via transmission electron microscope studies; and verification of yield strength design maps via novel micropillar testing and film/substrate bend testing. Yield strength design maps embody a fundamental understanding of how bulk, area, and line energies drive interfacial structure and internal stress state and further, how these features confine crystallographic slip to small volumes. The comparison of modeling and experimental results will provide values of dislocation line energies, internal stress magnitudes, and interfacial barrier strengths that are not currently available. Several scientific premises will be examined, including specific strategies for yield strength optimization.Non-technical: The proposed work will advance the field of nanolayered thin-film materials. It will also strengthen the mission of teaching, training, and learning and promote the participation and professional development of underrepresented groups.