CAREER: Understanding Microstructure Evolution and Deformation Mechanism of Strong yet Ductile Nanolamellar High-Entropy Alloys Produced by Additive Manufacturing

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NON-TECHNICAL SUMMARYAdditive manufacturing, also called 3D printing, is a new paradigm to produce net-shaped components layer by layer for a broad range of technological applications in automotive, aerospace, biomedical and other industries. In addition to vast design freedom, the rapid laser melting during additive manufacturing can produce highly refined structures at the nanoscale in metals for achieving high strength. However, high-strength nanostructured metals often suffer from limited ductility, which is an ability to be stretched without breaking. This strength-ductility tradeoff has been a long-standing challenge in materials science and the quest for materials that can simultaneously enhance strength and ductility has been a long-sought-after goal. High-entropy alloys (HEAs) are a new class of materials that contain high concentrations of five or more different elements in near equal atomic proportions, in contrast to traditional alloys that are primarily based on one major element with some minor alloying elements added. This Faculty Early Career Development (CAREER) award supports fundamental investigations into additive manufacturing of HEAs towards strength-ductility synergy beyond current benchmarks. This project is helping to understand the microstructural origin and deformation mechanism that govern the mechanical properties of 3D-printed HEAs by integrating microstructural characterization, mechanical testing, and computational modeling. The knowledge being established in this project will guide the development of strong yet tough metal alloys for various applications such as advanced energy systems, transportation, and defense. This CAREER award also includes a significant educational component that engages students in research across high school, undergraduate and graduate levels. Through broadening participation of underrepresented groups, this project is diversifying the next generation of researchers and STEM leaders in materials science and advanced manufacturing. TECHNICAL SUMMARY3D-printed metal alloys usually involve highly localized melting processes, strong temperature gradients, and fast cooling rates. These extreme printing conditions result in far-from-equilibrium states that enable microstructural refinement to the nanoscale for achieving high strength. However, high-strength nanostructured metal alloys often suffer from limited ductility, known as the strength-ductility tradeoff. Through harnessing the extreme printing conditions of laser additive manufacturing and favorable compositional effect of HEAs, a unique type of hierarchical microstructure in the form of dual-phase nanolamellae embedded in microscale eutectic colonies is achieved in 3D-printed eutectic HEAs. This process gives rise to an exceptional combination of strength and ductility. This CAREER award is investigating the fundamental processing-structure-property relationship in these strong yet ductile nanolamellar EHEAs produced by additive manufacturing. The scientific objectives in this study are to: 1) Understand how laser printing protocols affect the solidification microstructure and mechanical properties of 3D-printed eutectic HEAs. Process-sensitive thermal modeling will be developed to unveil the physical link between the complex printing parameters and the solidification microstructure and resulting mechanical properties. 2) Unravel the deformation mechanism and micromechanical response of 3D-printed eutectic HEAs by in situ neutron diffraction and transmission electron microscopy. 3) Elucidate the phase transformation pathways in 3D-printed eutectic HEAs upon post-printing heat treatment. A fundamental investigation of the phase transformation pathways and kinetics during annealing of the far-from-equilibrium 3D-printed HEAs are being performed to expand the palette for materials design. The mechanistic insights and design motifs being provided by this CAREER project have broad implications for the development of hierarchical, multi-phase, nanostructured alloys with excellent mechanical properties. This award also encompasses an educational and outreach plan to advance research training and education of next-generation students and underrepresented groups. New outreach initiatives such as 3D printing workshops and a summer-enrichment program will be developed to inspire women and underrepresented minorities and increase the diversity of our future workforce in materials science and advanced manufacturing.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.