CAREER: Understanding the Anomalous Ductility of Select B2 Intermetallic Compounds: Polycrystal Plasticity Modeling and Validation by In-Situ Diffraction Techniques

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TECHNICAL: Intermetallic alloys have tremendous technological potential due to their exceptional physical properties. However, the vast majority of intermetallics suffer from low ductility at room temperature. A new class of fully ordered intermetallics was recently discovered which exhibits significant polycrystalline ductility. For example, YAg exhibits greater than 20% elongation in tension at room temperature in laboratory air. A scientific explanation for this surprising behavior remains to be elucidated, and will be studied. The new class of the ductile intermetallics includes dozens of stoichiometric line compounds, comprised of a rare earth element (R) and a metal (M) from groups 2 or 8-13. While it is often stated that 5 independent slip systems are required for polycrystalline ductility (the von Mises condition), single crystal trace analyses performed on the ductile RM compounds provide only 3 independent systems. The research will determine how these select alloys sustain ductile plastic flow, while apparently violating the von Mises condition. Polycrystal plasticity simulation of crystallographic texture evolution and plastic anisotropy will be used to predict the deformation mechanisms that sustain plastic flow. Parameters describing the grain-level mechanisms of dislocation slip and potential mechanical twinning and martensitic phase transformations will be obtained by modeling data obtained experimentally. In-situ neutron diffraction will be used to directly examine evidence of twinning or stress-induced phase transformation. Diffraction measurement of the internal strain development under load would also provide insight regarding dislocation slip mechanism. Transmission electron microscopy (TEM) will be used to explore the details of the dislocation microstructure such as stacking faults and anti-phase boundaries. Planned TEM experiments also involve in-situ straining in order to determine the source mechanisms and kinetics of dislocation motion. NON-TECHNICAL: Research will impact a number of areas including magnetic storage, superconductivity, shape-memory alloys, fuel cells, and various structural applications. Research will also expose a broad segment of the population to the opportunities and challenges of a career in science and engineering. A summer research experience for teachers (RET) program will expose teachers from rural schools to the processing-structure-property paradigm underpinning materials science and engineering. Demonstrations will be developed to introduce students to the fascinating properties of intermetallic alloys. Some of these demonstrations will be designed to enhance undergraduate materials science and engineering classes, while others will be presented to pre-college students in the RET participants class rooms and at Engineering Open House events.