CAREER: Reversible plasticity in nanocrystalline metals and alloys for shape memory applications

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NON-TECHNICAL DESCRIPTION: Nanocrystalline materials are composed of crystallites (grains) that have an average size of about 100 nanometers or less. The small grain size leads to very high strength, which has led to significant interest in using these materials for structural applications. But in addition to their exceptional strength, nanocrystalline materials also exhibit several intriguing properties including the ability to recover plastic deformation, which is typically considered to be irrecoverable. This CAREER award supports fundamental research that aims to exploit this unusual deformation recovery in nanocrystalline metals and alloys for shape memory related applications. In essence, the project lays the foundation to employ nanocrystalline metals and alloys as smart, functional materials that have applications in aerospace, medicine and robotics. The research activity is integrated with a broad effort to promote materials science education at multiple levels. These education and outreach efforts include a new mentoring program and scientific demonstrations for high school students, teacher workshops, course enhancements, and training of undergraduate and graduate students in multidisciplinary materials research. TECHNICAL DESCRIPTION: Conventionally, plastic deformation is considered to be permanent. However, nanocrystalline materials can recover a large fraction of plastic strain after unloading by thermal activation. This project explores the fundamental aspects of this unusual strain recovery and seeks to determine whether complete strain recovery can be repeatedly obtained in nanocrystalline metals and alloys by controlling their microstructure. The research efforts are directed towards two distinct goals: 1) Characterizing the repeatability and temperature dependence of strain recovery in nanocrystalline face centered cubic metals and alloys over multiple cycles and 2) Understanding how the interplay between material properties (elastic and plastic anisotropy, stacking fault energy) and microstructural heterogeneity (variation in size and orientation of grains) affects the strain recovery characteristics and mechanisms. These goals are pursued using novel, micro-electro-mechanical systems based thermomechanical testing and in situ transmission electron microscopy straining experiments on nanocrystalline metal and alloy films with controlled microstructures.