The Mechanics of Ultra-Strength Materials

JU LI
Department of Materials Science and Engineering
University of Pennsylvania


Abstract:
Recent experiments on nanoscale materials, including nanowires, nanopillars, nanoparticles, thin films and
nanocrystals, have revealed a host of “ultra-strength” phenomena, defined by sample-wide stresses rising
up to a significant fraction (>1/10th) of the ideal strength - the highest achievable strength of a defect-free
crystal. While conventional materials deform or fracture at sample-wide stresses far below the ideal strength,
rapid development of nanotechnology has brought about needs to understand ultra- strength behavior, which
not only have to do with the forming and shape stabilities of a material component, but also its functional
properties. Reaching ultra-strength enables “elastic strain engineering”, where by controlling the local elastic
strain one achieves desired electronic, magnetic, optical, catalytic etc. properties in the component, and greatly
expands the materials selection space. Distinctive deformation mechanisms in ultra-strength materials will be
reviewed, where the competition between displacive and diffusive processes is highlighted. The critical role of
the activation volume is explained, as well as the size, temperature, and strain rate dependence of ultra strength.
Important unresolved issues are identified.

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T. Zhu, J. Li, S. Ogata and S. Yip, MRS Bulletin 34 (2009) 167