What is the Limit of Nanoparticle Strengthening?
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of Nanoparticle Strengthening?
D.C. Chrzan, J.W. Morris, Jr., Y.N. Osetsky, R.E. Stoller, and S.J. Zinkle Abstract The stress required to deform a perfect crystal to its elastic limit while maintaining perfect periodicity, the so-called ideal strength, sets the gold standard for the strength of a given material. Materials this strong would be of obvious engineering importance, potentially enabling more efficient turbines for energy production, lighter materials for transportation applications, and more reliable materials for nuclear reactor applications. In practice, the strength of engineering materials is often more than two orders of magnitude less than the ideal strength due to easily activated deformation processes involving dislocations. For many materials, precipitate strengthening is a promising approach to impede dislocation motion and thereby improves strength and creep resistance. This observation begs the question: What are the limits of nanoparticle strengthening? Can the ideal strength of a matrix material be reached? To answer these questions, we need a detailed, atomic scale understanding of the interactions between dislocations and obstacles. Fortunately, simulations are beginning to explore this interaction.
and semiconductors,9 and since then, discussions of mechanical properties beyond the elastic limit have focused on dislocations and fracture. Computational materials scientists, however, have begun to reconsider the importance of ideal strength calculations and have made some remarkable discoveries. For example, accurate ideal strength calculations for Mo indicate that it can yield very near its ideal strength during nanoindentation experiments.10 At the nanoscale, a material can reach ideal strength. From an engineer’s perspective, ideal strength sets the absolute engineering limit for the strength of a material. It is not possible to design a microstructure that will enable a material to exceed its ideal strength. However, the fact that ideal strength can be reached at the nanoscale begs the question: Can ideal strength be reached in a bulk material? There are at least two criteria that must be met for an engineering material to achieve a strength that approaches its ideal value.11 First, from engineering reliability considerations, the material must be intrinsically ductile: When the material is pulled in tension, it must ultimately fail in shear, regardless of the tensile axis. Second, dislocations, if present, must be immobile at all stresses below the ideal strength.
Continuum Theory Introduction Today, the need for improved structural materials remains strong, particularly if one considers the need to develop materials well suited for energy generation. We need radiation tolerant materials for nuclear reactors; materials for turbines that can withstand higher operating temperatures; and materials for transportation that are lighter, tougher, and stronger than those presently available. Computational hardware and theoretical advances give one hope that genuine prediction of the mechan
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