When 10:30 AM - 11:30 AM Jan 29, 2016
Where 1670 Beyster Building
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If smaller is stronger, is nothing weaker?

David Bahr
School of Materials Engineering, Purdue University

Since the 1950’s there has been experimental evidence that structures on the sub-micron scale can exhibit strengths approaching the theoretical strength of the material. The general hypothesis taken is that if dislocation sources are missing, then the nucleation, rather than propagation, of these defects must control deformation. In small samples it is possible to create nominally dislocation free solids, which provides a possible platform for both fundamental measurements of materials properties as well as insight into designing strong structures. Two common ways these extraordinary strengths are being examined experimentally are with nanoindentation and micro-machined structures such as tensile and compression samples, both in ambient conditions as well as during in situ electron microscopy. However, defects other than existing dislocations are either unavoidable (vacancies) or often introduced during the sample preparation process (FIB machining can add self interstitials). This presentation will examine the effects of point defects, surface defects, and other impurities on the onset of plastic deformation in metals and molecular organic crystals. In the case of metallic systems, it will be demonstrated experimentally that defects such as vacancies can lower the yield strength; simulations of these structures using molecular dynamics suggest that decreases in strength of up to 50% are possible in the presence of a variety of “hard to observe” defects. The simulations, when compared with positron studies of free volume of indented samples, match the trends observed experimentally. Impurities, such as solute hydrogen, also alter the onset of deformation, albeit in a different manner, retarding the multiplication of dislocations at high stresses. From this a methodology of demonstrating the sensitivity of the distribution of yield behavior in nanoindentation to defects will be shown. Finally, this will be extended to molecular crystals for pharmaceutical and explosive applications to demonstrate that a mechanical probe can be used to provide insight into the likely defects which are present in this challenging class of materials, and the resulting implied defect distribution will be used to comment on the importance of mechanical properties in explosive composite materials.

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