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Christopher A. Schuh. Rupert, Timothy J. (Timothy John) Massachusetts Institute of Technology. Dept. of Materials Science and Engineering. Massachusetts Institute of Technology. Department of Materials Science and Engineering 2012-03-16T16:03:40Z 2012-03-16T16:03:40Z 2011 2011 http://hdl.handle.net/1721.1/69793 777367788 Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2011. Cataloged from PDF version of thesis. Includes bibliographical references (p. 127-132). Nanocrystalline materials have experienced a great deal of attention in recent years, largely due to their impressive array of physical properties. In particular, nanocrystalline mechanical behavior has been of interest, as incredible strengths are predicted when grain size is reduced to the nanometer range. The vast majority of research to this point has focused on quantifying and understanding the grain size-dependence of strength, leading to the discovery of novel, grain boundary-dominated physics that begin to control deformation at extremely fine grain sizes. With the emergence of this detailed understanding of nanocrystalline deformation mechanisms, the opportunity now exists for studies that explore how other structural features affect mechanical properties in order to identify alternative strengthening mechanisms. In this thesis, we seek to extend our current knowledge of nanocrystalline structure-property relationships beyond just grain size, using combinations of structural characterization, mechanical testing, and atomistic simulations. Controlled experiments on Ni-W are first used to show that solid solution addition and the relaxation of nonequilibrium grain boundary state can dramatically affect the strength of nanocrystalline metals. Next, the sliding wear response of nanocrystalline Ni-W is investigated, to show how alloying and grain boundary structural state affect a more complex mechanical property. This type of mechanical loading also provides a strong driving force for structural evolution, which, in this case, is found to be beneficial. Mechanically-driven grain growth and grain boundary relaxation occur near the surface of the Ni-W samples during sliding, leading to a hardening effect that improves wear resistance and results in a deviation from Archard scaling. Finally, molecular dynamics simulations are performed to confirm that mechanical cycling alone can indeed relax grain boundary structure and strengthen nanocrystalline materials. In all of the cases discuss above, our observations can be directly connected to the unique deformation physics of nanocrystalline materials. by Timothy J. Rupert. Ph.D. 132 p. eng Massachusetts Institute of Technology M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission. http://dspace.mit.edu/handle/1721.1/7582 Materials Science and Engineering. Nanocrystalline alloys : enhanced strengthening mechanisms and mechanically-driven structural evolution Thesis application/pdf Publication unknown unknown <Publication xmlns="https://www.openaire.eu/cerif-profile/1.1/" id="c47eac05-e29e-49d1-ae86-74a0342bf4dc"> <Type xmlns="https://www.openaire.eu/cerif-profile/vocab/COAR_Publication_Types">http://purl.org/coar/resource_type/c_1843</Type> <Language>eng</Language> <Title>Nanocrystalline alloys : enhanced strengthening mechanisms and mechanically-driven structural evolution</Title> <PublishedIn> <Publication> </Publication> </PublishedIn> <PublicationDate>2011</PublicationDate> <Authors> <Author> <DisplayName>Rupert, Timothy J. (Timothy John)</DisplayName> <Affiliation> <OrgUnit> </OrgUnit> </Affiliation> </Author> </Authors> <Editors> </Editors> <Publishers> <Publisher> <DisplayName>Massachusetts Institute of Technology</DisplayName> <OrgUnit /> </Publisher> </Publishers> <License>http://dspace.mit.edu/handle/1721.1/7582</License> <Keyword>Materials Science and Engineering.</Keyword> <Abstract>Nanocrystalline materials have experienced a great deal of attention in recent years, largely due to their impressive array of physical properties. In particular, nanocrystalline mechanical behavior has been of interest, as incredible strengths are predicted when grain size is reduced to the nanometer range. The vast majority of research to this point has focused on quantifying and understanding the grain size-dependence of strength, leading to the discovery of novel, grain boundary-dominated physics that begin to control deformation at extremely fine grain sizes. With the emergence of this detailed understanding of nanocrystalline deformation mechanisms, the opportunity now exists for studies that explore how other structural features affect mechanical properties in order to identify alternative strengthening mechanisms. In this thesis, we seek to extend our current knowledge of nanocrystalline structure-property relationships beyond just grain size, using combinations of structural characterization, mechanical testing, and atomistic simulations. Controlled experiments on Ni-W are first used to show that solid solution addition and the relaxation of nonequilibrium grain boundary state can dramatically affect the strength of nanocrystalline metals. Next, the sliding wear response of nanocrystalline Ni-W is investigated, to show how alloying and grain boundary structural state affect a more complex mechanical property. This type of mechanical loading also provides a strong driving force for structural evolution, which, in this case, is found to be beneficial. Mechanically-driven grain growth and grain boundary relaxation occur near the surface of the Ni-W samples during sliding, leading to a hardening effect that improves wear resistance and results in a deviation from Archard scaling. Finally, molecular dynamics simulations are performed to confirm that mechanical cycling alone can indeed relax grain boundary structure and strengthen nanocrystalline materials. In all of the cases discuss above, our observations can be directly connected to the unique deformation physics of nanocrystalline materials.</Abstract> <Access xmlns="http://purl.org/coar/access_right" > </Access> </Publication>