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Christopher A. Schuh. Huang, Ting-Yun Sasha Massachusetts Institute of Technology. Department of Materials Science and Engineering. Massachusetts Institute of Technology. Department of Materials Science and Engineering 2016-09-13T18:05:39Z 2016-09-13T18:05:39Z 2016 2016 http://hdl.handle.net/1721.1/104107 958135441 Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2016. This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. Cataloged from student-submitted PDF version of thesis. Includes bibliographical references (pages 113-122). Nanocrystalline alloys have attracted interest for decades because of their improved mechanical strength without sacrificing ductility, but structural stability has always been an issue. In this work, bulk aluminum-manganese (Al-Mn) nanocrystalline alloys have been synthesized using room temperature ionic liquid electrodeposition, by which various nanostructures and dual-phase structures can be created by controlling the Mn solute incorporation level. The manganese exhibits grain boundary segregation in the Al-Mn solid solution in the as-deposited condition, which contributes to enhanced stability of the nanostructure. The grain boundary properties of the nanostructured alloys were studied via three dimensional atom probe tomography and aberration-corrected scanning electron microscopy. The segregation energies were calculated based on the experimental results and compared with the values calculated from a thermodynamic-based segregation model. Upon heating of the nanostructured and dual-phase alloys, a variety of complex phase transformations occur. A combination of X-ray diffraction, transmission electron microscopy, as well as differential scanning calorimetry were employed to understand the phase transformation mechanisms and grain growth processes. A Johnson-Mehl-Avrami-Kolmogorov analytical model was proposed as a descriptive method to explain the phase transformation sequence. Using the parameters extracted from the analytical model, predictive time-temperature transformation diagrams were constructed. The stability region of the alloy in time-temperature space is thus established, providing a simple way to evaluate nanostructure stability. by Ting-Yun Sasha Huang. Ph.D. 122 pages 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. Stability of nanostructured : amorphous aluminum-manganese alloys Thesis application/pdf Publication unknown unknown <Publication xmlns="https://www.openaire.eu/cerif-profile/1.1/" id="f15f36c4-38f4-4dbe-b9da-fde5ac6ad62e"> <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>Stability of nanostructured : amorphous aluminum-manganese alloys</Title> <PublishedIn> <Publication> </Publication> </PublishedIn> <PublicationDate>2016</PublicationDate> <Authors> <Author> <DisplayName>Huang, Ting-Yun Sasha</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 alloys have attracted interest for decades because of their improved mechanical strength without sacrificing ductility, but structural stability has always been an issue. In this work, bulk aluminum-manganese (Al-Mn) nanocrystalline alloys have been synthesized using room temperature ionic liquid electrodeposition, by which various nanostructures and dual-phase structures can be created by controlling the Mn solute incorporation level. The manganese exhibits grain boundary segregation in the Al-Mn solid solution in the as-deposited condition, which contributes to enhanced stability of the nanostructure. The grain boundary properties of the nanostructured alloys were studied via three dimensional atom probe tomography and aberration-corrected scanning electron microscopy. The segregation energies were calculated based on the experimental results and compared with the values calculated from a thermodynamic-based segregation model. Upon heating of the nanostructured and dual-phase alloys, a variety of complex phase transformations occur. A combination of X-ray diffraction, transmission electron microscopy, as well as differential scanning calorimetry were employed to understand the phase transformation mechanisms and grain growth processes. A Johnson-Mehl-Avrami-Kolmogorov analytical model was proposed as a descriptive method to explain the phase transformation sequence. Using the parameters extracted from the analytical model, predictive time-temperature transformation diagrams were constructed. The stability region of the alloy in time-temperature space is thus established, providing a simple way to evaluate nanostructure stability.</Abstract> <Access xmlns="http://purl.org/coar/access_right" > </Access> </Publication>