Dynamic Microstructural Characterization of High Strength Aluminum Alloys

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dc.contributor.advisor Alina Chertock, Committee Member en_US
dc.contributor.advisor Kara Peters, Committee Member en_US
dc.contributor.advisor Mohammed A. Zikry, Committee Chair en_US
dc.contributor.author Lee, William Morgan en_US
dc.date.accessioned 2010-04-02T18:03:11Z
dc.date.available 2010-04-02T18:03:11Z
dc.date.issued 2008-05-09 en_US
dc.identifier.other etd-04302008-114019 en_US
dc.identifier.uri http://www.lib.ncsu.edu/resolver/1840.16/1317
dc.description.abstract The use of aluminum alloys for commercial and military applications has increased substantially due to the alloys' low areal density, toughness, and processability. It has recently been shown that an aluminum alloy, Al 2139, with copper, magnesium, and silver can be significantly toughened and strengthened by combinations of θ' and Ω precipitates and dispersed manganese particles. What has not been quantified are how these precipitates and dispersed particles affect behavior and what the material mechanisms and microstructural characteristics are that control the behavior of Al 2139 for strain-rates that span the quasi-static to high rates of strain. Hence, in this investigation, detailed transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), scanning electron microscopy (SEM), orientation imaging microscopy (OIM), and optical microscopy (OM) were used to delineate the different physical scales that range from the nano for the precipitates and dislocations to the micron for the dispersed particles, grain orientations and texture, grain-sizes, slip-bands, and grain-boundary orientations. The deformed specimens were from an Al 2139 plate that was impacted by 4340 steel fragmentation stimulating projectiles (FSPs) at impact velocities ranging from 813 to 1043 m⁄s. The majority of the projectiles were defeated by the Al 2139 plate, which is another indication of the alloy's potential for damage mitigation and projectile defeat and resistance. Based on this detailed microstructural characterization, mechanisms for projectile defeat and full penetration are proposed. Deformation and damage modes include petalling on the impact face, shear cracking through the middle section of the plate due to projectile penetration, and discing due to bending stresses at a spall plane near the back of the plate. Shear cracking appears to be GB related, and the discing is dependent on the rolling direction. The extent of these modes for cross-sections where the target was penetrated was greater than that in regions where the projectile was defeated. For projectile defeat, large and elongated grains and precipitate deformation due to dislocation interaction can lead to highly ductile performance, which resists discing failure and plate penetration. Large grains significantly reduce the fraction of GBs, which then reduces the amount of GB cracking due to intense shear accumulation and spall. The elongation of the grains due to rolling also increased the dislocation densities, and subsequently the ductility of the grains, which reduced tensile failure due to the bending in the discing regions. High angle GB's can also limit heterogeneous θ' precipitation at the GB's, which would reduce intergranular fracture. Precipitation of Ω also increases the spall strength and decreases localized shear through its multiple cutting interactions with dislocations at the matrix interface. Dispersed particles also increase the strength of the alloy in high strain-rate applications by resisting localized shear. The results of this study are a first step in developing a tailored methodology that can be used to optimize microstructural characteristics and behavior of aluminum alloys for optimal strength and toughness. en_US
dc.rights I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dis sertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to NC State University or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. en_US
dc.subject precipitate en_US
dc.subject omega en_US
dc.subject impact en_US
dc.subject aluminum en_US
dc.title Dynamic Microstructural Characterization of High Strength Aluminum Alloys en_US
dc.degree.name MS en_US
dc.degree.level thesis en_US
dc.degree.discipline Mechanical Engineering en_US

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