Theses and Dissertations - Department of Physics & Astronomy

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Browsing Theses and Dissertations - Department of Physics & Astronomy by Author "Acoff, Viola L."

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    Influence of zirconium additions on nitinol shape memory phase stability, transformation temperatures, and thermo-mechanical properties
    (University of Alabama Libraries, 2016) Kornegay, Suzanne Marilyn; Thompson, Gregory B.; University of Alabama Tuscaloosa
    This research focuses on exploring the influence of Zr additions in Ni-rich Nitinol alloys on the phase stability, transformation temperatures, and thermo-mechanical behavior using various microanalysis techniques. The dissertation is divided into three major bodies of work: (1) The microstructural and thermo-mechanical characterization of a 50.3Ni-32.2Ti-17.5Zr (at.%) Zr alloy; (2) The characterization and mechanical behavior of 50.3Ni-48.7Ti-1Zr and 50.3Ni-48.7Ti-1Hf alloys to determine how dilute additions alter the phases, transformation temperatures, and thermo-mechanical properties; and (3) The microstructural evolution and transformation behavior comparison of microstructure and transformation temperature for 50.3Ni-(49.7-X)Ti-XZr alloys, where X is 1,7, or 17.5% Zr aged at either 400°C and 550°C. The major findings of this work include the following: (1) In the dilute limit of 1% Zr, at 400°C aging, a spherical precipitate, denoted as the S-phase, was observed. This is the first report of this phase. Further aging resulted in the secondary precipitation event of the H-phase. Increasing the aging temperature to 550°C, resulted in no evident precipitation of the S- and H-phase precipitates suggestive this temperature is above the solvus boundary for these compositions. (2) For the 7% and 17.5% Zr alloys, aging at 400°C and 550°C resulted in the precipitation of the H-phase. For the lower temperature anneal, this phase required annealing up to 300 hours of aging to be observed for the 17.5% Zr alloy. Upon increasing the aging temperature, the H-phase precipitation was present in both alloys. The transformation behavior and thermo-mechanical properties are linked to the precipitation behavior.
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    Laser assisted cold spray of ferritic alloys: oxide dispersion strengthened fe91ni8zr1 and aisi 4340 steel
    (University of Alabama Libraries, 2020-08) Barton, Dallin James; Thompson, Gregory B.; Brewer, Luke N.; University of Alabama Tuscaloosa
    Oxide dispersion strengthened (ODS) Fe91Ni8Zr1 (at. %) and AISI 4340 steel were successfully deposited via laser assisted cold spray. The laser assisted cold spray technique includes accelerating powder particles such that they strike a substrate at supersonic speeds and metallurgically bond. A high-powered laser irradiated the surface of the deposition area making the substrate surface thermally softer promoting deposition. In situ laser heating of the substrate increased the deposition efficiency of the high strength 4340 steel from 48 % to 72 %. The increased surface temperature from 400 C to 950 C also increased the median ferrite grain size. As the ferrite grain size increased, the hardness decreased; however, at higher surface temperatures, the steel transitioned to martensite and compensated the lost hardness due to grain size with the hardness returning to the same values as the cold spray deposits. The lowest viable surface temperature achieved for multi-layered LACS deposition of ODS materials is 650 C. Increased surface temperatures led to an increase in deposition efficiency up to 32 % at 950 C and resulted in a lower hardness material. Grain sizes and particle sizes increased from the elevated temperatures as well. However, the grains did not grow the same throughout the thickness of the material. Grains near the surface of the deposit are several times larger than grains near the deposit-substrate interface. In addition to LACS processing and microstructure, this work reports compressive surface residual stresses of LACS deposits, dynamic strain aging of ODS materials, and a commentary for improving cluster analysis of nano-scale oxides measured through atom probe tomography.
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    Microstructural evolution and oxidation behavior of AlNiCrCoSi multi-component alloys
    (University of Alabama Libraries, 2013) Alfano, Joel; Weaver, Mark Lovell; University of Alabama Tuscaloosa
    Equiatomic or near-equiatomic multicomponent alloys, often termed high-entropy alloys (HEAs), are an emerging class of metallic materials that are being investigated for a wide range of technical applications. Unlike conventional engineering alloys, HEAs lack a principal component and are instead composed of at least five major metallic elements, each with compositions generally ranging from 5 to 35 at. %. Unlike conventional alloys, HEAs have high entropies of mixing that are said to enhance the formation of solid solutions and other metastable phases resulting in materials exhibiting unique combinations of properties including high thermal stability, high strength/hardness, and high corrosion/oxidation resistance. Most studies of HEAs have focused on optimizing microstructures or mechanical properties. Relatively few have designed an alloy for or investigated oxidation behavior of HEAs. This dissertation describes the microstructural evolution and oxidation behavior of three HEA alloys that were designed with the intent to provide high oxidation resistance whilst offering the possibility for high temperature precipitation hardening. The alloys were intentionally designed to contain four elements that are commonly utilized in high temperature high-temperature Ni-based alloys, Al, Ni, Cr, and Co. A fifth element, Si, was also added. Though not a typical addition to structural Ni-based superalloys, it has been show to improve their resistance to oxidation, hot corrosion, and wear, particularly when incorporated into surface layers or protective coatings. All of the alloys investigated were designed using the phase selection rules proposed by Yang and Zhang which predicted that the alloys would consist of a mixture of solid solution and intermetallic phases in the as-cast or as-deposited state. Two alloys were designed to have higher concentrations of Al and Cr, thus maximizing the possibility of forming disordered BCC phases, while one was designed to have lower concentrations of Al and Cr, thus maximizing the chances of forming disordered FCC phases. Detailed microstructural and chemical analysis showed that characterizations have confirmed the presence of the expected solid solution phases, however, the majority phases in all of the alloys were found to be ordered intermetallics which increased in volume fraction upon annealing. The majority of the phases that formed were consistent with thermodynamic predications made using the Thermo-Calc software package; however the alloys were subject to phase transformations that could not be predicted using existing thermodynamic data. Isothermal and cyclic oxidation tests demonstrated that both sets of alloys were capable of forming aluminum oxide (aka alumina) surface scales, but that alloys containing higher Al and Cr were more stable. Coatings with similarly high Al and Cr levels were found to significantly improve the oxidation resistance of pure Ni and to exceed the 250 h oxidation upon the already excellent oxidation resistance of beta-NiAl based coatings. This result was surprising in that the HEA coatings produced in this study were thinner and were deposited on a less oxidation resistant substrate than the beta-NiAl based coatings. Collectively these results have confirmed that complex multicomponent HEAs that are capable of forming protective alumina scales can be designed and processed using existing phase selection rules. These results also reiterate the need for refinement of the phase selection rules for HEA formation and improved thermodynamic databases to facilitate the design of better HEAs.