ASU Electronic Theses and Dissertations
This collection includes most of the ASU Theses and Dissertations from 2011 to present. ASU Theses and Dissertations are available in downloadable PDF format; however, a small percentage of items are under embargo. Information about the dissertations/theses includes degree information, committee members, an abstract, supporting data or media.
In addition to the electronic theses found in the ASU Digital Repository, ASU Theses and Dissertations can be found in the ASU Library Catalog.
Dissertations and Theses granted by Arizona State University are archived and made available through a joint effort of the ASU Graduate College and the ASU Libraries. For more information or questions about this collection contact or visit the Digital Repository ETD Library Guide or contact the ASU Graduate College at gradformat@asu.edu.
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- Creators: Kannan, Arunachala Mada
- Creators: Mobasher, Barzin
Description
High-temperature mechanical behaviors of metal alloys and underlying microstructural variations responsible for such behaviors are essential areas of interest for many industries, particularly for applications such as jet engines. Anisotropic grain structures, change of preferred grain orientation, and other transformations of grains occur both during metal powder bed fusion additive manufacturing processes, due to variation of thermal gradient and cooling rates, and afterward during different thermomechanical loads, which parts experience in their specific applications, could also impact its mechanical properties both at room and high temperatures. In this study, an in-depth analysis of how different microstructural features, such as crystallographic texture, grain size, grain boundary misorientation angles, and inherent defects, as byproducts of electron beam powder bed fusion (EB-PBF) AM process, impact its anisotropic mechanical behaviors and softening behaviors due to interacting mechanisms. Mechanical testing is conducted for EB-PBF Ti6Al4V parts made at different build orientations up to 600°C temperature. Microstructural analysis using electron backscattered diffraction (EBSD) is conducted on samples before and after mechanical testing to understand the interacting impact that temperature and mechanical load have on the activation of certain mechanisms. The vertical samples showed larger grain sizes, with an average of 6.6 µm, a lower average misorientation angle, and subsequently lower strength values than the other two horizontal samples. Among the three strong preferred grain orientations of the α phases, <1 1 2 ̅ 1> and <1 1 2 ̅ 0> were dominant in horizontally built samples, whereas the <0 0 0 1> was dominant in vertically built samples. Thus, strong microstructural variation, as observed among different EB-PBF Ti6Al4V samples, mainly resulted in anisotropic behaviors. Furthermore, alpha grain showed a significant increase in average grain size for all samples with the increasing test temperature, especially from 400°C to 600°C, indicating grain growth and coarsening as potential softening mechanisms along with temperature-induced possible dislocation motion. The severity of internal and external defects on fatigue strength has been evaluated non-destructively using quantitative methods, i.e., Murakami’s square root of area parameter model and Basquin’s model, and the external surface defects were rendered to be more critical as potential crack initiation sites.
ContributorsMian, Md Jamal (Author) / Ladani, Leila (Thesis advisor) / Razmi, Jafar (Committee member) / Shuaib, Abdelrahman (Committee member) / Mobasher, Barzin (Committee member) / Nian, Qiong (Committee member) / Arizona State University (Publisher)
Created2022
Description
Gas Diffusion Layers (GDL) based on PUREBLACK® carbon and VULCAN® (XC72R) carbon along with catalyst coated membranes were used to fabricate the membrane electrode assemblies for use in proton exchange membrane fuel cells (PEMFCs). Polyethylene glycol was used as the pore-forming agent on the microporous layer to improve the lower and higher relative humidity performance of the fuel cells. Accelerated stress tests based on the dissolution effect of GDLs were conducted and the long-term performance of the GDLs was evaluated. A single-cell fuel cell was used to evaluate the effect of porosity of the micro-porous layer and the effect of different types of carbon powder on the performance of the fuel cell at different operating relative humidity conditions and compared with commercial GDLs.Both PUREBLACK® and VULCAN® (XC72R) based GDLs show crack-free surface morphology in the Scanning electron microscopy and hydrophobic characteristics in the contact angle measurements. The fuel cell performance is evaluated under relative humidity conditions of 60 and 100 % using H2/O2 and H2/Air at 70 ℃ and the durability is also evaluated for the sample with and without 30% PEG for both carbons. The pristine PUREBLACK® based GDL sample with 30% pore-forming agent (total pore volume of 1.72 cc.g-1) demonstrated the highest performance (peak power densities of 432 and 444 mW.cm-2 at 100 and 60 % RH respectively, using H2/Air). There was a significant increase in the macropores when GDLs are aged in H2O2 and the contact angle dropped to about 14 and 95° for PUREBLACK® and VULCAN® carbon, respectively. Overall PUREBLACK® based GDLs performed the best after ageing both in H2O2 and H2O (average performance degradation of 8% in H2O2 and 8.25% in H2O).
ContributorsChauhan, Nitin (Author) / Kannan, Arunachala Mada (Thesis advisor) / Phelan, Patrick (Committee member) / Nian, Qiong (Committee member) / Arizona State University (Publisher)
Created2021