Matching Items (10)
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- All Subjects: Materials Science
- All Subjects: Social Research
- Genre: Masters Thesis
- Creators: Chawla, Nikhilesh
Description
Aluminum alloys and their composites are attractive materials for applications requiring high strength-to-weight ratios and reasonable cost. Many of these applications, such as those in the aerospace industry, undergo fatigue loading. An understanding of the microstructural damage that occurs in these materials is critical in assessing their fatigue resistance. Two distinct experimental studies were performed to further the understanding of fatigue damage mechanisms in aluminum alloys and their composites, specifically fracture and plasticity. Fatigue resistance of metal matrix composites (MMCs) depends on many aspects of composite microstructure. Fatigue crack growth behavior is particularly dependent on the reinforcement characteristics and matrix microstructure. The goal of this work was to obtain a fundamental understanding of fatigue crack growth behavior in SiC particle-reinforced 2080 Al alloy composites. In situ X-ray synchrotron tomography was performed on two samples at low (R=0.1) and at high (R=0.6) R-ratios. The resulting reconstructed images were used to obtain three-dimensional (3D) rendering of the particles and fatigue crack. Behaviors of the particles and crack, as well as their interaction, were analyzed and quantified. Four-dimensional (4D) visual representations were constructed to aid in the overall understanding of damage evolution. During fatigue crack growth in ductile materials, a plastic zone is created in the region surrounding the crack tip. Knowledge of the plastic zone is important for the understanding of fatigue crack formation as well as subsequent growth behavior. The goal of this work was to quantify the 3D size and shape of the plastic zone in 7075 Al alloys. X-ray synchrotron tomography and Laue microdiffraction were used to non-destructively characterize the volume surrounding a fatigue crack tip. The precise 3D crack profile was segmented from the reconstructed tomography data. Depth-resolved Laue patterns were obtained using differential-aperture X-ray structural microscopy (DAXM), from which peak-broadening characteristics were quantified. Plasticity, as determined by the broadening of diffracted peaks, was mapped in 3D. Two-dimensional (2D) maps of plasticity were directly compared to the corresponding tomography slices. A 3D representation of the plastic zone surrounding the fatigue crack was generated by superimposing the mapped plasticity on the 3D crack profile.
ContributorsHruby, Peter (Author) / Chawla, Nikhilesh (Thesis advisor) / Solanki, Kiran (Committee member) / Liu, Yongming (Committee member) / Arizona State University (Publisher)
Created2014
Description
Increasing demand for high strength powder metallurgy (PM) steels has resulted in the development of dual phase PM steels. In this work, the effects of thermal aging on the microstructure and mechanical behavior of dual phase precipitation hardened powder metallurgy (PM) stainless steels of varying ferrite-martensite content were examined. Quantitative analyses of the inherent porosity and phase fractions were conducted on the steels and no significant differences were noted with respect to aging temperature. Tensile strength, yield strength, and elongation to fracture all increased with increasing aging temperature reaching maxima at 538oC in most cases. Increased strength and decreased ductility were observed in steels of higher martensite content. Nanoindentation of the individual microconstituents was employed to obtain a fundamental understanding of the strengthening contributions. Both the ferrite and martensite hardness values increased with aging temperature and exhibited similar maxima to the bulk tensile properties. Due to the complex non-uniform stresses and strains associated with conventional nanoindentation, micropillar compression has become an attractive method to probe local mechanical behavior while limiting strain gradients and contributions from surrounding features. In this study, micropillars of ferrite and martensite were fabricated by focused ion beam (FIB) milling of dual phase precipitation hardened powder metallurgy (PM) stainless steels. Compression testing was conducted using a nanoindenter equipped with a flat punch indenter. The stress-strain curves of the individual microconstituents were calculated from the load-displacement curves less the extraneous displacements of the system. Using a rule of mixtures approach in conjunction with porosity corrections, the mechanical properties of ferrite and martensite were combined for comparison to tensile tests of the bulk material, and reasonable agreement was found for the ultimate tensile strength. Micropillar compression experiments of both as sintered and thermally aged material allowed for investigation of the effect of thermal aging.
ContributorsStewart, Jennifer (Author) / Chawla, Nikhilesh (Thesis advisor) / Jiang, Hanqing (Committee member) / Krause, Stephen (Committee member) / Arizona State University (Publisher)
Created2011
Description
Pb-free solder joints are commonly used as interconnects in semiconductor packaging. One of the major defects affecting the mechanical performance of solder joints are reflow pores that form during processing. These pores exhibit significant variability in size and distribution, and understanding the effects of pore geometry on failure is an important reliability concern. In this thesis, the pore microstructures of solder joint samples and the localized plastic deformation around individual pores was characterized in 3D using lab scale X-ray Microtomography. To observe the deformation of a solder joint in 3D, a solder joint was imaged with Microtomography after reflow and then deformed in shear in several loading steps with additional tomography data taken between each. The 3D tomography datasets were then segmented using the 3D Livewire technique into regions corresponding to solder and pores, and used to generate 3D models of the joint at each strain value using Mimics software. The extent of deformation of individual pores in the joint as a function of strain was quantified using sphericity measurements, and correlated with the observed cracking in the joint. In addition, the error inherent in the data acquisition and 3D modeling process was also quantified. The progression of damage observed with X-ray Microtomography was then used to validate the deformation and failure predicted by a Finite Element (FE) simulation. The FE model was based on the as-reflowed tomography data, and incorporated a ductile damage failure model to simulate fracture. Using the measured sphericity change and cracking information obtained from the tomography data, the FE model is shown to correctly capture the broad plastic deformation and strain localization seen in the actual joint, as well as the crack propagation. Lastly, Digital Image Correlation was investigated as a method of obtaining improved local strain measurements in 3D. This technique measures the displacement of the inherent microstructural features of the joint, and can give localized strain measurements that can be directly comparable to that predicted by modeling. The technique is demonstrated in 2D on Pb-Sn solder, and example 3D data is presented for future analysis.
ContributorsPadilla, Erick (Author) / Chawla, Nikhilesh (Thesis advisor) / Alford, Terry (Committee member) / Krause, Stephen (Committee member) / Arizona State University (Publisher)
Created2012
Description
This project seeks to explore how organizations work toward refugee and immigrant integration through forming different types of coalitions and strategic networks. Semi-structured interviews were conducted to identify when coalitions emerge between refugee organizations and immigrant rights groups in order to examine their development, from how the coalitions broadly conceive of refugee and immigrant rights, to how they organize resources and information sharing, service provision, policy advocacy, and policy implementation. The project is guided by the question: What explains the formation of coalitions that advocate for both immigrant rights and refugee rights? Through examining the formation and development of these coalitions, this thesis engages at the intersections of immigration federalism, refugee studies and human rights scholarship to reveal deeper complexities in the politics of immigrant integration. The project sharpens these three scholarly intersections by three multi-level jurisdictions – California and Arizona in the United States and Athens in Greece – and by employing comparative analysis to unpack how national governments and federalism dynamics shape coalition building around immigrant integration.
ContributorsAmoroso-Pohl, Melanie Hope (Author) / Colbern, Allan (Thesis advisor) / Keahey, Jennifer (Committee member) / Walker, Shawn (Committee member) / Arizona State University (Publisher)
Created2019
Description
The thesis for this study is that structural racism within the U.S. criminal system causes Black mothers to assume the emotional work of caring for incarcerated sons. This project was designed using an interpretive approach that employed a combination of qualitative and auto-ethnographic methods, drawing on grounded theory principle. Six interviews were conducted with mothers in order to gain in-depth insight into their lived experiences. An auto-ethnographic method was used to analyze the author’s own personal experiences as a family member of the incarcerated in dialogue with the experiences of the broader research population. Studies on the key finding of the psycho-social impacts on mothers with incarcerated sons have explored the relationship between the mental depression of mothers and their son’s incarceration. They have found that financial challenges, dwindling social connections, lousy parenting evaluations, as well as the burden of care of the grandchildren of the incarcerated sons are some of the mediating factors of this relationship. A second key finding also showed that incarceration have had social-economic effects on the prisoner’s families. These families experience extreme financial hardship as a result of incarcerated loved ones. Another finding showed the unique coping strategies for mothers included assuming care taking responsibility, maintaining family relationships, and budget control. Finally, this study found that there are challenges to re-entry experienced by mothers with incarcerated sons when their released. Research findings and original contribution to scholarly knowledge uncovered that Black mothers of the incarcerated in addition to working the Second Shift, are experiencing the phenomena of what is coined to be the “Third Shift.”
ContributorsWhite, LaTonya C (Author) / Keahey, Jennifer (Thesis advisor) / Colbern, Allan (Committee member) / Murphy-Erfani, Julie (Committee member) / Arizona State University (Publisher)
Created2019
Description
ABSTRACT
Elite experience and careers in judged female sports complicate the binary categories of retirement while they are especially exposed to cultures of abuse, pressure and subjectivity. This thesis is comprised of multiple voices and experiences from the elite female athletic perspective, including my autoethnographic narrative. Highlighted and discussed are the topics of sexual assault and abuse, family pressure on children to do and excel at sport, the National Team experience representing the United States and subjected bodies and judging. It is an aim of this thesis to culminate all of those factors in the final chapter and hold that the experience and the cultures of athletic identity within synchronized swimming, gymnastics and figure skating not only cannot be explained by current research on athletic identity through retirement but have the capability to retire undeveloped young women by overdeveloped athletic identities. Through a sampling of voices and experiences across different female judged sports, over three decades, the reader will observe similarities that cause these sports to have a culture of solidarity through the aspects they hold in common with each other. The narrative highlights pivotal moments in the lives of the elite female athlete within these sports, which add to the calculation of their athletic identities and the lack of their personal identities. Through reflection and analyses of not only my story, but the interviewees from my original research and that of Joan Ryan’s as well, I aim to voice a mutual experience of elite athletes. Consisting of multiple factors throughout many years we will see through my autoethnography, paralleling with other voices and experiences, how it all intersects and contributes to this: Who am I now and where do I go from here?
Elite experience and careers in judged female sports complicate the binary categories of retirement while they are especially exposed to cultures of abuse, pressure and subjectivity. This thesis is comprised of multiple voices and experiences from the elite female athletic perspective, including my autoethnographic narrative. Highlighted and discussed are the topics of sexual assault and abuse, family pressure on children to do and excel at sport, the National Team experience representing the United States and subjected bodies and judging. It is an aim of this thesis to culminate all of those factors in the final chapter and hold that the experience and the cultures of athletic identity within synchronized swimming, gymnastics and figure skating not only cannot be explained by current research on athletic identity through retirement but have the capability to retire undeveloped young women by overdeveloped athletic identities. Through a sampling of voices and experiences across different female judged sports, over three decades, the reader will observe similarities that cause these sports to have a culture of solidarity through the aspects they hold in common with each other. The narrative highlights pivotal moments in the lives of the elite female athlete within these sports, which add to the calculation of their athletic identities and the lack of their personal identities. Through reflection and analyses of not only my story, but the interviewees from my original research and that of Joan Ryan’s as well, I aim to voice a mutual experience of elite athletes. Consisting of multiple factors throughout many years we will see through my autoethnography, paralleling with other voices and experiences, how it all intersects and contributes to this: Who am I now and where do I go from here?
ContributorsHaylor, Alyson (Author) / Colbern, Allan (Thesis advisor) / Mean, Lindsey (Committee member) / Kassing, Jeffrey (Committee member) / Arizona State University (Publisher)
Created2019
Description
Electromigration (EM) has been a serious reliability concern in microelectronics packaging for close to half a century now. Whenever the challenges of EM are overcome newer complications arise such as the demand for better performance due to increased miniaturization of semiconductor devices or the problems faced due to undesirable properties of lead-free solders. The motivation for the work is that there exists no fully computational modeling study on EM damage in lead-free solders (and also in lead-based solders). Modeling techniques such as one developed here can give new insights on effects of different grain features and offer high flexibility in varying parameters and study the corresponding effects. In this work, a new computational approach has been developed to study void nucleation and initial void growth in solders due to metal atom diffusion. It involves the creation of a 3D stochastic mesoscale model of the microstructure of a polycrystalline Tin structure. The next step was to identify regions of current crowding or ‘hot-spots’. This was done through solving a finite difference scheme on top of the 3D structure. The nucleation of voids due to atomic diffusion from the regions of current crowding was modeled by diffusion from the identified hot-spot through a rejection free kinetic Monte-Carlo scheme. This resulted in the net movement of atoms from the cathode to the anode. The above steps of identifying the hotspot and diffusing the atoms at the hot-spot were repeated and this lead to the initial growth of the void. This procedure was studied varying different grain parameters. In the future, the goal is to explore the effect of more grain parameters and consider other mechanisms of failure such as the formation of intermetallic compounds due to interstitial diffusion and dissolution of underbump metallurgy.
ContributorsKarunakaran, Deepak (Thesis advisor) / Jiao, Yang (Committee member) / Chawla, Nikhilesh (Committee member) / Rajagopalan, Jagannathan (Committee member) / Arizona State University (Publisher)
Created2016
Description
Miniaturization of microdevices comes at the cost of increased circuit complexity and operating current densities. At high current densities, the resulting electron wind imparts a large momentum to metal ions triggering electromigration which leads to degradation of interconnects and solder, ultimately resulting in circuit failure. Although electromigration-induced defects in electronic materials can manifest in several forms, the formation of voids is a common occurrence. This research aims at understanding the morphological evolution of voids under electromigration by formulating a diffuse interface approach that accounts for anisotropic mobility in the metallic interconnect. Based on an extensive parametric study, this study reports the conditions under which pancaking of voids or the novel void ‘swimming’ regimes are observed. Finally, inferences are drawn to formulate strategies using which the reliability of interconnects can be improved.
ContributorsVemulapalli, Sree Shivani (Author) / Ankit, Kumar (Thesis advisor) / Chawla, Nikhilesh (Committee member) / Singh, Arunima (Committee member) / Arizona State University (Publisher)
Created2020
Description
The increasing job opportunities abroad as spa therapists attract significant numbers of young Indonesian women. Although the placement process is conducted by licensed recruitment agents and supervised by government officials, migrant workers might be at high risk of experiencing work exploitation and physical or sexual abuse. To investigate the phenomenon of documented, yet still vulnerable, female migrant workers, this research conducts interviews with several former spa therapists who were working in Malaysia and some civil servants. This study highlights that individual or personal resistances could be a collective political struggles. Specifically, this research connects individual experiences with the bigger picture of social, economic, and political condition, which, together, constitutes a gender-based labor migration system. To do this, the research employs qualitative-interpretive research methods through discourse analysis and in-depth and open-ended interviews. It also employs an intersectional feminist approach to data analysis to reveal how Indonesian female migrant workers are marginalized and oppressed and the power dynamics at play.
ContributorsNabila, Asma Zahratun (Author) / Colbern, Allan (Thesis advisor) / Behl, Natasha (Thesis advisor) / Goksel, Nisa (Committee member) / Arizona State University (Publisher)
Created2021
Description
This paper assesses the obstacles faced by immigrant aid groups on the U.S./ Mexico border and the resiliency used to challenge these obstacles. The borderlands of the United States and Mexico is a unique landscape for activists and humanitarians to work given the prevalence and amount of entities that police the area and the suspension of certain constitutional protections. The criminalization of activists on the border provides a unique lens in understanding how social movements and nation-building are linked to immigration in the United States. This research aims to provide a rich description of what criminalization is and how it plays out between the government and activist groups along the border. My findings critique the United States and its claim that it is a liberal democracy because it breaks norms and international laws in its assault against activists and humanitarians, many of whom are U.S. citizens. This attack further demonstrates that the violence migrants endure on the border is not just an unfortunate side effect of border policies but very much intentional and by design. In addition to criminalizing activists, this thesis examines the activists’ mental health and exhaustion as they relate to their humanitarian work and how this is also intentional violence the U.S. Government inflicts in order to maintain itself as a nation-state.
ContributorsRoether, Nichole (Author) / Colbern, Allan (Thesis advisor) / Firoz, Malay (Committee member) / Redeker-Hepner, Tricia (Committee member) / Arizona State University (Publisher)
Created2021