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Wilmoth, Nathan GDetermining the Mechanical Properties of Lattice Block Structures
Master of Science in Mechanical Engineering, Cleveland State University, 2013, Fenn College of Engineering
Lattice block structures and shape memory alloys possess several traits ideal for solving intriguing new engineering problems in industries such as aerospace, military, and transportation. Recent testing at the NASA Glenn Research Center has investigated the material properties of lattice block structures cast from a conventional aerospace titanium alloy as well as lattice block structures cast from nickel-titanium shape memory alloy. The lattice block structures for both materials were sectioned into smaller subelements for tension and compression testing. The results from the cast conventional titanium material showed that the expected mechanical properties were maintained. The shape memory alloy material was found to be extremely brittle from the casting process and only compression testing was completed. Future shape memory alloy lattice block structures will utilize an adjusted material composition that will provide a better quality casting. The testing effort resulted in baseline mechanical property data from the conventional titanium material for comparison to shape memory alloy materials once suitable castings are available.

Committee:

Stephen Duffy, PhD, PE, F. ASCE (Committee Chair); Jerzy Sawicki, PhD, PE, F. ASME (Committee Member); Surendra Tewari, PhD (Committee Member)

Subjects:

Aerospace Materials; Engineering; Mechanical Engineering

Keywords:

lattice block structure; LBS; Ti-6-4; shape memory alloy; NiTi

Polasik, Alison KThe Role of Microstructure on High Cycle Fatigue Lifetime Variability in Ti-6Al-4V
Doctor of Philosophy, The Ohio State University, 2014, Materials Science and Engineering
The microstructural sources of fatigue lifetime variability were investigated for four different microstructural variations of Ti-6Al-4V. Specimens were tested at lower stresses to investigate the behavior in the HCF (high cycle fatigue) regime, which is characterized by lifetimes near or in excess of 10^6 cycles. Fractography and replication analyses confirmed that the lifetime was dominated by crack nucleation, and thus variations in the lifetime between individual test specimens are primarily attributed to variability in the time to nucleate a dominant crack. Stereology was used to quantify key microstructural features for each tested specimen. These values were used as inputs for a series of microstructurally-based fuzzy logic neural network models. Using these models, virtual experiments were conducted to investigate the individual effect of each microstructural feature on the lifetime, an investigation that is impossible to conduct empirically because of the complex microstructure in these alloy systems. These virtual experiments demonstrated that colony size and alath thickness have the greatest effect on HCF lifetime of ß-processed Ti-6Al-4V alloys, and that colony size is more important that a lath thickness. For the a/ß – processed microstructures, the volume fraction of primary a and the a lath thickness were shown to affect the lifetime, while the ap grain size was not. Defect analyses of failed specimens indicated that damage accumulation is often localized during cyclic loading, with dislocation densities varying from one a lath to another. For all specimens, a-type dislocations are seen and c+a - type dislocations were observed only in regions of localized plastic strain. Investigation of site-specific TEM foils extracted from the crack nucleation region of a/ß – processed specimens provided information about the nature and behavior of dislocations during the crack nucleation event. A comparison of short- and long- life specimens provides information about differences in the evolution of the dislocation structure prior to crack nucleation. The potential of this combinatorial approach for future fatigue lifetime investigations is discussed. In particular, the project demonstrates that such an approach could be useful in developing a quantitative understanding of the role variations in microstructural features have on variations in HCF lifetime.

Committee:

Hamish Fraser, PhD (Advisor); Michael Mills, PhD (Committee Member); Stephen Niezgoda, PhD (Committee Member)

Subjects:

Aerospace Materials; Engineering; Materials Science

Keywords:

Fatigue; Titanium; Fuzzy Logic Modeling; Ti-6-4; stereology; microstructure modeling

Collins, Peter ChancellorA combinatorial approach to the development of composition-microstructure-property relationships in titanium alloys using directed laser deposition
Doctor of Philosophy, The Ohio State University, 2004, Materials Science and Engineering
The Laser Engineered Net Shaping (LENS™) system, a type of directed laser manufacturing, has been used to create compositionally graded materials. Using elemental blends, it is possible to quickly vary composition, thus allowing fundamental aspects of phase transformations and microstructural development for particular alloy systems to be explored. In this work, it is shown that the use of elemental blends has been refined, such that bulk homogeneous specimens can be produced. When tested, the mechanical properties are equivalent to conventionally prepared specimens. Additionally, when elemental blends are used in LENS™ process, it is possible to deposit compositionally graded materials. In addition to the increase in design flexibility that such compositionally graded, net shape, unitized structures offer, they also afford the capability to rapidly explore composition-microstructure-property relationships in a variety of alloy systems. This research effort focuses on the titanium alloy system. Several composition gradients based on different classes of alloys (designated a, a+b, and b alloys) have been produced with the LENS™. Once deposited, such composition gradients have been exploited in two ways. Firstly, binary gradients (based on the Ti-xV and Ti-xMo systems) have been heat treated, allowing the relationships between thermal histories and microstructural features (i.e. phase composition and volume fraction) to be explored. Neural networks have been used to aid in the interpretation of strengthening mechanisms in these binary titanium alloy systems. Secondly, digitized steps in composition have been achieved in the Ti-xAl-yV system. Thus, alloy compositions in the neighborhood of Ti-6Al-4V, the most widely used titanium alloy, have been explored. The results of this have allowed for the investigation of composition-microstructure-property relationships in Ti-6-4 based systems.

Committee:

Hamish Fraser (Advisor)

Subjects:

Engineering, Materials Science

Keywords:

combinatorial method; combinatorial approach; laser deposition; directed laser deposition; LENS; titanium; molybdenum; Ti-6-4; Ti-6Al-4V; Timetal 21S; composition; microstructure; property; relationships; neural network; fuzzy logic