Theses and Dissertations - Department of Metallurgical and Materials Engineering

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Advanced characterization of the oxidation behavior of grain refined NiAl
(University of Alabama Libraries, 2019) White, Rachel Ellen; Weaver, Mark Lovell; University of Alabama Tuscaloosa
Reactive element doped β-NiAl is one of the most oxidation resistant materials available for high temperature use. It has been extensively studied to create the most adherent, slow growing, and passive layer possible. One recent area of interest is grain refinement, whereby the reduced metal grain size improves mechanical properties, transports reacting elements rapidly to the oxidizing surface, and facilitates the growth of a more adherent scale. This research focused on the effect of substrate grain refinement on the microstructure of its thermally grown oxide, in comparison to the oxide grown on extruded and single crystal NiAl alloys. The oxidation behavior of grain refined materials produced by via sputter deposition, ball milling, and cryomilling was found to vary significantly. Sputter deposition was shown to significantly increase the parabolic steady state oxidation rate constant, while decreasing the length of transient oxidation. Ball milling did not result in an increase in oxidation rate, but did show increased interfacial void formation as a result of the Al2O3 dispersions incorporated during the milling process. Last, cryomilling resulted in an increase in steady state oxidation rate and increased interfacial void formation that was correlated to AlN dispersions incorporated during milling. All three grain refinement methods were found to decrease the oxide grain size approximately three-fold in comparison with the oxide grown on extruded NiAl, though a consistent relationship between oxide grain size and steady state oxidation rate was not observed. This suggests that microstructural features other than substrate and oxide grain size dominate the oxidation behavior.
Experimental and theoretical analyses on the ultrasonic cavitation processing of al-based alloys and nanocomposites
(University of Alabama Libraries, 2015) Jia, Shian; Nastac, Laurentiu; University of Alabama Tuscaloosa
Strong evidence is showing that microstructure and mechanical properties of a casting component can be significantly improved if nanoparticles are used as reinforcement to form metal-matrix-nano-composite (MMNC). In this paper, 6061/A356 nanocomposite castings are fabricated using the ultrasonic stirring technology (UST). The 6061/A356 alloy and Al2O3/SiC nanoparticles are used as the matrix alloy and the reinforcement, respectively. Nanoparticles are injected into the molten metal and dispersed by ultrasonic cavitation and acoustic streaming. The applied UST parameters in the current experiments are used to validate a recently developed multiphase Computational Fluid Dynamics (CFD) model, which is used to model the nanoparticle dispersion during UST processing. The CFD model accounts for turbulent fluid flow, heat transfer and the complex interaction between the molten alloy and nanoparticles using the ANSYS Fluent Dense Discrete Phase Model (DDPM). The modeling study includes the effects of ultrasonic probe location and the initial location where the nanoparticles are injected into the molten alloy. The microstructure, mechanical behavior and mechanical properties of the nanocomposite castings have been also investigated in detail. The current experimental results show that the tensile strength and elongation of the as-cast nanocomposite samples (6061/A356 alloy reinforced by Al2O3 or SiC nanoparticles) are improved. The addition of the Al2O3 or SiC nanoparticles in 6061/A356 alloy matrix changes the fracture mechanism from brittle dominated to ductile dominated.
Experimental and theoretical investigation of ultrasonic cavitation processing of Al-based alloys and nanocomposites
(University of Alabama Libraries, 2018) Xuan, Yang; Nastac, Laurentiu; University of Alabama Tuscaloosa
Ultrasonic Treatment (UST) is one of the most promising manufacturing methods to refine the microstructure of casting alloys by transforming the morphology of the grains from dendritic to globular, decreasing the grain size, and modifying the precipitates. The applied temperature and/or temperature range during the ultrasonic and solidification processing are the key parameters that will influence the grain refinement. In this study, the effects of the temperature and/or temperature range applied during the ultrasonic and solidification processing on the microstructure and nano-particles distribution of the metal-matrix-nano-composites (MMNCs) have been investigated in detail. Aluminum alloy A356 and Al2O3/SiC nano-particles are used as the matrix alloy and the reinforcement, respectively. UST is applied during the solidification of the molten alloy. Experimental results indicated that the application of UST during solidification has positive effects on the microstructure of the as-cast ingots. Different UST application temperature/temperature range causes different refinement results. Moreover, the added nanoparticles refined the microstructure of the ingot section that is located adjacent to the immersed cylindrical face of the probe. Al-Si-Cu alloys have been widely used in the automotive industry. Fe-rich intermetallics are regarded as the most detrimental impurities that diminish the mechanical properties of alloys. In this study, the effect of ultrasound application temperature/temperature range on the pre-dendritic Fe-rich intermetallics (i.e, sludge) has been also investigated. Aluminum alloy A383 is used as the base alloy. Experimental results indicated that by applying UST on the melt highly influences the morphology and distribution of the precipitated Fe-rich intermetallics. Different UST application temperature/temperature range causes different modification and distribution results of the Fe-rich intermetallics. To create various temperature gradients in the laboratory scale ingot, an innovative two-zone furnace -ultrasound system has been set up in this study. A numerical model for simulation of the temperature-output power correlation that was validated by using experimental measurements has been built as well. The specific ultrasonic zone that will strongly affect the ingot microstructure has been identified and the ultrasonic attenuation coefficient of aluminum A356 melt has been determined.
The investigation of accumulative roll bonding for processing TI/Al multilayered composite targets for perforation testing
(University of Alabama Libraries, 2015) Stokes, Derrick D.; Acoff, Viola L.; University of Alabama Tuscaloosa
Multilayered composites (MLCs) processed using accumulative roll bonding (ARB) have great potential as candidates for perforation testing. In the current study, multilayered composites comprised of alternating layers of titanium and aluminum have been investigated. Since the ARB process has been shown to induce anisotropy, the Ti/Al MLCs were first subjected to quasi-static loading to determine the effects of anisotropy. The MLCs were then subjected to perforation testing using projectiles with various apex angles. The effects of perforation testing were studied in terms of varying ballistic parameters and characterization of the fracture surfaces of the MLCs. The results of this study show that ARB-processed Ti/Al MLCs are promising for use in ballistic and impact applications.
Investigations into the elevated temperature slip behavior of zirconium diboride
(University of Alabama Libraries, 2015) Hunter, Brett; Thompson, Gregory B.; University of Alabama Tuscaloosa
Ultra high temperature ceramics (UHTCs), which typically comprise carbides, nitrides, and borides, are a class of materials associated with high melting temperatures and high hardness. These materials offer a range of mechanical responses, from being very brittle to exhibiting significant plasticity as a function of composition and loading temperature. The purpose of this investigation is to characterize the slip mechanisms in ZrB2, where slip has been inferred but not definitively quantified. This work confirmed prior studies that dense dislocation arrays, with straight dislocation lines, exist under room temperature indents and demonstrates that such networks are highly localized to the load region. For elevated temperature deformation, ZrB2 has been reported to have a drop in flexural strength from 390 MPa at 1200 °C to 110 MPa at 1600 °C. Dynamical electron diffraction and image analysis confirmed basal, pyramidal, and prismatic slip which is rationalized by ZrB2’s hexagonal close packed c/a lattice parameter ratio of 1.11, which is less than the ideal ratio of 1.63. The dislocation densities prior to and after this flexural strength drop were 1.3 x 1013 m-2 and 1.0 x 1013 m-2, respectively. This indicates that the reduction in strength was not significantly associated with increased dislocation nucleation but rather stress relaxation.
Mathematical modeling of the fluid flow, multicomponent slag-metal reactions and desulfurization efficiency in gas-stirred ladles
(University of Alabama Libraries, 2018) Cao, Qing; Nastac, Laurentiu; University of Alabama Tuscaloosa
A three-dimensional, full-scale, transient computational fluid dynamics (CFD) model was developed to simulate the fluid flow, phase interfaces and slag-eye characteristics as well as the slag-metal reactions and desulfurization efficiency in the gas-stirred ladles. The volume of fluid (VOF) model, discrete phase model (DPM), and realizable k-ε model were applied to describe the argon/steel/slag/air multiphase evolution and the turbulent flow. The model was validated by comparing the simulation results with the experimental data based on a water model and an industrial gas-stirred ladle. The multiphase VOF model and the DPM-VOF model were applied to simulate the multiphase flow in the water model and gas-stirred ladle, respectively. The comparisons of the simulated and experimental results demonstrated that the DPM-VOF coupled model is more accurate for predicting the gas-liquid multiphase flow phenomena. The effects of the gas flow rate, bubble characteristics, slag layer thickness, slag viscosity and plug configurations on the flow characteristics and mixing phenomena in the ladles were also investigated. A CFD-reaction kinetics fully coupled model was developed and validated to predict the slag-metal reactions and desulfurization behavior in an industrial gas-stirred ladle. A quick modeling approach was developed by uncoupling the slag-metal reaction kinetics computations from the CFD simulation. It was shown that the computational time of this uncoupled predictive approach was decreased by at least 100 times for each case study in comparison with the CFD-reaction kinetics fully coupled model. The validated model was also applied to investigate the effects of the steel and slag compositions on the slag-metal reactions and on the desulfurization efficiency. Finally, a CFD modeling approach capable of predicting the transport and removal behavior of inclusions in a gas-stirred ladle was developed with considering a fluctuant top slag layer. The inclusions were tracked by DPM model, and the movement of individual inclusion was traced through computing their particle trajectories. The effects of the inclusion size, gas flow rates, and injected bubble diameters as well as various removal mechanisms including slag capture, bubble attachment, and ladle wall adhesion on the removal of inclusions were also investigated.
Microstructural analysis of TI-6Al-4V components made by electron beam additive manufacturing
(University of Alabama Libraries, 2017) Coleman, Rashadd; Acoff, Viola L.; University of Alabama Tuscaloosa
Electron Beam Additive Manufacturing (EBAM) is a relatively new additive manufacturing (AM) technology that uses a high-energy electron beam to melt and fuse powders to build full-density parts in a layer by layer fashion. EBAM can fabricate metallic components, particularly, of complex shapes, in an efficient and cost-effective manner compared to conventional manufacturing means. EBAM is an enabling technology for rapid manufacturing (RM) of metallic components, and thus, can efficiently integrate the design and manufacturing of aerospace components. However, EBAM for aerospace-related applications remain limited because the effect of the EBAM process on part characteristics is not fully understood. In this study, various techniques including microhardness, optical microscopy (OM), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and electron backscatter diffraction (EBSD) were used to characterize Ti-6Al-4V components processed using EBAM. The results were compared to Ti-6Al-4V components processed using conventional techniques. In this study it is shown that EBAM built Ti-64 components have increased hardness, elastic modulus, and yield strength compared to wrought Ti-6Al-4V. Further, it is also shown in this study that the horizontal build EBAM Ti-6Al-4V has increased hardness, elastic modulus, and yield strength compared to vertical build EBAM due to a preferential growth of the β phase.
Multi-scale modeling of spatial heterogeneity effect on the shear banding behaviors in metallic glasses
(University of Alabama Libraries, 2018) Wang, Neng; Li, Lin; University of Alabama Tuscaloosa
Stronger than steels but able to be shaped and molded like plastics, the mechanical properties of metallic glasses (MGs) are proven to be scientific interest and potential applications in industry. However, MGs suffer from negligible plasticity prior to catastrophic failure in the form of a single shear band at room temperature, which precludes their immediate application as structural components. The structural and property heterogeneity have been found recently in MG. These nanoscale heterogeneities may influence the initiation and propagation of the shear band, which would thus improve the plasticity. To understand the structural and property heterogeneity effect on the shear band behaviors, this thesis is divided into three sections. First, Activation-relaxation technique (ART) and dynamic atomic force microscopy (DAFM) are employed to prove the existence of the nanoscale heterogeneity. ART discovers that the activation energy possesses a normal distribution, reflecting the non-uniform local structure. This non-uniformity should correspond to the different motifs found in the molecular dynamics simulation. The energy dissipation resulted from the DAFM also exhibits a normal distribution, thus the inelastic spatial heterogeneity is confirmed. Furthermore, the correlation length of the inelasticity is identified based on the 2D scanning figures from DAFM. Second, a mesoscale modeling technique, shear transformation zone dynamics (STZD) will be employed. A series of configurations with the spatial elastic heterogeneity will be built up. We find that the organization of such nanometer-scale shear transformation events into shear-band patterns is dependent on the spatial heterogeneity of the local shear moduli. A critical spatial correlation length of elastic heterogeneity is identified for the simulated MGs to achieve the best tensile ductility, which is associated with a transition of shear-band formation mechanisms, from stress-dictated nucleation and growth to structure-dictated strain percolation, as well as a saturation of elastically soft sites participating in the plastic flow. Third, a state variable, excess free volume, is incorporated into STZD model in order to introduce the strain softening which is typical during MG deformation. The stress ‘overshoot’ and cyclic hardening of MGs have been successfully captured by the model. We found that it is the dynamic competition between free volume creation and annihilation that give rise to the signature stress overshoot in the homogeneous deformation regime at elevated temperatures during tension and cause the removal of large free volume sites in the confined deformation region during nanoindentation.
Numerical modeling of fluid flow and solidification phenomena during ultrasonic processing of metal-matrix-nanocomposites
(University of Alabama Libraries, 2016) Zhang, Daojie; Nastac, Laurentiu; University of Alabama Tuscaloosa
In present study, 6061 and A356 based nano-composites are fabricated by using the ultrasonic stirring technology (UST) in a coreless induction furnace. SiC nanoparticles are used as the reinforcement. Nanoparticles are added into the molten metal and then dispersed by ultrasonic cavitation and acoustic streaming assisted by electromagnetic stirring. The applied UST parameters in the current experiments are used to validate a recently developed magneto-hydro-dynamics (MHD) model, which is capable to model the cavitation and nanoparticle dispersion during UST processing. The MHD model accounts for turbulent fluid flow, heat transfer and solidification, and electromagnetic field, as well as the complex interaction between the nanoparticles and both the molten and solidified alloys by using ANSYS Maxwell and ANSYS Fluent. Molecular dynamics (MD) simulations are conducted to analyze the complex interactions between the nanoparticle and the liquid/solid interface. The current modeling results demonstrate that a strong flow can disperse the nanoparticles relatively well during molten metal and solidification processes. Molecular dynamics simulation results prove that ultrafine particles (<< 1 µm) will be engulfed by the solidification front instead of being pushed, which is beneficial for nano-dispersion. Experimental results confirm that the nanoparticles are dispersed reasonably well in the metal matrix, but some insignificant agglomeration still occurs. Besides, SEM/EDS results show that C element tends to gather around the grain boundary area where the Si eutectic phase is located.
Phase stability and oxidation behavior of al-ni-co-cr-fe based high-entropy alloys
(University of Alabama Libraries, 2016) Butler, Todd M.; Weaver, Mark Lovell; University of Alabama Tuscaloosa
In recent years, multi-component, high entropy alloys (HEAs) have been proposed as potential alternatives for high temperature structural materials and coatings due to their reportedly favorable combinations of high melting point, high strength, high ductility, and high resistance to oxidation and/or corrosion. HEAs are loosely defined as alloys containing five or more principal elements, each with a concentration between 5-35 at. %. This complex chemical arrangement has been reported to facilitate the formation of solid solution phases consisting of simple FCC and/or BCC crystal structures. Although their potential applications are vast, a fundamental understanding of their high-temperature phase stabilities and oxidation mechanisms, along with effective models to predict their behaviors is deficient. To aid in this gap of knowledge, this dissertation work systematically investigates the phase equilibria and oxidation behaviors of a series of transition metal based HEAs. The phase stability and oxidation studies will encompass both as-melted and annealed HEAs. To critically assess the merit and usefulness of existing thermodynamic databases for predicting complex phase equilibria, the experimental observations will be directly compared with predictions based on the CALPHAD method using ThermoCalcTM. The modeling simulations are applied to both the phase stabilities and the relative oxidation behaviors. The active oxidation mechanisms will also be addressed relative to existing oxide formation models for predicting the oxide growth in alloys with similar elements.
Processing and deformation studies in high temperature ceramics
(University of Alabama Libraries, 2018) Vinson, Katherine; Thompson, Gregory B.; University of Alabama Tuscaloosa
Ceramics are high hardness materials with high melting temperatures, above 2000 C. As a consequence of those unique properties, they have found a niche in several extreme-environment applications. Their properties also present challenges in fabricating fully dense parts and understanding the underlying mechanisms that governing plasticity in the intended operating temperature range. Here, each of these topics is addressed using a range of case study materials. Specifically, this research will describe the fabrication of small-diameter C and SiC fibers for ceramic matrix composites. These fibers are derived using a hyperbaric pressure-laser chemical vapor deposition (HP-LCVD) method in which a laser is used to deposit a fiber directly from the vapor phase. Then, the topic of HP-LCVD will be explored further by a study in the phase and microstructure stability of tetramethylsilane derived fibers under an array of process conditions. Similarly, this dissertation will address the complications of incorporating SiC into HfB2 coatings by vacuum plasma spraying (VPS) and its effect on phase stability. Finally, the dissertation explores the role of intrinsic stacking faults in governing deformation mechanisms in HfN. The collective work reveals the link of structure in controlling either phase stability or mechanical deformation in these specific ceramics and their processing routes.
Processing-microstructure relationships in Al-Cu alloys produced by the cold spray deposition process
(University of Alabama Libraries, 2018) Liu, Tian; Brewer, Luke N.; University of Alabama Tuscaloosa
This dissertation investigates the processing-microstructure relationships during cold spray deposition of binary aluminum-copper alloys. While cold spray has been successfully applied using a variety of engineering alloys, systematic investigations that relate powder particle microstructure to spray deposition characteristics and deposited material microstructure have not yet been performed. In this dissertation, the Al-Cu binary alloy will be used as a model system due to its well-known attributes in the physical metallurgy literature and its technological importance as the parent alloys for alloys such as AA2014, AA2024, and AA2219. First, the effect of systematic copper alloy additions (2-5wt% Cu) on the microstructure and properties of the gas atomized, feedstock powders was investigated using electron microscopy, X-ray diffraction, and nanoindentation. Second, the microstructural evolution and texture development during the cold spray process was studied using electron microscopy, and electron backscatter diffraction (EBSD). Third, the impact deformation of individual cold sprayed particles was quantified, and the deformation microstructure in the deposited particles was characterized using focused ion beam, transmission Kikuchi diffraction (TKD), and precession electron diffraction (PED). Lastly, the influence of heat treatment of the feedstock powders on the spray characteristics and microstructure of the cold sprayed material was examined using electron microscopy, and EBSD.

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Browsing Theses and Dissertations - Department of Metallurgical and Materials Engineering by Author "Allison, Paul Galon"

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