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.
Correlation between heat input and residual stresses from friction stir welding of AA5052 plate
(University of Alabama Libraries, 2018) Zhu, Ning; Brewer, Luke N.; University of Alabama Tuscaloosa
This thesis investigates the connections between friction stir welding (FSW) parameters, simple energy/heat input metrics, and the resultant residual stresses on AA5052-H32 plates. A range of weldments were produced with different tool rotational and traverse speeds to produce the same values of the pseudo heat index (PHI). Average residual stresses inside the stir zone and peak residual stress in the thermo-mechanically affected zone were systematically recorded using laboratory x-ray diffraction. In addition, thermal cycles on the advancing side of the welds were collected and analyzed for comparison with the predictions of heat input based upon FSW parameters. Based upon these results, the PHI is not a good predictor of the peak residual stress for welding conditions which produced sound welds. Increasing traverse speed, V, with fixed rotational speed does increase the residual stresses inside the stir zone. The data in this thesis suggests that there is a complex relationship between frictional heating and mechanical stirring of the material. As a result, there is a rotational speed, which requires minimum torque during welding.
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 influence of solute additions on intrinsic stress in thin films
(University of Alabama Libraries, 2018) Kaub, Tyler; Thompson, Gregory B.; University of Alabama Tuscaloosa
During physical vapor deposition of thin films, strong intrinsic stresses develop during film growth. These stresses along with the film microstructure are typically controlled through altering the film’s processing conditions. A possible technique to predict how processing influences film stress uses a proposed kinetic model for thin film stress evolution, where contributions to the total stress of the film from individual mechanisms can be elucidated. To determine the application space of the model a series of Cu films was sputter deposited under various deposition conditions to fit the intrinsic stress measurements to the model. Pressure and growth rate stress dependence emerged, which fit well with the model predictions. Expanding upon elemental Cu the influence of a solute additions on the intrinsic stress evolution was examined. Utilizing the Cu(Ni) system as the first case study revealed small Ni additions of 5 at. % resulted in increased compressive stress with further Ni additions showing a reduction in the compressive stress. Noting the stress dependence on solute additions two strongly segregating systems Cu(Ag) and Cu(V) were deposited under similar processing parameters to explore the effect of solute mobility on stress. The addition of Ag, the high atomic mobility solute, or V the low atomic mobility solute, both resulted in the alloy films undergoing grain refinement that scaled with solute content. This grain refinement was attributed to solute segregation and was associated with increased tensile stresses in both systems. Noting the role of solutes on stress in these Cu based alloy systems, further study in another alloy system was conducted to determine if similar mechanisms are present. Using W(Ti) the addition of Ti was observed to reduce the compressive stress of W. Upon examination of the microstructure, Ti additions did not alter the film’s grain size, but increased the fraction of low angle grain boundaries. Collectively, these studies demonstrate solute additions can be used to control the residual stresses, specific grain boundary formations, grain sizes and phase transformations in thin films. This indicates that solutes can be used as another processing tool to tune a thin film to the desired microstructure and stress state.
Influence of solute segregation on stress and structure evolution in nanocrystalline thin films
(University of Alabama Libraries, 2018) Zhou, Xuyang; Thompson, Gregory B.; University of Alabama Tuscaloosa
Alloy thin films offer several tantalizing opportunities to engineer the growth and subsequent stress conditions in thin films. This can be achieved by the mixed interaction of two or more species. For example, grain size is a known quantity in regulating the stress and is most often controlled through processing via pressure, temperature, deposition rate, or a combination of each of these variables. In an alloy, where one species is highly segregating, the solute can also control the grain size and therefore the stress. Based on this hypothesis, this work has elucidated how alloying effects in the residual growth stresses for a series of W-Cr, W-Fe, and Fe-Cr thin films. In addition, a molecular dynamics (MD) model for the growth of elemental W and Fe films was constructed to understand how different microstructures and processing variables control stress.
Investigation of high-performance lithium-ion batteries based on highly conductive Li7La3Zr2O12 solid-state electrolyte and stable electrode-electrolyte interface
(University of Alabama Libraries, 2021) Li, Junhao; Wang, Ruigang; University of Alabama Tuscaloosa
With the merits of high Li+ conductivity, wide potential window, and electrochemical stability against metallic lithium anode (the highest theoretical capacity: 3,860 mAh g−1), cubic phase garnet-type Li7La3Zr2O12 (LLZO) solid-state electrolyte has attracted much attention for developing solid-state batteries with increased safety, higher energy density, and longer lifespan. Besides, the solid-state or liquid electrolyte/electrode interface stability and low resistance are important to their optimized electrochemical performance of lithium-ion batteries. The goal of this dissertation is to develop high-performance lithium-ion batteries based on LLZO solid-state electrolyte and stable/low resistance electrode-electrolyte interface via (1) low-temperature synthesis/densification of Al/Bi-doped cubic LLZO electrolytes, (2) surface modification of LiNi1/3Co1/3Mn1/3O2 cathode particles, (3) composite polymer electrolytes, and (4) application of plastic-crystal interfacial modification.Chapter 3 explores a low-temperature synthesis strategy to obtain cubic LLZO powders via a combination of sol-gel method and ball milling induced tetragonal to cubic phase transition, which is ~200 °C lower than the thermally induced phase transition temperature. Chapter 4 investigates the role of a facial B2O3 surface modification of LiNi1/3Co1/3Mn1/3O2 cathode particles to achieve a stable cathode-electrolyte interface, which enables improved high-rate discharge performance and enhanced cycling stability of the batteries. Chapter 5 reveals the effects of LLZO ceramic filler distribution and doping elements (Al and Bi) on the ambient-temperature ionic conductivity, Li+ transference number, electrochemical stability window, and ability to suppress lithium dendrite growth of poly(vinylidene fluoride) based composite polymer electrolytes, as well as solid-state battery performance based on these composite polymer electrolytes. In Chapter 6, cubic Bi-doped LLZO ceramic pellets with a high relative density (>90%) and ionic conductivity (~1.32×10^(-4) S cm-1 at 20 °C) were achieved with a sintering temperature as low as 900 °C. A succinonitrile-based plastic-crystal interlayer at the Li/LLZO interface was demonstrated to be very effective to reduce interfacial resistance and enable stable cycling of a Li/LLZO/Li symmetric cell. With the help of the plastic-crystal interlayer and a composite cathode, a Li/LLZO/LiCoO2 all-solid-state battery was fabricated, which displayed a stable cycling at 0.1C for 40 times at 20 °C with a discharge capacity of ~115 mAh g-1 and a Coulombic efficiency of ~99%.
Investigation of influencing factors in liquid metal embrittlement of advanced high strength steel
(University of Alabama Libraries, 2019) Massie, Daniel Joseph Woodson; Brewer, Luke N.; University of Alabama Tuscaloosa
This thesis explored the influence of temperature, steel type, galvanization method, and macro-strain level on the sensitivity of advanced high strength steels (AHSS) to zinc-based liquid metal embrittlement (LME). It is critical to understand the influencing factors of LME because zinc coatings are commonly used to protect steel parts from corrosion, and the use of advanced high strength steel in the automotive industry is increasing. Electro-galvanized and zinc free samples of a transformation induced plasticity steel, TBF1180, and a complex phase steel, CP1200, were studied to examine the sensitivity of each to LME. Hot-dip galvanized samples of CP1200 were examined alongside the electro-galvanized samples to investigate the effect of coating method on the LME effect. Hot tension tests were performed and ductility trough graphs were created for all samples to examine the effect of these factors on LME during fracture. Additionally, small-strain tensile tests were designed and performed on the steels to examine LME crack nucleation. From the results it was determined that LME response is temperature and steel dependent. It was shown that TBF 1180 nucleated LME cracks at 600 °C while CP1200 did not. It was also determined that hot-dip galvanized coatings more readily nucleate LME cracks than electro-galvanized coatings. Finally, these results suggest that macro-plastic deformation may not be required to initiate an LME response.
Investigation of liquid metal embrittlment in advanced high strength steels
(University of Alabama Libraries, 2018) Briant, Nathaniel P.; Brewer, Luke N.; University of Alabama Tuscaloosa
This thesis explored the susceptibility of ferritic/bainitic, advanced high strength steels to zinc-based liquid metal embrittlement (LME). The understanding of the causes and effects of LME is critical because of the common use of zinc coatings for corrosion protection and an increased used of advanced high strength steels in industries such as automotive. Transformation induced plasticity steels, TBF 1180 and TRIP 700, and a complex phase steel, CP 1200, were studied to assess their susceptibility to LME during hot tensile tests. Comparisons between stress versus strain curves collected with and without zinc coatings at various temperatures and strain rates were used to determine the effect of the zinc-based embrittlement on the mechanical properties of these steels. The stress versus strain curves were collected using uniaxial hot tensile tests in a Gleeble 1500D thermal-mechanical simulator from Dynamic Systems Inc. The fracture modes were studied using scanning electron microscopy and energy-dispersive X-ray spectrometry (EDS). From the results it was determined that all three steels demonstrated LME at temperatures between 800 C and 900 C and nominal strain rates of 1.3 and 0.13 strain per second. The fracture mode for all LME cracks was found to be intergranular.
Oxidation behavior of refractory complex concentrated alloys: computational and experimental studies
(University of Alabama Libraries, 2019) Hunter, Brett Matthew; Weaver, Mark L.; University of Alabama Tuscaloosa
In recent years, high entropy alloys (HEAs) and, more specifically, refractory complex concentrated alloys (RCCAs) have been of increased interest due to their potential as replacements in high temperature environments. This dissertation work has systematically investigated the phase equilibria and oxidation resistance of an alloy system with the basis AlHfNbTiZr. The three alloys investigated in the five-component system were all found to contain a single phase microstructure composed of B2 while the seven component alloy was comprised of a B2 and C14 Laves phase. While the oxidation behavior was parabolic for all alloys studied, the five component alloys exhibited 2-stage parabolic behavior compared to a single stage in the seven-component alloy. The oxidation behavior was governed by a combination of thermodynamics and kinetics with regards to diffusion of oxygen into the system. The five component alloys were found to form an external scale comprised of a ZrTiO4-based structure containing all five elements in their respective stoichiometry. Internal oxidation also occurred in these alloys and exhibited a relationship between diffusion of cations out of the alloy and diffusion of anions into the alloy. However, the seven-component alloy did not form an external scale and was governed completely by oxygen diffusion into the alloy and thermodynamic factors as to the composition of the scale. This work has furthered the fundamental understanding of the oxidation behavior of RCCAs and the accuracy of modelling on the phase equilibria of these alloys.
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.
Phase transformations during cooling of automotive steels
(University of Alabama Libraries, 2017) Padgett, Matthew Chase; Brewer, Luke N.; University of Alabama Tuscaloosa
This thesis explores the effect of cooling rate on the microstructure and phases in advanced high strength steels (AHSS). In the manufacturing of automobiles, the primary joining mechanism for steel is resistance spot welding (RSW), a process that produces a high heat input and rapid cooling in the welded metal. The effect of RSW on the microstructure of these material systems is critical to understanding their mechanical properties. A dual phase steel, DP-600, and a transformation induced plasticity bainitic-ferritic steel, TBF-1180, were studied to assess the changes to their microstructure that take place in controlled cooling environments and in uncontrolled cooling environments, i.e. resistance spot welding. Continuous cooling transformation (CCT) diagrams were developed using strip specimens of DP-600 and TBF-1180 to determine the phase transformations that occur as a function of cooling rate. The resulting phases were determined using a thermal-mechanical simulator and dilatometry, combined with light optical microscopy and hardness measurements. The resulting phases were compared with RSW specimens where cooling rate was controlled by varying the welding time for two-plate welds. Comparisons were drawn between experimental welds of DP-600 and simulations performed using a commercial welding software. The type and quantity of phases present after RSW were examined using a variety of techniques, including light optical microscopy using several etchants, hardness measurements, and x-ray diffraction (XRD).
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.
Processing-microstructure-property relations in high pressure cold spray of AA2024 and AA7075
(University of Alabama Libraries, 2018) Story, William Andrew; Brewer, Luke N.; University of Alabama Tuscaloosa
In this dissertation work, the processing-microstructure-property relations are examined for cold spray deposited AA2024 and AA7075. Unlike traditional thermal spray technologies, powders are never melted during the cold spray process. This approach allows for heat-sensitive materials such as 2000 and 7000 series aluminum alloys to be used for repair of components damaged by corrosion or fatigue. While a small body of literature for cold spray of AA7075 and AA2024 exists, further understanding on processing-microstructure relationships is needed and improvements to deposition characteristics, microstructure and mechanical properties are necessary for successful application. This dissertation examines four aspects of the cold spray process applied to AA2024 and AA7075. First the effects of processing parameters on process efficiency, deposit microstructure, and deposit properties of AA2024 and AA7075 are examined. Spraying with helium is found to be more cost effective than nitrogen or any mixture of nitrogen and helium and generally results in higher quality deposits. A novel heat treatment method is developed to solution heat treat gas-atomized AA2024 and AA7075 powders prior to deposition. This approach is found to significantly improve deposition efficiency of powders and homogenize the deposit microstructure by solutionization of segregated solute. The influence of the deposit geometry on the development of residual stresses are investigated for AA2024 and AA7075 using neutron diffraction. Generally compressive residual stresses are found in deposits and deposit geometry is found to significantly effect to magnitude of residual stresses evolved. Finally, the effects of heating the substrate/deposit using an in situ laser during deposition are assessed. Laser assisted cold spray is found to improve deposition efficiency and improving ductility of AA7075 deposits but can result in significant heat damage to the substrate material.
Theoretical and experimental studies of dissimilar secondary metallurgy methods for improving steel cleanliness
(University of Alabama Libraries, 2017) Pitts-Baggett, April Danielle; Nastac, Laurentiu; University of Alabama Tuscaloosa
Due to a continual increasing industry demand for clean steels, a multi-depth sampling approach was developed to gain a more detailed depiction of the reactions occurring in the ladle throughout the Ladle Metallurgy Furnace (LMF) processing. This sampling technique allows for the ability for samples to be reached at depths, which have not been able to be captured before, of approximately 1.5 m below the slag layer. These samples were also taken in conjunction with samples taken just under the slag layer as well as in between those samples. Additional samples were also taken during the processing including multi-point slag sampling. The heats were divided in to five key processing steps: Start of heat (S), after Alloying (A), after desulfurization/start of pre-Rinse (R), prior to Ca treatment (C), and End of heat (E). Sampling sets were collected to compare the effects of silicon, desulfurization rates, slag emulsification, slag evolution and inclusion evolution. By gaining the ability to gather multiple depths, it was determined that the slag emulsification has the ability to follow the flow pattern of the ladle deeper into the ladle than previously seen in literature. Inclusion evolution has been shown by numerous researchers; however, this study showed differences in the inclusion grouping and distribution at the different depths of the ladle through Automated Feature Analysis (AFA). Also, the inclusion path was seen to change depending on both the silicon content and the sulfur content of the steel. This method was applied to develop a desulfurization model at Nucor Steel Tuscaloosa, Inc. (NSTI). In addition to a desulfurization model, a calcium (Ca) model was also developed. The Ca model was applied to target a finished inclusion region based on the conditions up to the wire treatment. These conditions included time, silicon content, and sulfur concentration. Due to the inability of this model to handle every process variable, a new procedure was created to provide a real time feedback via SparkDat © software installed in a ThermoFisher 4460 spectrometer.

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