MS, University of Cincinnati, 2024, Engineering and Applied Science: Materials Science

Refractory complex concentrated alloys (RCCAs) are known for their high-temperature mechanical properties, but less attention has been given to their low-temperature thermophysical behavior. High-throughput calculation of phase diagrams (CALPHAD) was used to study the body centered cubic (BCC) phase dominance within the Nbx(MoTi2V4)100-x alloy system by adding Niobium (Nb) at atomic concentrations of x=9%,23%,and 37%, and two additional alloys where vanadium (V) is substituted with hafnium (Hf) and zirconium (Zr) to form Hf10Mo24Nb38Ti28 and Mo24Nb38Ti28Zr10 alloys. Alloys were tested to see the effect of Nb content on the superconducting transition temperature (TC) and the effect of substituting V with Hf or Zr on TC. This approach tests the common assumption of the correlation between VEC with TC, to clarify the role of composition relative to VEC. Button specimens were cast via vacuum arc melting and were measured for electrical resistivity at different temperatures using a Quantum Design DynaCool system. X-ray diffraction (XRD) was used to confirm the formation of the phases predicted by CALPHAD. Five out of the six alloy samples exhibited superconductivity, with some anomalies observed and addressed in this study. These results are compared to literature to enhance understanding of the results.

Committee: Eric Payton Ph.D. (Committee Chair); Matthew Steiner Ph.D. (Committee Member); Sarah Watzman Ph.D. (Committee Member) Subjects: Materials Science

MS, University of Cincinnati, 2024, Engineering and Applied Science: Materials Science

Refractory high entropy alloys (RHEAs) have received significant attention for their remarkable strength at very high temperatures. However, even for ductile compositions, conventional thermomechanical processing remains extremely challenging due to the very same elevated temperature strength properties that make this family of alloys interesting for extreme environmental applications. Many researchers have proposed additive manufacturing as a potential solution for fabrication of components as a possible route for taking advantage of RHEA properties. This research thesis investigates screening metrics through which the additive manufacturability of RHEAs can be assessed and their hot crack susceptibility. Crack susceptibility criteria are calculated using parameters derived from a calculation of phase diagrams (CALPHAD) approach, using commercially available databases. Solidification predictions are used to predict possible phases, alloy compositions, and freezing ranges for a database of alloys published in the literature. It is found that different castability metrics can differ substantially from one another. The computational predictions are then assessed using laser glazing at two laser powers and eight laser scan speeds for arc-melted alloy pairs at opposite extremes of the solidification crack susceptibility and castability metrics. Cracking susceptibility differences and melt pool behavior are quantified and compared for key alloy pairs, and the correlation between crack propagation and microstructure are analyzed using scanning electron microscopy, and energy dispersive spectroscopy. An assessment of prospects for additive manufacturing processability prediction for refractory high entropy alloys will be presented, along with observations relevant to alloy design strategies for solidification crack tolerant complex compositions of refractory alloys.

Committee: Eric Payton Ph.D. (Committee Chair); Matthew Steiner Ph.D. (Committee Member); Ashley Paz y Puente Ph.D. (Committee Member) Subjects: Materials Science

Doctor of Philosophy, The Ohio State University, 2024, Welding Engineering

Improved die materials and repair procedures are needed to improve costs in high pressure die casting. Conventioanl die materials center around hot work tool steels which make up the bulk of high pressure die casting dies, but these materials accumulate damage rapidly in aggressive environments. Refractory based alloys provide superior die casting performance, but processing difficulties and high cost limit applications to small die regions / inserts. Direct cladding of die steels with refractory alloys has historically been limited by intermetallic formation and a lack of wire consumables for high deposition rate automated processes. A new refractory based tungsten heavy alloy known as Anviloy Wire was recently developed for use as a die hardfacing consumable, but welding metallurgy and die casting performance were unknown. To implement Anviloy wire cladding technology, this study investigated the arc weldability of Anviloy wire clads, the effect of H13 tool steel dilution on clad microstructure and thermal stability, feasibility of temperbead repair of H13 dies, effect of molten aluminum exposure on clad surface microstructure, and the effect of rapid solidification on Anviloy wire clad microstructure. Arc weldability was investigated using hot wire gas tungsten arc welding (HW-GTAW), pulsed gas metal arc welding (GMAW-P), and reciprocating wire feed gas metal arc welding (RWF-GMAW) processes using bead on plate experiments where weld quality and dilution minimization were optimized. A cladding procedure was developed to prepare an Anviloy wire clad H13 shot block for in-plant trial service evaluation using RWF-GMAW based on high weld quality and low dilution. Effect of clad dilution levels on microstructure and thermal stability was analyzed using arc-crucible melted samples aged isothermally at 600 and 725°C. Experiments involving static immersion of Anviloy wire calds and H13 in molten A380 aluminum were undertaken to better understand chemical soldering mechan (open full item for complete abstract)

Committee: Dennis Harwig (Advisor); Boian Alexandrov (Committee Member); Antonio Ramirez (Committee Member) Subjects: Materials Science

Doctor of Philosophy, The Ohio State University, 2023, Welding Engineering

Duplex stainless steels (DSS) are extensively used in heavy industry, such as Oil and gas, pulp and paper, and chemical, due to their remarkable corrosion resistance, yield strength, and toughness. The most corrosion-resistant DSS subgroups, super duplex stainless steels (SDSS) with Pitting Resistance Equivalent numbers (PREn) of 40-48, and the hyper duplex stainless steels (HDSS) with a PREn over 48, are highly alloyed. Additions of Cr and Mo provide better PREn but also promote intermetallic phases such as the chi and sigma phases. These intermetallics form when the material is exposed to temperatures between 600oC – 1100oC. It is known that even small volumetric fractions of the sigma phase severely reduce the material's corrosion resistance and mechanical performance. A dedicated study on sigma phase formation kinetics was developed to control sigma phase presence in these specific alloys. A field studied but not yet completely connected between scientific research and industrial applications. Fundamental aspects of sigma phase kinetics were analyzed, computationally modeled, and experimentally validated. As a result of these efforts, the interface area per unit of volume was revealed as a critical microstructure factor for the sigma phase kinetics. The resultant model's efficacy was further evaluated by building GTAW cladded mockups, and investigation into this material's mechanical and corrosion performance further expanded on the impacts of the sigma phase. A Gleeble® system was used to develop experimental time temperature transformation (TTT) maps on SDSS and HDSS filler metals for sigma phase precipitation kinetics. Classical nucleation theory was then implemented on the CALPHAD-based kinetics model. In this model, the interfacial energy and nucleation sites were identified as the kinetics parameters to adjust the model based on experimental data. The sigma phase kinetics continuous cooling transformation CCT curves were calculated using the additiv (open full item for complete abstract)

Committee: Antonio Ramirez (Advisor); Stephen Niezgoda (Committee Member); Carolin Fink (Committee Member) Subjects: Engineering; Materials Science; Mechanical Engineering

Doctor of Philosophy, Case Western Reserve University, 2022, Materials Science and Engineering

Nickel-based superalloys are strategically important materials for all high-value industries that support a sustainable safe society, amounting to an annual $7 billion global market. They are uniquely qualified to operate at intermediate temperatures (<900°C) and in corrosive environments, making them a choice material for turbines for propulsion and energy generation, as well as pressure vessels for reactors that manufacture radioactive therapies for cancer treatments. Their performance hinges on the careful design of processing routes that govern microstructure features; primarily nanoscale precipitates. The exploitation of processing-structure-property (PSP) models to tailor materials for device performance represent the heart of materials science and engineering. The development of the quantitative PSP model requires the collection of disparate datasets that involves standard experiments (data-driven methods) or physics-based computational methods that simulate required output data. Data-driven approaches involve an immense amount of time and cost-consuming data collection processes. Physics-based computational simulations are not capable of simulating property metrics and are also required to be experimentally calibrated due to their less fidelity. Because of these reasons, both approaches are challenging and motivate an enormous demand for accelerated, high-throughput, approaches. Further, in previous studies, both approaches (data-driven and simulations) functioned as two separate branches and were not integrated. Materials informatics, enabled by high-throughput approaches could overcome these challenges and corporate the data-driven and computational simulations that allow engineers to quantitatively measure the critical microstructure properties and design the processing routes to improve the material performance of Ni-based superalloys. This approach effectively provides a value-added proposition to be better stewards of the manufacturing digital thread (open full item for complete abstract)

Committee: Jennifer Carter (Committee Chair); Roger French (Committee Member); Francis Merat (Committee Member); John Lewandowski (Committee Member) Subjects: Engineering; Materials Science

Doctor of Philosophy, The Ohio State University, 2022, Materials Science and Engineering

In the last 20 years, research in magnesium alloys has greatly expanded with demand for high-strength lightweight alloys in the transportation industry. Mg-RE (rare earth) alloys have been of particular interest due to the formation of two strengthening phase types: long period stacking order (LPSO) phases and β-series precipitates. This work focuses on the development of high-strength cast Mg-RE multicomponent alloys that combine LPSO and β-series phases using a CALPHAD (CALculation of PHAse Diagrams)-based design approach. This work began by using CALPHAD modeling to study the effects of maximizing the LPSO phase fractions. Experimental samples demonstrated there was a slight increase in mechanical properties with high LPSO volume fractions, but the properties were below those obtained through β' precipitation in the commercial alloy WE43 (Mg-4Y-3.4RE-0.7Zr, all in wt%). It was also found that the CALPHAD model was underpredicting the LPSO phase fractions by ~20 vol%. Improvements were made to the Pandat database to bring the predictions within ~5 vol% of experimental values. In the second stage of this work, small-angle scattering (SAS) was used to quantitatively explore the effects of micro-alloying in the Mg-Nd system on β-series precipitates. Two SAS techniques were used in addition to transmission electron microscopy (TEM) to study the effects of micro-alloying: small-angle neutron scattering (SANS) and small-angle x-ray scattering (SAXS). It was found SAXS was a better technique to quantify the change in precipitate size with micro-alloying and aging, but more understanding of the system is needed to extract phase fraction changes. In the final stage of this work, the LPSO and β-series strengthening mechanisms were combined in an attempt to produce an Mg-Y-Nd-Zn-Zr alloy with properties superior to WE43. Nd does not form any LPSO phase, so there is less competition between the phases during formation. CALPHAD modeling is used to tailor the phase fracti (open full item for complete abstract)

Committee: Alan Luo (Advisor); Steve Niezgoda (Committee Member); Jenifer Locke (Committee Member) Subjects: Engineering; Materials Science

Doctor of Philosophy, The Ohio State University, 2022, Materials Science and Engineering

Since its discovery on early 2000's, Multi-principal element alloys (MPEAs) or High Entropy Alloys (HEAs) has gained a lot of research interest owing to the extensive opportunities for new materials discovery. MPEAs contain multiple elements at relatively high concentration. While the original efforts in MPEA development were focused on designing single-phase solid solutions, recent works are directed towards multi-phase MPEAs. The multi-phase MPEAs have precipitation hardening as an additional strengthening mechanism making it suitable for high temperature structural applications. Phase separation mechanism in many MPEA systems such as AlMo0.5NbTa0.5TiZr, Al0.5NbTa0.8Ti1.5V0.2Zr, and AlxCoCrCuFeNi are believed to occur through spinodal-mediated pathways. Understanding spinodal decomposition in multi-component alloys and the possible phase transformation pathways are crucial for the microstructural design of MPEAs. Firstly, a methodology was developed to calculate the spinodal driving force and initial concentration modulation in multicomponent alloys. Secondly, a phase-field model (PFM) with two order parameters was formulated to simulate the microstructural evolution for various free energy surfaces. Our PFM incorporates spinodal decomposition, order↔disorder transition, nucleation and growth, modulus anisotropy, modulus inhomogeneity, lattice misfit, and the effect of asymmetry in the free energy curves of the individual phases. Lastly, a software module (called PanPhasefield) was developed for simulating the microstructural evolution in MPEAs and other commercial alloys using the CALPHAD databases. The module was successfully commercialized and is available in the latest version of PandatTM, a CALPHAD software (https://computherm.com/).

Committee: Yunzhi Wang (Advisor); Hamish Fraser (Committee Member); Stephen Niezgoda (Committee Member) Subjects: Condensed Matter Physics; Materials Science

Doctor of Philosophy, The Ohio State University, 2021, Materials Science and Engineering

The primary solidification mode of austenitic stainless steel weld metals strongly dictates solidification cracking susceptibility. Provided that primary solidification mode selection is highly dependent on chemical composition, predictive tools such as the WRC-1992 diagram are often used to assess risk and/or design around potential solidification cracking concerns. In recent years, solidification simulations are becoming more commonplace with the advent and ever-growing adoption of CALPHAD methodologies, providing an additional avenue to predict primary solidification mode in austenitic stainless steels. In this work, high-throughput computational thermodynamic calculations have been used to develop a diagram to predict primary solidification mode for austenitic stainless steel weld metals. By simulating the stable and metastable liquidus temperatures for randomly generated austenitic stainless steel chemistries, a new set of nickel and chromium equivalency relationships have been developed that provide a sharp delineation between primary austenite and primary ferrite solidification modes under equilibrium conditions. Comparisons between legacy experimental data and computational thermodynamic calculations suggest that undercooling at the solid-liquid interface promotes metastable primary austenite solidification in stainless steel chemistries that fall near the equilibrium austenite-ferrite transition during conventional arc welding solidification conditions. Multicomponent dendrite growth theory has also been applied to help rationalize the occurrence of metastable primary austenite solidification. Using this information, a correction scheme has been established to modify the new primary solidification mode diagram to account for dendrite growth kinetics. A series of controlled gas tungsten arc spot welds have been performed on various arc-cast alloy chemistries that fall near the austenite-ferrite transition to assist with validation of the new primar (open full item for complete abstract)

Committee: Antonio Ramirez (Advisor); Jeffrey Sowards (Committee Member); Alan Luo (Committee Member); Jenifer Locke (Advisor) Subjects: Engineering; Materials Science; Metallurgy

Doctor of Philosophy, The Ohio State University, 2021, Materials Science and Engineering

The introduction of additional phases is one of the most effective ways to improve the mechanical strength of an alloy. However, such modifications result in electrochemical heterogeneities that lead to a degradation in the overall corrosion resistance of the microstructure. As the demand for alloys that can withstand harsher environments increases, a different approach towards design of alloys that are mechanical strong as well as corrosion resistant is called for. In recent years, High Entropy Alloys (HEAs), and more broadly Multi-Principal Element Alloys (MPEAs) have become a popular topic of research in the field of Materials Science and Engineering. MPEAs can be considered as a new class of materials where constituents are present in equimolar or near-equimolar concentrations. Alloys designed with this concept have been reported to possess unique properties with respect to mechanical strength as well as corrosion behavior, even those that contain multiple phases. The vast composition space opened up with the MPEA concept provides opportunities to design alloys that have multiple phases in their microstructure while being corrosion resistant at the same time. In this study, we explore the possibility of creating multi-phase MPEAs that are hard while being as corrosion resistant as their single-phase counterparts. In the first part of the study, a single-phase non-equimolar MPEA containing 13 at.% Ru (MPEA1) was annealed at 800 °C for 160 h to allow precipitation of secondary phases. After the heat treatment, the hardness of the alloy increases from 259 HV100 to 420 HV100 owing to the precipitation of hard plate-like σ-phase rich in Cr, Mo, Ru and W. Like the single-phase solutionized form of the alloy, the heat-treated form of the alloy was found to be immune to localized corrosion at ambient temperatures and did not pit below 80 °C. These results show that multi-phase alloys that are hard as well as corrosion-resistant are possible. The second part of the (open full item for complete abstract)

Committee: Gerald Frankel (Advisor); Christopher Taylor (Committee Member); Narasi Sridhar (Committee Member) Subjects: Materials Science; Metallurgy

Doctor of Philosophy, The Ohio State University, 2021, Materials Science and Engineering

For decades, traditional alloys always consist of one principal element and various number of alloying elements. Recently the introduction of high entropy alloys (HEAs) draw people's attention to the center region of the multicomponent spaces which was never explored, and created new opportunities for designing new alloys. However, the enormous number of potential combinations of elements poses a daunting challenge to the traditional trial-and-error alloy development approach. Thankfully, in recent years the progress in CALculation of PHAse Diagram (CALPHAD) method through the development of models to represent thermodynamic properties for various phases and optimization of thermodynamic and kinetic databases which permit prediction of phase equilibrium in multicomponent systems from those of binary and ternary subsystems. Two HEA systems, Al-Cr-Ti-V and Cu-Fe-Mn-Ni, are studied in this research. CuFeMnNi-based HEAs have a single FCC matrix after solution heat treatment. Its microstructure and mechanical properties after thermomechanical processing are investigated. AlCrTiV-based HEAs have a BCC matrix, which go through an order-disorder transformation at intermediate temperature. Due to the high hardness and high elastic modulus of AlCrTiV alloy, the feasibility of using AlCrTiV particles as the reinforcement phase in Al matrix composites are explored. In addition, a CALPHAD-based program was developed using Thermo-Calc Software Development Kits (SDKs) to perform automated composition screening based on phase constituents.

Committee: Alan Luo (Advisor); Stephen Niezgoda (Committee Member); Glenn Daehn (Committee Member) Subjects: Materials Science

Doctor of Philosophy, The Ohio State University, 2020, Materials Science and Engineering

In the past several decades, there has been an unprecedented demand for the discovery and design of new materials to support rapidly advancing technology. This demand has fueled a push for Integrated Computational Materials Engineering (ICME), an engineering approach whereby model linkages as well as experimental and computational integration are exploited in order to efficiently explore materials processing-to-performance relationships. Tailored simulations allow for the reduction of expensive and lengthy experiments, emphasizing the need to establish a statistical confidence in component designs and manufacturing processes from the simulations, rather than experiments, in a principled way. Since many materials models and simulations are deterministic in nature, the use of sophisticated tools and techniques are required. Achieving a statistical confidence in a simulation output requires, first, the identification of the various sources of error and uncertainty affecting the simulation results. These sources include machine and user error in collecting calibration data, uncertain model parameters, random error from natural processes, and model inadequacy in capturing the true material property or behavior. Statistical inference can then be used to recover information about unknown model parameters by conditioning on available data while taking into account the various sources of uncertainty. In this work, Bayesian inference is used to quantify and propagate uncertainty in simulations of material behavior. More specifically, a random effects hierarchical framework is used since it provides a way to account for uncertainty stemming from random natural processes or conditions. This is especially important in many materials modeling applications where the random microstructure plays an important role in dictating material behavior. In addition to this, in many cases experiments are quite costly, so in order to obtain sufficient data for calibration, a compilation (open full item for complete abstract)

Committee: Stephen Niezgoda (Advisor); Oksana Chkrebtii (Committee Member); Yunzhi Wang (Committee Member); Alan Luo (Committee Member) Subjects: Materials Science; Statistics

Doctor of Philosophy, The Ohio State University, 2019, Welding Engineering

During solidification of multi-component systems, composition partition between the liquid and solid phases is inevitable in most practical applications. This partition induces a segregation profile in the solid phase, which may affect solid-state transformation behavior. Several models exist both to address the formation of this segregation profile and solid-state phase transformations. The ability to model the full history of the material allows for digital testing of manufacturing process parameters, new alloy compositions and post solidification heat treatments. In this work, we investigate a modelling approach which uses several CalPhaD based models to follow the materials history and the phase transformations taking place during it. The basis and outline are presented and the modelling approach is applied to follow two cases, Cu phase formation in a Grade 91 Creep Strength Enhanced Ferritic Steel and solidification and phase transformations in a non-homogenized alloy 718 material produced through Laser powder bed additive manufacturing. The results of the model application are presented in article form. The first article correlates creep damage to second phase particles coming from solidification. The second article establishes a mechanism for the formation of a Cu phase layer around the second phase particles. The third article compares results of the Scheil model to segregation maps in as-built additively manufactured alloy 718. The fourth article uses a modelled segregation profile in alloy 718 to model δ phase formation in a heat treatment of the non-homogenized sample, and compares results with in-situ x-ray diffraction quantification of the δ phase. The document concludes with an analyses of the accuracy of the modelling approach and a discussion of the sensibility of the model to its inputs.

Committee: Antonio Ramirez (Advisor); Carolin Fink (Committee Member); Wei Zhang (Committee Member) Subjects: Materials Science

Master of Science, The Ohio State University, 2019, Welding Engineering

High Entropy Alloys (HEA) are a new class of alloys that was introduced in the early 2000's. These alloys are composed of five or more elements in near equiatomic ratios, with no single base element. HEAs have gained a lot of attention due to their unique or superior properties as compared to conventional alloys. However, there has been little attention paid to the welding metallurgy and weldability of HEAs. However, welding for manufacturing and repair is a key issue for structural engineering applications. This work aimed to establish an initial understanding of the welding metallurgy of HEAs, and identify any potential weldability issues with regard to weld cracking susceptibility in fusion welds. The outcomes of this initial evaluation were used to develop a methodology for rapidly screening the large compositional space of HEAs in order to find promising alloy compositions for weld applications, and ultimately to implement weldability in the early stages of HEA development. The most commonly studied equiatomic AlCoCrCuFeNi HEA was determined to have very poor weldability, due to the positive mixing enthalpy of copper and a high hardness microstructure promoted by aluminum. An improved weldability was achieved by modifying the composition to Al0.5CoCrCu0.1FeNi. Pulsed laser welding was shown to eliminate HAZ liquation cracking for AlCoCrFeNiTi HEA and reduces softening in the HAZ of Al0.5CoCrCu0.1FeNi HEA. Refractory HEA AlMo0.5NbTa0.5TiZr showed a very high susceptibility to porosity and brittle fracture, but a unique fusion zone microstructure with no cracking. A high-throughput screening based on thermodynamic modeling and experimental testing was developed in order to identify HEA compositions with promising weldability, and quickly reject alloy compositions with detrimental properties towards weldability.

Committee: Carolin Fink (Advisor); Antonio Ramirez (Committee Member) Subjects: Aerospace Materials; Engineering; Materials Science; Metallurgy

Doctor of Philosophy, The Ohio State University, 2018, Materials Science and Engineering

The development of new magnesium alloys with improved mechanical properties is important for lightweighting applications, since the current high pressure die cast (HPDC) magnesium alloys, i.e. AM50/60 (Mg-5/6wt.%Al-0.2wt.%Mn) and AZ91 (Mg-9wt.%Al-1wt.%Zn), have limited mechanical properties. Two magnesium alloy systems, Mg-Al-Sn (AT) and Mg-Al-Sn-Si (ATS), were investigated for potential automotive applications. A CALPHAD (CALculation of PHAse Diagrams) approach was used in the development of AT and ATS alloys and to aid in the design of heat treatment schedules. Scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), and transmission electron microscopy (TEM) techniques were used to characterize the microstructure of the alloys in the as-cast and heat treated conditions. Mechanical testing was performed on cast specimens, as well as samples cut from thin-wall HPDC components to compare the strength and ductility of these alloys to currently used magnesium alloys. To expand the applications of HPDC components in the transportation industries, further development and optimization of the process is needed. The development of super vacuum die casting (SVDC) process for aluminum and magnesium thin-wall castings were explored using process simulation and experimental validation. Two experimental dies, i.e., a test specimen die and a fluidity die, were designed to evaluate the castability of several new aluminum alloys and optimize process parameters for these alloys. The process conditions were successfully validated in industrial castings such as an automotive door inner and a side impact beam.

Committee: Alan Luo (Advisor); Michael Mills (Committee Member); Glenn Daehn (Committee Member); Gary Kennedy (Committee Member) Subjects: Materials Science

Doctor of Philosophy, Case Western Reserve University, 2017, Materials Science and Engineering

Increased use of second-generation duplex stainless steels is expected as demands for tougher, more economical, and corrosion resistant alloys increase. A novel gas-phase carburizing and nitriding procedure operating in the temperature range of 325 °C to 440 °C was utilized with the intent of improving both the tribological and electrochemical performance of the 2205 duplex alloy (22Cr–5Ni–3Mo–0.15N) under so-called paraequilibrium conditions. A suite of state-of-the-art microchemical and structural characterization tools were employed following each treatment, and performance of the treated alloys assessed by chloride-based cyclic polarization and nano-indentation hardness measurements. Particular emphasis was placed on understanding the response of the ferritic phase, which has been a source of speculation and confusion since the development of such treatments. CALPHAD-based thermodynamic modeling was also used to predict phase stability in the 2205 system subjected to gas-phase paraequilibrium nitridation or carburization. Analysis of the interstitially-hardened layer in the austenitic phase of 2205 provides results consistent with similar surface hardening treatments of single-phase austenitic stainless steels: a colossally supersaturated and precipitate-free hardened layer of expanded austenite is formed. The interstitial concentration, case depth, and concomitant mechanical properties can be tailored through control of the temperature, duration, and chemical activity with the gas-phase process. Spatially-resolved chemical and structural analysis within the d-ferrite of 2205 revealed two competitive transformation behaviors following nitridation, while carburization led to only one response. For both carburization and nitridation, carbon or nitrogen supersaturations in ferrite on the order of 20 at.% and 25 at.%, respectively, were observed—greater than 10^6 times the equilibrium concentration at room temperature, yet remarkably with unmeasurable expansion or d (open full item for complete abstract)

Committee: Arthur Heuer Prof. (Advisor); Frank Ernst Prof. (Committee Member); Matthew Willard Prof. (Committee Member); Farrel Martin Prof. (Committee Member) Subjects: Materials Science

Master of Science in Engineering (MSEgr), Wright State University, 2015, Materials Science and Engineering

High entropy alloys (HEAs) are a recent area of research in materials science. Their namesake is because their high entropy of mixing due to multiple metallic elements in a near equimolar ratio. The high entropy of mixing is supposed to suppress unwanted, brittle, ordered intermetallic phases, and form a single, randomly mixed solid solution phase. This phenomena makes HEAs a good candidate for structural applications. However, this entropy of mixing may not be enough to suppress all intermetallic phases. For this reason, the CALculation of PHAse Diagrams (CALPHAD) approach is being explored to predict phase equilibrium in HEAs. This study seeks to experimentally validate the current CALPHAD approach when applied to HEAs. Five alloy compositions were characterized with SEM, EDS, and XRD in the as-cast condition and after equilibrium heat treatments of 500hr at 1000°C and 1000hr at 750°C. Phases detected in the experimental alloys were compared with the CALPHAD predicted equilibrium phases. When considerations are taken, the current CALPHAD approach is adequate in predicting phase equilibrium for HEAs.

Committee: Raghavan Srinivasan Ph.D. (Advisor); Allen Jackson Ph.D. (Committee Member); Daniel Miracle Ph.D. (Committee Member) Subjects: Aerospace Materials; Materials Science; Metallurgy

Doctor of Philosophy, Case Western Reserve University, 2014, Materials Science and Engineering

Advanced high-strength steels (AHSS) have been widely used in automotive industry to improve safety and fuel economy. However, unintentional selective oxidation of alloying elements of AHSS during the thermal cycles employed on a continuous galvanizing line (CGL) complicates the coating process. Amongst the observed effects, external oxides can cause incomplete reactive wetting, resulting in bare spot defects in the zinc coating. This study focuses on developing and validating “oxidation maps” to define regions of selective oxidation for alloying elements in PO2-T space. Oxidation maps use CALPHAD method in combination with the Wagner’s model to determine regions where no oxidation, internal oxidation or external oxidation will occur. Experiments were carried out in an attempt to validate the predictions. Three types of AHSS grade steels, with variation in Mn, Si, Al contents, were used for experiments to validate the oxidation maps. Samples of selected steels were subjected to four different simulated CGL thermal cycles with two heating rates and two hold times, that bracket much of the range expected in industrial practice. Experiments were also undertaken for two of the steels under much higher dew point annealing conditions to probe the predicted boundary between external and internal oxidation. Samples were characterized by SEM, AES, XPS, and TEM to determine the oxide phases as well as the oxidation modes. Analysis results demonstrated that the formation of oxide phase on or in each steel is consistent with the thermodynamic modeling. A calculated Ellingham diagram clearly illustrates the formation sequence of oxide phases for the steels studied. For the prediction of oxidation mode, the oxidation maps are generally consistent with the analysis result. Inconsistency was observed only in one circumstance, and this is attributed to the highly heterogeneous surface caused by cold rolling. Otherwise, a kinetic model was studied to simul (open full item for complete abstract)

Committee: James McGuffin-Cawley (Advisor); Mark De Guire (Committee Member); Frank Ernst (Committee Member); Jay Mann Jr. (Committee Member) Subjects: Automotive Materials; Engineering; Materials Science; Metallurgy

Master of Sciences, Case Western Reserve University, 2014, Materials Science and Engineering

Owing to its thermodynamic control and conformal nature, gas-phase nitriding and carburizing of steels has for over a century been a popular method for increasing the hardness, wear-, and fatigue-resistance of ferrous components. Only in recent years (~3 decades) have such processes been successfully applied to stainless steels, under so-called paraequilibrium conditions whereby a truly colossal (>10^5 times the equilibrium concentration) supersaturation of interstitials can be achieved, thus imparting extraordinary improvements in the mechanical and electrochemical performance of the alloy. Such novel thermochemical techniques are relatively low-cost, industrially viable, and represent one of few value-added processes of a material that results in all gain and no loss. The thermodynamic origins of this metastable supersaturation, in particular carbon in austenitic 316, have been previously discussed within the context of the CALPHAD based multi-sublattice model for solid solutions. It is the present e ort of this work to review the complete thermodynamic database as applied to stainless steels, and develop a generalized approach for modeling metastable paraequilibria in both fcc austenitic and bcc ferritic (or martensitic) alloys upon low-temperature gas-phase nitriding or carburizing. A flexible and user-friendly program was developed to allow for predictions of the paraequilibrium carbon and nitrogen solubility in model binary to senary systems exposed to some carbon/nitrogen-rich ambient, as well as to model the solubility of carbon or nitrogen when a paraequilibrium, i.e. partitionless, carbide or nitride has formed. Changes in the paraequilibrium eutectoid temperature as a function of alloy content were also determined to predict the feasibility of an isothermal nitrogen- or carbon-induced ferrite (or martensite) to austenite (a (a') ¿ ¿) phase transformation. Such thermodynamic calculations can be used in optimizing interstitial hardening treatments of pre-e (open full item for complete abstract)

Committee: Gary Michal Prof. (Advisor); Arthur Heuer Prof. (Committee Chair); Frank Ernst Prof. (Committee Member); James McGuffin-Cawley Prof. (Committee Member) Subjects: Materials Science

Master of Science, Miami University, 2012, Chemical, Paper and Biomedical Engineering

Gallium nitride semiconductors are of great interest as high power/temperature transistors due to their wide band gaps and high electron mobility. However, AlGaN/GaN transistors have shown device instability at higher temperatures. In this thesis, Thermo Calc© and DICTRA© software were used to investigate the defect chemistry of the Al-Ga-N material system and the diffusion kinetics of nickel into the AlGaN layer of the device by the Computer Coupling of Phase Diagrams and Thermochemistry methodology. Using this methodology, both a thermodynamic and kinetic database need to be developed. A Ga-N thermodynamic database was first built and the phase diagram and defect concentration were calculated to ensure its accuracy in diffusion simulations. The kinetic simulation results indicated temperature activated diffusion of nickel as a possible mechanism for device failure.

Committee: Lei Kerr PhD (Advisor); Doug Coffin PhD (Committee Member); Shashi Lalvani PhD (Committee Member) Subjects: Chemical Engineering; Electrical Engineering; Engineering; Materials Science; Mechanical Engineering; Nanoscience; Nanotechnology

Master of Sciences, Case Western Reserve University, 2012, Materials Science and Engineering

CALPHAD based interstitial solid solution thermodynamic modeling was used to determine carbon solubility in the presence of select carbide phases and the eutectoid temperatures of 15-5 PH and 17-7 PH stainless steels under paraequilibrium conditions. Predictions using CALPHAD parameters from different sources in the literature were compared. Resulting values for the eutectoid temperature in 15-5 PH stainless steel varied nominally 100K depending upon the choice of Cr-Fe-C CALPHAD parameters employed. By converting from a dilute solution to a CALPHAD model, Cu-C interaction parameters were derived and applied to the numerical prediction. CALPHAD based thermodynamic modeling also was used to predict the effects of Cr and Ni on the solubility in Fe-based bcc and fcc matrices of graphite and paraequilibrium M3C, M7C3, and M23C6 carbides. The solubility of graphite increases with increasing Cr, and decreases with increasing Ni contents in both bcc and fcc matrices. For the Fe-Ni-C system, CALPHAD modeling of compositions up to 40 wt. pct. Ni found that the formation of paraequilibrium conbides increases the solubility of carbon relative to graphite for both bcc and fcc matrices at 700K. The Swagelok low-temperature carburization process was applied to 15-5 PH stainless steel over a temperature range from 380 to 450C. Through this process, hardened cases 8-12 µm thick were produced having a carbon content of 8-10 at. pct. as determined using Auger electron spectroscopy (AES). Near surface microstructures were examined using optical microscopy and scanning electron microscopy (SEM). Microhardness testing measured case hardness values of 950-1100 HV which are much higher than the core hardness values of approximately 500 HV.

Committee: Gary M. Michal PhD (Committee Chair); Arthur H. Heuer Phd (Committee Member); Frank Ernst Phd (Committee Member); Mark R. DeGuire Phd (Committee Member) Subjects: Materials Science