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Ghods, MasoudEffect of Convection Associated with Cross-section Change during Directional Solidification of Binary Alloys on Dendritic Array Morphology and Macrosegregation
Doctor of Engineering, Cleveland State University, 2017, Washkewicz College of Engineering
This dissertation explores the role of different types of convection on macrosegregation and on dendritic array morphology of two aluminum alloys directionally solidified through cylindrical graphite molds having both cross-section decrease and increase. Al- 19 wt. % Cu and Al-7 wt. % Si alloys were directionally solidified at two growth speed of 10 and 29.1 µm s-1 and examined for longitudinal and radial macrosegregation, and for primary dendrite spacing and dendrite trunk diameter. Directional solidification of these alloys through constant cross-section showed clustering of primary dendrites and parabolic-shaped radial macrosegregation profile, indicative of “steepling convection” in the mushy-zone. The degree of radial macrosegregation increased with decreased growth speed. The Al- 19 wt. % Cu samples, grown under similar conditions as Al-7 wt. % Si, showed more radial macrosegregation because of more intense “stepling convection” caused by their one order of magnitude larger coefficient of solutal expansion. Positive macrosegregation right before, followed by negative macrosegregation right after an abrupt cross-section decrease (from 9.5 mm diameter to 3.2 mm diameter), were observed in both alloys; this is because of the combined effect of thermosolutal convection and area-change-driven shrinkage flow in the contraction region. The degree of macrosegregation was found to be higher in the Al- 19 wt. % Cu samples. Strong area-change-driven shrinkage flow changes the parabolic-shape radial macrosegregation in the larger diameter section before contraction to “S-shaped” profile. But in the smaller diameter section after the contraction very low degree of radial macrosegregation was found. The samples solidified through an abrupt cross-section increase (from 3.2 mm diameter to 9.5 mm diameter) showed negative macrosegregation right after the cross-section increase on the expansion platform. During the transition to steady-state after the expansion, radial macrosegregation profile in locations close to the expansion was found to be “S-shaped”. This is attributed to the redistribution of solute-rich liquid ahead of the mushy-zone as it transitions from the narrow portion below into the large diameter portion above. Solutal remelting and fragmentation of dendrite branches, and floating of these fragmented pieces appear to be responsible for spurious grains formation in Al- 19 wt. % Cu samples after the cross-section expansion. New grain formation was not observed in Al-7 wt. % Si in similar locations; it is believed that this is due to the sinking of the fragmented dendrite branches in this alloy. Experimentally observed radial and axial macrosegregations agree well with the results obtained from the numerical simulations carried out by Dr. Mark Lauer and Prof. David R. Poirier at the University of Arizona. Trunk Diameter (TD) of dendritic array appears to respond more readily to the changing growth conditions as compared to the Nearest Neighbor Spacing (NNS) of primary dendrites.

Committee:

Surendra Tewari, Ph.D. (Advisor); Jorge Gatica, Ph.D. (Committee Member); Orhan Talu, Ph.D. (Committee Member); Rolf Lustig, Ph.D. (Committee Member); Kiril Streletzky, Ph.D. (Committee Member)

Subjects:

Aerospace Materials; Automotive Materials; Chemical Engineering; Condensed Matter Physics; Engineering; Fluid Dynamics; High Temperature Physics; Materials Science; Metallurgy

Keywords:

Directional Solidification; Natural Convection; Fluid Flow; Binary Alloys; Macrosegregation; Dendritic Array; Dendrite Morphology; Solutal Remelting; Thermosolutal Convection; Aluminum Alloy; Cross section Change

Fischdick Acuna, Andres FabricioHybrid Laser Welding in API X65 and X70 Steels
Master of Science, The Ohio State University, 2016, Materials Science and Engineering
Hybrid laser welding presents an important advance in productivity due to high welding speeds. However, fast cooling rates are inherent to the process, affecting the resultant microstructures and joint performance. In this research, three API steels were welded using hybrid laser welding with three distinct preheating conditions. The specimens, which were obtained using one hybrid laser root pass and two other GMAW filling passes, were subjected to microstructural characterization and performance evaluation using hardness and toughness measurements. Incomplete joints with only the hybrid root pass and completed joints (root and filling passes) were evaluated. Hardness mapping revealed as the critical area the top portion of hybrid laser fusion zone, which was subsequently reheated by the GMAW filling pass. Optical and scanning electron microscopy revealed a bainitic-martensitic microstructure with the proportion of those two phases varying as a function of the preheating. Miniaturized Charpy V-notch testing was used to evaluate the local toughness and ductile-to-brittle transition of several regions within the joint. Fractographic analysis confirmed the abrupt transition from ductile-to-brittle behavior. The localized fracture toughness testing showed an adequate joint performance for all tested conditions. Nevertheless, the hardness values meet the requirements only for higher preheating temperature conditions.

Committee:

Antonio Ramirez (Advisor); John Lippold (Committee Member)

Subjects:

Engineering; Materials Science; Metallurgy; Petroleum Engineering

Keywords:

Hybrid Laser, Pipeline, GMAW, HLAW, Steel, Hardness, Toughness, KLST, MCVN, Miniaturized Charpy, Ductile-to-Brittle Transition Temperature, DBTT, Bainite, Martensite

Park, ChangKyooDevelopment of Precise Femtosecond Laser Micromachining Processes for Metals and Electrospun Nanofibers
Doctor of Philosophy, The Ohio State University, 2015, Materials Science and Engineering
Femtosecond pulse lasers have proven to be versatile for micro-scale ablation of a variety of materials with high quality machining due to minimal residual stress, heat affected zone, and melting. In addition, femtosecond laser is one of the non-cleanroom techniques that does not require masking, chemical reagents, and multiple steps. This simple and convenient micromachining technique enables machining of various materials in 3-dimensional geometry. However, some factors such as optical scattering, beam shape, and debris accumulation hinder the high quality of ablation. In this dissertation, femtosecond laser was employed for the micromachining of electrospun nanofibers and metals. Optimization of a process for the high quality femtosecond laser machining was investigated. Femtosecond laser and electrospun poly(e-caprolactone) (PCL) nanofibers mesh interaction was analyzed by optical property measurements and the optical absorption and scattering coefficients were estimated. The specific energy required for ablating a unit volume of pure PCL nanofibers and polydimethylsiloxane-poly(e-caprolactone) (PDMS-PCL) core-shell nanofibers was measured. Material inherent optical properties including the ablation threshold fluence and the incubation coefficient of PDMS and PCL were estimated. Circular grooves were fabricated on aluminum, stainless steel 316, and Stellite 6 and circular disks were successfully machined from a thick section of Stellite. The tapered cross-section was detected from the Stellite disk and the tapering was minimized by varying pulse energy during ablation process. Moreover, a novel debris removal technique based on DC-dielectrophoresis (DEP) force was used to machine the linear and circular grooves on aluminum and the ablation depth and precision were compared with the gas jet debris removal technique.

Committee:

Dave Farson (Advisor); John Lannutti (Committee Member); Antonio Ramirez (Committee Member)

Subjects:

Materials Science; Metallurgy; Optics; Polymers

Keywords:

Femtosecond laser, Ablation, Metal, Polymer, Electrospun nanofiber, Scattering, Tapering, Debris removal, DC-dielectrophoresis

Wang, DanqiLOW-TEMPERATURE GAS-PHASE CARBURIZING AND NITRIDING OF 17-7 PH STAINLESS STEEL
Doctor of Philosophy, Case Western Reserve University, 2014, Materials Science and Engineering
Low-temperature carburization and low-temperature nitridation were successfully applied on 17-7 PH stainless steel and significantly improved the surface hardness. Via an isothermal martensite-to-austenite phase transformation, carbon- and nitrogen-supersaturated expanded austenite was achieved. Delta ferrite grains with astonishing amount (greater than 18 at.%) of carbon (nitrogen) were observed after carburization (nitridation). The interstitially-supersaturated ferrite shows a uniform contrast, i. e. no diffraction contrast from extended structural defects under transmission electron microscopy. Plates with uniform contrast were observed in ferrite grains near the interface between the carburized layer and bulk material after carburization. These plates are enriched in carbon but do not form any carbide. A model based on segregation of carbon interstitials to dislocation cores is proposed, the featureless appearance being ascribed to strain field overlap of a massive dislocation network. As the carbon-dislocation binding energy is higher than that of carbon to iron in cementite, carbon atoms are able to segregate to the dislocations cores. With an extremely high dislocation density (1013/cm2), ferrite can take up to several wt. pct. carbon without any phase transformation. Similar mechanism holds for featureless nitrogen-supersaturated ferrite. But after nitriding at high temperature (713 K), rocksalt-structured nitrides MN1-x (M being Fe, Cr, Ni and Al) were observed in a Bain orientation relationship with respect to ferrite matrix. Low nitriding temperatures (623 K and 653 K) prefer M2N1-x plate formation in ferrite. Continuing nitriding tends to dissolve M2N1-x, forming featureless grain with MN1-x. In spite of processing condition designed to eliminate long-range diffusion of substitutional solute atoms, decomposition of austenite into ferrite and nitride was observed to occur during low-temperature nitridation realized by enhanced diffusion along the austenite–ferrite interphase interface. NiAl formed in ferrite during both carburization and nitridation, but lost ordering afterwards in carbon- and nitrogen- supersaturated ferrite grains.

Committee:

Arthur Heuer (Advisor)

Subjects:

Engineering; Materials Science; Metallurgy

Keywords:

Stainless steel, carburization, nitridation, transmission electron microscopy, atom probe tomography, dislocation

Dunleavy, Joseph GordonCorrelation of Charpy "V" Notch and Mesnager "U" Notch Impact Values for Selected High=Strength Constructional Alloys
Master of Science, The Ohio State University, 1968, Materials Science and Engineering

Committee:

J.W. Stretnak (Advisor)

Subjects:

Metallurgy

Lin, Muh-RenExperimental Investigation of Temperature Effect on Uniaxial Tensile Test
Doctor of Philosophy, The Ohio State University, 1986, Materials Science and Engineering
An experimental investigation of the temperature effect on uniaxial tensile test is presented for Armco interstitial-free steel (I.F. steel) and stainless steel type 310 (310SS). This study is based on developing constitutive equations incorporating the effects of strain, strain rates, and temperatures and measuring the temperature distributions and tensile elongations under different thermal constraints. Constitutive equations were developed from a series of isothermal tensile tests at strain rates varying from 10-1 to 10-5/s, temperature ranging from 0°C to 90°C, and strains up to 0.30; followed by graphical inspection and non-linear least-squares curve fitting. Temperature distributions were measured using a high speed data-acquisition system at a transfer frequency of 104 Hz. For tests in air, temperature rises varied from 118 to 25 °C and 75 to 3 °C for 310SS and I.F. steel as strain rate changed from 10-l/s to 10-5/s. The results reveal that temperature plays an important role on tensile behavior. Comparisons between the temperature distributions measured in air, in stirred water, and with insulation show that tensile tests in air approach the adiabatic condition at strain rates higher than 10-1/s and approach isothermal at strain rates lower than 10-4/s for I.F. steel. For 310SS, the adiabatic condition could be approached at rates higher than 10-2/s; for approaching an isothermal condition, the rate must be lower than 10-5/s. These results were confirmed by comparing tensile elongations measured under the three heat flow conditions. Good agreement was found for I. F. steel at extreme cases (isothermal and adiabatic) where the total engineering strain approaches a constant of 45 % (isothermal) and 40% (adiabatic). For 310SS, the total engineering strain was measured ranging from 42% (adiabatic) to 60% (isothermal). The tests in air were not close to isothermal even at a rate of lxl0-4/s. The large effect of temperature on total elongation in this case results from a high flow stress, a low thermal diffusivity, and a reduced work hardening coefficient at higher temperatures.

Committee:

R.H. Wagoner (Advisor); G.W. Powell (Committee Member); Y. Sahai (Committee Member)

Subjects:

Metallurgy

Deutchman, Hallee ZoxON THE CREEP BEHAVIOR AND DEFORMATION MECHANISMS FOUND IN AN ADVANCED POLYCRYSTALLINE NICKEL-BASE SUPERALLOY AT HIGH TEMPERATURES
Doctor of Philosophy, The Ohio State University, 2013, Materials Science and Engineering
Polycrystalline Ni-base superalloys are used as turbine disks in the hot section in jet engines, placing them in a high temperature and stress environment. As operating temperatures increase in search of better fuel efficiency, it becomes important to understand how these higher temperatures are affecting mechanical behavior and active deformation mechanisms in the substructure. Not only are operating temperatures increasing, but there is a drive to design next generation alloys in shorter time periods using predictive modeling capabilities. This dissertation focuses on mechanical behavior and active deformation mechanisms found in two different advanced polycrystalline alloy systems, information which will then be used to build advanced predictive models to design the next generation of alloys. The first part of this dissertation discusses the creep behavior and identifying active deformation mechanisms in an advanced polycrystalline Ni-based superalloy (ME3) that is currently in operation, but at higher temperatures and stresses than are experienced in current engines. Monotonic creep tests were run at 700°C and between 655-793MPa at 34MPa increments, on microstructures (M1 and M2) produced by different heat treatments. All tests were crept to 0.5% plastic strain. Transient temperature and transient stress tests were used determine activation energy and stress exponents of the M1 microstructure. Constant strain rate tests (at 10-4s-1) were performed on both microstructures as well. Following creep testing, both microstructures were fully characterized using Scanning Electron Microscopy (SEM) for basic microstructure information, and Scanning Transmission Electron Microscopy (STEM) to determine active deformation mechanism. It was found that in the M1 microstructure, reorder mediated activity (such as discontinuous faulting and microtwinning) is dominant at low stresses (655-724 MPa). Dislocations in the ¿ matrix, and overall planar dislocation activity were also present. At higher stresses (758-793MPa), there is still planar activity present, but now non-planar “wavy” slip appears. Wavy slip was also present in the constant strain rate sample of the M1 microstructure. M2 similar activity as M1, but wavy slip was not present in either the high stress creep conditions or the constant strain rate conditions. The second part of this dissertation focused on Alloy X, which is a next generation alloy system. Three variants were examined (decreased levels of Cr and Co, decreased levels of Cr and Co with added Hf, and decreased levels of Cr and Co with added Hf and a faster cooling rate). The variants were all tested at 700°C/690MPa, 760°C/490MPa, and 815°C/345MPa, to various strains. In Alloy X, the dominant mechanisms that appear throughout all strains, stresses, and temperatures are unpaired dislocations and discontinuous stacking faults. In the Hf bearing variants, unpaired dislocations appear to start at grain boundaries and move in to the grains with increasing strain and temperature. Through 3D stereo imaging using STEM, it appears that the dislocations are able to climb over precipitates at larger strains. It is postulated that the decreasing levels of Cr and Co have changed the stacking fault energy so it is unfavorable to form microtwins anymore. This was discussed in terms of the Dislocation Activity Diagram (DAD).

Committee:

Michael Mills, PhD (Advisor); Glenn Daehn, PhD (Committee Member); Yunzhi Wang, PhD (Committee Member)

Subjects:

Materials Science; Metallurgy

Keywords:

Nickel-base Superalloys, polycrystalline, STEM, creep

Schaser, Matt SaxonMaterial Specific Load Combination Factors for Option 2 FAD Curves
Master of Science in Engineering Mechanics, Cleveland State University, 2013, Fenn College of Engineering
The use of failure assessment diagrams (FAD) for evaluating the integrity of components containing crack-like flaws has developed a well-defined methodology over the years that includes a correction factor to account for combined loading effects that are a result of primary and secondary stresses. The load combination factor, Ψ, is based on the Option 1 FAD currently in use in the Central Electricity Generating Board’s (CEGB) report No. R/H/R6 (R6) and the API-579-1/ASME FFS-1 fitness-for-service standard. The Ψ factors for the Option 2 FAD based on ASME B&PV Code Section VIII, Division 2 material stress-strain curves are developed and tabulated here for a wide range materials used for the construction of pressure vessels. The Ψ factors based on the Option 1 FAD are recalculated here and compared to current published data. The approach utilizing Option 1 FAD methods is evaluated here with regard to its conservatism and applicability to material models other than the Ramberg-Osgood model. In addition, a sensitivity analysis is performed to estimate Ψ factor errors due to uncertainty in material property parameters. A critical review of the tabulated data in API-579 is performed and errors are identified along with suggested solutions to correct the data.

Committee:

Stephen Duffy, PhD (Committee Chair); Paul Lin, PhD (Committee Member); Norbert Delatte, PhD (Committee Member)

Subjects:

Engineering; Materials Science; Mechanical Engineering; Metallurgy; Nuclear Engineering; Petroleum Engineering

Keywords:

Failure Assessment Diagram; FAD; Load Combination Factor; FFS; Fitness-for-Service; Fitness for Service; Psi Factor; Phi Factor; API-579; API-579-1; ASME FFS-1; Prager Stress Strain Model; Prager Stress-Strain Model; Crack-like Flaw

Meyendorf, RobertNondestructive Determination of Case Depth in Surface Hardened Steels by Combination of Electromagnetic Test Methods
Doctor of Philosophy (Ph.D.), University of Dayton, 2011, Materials Engineering
The objective of this study was to improve the accuracy and reliability of nondestructive case depth determination from the current state of the art for application in an industrial environment. In the current state of the art only a single test method is used. In the present study simultaneous measurements were made with four independent electromagnetic test methods. The test methods used were measurement of tangential component of the magnetizing field, magnetic Barkhausen noise analysis, incremental permeability and multi-frequency eddy current measurement. The methods have different penetration depth and sensitivity to microstructure variations, thus complementing each other. The method that measures the tangential component of the magnetizing field is the only method that has a sufficiently deep depth of penetration to be useful for case depth testing. However, measurements with this method can be distorted by material variations other than the case depth. This distortion can be corrected by combining the measurement of the tangential component of the magnetizing field with a method that is mainly sensitive to the distorting effect. Such distorting effects can for example be a thin martensite layer on top of the case or a different quench oil temperature. From the 4 test methods used here 41 parameters were derived that describe the measurement signals. Multiple regression methods were used to select the most suitable parameters and build models from them. This procedure is called calibration. A separate calibration has to be performed for each different material. Models were built and the case depth testing accuracy was evaluated at 2 case hardened specimen groups. For one group the average case depth test error was in a range of ±15µm. For another group the average test error was in a range of ±100µm for case depths ranging from 1 – 2.5mm. The results of the study show that the case depth testing accuracy and robustness could be improved by combining several independent electromagnetic test methods, thus providing industry with an effective quality control method for fast and reliable quality assurance.

Committee:

Daniel Eylon, D.Sc. (Committee Chair); P. Terrence Murray, PhD (Committee Member); James Malas, PhD (Committee Member); James Snide, PhD (Committee Member); Jürgen Schreiber, PhD (Committee Member); Gerald Shaughnessy, M.S. (Committee Member)

Subjects:

Electromagnetism; Materials Science; Metallurgy

Keywords:

case depth; steel; electromagnetic; nondestructive; carburized; surface hardened

Cao, LiuCorrosion and Stress Corrosion Cracking of Carbon Steel in Simulated Fuel Grade Ethanol
Doctor of Philosophy, The Ohio State University, 2012, Materials Science and Engineering

Carbon steel is susceptible to stress corrosion cracking (SCC) in fuel grade ethanol. Dissolved oxygen and corrosion potential have been identified as the critical factors. The threat of SCC prevents the use of the cost-efficient pipeline system for long distance transport of ethanol. Simulated fuel grade ethanol (SFGE) was used in the laboratory. Due to the high electrical resistivity of SFGE, adding non-complexing supporting electrolyte is considered to be the most practical method for accurate potential control. TBA-TFB was found to be the best suitable supporting electrolyte in deaerated FGE among the salts that were tested.

Carbon steel exhibits passivity and high corrosion potential in aerated SFGE. Deaerated conditions are of interest so that the effects of high potential on SCC susceptibility can be determined separately from other possible effects of dissolved oxygen. In slow strain rate (SSR) tests, cracking was reproduced at applied potential without oxygen in the deaerated SFGE + TBA-TFB, which indicates that the role of oxygen in ethanol SCC might be a simple oxidizing agent. However, the passivation effect of oxygen is also required to prevent lateral corrosion at crack tip. A potential range for ethanol SCC was determined by SSR tests at different potentials. No experimental evidence was found to support the proposed role of oxygen to react with ethanol to form an aggressive oxidation product.

The presence of chloride causes decreasing corrosion potential and enhanced pitting corrosion. The chloride effect on ethanol SCC was investigated both in aerated SFGE at open circuit and deaerated SFGE at applied anodic potentials over a wide range of chloride concentration. A minimum concentration of chloride is required for SCC of carbon steel, but it is not the controlling factor for crack growth. There is no upper limit of chloride concentration for cracking in aerated SFGE, but a window of chloride concentration in deaerated SFGE at applied potential.

The dissolution based SCC mechanism has been identified for carbon steel in ethanol environment. SSR testing with periodic potentiodynamic scans at different stains and strain rates show that the anodic current difference between plastic and elastic region has a peak in the cracking potential range. The notched SSR testing is more sensitive to ethanol SCC susceptibility, and it generates more consistent results of current evolution. A high R-ratio and low cyclic frequency crack growth rate (CGR) test was intended to mimic the actual loading condition of pipelines in service at accurately-controlled fracture mechanics conditions. The measured CGR at applied potentials matches the cracking susceptible potential region.

An oxygen depletion induced dissolution model and the traditional film rupture induced dissolution model were proposed to explain the mechanism of ethanol SCC. A few models based on oxygen diffusion and consumption were considered in an attempt to explain the differences of the two proposed mechanisms. Complete oxygen depletion is less likely to occur at crack tip.

Committee:

Gerald Frankel, Dr. (Advisor); Narasi Sridhar, Dr. (Committee Co-Chair); Rudolph Buchheit, Dr. (Committee Member); Glenn Daehn, Dr. (Committee Member)

Subjects:

Engineering; Materials Science; Metallurgy

Keywords:

biofuel; fuel grade ethanol; pitting corrosion; stress corrosion cracking; carbon steel; IR drop; supporting electrolyte; crack growth rate; dissolved oxygen; corrosion potential; chloride

Kuruvilla, MithunAn Understanding of the Quasi-static Behavior, High Cycle Fatigue and Final Fracture Behavior of a Titanium (Ti- 4 Al-2.5 V-1.5 Fe-0.25 O2) Alloy
Master of Science, University of Akron, 2008, Mechanical Engineering

Titanium, which is referred as a “wonder metal” has been in use for structural application for more than 50 years both in the form of commercially pure titanium and alloys. The wide range of mechanical properties exhibited by titanium led to the development of various alloys tailored for specific application in areas spanning aerospace to sports. The innovatively engineered titanium alloy ATI 425TM is an emerging high performance, high strength alloy and a viable replacement to the work horse and most commercially popular titanium alloy Ti-6Al-4V. This newly emerged alloy offers the inherent advantage of being receptive to mechanical deformation by cold working. Initially this alloy was developed and put forth for use as armor plate for ballistic protection. This alloy also shows promise for use in aerospace-related applications. In this thesis report is presented and discussed the final fracture behavior of the alloy deformed under both quasi static and cyclic fatigue loading conditions, highlighting the role of product form in governing the mechanical deformation and fracture behavior. Samples of the alloy were prepared from both rod stock and sheet stock, and deformed with stress axis both parallel and perpendicular to the longitudinal direction for the sheet stock and along the longitudinal axis for the rod stock.

The intrinsic influence of processing on microstructure of the rod revealed alpha grains of varying size and shape being well distributed through the transformed beta matrix. The hardness measurements were consistent and the macrohardness was found to be about half the value of the microhardness. Tensile properties of this alloy are comparable with the commercial alloy Ti-6Al-4V, within the limits of experimental scatter. The tensile deformed fracture surface was macroscopically rough and microscopically, reminiscent of locally ductile and brittle failure mechanisms. The influence of intrinsic microstructural features of the alloy product and nature of loading on final fracture behavior is discussed. The high cycle fatigue resistance of the chosen titanium alloy revealed a trend shown by most non- ferrous metallic materials. The final fracture behavior of the alloy under cyclic loading conditions showed differences in the nature and volume fraction of the features with maximum stress at a given load ratio.

The processing on the sheet stock of both orientations resulted in alpha plus beta microstructure. The microhardness and macrohardness data reveals the alloy to be harder in the transverse orientation than in the longitudinal orientation. The tensile properties of the sheet stock with transverse orientation, when compared to the commercial alloy Ti-6Al-4V were observed to be better than sheet stock with longitudinal orientation. The tensile fracture surface of the alloy sheet along the longitudinal orientation revealed at the macroscopic level a fairly rough transgranular region and at microscopic level a healthy population of microscopic voids and shallow dimples of varying size and shape. For the transverse test specimen, the tensile fracture surface was macroscopically rough and globally at an inclination to the far field stress axis and microscopically it was rough and covered with a healthy population of voids of varying and dimples of varying size and shape. The high cycle fatigue resistance of the chosen titanium alloy revealed that the transverse oriented specimens showed more fatigue resistance compared to the longitudinal ones. Cyclic fatigue fracture surfaces revealed differences in the nature and volume fraction of the features with maximum stress at a given load ratio. The region of crack initiation and early crack growth and stable crack growth was essentially flat and transgranular.

Committee:

Dr. Tirumalai S. Srivatsan, PhD (Advisor)

Subjects:

Aerospace Materials; Automotive Materials; Biomedical Research; Engineering; Materials Science; Mechanical Engineering; Metallurgy

Keywords:

Titanium Alloy; ATI 425; Tension; High Cycle Fatigue

Sutton, Benjamin JamesSolidification Behavior and Hot Cracking Susceptibility of High Manganese Steel Weld Metals
Master of Science, The Ohio State University, 2013, Welding Engineering
Recent attention has been given to developing austenitic high-Mn steels for cryogenic service conditions. Specifically, the austenite stabilizing capacities of Mn and C are being exploited to create lower cost alternatives to other cryogenic materials (9Ni steel, Invar, 304 SS, etc.) which are commonly used during the construction of liquefied natural gas (LNG) tanks. The proposed steel alloys contain Mn levels in the range of 20 to 28 wt% and C additions on the order of 0.4 wt%. Although austenite stability is beneficial from a low temperature mechanical property standpoint, the presence of such high concentrations of austenite stabilizing elements causes concern with regard to hot cracking during welding. The solidification cracking susceptibility of a wide range of high-Mn steel weld metal compositions was assessed through cast pin tear (CPT) testing. The tested compositions fell within the following ranges (wt%): Mn (14-34), C (0-0.7), and Al (0-3). Impurity elements were also present in the following ranges (wt%): S (0.005-0.011) and P (0.003-0.026). A total of 12 compositions were tested. It was found that C and P controlled the solidification cracking susceptibility of these alloys. In addition to solidification cracking testing, solidification temperature range (STR) analysis was performed on the test matrix using single-sensor differential thermal analysis (SS-DTA) and modified Scheil solidification simulations. STR analysis has previously been related to the weld solidification cracking tendencies of austenitic alloy systems, with large STR values relating to an increased susceptibility to solidification cracking. The measured and calculated STR values in this investigation exhibited a similar relationship. Results indicate that the tested alloys all exhibit primary austenite solidification. It was determined that C and P segregation were primarily responsible for STR expansion. Optical and scanning electron microscopy (OM and SEM) were used to characterize specimens from both CPT testing and SS-DTA analysis. Weld metal samples demonstrated microstructures of martensite and austenite in varying proportions. Increasing Mn and C contents tended to stabilize austenite, as expected. Solute segregation tended to stabilize austenite along interdendritic regions of alloys containing martensite. Energy-dispersive X-ray spectroscopy (EDS) analysis was utilized to analyze Mn, S, and P segregation in various weld metal samples. It was found that S impurities tended to form MnS inclusions prior to terminal solidification, indicating that the presence of S is less detrimental to solidification cracking in high-Mn steels than other austenitic materials. Evidence of a P-rich eutectic constituent was found along interdendritic regions in the alloy which demonstrated the highest susceptibility to solidification cracking. The same alloy showed apparent interdendritic liquation cracking within the heat affected zone (HAZ) immediately adjacent to the fusion boundary (FB) in a sample having an as-solidified microstructure prior to welding. In general, it was determined that the solidification cracking susceptibility of the high-Mn steel weld metal compositions explored in this investigation was relatively low. This conclusion was reached based on findings from CPT results that were obtained at the upper threshold of restraint conditions of the test.

Committee:

John Lippold (Advisor); Sudarsanam Babu (Committee Member)

Subjects:

Engineering; Materials Science; Metallurgy

Keywords:

high manganese steel; welding; solidification; solute redistribution; hot cracking

Sowards, Jeffrey WilliamDevelopment of a chromium-free consumable for joining stainless steel
Doctor of Philosophy, The Ohio State University, 2009, Welding Engineering

Conventional welding consumables used to join stainless steels are alloyed with Cr to produce welds with adequate corrosion resistance by promoting a passive oxide surface layer. Vaporization of Cr results in the formation of hexavalent chromium compounds in stainless steel welding fume. Government regulations in the United States and abroad are decreasing allowable exposure levels of hexavalent chromium to welding related personnel. A 2006 OSHA ruling reduced the permissible exposure limit of airborne hexavalent chromium from 52 to 5 micrograms/cubic meter. Achieving the new level may not be practical from an engineering controls standpoint during the fabrication of tightly enclosed stainless steel components. One method of addressing this problem is to implement a chromium-free welding consumable that provides equivalent mechanical performance and corrosion characteristics to current stainless steel welding consumables. This project was aimed at developing such a consumable and evaluating its suitability for replacement of current stainless steel consumables such as E308L-16. A new shielded metal arc welding consumable based on the Ni-Cu-Ru system was developed for austenitic stainless steel welding.

Mechanical properties of welds deposited with the new consumable were found to exceed minimum values of Type 304 stainless steel based on tensile testing. Hot ductility testing revealed a narrow crack susceptible region (33 to 54°C) indicating a low susceptibility to weld metal liquation cracking. A low ductility region was found at intermediate temperatures in the range of 800 to 1100°C. This ductility trough is believed to result in the weld cracking phenomenon known as ductility dip cracking (DDC). Threshold strain levels to initiate DDC were approximately 2 to 3% as determined by strain-to-fracture testing. Varestraint testing revealed that weld deposits have a higher solidification cracking susceptibility than stainless steel consumables used to join Type 304. This was attributed to fully austenitic solidification of the weld metal resulting in increased weld segregation and stabilization of a TiC eutectic reaction at the end of solidification. Solidification cracking susceptibility was shown to increase with dilution by Type 304L base metal.

Fume generation rates (FGR) of the new consumable were measured and bulk fume phases were analyzed with X-ray diffraction. FGR values were found to be similar to current SMAW and flux cored arc welding consumables. No chromium bearing compounds were observed during X-ray diffraction measurements, and the bulk fume consisted primarily of halides and metallic-oxides. Hexavalent Cr content (0.02 wt-%) was reduced by two orders of magnitude compared to E308-16 (2.6 wt-%). The source of this hexavalent chromium was from evaporation of the base metal due to the welding heat source. Fume produced by the new consumable was characterized with an Electrical Low Pressure Impactor to analyze the particle size and mass distributions for comparison to E308-16. This system showed that the fume size and mass distributions were dominated by particle agglomeration. Individual particles collected with this system were subjected to analysis with scanning and transmission electron microscopy. Cr-rich particles were not observed during fume analysis.

Committee:

John C. Lippold (Advisor); Gerald S. Frankel (Committee Member); S. Suresh Babu (Committee Member)

Subjects:

Engineering; Metallurgy

Keywords:

stainless steel; monel; Ni-Cu; dissimilar welding; shielded metal arc welding; SMAW; weldability; solidification cracking; ductility dip cracking; liquation cracking; welding fume; hexavalent chromium

Banik, Kristin ElizabethFACTORS EFFECTING ELECTROMAGNETIC FLAT SHEET FORMING USING THE UNIFORM PRESSURE COIL
Master of Science, The Ohio State University, 2008, Materials Science and Engineering

Electromagnetic forming is a possible alternative to sheet metal stamping. There are multiple limitations to the incumbent stamping methods including: complex alignment, changes to component shapes, and ductility issues, which often limits available formed geometry. Electromagnetic forming allows for the avoidance of some of these issues, but introduces a few other issues.

In this thesis, the issues with electromagnetic forming will be discussed in conjunction with the application of the uniform pressure coil. Also, the effects on properties of the electromagnetically formed samples in comparison to the traditional samples will be presented. These properties include hardness, formability and interface issues. Lastly, discussed in this paper is the implementation of the Photon Doppler Velocimetry (PDV) system, a velocity measurement system used to determine the velocity of the workpiece and compare it to physics-based models of the process.

Committee:

Glenn Daehn (Advisor); Katherine Flores (Committee Member)

Subjects:

Materials Science; Metallurgy

Keywords:

Electromagnetic Forming; Uniform Pressure Coil

Unocic, Raymond RobertOn the Creep Deformation Mechanisms of an Advanced Disk Ni-base Superalloy
Doctor of Philosophy, The Ohio State University, 2008, Materials Science and Engineering

Ni-base superalloys are an important class of high temperature structural materials that are used in the hot section of aircraft gas turbine engines since they possess the inherent capability to retain strength and resistance to creep, fatigue, and oxidation at elevated temperature. These components are subjected to elevated temperatures and complex stress states where time dependent creep deformation is of utmost concern and needs to be accounted for from a design criteria point of view. In this study, the creep deformation mechanisms of a newer generation turbine disk alloy, Rene 104, was investigated using transmission electron microscopy characterization techniques. The creep deformation behavior and underling creep deformation mechanisms were found to be highly dependent upon stress, temperature and microstructure.

Microtwinning was found to be the dominant deformation mechanism following creep at an intermediate temperature and stress regime. Microtwins form by the motion of paired a/6<112> Shockley partial dislocations that shear both the γ matrix and γ' precipitates. The rate limiting process in this mechanism is diffusion mediated atomic reordering that occurs in the wake of the shearing, twinning partial dislocations in order to maintain the ordered L12 structure of the γ' precipitates. To determine the salient microstructural features that lead to microtwinning specimens with varying γ' precipitate size scale, volume fraction and γ channel width spacing were crept at the same temperature and stress (677°C and 724MPa). The most creep resistant microstructure consisted of a bimodal distribution of γ' precipitates with a finer secondary γ' precipitate size, low volume fraction of γ' and narrow γ channel width spacing. Due to the combined effects of narrow γ channel width spacing, stacking fault energy, and resolved shear stress the a/2<110> dislocations dissociate into leading and trailing a/6<112> Shockley partial dislocations at low strain, which was determined to be a necessary precursor microtwinning at higher strains. Based on post mortem and in-situ TEM straining characterization a model was proposed which describes how a/6<112> Shockley partial dislocations can layer atop one another on adjacent {111} glide planes which could then cooperatively shear both γ matrix and γ' precipitates thereby creating a microtwin.

Committee:

Michael Mills, PhD (Advisor); Glenn Daehn, PhD (Committee Member); James Williams, PhD (Committee Member); Robert Hamlin, PhD (Committee Member)

Subjects:

Engineering; Materials Science; Metallurgy

Keywords:

Ni-base Superalloy; Creep; Deformation Mechanisms; Transmission Electron Microscopy

Pallikonda, Mahesh KumarFORMING A METAL MATRIX NANOCOMPOSITE (MMNC) WITH FULLY DISPERSED AND DEAGGLOMERATED MULTIWALLED CARBON NANOTUBES (MWCNTs)
Master of Science in Mechanical Engineering, Cleveland State University, 2017, Washkewicz College of Engineering
Carbon Nanotubes (CNTs) with their exceptional properties will facilitate the Metal matrix composites (MMC) to exhibit good mechanical properties, thermal and electrical conductivities, corrosion resistance, etc. The critical factor that holds the development of the Metal matrix Nanocomposites (MMNC) by using CNTs is the tendency of CNTs to form clusters (agglomerations) due to their high Van der Waals attractions. Due to this factor, low density and other properties of the CNTs, there has been a delay in harnessing their ultimate potential. Existing literature in contemporary times from the works of few researches in Nanocomposites shows the prevalence of using surfactants / dispersing agents for dispersing CNTs in the metal matrix. But the addition of these dispersing agents will form inclusions in the metal thus closing the avenue for developing ballistic electrical conductors and high purity MMNCs. Also the vol% of CNTs is limited to 1% in many cases and further increase reduces the mechanical strength. The reason for decreasing the strength is attributed to the agglomeration of CNTs and their disorderly alignment. In this work we developed a process where total dispersion and deagglomeration of CNTs up to 5 vol% is achieved without the addition of any surfactants / dispersing agents in the Magnesium Metal matrix. The process developed in this work can be applied to other metals with proper process parameters to develop various MMNCs with exceptional properties relative to the base metal. This process will open doors for the future works for developing high strength, High electrical and thermal conductive Metal Matrix Nanocomposites.

Committee:

Taysir Nayfeh, Ph.D. (Committee Chair); Jessica Bickel, Ph.D. (Committee Member); Tushar Borkar, Ph.D. (Committee Member)

Subjects:

Materials Science; Mechanical Engineering; Metallurgy; Nanotechnology

Keywords:

Metal Matrix Nano Composites, Carbon Nanotubes, MWCNTs, Vacuum Stirring, Ballastic Conductance, Low porosity, Vacuum Hot Consolidation, Casting Magnesium near vacuum, Composite wire, MMNC, MMC, Deagglomeration

Alexandre, Rex TInduction Bending of Internally Clad Steel Pipes: Failure Mechanisms & Processing Parameter Optimization in Ni-base Alloy Weld Overlays
Master of Science, The Ohio State University, 2016, Welding Engineering
Cracking in corrosion resistant clad overlays on low alloy carbon steel pipes made with Alloy 825 has been experienced in the industry in an effort to reduce production costs by changing the cladding material from the more costly Alloy 625. A detailed metallurgical investigation was carried out to understand the root cause of the cracking phenomenon. Analysis consisting of optical and scanning electron microscopy along with hardness traverses and mapping revealed weld metal heat-affected zone liquation cracks in the second overlay after welding, as well as a region of high hardness in the planar growth region of the weld metal directly adjacent to the dissimilar metal weld interface. Serial sectioning shows that ductility-dip cracks form between the pre-existing weld metal liquation cracks and microcracks forming in the embrittled planar growth region, ultimately leading to through-thickness cracks of the overlay during induction bending. The strain-to-fracture test was modified to replicate the bending procedure, and an optimal parameter window consisting of bending temperature, total strain, and strain rate was identified based on test results. ThermoCalc pseudo-binary phase diagrams were created using both the equilibrium and Scheil models. Neither diagram predicts the formation of any low melting eutectic constituents that could lead to liquation during welding. EDS results show spikes of titanium in the bulk weld metal, presumably due to the presence of titanium carbide particles. A plot of solidus temperature versus weight percent titanium created in Thermocalc reveals severe melting point depression in the Alloy 825 matrix as the titanium content increases. It is hypothesized that the weld metal heat-affected zone liquation cracking occurs via constitutional liquation of titanium carbide particles in close proximity to the fusion zone during welding of the second overlay. The region of high hardness at the DMW interface was observed to correlate with microcracks forming in that region as initial straining began during induction bending. Gleeble testing showed that avoidance of microcrack formation by manipulation of bending parameters is not possible. A study on hardness at the interface during processes steps was performed, revealing that the hardness increase occurs during the normalizing post-weld heat treatment before bending. A DICTRA diffusion model was carried out to further understand the mechanism behind the increased hardness in the planar growth region. Results show a pile-up of carbon extending approximately 100-150 microns into the weld metal at the DMW interface. It is theorized that avoidance of microcracks at the DMW interface is best achieved by elimination of the PWHT. Replication of the induction bending process in the Gleeble thermo-mechanical simulator was achieved by modification of the strain-to-fracture test. Results show that reducing the strain rate opens the safe bending parameter window in terms of temperature and total strain. A bending temperature of 975 ± 25 °C is suggested to successfully induction bend pipes without causing ductility-dip cracking.

Committee:

Boian Alexandrov, PhD (Advisor); Antonio Ramirez, PhD (Committee Member)

Subjects:

Materials Science; Metallurgy

Keywords:

induction bending; weld overlay; ni-base alloy; alloy 825; dissimilar metal weld;

Suh, SanghyunWeldability Evaluation in Autogenous Welds of Alloys 230, 800H, and 825
Master of Science, The Ohio State University, 2016, Welding Engineering
Pipes of solid solution strengthened Ni-based alloys, as Alloy 826 and Alloy 800H, have been used for high temperature service in once through steam generators (OSTGs) on off-shore platforms. The oil and gas industry is seeking to increase service temperature, improve service reliability, and extend service life to 40 years of such installations. Alloy 230 has better-high temperature stability and mechanical properties, and higher service temperature than Alloys 825 and 800H, and is therefore considered as a potential replacement of these alloys in newly built OTSGs. However, the weldability and the high temperature service behavior in welds of Alloy 230 have not been thoroughly investigated yet. This study is a comprehensive comparative research focused on susceptibility to solidification cracking and stress relief cracking in Alloys 800H, 825, and 230. To evaluate the solidification behavior and solidification cracking susceptibility in these alloys, the Cast Pin Tear Test (CPTT), thermodynamic simulations with Thermo-Calc, and the technique of Single-Sensor Differential Analysis (SS-DTA) were used. The results revealed that Alloy 230 and Alloy 825 were more resistant to solidification cracking than Alloy 800H, due to narrower solidification temperature range and crack back filling with eutectic constituents. The OSU Stress Relief Cracking (SRC) Test was applied to evaluate the susceptibility to stress relief cracking in autogenous gas tungsten arc welds of the investigated alloys. None of the three alloys failed by stress relief cracking mechanism while loaded at stress equal to 90% of the high temperature yield strength at 650 oC for 8 hours.. Alloys 825 and 800H showed significant amount of stress relief, while Alloy 230 sustained the applied load at 650C with almost no stress relief. Tensile testing at 650 oC after the 8 hours SRC test showed that the autogenous weld in Alloy 230 had significantly higher yield and tensile strength and slightly lower elongation at failure compared to the Alloy 825 and 800H welds. The fracture surface in Alloy 230 autogenous welds exhibited partially brittle interdendritic failure, while welds in the other two alloys failed in completely ductile micro-void coalescence mode.

Committee:

Boian Alexandrov (Advisor); Avraham Benatar (Advisor)

Subjects:

Materials Science; Metallurgy

Keywords:

solidification cracking; hot cracking; cast pin tear test; ss-dta; thermo-calc; stress relief cracking; gleeble;nickel based alloys; ni based alloys; autogenous weld; tensile test; characterization

Kuntz, Sarah LouiseFeasibility of Attaining Fully Equiaxed Microstructure through Process Variable Control for Additive Manufacturing of Ti-6Al-4V
Master of Science in Mechanical Engineering (MSME), Wright State University, 2016, Mechanical Engineering
One of the greatest challenges in additive manufacturing is fabricating titanium structures with consistent and desirable microstructure. To date, fully columnar deposits have been achieved through direct control of process variables. However, the introduction of external factors appears necessary to achieve fully equiaxed grain morphology using existing commercial processes. This work introduces and employs an analytic model to relate process variables to solidification thermal conditions and expected beta grain morphology at the surface of and at the deepest point in the melt pool. The latter is required in order to ensure the deposited microstructure is maintained even after the deposition of subsequent layers and, thus, the possibility of equiaxed microstructure throughout. By exploring the impact of process variables on thermal, morphological, and geometric trends at the deepest point in the melt pool, this work evaluates four commercial processes, estimates the range of process variables capable of producing fully equiaxed microstructure, and considers the expected size of the resultant equiaxed melt pool.

Committee:

Nathan Klingbeil, Ph.D. (Advisor); Joy Gockel, Ph.D. (Committee Member); Raghavan Srinivasan, Ph.D. (Committee Member)

Subjects:

Aerospace Materials; Engineering; Materials Science; Mechanical Engineering; Metallurgy; Morphology

Keywords:

additive manufacturing; 3D printing; titanium alloy; Ti64; Ti-6Al-4V; equiaxed morphology; equiaxed beta grains; solidification maps; process variable control; process parameter control; power, velocity, preheat; Rosenthal; Hunts criterion curve equations

Johnson, Jason RDeveloping the Axisymmetric Expanding Ring: A High Strain-Rate Materials Characterization Test
Doctor of Philosophy, The Ohio State University, 2014, Materials Science and Engineering
Towards the end goal of high rate tensile characterization of any material, existing ideas and newly developed technology have been combined in the form of a test platform dubbed the FIRE system. The acronym stands for Fully Instrumented Ring Expansion, a concept that is capable of evaluating the dynamic behavior of a wide range of materials in tension at strain rates well in excess of 1000/s. At the center of the design is a collection of techniques used to impulsively drive ring shaped samples radially outward in a highly symmetric fashion. This geometry avoids many of the traditional pitfalls associated with high rate testing such as end effects and critical extension speeds. Precision velocimetry has been adapted to the system utilizing state of the art optical and electronic equipment via a subassembly known as PDV (Photon Doppler Velocimetry). The PDV capabilities at present include determination of sample velocities up to 3.2 km/s with simultaneous displacement resolution on the order of 1-10 microns. As validation of the techniques developed, numerous representative material studies were carried out and compared to established data from other sources. Results were found to be in favorable agreement, verifying the efficacy of the methods. Additionally, the expanding ring test has been applied in conjunction with sample types and actuator technologies not reported previously. This provides an expanded usefulness to the test, which has been developed to the point now of being user friendly.

Committee:

Glenn Daehn, Dr. (Advisor); Michael Mills, Dr. (Committee Member); John Lippold, Dr. (Committee Member); Alan Hirvela, Dr. (Committee Member)

Subjects:

Electromagnetics; Engineering; Experiments; Materials Science; Mechanical Engineering; Mechanics; Metallurgy; Physics

Keywords:

Ring expansion; axisymmetric expanding ring test; high rate testing; high strain rate; OFHC copper; FIRE system; photon doppler velocimetry; PDV

Bryant, Nathan J.EXPERIMENTAL VALIDATION OF THE CALPHAD APPROACH APPLIED TO MULTI-PRINCIPLE ELEMENT ALLOYS
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

Keywords:

CALPHAD; High Entropy Alloys

Maya Visuet, EnriqueElectrolyte Transport And Interfacial Initiation Mechanisms Of Zinc Rich Epoxy Nanocoating/Substrate System Under Corrosive Environment
Doctor of Philosophy, University of Akron, 2015, Chemical Engineering
Zinc rich epoxy primers are an effective corrosion protection method to protect the steel structures against an aggressive environment. An aggressive environment like marine environment (high content in chloride ions) degrades the metal structure until a complete failure of the metal system. A way to avoid the degradation is to isolate the system form the environment. Zinc rich primers protect the steel by the sacrificial action of the zinc dust particles. A zinc corrosion products barrier layer protects the steel by blocking the passage of the aggressive species like oxygen and chlorides that favor the cathodic reaction. In this thesis, the corrosion protection properties of a commercial formulation (zinc rich primer with CNTs) system and prototype formulations (Zn/CNTs ratios) were studied as a function of the immersion time. Electrochemical corrosion measurements were implemented in non-dearated electrolytes of 0.6M NaCl (3.5 wt% NaCl) in distilled water with a pH=6. Besides the NaCl solution, alkali-halides solution identify the chloride effect with the change in the cation. The transport properties of these solutions defined the 0.6M NaCl the more aggressive solution to the bare metal and the coating-metal system.. Electrochemical Impedance Spectroscopy is the main AC technique to characterize the system and the DC Polarization curves, potentiostatic (galvanic couple) and OCP. Localized electrochemical impedance characterizes the commercial system to define the anodic and cathodic zone once the sample is immerse in the solution. ASTM-B117 was employed to evaluate the commercial and the prototye system. The samples were divided into two sections and one with 1 inch long indentation and tested by 30 days. High resolution techniques like an atomic force microscope (AFM) and scanning electron microscopy (SEM), characterizes the surface before and after immersion. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) identify the corrosion product formation to validate the corrosion protection properties. A long-term immersion of 360 days was followed for the commercial samples. Intact and pre-damage samples compared the corrosion protection mechanism of the zinc rich primers with CNTs. The potential change after the final day was close enough to consider the two types of samples were protected by means of the coating system. EIS shows the system with the pre-damage develop the cathodic protection mechanism once immersed in the electrolyte. On the other hand, the intact coating follows a barrier effect and then the cathodic protection system. A short-term immersion of 10 days was followed for the prototype systems. A barrier and cathodic effect are in effect once the electrolyte touches the system. This mixed mechanism of protection changes with the Zn/CNTs ratio. Low or non CNTs with high zinc protects the system differently than the non CNTs and low zinc and the high CNTs and high zinc. The differences reside in the Zn/CNTs matrix network they created. This network works as a physical and electrochemical barrier. Physical that repels water from the bare metal and electrochemical because the CNTs might act as a second cathode (instead of the carbon steel). This mixed effect protects the system with a barrier and a cathodic effect.

Committee:

Homero Castaneda, Dr. (Advisor); Scott Lillard, Dr. (Committee Member); Bi-min Zhang Newby, Dr. (Committee Member); Mark Soucek, Dr. (Committee Member); Chelsea Monty, Dr. (Committee Member)

Subjects:

Chemical Engineering; Materials Science; Metallurgy

Keywords:

Zinc rich epoxy coating; EIS; LEIS; CNT; cathodic protection; polarization; barrier protection; Zn-CNTs mixed effect; corrosion; aggressive environment; XPS; EDS; XRD; AFM; damage evolution; carbon steel; corrosion potential; blister

Noble, Margaret LauraDetermining the Variance and Distribution of Quantified Microstructure in α+β Processed Ti-6Al-4V
Master of Science, The Ohio State University, Materials Science and Engineering
The use of quantified microstructures as inputs to neural network models for property prediction has been pioneered by Center for the Accelerated Maturation of Materials (CAMM) at The Ohio State University. Through microstructure-property correlations, neural network models provide predictive tools for mechanical properties in titanium alloys while concurrently developing phenomenological models. The output accuracy (mechanical property prediction) of such models is therefore dependent on the variance and distribution of the input data (quantified microstructures). An estimation of the true variance and distribution can be calculated if a sufficiently large sampling volume of quantifiable microstructural features is available; however, current manual image processing and segmentation techniques made attainment of large-dataset image-processing unfeasible. In this work, a new generation of automated tools has been developed by CAMM which have reduced the total analysis time, including image capture, processing, and characterization to less than 30 seconds per micrograph for optically captured micrographs. Using a comparable SEM-based technique requires less than 6 minutes per micrograph due to extended image capture times. Serial sectioning of a meso-scale 3D volume (mm3) of α+β processed Ti-6Al-4V was collected for direct 3D quantification. Images were captured two ways: (1) using a Leica optical microscope in conjunction with Clemex image analysis software and (2) using a FEI Sirion SEM. In both cases, CAMM developed image processing package MIPAR was used to calculate the spatial variation in globular a area fraction. Comparisons between the two image capturing methods reveal similar trends in spatial variation indicating SEM-based imaging is only necessary if required by the scale of the particular microstructural feature of interest. A total of 37,800 micrographs were captured and processed. The large number of micrographs allows for accurate quantification of the variance and distribution of globular a volume fraction in α+β processed Ti-6Al-4V via application of bootstrap confidence intervals. The globular a volume fraction will be combined with other quantified microstructural features in the future to provide inputs for neural network models.

Committee:

Hamish Fraser (Advisor); William Clark (Committee Member)

Subjects:

Engineering; Materials Science; Metallurgy

Keywords:

Titanium; Quantified Microstructure; Optical Microscopy; SEM

Smith, Anthony JustinProcedure and Results for Constitutive Equations for Advanced High Strength Steels Incorporating Strain, Strain Rate, and Temperature
Master of Science, The Ohio State University, 2012, Mechanical Engineering
A wide range of advanced high strength steel (AHSS) sheets with nominal ultimate tensile strengths (UTS) from 590 to 1180 MPa and engineering strains to failure from 0.09 to 0.51 were tested to obtain the tensile flow stress under various combinations of strain, strain-rate, and temperature. Procedures were developed for selecting suitable constitutive forms corresponding to a generalized framework proposed in the literature by Sung et al. (Sung et al., 2010). Using the selected forms, the least-squares coefficients were obtained using an efficient optimization procedure that was also developed. The fit accuracy in all cases was similar to the test-to-test experimental scatter. Comparison with results from standard fitting schemes showed that the optimum coefficient values were found using the proposed, more time-efficient, methodology. The resulting constitutive models were compared with novel balanced biaxial bulge test measurements at strains up to several times larger than those accessible to tensile testing. The results show that constitutive models obtained by such procedures from standard tensile data can be extrapolated accurately to strain ranges of interest in bending-affected plastic localization (“shear fracture”).

Committee:

Dr. Robert H. Wagoner (Advisor); Dr. Rebecca B. Dupaix (Committee Member)

Subjects:

Materials Science; Metallurgy

Keywords:

Constitutive Equation; AHSS; Advanced High Strength Steel; Constitutive

Nunez Moran, Emerson OsvaldoEvaluation of the Localized Corrosion Resistance of 21Cr Stainless Steels
Master of Science, The Ohio State University, 2010, Materials Science and Engineering
Ferritic stainless steels have good corrosion resistance properties and lower cost than austenitic steels due to the lack of nickel. However, they have a lower formability than that of austenitics, and they show brittleness at low temperatures, near 475°C, and of welds. Pohang Steel Company (POSCO) has interest in a 21% Cr ferritic stainless steel, which is a concentration that is relatively unexplored. Pitting corrosion of stainless steels is associated with surface defects and heterogeneities in the matrix, in particular inclusions, however, there is little information about the initiation sites in clean steels, with low S content. Therefore, it is of interest to investigate where the most susceptible sites for pitting initiation are located, and the role they play. The corrosion resistance of the ferritic alloys was evaluated and compared to the performance of austenitic SS304 steel using crevice corrosion tests and cyclic polarization tests in chloride solution to determine the pitting and repassivation potentials of the alloys. The role that inclusions play during pitting was evaluated for the ferritic stainless steels through a chemical attack experiment where the alloys were exposed to an acidic chloride solution and the progression of the attack was assessed at defined inclusions and discrete time intervals. The pitting potential (Epit) distribution of the ferritic alloys shows values ranging from 100 mV to 450 mV higher than those observed on SS304, indicating a higher resistance to pit initiation for the ferritic steels. In the crevice corrosion test, SS304 showed higher repassivation potentials (Erep) than the ferritic steels and in the pitting corrosion test the Erep values were higher for the ferritic steels. In both cases, however, the difference in Erep was about 100 mV. The differences in Erep between crevice and pitting may be caused by a strong dependence of Erep on the charge density in the low charge density region associated to pitting. The attack under a crevice former has larger dimensions than a pit, and thus the crevice repassivation potential might be different than that for pits. The higher repassivation potential for deep crevices found for SS304 indicates a better resistance to localized corrosion propagation. The combination of this result with the higher pitting potentials for the ferritic stainless steels suggests that the localized corrosion resistance of the ferritic steels is about the same as for SS304. The inclusions present in the ferritic stainless steels acted as pit initiation sites, due to their cathodic behavior compared to the metal matrix, promoting the dissolution of the metal matrix surrounding them.

Committee:

Gerald Frankel (Advisor); Rudolph Buchheit (Committee Member)

Subjects:

Automotive Materials; Engineering; Materials Science; Metallurgy

Keywords:

Corrosion; pitting; NMI; Stainless steel; ferritic; inclusion

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