Engineering Commons^™

Characterization Of The Fine-Scale Weld Deposit Microstructure And Its Influence On The Elevated Temperature Properties Of 2.25cr - 1mo - 0.25v Weldments In Heavy Wall Pressure Vessels, Dewey Joshua Burgess

Doctoral Dissertations

The research herein was conducted to characterize the fine-scale microstructure of 2.25Cr‑1Mo‑0.25V (22V) submerged arc weld deposits and to study the influence of the microstructure on creep behavior and reheat cracking susceptibility. Scanning electron microscopy and transmission electron microscopy examinations concentrated on carbide morphology and evolution as a function of time and temperature, since the majority of properties that are associated with 22V weld deposits are attributable to the carbide type and location throughout the microstructure.

Five distinct carbides were observed in the range of heat treatments studied: MC, M2C, M23C6, M7C3, and M6C. It was shown that each carbide …

Fatigue, Fracture, And Environmentally-Assisted Behavior Of Advanced Engineering Materials, Zhi Tang

Doctoral Dissertations

The objective of the present study is to provide a fundamental understanding of fatigue, fracture, and environmentally-assisted behavior of high-entropy alloys (HEAs). The work involves fatigue, fracture, and environmentally-assisted behavior of a new kind of advanced engineering materials, called HEAs. Three tasks are studied: (1) microstructures and fracture mechanisms of HEAs, (2) fatigue failure and life prediction of HEAs, and (3) corrosion and environmentally-assisted behavior of HEAs.

In the first task, microstructural stability and fracture mechanism of the AlCoCrFeNi alloy are studied and compared with thermodynamic calculations. In the second task, high-cycle fatigue-failure mechanisms of the cold-rolled Al0.5CoCrCuFeNi alloy are …

Investigation Of The Phase Transformation Kinetics And Texture Evolution In A Trip Steel Under Complex Loads, Ercan Cakmak

Doctoral Dissertations

The martensitic phase transformation kinetics and its relation with texture evolution and deformation/transformation microstructures under complex loading conditions were investigated in a 304L stainless steel that exhibits the transformation induced plasticity (TRIP) effect when strained at ambient temperature. The applied load paths included torsional and biaxial deformation scenarios including simultaneous biaxial torsion/tension and torsion/compression as well as stepwise deformation of tension followed by torsion. Synchrotron x-ray and electron back-scatter diffraction techniques were used to investigate the phase transformation-microstructure-texture evolutions relations.

Under torsional deformation, the inhomogeneous distribution of martensite phase fractions were recorded through the radius of solid cylindrical specimens consistent …

High Energy Synchrotron X-Ray Study Of Fatigue Damage In Amorphous And Polycrystalline Engineering Alloys, Chih-Pin Chuang

Doctoral Dissertations

The dissertation summarizes a series of studies on the fatigue damage of both amorphous and crystalline engineering alloys. The attention focuses on the utilization of synchrotron x-ray scattering related techniques for advanced material characterizations. In the first part of the research, critical issues regarding the mechanical response and structural evolution of Bulk-Metallic Glasses (BMGs) in the elastic region are addressed. The effects of cyclic-loading on the microstructures of an amorphous alloy are investigated, aiming to provide better mechanistic understandings of fatigue damage in BMGs. The second part of the research focuses on the characterization method based on two-dimensional X-ray diffraction …

Understanding Mechanical And Fatigue Behavior Of A Wrought Magnesium Alloy Using In-Situ Neutron And Synchrotron X-Ray Diffraction, Wei Wu

Doctoral Dissertations

Magnesium (Mg) alloys have received great attentions in the past several decades, due to their unique properties of low density, high strength-to-weight ratio, and high specific stiffness. Previous work on fatigue behavior of Mg alloys typically relies on ex situ microstructural characterization and crack-growth monitoring with replica techniques. The primary challenge is thus the lack of an in situ, non-destructive measurement on microstructural length scales, which prevents us from linking the stress analyses (top-down point of view) to the failure mechanisms on inter- and intra-granular scales (bottom-up point of view). A unique opportunity that resolves many of these difficulties …

Metallurgical Effects On Long-Term Creep-Rupture In A New Nickel-Based Alloy, John P Shingledecker

Doctoral Dissertations

A series of creep-rupture studies were conducted on a new nickel-based alloy, Inconel Alloy 740, for use in Advanced Ultrasupercritical (A-USC) Steam boiler applications. The research quantitatively showed, for the first time, that the formation of a small amount of eta phase during long-term creep testing at 750 degrees Celsius does not degrade the rupture strength or ductility of the alloy. A unique full-scale pressurized tubular creep test was conducted, and a new analysis methodology was developed to evaluate the effect of cold-straining on the creep performance of tube bends made from the alloy. The results showed that 15% cold-work …

Processing, Microstructures, And Properties Of Aluminide-Strengthened Ferritic Steels, Zhenke Teng

Doctoral Dissertations

NiAl-strengthened ferritic steel is a material of interest for steam turbine applications due to their promising creep resistance, high thermal conductivity, low cost and thermal expansion coefficient. However, there are two critical issues restricting their applications: low room temperature ductility and limited creep resistance at temperatures higher than 600°C. The reasons for these behaviors have not been fully understood, especially the processing-microstructure-property relationship has not been clearly identified.

The alloys were designed, fabricated, and processed, according to the required microstructures and properties. The matrix and precipitates can be thoroughly characterized by a combination of techniques, including atom probe tomography, neutron …

Computational And Experimental Study Of Structure-Property Relationships In Nial Precipitate-Strengthened Ferritic Superalloys, Shenyan Huang

Doctoral Dissertations

Ferritic superalloys strengthened by coherent ordered NiAl B2-type precipitates are promising candidates for ultra-supercritical steam-turbine applications, due to their superior resistance to creep, coarsening, oxidation, and steam corrosion as compared to Cr ferritic steels at high temperatures. Combined computational and experimental tools have been employed to investigate the interrelationships among the composition, microstructure, and mechanical behavior, and provide insight into deformation micromechanisms at elevated temperatures.

Self and impurity diffusivities in a body-centered-cubic (bcc) iron are calculated using first-principles methods. Calculated self and impurity diffusivities compare favorably with experimental measurements in both ferromagnetic and paramagnetic states of bcc Fe. The calculated …

Shear-Band Formation And Thermal Activation In Metallic Glasses, Lu Wang

Doctoral Dissertations

Metallic glasses (MGs) usually have high strength, high hardness and high elastic strain limit. However, the deformation mode and mechanism in metallic glasses are radically different from those in conventional crystalline materials with a long-range ordered structure. For crystalline materials, the intrinsic relationship between their mechanical properties and crystal structures has been well described by dislocation theory. In contrast, for amorphous materials, theories on the structures and controlling factors of localized shear-band formation are far from being complete.

In this thesis, shear-banding behavior of MGs under nanoindentation was first reviewed. The hardness of MGs was found to be independent on …

Micromechanical Studies Of Intergranular Strain And Lattice Misorientation Fields And Comparisons To Advanced Diffraction Measurements, Lili Zheng

Doctoral Dissertations

Inhomogeneous deformation fields arising from the grain-grain interactions in polycrystalline materials have been evaluated using a crystal plasticity finite element method and extensively compared to neutron diffraction measurements under fatigue crack growth conditions. The roles of intergranular deformation anisotropy, grain boundary damage, and non-common deformation mechanisms (such as twinning for hexagonal close packed crystals) are systematically evaluated. The lattice misorientation field can be used to determine the intragranular deformation behavior in polycrystals or to describe the deformation inhomogeneity due to dislocation plasticity in single crystals. The study of indentation-induced lattice misorientation fields in single crystals sheds lights on the understanding …

Study Of Mechanical Behaviors And Structures Of Bulk Metallic Glasses With High-Energy Synchrotron X-Ray Diffraction, Feng Jiang

Doctoral Dissertations

This dissertation addresses two critical issues in the mechanical behaviors and structures of bulk-metallic glasses (BMGs): (1) the effect of composition, fabrication method, and pretreatment of plastic deformation on mechanical properties and structures of BMGs; (2) the mechanical response and structural evolution of BMGs in the elastic and plastic region.

(Cu₅₀Zr₅₀)₉₄Al₆ and (Cu₅₀Zr₅₀)₉₂Al₈ amorphous alloys were used to study the effect of composition on mechanical properties and structures of BMGs. The (Cu₅₀Zr₅₀)₉₄Al₆ alloy exhibits lower yield stress and Young’s …

Energetics And Kinetics Of Dislocation Initiation In The Stressed Volume At Small Scales, Tianlei Li

Doctoral Dissertations

Instrumented nanoindentation techniques have been widely used in characterizing mechanical behavior of materials in small length scales. For defect-free single crystals under nanoindentation, the onset of elastic-plastic transition is often shown by a sudden displacement burst in the measured load-displacement curve. It is believed to result from the homogeneous dislocation nucleation because the maximum shear stress at the pop-in load approaches the theoretical strength of the material and because statistical measurements agree with a thermally activated process of homogeneous dislocation nucleation. For single crystals with defects, the pop-in is believed to result from the sudden motion of pre-existing dislocations or …

Dependence Of Microstructure Evolution, Texture, And Mechanical Behavior Of A Mg Alloy On Thermo-Mechanical Input During Friction Stir Processing, Zhenzhen Yu

Doctoral Dissertations

In this thesis, the relationship among friction stir processing (FSP) parameters, microstructure evolution, texture development, and mechanical hehavior of AZ31B Mg alloy was investigated.

First of all, in order to reveal the correlation among the deformation conditions, dynamic recrystallization (DRX) mechanisms, and microstructure evolution in the Mg alloy, hot compression tests at a wide range of Zener-Hollomon parameter (Z) values were conducted. Through optical microscopic examination, it was found out that above a critical Z value, twinning influences the DRX process resulting in a more effective grain refinement, which is manifested in a significant change in the slope of the …

Laser Textured Calcium Phosphate Bio-Ceramic Coatings On Ti-6al-4v For Improved Wettability And Bone Cell Compatibility, Sameer R. Paital

Doctoral Dissertations

The interaction at the surfaces of load bearing implant biomaterials with tissues and physiological fluids is an area of crucial importance to all kinds of medical technologies. To achieve the best clinical outcome and restore the function of the diseased tissue, several surface engineering strategies have been discussed by scientific community throughout the world. In the current work, we are focusing on one such technique based on laser surface engineering to achieve the appropriate surface morphology and surface chemistry. Here by using a pulsed and continuous wave laser direct melting techniques we synthesize three dimensional textured surfaces of calcium phosphate …

Deformation Study Of Nanocrystalline Ni-Fe Alloy Using Synchrotron Diffraction, Li Li

Doctoral Dissertations

This dissertation addresses two critical issues in the deformation of nc metals and alloys: (1) A stress-induced genuine grain growth after the plastic deformation rather than just a change of the grain shape; (2) A systematically quantitative study of micrsostructural evolution during the plastic deformation.

These two critical issues point to the deformation of nc materials with the average-grain sizes within the range of 10 to 50 nm, which is the most interesting and controversial region in the current time. The current study provides a systematic and detailed microstructural evolution for this region, which is definitely beneficial for the investigation …

An Experimental Investigation Of Creep And Viscoelastic Properties Using Depth-Sensing Indentation Techniques, Barry Neal Lucas

Doctoral Dissertations

Broad band, quasi-static and frequency specific dynamic techniques adapted to depth-sensing indentation testing have been utilized to measure both time dependent plasticity (creep) at both room and elevated temperatures as well as time dependent elasticity (viscoelasticity) at room temperature.

Indentation Creep

Using a variety of depth-sensing indentation techniques at both room and elevated temperatures, the dependency of the indentation hardness on the variables of indentation strain rate (stress exponent for creep, n) and temperature (apparent activation energy for creep, Q), and the existence of a steady state behavior in an indentation creep test with a Berkovich indenter were investigated. The …

Contributions To The Specific Heat Capacity Of Nickel, Iron, And The Alloy Ni3Fe, Thomas Gilman Kollie

Doctoral Dissertations

The specific heat capacity at constant pressure C_p of a ferromagnetic solid is assumed separable into six contributions - that due to the dilation of the lattice C_d, that due to the harmonic oscillations of the atoms C_vh, that due to the anharmonic oscillations of the electrons C_va, that due to changes in energy distributions of the electrons C_ve, that due to changes in the magnetic coupling among the electrons C_vm, and that due to atomic rearrangements C_vw. Expressions appearing in the literature for these contributions, which …

The Application Of Dynamic Adiabatic Calorimetry To The Copper-Nickel System From 50 To 620 °C, Richard E. Pawel

Doctoral Dissertations

Introduction: The accurate measurement of changes in thermal energy in the solid state has long been the subject of investigators in many fields. Knowledge of thermal effects can contribute greatly to fundamental studies and the general understanding of many phenomena, as well as being the basis of thermodynamic and theoretical work leading to a better comprehension of the behavior of metals and alloys. High temperature calorimetric methods for the solid state may be naively considered to be identical, at least in principle, with the familiar adiabatic or isothermal type calorimeters used extensively in the vicinity of room temperature. A wealth …