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- Department of Mechanical and Materials Engineering: Faculty Publications (10)
- Department of Mechanical and Materials Engineering: Dissertations, Theses, and Student Research (3)
- Department of Civil and Environmental Engineering: Dissertations, Theses, and Student Research (1)
- Department of Engineering Mechanics: Dissertations, Theses, and Student Research (1)
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Articles 1 - 17 of 17
Full-Text Articles in Engineering
Twin-Solute, Twin-Dislocation And Twin-Twin Interactions In Magnesium, Materials Yue, Jian Wang, Jian-Feng Nie
Twin-Solute, Twin-Dislocation And Twin-Twin Interactions In Magnesium, Materials Yue, Jian Wang, Jian-Feng Nie
Department of Mechanical and Materials Engineering: Faculty Publications
Magnesium alloys have received considerable research interest due to their lightweight, high specific strength and excellent castability. However, their plastic deformation is more complicated compared to cubic materials, primarily because their low-symmetry hexagonal closepacked (hcp) crystal structure. Deformation twinning is a crucial plastic deformation mechanism in magnesium, and twins can affect the evolution of microstructure by interacting with other lattice defects, thereby affecting the mechanical properties. This paper provides a review of the interactions between deformation twins and lattice defects, such as solute atoms, dislocations and twins, in magnesium and its alloys. This review starts with interactions between twin boundaries …
Crystalline–Amorphous Nanostructures: Microstructure, Property And Modelling, Binqiang Wei, Lin Li, Lin Shao, Jian Wang
Crystalline–Amorphous Nanostructures: Microstructure, Property And Modelling, Binqiang Wei, Lin Li, Lin Shao, Jian Wang
Department of Mechanical and Materials Engineering: Faculty Publications
Crystalline metals generally exhibit good deformability but low strength and poor irradiation tolerance. Amorphous materials in general display poor deformability but high strength and good irradiation tolerance. Interestingly, refining characteristic size can enhance the flow strength of crystalline metals and the deformability of amorphous materials. Thus, crystalline–amorphous nanostructures can exhibit an enhanced strength and an improved plastic flow stability. In addition, high-density interfaces can trap radiation-induced defects and accommodate free volume fluctuation. In this article, we review crystalline–amorphous nanocomposites with characteristic microstructures including nanolaminates, core–shell microstructures, and crystalline/amorphous-based dual-phase nanocomposites. The focus is put on synthesis of characteristic microstructures, deformation …
Closed-Loop Control Of Meltpool Temperature In Directed Energy Deposition, Ziyad M. Smoqi, Ben Bevans, Aniruddha Gaikwad, James Craig, Alan Abul-Haj, Brent Roeder, Bill Macy, Jeffrey E. Shield, Prahalada K. Rao
Closed-Loop Control Of Meltpool Temperature In Directed Energy Deposition, Ziyad M. Smoqi, Ben Bevans, Aniruddha Gaikwad, James Craig, Alan Abul-Haj, Brent Roeder, Bill Macy, Jeffrey E. Shield, Prahalada K. Rao
Department of Mechanical and Materials Engineering: Faculty Publications
The objective of this work is to mitigate flaw formation in powder and laser-based directed energy deposition (DED) additive manufacturing process through close-loop control of the meltpool temperature. In this work, the meltpool temperature was controlled by modulating the laser power based on feedback signals from a coaxial two-wavelength imaging pyrometer. The utility of closed-loop control in DED is demonstrated in the context of practically inspired trapezoid-shaped stainlesssteel parts (SS 316L). We demonstrate that parts built under closed-loop control have reduced variation in porosity and uniform microstructure compared to parts built under open-loop conditions. For example, post-process characterization showed that …
Part-Scale Thermal Simulation Of Laser Powder Bed Fusion Using Graph Theory: Effect Of Thermal History On Porosity, Microstructure Evolution, And Recoater Crash, Reza Yavari, Ziyad Smoqi, Alex Riensche, Ben Bevans, Humaun Kobir, Heimdall Mendoza, Hyeyun Song, Kevin Cole, Prahalada Rao
Part-Scale Thermal Simulation Of Laser Powder Bed Fusion Using Graph Theory: Effect Of Thermal History On Porosity, Microstructure Evolution, And Recoater Crash, Reza Yavari, Ziyad Smoqi, Alex Riensche, Ben Bevans, Humaun Kobir, Heimdall Mendoza, Hyeyun Song, Kevin Cole, Prahalada Rao
Department of Mechanical and Materials Engineering: Faculty Publications
Flaw formation in laser powder bed fusion (LPBF) is influenced by the spatiotemporal temperature distribution – thermal history – of the part during the process. Therefore, to prevent flaw formation there is a need for fast and accurate models that can predict the thermal history as a function of the part shape and processing parameters. In previous work, a thermal modeling approach based on graph theory was used to predict the thermal history in LPBF parts in less-than 20% of the time required by finite element-based models with error within 10% of experimental measurements. The present work transitions toward the …
Process-Structure Relationship In The Directed Energy Deposition Of Cobalt-Chromium Alloy (Stellite 21) Coatings, Ziyad M. Smoqi, Joshua Toddy, Harold (Scott) Halliday, Jeffrey E. Shield, Prahalada K. Rao
Process-Structure Relationship In The Directed Energy Deposition Of Cobalt-Chromium Alloy (Stellite 21) Coatings, Ziyad M. Smoqi, Joshua Toddy, Harold (Scott) Halliday, Jeffrey E. Shield, Prahalada K. Rao
Department of Mechanical and Materials Engineering: Faculty Publications
In this work, we accomplished the crack-free directed energy deposition (DED) of a multi-layer Cobalt- Chromium alloy coating (Stellite 21) on Inconel 718 substrate. Stellite alloys are used as coating materials given their resistance to wear, corrosion, and high temperature. The main challenge in DED of Stellite coatings is the proclivity for crack formation during printing. The objective of this work is to characterize the effect of the input energy density and localized laser-based preheating on the characteristics of the deposited coating, namely, crack formation, microstructural evolution, dilution of the coating composition due to diffusion of iron and nickel from …
Anomalous Eutectic Microstructures In Mg-Al Structural Alloy Prepared By Rapid Solidification, Soodabeh Azadehranjbar, Jian Wang, Jeffrey E. Shield
Anomalous Eutectic Microstructures In Mg-Al Structural Alloy Prepared By Rapid Solidification, Soodabeh Azadehranjbar, Jian Wang, Jeffrey E. Shield
Department of Mechanical and Materials Engineering: Dissertations, Theses, and Student Research
Magnesium is the lightest engineering metal 1 However, conventional Mg alloys typically suffer from low strength and poor deformability due to very few slip systems and easy twinning 3 Alloying Mg with other materials and microstructural engineering are promising approaches to increase ductility and strength of Mg In the current work, non equilibrium solidification conditions were applied to induce a transition from regular to anomalous eutectic in Mg Al eutectic alloy such that four distinguished microstructures were acquired and the corresponding formation mechanisms were investigated
An Integrated Microstructural-Nanomechanical-Chemical Approach To Examine Material-Specific Characteristics Of Cementitious Interphase Regions, Mahdieh Khedmati, Yong-Rak Kim, Joseph A. Turner, Hani Alanazi, Charles Nguyen
An Integrated Microstructural-Nanomechanical-Chemical Approach To Examine Material-Specific Characteristics Of Cementitious Interphase Regions, Mahdieh Khedmati, Yong-Rak Kim, Joseph A. Turner, Hani Alanazi, Charles Nguyen
Department of Mechanical and Materials Engineering: Faculty Publications
Effective properties and structural performance of cementitious mixtures are substantially governed by the quality of the interphase region because it acts as a bridge transferring forces between aggregates and a binding matrix and is generally susceptible to damage. As alternative binding agents like alkali-activated precursors have obtained substantial attention in recent years, there is a growing need for fundamental knowledge to uncover interphase formation mechanisms. In this paper, two different types of binding materials, i.e., fly ash-based geopolymer and ordinary portland cement, were mixed with limestone aggregate to examine and compare the microstructures and nanomechanical properties of interphase region. To …
An Integrated Microstructural-Nanomechanical-Chemical Approach To Examine Material-Specific Characteristics Of Cementitious Interphase Regions, Mahdieh Khedmati, Yong-Rak Kim, Joesph A. Turner, Hani Alanazi, Charles Nguyen
An Integrated Microstructural-Nanomechanical-Chemical Approach To Examine Material-Specific Characteristics Of Cementitious Interphase Regions, Mahdieh Khedmati, Yong-Rak Kim, Joesph A. Turner, Hani Alanazi, Charles Nguyen
Department of Mechanical and Materials Engineering: Faculty Publications
Effective properties and structural performance of cementitious mixtures are substantially governed by the quality of the interphase region because it acts as a bridge transferring forces between aggregates and a binding matrix and is generally susceptible to damage. As alternative binding agents like alkali-activated precursors have obtained substantial attention in recent years, there is a growing need for fundamental knowledge to uncover interphase formation mechanisms. In this paper, two different types of binding materials, i.e., fly ash-based geopolymer and ordinary portland cement, were mixed with limestone aggregate to examine and compare the microstructures and nanomechanical properties of interphase region. To …
Multiple Scattering Theory For Polycrystalline Materials With Strong Grain Anisotropy: Theoretical Fundamentals And Applications, Huijing He
Department of Mechanical and Materials Engineering: Faculty Publications
This work is a natural extension of the author’s previous work: “Multiple scattering theory for heterogeneous elastic continua with strong property fluctuation: theoretical fundamentals and applications” (arXiv:1706.09137 [physics.geo-ph]), which established the foundation for developing multiple scattering model for heterogeneous elastic continua with either weak or strong fluctuations in mass density and elastic stiffness. Polycrystalline material is another type of heterogeneous materials that widely exists in nature and extensively used in industry. In this work, the corresponding multiple scattering theory for polycrystalline materials with randomly oriented anisotropic crystallites is developed. To validate the theory, the theoretical results for a series of …
Micro/Nanostructures Formation By Femtosecond Laser Surface Processing On Amorphous And Polycrystalline Ni60nb40, Edwin Peng, Alfred Tsubaki, Craig A. Zuhlke, Meiyu Wang, Ryan Bell, Michael J. Lucis, Troy P. Anderson, Dennis R. Alexander, George Gogos, Jeffrey E. Shield
Micro/Nanostructures Formation By Femtosecond Laser Surface Processing On Amorphous And Polycrystalline Ni60nb40, Edwin Peng, Alfred Tsubaki, Craig A. Zuhlke, Meiyu Wang, Ryan Bell, Michael J. Lucis, Troy P. Anderson, Dennis R. Alexander, George Gogos, Jeffrey E. Shield
Department of Mechanical and Materials Engineering: Faculty Publications
Femtosecond laser surface processing is a technology that can be used to functionalize many surfaces, imparting specialized properties such as increased broadband optical absorption or superhydrophilicity/superhydrophobicity. In this study, two unique classes of surface structures, below surface growth (BSG) and above surface growth (ASG) mounds, were formed by femtosecond laser surface processing on amorphous and polycrystalline Ni60Nb40 with two different grain sizes. Cross sectional imaging of these mounds revealed thermal evidence of the unique formation processes for each class of surface structure. BSG mounds formed on all three substrates using the same laser parameters had similar surface …
Coercivity Enhancement And Gamma Phase Avoidance Of Alnico Alloys, Li Zhang
Coercivity Enhancement And Gamma Phase Avoidance Of Alnico Alloys, Li Zhang
Department of Engineering Mechanics: Dissertations, Theses, and Student Research
The promotion of social progress requires greater levels of energy efficiency, quality and productivity. However, these developments usually come at the cost of the environment. Green technologies such as electric vehicles, wind turbine and solar panels are ironically overshadowed by supply limitations and high prices of rare earth elements. Therefore, it is important to find alternative materials to replace those that contain critical elements. Alnico alloys show high magnetization, high Curie temperature (800°C) and good corrosion resistance, making it one of the best candidates to replace neodymium-based magnets used in electric vehicles.
In this thesis, methods controlling shape anisotropy and …
Structural, Magnetic And Microstructural Studies Of Composition-Modified Sm-Co Ribbons, Xiujuan Jiang
Structural, Magnetic And Microstructural Studies Of Composition-Modified Sm-Co Ribbons, Xiujuan Jiang
Department of Mechanical and Materials Engineering: Dissertations, Theses, and Student Research
There is an increasing interest in developing desirable microstructures in hard magnetic materials. Sm-Co-based magnets, bearing superior intrinsic magnetic properties, are good candidates for further development. Two Sm-Co-based alloys, (Sm12Co88)100-x-yCryCx (taking SmCo7 phase) and SmCo4-xFexB (a derivative of SmCo5 phase), were produced using melt-spinning technique. The magnetic properties are correlated to the structural and microstructural properties.
Within the SmCo7 stoichiometry, cumulative effects of Cr and C additions on the structural and magnetic properties have been investigated. Experimental results have shown that these additions along …
Microstructure Selection Of Sm-Co-Al Alloys To Increase Magnetization, Brian Dick
Microstructure Selection Of Sm-Co-Al Alloys To Increase Magnetization, Brian Dick
Department of Mechanical and Materials Engineering: Dissertations, Theses, and Student Research
An ever increasing demand for higher performing magnets drives the need for new and innovative methods to achieve this goal. Sm-Co rare earth permanent magnets have a unique eutectic microstructure that, through refinement, could become a two-phase magnet which would significantly increase their energy product. The eutectic structure of Sm8Co92 is comprised of αCo rods embedded within a Sm2Co17 matrix. If the rods are small enough to encourage exchange coupling and the matrix is smaller than the single domain limit, then an efficient two-phase magnet is created.
Refining the Co rods and matrix size …
Virtual Microstructure Generation Of Asphaltic Mixtures, Mohammad Haft-Javaherian
Virtual Microstructure Generation Of Asphaltic Mixtures, Mohammad Haft-Javaherian
Department of Civil and Environmental Engineering: Dissertations, Theses, and Student Research
This thesis describes the development and application of a virtual microstructure generator incorporated with post-processing image analysis methods that can be used to fabricate a virtual, two-dimensional microstructure of asphaltic mixtures. In the generator, geometrical characteristics such as aggregate gradation, aggregate area fraction, angularity, orientation, and elongation were used to transform data from a three-dimensional (3D) mixture into its two-dimensional (2D) microstructure. The 2D virtual microstructures were generated from real 3D mixture information of asphaltic composites. Resulting virtual microstructures were then compared to real cross-sectional microstructure images obtained from actual samples for validation. Comparison presented a good agreement between the …
Synthesis And Characterization Of Highly Textured Pt–Bi Thin Films, Xingzhong Li, Parashu Kharel, Shah R. Valloppilly, David J. Sellmyer
Synthesis And Characterization Of Highly Textured Pt–Bi Thin Films, Xingzhong Li, Parashu Kharel, Shah R. Valloppilly, David J. Sellmyer
Nebraska Center for Materials and Nanoscience: Faculty Publications
Pt–Bi films were synthesized on glass and thermally oxidized silicon substrates by e-beam evaporation and annealing. The structures were characterized using X-ray diffraction (XRD) and transmission electron microscopy/selected area electron diffraction (TEM/SAED) techniques. Single-phase PtBi was obtained at an annealing temperature of 300°C, whereas a higher annealing temperature of 400°C was required to obtain the highly textured γ-PtBi2 phase. TEM/SAED analysis showed that the films annealed at 400°C contain a dominant γ-PtBi2 phase with a small amount of β-PtBi2 and α-PtBi2 phases. Both the PtBi and γ-PtBi2 phases are highly textured in these two kinds of film: the c-axis of …
Microstructure And Mechanical Properties At Different Length Scales And Strain Rates Of Nanocrystalline Tantalum Produced By High-Pressure Torsion, Q. Wei, Z. L. Pan, X. L. Wu, B. E. Schuster, L. J. Kecdkes, R. Z. Valiev
Microstructure And Mechanical Properties At Different Length Scales And Strain Rates Of Nanocrystalline Tantalum Produced By High-Pressure Torsion, Q. Wei, Z. L. Pan, X. L. Wu, B. E. Schuster, L. J. Kecdkes, R. Z. Valiev
US Army Research
Fully dense, nanocrystalline tantalum (average grain size as small as ~40 nm) has been processed for the first time by high-pressure torsion. High-resolution transmission electron microscopy reveals non-equilibrium grain boundaries and grains decorated with high-density dislocations. Microhardness measurements and instrumented nanoindentation experiments indicate that the mechanical property is quite uniform except for the central area of the disks. Nanoindentation experiments at different strain rates suggest that the strain rate sensitivity of nanocrystalline tantalum is increased compared to the coarse- and ultrafine-grained counterparts and is accompanied by an activation energy of the order of a few ~b3 (b is the magnitude …
Chemcial Vapor Deposition Precursor Chemistry. 5. The Photolytic Laser Deposition Of Aluminum Thin Films By Chemical Vapor Deposition, John A. Glass, Seong-Don Hwang, Saswati Datta, Brian W. Robertson, James T. Spencer
Chemcial Vapor Deposition Precursor Chemistry. 5. The Photolytic Laser Deposition Of Aluminum Thin Films By Chemical Vapor Deposition, John A. Glass, Seong-Don Hwang, Saswati Datta, Brian W. Robertson, James T. Spencer
Department of Mechanical and Materials Engineering: Faculty Publications
Thin films of very high purity aluminum were formed from the laser photolysis of trimethylamine alane (TMAA) using both ultraviolet (pulsed nitrogen) and visible (argon ion) laser irradiation on a variety of substrates including gold. Si (111), GaAs (110) and Teflon (PTFE). At thicknesses of up to 1 μm, nearly linear growth rates of 377 Ås-1 and 112 Ås-1 were observed. The formation of volatile species formed during the deposition of aluminum from TMAA was investigated by quadrupole mass spectrometry (QMS) of the reactant gas stream. The highest intensity post-deposition mass fragments were observed at m/z 58, …