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Continuum Dislocation Dynamics Modeling of Mesoscale Crystal Plasticity at Finite DeformationKyle R Starkey (12476760) 29 April 2022 (has links)
<p>Over the past two decade, there have been renewed interests in the use of continuum models of dislocation to predict the plastic strength of metals from basic properties of dislocations. Such interests have been motivated by the unique self-organized dislocation microstructures that develop during plastic deformation of metals and the need to understand their origin and connection with strength of metals. This thesis effort focuses on the theoretical development of a vector-density based representation of dislocation dynamics on the mesoscale accounting for the kinematics of finite deformation. This model consists of two parts, the first is the development of the transport-reaction equations governing dislocation dynamics within the finite deformation setting, and the second focuses on the computational solution of the resulting model. The transport-reaction equations come in the form of a set of hyperbolic curl type transport equations, with reaction terms that nonlinearly couple these equations. The equations are also geometrically non-linear due to finite deformation kinematics and by their constitutive closure. The solution of the resulting model consists of two parts that are coupled in a staggered fashion, the crystal mechanics equations are lumped in the stress equilibrium equations, and the dislocation transport-reactions equations. The two sets of equations are solved by the Galerkin and First-Order System Least-Squares (FOSLS) finite element methods. A special attention is given to the accurate modeling of glissile dislocation junctions using de Rahm currents and graph theory ideas. The introduction of these measures requires the derivation of further transport relations. Using homogenization theory, we specialize the proposed model to a mean deformation gradient driven bulk plasticity model. Lastly, we simulate bulk plasticity behavior and compare our results against experiments.</p>
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SURFACE ROUGHNESS AND SUPERHYDROPHOBICITY BEHAVIOR IN ELECTROCHEMICALLY-ETCHED FE- AND NI-BASED ALLOYSBenjamin P Smith (11820377) 09 December 2021 (has links)
<blockquote><div><div>Methods and techniques for tailoring the surface morphology of metallic surfaces are determined in part by the complex behavior of elemental interactions in conjunction with electrochemical reactions. In this work, we show how the surface morphology can be predicted based on experimental data resulting from polarization curves and compositional differences of Fe- and Ni-based superalloys. Electrochemical treatments utilizing NaCl as the electrolyte were adapted using parameters such as the pitting resistance equivalent (PRE) number and polarization curves to obtain both rough and smooth surfaces. Utilizing these metrics, we electrochemically etched Inconel 600, SS304, Inconel 718 and Inconel 625 obtaining average surface roughness values that ranged from 0.05 to 57.4 μm indicating the success of tailoring the technique to obtaining rough and smooth surfaces. The effect of current density, current pulsing, and temperature were varied to elucidate roughness and pitting behavior, and strong correlations to the PRE number and polarization curve properties of the alloy were observed. Heat treatments and subsequent evolution to the microstructure in the form of grain growth and precipitation altered the etching behavior. These techniques can be used in preventing corrosion failure and enhancing electrochemical machining</div></div></blockquote>
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<strong>Computational Modeling of Dislocation Microstructure Patterns at Small Strains Using Continuum Dislocation Dynamics</strong>Vignesh Vivekanandan (14047986) 25 July 2023 (has links)
<p> Self-organized dislocation structures in deforming metals have a strong influence on the mechanical response of metals. However, accurate prediction of these patterns remains a challenge due to the complex dynamic and multiscale nature of the underlying process. This dissertation focuses on the development of a theoretical framework for continuum dislocation dynamics (CDD) models to predict dislocation microstructure formation at small strains, along with corresponding numerical simulation results. CDD models have the capability to incorporate plasticity physics spanning different time and length scales while capturing the dislocation motion explicitly within reasonable computational time. A typical model consists of two components: crystal mechanics, formulated as an eigenstrain problem, and dislocation dynamics, treated as a transport-reaction problem. In the first part of the thesis, a novel framework is introduced to solve the dislocation transport by decoupling the system of transport-reaction equations and enforcing the dislocation continuity constraint on individual slip systems. The results obtained from this framework demonstrate high accuracy and computational efficiency, significantly enhancing the predictive capabilities of the model. Building upon the framework, a statistical analysis of stress fluctuations in discrete dislocation dynamics (DDD) simulations is conducted to understand the relationship between coarse-grained average stress and local stress states. This analysis is motivated by the need to accurately capture dislocation reactions, such as cross-slip, which strongly depend on the local stress state, using the coarse-grained approach in CDD. The results revealed that the difference between the local and the coarse-grained states can be characterized using a Cauchy distribution. Consequently, a novel strategy is proposed to incorporate these statistical characteristics into the CDD model, yielding cross-slip rate predictions that align well with DDD results. In the final part of the study, the developed framework is applied to investigate the dislocation pattern formation during the early stages of cyclic loading. The simulation results successfully capture the formation of dislocation vein like structure and provide insights regarding the formation of labyrinth structure observed in experiments during cyclic loading at saturated state. </p>
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Role of Dislocations on Martensitic Transformation and Microstructure through Molecular Dynamic SimulationsDavid Enrique Farache (16623762) 20 July 2023 (has links)
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<p>Martensitic transformation underlies the phenomena of super-elasticity within shape memory alloys and the production of advanced steels. Experimentation has demonstrated that defects and microstructural changes strongly influence this process. With simulations granting up to an atomic-level understanding of the impact that grain boundaries and precipitates have upon the solid-to-solid phase transformation. Yet the role that dislocations partake in the martensitic transformation and its microstructures remains unclear or disputed. </p>
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<p>Therefore, we utilize large-scale molecular dynamics (MD) simulations to study the forward and reverse transformation of martensitic material modeled after Ni63Al37 shape via thermal cycling loading. The simulations indicate that dislocations retain martensite well above the martensite start temperature and behave as nucleation sites for the martensite. We found that a reduction in dislocation density with cycle correlated with a decrement in the Ms and As transition temperatures, in agreement with the experiment. It was found that competing martensite variants could develop stable domains as dislocation density reduced sufficiently which resulted in multi-domain structures. Furthermore, the critical nuclei size of the martensite variant was able to be extracted from our results. </p>
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IRRADIATION BEHAVIORS IN MOLYBDENUM AND URANIUM-10WT.%MOLYBDENUM ALLOYS FROM THE ATOMISTIC SCALE TO THE MICROSCALEPark Gyuchul (12155445) 28 July 2022 (has links)
<p>Low enriched uranium (LEU, < 20 % 235U)-molybdenum (U-Mo) alloy is the primary nuclear fuel candidate for research and test reactors, and it is also considered one of the fuel candidates for fast reactors. Furthermore, U-Mo monolithic fuel is currently undergoing a qualification process to replace highly-enriched uranium (≥ 20 % 235U) fuel for high-performance research and test reactors. As part of the fuel qualification process, it is critical to examine the microstructural evolution in the final form of U-Mo fuel under irradiation. However, there is a lack of knowledge on the microstructural evolution of the rolled U-Mo alloy foil, which is a proposed geometry for research and test reactors/high-performance research and test reactors, as well as the U-Mo alloy fuel that is cast into a slug form without rolling, which is a more suitable geometry for other advanced reactor fuel types. The effects of the fabrication methods, specifically arc-casting and cold-rolling, on the phase decomposition in U-10Mo alloy subjected to low neutron fluence (0.01 displacements per atom (dpa) and 0.1 dpa) in the temperature range of 150–350oC are evaluated using synchrotron X-ray techniques and scanning electron microscopy (SEM). The as-cast U-10Mo alloys demonstrated better irradiation performance than the U-10Mo alloy foils in research reactor conditions for the investigated regimes. This study will help optimize fuel fabrication techniques to tune phase decomposition under irradiation. </p>
<p>Additionally, irradiation behavior in Mo, a critical element in U-Mo fuel, as well as a candidate for cladding material for the next generation of nuclear power plants, is investigated at the atomistic scale following low neutron fluence regimes (0.01 dpa and 0.1 dpa) in the temperature range of 150–800oC using synchrotron X-ray techniques. Lattice contraction was observed in irradiated Mo by synchrotron XRD, indicating that interstitial diffusion is faster than vacancy diffusion in Mo. More interstitials diffuse into the sinks, such as grain boundaries, while fewer vacancies diffuse into the sinks due to slow diffusion, resulting in a higher steady-state concentration of vacancies than that of interstitials under irradiation. The synchrotron PDF also supported the synchrotron XRD results by demonstrating a decrease in the atomic distances under irradiation. </p>
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Radiation response and mechanical properties of FeCrAl alloyTianyi Sun (13163040) 27 July 2022 (has links)
<p>Nuclear fission energy has developed for more than five decades and become one of the most important low-carbon energy forms. The extreme environment in the advanced reactors, including high operating temperature and high neutron radiation dose, raises new challenges for structural materials. To date, no materials are immune to radiation damage. Bombardment by energetic particles displaces atoms from their original sites, leaves various forms of defect aggregates after cascade, and degrades the properties of the irradiated materials. FeCrAl alloys, known for their excellent high-temperature oxidation resistance, were developed under the accident tolerant fuel program in hope to replace the Zr cladding alloys in future reactors. The radiation response and mechanical properties of FeCrAl alloy have attracted great attention. The objective of this thesis is two-fold. First, investigate the high temperature mechanical behavior of coarse-grained FeCrAl alloys with and without irradiation from the perspective of small-scale testing. Second, develop a fine-grained FeCrAl alloy variant and evaluate its mechanical properties and radiation tolerance.</p>
<p>Critical resolved shear stress of pristine and proton irradiated CG FeCrAl alloy was quantified at elevated temperatures. {112}<111> slip system exhibited higher irradiation induced hardening compared with the {110}<111> slip system. Gradient FeCrAl alloy was fabricated through surface mechanical grinding treatment. In situ pillar compression tests revealed an excellent combination of strength and deformability of ultra-fine-grained (UFG) FeCrAl alloys. The activation energy for plastic deformation of a nanolaminate (NL) FeCrAl alloy was determined through strain rate jump tests. Ex situ Fe-ion irradiation showed the interplay between dislocation loops and grain coarsening and their contributions to the mechanical properties of the irradiated UFG FeCrAl alloys. In situ Kr ion irradiation studies on the helium pre-injected NL FeCrAl and CG FeCrAl show that the helium induced swelling was effectively reduced in NL alloy due to their abundant grain boundaries serving as defect sinks. The findings in this thesis may provide innovative perspectives on the design and manufacture of novel FeCrAl alloys with outstanding mechanical properties and radiation tolerance.</p>
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<b>3D Correlative Microscopy to Understand Processing-Structure Relationships in Laser Powder Bed Fusion Aluminum Refined by In Situ Reactions</b>Daniel Ritchie Sinclair (19200673) 23 July 2024 (has links)
<p dir="ltr">The production of aluminum components by laser powder bed fusion additive manufacturing (LPBF-AM) offers simultaneous weight reduction benefits through low material density and topology optimization. The primary limitation of the method – hot cracking in high-strength compositions – is addressed by the reactive additive manufacturing (RAM) process, which introduces ceramic-forming metallic particles to powder feedstock. <i>In situ</i> reactions subsequently inoculate equiaxed grains, prevent cracking, and strengthen the resulting alloy. The adoption of RAM alloys in aerospace applications requires the elimination of heterogeneous defects, requiring an understanding of laser processing effects and feedstock quality. To meet these needs, the collected work presents characterization methods based on x-ray tomography, seeking to establish novel descriptors for RAM feedstock and microstructures.</p><p dir="ltr">In the first two chapters, x-ray microscopy (XRM) is applied to produce multi-dimensional particle measurements for feedstock powder qualification. Evolving existing measure-and-classify processes, a method is described to characterize AA7050-RAM2 feedstock that is rapid, interpretable, and descriptive of the highly deformed particles observed. Applying the developed methodology to an analysis of recycled AA7050-RAM2 rationalizes decreasing particle sizes by identifying the selective removal of specific shape classes. Combined with quantitative electron microscopy of particle microstructures, sieving and heat effects are comprehensively reported, demonstrating a modernized powder analysis workflow.</p><p dir="ltr">In the second two chapters, the characteristic reactions seen in LPBF of AA7050-RAM2 are characterized. Correlative SEM/EDS and nanoindentation identified reactive phases and their mechanical properties and found a correlation between the extent of the Al-Ti reaction and the degree of particle remelting. Using 3D XRM measurements, the populations and distributions of low- and high-reaction particles were quantified, raising questions regarding homogenization mechanisms in laser-processed, particle-reinforced alloys. Thus, thin wall samples were produced and characterized to visualize convective and thermal history effects within symmetrical tracks. Novel observed mechanisms include thermal grain coarsening, keyhole-induced convection, and pore segregation by size. The accumulated microstructural quantification and novel perspective on pore movement provide a basis to improve contouring processes in RAM alloys and to better align fluid dynamics models of printing with experimental data.</p>
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MECHANICAL PROPERTIES AND RADIATION RESPONSE OF NANOSTRUCTURED FERRITIC-MARTENSITIC STEELSZhongxia Shang (9171533) 17 November 2022 (has links)
<p>Structural metallic materials
exposed to energetic particle bombardments often experience various types of
irradiation-induced microstructural damage, thus degrading the mechanical
properties of the materials in form of irradiation hardening and embrittlement.
Nanostructured materials have shown better radiation resistance than their
coarse-grained (CG) counterparts due to the existence of abundant defect sinks,
such as grain boundaries, twin boundaries, and phase boundaries. However,
recently developed nanocrystalline (NC) steels show limited room-temperature
tensile ductility (< 1%), which may become a concern for their future
application for nuclear reactors. The focus of this thesis is to explore the
strength-ductility dilemma in modified 9Cr1Mo (T91) ferritic/martensitic (F/M)
steel processed by thermomechanical treatment (TMT) and surface severe plastic
deformation (SSPD) with an attempt to fabricate strong, ductile and radiation
resistant F/M steels. </p>
<p><b>Carbon partitioning</b>
between the quenched martensite and the other phases (bainitic ferrite or
retained austenite) is critical for enhancing the strength and ductility of T91
steel. The tensile properties of partially tempered (PT) T91 steel can be
tailored through introducing bainitic ferrite with high-density nanoscale
transition carbides and refined lath martensite. In addition, retained
austenite was introduced by increasing the carbon concentration of T91 steel to
0.6 wt.%. The carbon-modified steel processed by quenching partitioning (Q-P)
treatment exhibits an ultrahigh strength, ~ 2 GPa, with a uniform strain of ~
5% due to the existence of coherent carbides, ultrafine martensite and retained
austenite. </p>
<p>Meanwhile, surface mechanical
grinding treatment (SMGT) on T91 steel reveals that introducing <b>gradient
structures</b> on the sample surface contributes to a higher strength and an
improved plasticity than its homogeneously structured counterpart. The
deformation mechanism of the gradient structures was investigated with the
assistance of quasi <i>in situ</i> crystal orientation analyses. Furthermore, <i>ex
situ</i> He ion irradiation on the gradient T91 steel indicates that
radiation-induced damage, such as bubble-induced swelling and irradiation
hardening, were gradually mitigated by grain refinement from the sample surface
to the center, resulting in superior radiation resistance. The results obtained
from this thesis may facilitate the design and fabrication of strong, ductile
and radiation-tolerant F/M steels.</p>
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THERMOMECHANICAL MEASUREMENTS OF ZIRCALOY-4: APPLICATION OF RAMAN THERMOMETRY AND NANO-MECHANCIAL TESTING TECHNIQUESHao Wang (7486526) 17 October 2019 (has links)
Zirconium alloys (zircaloy) have been widely used in light water reactors due to their good thermomechanical properties, corrosion resistance, and low thermal neutron absorption rate. As one of the most important safety barriers, cladding is not only used to encapsulate nuclear fuel, but also to prevent the nuclear fission products from leaking into the coolant. During the operation of nuclear reactors, hydride will form in zircaloy and significantly degrade the tensile strength, ductility, fracture toughness, and creep behavior of the cladding, and eventually leading to the failure of cladding. Therefore, understanding the material properties of zircaloy and its hydrides is crucial to the safety of power plants. In this study, the mechanical Raman spectroscopy and nano-mechancial testing techniques were used to perform thermomechanical measurements and damage analysis of zircaloy-4. The Raman thermometry method was used to measure localized spatially resolved thermal conductivity and establish the potential linkage of microstructure to thermal and mechanical properties of zircaloy-4. The local thermal conductivity values showed to increase with increase in grain size. Nanoindentation and nano-scale impact techniques were used to obtain the viscoplastic constitutive relation of hydrides at elevated temperatures. Based on the obtained viscoplastic model, fracture strength of hydrides was predicted by using finite element method (FEM) simulations. An extended Gurson-Tvergaard-Needleman (GTN) model was used to study the macro-scale fracture behavior of hydrided zircaloy-4 structures. Good agreement between calculated and experimental results was obtained for various boundary conditions.
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FABRICATION, PLASTICITY AND THERMAL STABILITY OF NANOTWINNED AL ALLOYSQiang Li (7041092) 12 October 2021 (has links)
<p>Applications of Aluminum (Al) alloys in harsh
environments involving high stress and high temperatures are often hindered
because of their inherently low strength and poor performance at high
temperatures. The strongest commercial Al alloys reported up to date have a
maximum strength less than 700 MPa. Although ultrafine grained Al alloys
prepared by severe plastic deformation have higher strength, they encounter
grain growth at moderate temperatures. </p>
<p>This thesis focuses on adopting transition metal
solutes and non-equilibrium approach to fabricate high-strength, thermally
stable nanotwinned Al alloys. To understand the underlying deformation
mechanisms of nanotwinned Al alloys, <i>in-situ</i>
micromechanical tests, high resolution and analytical transmission microscopy
and atomistic simulations were used. Our studies show that nanotwinned
supersaturated Al-Fe alloys have a maximum hardness and flow stress of ~ 5.5
GPa and 1.6 GPa, respectively. The apparent directionality of the vertical
incoherent twin boundaries renders plastic anisotropy and compression-tension
asymmetry in the nanotwinned Al-Fe alloys, revealed by systematic <i>in-situ</i> tensile and compressive
micromechanical experiments conducted from both in-plane and out-of-plane
directions. Moreover, the nanotwinned Al-Fe alloys experience no apparent
softening when tested at 200 °C. When selectively incorporating with one
additional solute as stabilizer, the ternary nanotwinned Al alloys can preserve
an exceptionally high flow stress, exceeding 2 GPa, prior to precipitous
softening at an annealing temperature of > 400 °C. The thesis offers a
new perspective to the design of future strong, deformable and thermally stable
nanostructured Al alloys. </p>
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