Micromechanics in additively manufactured metals using electron beam-based powder bed fusion

Els metalls obtinguts mitjançant fusió de llits de pols (PBF, Powder Bed Fusion), concretament per la tècnica de fusió per feix d'electrons (EB, Electron Beam), poden tenir propietats diferents, ja siguin físiques o mecàniques, depenent de la deposició d'energia del feix d'electrons i d'altres paràmetres de fabricació. El fet de conèixer les propietats mecàniques dels materials permet explicar el seu comportament davant d'estímuls externs, i una tècnica molt utilitzada per fer-ho és la de realitzar assajos de nanoindentació. En el present estudi, s'han analitzat una sèrie de mostres de metalls produïts mitjançant PBF-EB utilitzant diferents paràmetres de manufactura mitjançant assaigs de nanoindentació amb l'objectiu de conèixer els canvis que es produeixen a les propietats mecàniques (concretament, la duresa i el mòdul elàstic), i correlacionar-los amb la microestructura. Paral·lelament, s'han dut a terme anàlisis estadístics de les dades obtingudes als assajos mitjançant algoritmes d'aprenentatge automàtic (Machine Learning). Concretament, s'han aplicat mètodes d'agrupació (clustering), amb la intenció de crear un mètode robust que sigui capaç de, a partir de les dades dels assaigs de nanoindentació, identificar determinades característiques del material, concretament quines fases conté, com estan distribuïdes i les orientacions cristal·lines que tenen.Los metales obtenidos mediante fusión de lechos de polvo (PBF, Powder Bed Fusion), concretamente por la técnica de fusión por haz de electrones (EB, Electron Beam), pueden tener propiedades diferentes, ya sean físicas o mecánicas, dependiendo de la deposición de energía del haz de electrones y otros parámetros de fabricación. El hecho de conocer las propiedades mecánicas de los materiales permite explicar su comportamiento frente a estímulos externos, y una técnica muy utilizada para ello es la de realizar ensayos de nanoindentación. En el presente estudio, se han analizado una serie de muestras de metales producidos mediante PBFEB usando diferentes parámetros de manufactura mediante ensayos de nanoindentación con el objetivo de conocer los cambios que se producen en las propiedades mecánicas (concretamente, la dureza y el módulo elástico), y correlacionarlos con la microestructura. Paralelamente, se han llevado a cabo análisis estadísticos de los datos obtenidos en los ensayos mediante algoritmos de aprendizaje automático (Machine Learning). Concretamente, se han aplicado métodos de agrupación (clustering), con la intención de crear un método robusto que sea capaz de, a partir de los datos de los ensayos de nanoindentación, identificar determinadas características del material, concretamente qué fases contiene, cómo están distribuidas y sus orientaciones cristalinas.Metals obtained by Powder Bed Fusion (PBF), specifically by Electron Beam (EB), can have different properties, either physical or mechanical, depending on the electron beam energy deposition and other manufacturing parameters. The knowledge on the mechanical properties of materials makes it possible to explain their behavior in the face of external stimuli, and a widely used technique for this is to perform nanoindentation tests. In the present study, a set of metallic samples produced by PBF-EB using different manufacturing parameters have been analyzed performing nanoindentation tests with the aim of understanding the changes that occur in the mechanical properties (specifically, hardness and elastic modulus), and correlating them with the microstructure. At the same time, statistical analysis of the data obtained in the tests was carried out by Machine Learning algorithms. Specifically, clustering algorithms have been applied, with the intention of creating a robust method capable of identifying, from the nanoindentation test data, identify certain characteristics of the material, specifically which phases it contains, how they are distributed and the crystalline orientations they have

A Comparative Study on Representativeness and Stochastic Efficacy of Miniature Tensile Specimen Testing

In this article, a miniature dog bone tensile coupon design was tested against the existing ASTM standard specimen design. Specimens were prepared from commercially sourced austenitic stainless steel 304 alloy, and a defect-ridden additively manufactured 304L alloy was studied. By utilizing a tensile specimen design that is 1/230th volume of the smallest ASTM E8-04 (2016), Standard Test Methods for Tension Testing of Metallic Materials, dog bone specimen, coupled to a digital image correlation (DIC) setup, case studies were performed to compare tensile property measurements and strain field evolution. Whereas yield strength measurements were observed to be similar, post-yield, the ultimate strength measurements and ductility measurements from the miniature specimens were observed to be higher than the ASTM specimen design. Although the strength measurements were comparable, the strain evolution was found to differ in the miniature specimens. Studies to assess effects of varying thickness and defect population were also pursued on the miniature tensile specimen. From the DIC strain field estimations, the peak local strain values at ultimate tensile strength were observed to be increasing with reducing specimen thickness. Testing of defect ridden stainless steel revealed the sensitivity to failure through strain localization and the influence of defect size was captured in the strength measurements

Symmetry Breaking in Few Layer Graphene Films

Recently, it was demonstrated that the quasiparticle dynamics, the layer-dependent charge and potential, and the c-axis screening coefficient could be extracted from measurements of the spectral function of few layer graphene films grown epitaxially on SiC using angle-resolved photoemission spectroscopy (ARPES). In this article we review these findings, and present detailed methodology for extracting such parameters from ARPES. We also present detailed arguments against the possibility of an energy gap at the Dirac crossing ED.Comment: 23 pages, 13 figures, Conference Proceedings of DPG Meeting Mar 2007 Regensburg Submitted to New Journal of Physic

Manufacturing Metrology

Metrology is the science of measurement, which can be divided into three overlapping activities: (1) the definition of units of measurement, (2) the realization of units of measurement, and (3) the traceability of measurement units. Manufacturing metrology originally implicates the measurement of components and inputs for a manufacturing process to assure they are within specification requirements. It can also be extended to indicate the performance measurement of manufacturing equipment. This Special Issue covers papers revealing novel measurement methodologies and instrumentations for manufacturing metrology from the conventional industry to the frontier of the advanced hi-tech industry. Twenty-five papers are included in this Special Issue. These published papers can be categorized into four main groups, as follows: Length measurement: covering new designs, from micro/nanogap measurement with laser triangulation sensors and laser interferometers to very-long-distance, newly developed mode-locked femtosecond lasers. Surface profile and form measurements: covering technologies with new confocal sensors and imagine sensors: in situ and on-machine measurements. Angle measurements: these include a new 2D precision level design, a review of angle measurement with mode-locked femtosecond lasers, and multi-axis machine tool squareness measurement. Other laboratory systems: these include a water cooling temperature control system and a computer-aided inspection framework for CMM performance evaluation

Anisotropic Response of Laser Additively Manufactured Nuclear Alloys to Radiation Damage

The impact of radiation-induced effects on the properties of alloys fabricated using additive manufacturing (AM) was evaluated through the implementation of ion beam irradiation testing followed by electron backscatter diffraction (EBSD), scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), nanoindentation, scanning probe microscopy (SPM), and transmission electron microscopy (TEM). Inconel 600 (I600) and 316L stainless steel (316L) rods were fabricated by Quad City Manufacturing Laboratory in collaboration with Lockheed Martin for this study. The rods were produced in three distinct orientations (vertical, horizontal, and 45°) using laser additive manufacturing (LAM). Conventionally manufactured I600 and 316L rods were purchased from Metal Samples, Inc. to enable comparative studies. The I600 and 316L LAM specimens were heat treated to 900 °C and 650 °C in argon with no cold working, respectively. Similarly, the conventionally manufactured I600 and 316L control specimens were cold rolled and annealed at 980 °C and 1040 °C in argon with no cold working, respectively. XRD of unirradiated specimens showed differences in peak ratios between build orientations, indicating anisotropic grain structures for samples fabricated by LAM. All LAM rods contained significantly fewer coincidence site lattice (CSL) boundaries and more residual strain compared to the controls before and after irradiation, regardless of build direction, as determined by EBSD. Material performance parameters such as resistance to radiation-enhanced embrittlement, corrosion, creep, intergranular stress corrosion cracking, and hydrogen-induced cracking were inferred from CSL theory, which suggests that all LAM rods are more susceptible to grain boundary-related failure mechanisms than their conventionally manufactured counterparts. All alloys built by LAM are strongly textured with parallel to the build direction before and after irradiation. Directionally dependent Taylor Factor distributions suggest that resistance to slip depends on build direction where, from highest to lowest resistance: horizontal > 45° > vertical. All I600 samples experienced radiation-induced segregation which, according to SEM/EDS and SPM studies, resulted in the formation of chromium carbide precipitates on to the irradiated surfaces. Strong anisotropic mechanical behavior was observed in the LAM rods, as measured by nanoindentation and bulk tensile testing. The hardness of the unirradiated as-annealed specimens, from greatest to least, is: horizontal > 45° > vertical. The radiation-induced hardening of LAM specimens, from greatest to least, is: horizontal > 45° > vertical. The orientation dependence of radiation-induced segregation and hardening mechanisms is discussed. The ultimate outcome of this work is a first-of-a-kind high-dose radiation damage study of alloys fabricated by LAM, revealing that the radiation-induced changes in material properties for these alloys is dependent upon build orientation

Microstructural and Interface Engineering of Garnet-Type Fast Ion-Conductor for Use in Solid-State Batteries

Large-scale adoption of electric vehicles requires batteries with higher energy density, lower cost, and improved safety compared to state-of-the-art (SOA) Li-ion batteries. This dissertation addresses the great-unmet need to develop beyond Li-ion batteries to facilitate the transition to electric power trains. The successful integration of metallic Li anodes into rechargeable batteries will enable a step increase in energy density compared to SOA Li-ion technology. However, the unstable nature of the electrode-electrolyte interface has limited the use of metallic Li anodes when paired with conventional organic solvent-based electrolytes. One approach to stabilize the metallic Li anode interface involves the integration of a solid-state electrolyte (SSE). Theoretical predictions suggest that Li dendrites will not form if a SSE exhibits a shear modulus that is approximately twice the shear modulus of metallic Li (GLi= 4.2 GPa) or higher. This criterion indicates that ceramic Li-ion conducting solid-state electrolytes (SSE) can prevent dendrites. Thus, the development of solid-state batteries using SSE has been overlooked as a potential means to stabilize the metallic Li anode during cycling. The garnet-type Li-ion conductor, Li7La3Zr2O12 (LLZO), is an example of a SSE that exhibits the unique combination of high Li-ion conductivity (1 mS.cm-1 at 298 K) and stability against metallic Li. Additionally, LLZO has a shear modulus 14 times higher than metallic Li, thus should act as a physical barrier to prevent Li dendrite formation according to computational analysis. However, despite satisfying the shear modulus criterion, Li metal propagation has been observed in polycrystalline LLZO. This dissertation hypothesizes that atomistic and microstructural defects such as porosity, grain boundaries, interfaces, and surface impurities govern the stability of the Li-LLZO interface. The effect of each defect was isolated through ceramic processing and analyzed using a suite of characterization tools such as X-ray diffraction, electron backscatter, scanning and transmission electron microscopy, electron energy loss, acoustic, and Raman spectroscopy, direct current cycling and complex impedance, Vickers indentation, and fracture toughness measurements. The overarching goal of this dissertation was to better understand the phenomena that control the stability of the Li-LLZO interface, quantify the contributions of each defect, and develop engineering approaches to tailor the LLZO microstructure and interface for maximum resistance to Li metal propagation during cycling. The implications of this dissertation could accelerate the development of high energy density solid-state batteries.PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/140865/1/sharafi_1.pd

A holistic review on fatigue properties of additively manufactured metals

Additive manufacturing (AM) technology is undergoing rapid development and emerging as an advanced technique that can fabricate complex near-net shaped and light-weight metallic parts with acceptable strength and fatigue performance. A number of studies have indicated that the strength or other mechanical properties of AM metals are comparable or even superior to that of conventionally manufactured metals, but the fatigue performance is still a thorny problem that may hinder the replacement of currently used metallic components by AM counterparts when the cyclic loading and thus fatigue failure dominates. This article reviews the state-of-art published data on the fatigue properties of AM metals, principally including $S$--$N$ data and fatigue crack growth data. The AM techniques utilized to generate samples in this review include powder bed fusion (e.g., EBM, SLM, DMLS) and directed energy deposition (e.g., LENS, WAAM). Further, the fatigue properties of AM metallic materials that involve titanium alloys, aluminum alloys, stainless steel, nickel-based alloys, magnesium alloys, and high entropy alloys, are systematically overviewed. In addition, summary figures or tables for the published data on fatigue properties are presented for the above metals, the AM techniques, and the influencing factors (manufacturing parameters, e.g., built orientation, processing parameter, and post-processing). The effects of build direction, particle, geometry, manufacturing parameters, post-processing, and heat-treatment on fatigue properties, when available, are provided and discussed. The fatigue performance and main factors affecting the fatigue behavior of AM metals are finally compared and critically analyzed, thus potentially providing valuable guidance for improving the fatigue performance of AM metals.Comment: 201 pages, 154 figure

X-ray scattering studies of compound semiconductors

In this thesis, techniques of high resolution x-ray diffraction, topography and grazing incidence reflectivity have been employed in order to gain information on compound semiconductors. A recent growth technique. Vertical Gradient Freeze (VGF), has been investigated for 2" InP wafers, and been found to produce virtually dislocation- free crystals. In the one wafer where dislocations have been imaged, they have a density of ~200cm(^-2), with Burgers vectors lying in the plane of the wafer. This is in contrast to topographs of Liquid Encapsulated Czochralski (LEG) InP, where a dislocation density of up to 6.10'(^4)cm(^-2) was observed at the wafer periphery. No growth striations were observed in VGF samples, implying a planar solid-melt interface. VGF GaAs wafers are seen to be virtually dislocation free when Si doped, have dislocation densities of 900cm(^-2) when undoped and 1200cm(^-2) when Zn doped. Triple axis diffraction measurements showed a variation in tilts between/samples, but no strain variation. The tilt variation was attributed to the polishing process. Asymmetric scans showed a variation in strain at high tilts. These data have been used to form a model for the crystal surface: mosaic blocks of perfect crystal surrounded by low angle boundaries consisting of rows of edge dislocations. During the polishing process, these blocks are physically rotated, evidence for which is supplied from analysis of the specular part of reflectivity scans. Specular and diffuse reflectivity scans on InP substrates have been simulated using the Distorted Wave Born Approximation (DWBA). In all cases a 30Å thick oxide layer was identified on the sample surface. In order to obtain a good simulation for transverse scans at two values of q(_z), it was necessary to include a grading in electron density at the top surface. Epitaxial layers of Hg(_1-x)Mn(_x)Te (MMT) grown by the Interdiffused Multilayer Process (IMP) on GaAs with a CdTe buffer layer have been characterised using double and triple axis diffraction. Although reasonable compositional uniformity was observed across the wafers (from 0.3%mm(^-1)), dynamical simulations of pseudo-triple axis scans showed a grading in composition with depth. It was observed that the crystalline perfection deteriorated with increased Mn fraction. The MMT and CdTe layers were almost fully relaxed, and were found to have dislocation densities of l0(^7)-l0(^9)cm(^-2). In one sample the presence of zinc blende MnTe was established using double axis diffraction. Finally, the high intensity of the European Synchrotron Radiation Facility (ESRF) has been exploited in order to topograph highly absorbing materials. The effect of heater failure in the growth of GaAs in space has been shown to produce high levels of strain and twinning. It has also been shown topographically that contact with the crucible during the growth of GaInSb from the melt leads to increased strain, so de-wetting phenomena improve crystalline growth

Synthesis, Characterization, and Device Applications of Two-Dimensional Materials

Two-dimensional (2D) materials have attracted tremendous research interest, as they offer novel physics, facile visualization by electron and scanning probe microscopy, and the potential to become next-generation electronic materials, all due to reduced dimensionality. Large-area 2D single crystals are needed for both fundamental scientific experiments and electronic device applications. New methods need to be developed to exploit state-of-the-art microscopy in the scientific investigation of 2D materials. Mechanisms behind the behavior of 2D-material based devices need to be resolved and new device concepts and applications need to be explored. This dissertation addresses these three aspects of 2D materials research.Using chemical vapor deposition growth of graphene on copper as a platform, the first part of my research in this thesis demonstrates a facile method involving a simple in-situ treatment of the copper catalytic substrate right before the growth that results in mm-sized graphene single crystals, elucidating the key factors of achieving large-area 2D single crystals.The second part of this work developed experimental methods to resolve important issues in 2D materials research by employing modern transmission electron microscopy. Here, a method has been developed to determine the edge orientation and termination without imaging the edge down to the atomic scale of monolayer hexagonal boron nitride (h-BN), enabling a direct comparison to theoretical predictions. Another important issue in 2D materials research is the determination of the layer count and its lateral spatial uniformity. In this work, a method is developed to map the layer count of a 2D material at nanometer-scale lateral resolution over extended areas, utilizing a combination of mass-thickness mapping offered by STEM and element-specific quantization afforded by electron energy loss spectrum (EELS) mapping.The last part of this thesis work unravels the multiple mechanisms behind the behavior of field effect transistors (FETs) based on PdSe2. The change in device behavior in early reports from ambipolar to n channel was puzzling. As commonly encountered in device research, many factors, including channel material properties, defects, contaminants, and contact effects, are almost always entangled. Here, I use multiple control devices to unravel various mechanisms and provide consistent explanations for device behvior variations

Surface characterisation of semiconductor materials

Several well-established x-ray characterisation techniques have been developed to obtain high resolution for applications where high strain sensitivity and surface sensitivity are important X-ray methods are compared with other characterisation methods and a range of x-ray techniques is reviewed. The double-axis diffractometer and its capabilities are described. Dynamical x-ray diffraction theory for distorted crystals, and the theory of diffuse scattering from randomly distributed defects are reviewed. X-ray reflectivity theory is also covered. Several complementary characterisation techniques have been developed: Double-axis diffiactometry using a four-reflection beam conditioner to measure surface scattering in the rocking-curve tails, topography using highly strain sensitive conditions at grazing incidence, with both a conventional x-ray source and synchrotron radiation, and energy dispersive reflectometry using a high-energy x-ray source. A range of samples has been characterised, including silicon wafers machined and polished under different conditions and from different manufacturers, silicon epiwafers, and ion implanted silicon. In the rocking-curve analysis, modelling and simulation were used to determine the residual surface strain-depth profiles. Silicon wafers polished using a mechanical- chemical technique were found to have a lattice expansion of 4 to 8 parts per million near the surface, decreasing linearly to zero at up to one micron depth. Topography was used to detect strains of order 10-7 in polished silicon wafers. Strains were measured at the edges of polished areas which had been etched away, enabling strain relaxation. Energy dispersive reflectometry enabled determination of surface roughness of polished silicon wafers, down to Angstrom resolution. The techniques developed can be used widely in the characterisation of semiconductor materials. Rocking-curve analysis in particular is an extremely useful tool for the assessment of wafer quality and monitoring and development of the wafer production process