Advanced scattering techniques for the investigation of radiation-induced damage in nuclear materials

Harsh fission and fusion reactor environments lead to radiation damage in nuclear materials in the form of nanoscale precipitation events. Such microstructural changes can ultimately compromise macroscopic material properties. Understanding the evolution of nuclear material properties over their operational lifetime is crucial for the provision of safe, reliable nuclear power generation. This thesis aims to demonstrate how advanced scattering techniques can contribute to the understanding of nanoscale transitions in nuclear materials systems. It focuses on materials of interest in the current nuclear landscape where knowledge gaps and contradictions exist on the structural and compositional properties of both thermal ageing induced and irradiation induced precipitates. Specifically, it presents novel applications of small angle scattering (SAS) techniques to the study of both thermal ageing induced and irradiation induced precipitation damage to showcase the versatility of SAS for characterising the differences between the two mechanisms of precipitation damage. The systems of interest are life-limiting nuclear materials such as low alloy reactor pressure vessel (RPV) steels and tungsten as a fusion plasma-facing material. These material systems were chosen as they are critical for the future of nuclear power generation (advanced light water fission reactors and commercial fusion reactors) where there is an industry requirement to increase the understanding of how thermal ageing induced and irradiation induced precipitation damage can lead to adverse material property evolution. A high Ni RPV weld thermally aged for 100 000 hours at 330 ◦C was studied using SAS to complement previous findings from other investigative techniques. Magnetic precipitates with mean radius 2.01 ± 0.06 nm were observed using small angle neutron scattering. The first use of anomalous small angle x-ray scattering on this RPV material system allowed for the explicit consideration of the role of iron and vacancies on thermal ageing induced precipitation which is often omitted in literature. A series of model Fe-Cu-Mn-Ni RPV alloys were thermally aged to elucidate the role of alloying elements on precipitate nucleation and growth. SAS was used to extract precipitate properties and also for a novel in-situ ageing study on the kinetics of precipitation. The structure and likely composition of thermal ageing induced precipitates was confirmed. It was found that Mn suppresses precipitate growth in the presence of Cu and Ni during thermal ageing. A comparison between the mechanisms of thermal ageing and irradiation induced precipitation was made. A novel proton irradiation configuration was used to investigate transmutation reactions and precipitation damage in pure tungsten. Displacement damage up to 0.414 dpa was reached, with small fractions of rhenium, osmium and tantalum transmuted during irradiation. Due to the limited transmutation, the matrix damage was identified, using SANS, as a network of spherical nanoscale voids that increase in size and volume fraction with increasing damage in irradiated tungsten. Material hardening and embrittlement due to the presence of nanoscale precipitates has been quantified for each material system using microindentation and nanoindentation hardness techniques

Assessment of thermally activated dislocation mechanisms via novel indentation approaches

The efficiency of gas turbines and jet engines used for energy generation and transportation can be increased by raising their combustion temperature. However, this is often limited by the materials used. For the development of new high-temperature materials, knowledge of the local mechanical properties of, for instance, individual phases in Ni-based superalloys is therefore of great importance. These properties are largely unknown, as they are not accessible with conventional macroscopic test methods. In the present work, the depth-sensing indentation testing technique was applied to assess the thermally activated deformation mechanisms on a local scale. For this purpose, a new in-situ indentation device was developed, which for the first time allows dynamic indentation experiments to be carried out on a small scale at temperatures of up to 1100 °C. Furthermore, a new indentation creep loading protocol was developed using a constant contact pressure approach similar to conventional uniaxial creep experiments. For indentation testing at high temperatures, a new step load method has been presented that allows a significant reduction of the contact time, thus minimizing the wear of the indenter tips. The method is suitable for the investigation of transients in material behavior at high to medium strain rates. In addition, a new approach for determining the brittle-ductile-transition temperature of body centered cubic metals was presented. In this approach, the change in the temperature-dependent activation volume was used to determine an intersection temperature that agrees well with the brittle-to-ductile-transition temperature from conventional Charpy pendulum impact tests

Improved micro-contact resistance model that considers material deformation, electron transport and thin film characteristics

This paper reports on an improved analytic model forpredicting micro-contact resistance needed for designing microelectro-mechanical systems (MEMS) switches. The originalmodel had two primary considerations: 1) contact materialdeformation (i.e. elastic, plastic, or elastic-plastic) and 2) effectivecontact area radius. The model also assumed that individual aspotswere close together and that their interactions weredependent on each other which led to using the single effective aspotcontact area model. This single effective area model wasused to determine specific electron transport regions (i.e. ballistic,quasi-ballistic, or diffusive) by comparing the effective radius andthe mean free path of an electron. Using this model required thatmicro-switch contact materials be deposited, during devicefabrication, with processes ensuring low surface roughness values(i.e. sputtered films). Sputtered thin film electric contacts,however, do not behave like bulk materials and the effects of thinfilm contacts and spreading resistance must be considered. Theimproved micro-contact resistance model accounts for the twoprimary considerations above, as well as, using thin film,sputtered, electric contact

Design of high temperature cobalt-based alloys processed by powder metallurgy route

Mención Internacional en el título de doctorThe superalloys are a family of alloys with great interest for their application in severe conditions due to their high specific resistance at high temperatures up to 800 °C. Traditionally, different industries have focused their efforts on implementing nickel-based superalloys due to their excellent performance in extreme environments as for example in hot gas turbine parts of aerospace industry. The discovery of Sato et al. [1] marked a milestone, when a Co-Al-W ternary system with a γ/γ’ dual phase microstructure was developed. The resulted precipitate phase has a stable stoichiometry L12 at high temperatures. This new family of cobalt-based alloys allowed to develop an alternative to conventional nickel-based. During the last decade, many investigations have confirmed its excellent versatility, suitable corrosion resistance and good mechanical properties and in severe conditions at high temperature. On the other hand, the processing by powder metallurgy route of this family of superalloys, has been advanced, in general terms, to improve the mechanical properties of compositions already developed. This type of technology has made the powder metallurgy route, a very attractive alternative for the design and development of new superalloys with a high degree of distribution elements and good mechanical properties. The main objective of this research work has been to design a new cobalt-based superalloy by powder metallurgy route with a dual γ/γ’ microstructure, with a nominal composition of Co- 12Al-10W (at.%) and Co-12Al-10W-2Ti-2Ta (at.%), as well as, the study of its mechanical properties at high strain rate conditions. To be suitable, the defined compositions were processed by Mechanical Alloying (MA) and consolidated by Field Assisted Sintering Techniques (FAST). To be able to perform a more precise comparison, both compositions were also processed by conventional casting route in a controlled atmosphere. Once the alloys were consolidated, different heat treatments were suggested to choose the most suitable γ/γ’ microstructure. All samples were characterized by Scanning Electron Microscope and Transmission Electron Microscope, as well as study composition by Energy Dispersive Spectrometer and the present phases by X-ray diffraction. The mechanical properties in terms of hardness were also studied by means of micro and nano indentation. Finally, the mechanical behavior under dynamic conditions (at high strain rate of 103 s-1), and simultaneously modifying the temperature in a range from room temperature to 850 °C was studied. A thermosvisco- plastic Johnson-Cook model was constituted to simulate the behavior the cobalt-based processed alloys in dynamic regimes.Las denominadas superaleaciones son una familia de aleaciones de gran interés para aplicaciones en condiciones severas, al proporcionar una elevada resistencia específica a unas temperaturas cercanas a 800 °C. Tradicionalmente, industrias como la aeroespacial, han focalizado sus esfuerzos en implementar las superaleaciones base níquel, debido a su excelente comportamiento en ambientes extremos como, por ejemplo, en zonas calientes de turbinas de gas. El descubrimiento por Sato et al. [1] marcó un hito, al constatar que en el sistema ternario Co-Al-W se podía conseguir una microestructura de tipo dual γ/γ’, donde la fase precipitada tenía una estequiometria L12 estable a altas temperaturas. Esta nueva familia de superaleación base cobalto permitió desarrollar una alternativa a las convencionales base níquel. Durante esta última década, diversas investigaciones han confirmado su excelente versatilidad, buenas propiedades mecánicas y resistencia a corrosión en condiciones extremas a alta temperatura. Por otro lado, el procesamiento por la ruta pulvimetalúrgica de esta familia de las superaleaciones, ha ido consiguiendo, en términos generales, mejorar las propiedades mecánicas de las composiciones ya desarrolladas anteriormente. Este tipo de tecnología ha hecho de la vía pulvimetalúrgica, una opción muy atractiva para el diseño y creación de nuevas superaleaciones con un alto grado de distribución de elementos y buenas propiedades mecánicas. El principal objetivo de esta tesis ha sido plantear a partir de la ruta pulvimetalúrgica una nueva superaleación base cobalto con una composición nominal Co-12Al-10W (at.%) y Co-12Al- 10W-2Ti-2Ta (at.%), así como el estudio de sus propiedades mecánicas en condiciones de alta velocidad de deformación. Para cumplir con él, las composiciones definidas se fabricaron en polvo mediante molienda mecánica de alta energía (MA del inglés Mechanical Alloying) y se consolidaron mediante técnicas asistidas por campo (FAST, del inglés Field Assisted Techniques). Para poder realizar de forma más precisa una comparativa con respecto a las mismas aleaciones obtenidas por moldeo, también fueron procesadas las dos composiciones por colada convencional en atmosfera controlada. Una vez consolidadas las aleaciones, se estudiaron diferentes tipos de tratamientos térmicos para poder elegir la microestructura más adecuada. Todas las muestras se caracterizaron mediante microscopia electrónica de barrido y transmisión, así como estudio de composición por EDS y de las fases presentes por difracción de Rayos X. También se estudiaron sus propiedades mecánicas mediante micro y nano identación. Finalmente se estudió el comportamiento mecánico en condiciones dinámicas (a alta velocidad de deformación, ε!=103 s-1), y modificando simultáneamente las temperaturas en un rango desde temperatura ambiente hasta 850 °C. Se constituyó así, un modelo termo-visco plástico de Johnson-Cook que simula el comportamiento del material en regímenes dinámicosThis work has been developed in the frame of a DIMMAT project funded by Madrid region under program S2013/MIT-2775.Programa Oficial de Doctorado en Ciencia e Ingeniería de MaterialesPresidente: Elena Gordo Oderiz.- Secretario: Álvaro Ridruejo Rodríguez.- Vocal: Borja Erice Echávarr

Nano to micrometric grain sized CVD diamond for turning hard and abrasive materials

Doutoramento em Ciência e Engenharia de MateriaisO presente trabalho consistiu no desenvolvimento de ferramentas de corte de diamante CVD (Chemical Vapour Deposition) obtido na forma de revestimento em materiais cerâmicos à base de nitreto de silício monolítico (Si3N4) ou compósitos nitreto de silício-nitreto de titânio (Si3N4-TiN). A adição de TiN acima de 23 vol.% conferiu conductividade eléctrica ao compósito, na ordem de 1×10-1 W−1.cm-1, possibilitando a sua maquinagem por electroerosão. Duas técnicas foram utilizadas para o crescimento dos filmes de diamante: deposição química em fase vapor por plasma gerado por microondas, MPCVD (Microwave Plasma Chemical Vapour Deposition), e por filamento quente, HFCVD (Hot Filament Chemical Vapour Deposition). Previamente os substratos cerâmicos sofreram uma preparação superficial por diversos métodos: rectificação por mós diamantadas; polimento com suspensão de diamante (15μm); ataque da superfície por plasma de CF4; riscagem manual ou por ultra-sons com pó de diamante (0.5-1.0 μm). A caracterização das ferramentas revestidas envolveu: o estudo da qualidade e tensões residuais dos filmes de diamante a partir da difracção dos raios X e espectroscopia Raman; a análise da respectiva microestrutura e medida da espessura por microscopia electrónica de varrimento (SEM); a determinação dos valores de rugosidade dos filmes por microscopia de força atómica (AFM); e a avaliação da adesão dos filmes aos substratos por indentação com penetrador Brale. Foram obtidos filmes com granulometria que variaram da gama do diamante nanométrico (< 100 nm) até ao micrométrico convencional (3-12 μm), com consequências na rugosidade superficial do filme. Os filmes de diamante CVD apresentaram espessuras de 15 a 150μm. Os revestimentos apresentaram elevada adesão ao substrato, sendo que o melhor resultado foi atingido pelo diamante micrométrico, suportando um limite de carga aplicada de até 1600 N. O estudo do comportamento em serviço das ferramentas foi efectuado na operação de torneamento de metal duro (WC-Co) e de eléctrodos de grafite, com medição de forças de corte em tempo real por meio de um dinamómetro. Os ensaios foram realizados num torno CNC, em ambiente industrial, na empresa Durit (Albergaria-a-Velha), produtora de metal duro. Os modos de desgaste das ferramentas foram avaliados por meio de observação em microscopia óptica e electrónica de varrimento e o grau de acabamento da superfície maquinada por rugosimetria. A influência destes parâmetros foi estudada em termos das forças envolvidas em operações de torneamento, desgaste das ferramentas e do acabamento conferido à peça maquinada. Os melhores resultados do torneamento de metal duro foram atingidos pelas ferramentas com geometria de aresta em quina-viva, recobertas com os filmes de diamante de 100-200 nm de tamanho de grão, correspondentes às menores forças de corte (<150N), melhor qualidade da peça maquinada (rugosidade aritmética igual a 0,2 μm) e menor desgaste (flanco igual a 110μm). No torneamento de eléctrodos de grafite, as forças de corte foram baixas (< 20N), sendo que o principal modo de desgaste foi a formação de cratera na superfície de ataque (valor máximo igual a 22 μm). O fio da aresta de corte permaneceu inalterado (devido ao mínimo desgaste de flanco), sendo que as diferentes granulometrias do diamante não tiveram influência significativa no comportamento geral das ferramentas.This work consisted on the development of CVD (Chemical Vapour Deposition) diamond cutting tools directly deposited on monolithic silicon nitride (Si3N4) based ceramics and silicon nitride-titanium nitride composites (Si3N4-TiN). A TiN content higher than 23 vol.% confers electric conductivity to the composite in the order of 1×10-1 W−1.cm-1, making possible its machinability by means of electrodischarge machining. Two techniques were used for diamond growth: Microwave Plasma Chemical Vapour Deposition (MPCVD) and Hot Filament Chemical Vapour Deposition (HFCVD). The substrate pre-treatment steps prior to diamond deposition were: grinding with diamond wheels; polishing with diamond suspension (15μm); chemical etching with CF4 plasma; manual scratching or ultrasonic bath scratching with diamond powder (0.5-1.0 μm) for seeding purposes. The diamond cutting tools characterization involved: study of the quality and the residual stress of the films by X ray diffraction and Raman spectroscopy; analysis of respective film microstructure and measurement of film thickness by scanning electron microscopy (SEM); quantification of film surface roughness by atomic force microscopy (AFM); evaluation of adhesion strength of the thin films to Si3N4 substrate by the indentation technique with a Brale indenter. The grain size of the films varied from nanometric (< 100 nm) to conventional micrometric (3-12 μm), therefore giving different surface roughness. The CVD diamond film thickness was in the range of (15-150 μm). The diamond films presented a high adhesion level to the Si3N4 ceramic substrates, the best results being achieved by the micrometric grain sized film, which undergo a normal load of until 1600N. The study of the cutting tool behaviour was performed in turning operations of hardmetal (WC-Co) and graphite electrodes, by real-time acquisition of the cutting forces using a dynamometer. The turning operations were carried out in a CNC lathe, at industrial environment of a hardmetal producer company, Durit (Albergaria-a-Velha). The wear modes of the tested cutting tools were analysed by optical and electronic microscopy observations and the finishing quality of the machined workpiece was measured by surface roughness measurements. The influence of these parameters was studied in terms of the cutting forces developed during turning operations, of tool wear and of the finishing quality of the machined workpieces. The best results attained in hardmetal turning were achieved by the cutting tools with sharp edges, covered with diamond films of 100-200 nm of grain size, which presented the lowest cutting forces (<150N), the best workpiece surface quality (Ra=0.2μm) and the lowest flank wear (110μm). In graphite turning, the cutting forces were very low (<20N) and the main wear mode was the crater one on the rake face (maximum value of 22μm). The cutting edge remained almost intact (due to the minimum flank wear) while the different diamond grain sizes did not have a significant influence on the overall cutting behaviour

Technology of Welding and Joining

In this book, you will find information on new materials and new welding technologies. Problems related to the welding of difficult-to-weld materials are considered and solved. The latest welding technologies and processes are presented. This book provides an opportunity to learn about the latest trends and developments in the welding industry. Enjoy reading

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

Batch-fabrication of novel nanoprobes for SPM

A micromachining method has been developed for fabricating 20µm tall silicon atomic force tips with flat tops less than 2µm wide suitable for defining nanosensors upon, and with low aspect ratio sides suitable for defining electrical connections to the sensor. Methods have been developed to allow flat substrate processing techniques to be applied to such non-planar micromachined substrates. This has necessitated the development of a novel resist-coating technique and the use of defocused electron-beam lithography. Methods for through-wafer alignment by electron-beam lithography and accurate alignment to the tips using micromachined alignment markers have also had to be developed. The fabrication process has been designed to enable a wide variety of sub-micron sensors to be defined on the atomic force probes, with little additional development beyond that of : sensors themselves. This flexibility has enabled very different sensors meant for very different scanning probe microscopy techniques to be designed without significant redevelopment of the underlying fabrication process. The main restrictions on the type of sensor that can be used are the physical dimensions of the sensor, the number of alignment levels necessary, the degree of alignment accuracy required and the choice of sensor materials. However, within these constraints it has been found that probes optimised for scanning near-field optical microscopy (SNOM), scanning thermal microscopy, modulation differential scanning calorimetry (MDSC) and scanning Hall-probe microscopy can be fabricated. For the SNOM probes three methods for fabricating sub-l00nm diameter apertures have been developed, analysed and compared with each other to evaluate both the process latitude. and, the size and reproducibility of apertures that can be fabricated, as a function of electron beam dose, pattern shape and size, and metallisation material and thickness. Two methods, both utilising multilayer 'resist' schemes have been found suitable for this purpose, one based on conventional electron-beam lithography with PMMA and a new dry etching process for titanium, and the other based on a novel electron-beam lithography technique utilising cross-linked PMMA for lifting off nichrome. A simple analytical model has also been developed for these probes allowing the effects of changes in the sensor design parameters on the light throughput to be compared qualitatively, if not quantitatively. For the scanning thermal probes a method for lifting-off sub-l00nm, thin-film thermocouple sensors on silicon tips without the loss of electrical continuity has been developed. For the MDSC probes, a similar method has been developed for defining thermal resistors. A method has also been presented for fabricating sensors for scanning Hall-probe microscopy based on an evaporated germanium sensing layer. This has been found to require annealing and optimisation of sensor design and geometry to reduce sensor resistance to acceptable levels

Crystal Plasticity (Volume II)

With the second volume, we continue our mission to providing theoretical and experimental research that contribute new insights and practical findings in the field of crystal plasticity-related topics. Once again, a completely new set of 26 original works (including 22 research articles, 3 communications, and 1 review) has been collected. As in the case of the first volume, here, a full spectrum of topics belonging to the field of crystal plasticity is represented, including both numerical simulations and experimental works. By taking into account the investigated materials, the papers can be assigned to the following thematic groups: Steels and iron-based alloys; Non-ferrous alloys with fcc- (Ni- and Cu-based), or hcp crystal structure (Mg- and Ti-based). Other examples include Zirconium, Bi-Sn alloy, or polycarbonate resins; Multicomponent and high-entropy alloys; General theoretical studies on crystal plasticity. Specifically, the reprint should be interesting for students of material science and engineering, Ph.D. candidates, and researchers dealing with various theoretical and practical aspects of plastic deformation in crystalline materials