The mechanical behavior of structural materials used in nuclear applications is significantly degraded as a result of irradiation, typically characterized by an increase in yield stress, localization of inelastic deformation along narrow dislocation channels, and considerably reduced strains to failure. Further, creep rates are accelerated under irradiation. These changes in mechanical properties can be traced back to the irradiated microstructure which shows the formation of a large number of material defects, e.g., point defect clusters, dislocation loops, and complex dislocation networks. Interaction of dislocations with the irradiation-induced defects governs the mechanical behavior of irradiated metals. However, the mechanical properties are seldom systematically correlated to the underlying irradiated microstructure. Further, the current state of modeling of deformation behavior is mostly phenomenological and typically does not incorporate the effects of microstructure or defect densities. The present research develops a continuum constitutive crystal plasticity framework to model the mechanical behavior and deformed microstructure of bcc ferritic/martensitic steels exposed to irradiation. Physically-based constitutive models for various plasticity-induced dislocation migration processes such as climb and cross-slip are developed. We have also developed models for the interaction of dislocations with the irradiation-induced defects. A rate theory based approach is used to model the evolution of point defects generated due to irradiation, and coupled to the mechanical behavior. A void nucleation and growth based damage framework is also developed to model failure initiation in these irradiated materials. The framework is used to simulate the following major features of inelastic deformation in bcc ferritic/martensitic steels: irradiation hardening, flow localization due to dislocation channel formation, failure initiation at the interfaces of these dislocation channels and grain boundaries, irradiation creep deformation, and temperature-dependent non-Schmid yield behavior. Model results are compared to available experimental data. This framework represents the state-of-the-art in constitutive modeling of the deformation behavior of irradiated materials.

Dual-phase (DP) steels consisting of a ferrite matrix with dispersed martensite particles have attracted a significant interest due to their combination of high work hardening and ductility. A great deal of experimental work has been done to obtain a better comprehension of the relation of their mechanical behaviour to their microstructural characteristics. In the present work, a micromechanical study of ferrite-martensite DP steels is conducted. The deformation of ferrite is described by a rate-dependent crystal plasticity theory, which relates the stress-strain field equations on the grain level to the macroscopic behaviour of the material. The crystal plasticity theory assumes that slip is the only deformation mechanism. Martensite, on the other hand, is considered an elastic-plastic isotropic solid. The interfaces of the grains are taken into account through an idealized form of grain boundaries. A FORTRAN program was coupled with the finite element method to solve the stress equations of the crystal plasticity. Including the grain boundaries made it possible to examine the effect of ferrite grain size on the strength of the material. It is shown that by decreasing the grain size, the yield stress increases according to Hall-Petch equation. Additionally, the effects of the volume fraction of martensite (Vm) on the onset strain, i.e. the strain at which martensite deforms plastically, and of the distribution of martensite on the stress are studied. The former showed that the onset strain of the DP steel declines linearly with increasing Vm up to 36%, beyond which the onset strain becomes independent of V m. The latter revealed that when martensite particles are formed as islands in the ferrite grains, the material exhibits higher strength and hardening rate; compared to when martensite is distributed as large blocks among the ferrite grains.

Submitted by Maria Medeiros (maria.dilva1@ufcg.edu.br) on 2018-08-17T15:03:40Z No. of bitstreams: 1 JAIME MATIAS DA SILVA NETO- DISSERTAÇÃO (PPGEM) 2018.pdf: 3496418 bytes, checksum: 6c6b6ae8738da4d0e77cbcedcb6bb9d6 (MD5) / Made available in DSpace on 2018-08-17T15:03:40Z (GMT). No. of bitstreams: 1 JAIME MATIAS DA SILVA NETO- DISSERTAÇÃO (PPGEM) 2018.pdf: 3496418 bytes, checksum: 6c6b6ae8738da4d0e77cbcedcb6bb9d6 (MD5) Previous issue date: 2018-02-27 / CNPq / O presente trabalho teve como objetivo avaliar a influência da aplicação de vibração mecânica durante o processo da soldagem sobre o refino de grãos em metais de solda obtidos na soldagem do aço ASTM A516 Gr.60, aplicado em caldeiras e vasos de pressão. Para o tratamento de vibração mecânica foi utilizado o processo VWC (Condicionamento de Soldagem Vibratória), e para a soldagem foram empregados dois tipos de processos: SMAW (Arco Elétrico com Eletrodo Revestido) e GMAW (Soldagem ao arco elétrico com proteção gasosa). Os consumíveis utilizados foram o arame AWS ER 70S-6 com 1,2 mm de diâmetro protegido com CO2 no processo GMAW e o eletrodo AWS E7018 com 2,25 mm de diâmetro no processo SMAW. Foi projetada e desenvolvida uma bancada vibratória para realização dos experimentos. As juntas foram executadas sem e com vibração, submetidas a amplitudes que variaram de 7,2 a 51,2 m/s2 e frequências de 205 a 220 Hz. A vibração promoveu uma melhor distribuição da microestrutura no metal de solda, promovendo um refino de até 113% para uma amplitude de 51,2 m/s2 e frequência de 220 Hz no processo GMAW, em relação a condição sem vibração. Já no processo SMAW a redução chegou a 25% com amplitude de 7,2 m/s2 e frequência de 205 Hz. As reduções nos desvios padrões médios de dureza chegaram a 71% para uma amplitude de 48,9 m/s2 e frequência de 215 Hz, no processo GMAW. Enquanto que no processo SMAW, o desvio padrão médio de microdureza reduziu 27%, com amplitude 7,2 m/s2 e frequência de 205 Hz, em relação a condição sem vibração. / The present work had the objective of evaluating the influence of mechanical vibration during the welding process on grain refining in brazing metals obtained in the welding of steel ASTM A516 Gr.60, applied in boilers and pressure vessels. For the mechanical vibration treatment VWC (Vibration Welding Conditioning) process was used.. , The welds were done using the SMAW (Electric Arc with Coated Electrode) and GMAW (Electric Arc Welding with Gaseous Protection) process. AWS ER 70S-6 wire with 1.2 mm of diameter protected with CO2 in the GMAW process and the electrode E7018 with 2.25 mm of diameter in the process SMAW were used as consumable. A vibratory bed was designed and developed to perform the experiments. The joints were performed without and with vibration, subjected to amplitudes ranging from 7.2 to 51.2 m/s2 and frequencies from 205 to 220 Hz. The vibration promoted a better distribution of the microstructure in the weld metal, promoting a refining of up to 113% for an amplitude of 51.2 m/s2 and a frequency of 220 Hz in the GMAW process, in relation to the condition without vibration. In the SMAW process the reduction reached 25% with amplitude 7.2 m/s2 and frequency of 205 Hz. The reductions in the average hardness standard deviations reached 71% for a range of 48.9 m/s2 and a frequency of 215 Hz, in the GMAW process. Using the SMAW process, the mean standard deviation of microhardness value decreased by 27%, with amplitude 7.2 m/s2 and a frequency of 205 Hz, in relation to the condition without vibration.

No description available.

The need to develop light-weight and high strength materials for car frames which improve fuel efficiency and provide increased passenger safety during dynamic events such as automobile crashes has been the focus of the steel and automobile industries for the past 30 years. In recent years, the development of high strength steels such as multi-phase TRIP (Transformation-Induced Plasticity)-aided steels have shown great promise due to their excellent combination of high strength and ductility. The savings in automobile weight is provided by the inherent strength of TRIP steels which allows for the use of thinner sections. The TRIP effect is characterized by the phenomenon known as strain-induced martensitic transformation (SIMT) which enhances the work hardenability of such steels as the austenite phase transforms to the much harder martensite phase during plastic straining. This results in a resistance to local necking which subsequently enhances the strength, ductility, and formability of such steels. However, various factors exist which affect the mechanical behaviour of TRIP steels. This study will aim, through the use of finite element models, to investigate the role and influence of each of these factors on the TRIP effect in type 304 austenitic and multi-phase TRIP steels. These factors include the rate at which the martensitic transformation proceeds, the state of stress to which the material is subjected to, the interaction between the surrounding matrix and embedded retained austenite islands in multi-phase TRIP steels, and the volume fraction and morphology of the retained austenite islands. Investigation of these factors will provide further insight on each of their contributions to the TRIP effect in order to exploit the potential benefits offered by these steels.

The ageing state of the world's nuclear power infrastructure, and the need to reduce humanity s dependency on fossil fuels, requires that this electrical energy generating capacity is replaced. Economic factors, and its physical and chemical properties, make high purity iron-chromium binary alloys a strong candidate for use in the construction of the pressure vessels of the next generation of nuclear reactors. This relatively inexpensive metal retains the oxidation resistance property of so-called stainless steel alloys, and has demonstrated dimensional stability and low degradation under harsh experimental environments of temperature and radiation. In this work, we consider radiation induced interstitial damage to the atomic lattices of iron-chromium binary alloys using the atomistic modelling methods, Molecular Dynamics and Adaptive Kinetic Monte Carlo, simulating collision cascade sequences, and the migration of defects in the aftermath. Variations in chromium content does not effect the initial damage production in terms of the number of Frenkel pairs produced, but iron and chromium atoms are not evenly distributed in defect atoms with respect to the bulk concentration. In simulations conducted at low temperature, chromium is under-represented, and at high temperature, a greater proportion of interstitial atoms are chromium than in the lattice overall. The latter phenomena is most strongly pronounced in systems of low bulk chromium content. During the simulation of post-cascade defect migration, interstitials atoms are observed to form temporary clusters and vacancies align along adjacent lattice sites, with the two types of defect also migrating to annihilate by recombination. Calculating the energy spectra of cascade events corresponding to an example experimental configuration using the SRIM package, we investigated the evolution of lattice systems in which a sequence of multiple cascade events occurred, both with and without a physically representative time gap between events. These simulations gave us the opportunity to observe the behaviour of cascades in the proximity of damage remaining from previous events, such as the promotion of defect clustering when this occurs.

No description available.

Irradiation of a reactor pressure vessel (RPV) causes a shift of the ductile to brittle transition region towards higher temperature regions. In the event of a pressurized thermal shock (PTS), where the temperature drops drastically, the ductile to brittle transition region might be entered for irradiated ferritic steel. Hence, there is a risk of brittle cleavage fracture. Cleavage fracture is a transgranular unstable fracture initiated by cracked second phase particles and rapidly propagated over grain boundaries. The warm pre-stressing (WPS) effect can be helpful as it increases the apparent fracture toughness of ferritic steel pre-loaded in the ductile temperature region, which is the case for a PTS. This effect has been proven effective for virgin material, but the impact of residual stress fields on the WPS effect have not been investigated thoroughly. Utilizing a finite element model of notched three-point bending specimens and a non-local probabilistic model for fracture prediction the effect of residual stresses on the WPS effect was investigated in this thesis. Regarding the crack tip state, expressed as J, the probability of fracture was alike for both material with and without residual stresses, however a significant loss of load bearing capacity was found comparing them two. The magnitude of this loss depends on pre-load level as well as specimen size. This loss however, was also found when not considering the WPS effect. / Bestrålning av ferritiska reaktortankar orakar en förskjutning av den duktil-spröda omslagstemperaturen till högre temperaturer. Vid en trycksatt termisk shock (TTS) sjunker temperaturen drastiskt i reaktorn och omslagstemperaturen kan nås. Därför uppstår en risk för klyvbrott. Klyvbrott är en transgranulär ostabil spricktillväxt initierad av sprickor i sekundärfaspartiklar som propagerar över korngränserna om spänningstillståndet är gynnsamt. Varm förbelastning, eller warm pre-stressing (WPS) kan vara fördelaktig eftersom den höjer den effektiva brottsegheten hos ferritiskt stål som förbelstats i den duktila temperaturregionen, som är fallet för TTS. Den här effekten har visats effektiv för material utan restspänningar, men för fallet med restspäninningar saknas utförliga undersökningar av WPS-effekten. Med hjälp av en finita element model av tre-punkt böjprovstavar och en icke-lokal sannolikhetsmodell för prediktion av brott så undersöktes vilken effekt restspänningar har på WPS-effekten. Brottsannolikheten visade sig vara lika för både material med och utan restspännigar om man syftar på sprickspetstillståndet, uttryckt som J. Däremot syns en tydlig förlust i lastbärande förmåga mellan de två fallen. Storleksordningen på förlustern beror både på förbelastningsnivå och provstavsstorlek. Den förlusten kunde dock finnas även för fall utan någon WPS effekt.

Made available in DSpace on 2014-12-17T14:07:10Z (GMT). No. of bitstreams: 1 JoseFSJ_TESE.pdf: 4970258 bytes, checksum: fcd6ec64fc7d0cdced502c9b8b4dd5f9 (MD5) Previous issue date: 2012-11-01 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / The 15Kh2MFA steel is a kind of Cr-Mo-V family steels and can be used in turbines for energy generation, pressure vessels, nuclear reactors or applications where the range of temperature that the material works is between 250 to 450?C. To improve the properties of these steels increasing the service temperature and the thermal stability is add a second particle phase. These particles can be oxides, carbides, nitrites or even solid solution of some chemical elements. On this way, this work aim to study the effect of addition of 3wt% of niobium carbide in the metallic matrix of 15Kh2MFA steel. Powder metallurgy was the route employed to produce this metallic matrix composite. Two different milling conditions were performed. Condition 1: milling of pure 15Kh2MFA steel and condition 2: milling of 15Kh2MFA steel with addition of niobium carbide. A high energy milling was carried out during 5 hours. Then, these two powders were sintered in a vacuum furnace (10-4torr) at 1150 and 1250?C during 60 minutes. After sintering the samples were normalized at 950?C per 3 minutes followed by air cooling to obtain a desired microstructure. Results show that the addition of niobium carbide helps to mill faster the particles during the milling when compared with that steel without carbide. At the sintering, the niobium carbide helps to sinter increasing the density of the samples reaching a maximum density of 7.86g/cm?, better than the melted steel as received that was 7,81g/cm?. In spite this good densification, after normalizing, the niobium carbide don t contributed to increase the microhardness. The best microhardness obtained to the steel with niobium carbide was 156HV and to pure 15Kh2MFA steel was 212HV. It happened due when the niobium carbide is added to the steel a pearlitic structure was formed, and the steel without niobium carbide submitted to the same conditions reached a bainitic structure / O a?o 15Kh2MFA, da fam?lia dos a?os CrMoV, pode ser utilizado em turbinas para gera??o de energia, vasos de press?o, reatores nuclear ou aplica??es, onde o material ? submetido a temperaturas de servi?o entre 250 e 450?C. Uma forma de melhorar as propriedades do a?o, para que ele trabalhe a temperaturas mais altas ou que se torne mais est?vel ? adicionar part?culas de segunda fase na sua matriz. Estas part?culas podem estar na forma de ?xidos, carbetos, nitretos ou at? mesmo em solu??o s?lida quando alguns elementos qu?micos s?o adicionados ao material. Neste contexto, este trabalho objetiva estudar o efeito da adi??o de 3% de carbeto de ni?bio na matriz met?lica do a?o 15Kh2MFA. Para isto a metalurgia do p? foi a rota empregada para a produ??o deste comp?sito de matriz met?lica. Para tal, duas moagens distintas foram realizadas. A primeira com o a?o 15Kh2MFA e a segunda com o a?o 15Kh2MFA com adi??o de 3% de carbeto de ni?bio. A moagem de alta energia foi realizada durante 5 horas. Em seguida, os dois p?s produzidos foram sinterizados em um forno a v?cuo (10-4torr) a temperaturas de 1150?C e 1250?C durante 60 minutos. Ap?s a sinteriza??o as amostras foram submetidas ao tratamento t?rmico de normaliza??o a 950?C. Os resultados mostraram que a adi??o do carbeto de ni?bio ajuda o processo de cominui??o das part?culas, quando comparado com o a?o sem o carbeto de ni?bio. O carbeto de ni?bio tem um papel fundamental na densifica??o das amostras durante a sinteriza??o, levando a densidade 7,86g/cm?, que ? maior do que a densidade do a?o fundido recebido que era de 7,81g/cm?. Apesar desta boa densifica??o, ap?s a normaliza??o, o NbC n?o contribuiu de forma significativa para aumento da dureza, onde a melhor dureza obtida para o a?o com NbC foi de 156HV e para o a?o puro foi de 212HV. Isto se deve ao fato de que, quando o NbC foi adicionado ao a?o, formou-se uma estrutura perl?tica, enquanto que, com o a?o sem adi??o de NbC, submetido as mesmas condi??es, obteve-se uma estrutura bain?tica