Boron carbide nano-powders, singly-doped over a range of compositions, were pressurelessly-sintered at identical temperature and atmospheric conditions in a dif- ferential dilatometer to investigate sintering behavior. Samples that achieved relative densities greater than 93% of theoretical density were post-HIPed. Post-HIPing re- sulted in an increase in relative density as well as an increase in Vicker's hardness. To optimize the sintering behavior, nano-powders with multiple dopants were prepared based on the results of single dopant experiments. These powders were studied using the same heating schedule as the single dopant samples. The powder with optimized composition was selected, and 44.45 mm diameter disks were pressed to determine the effects of sample size. Powder composition #166 with Al, Ti, W and Mg additions was processed using di¢çerent methods in order to create defect-free green bodies after uniaxial press- ing. The 44.45 mm diameter compacts were heat-treated to remove organics and B₂O₃coatings on particles and then encapsulated in an evacuated fused silica am- pule. Encapsulated samples were HIPed at temperatures below the coarsening region observed in the dilatometric traces of multiply-doped nano-powders. The E-HIPed sample showed a relative density of 96% with a limited extent of nano-sized grain microstructure.

<p>In this thesis 9-12 % Cr steels, used for high-temperaturecomponents in fossil-fired power plants are considered. Thecreep strength of thees steels depend on their microstructurethat consists of a matrix of tempered lath martensite withdensely distributed precipitates. The mechanical properties arestrongly influenced by precipitates present in the matrix andthe more densely distributed they are the higher is thehardening effect. These particles nucleate, grow and coarsenduring use in power plants, leading to a degradation ofmechanical properties. In this thesis the nucleation andcoarsening behaviour of the precipitates in the Cr steels aresimulated by new models and the results are compared withtransmission electron microscopy (TEM) observations of testedmaterials.</p><p>A model of the nucleation of MX, which mainly isvanadium-nitrides, is presented. MX precipitates are assumed tonucleate on dislocations during annealing. The model takes intoaccount the full multicomponent thermodynamical behaviour ofthe system as well as the strain energy caused by a puredilatoric strain and the shape of the nucleus. The calculationsyield a critical size and shape represented by an oblatespheroid with a radius of a few nanometers and a thickness ofan atomic layer. This is in agreement with TEM observations ofsupercritical particles.</p><p>A new coarsening model, which takes into account themulticomponent effects, is presented and it has been tested ondifferent systems to validate the model.</p><p>    Four different carbides, Cr₇C₃, Mo₆C, VC and NbC, in austenitic matrix as well asγ ' in ternary Ni-base super-alloy systems (Ni-Al-Mo)were compared with measurements from literature.</p><p>    TiC-particles in austenitic stainless steel, ASTM 316Ti,were considered. The simulations were performed taking intoaccount iron and 7 alloying elements. The measurements wereperformed with TEM on samples that had been heat-treated at900° C.</p><p>    MX and M₂₃C₆in a 9 % Cr steel were investigated with energyfiltering transmission electron microscopy (EFTEM) andcompared with simulations. The used samples had been heattreated for various periods of time at 600 and 650° Cfor up to 26 000 h.</p><p>The agreement of the simulations with the experiments wasgood in all cases when reasonable values of the interfacialenergy, the only adjustable parameter, were chosen.</p><p>Coarsening simulations were also performed to investigatethe influence of changes in composition on the coarsening rate.For MX, in a 9 % Cr steel, the coarsening rate is almostindependent of the V/Nb ratio but highly dependent on theN-content. Also the effect on the coarsening rate for M₂₃C₆by adding Co to a Cr steel was investigated bysimulation. Co is known to increase the resistance totempering. The results show that a final average radius of thecarbides after 30 000 h at 600° C decreases with 30 % witha Co addition of 10 mass %.</p><p><b>Keywords:</b>Cr steels, nucleation, coarsening, model,DICTRA, precipitates, carbides, carbo-nitrides, MX, VN, M₂₃C₆, TiC, TEM, EFTEM, Curie-temperature</p>

<p>Fatigue and ductility properties in various tool steels, produced by powder metallurgy, spray forming or conventionally ingot casting, have been analysed experimentally and successfully compared to developed models. The models are able to predict the fatigue limit and cause for fatigue fracture, and strain- and stress-development until fracture during the ductile fracture process respectively. Total fracture in a tool steel component, both in fatigue and ductility testing, is caused by a propagating crack initiated by particles, i.e. carbides or non-metallic inclusions. The models are based on experimentally observed size distributions.</p><p>The axial fatigue strength at two million cycles was determined for various tool steels. The fracture surface of each test bar broken was examined in SEM to determine the cause for fatigue failure, i.e. a single carbide or inclusion particle or a cluster of carbides, and the size of the particle. The particles act as stress concentrators where a crack is easily initiated when the material is subjected to alternating stresses. The developed models calculate the probability that at least one particle will be present in the material which is larger than the threshold level for crack initiation at a certain stress range.</p><p>The ductility testing was performed on various tool steels by four-point bending under static load. The load and displacement until total fracture were recorded and the maximum strain and stress acting in the material were calculated. The fracture surface of each broken test bar was examined in SEM, though the crack initiating area appears different compared to a fatigue failure. Ductile fracture is caused by a crack emanating from voids nucleated around many particles in a joint process and then linked together. By finite element modelling of void initiation and propagation in 2D of an experimentally observed carbide microstructure for each tool steel, successful comparisons with experiments were performed. Carbides were modelled as cracked when larger than a certain size, based on fracture surface observations, and the matrix cracked above a pre-defined plastic deformation level. The stresses and strains at total failure were in good agreement between model and experiments when evaluated.</p><p>The use of these developed models could be a powerful tool for optimisation of fatigue and ductility properties for tool steels. With good fatigue and ductility properties normal failures appearing during operation of a tool steel product could be minimised. By theoretical tests in the developed models of various carbide microstructures the optimum mechanical properties could be achieved with a minimum of experiments performed.</p>

Made available in DSpace on 2014-12-17T14:07:20Z (GMT). No. of bitstreams: 1 MariaRPF_TESE.pdf: 4797522 bytes, checksum: 8d9626466b01e2425f6ea6036398adcf (MD5) Previous issue date: 2014-01-24 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / Carbide reinforced metallic alloys potentially improve some important mechanical properties required for the overall use of important engineering materials such as steel and nickel. Nevertheless, improved performance is achieved not only by composition enhancement but also by adequate processing techniques, such as novel sintering methods in the case of powder metallurgy. The method minimizes energy losses in addition to providing uniform heating during sintering. Thus, the general objective of this study was to evaluate the density, hardness, flexural strength, dilatometric behavior and to analyze the microstructure of metal matrix composites based nickel with addition of carbides of tantalum and / or niobium when sintered in a conventional furnace and Plasma assisted debinding and sintering (PADS). Initially, were defineds best parameters of granulation, screening and mixing procedure. After, mixtures of carbonyl Ni and 5%, 10% and 15 wt.% NbC and TaC were prepared in a Y-type mixer under wet conditions during 60 minutes. The mixtures were then dried and granulated using 1.5 wt. % paraffin diluted in hexane. Granulates were cold pressed under 600 MPa. Paraffin was then removed from the pressed pellets during a pre-sintering process carried out in a tubular furnace at 500 ?C during 30 min. The heating rate was 3 ?C/min. The pellets were then sintered using either a plasma assisted reactor or a conventional resistive tubular furnace. For both methods, the heating rate was set to 8 ?C/min up to 1150 ?C. The holding time was 60 minutes. The microstructure of the sintered samples was evaluated by SEM. Brinell hardness tests were also carried out. The results revealed that higher density and higher hardness values were observed in the plasma-assisted sintered samples. Hardness increased with the concentration of carbides in the Ni-matrix. The flexural strength also increased by adding the carbides. The decline was larger for the sample with addition of 5% 5% TaC and NbC. In general, compositions containing added carbide 10% showed less porous and more uniform distribution of carbides in the nickel matrix microstructural appearance. Thus, both added carbide and plasma sintering improved density, hardness, flexural strength and microstructural appearance of the composites / Ligas met?licas refor?adas por carbetos melhoram potencialmente algumas propriedades mec?nicas necess?rias para a utiliza??o de importantes materiais de engenharia, tais como o a?o e o n?quel. No entanto, o desempenho ? conseguido n?o apenas pela melhoria de composi??o, mas tamb?m por t?cnicas de processamento adequadas, tais como, novos m?todos de sinteriza??o, no caso da metalurgia do p?. Desta forma, o objetivo do presente trabalho foi avaliar a dureza, resist?ncia ? flex?o, comportamento dilatom?trico e analisar a microestrutura de comp?sitos de matriz met?lica base n?quel com adi??o de carbetos de t?ntalo e/ou ni?bio quando sinterizados em forno tubular convencional e Plasma assisted debinding sintering (PADS). Os carbetos (5%, 10% e 15% em massa) foram misturados ao p? de n?quel carbonila via ?mido com aux?lio do misturador Y adaptado durante 1h. Ap?s secagem, as misturas foram submetidas ao processo denominado de granula??o em tambor. Utilizou-se 1,5% de parafina (% massa) dilu?da em hexano. Os p?s granulados foram compactados a frio utilizando press?o de 600 MPa. Antes da sinteriza??o a uma taxa de 8 ?C/min com patamar 1h na temperatura de 1150?C tanto em forno tubular quanto em reator PADS, as amostras foram pr?-sinterizadas em forno tubular para extra??o da parafina a uma taxa de 3 ?C/min com patamar 30 min em 500 ?C. A dureza avaliada foi a Brinell e a an?lise microestrutural por MEV. Os resultados mostraram que as amostras sinterizadas assistidas por plasma apresentaram dureza superior ?quelas sinterizadas em forno convencional. As imagens por MEV comprovaram esses maiores valores de durezas, uma vez que a matriz apresentou-se mais densificada. Com rela??o ? adi??o dos carbetos, a dureza aumentou com o aumento da concentra??o dos mesmos. A resist?ncia ? flex?o tamb?m aumentou ao adicionar os carbetos. A retra??o foi maior para a amostra com adi??o de 5% de TaC e 5% NbC. De maneira geral, as composi??es com adi??o de 10% de carbetos apresentaram um aspecto microestrutural menos poroso e com uma distribui??o mais uniforme dos mesmos na matriz de n?quel. Assim, tanto a adi??o de carbetos quanto a sinteriza??o com aux?lio do plasma melhoraram a dureza e o aspecto microestrutural dos comp?sitos

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