Magnetic measurements are useful tools for quality control of cemented carbides. Previous work at Sandvik Mining and Rock Technology has shown that the coercivity increases with increased cooling rate during sintering for a specific grade.  This study aims to investigate why the coercivity changes with the cooling rate and if this is true for other cemented carbide grades as well. Three different cemented carbide grades were sintered with different cooling rates and evaluated with coercivity, Cobalt-magnetic saturation and hardness measurements, and with microscopy and Electron Backscatter Diffraction analysis. It was found that the coercivity increased with increasing cooling rates for the previously studied grade, but not for the two other grades. It was expected that the increased coercivity would indicate a decrease in WC grain size, but the results showed that the WC grain size of the fastest and slowest cooling rate were the same. However, a change in size of the Co areas between the WC grains was found. The fast cooled sample showed smaller Co areas than the slow cooled sample. These Co/WC grain boundaries increase the coercivity. An increased fraction of hcp-Co/fcc-Co was also found for the fast cooled material which also increases the coercivity. No relationship between the hardness and the coercivity or the cooling rate was found.   The contiguity for the different grades was also calculated.  No significant difference in contiguity between the different cooling rates of each material was found but the contiguity values between the different materials differed.  This is probably mainly dependent on the different binder contents of the materials. / Magnetiska egenskaper är en viktig del av kvalitetskontrollen av hårdmetaller.  Tidigare forskning hos Sandvik Mining and Rock Technology har visat att koerciviteten hos en hårdmetallsort ökar med ökad kylhastighet vid sintring.  Målet med den här studien är att undersöka varför koerciviteten ökar med kylhastigheten och om detta även gäller andra hårdmetallsorter.  Tre olika hårdmetallsorter sintrades med olika kylhastigheter undersöktes med mätningar av koercivitet, magnetisk mättnat i koboltfasen och hårdhet,  samt med mikroskopi och Electron Backscatter Diffraction analys. Det  visade  sig  att  koerciviteten  ökade  med  ökad  kylhastighet  för  hårdmetallsorten  som  studerats tidigare, men inte för de andra två sorterna.  En ökad koercivitet förväntas visa på en minskad WC-kornstorlek, men resultaten visar att det långsamt kylda och snabbkylda provet hade samma WC-kornstorlek.  Däremot upptäcktes en ändring av storleken på Co-områdena mellan WC-kornen.  Det snabbkylda provet visade en mindre storlek av Co-områdena jämfört med det långsamt kylda provet. Fler Co/WC-korngränser i det snabbkylda provet leder till en ökad koercivitet.  Fraktionen av hcp-Co/fcc-Co-korngränser ökade också för det snabbkylda provet vilket också ökar koerciviteten. Inget samband mellan hårdheten och koerciviteten eller kylhastigheten upptäcktes.  Beräkning av kontiguiteten för de olika sorterna genomfördes också. Ingen signifikant skillnad i koercivitet mellan de olika kylhastigheterna för the olika sorterna hittades,  men kontiguiteten mellan de olika sorterna varierade. Det beror mest troligt på att dom olika sorterna har olika mängd matrismaterial.

Methods for predicting weld properties based on welding parameters are needed in friction stir welding (FSW). FSW is a joining process in which the resulting properties depend on the thermal cycle of the weld. Buckingham's Pi theorem and heat transfer analysis was used to identify dimensionless parameters relevant to the FSW process. Experimental data from Al 7075 and HSLA-65 on five different backing plate materials and a wide range of travel speeds and weld powers was used to create a dimensionless, empirical model relating critical weld parameters to the peak temperature rise and cooling rate of the weld. The models created have R-squared values greater than 0.99 for both dimensionless peak temperature rise and cooling rate correlations. The model can be used to identify weld parameters needed to produce a desired peak temperature rise or cooling rate. The model can also be used to explore the relative effects of welding parameters on the weld thermal response.

The aim of this study is to form a theoretical model for simulation of glass forming ability of Fe &amp / #65533 / Based bulk amorphous alloys, to synthesize Fe &amp / #65533 / based multicomponent glassy alloys by using the predictions of the theoretical study, and to analyze the influence of crystallization and solidification kinetics on the microstructural features of this amorphous alloys. For this purpose, first, glass forming ability of Fe &amp / #65533 / (Mo, B, Cr, Nb, C) &amp / #65533 / X ( X = various alloying elements, selected from the periodic table) ternary alloy systems were simulated for twenty different alloy compositions by using the electronic theory of alloys in pseudopotential approximation and regular solution theory. Then, by using the results of the theoretical study, systematic casting experiments were performed by using centrifugal casting method. The alloying elements were melted with induction under argon atmosphere in alumina crucibles and casted into copper molds of different shapes. Characterization of the cast specimens were performed by using DSC, XRD, SEM, and optical microscopy. Comparison of equilibrium and nonequilibrium solidification structures of cast specimens were also performed so as to verify the existence of the amorphous phase. Good agreement of the results of experimental work, with the predictions of the theoretical study, and the related literature was obtained.

The aim of this study is to synthesize and characterize new bulk amorphous alloys in the Ni- based systems. Theoretical studies on the basis of semi-empirical rules and the electronic theory of alloys in pseudopotential approximation has been provided in order to predict the impurity elements that will lead to an increase in the glass forming ability of Ni-based alloy systems. Glass forming ability of ten different compositions of alloys of Ni-Nb, Ni-Fe, Ni-B, Ni-Hf and Ni-Cr was simulated by using FORTRAN programs based on pseudopotential theory. In addition to the binary alloys, ternary alloys, which were formed by addition of 1 at% of third element to these systems, were also simulated. Since ordering energy is an indicator of glass forming ability, theoretical studies allowed to predict the effect of various third elements on the formation of amorphous phase. Furthermore, ordering energies were also used to calculate other parameters important for glass forming ability. In the second part of the study, on the basis of theoretical results, a series of casting experiments were done. Different compositions of Ni-Nb, Ni-Nb-Sn and Ni-Nb-Al alloys were cast in the centrifugal casting machine. Alloys were melted in alumina crucibles and cast into the copper moulds. Characterizations of cast alloys were done by the use of Metallography, SEM, XRD and DSC. Fully amorphous Ni52Nb41Al7 alloy was synthesized in bulk form with 0.8 mm thickness.

Orientadores: Rubens Caram Junior, Conrado Ramos Moreira Afonso / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-08-06T17:14:04Z (GMT). No. of bitstreams: 1 Aleixo_GiorgiaTaiacol_M.pdf: 13698827 bytes, checksum: 10117398b384c5a1eda6c62fbfc74507 (MD5) Previous issue date: 2006 / Resumo: Ligas de titânio formam a classe mais versátil de materiais metálicos utilizados para aplicações biomédicas. As ligas de Ti tipo ß usadas em dispositivos ortopédicos envolve, além de excelentes propriedades como baixa densidade, elevada resistência mecânica e excepcional resistência à corrosão, a biocompatibilidade com elementos de liga, tais como Nb, Ta, Zr e Mo. O objetivo deste trabalho foi analisar a estabilidade e metaestabilidade de fases em ligas tipo ß no sistema Ti-Nb através da verificação da influência do teor de nióbio sob diversas condições de tratamento térmico. Para tanto variou-se a adição de Nb de 5% a 30% em peso na liga, obtendo amostras que foram submetidas a condições distintas de resfriamento a partir do campo ß. Amostras obtidas foram caracterizadas no tocante à microestrutura via microscopia ótica, eletrônica de varredura e de transmissão, difração de raios-X, enquanto o comportamento mecânico foi preliminarmente avaliado através de ensaios de dureza Vickers e de medidas de módulo de elasticidade através de técnicas acústicas. Os resultados obtidos indicam que baixas taxas de resfriamento conduzem à microestrutura de equilíbrio (a+ß) enquanto que altas taxas de resfriamento levaram à formação de estruturas metaestáveis. À medida que o teor de Nb foi incrementado e sob altas taxas de resfriamento, obteve-se, respectivamente, martensita a¿ (hexagonal compacta), martensita a¿ (ortorrômbica), fase ? (trigonal) e finalmente, fase ß metaestável. Em relação ao módulo de elasticidade, observou-se que esse parâmetro decresce entre 0 e 15% de Nb e após este último valor, tal constante elástica se eleva como resultado da precipitação de fase ? bem como da possível alteração de dimensões da estrutura cristalina. As ligas Ti-Nb apresentaram uma variabilidade de microestruturas e propriedades que dependem fortemente da composição e condição de resfriamento / Abstract: Titanium alloys form one of the most versatile classes of metallic materials used for biomedical applications. ß Ti alloys for orthopaedic devices involves, besides excellent properties like low density, high mechanical strength and good corrosion resistance, great biocompatibility with alloying elements, such as Nb, Ta, Zr and Mo. The objective of this work was to analyze the stability and metastability of phases in ß Ti-Nb alloys through the evaluation of the influence of percentage of Nb obtained in different heat treatment conditions. In this way, Nb addition was varied from 5 to 30wt% in Ti-Nb system. Samples were obtained and microstructural characterization was made through optical microscopy, scanning and transmission electron microscopy and X-ray diffraction, while mechanical behavior was primarily evaluated by Vickers hardness and elasticity modulus through acoustic techniques. The results obtained indicate that lower cooling rates resulted in the equilibrium microstructures (a+ß), otherwise higher cooling rates led to formation of metastable structures. As the Nb content was increased and under greater cooling rates, it was obtained, respectively, martensite a¿ (hexagonal closed packed), martensite a¿ (orthorhombic), ? phase (trigonal) and finally, metastable ß phase. Regarding modulus of elasticity it was observed that it decreases as the %Nb increases up to 15% and beyond this percentage the modulus increases due the precipitation of ? phase as well as possible variation of crystalline structure dimensions. Ti-Nb alloys presented a great variety of combinations between microstructures and properties that strongly depends on composition and cooling conditions / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica

The magnetic Fe–6.5 wt% Si powder was produced by gas atomization and its microstructure was also investigated. The secondary dendritic arm spacing (SDAS) is related to the droplet size, λ = 0.29 · D⁰·⁵, and the numerical solidification model was applied to the system, giving rise to the correlation of microstructure to the solidification process of the droplet. It is found that the solid fraction at the end of recalescence is strongly dependent on the undercooling achieved before nucleation; the chances for the smaller droplets to form the grain-refined microstructures are less than the larger ones. Furthermore, the SDAS is strongly influenced by the cooling rate of post-recalescence solidification, and the relationship can be expressed as follows, λ = 74.2 · (T)⁻⁰·³⁴⁷. Then, the growth of the SDAS is driven by the solute diffusion of the interdendritic liquids, leading to a coarsening phenomenon, shown in a cubic root law of local solidification time, λ = 10.73 · (tf)⁰·²⁹⁶.

Potential parameters that can handle multi-component oxide glass systems especially boron oxide are very limited in literature. One of the main goals of my dissertation is to develop empirical potentials to simulate multi-component oxide glass systems with boron oxide. Two approaches, both by introducing the composition dependent parameter feature, were taken and both led to successful potentials for boron containing glass systems after extensive testing and fitting. Both potential sets can produce reasonable glass structures of the multi-component oxide glass systems, with structure and properties in good agreement with experimental data. Furthermore, we have tested the simulation settings such as system size and cooling rate effects on the results of structures and properties of MD simulated borosilicate glasses. It was found that increase four-coordinated boron with decreasing cooling rate and system size above 1000 atoms is necessary to produce converged structure. Another application of the potentials is to simulate a six-component nuclear waste glass, international simple glass (ISG), which was for first time simulated using the newly developed parameters. Structural features obtained from simulations agree well with the experimental results. In addition, two series of sodium borosilicate and boroaluminosilicate glasses were simulated with the two sets of potentials to compare and evaluate their applicability and deficiency. Various analyses on the structures and properties such as pair distribution function, total correlation function, coordination number analysis, Qn distribution function, ring size distribution function, vibrational density of states and mechanical properties were performed. This work highlights the challenge of MD simulations of boron containing glasses and the capability of the new potential parameters that enable simulations of wide range of mixed former glasses to investigate new structure features and design of new glass compositions for various applications.

High chromium cast iron (HCCI) is considered as one of the most useful wear resistance materials and their usage are widely spread in industry. The wear resistance and mechanical properties of HCCI mainly depend on type, size, number, morphology of hard carbides and the matrix structure (γ or α). The Hypereutectic HCCI with large volume fractions of hard carbides is preferred to apply in wear applications. However, the coarser and larger primary M7C3 carbides will be precipitated during the solidification of the hypereutectic alloy and these will have a negative influence on the wear resistance. In this thesis, the Ti-added hypereutectic HCCI with a main composition of Fe-17mass%Cr-4mass%C is quantitatively studied based on the type, size distribution, composition and morphology of hard carbides and martensite units. A 11.2μm border size is suggested to classify the primary M7C3 carbides and eutectic M7C3 carbides. Thereafter, the change of the solidification structure and especially the refinement of carbides (M7C3 and TiC) size by changing the cooling rates and Ti addition is determined and discussed. Furthermore, the mechanical properties of hypereutectic HCCI related to the solidification structure are discussed. Mechanical properties of HCCI can normally be improved by a heat treatment process. The size distribution and the volume fraction of carbides (M7C3 and TiC) as well as the matrix structure (martensite) were examined by means of scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). Especially for the matrix structure, EBSD is a useful tool to classify the fcc (γ) and bcc (α) phases. In conclusion, low holding temperatures close to the eutectic temperature and long holding times are the best heat treatment strategies in order to improve wear resistance and hardness of Ti-alloyed hypereutectic HCCI. / <p>QC 20121130</p>

This thesis work deals with the volume changes during the solidification of cast irons and peritectic steels. The volume changes in casting metals are related to the expansion and/or contraction of the molten metal during solidification. Often, different types of shrinkage, namely macro- and micro-shrinkage, affect the casting quality. In addition to that, exposure of the metal casting to higher contraction or expansion during the solidification might also be related to internal strain development in samples, which eventually leads to surface crack propagation in some types of steel alloys during continuous casting. In consequence, a deep understanding of the mechanisms and control of the solidification will improve casting quality and production. All of the experiments during the entire work were carried out on laboratory scale samples. Displacement changes during solidification were measured with the help of a Linear Variable Displacement Transformer (LVDT). All of the LVDT experiments were performed on samples inside a sand mould. Simultaneously, the cooling curves of the respective samples during solidification were recorded with a thermocouple. By combining the displacement and cooling curves, the volume changes was evaluated and later used to explain the influence of inoculants, carbon and cooling rates on volume shrinkages of the casting. Hypoeutectic grey cast iron (GCI) and nodular cast iron (NCI) with hypo-, hyper- and eutectic carbon compositions were considered in the experiments from cast iron group. High nickel alloy steel (Sandvik Sanbar 64) was also used from peritectic steel type. These materials were melted inside an induction furnace and treated with different types of inoculants before and during pouring in order to modify the composition. Samples that were taken from the LVDT experiments were investigated using a number of different  methods in order to support the observations from the displacement measurements:  Differential Thermal Analysis (DTA), to evaluate the different phase present; Dilatometry, to see the effect of cooling rates on contraction for the various types of alloys; metallographic studies with optical microscopy; Backscattered electrons (BSE) analysis on SEM S-3700N, to investigate the different types of oxide and sulphide nuclei; and bulk density measurements  by applying Archimedes' principle. Furthermore, the experimental volume expansion during solidification was compared with the theoretically calculated values for GCI and NCI. It was found that the casting shows hardly any shrinkage during early solidification in GCI, but in the eutectic region the casting expands until the end of solidification. The measured and the calculated volume changes are close to one another, but the former shows more expansion. The addition of MBZCAS (Si, Ca, Zr, Ba, Mn and Al) promotes more flake graphite, and ASSC (Si, Ca, Sr and Al) does not increase the number of eutectic cells by much. In addition to that, it lowers the primary austenite fraction, promotes more eutectic growth and decreases undercooled graphite and secondary dendritic arm spacing (SDAS). As a result, the volume expansion changes in the eutectic region. The expansion during the eutectic growth increase with an increase in the inoculant weight percentage. At the same time, the eutectic cells become smaller and increase in number. The effect of the inoculant and the superheat temperature shows a variation in the degree of expansion/contraction and the cooling rates for the experiments. Effective inoculation tends to homogenize the eutectic structure, reducing the undercooled and interdendritic graphite throughout the structure. In NCI experiments, it was found that the samples showed no expansion in the transversal direction due to higher micro-shrinkages in the centre, whereas in the longitudinal direction the samples shows expansion until solidification was complete.   The theoretical and measured volume changes agreed with each other. The austenite fraction and number of micro-shrinkage pores decreased with increase in carbon content. The nodule count and distribution changes with carbon content. The thermal contraction of NCI is not influenced by the variation in carbon content at lower cooling rates. The structural analysis and solidification simulation results for NCI show that the nodule size and count distribution along the cross-sections at various locations are different due to the variation in cooling rates and carbon concentration. Finer nodule graphite appears in the thinner sections and close to the mold walls. A coarser structure is distributed mostly in the last solidified location. The simulation result indicates that finer nodules are associated with higher cooling rate and a lower degree of microsegregation, whereas the coarser nodules are related to lower cooling rate and a higher degree of microsegregation. As a result, this structural variation influences the micro-shrinkage in different parts. The displacement change measurements show that the peritectic steel expands and/or contracts during the solidification. The primary austenite precipitation during the solidification in the metastable region is accompanied by gradual expansion on the casting sides. Primary δ-ferrite precipitation under stable phase diagram is complemented by a severe contraction during solidification. The microstructural analysis reveals that the only difference between the samples is grain refinement with Ti addition. Moreover, the severe contraction in solidification region might be the source for the crack formation due to strain development, and further theoretical analysis is required in the future to verify this observation. / <p>QC 20170228</p>

Desde meados do século XX, métodos analíticos, numéricos e experimentais foram utilizados para quantificar a eficiência térmica na soldagem a arco, isto é, a razão entre a energia do arco elétrico e a energia que é entregue ao material durante a soldagem. Conhecendo-se a eficiência térmica, as dimensões da chapa e as propriedades térmicas do metal de base, torna-se possível prever as taxas e tempos de resfriamento a partir de modelo analíticos. Consequentemente, permitindo a previsão das transformações de fase, garantindo que os valores das propriedades da junta soldada se enquadrem dentro de um intervalo desejado. Os modelos de previsão de taxa de resfriamento derivados a partir dos modelos analíticos de Rosenthal são validos apenas no regime quase estacionário. Todavia, estes modelos costumam ser utilizados com pouco critério, mesmo em situações em que este regime não é alcançado. Portanto, para o desenvolvimento de modelos mais precisos e acurados para previsões das taxas de resfriamento, torna-se necessário entender como os termos do balanço de energia se desenvolvem durante a soldagem, até atingir o regime quase estacionário. Este trabalho tem como objetivo desenvolver um entendimento mais abrangente dos fatores que impactam nos valores de taxas de resfriamento em juntas soldadas. Realizaram-se ensaios de calorimetria com nitrogênio líquido para diferentes tempos de soldagem, como também, foram coletados ciclos térmicos em diferentes posições ao longo do cordão, buscando entender como as variações no balanço de energia, ao longo da soldagem, impactam no material. A partir da Metodologia de Superfície de Resposta, foram obtidos os modelos empíricos da energia entregue ao material e da eficiência térmica do arco. Um novo entendimento acerca do balanço de energia na soldagem foi alcançado, servindo de base para o desenvolvimento e validação de um modelo preditivo de taxa de resfriamento, válido para diferentes condições de fluxo de energia no material. / Since the early 20th century, numerical and experimental methods have been used to quantify the thermal arc efficiency in welding, i.e., the ratio between the electric arc energy and the energy delivered to the material during welding. Knowing the thermal arc efficiency, the sample dimensions and the thermal properties of the base metal, it is possible to predict the cooling rates by analytical models. Consequently, allowing the prediction of phase transformations in the material, ensuring that values of welded joint properties are within a desired range. Cooling rate prediction models derived from Rosenthal\'s analytical models are valid only in quasi-stationary state. However, these models are used with little criterion, even in situations where this state is not achieved. Therefore, to development of more accurate and precise models for the prediction of cooling rate it become necessary to understand the evolution of the energy balance during the welding until the quasi-stationary state was reached. This work aims to develop a more comprehensive understanding of the factors that affect the cooling rate in welded joints. Calorimetry tests with liquid nitrogen were carried out for different welding times, and thermal cycles were collected at different positions along the weld bead, to understand how the variations in the energy balance during the welding affect the material. From the Response Surface Methodology, the empirical models of delivered energy and the thermal arc efficiency were obtained. A new understanding about the energy balance in the welding was found, that was used for the development and validation of a predictive model of cooling rate, valid for different condition of energy flow in the material.