As ligas de cobre têm grande importância na fabricação de produtos metálicos que necessitam de boa resistência à corrosão e/ou alta condutividade térmica e elétrica. A sua estrutura de grãos bruta de solidificação define diversas propriedades requeridas em serviço/processamento e o refino desta estrutura pode ser realizado a partir da inoculação do banho metálico antes do início da solidificação, aumentando o número de núcleos sólidos formados. A inoculação e os mecanismos de refino de grão em ligas de cobre foram pouco investigados e não são entendidos suficientemente para permitir o projeto de inoculantes. O presente trabalho tem como principal objetivo identificar os mecanismos fundamentais de inoculação e refino de grão das ligas do sistema Cu-Sn a partir de adições de Zr aos banhos líquidos antes da solidificação. Amostras cilíndricas foram produzidas a partir da solidificação das ligas a uma taxa de resfriamento de ~ 0,8 ?C/s. Estas amostras foram analisadas por diferentes técnicas: (a) observação da macroestrutura de grãos; (b) análise térmica das curvas de resfriamento obtidas durante a solidificação; (c) análise química; (d) exame da microestrutura em microscopia óptica, microscopia eletrônica de varredura (MEV) com emissão de campo (FEG), microscopia eletrônica de transmissão (MET) com e sem o feixe de íons (FIB) e (e) microanálise por espectroscopia de energia dispersiva (EDS). Observa-se que adições de Zr na faixa de 0,04 a 0,4% a banhos da liga Cu-8%Sn com cobertura de pó de grafita reduzem o tamanho médio de grão em até 90%, mas não causam efeito significativo sem a cobertura de grafita. Este efeito da diminuição do tamanho de grão com a adição de Zr torna-se significativo somente para teores de Sn >=4%. Esta redução de tamanho médio de grão é acompanhada do aparecimento de partículas facetadas na microestrutura, identificadas como ZrC a partir de padrões de difração de elétrons e microanálises EDS. Uma análise teórica mostra desajustes entre os reticulados do ZrC e do Cu? tão pequenos quanto 1,9%, indicando que estas partículas podem funcionar como centros efetivos para a nucleação heterogênea e causar o refino de grão em ligas de Cu. Parâmetros extraídos da recalescência das curvas de resfriamento, como a diferença entre as temperaturas mínima e máxima da recalescência, apresentaram boa correlação com o tamanho médio de grão final, indicando a possibilidade do controle do tamanho de grão em ligas Cu-Sn com adições de Zr a partir da análise térmica. / Copper alloys are important to the production of metallic parts with good corrosion resistance and high thermal and electrical conductivity. The as-cast grain structures of these alloys are closely related to the properties required in service or during processing. The refining of the grain structure can be achieved by inoculation of the metallic melt, before the beginning of solidification, increasing the number of solid nuclei. The inoculantion and grain refining of copper alloys have been poorly investigated and are not sufficiently understood to enable the design of inoculants. The objective of the present work is to identify the fundamental mechanisms underlying the inoculation and grain refining of Cu-Sn alloys by additions of Zr to the melt prior to solidification. Cylindrical samples of these alloys were cast at a cooling rate of ~0,8 ?C/s. These samples were analyzed using different techniques: (a) observation of the grain macrostructure; (b) thermal analysis of the cooling curves collected during solidification; (c) chemical analysis; (d) observation of the microstructure in the optical microscope, scanning electron microscope (SEM) with field emission gun (FEG), trasmission electron microscope (TEM) with and without the focused ion beam (FIB) for sample preparation and (e) energy dispersive spectroscopy (EDS) microanalysis. The results show that additions of Zr in the range between 0,04 and 0,4% to melts of a Cu-8%Sn alloy with a graphite powder cover decrease the average grain size by up to 90%, but have no effect without the graphite cover. The decrease in grain size obtained with Zr additions become significant only for alloys with %Sn >==4 and is followed by the appearance of faceted particles in the microstructure, identified as ZrC after examining electron difraction patterns and EDS microanalyses of the particles.

As propriedades mecânicas, de corrosão e a condutividade térmica e elétrica, aliadas à relativa facilidade de fundir atribuem grande relevância ao cobre e suas ligas. Algumas destas propriedades podem ser significativamente melhoradas com a diminuição do tamanho de grão da estrutura bruta de solidificação a partir do tratamento de inoculação do banho líquido. Este tratamento aumenta o número de núcleos sólidos, diminuindo assim o tamanho médio de grão. Neste trabalho, foi realizado um estudo do refino de grão da estrutura bruta de solidificação a partir da inoculação com adições de C, P e Zr a ligas do sistema Cu-Zn com teores de até 30% de Zn. Três séries de experimentos principais foram realizadas. Na primeira, o teor de Zr na liga Cu- 30%Zn foi gradativamente aumentado até 0,4% para mostrar o efeito do Zr. Na segunda, o teor de Zr foi mantido constante em 0,08%, enquanto o teor de Zn foi elevado gradativamente até 30% para mostrar o efeito do Zn. Na terceira foram testadas adições isoladas e conjuntas de C, P e Zr para verificar o efeito de cada elemento no refinamento de grãos. As amostras foram caracterizadas e analisadas por diferentes técnicas: (a) observação da macroestrutura de grãos; (b) análise química; (c) análise térmica; (d) microscopia óptica; (e) microscopia eletrônica de varredura; (f) microanálise EDS; g) microanálise WDS e h) difração de raios X. Os resultados mostram que o tamanho médio de grão diminui significativamente nas ligas do sistema Cu-Zn (6% <= Zn <= 30%) quando Zr e P são adicionados simultaneamente. Isoladamente, estes elementos quando adicionados, não reduzem significativamente o tamanho de grão. Nota-se também que o tamanho de grão é cada vez menor à medida que o teor de Zn aumenta de 0 a 30% com a adição de 0,08%Zr, porém não se observa uma correlação entre tamanho de grão e teor de Zn quando Zr não é adicionado. Uma análise da microestrutura dos lingotes utilizando as técnicas mencionadas mostra que a diminuição do tamanho médio de grão a partir de adições de Zr e P coincide com o aparecimento de precipitados facetados ricos em Zr e P. Esta análise indica que um possível mecanismo de inoculação dos latões é a nucleação heterogênea da fase sólida Cu-µ, com estrutura cristalina cúbica de faces centradas, sobre partículas sólidas de ZrP, com estrutura hexagonal compacta, formadas no banho líquido após as adições de Zr e P. / Good mechanical and corrosion properties, large thermal and electrical conductivities, as well as relatively large castability are characteristics that contribute to the importance of copper and copper alloys. Some of these properties are enhanced when the average grain size of the as-cast structure decreases by inoculation of the melt. The inoculation treatment increases the number of solid nuclei, causing a reduction in the average grain size. In the present work, an investigation of the grain refinement by inoculation with Zr and P additions to Cu-Zn melts with contents of Zn up to 30%(mass) was carried out. Three series of experiments were conducted. In one the Zr content of the Cu-30%Zn alloy was gradually increased up to 0.4% to reveal the effects of Zr, while in the second series, the concentration of Zr was constant, equal to 0.08%, but the Zn content was gradually increased to 30% to show the effect of Zn. The third one was to test separate and conjunct addition of C, P and Zr in order to understand each elements effect on grain refinement. The resulting samples were characterized and analyzed by several techniques: (a) visual observation of the grain macrostructure; (b) chemical analysis; (c) thermal analysis; (d) optical microscopy; (e) scanning electron microscopy; (f) EDS microanalysis; (g) WDS microanalysis; and (h) X-Ray diffraction. The results show that the average grain size of the Cu-Zn alloys (6% <= Zn <= 30% mass) decreases significantly when Zr and P are both added to the melt. Nevertheless, average grain size is not reduced significantly when either Zr or P are added separately to the melt. When 0.08%Zr is added, the average grain size decreases as the concentration of Zn increases from 0 to 30%Zn, but without Zr additions, the Zn content and the average grain size are uncorrelated. The microstructural analysis with the techniques listed before show that the decrease in average grain size caused by Zr and P additions coincide with the observation of faceted precipitates rich in Zr and P. This analysis indicates that the inoculation mechanism is probably the heterogeneous nucleation of solid Cu-µ, of face-centered-cubic lattice structure, on solid particles of ZrP, with hexagonal compact structure, formed in the melt after Zr and P additions.

β-grain size is an influential microstructural parameter on the properties of titanium components. A reduction of β-grain size is generally associated with improvements to ductility, strength, corrosion and fatigue resistance of many α, α/β and β titanium alloys. During production of wrought titanium components, the β-grain size is carefully controlled during thermomechanical processing but there is currently no control of the β-grain size during solidification of cast components. As such, this inability to control the β-grain structure during solidification may limit the applications for solidification based technologies including casting, welding and direct metal deposition. Due to the limited knowledge of grain refinement practices and the lack of commercial grain refiners for the titanium system, this thesis investigates the mechanisms of β-grain refinement during solidification of cast titanium alloys. In this thesis, generalized theories for grain refinement that have been developed from research into other metallic systems are applied to the titanium system. Similar to the findings from aluminium and magnesium research, it is shown that grain refinement of cast titanium alloys requires the addition of growth restricting solutes which provide constitutional undercooling as well as the presence of potent nucleant particles. It is demonstrated that commercially pure titanium contains a natural distribution of nuclei particles which may originate from the mould wall and when powerful growth restricting solutes are introduced, significant prior-β grain refinement is achievable. All solutes investigated do not interact or poison the naturally occurring nucleants enabling the grain size of the titanium alloys to be predicted by an empirically determined relationship based on the growth restriction factor. A full list of growth restriction factors for various elements in titanium is determined and it is proven that growth restriction theory is valid in the titanium system. A further reduction in β-grain size is achievable by introducing additional nucleant particles to titanium castings in conjunction with growth-restricting solutes. Using a novel technique, titanium powder was introduced to the melt stream prior to solidification and was mixed throughout the liquid. The powder particles partially melted and the oxide surface layer dissolved allowing intimate substrate-liquid contact, enabling the titanium substrates to act as sites for heterogeneous nucleation. Using this technique, it was possible to grain refine commercially pure titanium without foreign elemental addition and when growth restricting solutes were present it was possible to obtain approximately an order of magnitude grain size reduction. The results and concepts developed from this work may aid the future development of a commercial grain refiner for titanium. If a grain refiner is developed, its application will not just be limited to the titanium casting industry but may also benefit other solidification based technologies such as welding, direct metal deposition and wrought billet production.

Ultrafine-grained(UFG) AZ31 Mg alloy was obtained by equal-channel angular extrusion(ECAE) and subsequent annealing at elevated temperatures. The basal texture component for ECAEed material is located on the Z plane of the ECAEed billets. Tensile tests were performed at temperatures between room temperature and 125¢J, and strain rates used ranging from 3*10-5 to 6*10-2 s-1. The experimental results showed that a high tensile yield stress of 394 MPa was obtained at room temperature under a strain rate of 3*10-3 s-1. Strengths of UFG AZ31 specimens were greatly improved due to grain refinement. It was found that strain rate sensitivity of UFG AZ31 alloy increased significantly from 0.024 to 0.321 with increasing temperature. The constant k of Hall-Petch equation, £m=£m0 +kd-1/2, decreased with increasing temperature, and decreasing strain rate. Negative k values were ontained at 75¢J and 100¢J under a strain rate 3*10-5 s-1. When compressed along X, Y and X45Z billet orientations, strain localization within shear bands was found in UFG AZ31 specimens. Shear bands are formed inclined near 45 to the compression axis. The smaller the grain size, the thinner the shear band. Different Hall-Petch constant k were found in specimens deformed along different orientations, which is caused by different deformation mechanisms. The formation of tension twins is the primary deformation mechanism for compressed X and Y samples, and basal slip is responsible for the deformation of X45Z sample. tension twins were found in 0.46 £gm grain size specimens.

In this study, firstly, in order to achieve fine grains in solid solution strengthened AZ31 magnesium alloy by friction stir processing (FSP), various efforts have been made. It has found that with a newly designed cooling system, the microstructure of commercial AZ31 alloy can be refined dramatically by carefully controlling the FSP parameters. It is of scientific interest that nanometer grains have been observed in the resultant microstructure for the AZ alloy experienced by two-pass FSP. Besides, in order to modify the microstructure and mechanical properties, FSP is also applied to incorporate AZ31 Mg alloy with nano-ZrO2 particles, nano-SiO2 particles and different fractions of Al and Zn elements. The microstructure and mechanical properties of the modified alloy and composite samples are investigated and compared. By one-pass FSP coupled with rapid heat sink from liquid nitrogen cooling approach, the ultrafine grain size in AZ31 Mg alloy is successfully achieved. The grain boundaries are well defined and the mean grain size can be refined to 100~300 nm from the initial 75 £gm of commercial AZ31 Mg alloys sheets. The ultrafine grained structure can drastically increases the microhardness from the initial 50 up to 120 Hv, or an increment factor of 2.4 times. Furthermore, the nanometer grains can be even achieved by two passes FSP coupled with rapid heat sink. The resulting microstructure exhibits equiaxed grains ranging from 40 nm to 200 nm with an average grain size of less than 100 nm. The nanocrystalline grains can be characterized by the TEM observations and the diffraction rings in SAD patterns. The highest hardness point can reach ~150 Hv which is equal to triple of the AZ31 matrix, and the mean hardness also increases up to around 134 Hv. Bulk Mg-AZ31 based composites with 10~20 vol% of nano-ZrO2 particles and 5~10 vol% of nano-SiO2 particles are also successfully fabricated by FSP. The average grain size of the resultant composites could be effectively refined to 2~4 £gm, and it demonstrates much higher hardness values compared to commercial AZ31 billet. Moreover, for the Mg/ZrO2 composite fabricated by one pass and subsequent cooling pass FSP, the recrystallized grain size could be further refined to 0.4 £gm with the hardness value of 135 Hv. As for multi-element Mg base alloys fabricated by FSP, high fractions of Al and Zn elements can result in apparent grain refinement, this can be proved by the broadening of diffraction peaks. Multi-passes FSP can induce the appearance of intermetallic compounds, however, some of them are quasi-crystals with icosahedral point group symmetry. The average hardness of the resultant alloys reachs nearly 350 in Hv scale due to the generation of intermetallic compounds and grain refinement.

Aluminium alloys, including both foundry and wrought alloys, have been extensively used for light-weight structural and functional applications. A grain refined as-cast microstructure is generally highly desirable for either subsequent processing ability or mechanical properties of the finished components. In this thesis, the grain refined microstructures in Al alloys have been achieved by intensive melt shearing using the melt conditioning by advanced shearing technology (MCAST) without deliberate grain refiner additions. Such grain refinement has been attributed to the enhanced heterogeneous nucleation on the dispersed oxide particles. It has been established that the naturally occurring oxides in molten Al alloys normally have a good crystallographic match with the a-Al phase, indicating the high potency of oxide particles as the nucleation sites of the a-Al phase. The governing factors for these oxide particles to be effective grain refiners in Al alloys have been proposed, including the achievement of good wetting between oxide particles and liquid aluminium, a sufficient number density and uniform spatial distribution of the dispersed oxide particles, and near equilibrium kinetic conditions in liquid alloys. In the present study, near equilibrium kinetic conditions can be achieved by intensive melt shearing using a twin screw mechanism, which has been confirmed by the observed equilibrium a-AlFeSi phase in a cast Al alloy and the transformation from g- to a-Al2O3 at 740±20oC under intensive shearing. For different alloy systems, depending on the alloy system, and melting conditions, due to the particular types of oxide formed and its crystallographic and chemical characteristics, the nucleation site of the nucleated phase is different. Specifically, MgAl2O4 relative to MgO, and a-Al2O3 relative to g-Al2O3, have higher potency as heterogeneous nucleation sites of a-Al phase in Al alloys. In future, the modification of the crystallographic match, and of the other surface characteristics related to the interfacial energy between the specific oxides and nucleated phase by trace alloying addition through segregation to the interface between oxides and nucleated phases combined with physical melt processing (such as intensive shearing in the present study) should be investigated in more detail.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第18987号 / 工博第4029号 / 新制||工||1620 / 31938 / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 辻 伸泰, 教授 白井 泰治, 教授 松原 英一郎 / 学位規則第4条第1項該当

A series of AlSi9Mg alloys were prepared and tested to reveal the effect of addition sequence and timing of grain refiner and eutectic modifier. AlSr10 master alloy was used as an modification reagent, and also for grain refiner AlTi5B master alloy was used. The depression at the eutectic temperature due to the addition of modifier and decrease in the amount of undercooling at the liquidus due to the presence of grain refiner were examined by the cooling curves which were obtained by the Alu-Therm instrument, which is the aluminum thermal analyzer of the Heraeus Electro-Nite. The alloys that were both modified and grain refined were subsequently poured as tensile test specimen shapes in permanent die casting mould for four times at 60 minutes time intervals, meanwhile thermal analysis of the alloys were also made. In this work the effect of grain refinement and modification agent, also the determination of the optimum time to pour after adding these agents were studied by aluminum thermal analyzer. The parameters obtained from this analyzer are compared with the microstructures / to see the effect of these agents on mechanical properties, hardness, tensile strength and percent elongation values were investigated. In this study the possibility of predicting the mechanical properties prior to casting by thermal analysis method was examined by regression analysis method. By this method relationship between thermal analysis parameters and mechanical properties was established.

Bulk nanocrystalline 304L and 316L austenitic stainless steels (SS) were produced by equal channel angular pressing(ECAP) at elevated temperature. The average grain size achieved in 316L and 304 L SS is ~ 100 nm, and grain refinement occurs more rapid in 316 L SS than that in 304L. Also the structures are shown to retain a predominant austenite phase. Hardness increases by a factor of about 2.5 in both steels due largely to grain refinement and an introduction of a high density of dislocations. Tensile strength of nanocrystalline steels exceeds 1 GPa with good ductility in both systems. Mechanical properties of ECAPed 316L are also shown to have less dependence on strain rate than ECAPed 304L. ECAPed steels were shown to exhibit thermal stability up to 600oC as indicated by retention of high hardness in annealed specimens. Furthermore, there is an increased tolerance to radiation-induced hardening in the nanocrystalline equiaxed materials subjected to 100 keV He ions at an average dose of 3-4 displacement-per-atom level at room temperature. The large volume fraction of high angle grain boundaries may be vital for enhanced radiation tolerance. These nanocrystalline SSs show promise for further research in radiation resistant structural materials for next-generation nuclear reactor systems.

Submitted by GILSON SILVA JUNIOR null (gilson_feg@yahoo.com.br) on 2017-12-18T12:27:06Z No. of bitstreams: 1 Tese Doutorado Defesa - Versão Final.pdf: 8602731 bytes, checksum: 8f3cbfc632bdb7f8998d8a2a7aa87243 (MD5) / Approved for entry into archive by Pamella Benevides Gonçalves null (pamella@feg.unesp.br) on 2017-12-18T13:21:51Z (GMT) No. of bitstreams: 1 silvajunior_g_dr_guara.pdf: 8602731 bytes, checksum: 8f3cbfc632bdb7f8998d8a2a7aa87243 (MD5) / Made available in DSpace on 2017-12-18T13:21:51Z (GMT). No. of bitstreams: 1 silvajunior_g_dr_guara.pdf: 8602731 bytes, checksum: 8f3cbfc632bdb7f8998d8a2a7aa87243 (MD5) Previous issue date: 2017-11-10 / A obtenção de granulometria ultrafina em aços com baixo teor de carbono pode contribuir para ampliação de suas aplicações na indústria, devido as propriedades mecânicas superiores que podem ser alcançadas com o refinamento de grãos, tais como: resistência mecânica, dureza, e tenacidade. O processo conhecido como Equal Channel Angular Pressing (ECAP) induz deformações plásticas severas suficientes para alterar as características microestruturais dos metais reduzindo seu tamanho de grão, e consequentemente melhorando algumas propriedades mecânicas, sem alterar a composição química dos materiais, ao utilizar temperaturas abaixo do ponto de recristalização dos metais. Neste trabalho o processo ECAP foi conduzido com corpos de prova na temperatura de 550°C utilizando como material de estudo aço SAE 1020. Os corpos de prova foram separados em três grupos. No primeiro grupo as amostras não foram submetidas a nenhum tratamento térmico entre os passes, no segundo grupo foi aplicado tratamento de alívio de tensões após os passes, e por fim, no terceiro grupo e foi aplicado um tratamento de recozimento intercrítico após o primeiro passe. Ensaios mecânicos de tração, dureza e charpy foram realizados com objetivo de verificar a influência do processo ECAP no comportamento mecânico do aço. Com intuito de verificar as alterações microestruturais causadas pelo processo ECAP foram utilizadas as técnicas de microscopia óptica e eletrônica de varredura. O trabalho tem como objetivo principal induzir o refinamento dos grãos por meio do processo ECAP em matriz bipartida elaborada neste trabalho. Os resultados das análises microestruturais e dos ensaios mecânicos demonstraram que os tratamentos térmicos utilizados combinados ao processo ECAP influenciaram diretamente no comportamento do aço SAE 1020. Conforme o número de passes pela matriz ECAP ocorreu uma redução do tamanho dos grãos, assim como aumento do limite de resistência a tração e dureza do aço 1020. Com relação ao tratamento térmico de alívio de tensão, uma melhor combinação entre resistência mecânica e ductilidade foi encontrada. O tratamento de recozimento intercrítico foi suficiente para induzir a transformação de fases no aço SAE 1020, o qual proporcionou resultados positivo no que diz respeito a ductilidade e resistência mecânica. Por fim, a consistência das investigações da evolução microestrutural permitiu compreender os efeitos do ECAP no aço SAE 1020. / Ultrafine grained microstructures obtaining in low carbon steels may contribute to enlarge the application of this material in industry, due to superior properties that can be achieved, such as: mechanical strength, hardness, and toughness. The process known as Equal Channel Angular Pressing (ECAP) induces severe plastic deformation sufficient to modify metals microstructures characteristics, reducing its grains size, and consequently improve its mechanical properties without materials chemistry composition changes, under temperatures below to recrystallization point. At this work ECAP process was carried out with specimens at 550° C using steel SAE 1020 as material. The specimens were divided into three groups. The specimens in the first group none heat treatment was applied between and after ECAP passes, in the second group the specimens were submitted under stress relief heat treatment after ECAP passes, and in the third group, specimens were submitted under intercritical annealing after first pass. Mechanical tensile strength, hardness and charpy impact tests were used with aim to verify the ECAP influence in steel mechanical behavior. In order to verify microstructures evolution caused by ECAP were applied optical and scanning electron microscopy. The aim of this work is grain refining by means of ECAP process with two parts tool elaborated in this study. The microstructure analysis and mechanical tests results shown that the heat treatments applied, combined with ECAP process directly influenced on steel SAE 1020 behavior. According to the number of passes grains sizes were reduced, as well the ultima tensile strength and hardness were increased. In reference of stress relief heat treatment, better combination between mechanical strength and ductility was achieved. Intercritical annealing treatment was capable to induce phase transformation in steel SAE 1020, which provided positive results with respect to ductility and mechanical strength. In conclusion, the consistence of microstructure evolution investigation became possible to understand effects of ECAP in steel SAE 1020