O aumento no consumo energético mundial e a perspectiva de esgotamento das reservas de combustíveis fósseis têm estimulado o desenvolvimento de tecnologias e materiais para aplicações nos futuros reatores de fusão nuclear e reatores de fissão a nêutrons rápidos. O foco deste trabalho é avaliar a estabilidade microestrutural de um material tecnologicamente promissor por meio de diferentes técnicas de caracterização. O material estudado, o aço ODS-EUROFER, é um aço ferrítico-martensítico de atividade reduzida de composição 9%Cr-1%W (em massa) endurecido pela dispersão de 0,3% (em massa) de óxido de ítrio. Amostras foram laminadas até 80% de redução e submetidas a tratamentos isotérmicos em 800 °C por até 6 meses (4320 h) de duração para se investigar a ocorrência de fenômenos tais como recuperação, recristalização, crescimento de grão, precipitação e engrossamento de Ostwald. Outro conjunto de amostras foi recozido por 1 hora em temperaturas entre 900 e 1300 °C para se estudar a transformação martensítica neste material. Diversas técnicas de caracterização microestrutural complementares entre si (MEV, MET, DRX, EBSD, APT, ensaios de microdureza Vickers, dilatometria, DTA e magnetização) foram empregadas. Os resultados obtidos indicam que a dispersão de partículas nanométricas de Y2O3 confere uma grande resistência à recristalização primária, favorecendo a recuperação estática como principal mecanismo de amolecimento durante o recozimento prolongado deste aço. De modo similar, o crescimento de grão foi suprimido no campo austenítico em temperaturas tão altas quanto 1200 °C. / The rise in the world energy consumption and the possibility of depletion of the fossil fuel reserves have stimulated the development of new technologies and new materials for applications in the future nuclear fusion reactors and in the fast breeder fission reactors. The aim of this study is to evaluate the microstructural stability of a technologically promising material by using different characterization techniques. The investigated material, the ODS-EUROFER steel, is 9Cr-1W (%wt) reduced-activation ferritic-martensitic steel reinforced with a dispersion of 0.3%wt of yttrium oxide nanoparticles. Samples were cold rolled to 80% thickness reduction and subjected to isothermal annealing at 800 °C for up to 6 months (4,320 h) to investigate the occurrence of phenomena such as recovery, recrystallization, grain growth, precipitation, and Ostwald ripening. Another set of samples was annealed for 1 hour at temperatures between 900 and 1300 °C to study the martensitic transformation in this steel. Several complementary microstructural characterization techniques were employed (SEM, TEM, XRD, EBSD, APT, Vickers hardness, dilatometry, DTA and magnectic measurements). The results suggest that the dispersion of nanoscaled particles of Y2O3 provides a high resistance to discontinous recrystallization and favors static recovery as the main softening mechanism during long-term annealing in this steel. Similarly, grain growth was suppressed even in temperatures as high as 1200 °C in the austenitic field.

O aumento no consumo energético mundial e a perspectiva de esgotamento das reservas de combustíveis fósseis têm estimulado o desenvolvimento de tecnologias e materiais para aplicações nos futuros reatores de fusão nuclear e reatores de fissão a nêutrons rápidos. O foco deste trabalho é avaliar a estabilidade microestrutural de um material tecnologicamente promissor por meio de diferentes técnicas de caracterização. O material estudado, o aço ODS-EUROFER, é um aço ferrítico-martensítico de atividade reduzida de composição 9%Cr-1%W (em massa) endurecido pela dispersão de 0,3% (em massa) de óxido de ítrio. Amostras foram laminadas até 80% de redução e submetidas a tratamentos isotérmicos em 800 °C por até 6 meses (4320 h) de duração para se investigar a ocorrência de fenômenos tais como recuperação, recristalização, crescimento de grão, precipitação e engrossamento de Ostwald. Outro conjunto de amostras foi recozido por 1 hora em temperaturas entre 900 e 1300 °C para se estudar a transformação martensítica neste material. Diversas técnicas de caracterização microestrutural complementares entre si (MEV, MET, DRX, EBSD, APT, ensaios de microdureza Vickers, dilatometria, DTA e magnetização) foram empregadas. Os resultados obtidos indicam que a dispersão de partículas nanométricas de Y2O3 confere uma grande resistência à recristalização primária, favorecendo a recuperação estática como principal mecanismo de amolecimento durante o recozimento prolongado deste aço. De modo similar, o crescimento de grão foi suprimido no campo austenítico em temperaturas tão altas quanto 1200 °C. / The rise in the world energy consumption and the possibility of depletion of the fossil fuel reserves have stimulated the development of new technologies and new materials for applications in the future nuclear fusion reactors and in the fast breeder fission reactors. The aim of this study is to evaluate the microstructural stability of a technologically promising material by using different characterization techniques. The investigated material, the ODS-EUROFER steel, is 9Cr-1W (%wt) reduced-activation ferritic-martensitic steel reinforced with a dispersion of 0.3%wt of yttrium oxide nanoparticles. Samples were cold rolled to 80% thickness reduction and subjected to isothermal annealing at 800 °C for up to 6 months (4,320 h) to investigate the occurrence of phenomena such as recovery, recrystallization, grain growth, precipitation, and Ostwald ripening. Another set of samples was annealed for 1 hour at temperatures between 900 and 1300 °C to study the martensitic transformation in this steel. Several complementary microstructural characterization techniques were employed (SEM, TEM, XRD, EBSD, APT, Vickers hardness, dilatometry, DTA and magnectic measurements). The results suggest that the dispersion of nanoscaled particles of Y2O3 provides a high resistance to discontinous recrystallization and favors static recovery as the main softening mechanism during long-term annealing in this steel. Similarly, grain growth was suppressed even in temperatures as high as 1200 °C in the austenitic field.

In the present work, deformation behavior, texture and microstructure evolution of commercially pure titanium (CP Ti) are investigated by electron backscattered diffraction (EBSD) after compression tests at elevated temperatures. By analysing work hardening rate vs. flow stress, the deformation behaviour can be divided into three groups, viz. three-stage work hardening, two-stage work hardening and flow softening. A new deformation condition map is presented, dividing the deformation behavior of CP Ti into three distinct zones which can be separated by two distinct values of the Zener-Hollomon parameter. The deformed microstructures reveal that dynamic recovery is the dominant deformation mechanism for CP Ti during hot working. It is the first time that the Schmid factor and pole figures are used to analyse how the individual slip systems activate and how their activities evolve under various deformation conditions. Two constitutive equations are proposed in this work, one is for single peak dynamic recrystallization (DRX), the other is specially for CP Ti deformed during hot working. After the hot compression tests, some stress-strain curves show a single peak, leading to the motivation of setting up a DRX model. However, the examinations of EBSD maps and metallography evidently show that the deformation mechanism is dynamic recovery rather than DRX. Then, the second model is set up. The influence of the deformation conditions on grain size, texture and deformation twinning is systematically investigated. The results show that {10-12} twinning only occurs at the early stage of deformation. As the strain increases, the {10-12} twinning is suppressed while {10- 11} twinning appears. Three peaks are found in the misorientation frequency-distribution corresponding to basal fiber texture, {10-11} and {10-12} twinning, respectively. A logZ-value of 13 is found to be critical for both the onset of {10-11} compressive twinning and the break point for the subgrain size. The presence of {10-11} twinning is the key factor for effectively reducing the deformed grain size. The percentage of low angle grain boundaries decreases with increasing Z-parameter, falling into a region separated by two parallel lines with a common slope and 10% displacement. After deformation, three texture components can be found, one close to the compression direction, CD, one 10~30° to CD and another 45° to CD. / QC 20100819