Designing materials for fusion applications is a very challenging problem, requiring detailed understanding of the behaviour of materials under the kinds of extreme conditions expected in a fusion environment. During the lifetime of fusion-reactor components, materials will be subjected to high levels of neutron irradiation, but must still perform effectively at high operating temperatures and under significant loading conditions. Body-centred cubic (bcc) transition metals are some of the most promising candidates for structural materials in fusion because of their relatively high density, which allows for effective neutron-shielding with the minimum volume and mass of material. In this work we perform atomistic simulations on two of the most important of these, Fe and W. In this thesis we describe atomic-scale simulations of defects found in bcc systems. In part I we consider the vacancy and interstitial loop defects that are produced and accumulated as a result of irradiation-induced displacement cascades. We show that vacancy dislocation loops have a critical size below which they are highly unstable relative to planar void defects, and thus offer an explanation as to why they are so rarely seen in TEM observations of irradiated bcc metals. Additionally, we compare the diffusion rates of these vacancy loops to their interstitial counterparts and find that, while interstitial loops are more mobile, the difference in mobility is not as significant as might have been expected. In part II we study screw dislocations, which, as the rate limiting carriers of plastic deformation, are significantly responsible for the strength of materials. We present results from large-scale finite temperature molecular dynamics simulations of screw dislocations under stress and observe the thermally-activated kink-pair formation regime at low stress, which appears to be superseded by a frictional regime at higher stresses. The mobility functions fitted to the results are vital components in simulations of dislocation networks and other large-scale phenomena. Lastly, we develop a multi-string Frenkel-Kontorova model that allows us to study the core structure of screw dislocations. Subtle changes in the form of the interaction laws used in this model demonstrate the difference between the non-degenerate and degenerate core structures. We provide simple criteria to guarantee the correct structure when developing interatomic potentials for bcc metals.

Uma amostra de zoisita natural proveniente da região de Teófilo Otoni Minas Gerais foi caracterizada pelas técnicas de termoluminescência (TL), ressonância paramagnética eletrônica (EPR) e absorção óptica (AO). As curvas de emissão TL das amostras natural e sob irradiação gama apresentaram picos em 130, 150, 265, 350 e 435 ºC todos eles com uma forte superposição. Tratamentos térmicos (TT) entre 500 e 900 °C afetaram muito pouco a estrutura cristalina da zoisita, no entanto os nove picos em 135, 155, 175, 200, 225, 255, 285, 320 e 360 ºC, ajustados teoricamente por deconvolução à curva experimental, apresentaram um máximo de sensibilidade TL para TT entre 600 e 700 ºC. O espectro de emissão TL contém uma banda intensa em torno de 310 nm e outra fraca em torno de 270 nm indicando que existem dois centros de recombinação que participam do processo de TL, o primeiro devido ao alumínio e o segundo devido ao titânio respectivamente. Exposições de amostras com TT em 600 °C à luz ultravioleta (UV) revelaram uma alta sensibilidade dos picos TL até 300 ºC, o que torna a zoisita um potencial candidato para aplicações em dosimetria de luz UV. A resposta TL da zoisita sob irradiação de raios gama, beta e elétrons de 1,4 MeV é similar, a única diferença está no aparecimento do pico em 110 ºC na irradiação com fonte beta. O espectro de EPR apresentou as seis linhas hiperfinas típicas do íon de Mn2+, em torno de g = 2,0, sobrepostas à linha da transição -1/2 -- +1/2 do Fe3+ num ambiente octaédrico. Além disso, dois conjuntos de linhas entre 800 - 1500 Gauss e 1500 - 2000 Gauss foram atribuídos aos íons de Cr3+ e Fe3+ respectivamente. Ambos íons sob ação de um forte componente axial de campo cristalino (CC), onde o nível fundamental 4A2 do íon de Cr3+ é desdobrado em dois dubletos mS = ±1/2 e mS = ±3/2. Os parâmetros de campo cristalino, Delta = 15100 cm-1, B = 739,5 cm-1 e D/B = 2,19 foram calculados a partir das transições permitidas de spin 4A2 -- 4T1 e 4A2 -- 4T2 do íon de Cr3+ na região visível do espectro de AO. A quebra do nível 4T2 foi atribuída à redução da simetria do poliedro Al(Cr)-O e à transição proibida de spin 4A2 -- 2T1 que sugere a substituição do Al3+ pelo Cr3+ na posição M3 da estrutura da zoisita. Bandas de AO devido a íons hidroxila e água foram identificadas na região do infravermelho próximo, todas elas estáveis frente a TT até 800 ºC e doses gama adicionais até 50 kGy. Baseado nos comportamentos dos centros de alumínio, titânio e E1 foi proposto um mecanismo de emissão TL. / Natural zoisite from the locality of Teófilo Otoni in the state of Minas Gerais - Brazil was investigated utilizing the techniques of thermoluminescence (TL), electron paramagnetic resonance (EPR) and optical absorption (OA). Natural as well as gamma irradiated zoisite exhibited TL peaks at 130, 150, 265, 350 and 435 ºC with an overlap of one peak with the other. Crystal structure of zoisite was found to be little affected by heat treatments in the range 500 - 900 °C. Deconvolution analysis has shown nine TL peaks at 135, 155, 175, 200, 225, 255, 285, 320 and 360 ºC. High TL sensitization has been observed for heat treated samples at 600 and 700 °C. The TL emission spectrum has shown a strong band around 310 nm and a weak one at 270 nm indicating the existence of two recombination centers, the first one due to aluminum and second one to titanium are involved in the TL process. Heat treated zoisite at 600 ºC exhibited high TL sensitivity of all TL peaks up 300 ºC. This feature indicates that zoisite can be a strong candidate for applications in UV dosimetry. The TL response of zoisite to beta rays and 1.4 MeV accelerated electrons is similar to that of gamma rays, with the exception that the electrons produced a 110 ºC peak. This peak was not seen under gamma irradiation. The EPR spectrum has shown the typical Mn2+ six hyperfine lines around g = 2.0. 1/ 2 to +1/ 2 transition line of the Fe3+ ion in an octahedral environment is also seen and the Mn2+ lines are overlapped by the Fe3+ line. On the other hand, most significant results have been observed in the low magnetic field region. Cr3+ lines are seen in the 800 - 1500 Gauss region and lines attributable to Fe3+ ion are observed in the 1500 - 2000 Gauss region. Both ions are under strong axial component of the crystal field (CF), where 4A2 state is split in two doublets mS = ±1/2 and mS = ±3/2. The crystal field parameters, Delta = 15100 cm-1, B = 739.5 cm-1 and Dq/B = 2.19 were calculated from the allowed spin transitions 4A2 -- 4T1 and 4A2 -- 4T2 of the Cr3+ ion in the visible region. The lifting of the 4T2 level was attributed to the lower polyhedron symmetry Al(Cr)-O and to the forbidden spin transition 4A2 -- 2T1; this suggests a substitution of Al3+ by Cr3+ ion at the site known as M3 in the structure of zoisite. Absorption bands due to OH and water molecule have been identified in the near infrared region. These bands have been found to be stable up to 800 ºC heat treatment and gamma doses up to 50 kGy. A model for the observed TL emission has been proposed based on the thermal annealing behaviour Al, Ti and E1 centers.

Uma amostra de zoisita natural proveniente da região de Teófilo Otoni Minas Gerais foi caracterizada pelas técnicas de termoluminescência (TL), ressonância paramagnética eletrônica (EPR) e absorção óptica (AO). As curvas de emissão TL das amostras natural e sob irradiação gama apresentaram picos em 130, 150, 265, 350 e 435 ºC todos eles com uma forte superposição. Tratamentos térmicos (TT) entre 500 e 900 °C afetaram muito pouco a estrutura cristalina da zoisita, no entanto os nove picos em 135, 155, 175, 200, 225, 255, 285, 320 e 360 ºC, ajustados teoricamente por deconvolução à curva experimental, apresentaram um máximo de sensibilidade TL para TT entre 600 e 700 ºC. O espectro de emissão TL contém uma banda intensa em torno de 310 nm e outra fraca em torno de 270 nm indicando que existem dois centros de recombinação que participam do processo de TL, o primeiro devido ao alumínio e o segundo devido ao titânio respectivamente. Exposições de amostras com TT em 600 °C à luz ultravioleta (UV) revelaram uma alta sensibilidade dos picos TL até 300 ºC, o que torna a zoisita um potencial candidato para aplicações em dosimetria de luz UV. A resposta TL da zoisita sob irradiação de raios gama, beta e elétrons de 1,4 MeV é similar, a única diferença está no aparecimento do pico em 110 ºC na irradiação com fonte beta. O espectro de EPR apresentou as seis linhas hiperfinas típicas do íon de Mn2+, em torno de g = 2,0, sobrepostas à linha da transição -1/2 -- +1/2 do Fe3+ num ambiente octaédrico. Além disso, dois conjuntos de linhas entre 800 - 1500 Gauss e 1500 - 2000 Gauss foram atribuídos aos íons de Cr3+ e Fe3+ respectivamente. Ambos íons sob ação de um forte componente axial de campo cristalino (CC), onde o nível fundamental 4A2 do íon de Cr3+ é desdobrado em dois dubletos mS = ±1/2 e mS = ±3/2. Os parâmetros de campo cristalino, Delta = 15100 cm-1, B = 739,5 cm-1 e D/B = 2,19 foram calculados a partir das transições permitidas de spin 4A2 -- 4T1 e 4A2 -- 4T2 do íon de Cr3+ na região visível do espectro de AO. A quebra do nível 4T2 foi atribuída à redução da simetria do poliedro Al(Cr)-O e à transição proibida de spin 4A2 -- 2T1 que sugere a substituição do Al3+ pelo Cr3+ na posição M3 da estrutura da zoisita. Bandas de AO devido a íons hidroxila e água foram identificadas na região do infravermelho próximo, todas elas estáveis frente a TT até 800 ºC e doses gama adicionais até 50 kGy. Baseado nos comportamentos dos centros de alumínio, titânio e E1 foi proposto um mecanismo de emissão TL. / Natural zoisite from the locality of Teófilo Otoni in the state of Minas Gerais - Brazil was investigated utilizing the techniques of thermoluminescence (TL), electron paramagnetic resonance (EPR) and optical absorption (OA). Natural as well as gamma irradiated zoisite exhibited TL peaks at 130, 150, 265, 350 and 435 ºC with an overlap of one peak with the other. Crystal structure of zoisite was found to be little affected by heat treatments in the range 500 - 900 °C. Deconvolution analysis has shown nine TL peaks at 135, 155, 175, 200, 225, 255, 285, 320 and 360 ºC. High TL sensitization has been observed for heat treated samples at 600 and 700 °C. The TL emission spectrum has shown a strong band around 310 nm and a weak one at 270 nm indicating the existence of two recombination centers, the first one due to aluminum and second one to titanium are involved in the TL process. Heat treated zoisite at 600 ºC exhibited high TL sensitivity of all TL peaks up 300 ºC. This feature indicates that zoisite can be a strong candidate for applications in UV dosimetry. The TL response of zoisite to beta rays and 1.4 MeV accelerated electrons is similar to that of gamma rays, with the exception that the electrons produced a 110 ºC peak. This peak was not seen under gamma irradiation. The EPR spectrum has shown the typical Mn2+ six hyperfine lines around g = 2.0. 1/ 2 to +1/ 2 transition line of the Fe3+ ion in an octahedral environment is also seen and the Mn2+ lines are overlapped by the Fe3+ line. On the other hand, most significant results have been observed in the low magnetic field region. Cr3+ lines are seen in the 800 - 1500 Gauss region and lines attributable to Fe3+ ion are observed in the 1500 - 2000 Gauss region. Both ions are under strong axial component of the crystal field (CF), where 4A2 state is split in two doublets mS = ±1/2 and mS = ±3/2. The crystal field parameters, Delta = 15100 cm-1, B = 739.5 cm-1 and Dq/B = 2.19 were calculated from the allowed spin transitions 4A2 -- 4T1 and 4A2 -- 4T2 of the Cr3+ ion in the visible region. The lifting of the 4T2 level was attributed to the lower polyhedron symmetry Al(Cr)-O and to the forbidden spin transition 4A2 -- 2T1; this suggests a substitution of Al3+ by Cr3+ ion at the site known as M3 in the structure of zoisite. Absorption bands due to OH and water molecule have been identified in the near infrared region. These bands have been found to be stable up to 800 ºC heat treatment and gamma doses up to 50 kGy. A model for the observed TL emission has been proposed based on the thermal annealing behaviour Al, Ti and E1 centers.