In this work, STM measurements are employed to study the growth behaviors of alumina formed on the NiAl(110) surface under low coverage of oxygen dose (3 Langmuir and 120 Langmuir). Utilizing the capability of STM, the surface structures and the corresponding electronic properties of these alumina can be probed, respectively. The alumina nanoclusters are observed at sample bias 4V after the NiAl(110) surface is oxidized. However, when the bias is lower to 2V, the electronic states of the oxide are not accessible, so that the electrons can only tunnel between the tip and the metal support. In this case, the oxide just modulates the potential barrier as a function of position. Compared with the topographic image of the alumina formed by 3L and 120L oxygen dose, the growing behavior of the alumina/NiAl(110) at the initial stage could be measured.

Realizou-se um estudo dos efeitos das variáveis de moagem e dos moinhos de alta energia sobre a síntese do composto intermetálico NiAl. A influência do poder de moagem, tamanho de partícula inicial de Ni, agente controlador de processo e dimensões dos corpos moedores durante a síntese do composto NiAl no moinho Spex foi quantificada utilizando-se um planejamento fatorial. Verificou-se uma significativa influência do poder de moagem no tempo de ignição da reação exotérmica de formação do composto NiAl, além da influência da interação entre o poder de moagem, agente controlador de processo e dimensões dos corpos moedores no tamanho médio de partículas dos produtos das moagens. O rendimento no moinho Attritor demonstrou-se dependente do poder de moagem e também da quantidade de agente controlador do processo durante a moagem de misturas de pós de Ni e Al. A síntese por combustão de pós mecanicamente ativados no moinho Attritor levou à formação de NiAl3, Ni2Al3 e NiAl ou somente NiAl, dependendo da temperatura de síntese. Conduziu-se a tentativa de determinação experimental, através da técnica de calorimetria diferencial de varredura, das entalpias de formação dos compostos intermetálicos NiAl3, Ni2Al3, NiAl e Ni3Al; os valores encontrados para Ni2Al3 e Ni3Al foram, respectivamente, 167 kJ/mol e 93 kJ/mol. A formação de NiAl no moinho planetário, ao contrário do verificado nos moinhos Spex e Attritor, ocorreu de maneira gradual e o tempo necessário para a formação do composto foi dependente da quantidade de agente controlador de processo. Para os moinhos planetário e Attritor estimaram-se as respectivas energias de moagens transferidas ao longo do processo e o composto NiAl obtido nestes moinhos foi parcialmente ordenado. O método de Rietveld foi utilizado para a determinação da quantidade de NiAl formado após a moagem nos moinhos planetário e Attritor e também para a estimativa dos tamanhos de cristalitos e da deformação da rede. Amostras particuladas obtidas nos moinhos planetário e Attritor foram consolidadas, sinterizadas a 1300ºC e caracterizadas por ensaios de flexão e microdureza Vickers; as amostras apresentaram um comportamento frágil em flexão e microdureza Vickers inferior em comparação ao valor encontrado na literatura. / A study on the effect of milling variables and high-energy mills on the NiAl intermetallic compound synthesis was performed. The effect of ball-to-powder ratio, Ni particle initial size, the use of process control agent and milling media size on NiAl synthesis in a Spex mill was evaluated using a factorial design. It was shown that ball-to-powder ratio plays an important role in the ignition time of the mechanically induced reaction of NiAl intermetallic formation; moreover, an interaction of ball-to-powder ratio, process control agent and milling media size was found to affect the particle size of milling products. During milling of Ni and Al powder, the yield of an Attritor mill was found dependent on ball-to-powder ratio and the process control agent quantity. After combustion synthesis of powders, which were previously mechanically activated in the Attritor mill, monophasic NiAl or a mixture of NiAl3, Ni2Al3 and NiAl were obtained depending on synthesis temperature; however, swelling of samples was verified in both situations. The enthalpy of formation of Ni2Al3 and Ni3Al was measured using differential scanning calorimetry and the experimental values were, respectively, 167 kJ/mol and 93 kJ/mol. NiAl compound formation occurred through an exothermic reaction in Spex and Attritor mills, on the other hand, this intermetallic was gradually formed during milling in the planetary mill, and, in this milling device, the time for compound formation was dependent on process control agent quantity. Furthermore, the energy transferred to the powder during milling in planetary and Attritor mills was estimated, and the NiAl synthesized in these devices was partially disordered. Also, the Rietveld method was employed to determine the amount of NiAl synthesized in planetary and Attritor mills, and, also, estimate crystallite size and lattice strain of milling products. Finally, milling products of planetary and Attritor mills were consolidated and sintered at 1300ºC so that Vickers microhardness and flexion tests could be performed; these tests indicated a fragile behavior, and hardness lower than the value found on literature.

Realizou-se um estudo dos efeitos das variáveis de moagem e dos moinhos de alta energia sobre a síntese do composto intermetálico NiAl. A influência do poder de moagem, tamanho de partícula inicial de Ni, agente controlador de processo e dimensões dos corpos moedores durante a síntese do composto NiAl no moinho Spex foi quantificada utilizando-se um planejamento fatorial. Verificou-se uma significativa influência do poder de moagem no tempo de ignição da reação exotérmica de formação do composto NiAl, além da influência da interação entre o poder de moagem, agente controlador de processo e dimensões dos corpos moedores no tamanho médio de partículas dos produtos das moagens. O rendimento no moinho Attritor demonstrou-se dependente do poder de moagem e também da quantidade de agente controlador do processo durante a moagem de misturas de pós de Ni e Al. A síntese por combustão de pós mecanicamente ativados no moinho Attritor levou à formação de NiAl3, Ni2Al3 e NiAl ou somente NiAl, dependendo da temperatura de síntese. Conduziu-se a tentativa de determinação experimental, através da técnica de calorimetria diferencial de varredura, das entalpias de formação dos compostos intermetálicos NiAl3, Ni2Al3, NiAl e Ni3Al; os valores encontrados para Ni2Al3 e Ni3Al foram, respectivamente, 167 kJ/mol e 93 kJ/mol. A formação de NiAl no moinho planetário, ao contrário do verificado nos moinhos Spex e Attritor, ocorreu de maneira gradual e o tempo necessário para a formação do composto foi dependente da quantidade de agente controlador de processo. Para os moinhos planetário e Attritor estimaram-se as respectivas energias de moagens transferidas ao longo do processo e o composto NiAl obtido nestes moinhos foi parcialmente ordenado. O método de Rietveld foi utilizado para a determinação da quantidade de NiAl formado após a moagem nos moinhos planetário e Attritor e também para a estimativa dos tamanhos de cristalitos e da deformação da rede. Amostras particuladas obtidas nos moinhos planetário e Attritor foram consolidadas, sinterizadas a 1300ºC e caracterizadas por ensaios de flexão e microdureza Vickers; as amostras apresentaram um comportamento frágil em flexão e microdureza Vickers inferior em comparação ao valor encontrado na literatura. / A study on the effect of milling variables and high-energy mills on the NiAl intermetallic compound synthesis was performed. The effect of ball-to-powder ratio, Ni particle initial size, the use of process control agent and milling media size on NiAl synthesis in a Spex mill was evaluated using a factorial design. It was shown that ball-to-powder ratio plays an important role in the ignition time of the mechanically induced reaction of NiAl intermetallic formation; moreover, an interaction of ball-to-powder ratio, process control agent and milling media size was found to affect the particle size of milling products. During milling of Ni and Al powder, the yield of an Attritor mill was found dependent on ball-to-powder ratio and the process control agent quantity. After combustion synthesis of powders, which were previously mechanically activated in the Attritor mill, monophasic NiAl or a mixture of NiAl3, Ni2Al3 and NiAl were obtained depending on synthesis temperature; however, swelling of samples was verified in both situations. The enthalpy of formation of Ni2Al3 and Ni3Al was measured using differential scanning calorimetry and the experimental values were, respectively, 167 kJ/mol and 93 kJ/mol. NiAl compound formation occurred through an exothermic reaction in Spex and Attritor mills, on the other hand, this intermetallic was gradually formed during milling in the planetary mill, and, in this milling device, the time for compound formation was dependent on process control agent quantity. Furthermore, the energy transferred to the powder during milling in planetary and Attritor mills was estimated, and the NiAl synthesized in these devices was partially disordered. Also, the Rietveld method was employed to determine the amount of NiAl synthesized in planetary and Attritor mills, and, also, estimate crystallite size and lattice strain of milling products. Finally, milling products of planetary and Attritor mills were consolidated and sintered at 1300ºC so that Vickers microhardness and flexion tests could be performed; these tests indicated a fragile behavior, and hardness lower than the value found on literature.

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Molecular dynamics simulations of the diffusion process in ordered B2 compounds at high temperature were performed using an embedded atom interatomic potential developed to fit NiAl properties. Diffusion in the bulk occurs through a variety of cyclic mechanisms that accomplish the motion of the vacancy through nearest neighbor jumps restoring order to the alloy at the end of the cycle. The traditionally postulated six-jump cycle is only one of the various cycles observed and some of these are quite complex. Diffusion at the grain boundary mainly takes place through sequences of coordinated nearest neighbor jumps yielding to a rearrangement of the grain boundary structure. Two distinct mechanisms resulting in a structural unit migration of the vacancy are also identified. The results are analyzed in terms of the activation and configuration energies calculated using molecular statics simulations. / Master of Science

NiAl ist eine intermetallische Verbindung mit kubischer B2-Struktur. Unterhalb der Spröd-duktil-Übergangstemperatur, die bei Normaldruck bei etwa 300°C liegt, können nach dem Von-Mises-Kriterium nicht ausreichend viele unabhängige Gleitsysteme zur homogenen plastischen Verformung aktiviert werden, da die Bruchspannung unterhalb der kritischen Schubspannung zur Aktivierung des sekundären Gleitsystems liegt. Um NiAl trotzdem massiv plastisch verformen zu können, muss die Verformung bei hohem Druck erfolgen. Dies ist beispielsweise möglich bei der Verformung mittels Gleichkanal- Winkelpressen (ECAP) oder Hochdrucktorsion (HPT). Hierbei werden sehr hohe Verformungsgrade erreicht. In dieser Arbeit wird nun die Gefüge- und Texturentwicklung bei Verformung mittels HPT bei Temperaturen zwischen Raumtemperatur und 500°C in Abhängigkeit von Temperatur und hydrostatischem Druck sowie Verformungsgrad untersucht und mit den entsprechenden Größen bei der Verformung mit ECAP verglichen. Es zeigt sich, dass bei der HPT-Verformung die für kubisch-raumzentrierte Metalle typische Schertextur auftritt; zudem findet sich ab einer Verformungstemperatur von 100°C eine geneigte Würfellage, die durch dynamische Rekristallisation entstanden ist und deren Intensität mit der Verformungstemperatur ansteigt. Auch bei den ECAP-Proben findet man die typische Schertextur, eine Würfeltextur tritt nicht auf. Bei beiden Prozessen kommt es zur Kornfeinung, die beeinflusst wird von Verformungstemperatur, Druck und Verformungsgrad. Eine verbesserte Duktilität kann im Zugversuch nicht festgestellt werden, während eine gesteigerte Festigkeit durch Mikro- härtemessungen bestätigt wird. Bei genügend hoher Verformung findet man auch nach der Verformung von Einkristallen eine typische Schertextur. / NiAl is an intermetallic compound with a cubic B2 structure. Below the brittle-to-ductile transition temperature at about 300°C under ambient pressure the number of independent slip systems that have to be activated for a homogeneous plastic deformation is not sufficiently high according to the von Mises criterion. This is because the fracture stress is smaller than the critical yield stress for the activation of the secondary slip system. Despite this, severe plastic deformation is possible under a high pressure. It can be realized by deforming by equal channel angular pressing (ECAP) or high pressure torsion (HPT). Thereby a very high degree of deformation is reached. In this work, texture and microstructure evolution after deformation by HPT at different tem- peratures between room temperature and 500°C and at different hydrostatic pressures and degrees of deformation are analysed and compared to texture and microstructure after ECAP. After deformation by HPT, a texture typical for sheared body-centred metals is found; besides, for a deformation temperature of 100°C and higher an oblique cube component appears. It develops by dynamic recrystallization and grows with increasing deformation temperature. The shear texture also arises in the ECAP deformed samples, but no oblique cube texture is found. Both processes lead to grain refinement, which is influenced by deformation temperature, pressure, and degree of deformation. Tensile testing doesn’t show an enhanced ductility, but an improved strength is confirmed by microhardness measurements. If the degree of deformation is high enough, a shear texture can even be found after deforming single crystals.

No description available.

NiAl ist eine intermetallische Verbindung mit kubischer B2-Struktur. Unterhalb der Spröd-duktil-Übergangstemperatur, die bei Normaldruck bei etwa 300°C liegt, können nach dem Von-Mises-Kriterium nicht ausreichend viele unabhängige Gleitsysteme zur homogenen plastischen Verformung aktiviert werden, da die Bruchspannung unterhalb der kritischen Schubspannung zur Aktivierung des sekundären Gleitsystems liegt. Um NiAl trotzdem massiv plastisch verformen zu können, muss die Verformung bei hohem Druck erfolgen. Dies ist beispielsweise möglich bei der Verformung mittels Gleichkanal- Winkelpressen (ECAP) oder Hochdrucktorsion (HPT). Hierbei werden sehr hohe Verformungsgrade erreicht. In dieser Arbeit wird nun die Gefüge- und Texturentwicklung bei Verformung mittels HPT bei Temperaturen zwischen Raumtemperatur und 500°C in Abhängigkeit von Temperatur und hydrostatischem Druck sowie Verformungsgrad untersucht und mit den entsprechenden Größen bei der Verformung mit ECAP verglichen. Es zeigt sich, dass bei der HPT-Verformung die für kubisch-raumzentrierte Metalle typische Schertextur auftritt; zudem findet sich ab einer Verformungstemperatur von 100°C eine geneigte Würfellage, die durch dynamische Rekristallisation entstanden ist und deren Intensität mit der Verformungstemperatur ansteigt. Auch bei den ECAP-Proben findet man die typische Schertextur, eine Würfeltextur tritt nicht auf. Bei beiden Prozessen kommt es zur Kornfeinung, die beeinflusst wird von Verformungstemperatur, Druck und Verformungsgrad. Eine verbesserte Duktilität kann im Zugversuch nicht festgestellt werden, während eine gesteigerte Festigkeit durch Mikro- härtemessungen bestätigt wird. Bei genügend hoher Verformung findet man auch nach der Verformung von Einkristallen eine typische Schertextur. / NiAl is an intermetallic compound with a cubic B2 structure. Below the brittle-to-ductile transition temperature at about 300°C under ambient pressure the number of independent slip systems that have to be activated for a homogeneous plastic deformation is not sufficiently high according to the von Mises criterion. This is because the fracture stress is smaller than the critical yield stress for the activation of the secondary slip system. Despite this, severe plastic deformation is possible under a high pressure. It can be realized by deforming by equal channel angular pressing (ECAP) or high pressure torsion (HPT). Thereby a very high degree of deformation is reached. In this work, texture and microstructure evolution after deformation by HPT at different tem- peratures between room temperature and 500°C and at different hydrostatic pressures and degrees of deformation are analysed and compared to texture and microstructure after ECAP. After deformation by HPT, a texture typical for sheared body-centred metals is found; besides, for a deformation temperature of 100°C and higher an oblique cube component appears. It develops by dynamic recrystallization and grows with increasing deformation temperature. The shear texture also arises in the ECAP deformed samples, but no oblique cube texture is found. Both processes lead to grain refinement, which is influenced by deformation temperature, pressure, and degree of deformation. Tensile testing doesn’t show an enhanced ductility, but an improved strength is confirmed by microhardness measurements. If the degree of deformation is high enough, a shear texture can even be found after deforming single crystals.

The principal aim of this thesis is to characterise quantitatively the influence of high temperature creep upon the structure of ß-NiAl and ß-NiAl(Fe) single crystals. A non-destructive procedure is established following the classic line of X-ray structure analysis, namely controlling the chemical composition with the electron probe microanalysis, determining the unit cell contents from the combined lattice parameter and mass density measurements, and refining the structure parameters according to the X-ray reflection intensity. Specifically, two special single crystal X-ray diffraction methods, namely the back reflection Kossel technique and the back reflection Laue method, are applied for the determination of lattice parameter and for the collection of intensity data. All experimental measurements can be performed in non-destructive manner, which allows a direct comparison to be made between the crystal structure determined prior to and after a creep test.

The principal aim of this thesis is to characterise quantitatively the influence of high temperature creep upon the structure of ß-NiAl and ß-NiAl(Fe) single crystals. A non-destructive procedure is established following the classic line of X-ray structure analysis, namely controlling the chemical composition with the electron probe microanalysis, determining the unit cell contents from the combined lattice parameter and mass density measurements, and refining the structure parameters according to the X-ray reflection intensity. Specifically, two special single crystal X-ray diffraction methods, namely the back reflection Kossel technique and the back reflection Laue method, are applied for the determination of lattice parameter and for the collection of intensity data. All experimental measurements can be performed in non-destructive manner, which allows a direct comparison to be made between the crystal structure determined prior to and after a creep test.