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The climb of dissociated dislocations in FCC materials is known to be complex: Observations of climb under HVEM irradiation in CuAl suggest that it, proceeds via the nucleation of interstitial loops directly onto individual partials. In silver, however, dissociated dislocations appear to constrict at an early stage in the irradiation, and dense vacancy cluster damage is seen to form in their vicinity. This thesis presents results and analysis of a study aimed at the determination of the interaction of HVEM generated interstitials and vacancies with pre-existing dislocations in a range of Fe-17Cr-Ni ternary alloys (with the nickel content varying between 15 and 40%). Two quaternary alloys (Fe-15%Ni-17%Cr-l%Si and Fe-15%Ni-17%Cr-2%Mo) arc also studied. As with earlier studies in CuAl and Ag, pre-existing dislocations in a (111) orientated foil were first characterized at subthreshold voltages employing the weak-becim technique, then irradiated with IMeV electrons at temperatures in the range 400-430°C and finally returned to the low voltage microscope for postirradiation characterization of the observed damage. Analysis of the post-irradiation microstructures indicates that interstitial climb only occurs at particularly favourable sites, such as pre-existing jogs: For the ternary alloys, constrictions are removed along edge and mixed dislocations, whilst zig-zagging of screw and near-screw dislocations may also be attributable to jog climb. After the annihilation of constrictions evidence of climb is not seen and pipe diffusion is thought to be occurring. The precipitation of small clusters, many of which are identifiable as vacancy SFT, is reminiscent of observations in silver. The addition of silicon to the matrix apparently leads to the creation of favourable sites for interstitial climb, as evidenced by the formation of high densities of new jogs after irradiation. Loops are seen to precipitate close to dislocations in the Mo-doped material. The origin of these loops is unclear at this stage. The relevance of the results to the phenomenon of void swelling is discussed.

Understanding the relationships between microstructures and properties of materials is a key to developing new materials with more suitable qualities or employing the appropriate materials in special uses. In the present world of material research, the main focus is on microstructural control to cost-effectively enhance properties and meet performance specifications. This present work is directed towards improving the fundamental understanding of the microscale deformation mechanisms and mechanical behavior of metallic alloys, particularly focusing on face centered cubic (FCC) structured metals through a unique computational methodology called three-dimensional dislocation dynamics (3D-DD). In these simulations, the equations of motion for dislocations are mathematically solved to determine the evolution and interaction of dislocations. Microstructure details and stress-strain curves are a direct observation in the simulation and can be used to validate experimental results. The effect of initial dislocation microstructure on the yield strength has been studied. It has been shown that dislocation density based crystal plasticity formulations only work when dislocation densities/numbers are sufficiently large so that a statistically accurate description of the microstructure can be obtainable. The evolution of the flow stress for grain sizes ranging from 0.5 to 10 µm under uniaxial tension was simulated using an improvised model by integrating dislocation pile-up mechanism at grain boundaries has been performed. This study showed that for a same initial dislocation density, the Hall–Petch relationship holds well at small grain sizes (0.5–2 µm), beyond which the yield strength remains constant as the grain size increases.

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A methodology and infrastructure was established for the generation of images of embedded atom atomistic simulation data showing the deformed crystal lattice with one or more superimposed strain invariant fields. These methods were applied to data from simulations of B2 NiAl to study extended dislocation cores and how these cores react to nearby point defects and applied stress. Such images may be viewed as a series of images forming an animation such that a simulated quantity, such as applied stress, is cast into time. Three movies were made doing this. Invariant fields are seen to expand and/or contract before and as they slip depending upon their initial characteristics. Two different interatomic potentials were used to study the relationships between dislocation core structure and mobility for a variety of dislocations. Using the visualization techniques noted above, it was clearly seen that in some cases, the dislocation core transforms to a planar structure before the dislocation glides, whereas in some other cases the core retains the non-planar structure at stresses sufficient to sustain glide. The effects of stoichiometry deviations on the core structure and motion were also studied for two pure edge dislocations. A 2% deviation from stoichiometry affects the shapes of dislocation cores in agreement with the experimental results of high resolution electron microscopy. This deviation was also found to increase Peierls stress. / Master of Science

A systematic study of the core structures of (100), (110), and (111) dislocations in B2 NiAI has been conducted using atomistic simulations with an embedded atom method (EAM) potential. New flexible boundary conditions and a new method of graphic representation of dislocation core structure have been employed. The main findings are the following: Core structures: There are no planar core structures of the dislocations found in B2 NiAl. The core spreading of (100) dislocations in NiAl can occur along a variety of planes depending on dislocation slip plane and line orientation. Discrete lattice effects reduced the high strain levels from anisotropic elasticity solution at the dislocation core considerably and resulted in asymmetrical core structures. The core structure of the (110) dislocations is mutilayered with spreading on the {110} plane. The extent of the same strain level comparing with (100) and (111) dislocations is much larger. The complete (111) dislocations in NiAl are also highly non-planar and are stable with respect to splitting into exact 1/2(111) partials as well as to alternative splittings that correspond to the stable fault in the vicinity of the antiphase boundary (APB), in both {110} and {112} planes. Peierls stresses: Peierls stresses of the dislocations have been calculated and have been compared for their relative ease of motion. Local disordering effects: The local disordering effects on the core structure are found to be significant only in the immediate vicinity of the point defect. Compositional deviation from stoichiometry: The simulation results of (100), (110), and (111)dislocations in off stoichiometric NiAl show that the core structures became more extended than the ones in the stoichiometric NiAl. The core structures are not only dependent on the overall composition but also on their local atomic arrangement near the core region. When compositional deviation from stoichiometry is introduced, the response to the applied stress is different for the various slip systems. The Peierls stresses for the usually easiest moving (100){110} dislocations increased and for the (100){100} dislocations decreased, and the latter are expected to be more active in the deformation processes. The practical implications of these results are that it seems very difficult to modify the alloy behaviors through local changes in stoichiometry and ordering state. The best way to improve the ductility of B2 NiAl is to stabilize (111) slip through the addition of alloying elements that can lower the APB energy. / Ph. D.

[Truncated abstract] The study of NiTi shape memory alloys, although comprehensive and diverse, still encounters numerous uncertainties and misunderstandings that often jeopardise the effective use of these alloys in various applications. One such key area is the understanding of the micromechanics and thermodynamics of the deformation mechanisms, such that their deformation behaviour can be accurately predicted and modelled. Furthermore, most research involves polycrystalline NiTi of varying compositions and processing history, both of which complicate and damage the internal structure of the matrix even before deformation. This work aims to study the micromechanisms of deformation of near-equiatomic NiTi alloys, both in polycrystalline and single crystal forms, with particular attention given to the commonly observed phenomena of Luders-like deformation behaviour and deformation induced martensite stabilisation. This work was carried out in three sections. Firstly, the tensile deformation of polycrystalline NiTi samples via martensite reorientation and stress-induced martensitic transformations was carried out. The samples were deformed to various stages of deformation and then thermally cycled to study the thermomechanical response to deformation as a means to explore the various mechanisms of deformation. Next, the deformation and post-deformation transformation behaviour of NiTi single crystals were studied to verify the effect of grain boundaries and other hypotheses raised regarding the deformation mechanisms. The single crystal samples were deformed along three low-index axial orientations. Finally, microscopic analysis was carried out on as-annealed and the deformed polycrystal and single crystal samples by means of transmission electron microscopy. The microstructural analyses accompanied the thermodynamic study and provided evidences to support various hypotheses

Approved for public release, distribution is unlimited / This study evaluated dome test samples of a superplastic AA5083 aluminum alloy deformed at nominally constant strain rates under biaxial strain conditions. Dome test samples resulted from gas-pressure forming of sheet material; for this study, samples were deformed at strain rates corresponding either to grain boundary sliding or dislocation creep control of deformation. Orientation Imaging Microscopy was utilized to determine texture development, grain size and grain-to-grain misorientation angle distributions for locations located along a line of latitude of the dome samples. The goal was to identify the location of the transition from grain boundary sliding to dislocation creep. Grain boundary sliding, which dominates at lower strain rates, can be recognized by a randomized texture and a higher concentration of high disorientation angles. Dislocation creep, which dominates at higher strain rates, is characterized by fiber texture formation and development of a peak at lower angles in the grain-to-grain misorientation angle distribution. / Ensign, United States Navy

The principal objective of this work was to elucidate the effect of microstructural features on the intrinsic dislocation mechanisms in two FCC alloys. First alloy Al0.1CoCrFeNi was from a new class of material known as complex concentrated alloys, particularly high entropy alloys (HEA). The second was a conventional Al-Mg-Sc alloy in ultrafine-grained (UFG) condition. In the case of HEA, the lattice possess significant lattice strain due to the atomic size variation and cohesive energy differences. Moreover, both the lattice friction stress and the Peierls barrier height are significantly larger than the conventional FCC metals and alloys. The experimental evidences, so far, provide a distinctive identity to the nature and motion of dislocations in FCC HEA as compared to the conventional FCC metals and alloys. Hence, the thermally activated dislocation mechanisms and kinetics in HEA has been studied in detail. To achieve the aim of examining the dislocation kinetics, transient tests, both strain rate jump tests and stress relaxation tests, were conducted. Anomalous behavior in dislocation kinetics was observed. Surprisingly, a large rate sensitivity of the flow stress and low activation volume of dislocations were observed, which are unparalleled as compared to conventional CG FCC metals and alloys. The observed trend has been explained in terms of the lattice distortion and dislocation energy framework. As opposed to the constant dislocation line energy and Peierls potential energy (amplitude, ΔE) in conventional metals and alloys, both line energy and Peierls potential undergo continuous variation in the case of HEA. These energy fluctuations have greatly affected the dislocation mobility and can be distinctly noted from the activation volume of dislocations. The proposed hypothesis was tested by varying the grain size and also the test temperature. Activation volume of dislocations was a strong function of temperature and increased with temperature. And the reduction in grain size did not affect the dislocation mechanisms and kinetics. This further bolstered the hypothesis. The second part deals with deformation characteristics of Al-Mg-Sc alloy. The microstructure obtained from the severe plastic deformation (SPD) techniques differ in dislocation density, grain/cell size, and in the grain boundary character distribution. Therefore, it is vital to understand the deformation behavior of the UFG materials produced by various SPD techniques, as the microstructural features basically control the deformation mechanisms. In this study, a detailed analysis was made to understand the deformation mechanisms operative in various regimes of a stress-strain in UFG Al-Mg-Sc alloy produced via friction stir processing. The stress-strain curves exhibited serrations from the onset of yielding to the point of sample failure. The serration amplitude and frequency was higher in UFG material as compared to CG material. Furthermore, the microstructural features that result in the serrated flow were investigated along with the avalanche characteristics. The presence of both ultrafine grains and Al3Sc precipitates were the necessary conditions to reach the critical stress required to push the grain boundary into a critical state to set off an avalanche. The microstructural conditions that did not satisfy both the requirements did not exhibit deep serrations.