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.

A laser-driven flyer impact system was constructed, characterized, and validated for performing uniaxial-strain experiments to investigate the shock equation-of-state (EOS) and processes leading to reaction initiation in thin, fully-dense Ni-Al nanolayered foils. Additionally, various fully-dense Ni-Al mixtures with highly heterogeneous microstructures and widely varying length scales were investigated to understand influence of meso-scale features on the shock compression and reaction response. Ni-Al composites are a class of reactive materials also called Structural Energetic Materials (SEMs), which aim to combine stiffness and strength with the ability to release large amounts of energy through highly exothermic reactions when the constituents are intimately mixed during shock loading. While porous reactive materials have been studied extensively, the processes leading to reaction initiation in fully-dense mixtures consisting of phases with disparate mechanical properties is more ambiguous. A table-top, small-scale laser system was developed for studying shock-induced effects in extremely thin reactive materials. Laser accelerated thin foil impact experiments utilizing time-resolved interferometry allowed for measuring the Hugoniot of the nanolayered Ni-Al foil over a range of particle velocities/pressures. Separate recovery experiments were performed by shock-loading Ni-Al foils slightly below the reaction initiation threshold and performing post-mortem TEM/STEM analysis to identify the constituent mixing processes leading to reaction. Direct-shock experiments were performed on the different fully-dense Ni-Al mixtures and hydrodynamic simulations using real microstructures allowed direct correlations with the experiment results, which yielded an improved understanding of the effect of phase arrangement on the shock propagation and reaction initiation response. The EOS experiments performed at particle velocities > 200 m/s showed a deviation from the predicted inert trend and recovered targets showed complete reaction to the B2-NiAl intermetallic phase. The measured deviation from inert behavior and state of recovered material suggests the occurrence of a shock-induced chemical reaction. The shocked (but unreacted) Ni-Al materials contained distinct constituent mixing features (layer jets and intermixed zones), where significant elemental penetration occurred and are likely sources of reaction initiation. The observed results provide the first clear evidence of shock-induced reactions in fully-dense nanolayered Ni-Al foils.

Approved for public release, distribution is unlimited / The effect of friction-stir processing (FSP) on the microstructure of a cast nickel-aluminum bronze (NAB) material has been characterized by various micro-analytical methods including orientation imaging microscopy (OIM). Cast NAB is widely utilized in the production of propellers for the surface ships and submarines of the U.S. Navy due to excellent corrosion-resistance. New applications require improved mechanical properties that may be attainable using FSP to achieve localized microstructure modification. Friction between a rotating tool and the surface of the material results in a *stirring* action that, in turn, produces adiabatic heating and local softening of the material. The tool rotation results in very large shear deformations in the softened regions and thus microstructure refinement and homogenization; in effect FSP may convert an as-cast microstructure to a wrought condition in the absence of macroscopic shape change. In as-cast material, results of optical and scanning electron microscopy (using energy dispersive analysis) show an ` (FCC) matrix containing globular and particulate dispersions that correspond to the *I, *II and *IV second phases; these represent various morphologies of the Fe3Al intermetallic compound, which has a D03 structure. Also present are lamellar particles of *III, which is NiAl and has a B2 structure. The grain size in the ` matrix is ~ 1 mm. In OIM, the microtexture and microstructure in the ` (FCC) matrix may be readily obtained and analyzed. However, interatom distances in the Fe3Al and NiAl phases differ by only about one percent and so these phases are not distinguishable from one another during OIM. Altogether, microstructure and microtexture analysis showed that there are several regions in the thermomechanically affected zone (TMAZ) of a material subjected to FSP. From base material inward toward the TMAZ, these include: annealing effects in undeformed base material; a region just inside the TMAZ in which grain deformation and C-type shear deformation textures are observed; regions of highly refined and recrystallized grains further inside the TMAZ, wherein the grain size is < 5æm; and, finally, regions of elongated, banded and twinned grain structures that suggest grain growth following recrystalliztion. / Lieutenant, United States Navy

Thesis (Ph. D.)--University of Missouri-Columbia, 2003. / Typescript. Vita. Includes bibliographical references (leaves 160-167). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2003. / Typescript. Vita. Includes bibliographical references (leaves 160-167). Also available on the Internet.