The Digitization Centre has determined that page 49 was not published with the original copies of this thesis. / The validity of current theories of precipitate morphology is dependent on the microscopic nature of the precipitate-matrix interface. It is the purpose of this thesis to investigate, by electron microscopy interfacial structure and mobility in precipitation reactions pertaining to these theories. In particular the B->a Widmanstatten reaction in 60:40 brass is investigated and compared with the B->y reaction in a Cu-Zn-Sn alloy. / Thesis / Master of Science (MS)

We investigated the structure and defects associated with interphase boundaries between a γ (fcc) matrix and plate-shaped precipitates of the δ (orthorhombic) phase in Inconel 718. Based upon transmission electron microscopy (TEM) observations, the average habit plane was confirmed to be (111)_γ which is consistent with previous reports. A parallel array of misfit dislocations with Burgers vector b=1/6[112̅]_γ, (designated M1) are always observed lying along the [11̅0] direction. Another array of misfit dislocations appears in some regions of the interface with Burgers vector b=1/6[21̅1̅]_γ (designated M2). These dislocations also lie along the [11̅0] direction. Irregular ledges were identified on the interface and are believed to contribute to the thickening of δ plates. Dislocations in the matrix were also characterized. Most matrix dislocations have a 1/2[1̅01]_γ Burgers vector. The growth ledges in the habit plane of a single δ plate have a variety of effective Burgers vectors. A geometric matching approach based upon near-coincident sites was employed to explain the interfaces structure of interphase boundaries in Inconel 718, as well as fdc/bcc in Ni-45wt% Cr. In both cases, the conjugate plane is the plane with the highest areal density of near-coincident sites over a small region while the average habit plane is determined by the continuity of near-coincidence sites over a large area. The M1 interfacial dislocations in the γ/δ interface accommodate misfit in the habit plane whereas M2 dislocations do not and are probably a by-product of the dissociation of matrix dislocations. In the fcc/bcc system, the habit plane is not parallel to the conjugate plane and the partial dislocations associated with matrix stacking faults improve matching in the habit plane even though their Burgers vector lies out of this plane. / Ph. D.

The defect structure, atomic structure, and energy of the interphase boundaries between an fcc matrix and a lath-shaped bcc precipitate in Ni-45 wt% Cr were investigated. The interfacial structure on the side facet of the precipitate consists of regular structural ledges and misfit dislocations. No regular defect structure can be found on the habit plane, or broad face, of the lath except for atomic-scale structural ledges. High resolution electron microscopy (HREM) observations show the (12¯1)_f habit plane is coherent and is a good matching interface. Based upon conventional transmission electron microscopy (TEM) observations, the orientation of the habit plane results from advancing growth ledges on the conjugate plane of the Kurdjumov-Sachs orientation relationship. Using embedded atom method (EAM) simulations, the interfacial energy of the (12¯1)_f habit plane is calculated and the simulated interphase structure is compared with the HREM observations. The simulated interface represents a major portion of the observed interface. The calculated interfacial energy of the (12¯1)_f habit plane is 210 mJ/m², lower than typical grain boundary energies indicating this habit plane is a low-energy interphase boundary. A non-Bain lattice correspondence is identified and employed to predict the (12¯1)_f habit plane successfully, although a Bain correspondence is more successful at predicting the elongation direction for the precipitate. Geometric matching is proposed to be responsible for determining the orientation of the precipitate habit plane and the growth direction. Lattice correspondence-based approaches such as the invariant line model and the phenomenological theory of martensitic crystallography can mimic aspects of geometric matching, but they do not accurately reflect the transformation mechanism during precipitation of bcc laths from an fcc parent. / Ph. D.

Due to the extreme operating conditions present in the combustion sections of gas turbines, designers have relied heavily on specialized engineering materials. For blades, which must retain substantial strength and resistance to fatigue, creep, and corrosion at high temperatures, directionally-solidified (DS) nickel-base superalloys have been used extensively. Complex thermomechanical loading histories makes life prediction for such components difficult and subjective. Costly product inspection and refurbishment, as well as capital expense required in turbine forced outage situations, are significant drains on the resources of turbine producers. This places a premium on accurate endurance prediction as the foundation of viable long-term service contracts with customers. In working towards that end, this work characterizes the behavior of the blade material CM247LC DS subjected to a variety of in-phase (IP) and out-of phase (OP) loading cycles in the presence of notch stress concentrations. The material response to multiaxial notch effects, highly anisotropic material behavior, time-dependent deformation, and waveform and temperature cycle characteristics is presented. The active damage mechanisms influencing crack initiation are identified through extensive microscopy as a function of these parameters. This study consisted of an experimental phase as well as a numerical modeling phase. The first involved conducting high temperature thermomechanical fatigue (TMF) tests on both smooth and notched round-bar specimens to compile experimental results. Tests were conducted on longitudinal and transverse material grain orientations. Damage is characterized and conclusions drawn in light of fractography and microscopy. The influences of microstructure morphology and environmental effects on crack initiation are discussed. The modeling phase utilized various finite element (FE) simulations. These included an anisotropic-elastic model to capture the purely elastic notch response, and a continuum-based crystal visco-plastic model developed specifically to compute the material response of a DS Ni-base superalloy based on microstructure and orientation dependencies. These FE simulations were performed to predict and validate experimental results, as well as identify the manifestation of damage mechanisms resulting from thermomechanical fatigue. Finally, life predictions using simple and complex analytical modeling methods are discussed for predicting component life at various stages of the design process.