The grain boundary network of nanocrystalline Cu foils was modified by the application of cyclic loadings and elevated temperatures. Broadly, the changes to the boundary network were directly correlated with the applied temperature and accumulated strain, including a 300% increase in the twin length fraction. By independently varying each treatment variable, a matrix of grain boundary statistics was built to check the plausibility of hypothesized mechanisms against their expected temperature and stress/strain dependences. These comparisons allow the field of candidate mechanisms to be significantly narrowed. Most importantly, the effects of temperature and strain on twin length fraction were found to be strongly synergistic, with the combined effect being ∼150% that of the summed individual contributions. Looking beyond scalar metrics, an analysis of the grain boundary network showed that twin related domain formation favored larger sizes and repeated twin variant selection over the creation of many small domains with diverse variants.

The dynamic response of structured materials, such as regular lattices, is nontrivial partly due to the interaction of mechanical waves throughout the structure and free surfaces as the material is dynamically compressed. The existence of an elastic precursor wave in additively manufactured lattices was recently shown to match theoretical predictions and simulation results. Following up on this work, we have investigated the behavior of the elastic precursor with propagation distance, impact speed, and impact material. Through a series of gas gun experiments coupled to X-ray phase contrast imaging measurements and complementary simulations, the elastic precursor wavespeed appears to be nearly independent of impact speed and impact material. We observed evidence for the sustained elastic wave propagation through many unit cells at four significantly different impact conditions. We compared these results with direct numerical simulations of the experiments and found good agreement.

SnO2-Fe2O3 composite thin films were deposited on quartz substrate at 650 °C by pulsed laser deposition with Fe2O3 content varying between 0 and 50 wt.%. The effect of increasing Fe2O3 content on structural, optical and electrical properties of SnO2-Fe2O3 films were investigated. X-ray diffraction analysis indicates that the nature of deposited composite films can be tune from crystalline to amorphous with increase in the Fe2O3 content. X-ray photoelectron spectroscopy studies confirm the presence of SnO2 and Fe2O3 in the microstructure. Atomic force microscopy studies of the deposited films indicate that RMS roughness vary between 114.4 and 1.48 nm. The average optical transmittance decreases from 92 to 72% with increasing Fe2O3 content of the deposited films. The optical bandgap was decreased from 3.69 to 3.31 eV with increase in Fe2O3 content. The minimum resistivity was observed 3.71 × 10−2 Ω cm for the 30 wt.% Fe2O3 composite film. The low resistivity and high transmittance in the visible region enables these films suitable for optoelectronic applications.

The roughness scaling of fracture surfaces in two-dimensional grain boundary networks is stud- ied numerically. Grain boundary networks are created using a Metropolis method in order to mimic the triple junction distributions from experiments. Fracture surfaces through these grain boundary networks are predicted using a combinatorial optimization method of maximum flow — minimum cut type. We have preliminary results from system sizes up to N = 22500 grains suggesting that the roughness scaling of these surfaces follows a random elastic manifold scaling exponent ζ = 2/3. We propose a strong dependence between the energy needed to create a crack and the special boundary fraction. Also the special boundaries at the crack and elsewhere in the system can be tracked.

Grain boundary engineering technology has been successfully transferred from rolling and forging to back-extrusion, and back- extruded copper shaped-charge liners with engineered microstructures have been produced. The grain-boundary engineered SCLs have special boundary fractions of 0.60 and 0.66, compared to 0.48 in a conventionally processed SCL. It is hoped that these engineered SCLs will perform better through a combination of a delayed onset of plastic instability and an improved resistance to void nucleation and coalescence.

Previous work demonstrates that SCL performance is affected by microstructure, and there is significant evidence which shows that microstructure influences both the fracture and constitutive responses of a material. For SCLs, constitutive properties which affect both the strength and stability of a material are important to performance. In the work described here, the focus is on improving stability properties such as strain-hardening rate and strain-rate sensitivity through grain boundary engineering.

Initial mechanical tests of grain boundary engineered copper indicate that its yield strength is nearly the same as conventionally processed material, even though the engineered sample has a larger grain size. Yield strength certainly affects the strength of an SCL, but its effect on stability, in terms of delaying the onset of plastic instability in a jet, is still under investigation. Although we can measure constitutive properties such as yield strength and strain to failure in the laboratory, the true effects of grain boundary engineering on SCL performance can only be measured in actual tests of the liners. Therefore, conventionally processed and grain boundary engineered SCLs will be fielded in real shaped-charges.

Two-dimensional (2D) cross sections through three-dimensional (3D) polycrystalline materials present a biased picture of the statistical properties of grain boundary networks. These properties are essential to many practical applications such as grain boundary engineering. We show a simple correction that will partly correct for the sampling biases by removing the effect of the correlation between grain boundary type and grain boundary area. This correction alters number fraction estimates by as much as ∼60% for σ3 boundaries in the highly-twinned copper samples we consider. We also estimate the bias introduced by the correlation between boundary type and boundary shape, which for many materials represents perhaps a 10% shift in the measured statistics, so that the simple method we propose should correct for the majority of the bias in favorable cases.

This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or the University of California, and shall not be used for advertising or product endorsement purposes.

Interfaces control many properties in engineering materials, several of which are critical to the integrity of the engineering structure. In single phase, solid solution, austenitic alloys, grain boundaries are often the weak link, displaying susceptibility to creep, corrosion and stress corrosion cracking. As such, grain boundary structure control affords the opportunity to improve the overall performance of alloys in a variety of applications. The role of coincident site lattice boundary (CSLB) enhancement and grain boundary connectivity is examined for how it affects the response of an alloy to stress and the environment. Specifically, the effect of grain boundary character on creep, grain boundary sliding, intergranular stress corrosion cracking, and irradiation assisted stress corrosion cracking in austenitic nickel-base (high purity Ni-Cr-Fe and alloy 600) and iron-base (high purity Fe-Cr-Ni and 304 stainless steel) alloys and for ferritic- martensitic alloy T91 is discussed.

We present a method for assessing the relative vulnerabilites of distinct classes of grain boundaries to localized corrosion. Orientation imaging microscopy provides a spatial map which identifies and classifies grain boundaries at a metal surface. Once the microstructure of a region of a sample surface has been characterized, a sample can be exposed to repeated cycles of exposure to a corrosive environment alternating with topographic measurement by an atomic force microscope in the same region in which the microstructure had been mapped. When this procedure is applied to Ni and Ni-based alloys, we observe enhanced attack at random grain boundaries relative to special boundaries and twins in a variety of environments.

The phase stability of C-22 alloy (UNS #N06022) was studied by aging samples at 593, 649, 704 and 760°C for 2000 h (2.7 mo) and 16,000 h (1.8 yr). The tensile properties and the Charpy impact toughness of these samples were measured in the mill annealed condition as well as after aging. The microstructures of samples aged 16,000 hours were examined using scanning and transmission electron microscopy (SEM and TEM). Preliminary TEM results suggest that μse forms at all temperatures investigated. Discrete carbide particles in addition to a film with very uniform thickness which appears to be μ phase formed on grain boundaries in the sample aged at 593°C. The ordered Ni2(Cr, Mo) phase was also seen in this sample. At the higher aging temperatures, mainly μ phase forms covering all the grain boundaries and also distributed throughout the bulk. Although strength increased somewhat with aging. the ductility decreased due to the formation of these grain boundary precipitates and brittle intermetallics.

There exists a growing body of literature that correlates the fraction of “special” boundaries in a microstructure, as described by the Coincident Site Lattice Model, to properties such as corrosion resistance, intergranular stress corrosion cracking, creep, etc. Several studies suggest that the grain boundary character distribution (GBCD), which is defined in terms of the relative fractions of “special” and “random” grain boundaries, can be manipulated through thermomechanical processing. This investigation evaluates the influence of specific thermomechanical processing methods on the resulting GBCD in FCC materials such as oxygenfree electronic (ofe) copper and Inconel 600. We also demonstrate that the primary effect of thermomechanical processing is to reduce or break the connectivity of the random grain boundary network. Samples of ofe Cu were subjected to a minimum of three different deformation paths to evaluate the influence of deformation path on the resulting GBCD. These include: rolling to 82% reduction in thickness, compression to 82% strain, repeated compression to 20% strain followed by annealing. In addition, the influence of annealing temperature was probed by applying, for each of the processes, three different annealing temperatures of 400, 560, and 800°C. The observations obtained from automated electron backscatter diffraction (EBSD) characterization of the microstructure are discussed in terms of deformation path, annealing temperature, and processing method. Results are compared to previous reports on strainannealed ofe Cu and sequential processed Inconel 600. These results demonstrate that among the processes considered, sequential processing is the most effective method to disrupt the random grain boundary network and improve the GBCD.

The nature of dissociated superlattice dislocation cores in Al66Ti25Cr9, deformed at room temperature, has been characterized by weak-beam transmission electron microscopy (TEM) and comparison of experimental images with computer-simulated images. The displacement fields associated with narrowly dissociated APB- and SISF-dissociated ‹110› superdislocations were calculated to account for the asymmetry in dislocation contrast and led to a better understanding of the formation of images. Such calculations are a powerful aid, when coupled with image simulations, in distinguishing the “real” intensity peaks from the supplementary peaks that can be generated under experimental imaging conditions. While both APB- and SISF-dissociated superdislocations were identified, the vast majority of superdislocations were determined to be APB-dissociated. Corrected values of the fault energies (γAPB and γSISF ) have been measured for this alloy. These energies and the observed dissociations are shown to be self-consistent.

The Ni3Ge-Fe3Ge model system provides us with a unique opportunity to characterize the mechanisms of deformation in both anomalous and normal L12 intermetallic alloys. The elastic moduli of alloys in this system have been measured and used as benchmarks for first principles calculations. At 77K, increasing the Fe content has been found to result in a dramatic increase in flow stress. The Ni-rich alloys exhibit a yield strength anomaly, but as Ni is replaced by Fe, the anomalous temperature dependence gradually disappears, and no yield strength anomaly is observed for alloys with more than 25 at% Fe. At low temperatures and Fe contents, the deformation microstructure has been found to be dominated by Kear-Wilsdorf locking; but a transition from octahedral glide and Kear-Wilsdorf locking to cube glide is observed as either Fe content or temperature is increased. This transition is related to changes that occur in the core structures of dissociated superdislocations and planar fault energies measured through computer simulations of weak-beam TEM images.

A combination of transmission electron microscopy (TEM) and image simulations has facilitated a highly quantitative measure of superdislocation dissociations. The experimental observations have been corrected for image shifts within the framework of anisotropic elasticity by comparison with simulated images. This allows experimental quantification of planar fault energies, thus providing a benchmark for first principles and atomistic simulations and fundamental insight towards alloy modeling and design. Such measurements of superdislocation dissociations on the order of 1-15 nm have been recorded for the pseudobinary Ni3Ge-Fe3Ge alloy system. These detailed measurements of fault widths obtained by weak-beam TEM observations of deformation structures will be presented and discussed as a function of alloy composition. The transition from anomalous to normal temperature dependence of yielding behavior in these alloys will also be discussed in terms of observed dislocation structures and planar fault energies calculated using the above measurements.

The diffraction contrast from dissociated ½<112] superdislocations in γ-TiAl intermetallic alloy cannot always be analyzed using conventional rules of diffraction contrast. In particular, the configuration involving three similar Shockley partials on adjacent planes has often been ruled out due to the absence of fringes indicating the presence of stacking faults. In order to determine the dissociated configuration, weak-beam transmission electron microscope observations of edge-oriented ½<112] superdislocations have been correlated with computer simulated images. Dissociation of these superdislocations into three similar ⅙<112] partial dislocations bounding a superlattice extrinsic and intrinsic stacking fault pair has been consequently determined from these analyses. It has been found that diffraction contrast alone cannot distinguish between the various configurations that lead to the formation of the fault pair, but the formation of an antiphase boundary or complex stacking fault linked dissociation or locking by stair rod dislocations can be ruled out.

The transition from positive to negative temperature dependence of 0.2% yield stress is investigated m the model pseudo-binary Ni3Ge-Fe3Ge system. Ni3Ge and Fe3Ge, both Ll2 intermetallic alloys, show complete solid solubility as Fe is continuously substituted for Ni across the composition range. However, Ni3Ge exhibits the yield stress anomaly, whereas the yield stress of Fe3Ge shows a normal decline with temperature. Mechanical testing has verified this behavior, with the anomalous behavior gradually disappearing with increasing Fe content. It is proposed that this transition results from changes in the structure of dissociated superdislocation cores. Alloys with anomalous behavior from this system are characterized by the presence of screw superdislocations locked in the Kear-Wilsdorf (KW) configuration. Conversely, alloys with normal yield stress dependence are observed to contain curvilinear superdislocations that glide on the cube planes. Results from mechanical testing are presented and correlated with TEM observations of deformation structures. These results are discussed in light of planar fault energies determined through computer simulations of images.

The transformation from short-range↔long-range order in Haynes® Alloy 242, a nominal Ni-25Mo-8Cr (in wt.%) alloy, during both continuous heating and cooling and isothermal holding at temperatures between 550 and 750°C is reported using dilatometry and microhardness measurements, and optical and transmission electron microscopy The long-range to short-range order transformation temperature was observed to increase with an increase in the degree of prior order. Aging below the transformation temperature led to considerable hardening; this hardening was associated with the formation of a very high volume fraction of Ni2Mo domains/precipitates. On aging at 700°C and below, the hardness initially increases and then levels off, with no evidence of overaging for times to 2000 hours. The role of both short- and long-range order on the transformation temperature, thermal expansion and hardening behavior and the mechanisms of transformation from short-range to long-range order were established and will be discussed.