Megan Frary

Connectivity and percolation behaviour of grain boundary networks in three dimensions

Grain boundary networks are subject to crystallographic constraints at both triple junctions (first-order constraints) and quadruple nodes (second-order constraints). First-order constraints are known to influence the connectivity and percolation behaviour in two-dimensional grain boundary networks, and here we extend these considerations to fully three-dimensional microstructures. Defining a quadruple node distribution (QND) to quantify both the composition and isomerism of quadruple nodes, we explore how the QNDs for crystallographically consistent networks differ from that expected in a randomly assembled network. Configurational entropy is used to quantify the relative strength of each type of constraint (i.e., first- and second-order), with first-order triple junction constraints accounting for at least 75% of the non-random correlations in the network. As the dominant effects of constraint are captured by considering the triple junctions alone, a new analytical model is presented which allows the 3-D network connectivity to be estimated from data on 2-D microstructural sections. Finally, we show that the percolation thresholds for 3-D crystallographically consistent networks differ by as much as ±0.07 from those of standard percolation theory.

Simulation of Plasticity in Nanocrystalline Silicon

Molecular dynamics investigation of plasticity in a model nanocrystalline silicon system demonstrates that inelastic deformation localizes in intergranular regions. The carriers of plasticity in these regions are atomic environments, which can be described as high-density liquid-like amorphous silicon. During fully developed flow, plasticity is confined to system-spanning intergranular zones of easy flow. As an active flow zone rotates out of the plane of maximum resolved shear stress during deformation to large strain, new zones of easy flow are formed. Compatibility of the microstructure is accommodated by processes such as grain rotation and formation of new grains. Nano-scale voids or cracks may form if stress concentrations emerge which cannot be relaxed by a mechanism that simultaneously preserves microstructural compatibility.

Nonrandom percolation behavior of grain boundary networks in high-Tc superconductors

The grain boundary networks of tetragonal and orthorhombic superconductors are shown to be nonrandom through computer simulations of polycrystalline structures. For biaxially textured microstructures, the distribution of low-angle grain boundaries around triple junctions is measurably deviant from the expectations for a randomly constructed lattice. Percolation thresholds are calculated for the unhindered flow of superconducting current through a polycrystal, and are found to be significantly different from the standard random percolation threshold in two dimensions. These deviations are explained as a result of crystallographic constraints on the network topology.

Combination rule for deviant CSL grain boundaries at triple junctions

Abstract The well-known sigma combination rule governs the connectivity of coincidence site lattice (CSL) grain boundaries at triple junctions, but is only strictly applicable for ideal CSL boundaries. Here, we extend this relationship to boundaries that deviate from ideal CSL misorientations, and derive a complementary rule that relates their angular deviations. This “deviation limit rule” states that the angular deviation of one boundary at a triple junction is at most equal to the sum of the other two deviation angles. This analysis also provides insight into apparent violations of the sigma combination rule, which can arise when one boundary deviates beyond the allowable limit for CSL classification.

Strain ratchetting of titanium upon reversible alloying with hydrogen

Abstract During cyclic hydrogen charging (e.g., in metal–hydride systems), internal stresses and strains can be developed due to lattice swelling and/or phase transformation (e.g., allotropic transformation or hydride precipitation). We examine macroscopic plastic deformation due to such internal stresses (strain ratctetting) in the Ti–H system, where gaseous hydrogen is alloyed with Ti, causing the Ti α–β allotropic transformation, and subsequently removed, producing the β–α transformation. Cyclic hydrogen charging is found to cause macroscopic plastic shrinkage strains in directions normal to the hydrogen concentration gradient. Furthermore, increasing the charging time leads to larger ratchetting strains. A simple adaptation of diffusion theory is used to describe the kinetics of strain evolution, and the contributions to total ratchetting from both the α–β phase transformation and the lattice swelling strains are quantified.

Microstructural Effects during Chemical Mechanical Planarization of Copper

Die-stacking schema using through-wafer interconnects require vias to be filled with electroplated Cu, resulting in thick copper films and requiring an aggressive first-step chemical mechanical planarization (CMP). This work investigates the effects of microstructure on CMP of copper films, which are not presently well understood. Bulk and local removal rates are investigated for several different microstructures. Surface orientation maps are created, and the orientations of individual grains are correlated with topographical data to elucidate local removal behavior. Cu removal depends on the details of the microstructure, and certain microstructures allow for either faster or more uniform removal of thick Cu films.

A Comparative Study of Welded ODS Cladding materials for AFCI/GNEP Applications

This research project involved working on the pressure resistance welding of oxide dispersion strengthened (ODS) alloys which will have a large role to play in advanced nuclear reactors. The project also demonstrated the research collaboration between four universities and one nation laboratory (Idaho National Laboratory) with participation from an industry for developing for ODS alloys. These alloys contain a high number density of very fine oxide particles that can impart high temperature strength and radiation damage resistance suitable for in-core applications in advanced reactors. The conventional fusion welding techniques tend to produce porosity-laden microstructure in the weld region and lead to the agglomeration and non-uniform distribution of the neededoxide particles. That is why two solid state welding methods - pressure resistance welding (PRW) and friction stir welding (FSW) - were chosen to be evaluated in this project. The proposal is expected to support the development of Advanced Burner Reactors (ABR) under the GNEP program (now incorporated in Fuel Cycle R&D program). The outcomes of the concluded research include training of graduate and undergraduate students and get them interested in nuclear related research.

Modeling and Simulation of the Percolation Problem in High-T c Superconductors: Role of Crystallographic Constraints on Grain Boundary Connectivity

Superconductivity in high-T c materials is often modeled as a percolation problem in which grain boundaries are classified as strong or weak-links for current transmission based on their disorientation angle. Using Monte Carlo simulations, we have explored the topology and percolation thresholds for grain boundary networks in orthorhombic and tetragonal polycrystals where the grain boundary disorientations are assigned in a crystallographically consistent manner. We find that the networks are highly nonrandom, and that the percolation thresholds differ from those found with standard percolation theory. For biaxially textured materials, we have also developed an analytical model that illustrates the role of local crystallographic constraint on the observed nonrandom behavior.

Understanding the Effect of Grain Boundary Character on Dynamic Recrystallization in Stainless Steel 316L

Dynamic recrystallization (DRX) occurs during high-temperature deformation in metals and alloys with low to medium stacking fault energies. Previous simulations and experimental research have shown the effect of temperature and grain size on DRX behavior, but not the effect of the grain boundary character distribution. To investigate the effects of the distribution of grain boundary types, experimental testing was performed on stainless steel 316L specimens with different initial special boundary fractions (SBF). This work was completed in conjunction with computer simulations that used a modified Monte Carlo method which allowed for the addition of anisotropic grain boundary energies using orientation data from electron backscatter diffraction (EBSD). The correlation of the experimental and simulation work allows for a better understanding of how the input parameters in the simulations correspond to what occurs experimentally. Results from both simulations and experiments showed that a higher fraction of so-called “special” boundaries (e.g., Σ3 twin boundaries) delayed the onset of recrystallization to larger strains and that it is energetically favorable for nuclei to form on triple junctions without these so-called “special” boundaries.

Influence of Microstructure on Aggressive Chemical Mechanical Planarization Processes for Thick Copper Films

Novel die-stacking schema using through-wafer vias may require thick electrodeposited copper and aggressive first-step chemical mechanical planarization (CMP). However, the effect of microstructural parameters, including surface orientation and grain size, on the CMP behavior of thick electrodeposited copper is not well understood. Here we explore the relationship be-tween the surface orientation of copper grains and local CMP removal parameters using electron backscatter diffraction and topography correlation techniques. In the present work, solid copper disks are studied which are annealed to produce samples with differing grain sizes. In addition, aggressive CMP is performed on copper films (30 μm) electrodeposited on silicon. At the bulk level, the slurry composition is found to have the greatest effect on the removal rate and surface roughness. At the microstructural level, the nature of the grain boundaries (e.g. coincidence site lattice (CSL) vs. non-CSL boundaries) is shown to impact the depth of grooving at the grain boundaries. A relationship between surface orientation and local removal rate is found.