Megan Frary

Analysis of Precipitate Redistribution in Inconel 617 Using Integrated Electron Backscatter Diffraction and Energy Dispersive Spectroscopy

Megan Frary et al. — Wed, 07 Dec 2011 14:16:58 PST

Inconel 617 (IN617), a candidate alloy for applications in the Next Generation Nuclear Plant, derives its oxidation resistance and strength at temperatures above 900°C from both solid solution strengthening and the precipitation of carbides [1]. Cr-rich carbides (usually M23C6) reside primarily on grain boundaries, while Mo-rich carbides (usually M6C) tend to be within grains [1-4]. Both intragranular and intergranular carbides play an important role in the creep behavior of the alloy [1]. During creep, intragranular carbides can dissolve and re-precipitate at grain boundaries, especially on boundaries in tension [1]. While the precipitate distribution before and after creep deformation has been investigated, the role of grain boundary character has not been included in the analysis.

SPS Fabrication of Tungsten and Tungsten Rhenium Alloys in Support of NTR Fuels Development

Jonathan A. Webb et al. — Wed, 17 Aug 2011 15:09:24 PDT

Tungsten metal slugs were fabricated via Spark Plasma Sintering (SPS) of powdered metals at temperatures ranging from 1575 K to 1975 K and hold times of 5 minutes to 30 minutes, using powders with an average diameter of 7.8 µm. Sintered tungsten specimens were found to have relative densities ranging from 83% to 94% of the theoretical density for tungsten. Consolidated specimens were also tested for their Vickers Hardness Number (VHN), which were plotted as a function of relative density. Concurrently, tungsten and rhenium powders with average respective diameters of 0.5 µm and 13.3 µm were pre-processed either by High-Energy-Ball-Milling (HEBM) or by homogeneous mixing to yield W-25at.%Re mixtures. The powder batches were sintered at temperatures of 1975 K and 2175 K for hold times up to 60 minutes yielding relative densities in the range 94% to 97%. The combination of HEBM and sintering showed a significant decrease in the intermetallic phases compared to that of the homogenous mixing and sintering.

Pressure Resistance Welding of High Temperature Metallic Materials

Nathan Jerred et al. — Mon, 25 Jul 2011 10:13:01 PDT

Pressure Resistance Welding (PRW) is a solid state joining process used for various high temperature metallic materials (Oxide dispersion strengthened alloys of MA957, MA754; ferritic/martensitic alloy HT-9, and tungsten) for advanced nuclear reactor applications. A new PRW machine has been installed at the Center for Advanced Energy Studies (CAES) in Idaho Falls for conducting joining research for nuclear applications. The key emphasis has been on understanding processing-microstructure-property relationships. Initial studies have shown that sound joints can be made between dissimilar materials such as MA957 alloy cladding tubes and HT-9 end plugs, and MA754 and HT-9 coupons. Limited burst testing of MA957/HT-9 joints carried out at various pressures up to 400^oC has shown encouraging results in that the joint regions do not develop any cracking. Similar joint strength observations have also been made by performing simple bend tests. Detailed microstructural studies using SEM/EBSD tools and fatigue crack growth studies of MA754/HT-9 joints are ongoing.

Immuno-SEM Characterization of Developing Bovine Cartilage

Michelle Gerritsen et al. — Mon, 25 Jul 2011 10:12:59 PDT

Collagen is a vital material in the tissues of living organisms. Found almost everywhere in the human body, collagen is important in connective tissues, bone growth, and cartilage. Collagen XI makes up a very small portion of the cartilaginous tissue; however, it plays a key role in cartilaginous tissue. Collagen XI and two collagen XI isoforms, V1b and V2, are critical in the ossification process. The location of collagen XI, V1b, V2, and their specific functions in the ossification process within developing bovine cartilage are not well characterized. In this work, the location of collagens I, II, XI and two collagen XI isoforms, V1b and V2, present in developing bovine cartilage are investigated using the immuno-SEM technique. The results for the locations of collagen I and II indicate a high level of consistency with previous work, thus showing that the technique of immuno-SEM can be used with confidence to determine the location of various collagen types within cartilaginous and mineralized tissue. This work has shown that collagen XI is present in the lower hypertrophic region and also in a pericellular arrangement, within about two microns of cell walls, throughout the cartilaginous tissue. V1b is expressed in the articular surface, mineralized region, resting zone, and the distal edge of the diaphysis. The V2 isoform is most strongly expressed in areas of newly forming cartilage, and disappears with chondrocyte maturation. V2 is present in the distal edge of the epiphysis, as well as in mineralized tissue. Collagen XI and two of its isoforms, V1b and V2, are thought to play a critical role in the ossification process. However, this role is not well understood, and is still being characterized. The detection of collagen XI and two of its isoforms in the osteo-chondral junction as well as at a joint surface further point to collagen XI, V1b, and V2 playing a vital role in the ossification process, and warrants further research as to their specific function within the ossification process.

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

Patrick J. Andersen et al. — Mon, 25 Jul 2011 10:12:57 PDT

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 between 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.

Improving Engineering Students’ Cognitive and Affective Preparedness with a Pre-Instructional E-Learning Strategy

Seung Youn Chyung et al. — Mon, 25 Jul 2011 10:12:56 PDT

During the 2006–2007 academic year, five faculty members from the College of Engineering at Boise State University initiated a curriculum augmentation project using new instructional technologies with the intention to help improve undergraduate engineering students’ cognitive and affective preparedness for their classroom learning. The instructional technologies used in the project were a pre-instructional strategy and a self-paced e-learning method. The main question addressed in this project was: Will a pre-instructional e-learning strategy help engineering students cognitively and affectively prepare for their classroom learning? This paper is a report of the project, describing the analysis, design, and development of a multimedia e-learning module for an engineering curriculum, the implementation of the e-learning module as a pre-instructional strategy in two engineering courses, and the evaluation of the effectiveness of using the pre-instructional e-learning strategy on engineering students’ cognitive and affective preparedness for classroom learning. It also provides a list of lessons learned from the project.

Effects of Crystallographic Orientation on the Early Stages of Oxidation in Nickel and Chromium

Louis P. Bonfrisco et al. — Fri, 24 Sep 2010 15:23:59 PDT

Surface orientation plays an important role in the oxidation behavior of single crystals where studies have found the relative oxidation rates for surfaces with different orientations. However, most materials are polycrystalline and contain myriad orientations that contribute to the overall oxidation process. Here we determine the effects of orientation on the early stages of oxidation behavior as a function of surface orientation for polycrystalline nickel (face-centered cubic) and chromium (body-centered cubic). After high temperature oxidation, the oxide topography is characterized using optical profilometry and the underlying microstructure is characterized with electron backscatter diffraction (EBSD). By correlating results from EBSD and optical profilometry, the oxide height is determined for each crystallographic orientation. In both Ni and Cr, a strong relationship is observed between the oxidation rate and direction of the surface normal; for Ni, (111) surfaces oxidize slowest, while (100) surfaces in Cr have the lowest oxidation rates. Although orientation-dependent oxidation rates are observed at short times, the effect is diminished at longer oxidation times.

Precipitate Redistribution During Creep of Alloy 617

Scott Schlegel et al. — Fri, 24 Sep 2010 15:23:58 PDT

Nickel-based superalloys are being considered for applications within advanced nuclear power generation systems due to their high temperature strength and corrosion resistance. Alloy 617, a candidate for use in heat exchangers, derives its strength from both solid solution strengthening and the precipitation of carbide particles. However, during creep, carbides that are supposed to retard grain boundary motion are found to dissolve and re-precipitate on boundaries in tension. To quantify the redistribution, we have used electron backscatter diffraction and energy dispersive spectroscopy to analyze the microstructure of 617 after creep testing at 900 and 1000°C. The data were analyzed with respect to location of the carbides (e.g., intergranular vs. intragranular), grain boundary character, and precipitate type (i.e., Cr-rich or Mo-rich). We find that grain boundary character is the most important factor in carbide distribution; some evidence of preferential distribution to boundaries in tension is also observed at higher applied stresses. Finally, the results suggest that the observed redistribution is due to the migration of carbides to the boundaries and not the migration of boundaries to the precipitates.

Effect of Grain Boundary Engineering on Microstructural Stability during Annealing

Scott M. Schlegel et al. — Fri, 24 Sep 2010 15:23:58 PDT

Grain boundary engineering, which increases the special boundary fraction, may improve microstructural stability during annealing. Different processing routes are undertaken to establish the effectiveness of each and to better understand which microstructural features determine the resulting stability. We find that multiple cycles of grain boundary engineering result in a material that resists abnormal grain growth better than other processing routes despite similarities in special boundary fraction, grain size, and general boundary connectivity among as-processed materials.

Influence of Grain Boundary Character on Creep Void Formation in Alloy 617

Thomas Lillo et al. — Fri, 24 Sep 2010 15:23:57 PDT

Alloy 617, a high temperature creep-resistant, nickel-based alloy, is being considered for the primary heat exchanger for the Next Generation Nuclear Plant (NGNP) which will operate at temperatures exceeding 760^oC. Orientation imaging microscopy (OIM) is used to characterize the grain boundaries in the vicinity of creep voids that develop during high temperature creep tests (800-1000^oC at creep stresses ranging from 20-85 MPa) terminated at creep strains ranging from 5-40%. Observations using optical microscopy indicate creep rate does not significantly influence the creep void fraction at a given creep strain. Preliminary analysis of the OIM data indicates voids tend to form on grain boundaries parallel, perpendicular or 45^o to the tensile axis, while few voids are found at intermediate inclinations to the tensile axis. Random grain boundaries intersect most voids while CSL-related grain boundaries did not appear to be consistently associated with void development.

Role of Grain Boundary Character Distribution on Dynamic Recrystallization Using Monte Carlo Simulations

Jared Stein et al. — Fri, 03 Sep 2010 14:23:14 PDT

Monte Carlo simulations are commonly applied to study microstructural evolution, including abnormal grain growth and recrystallization. Here, Monte Carlo simulations are used to study the effects of grain boundary character distribution on dynamic recrystallization. The initial stored energy is uniformly distributed and a constant strain rate is achieved by incremental increases in stored energy. The simulation tracks the grain size, special boundary fraction, and recrystallization fraction. Recrystallization rates vary with initial special boundary fraction and are compared to kinetic models. Similar to experimental results, a high percentage of grains that nucleate early in the simulation have special-boundary relationships with surrounding grains. As the simulation progresses, the orientation of nuclei becomes nearly random compared to surrounding grains. The simulations also predicted that the final special boundary fraction can be controlled by varying the strain rate. Once correlated with experimental results, the simulation can be used to investigate and refine hot-deformation processes.

Comparison of Microstructural Evolution of Nickel during Conventional and Spark Plasma Sintering

Matthew Luke et al. — Fri, 03 Sep 2010 14:20:50 PDT

Spark plasma sintering (SPS) is a rapid powder consolidation technique that uses pulsed electric current to directly heat the powder. A comparison was made between the microstructural evolution of pure polycrystalline nickel powder processed by conventional (press and sinter) and SPS routes. The sintering temperature, dwell time, applied pressure and ramp rate were varied to affect the densification and microstructural evolution of the powders. Characterization of grain size, morphology, grain boundary character, and crystallographic texture was performed on SPS and traditionally-processed specimens using electron backscatter diffraction. The microstructure of SPS nickel was observed to vary between one that resembles a green body with some grain coarsening to one that is indistinguishable from a wrought nickel microstructure except for some porosity. Grain size distributions in press and sintered nickel were much wider than those for SPS nickel. SPS nickel has increased porosity at the edges, evidence of processing temperature variations.

Weldability Characteristics of Oxide Dispersion Strengthened Alloys: An Overview

Kalyan Chitrada et al. — Fri, 03 Sep 2010 14:18:51 PDT

Oxide dispersion strengthened (ODS) alloys are considered an important class of structural materials for various high temperature applications including advanced nuclear reactors. Conventional fusion welding methods lead to high porosity and agglomerated oxide particles in these alloys deteriorating critical properties of the joint. That is why solid state joining techniques are touted as the potential remedy for the welding issues encountered in these alloys. Research activities in an ongoing research program supported by the DOE (grant # DEFG07-08ID14925) are highlighted. In this program, friction stir welding and pressure resistance welding have been used to successfully weld various ODS alloys, such as MA956, MA957 and MA754. Microstructural characteristics of the welded alloys are studied using a host of microscopy techniques. Mechanical properties are evaluated using microhardness, miniaturized tensile and shear punch test techniques. Appropriate microstructure-property correlations are developed. An overview of the state-of-the-art based on the published literature is also presented.

The Effects of Stress and Temperature on the Fatigue Crack Growth Behavior and Microstructural Evolution of Alloy 230

Jatu Burns et al. — Fri, 03 Sep 2010 14:05:42 PDT

Alloy 230, a nickel-based superalloy, is a candidate material for heat exchangers in very high temperature reactors. The heat exchangers are expected to see a wide range of operating conditions, so it is important to understand the effects of both stress and temperature on mechanical behavior and microstructural evolution. We studied crack growth behavior under controlled stress intensity factor near the threshold regime between 650 and 800°C. The tests were performed in using static and cyclic loads with various frequencies and stress ratios. In the different temperature regimes, the damage mechanism is dominated by different phenomena. Electron backscatter diffraction was used to observe the cracking mode and to characterize microstructure and texture at the crack tip. Transgranular cracks were observed from fatigue cracking. In grains near the crack tip, the local orientation spread increased in to as much as 5°, an indication of local deformation.

Three-Dimensional Grain Boundary Networks: Modeling and Connections to Experimental Data

Megan Frary — Fri, 03 Sep 2010 14:03:54 PDT

The grain boundary network that makes up a microstructure plays an important role in determining the properties of the material. These networks are well-studied in two dimensional systems; however, most microstructures are inherently three dimensional, so an understanding of the role microstructure plays in determining properties must account for the full 3D microstructure. Here, percolation-based models are applied to 3D grain boundary networks with both regular and irregular grain shapes. The microstructures are characterized in terms of the cluster size distribution, mean cluster size, and radius of gyration; grain boundary area must be accounted for when calculating these properties in 3D. The analytical tools developed to characterize simulated microstructures can be applied to experimentally-determined data sets to extract the same metrics from those microstructures. Developing an understanding of 3D connectivity in real microstructures may elucidate the role of grain boundary character distribution on property improvement observed during grain boundary engineering.

Microstructural Effects During Chemical Mechanical Planarization of Copper

Patrick J. Andersen et al. — Fri, 03 Sep 2010 13:30:31 PDT

Novel 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 CMP. This work investigates the effects of microstructure on CMP of copper films, which are not presently well understood. Bulk and local removal rates were investigated for several different microstructures. Surface orientation maps were created and the orientations of individual grains were correlated with topographical data to elucidate local removal behavior. Cu removal depends on the details of the microstructure, and certain microstructures allowed for either faster or more uniform removal of thick Cu films.

Simulation of Plasticity in Nanocrystalline Silicon

M. J. Demkowicz et al. — Thu, 23 Apr 2009 18:14:58 PDT

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 that 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 there emerge stress concentrations that cannot be relaxed by a mechanism that simultaneously preserves microstructural compatibility.

Determination of Three-Dimensional Grain Boundary Connectivity from Two-Dimensional Microstructures

Megan Frary — Thu, 23 Apr 2009 18:14:58 PDT

The connectivity of so-called “special” and “general” grain boundaries at a quadruple node is known to be nonrandom as a result of crystallographic constraints. Although a quadruple node is a three-dimensional feature, there exist two-dimensional features which are topologically identical. Therefore, the distribution of these two-dimensional features may be used to determine the three-dimensional connectivity. Computer simulations of a three-dimensional microstructure which is virtually serial sectioned are used to validate the proposed approach.

Correlations Beyond the Nearest-Neighbor Level in Grain Boundary Networks

C. A. Schuh et al. — Thu, 23 Apr 2009 18:14:57 PDT

Correlations among ‘special’ and ‘general’ grain boundaries are studied on two-dimensional networks, by examining the configurational entropy of boundary structures as well as percolation thresholds. Consideration of crystallographic constraints at various length scales reveals that higher-order constraints play a role in boundary connectivity and network structure. Implications for grain boundary engineering are discussed and directions for future work highlighted.

Correlation-Space Description of the Percolation Transition in Composite Microstructures

Megan E. Frary et al. — Tue, 14 Apr 2009 09:55:15 PDT

We explore the percolation threshold shift as short-range correlations are introduced and systematically varied in binary composites. Two complementary representations of the correlations are developed in terms of the distribution of phase bonds or, alternatively, using a set of appropriate short-range order parameters. In either case, systematic exploration of the correlation space reveals a boundary that separates percolating from nonpercolating structures and permits empirical equations that identify the location of the threshold for systems of arbitrary short-range correlation states. Two- and three-dimensional site lattices with two-body correlations, as well as a two-dimensional hexagonal bond network with three-body correlations, are explored. The approach presented here should be generalizable to more complex correlation states, including higher-order and longer-range correlations.