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Two-stage sintering inhibits abnormal grain growth during beta to alpha transformation in SiC

Description: Free sintering of SiC with Al, B, and C additions in two successive stages, first under nitrogen and then under argon, produced a near full-density ceramic with equiaxed grain structure. The beta to alpha transformation proceeded to completion; however, the grain shape remained equiaxed due to the action of nitrogen present during the first stage of sintering. It is found that the beta to alpha transformation is necessary but not sufficient for producing the microstructure of interlocking plates found in high-toughness SiC.
Date: September 17, 2007
Creator: Kueck, Aaron M. & De Jonghe, Lutgard C.
Partner: UNT Libraries Government Documents Department

Evolution of 2D Potts Model Grain Microstructures from an Initial Hillert Size Distribution

Description: Grain growth experiments and simulations exhibit self-similar grain size distributions quite different from that derived via a mean field approach by Hillert [ 1]. To test whether this discrepancy is due to insufficient anneal times, two different two-dimensional grain structures with realistic topologies and Hillert grain size distributions are generated and subjected to grain growth via the Monte Carlo Potts Model (MCPM). In both cases, the observed self-similar grain size distributions deviate from the initial Hillert form and conform instead to that observed in MCPM grain growth simulations that start from a random microstructure. This suggests that the Hillert grain size distribution is not an attractor.
Date: October 19, 1998
Creator: Battaile, C.C. & E.A., Holm
Partner: UNT Libraries Government Documents Department

Phenomenological description of grain growth stagnation for nanocrystalline films and powders

Description: At fixed hold temperatures, grain growth usually stagnates indefinitely after sufficiently long hold times. The change in the growth behavior can be very abrupt, resulting in a sudden plateau in plots of grain size versus time at fixed temperature. Standard grain growth laws do not formally predict the rapid onset of growth stagnation, merely a slow down of grain growth to imperceptible rates. Therefore, the grain size in the plateau regions for long hold times is typically not in agreement with that predicted with kinetic variables derived from the size versus time curves for short hold times where there is pronounced curvature. Standard laws lead to endpoint grain sizes with strong dependences on the hold times. The experimental observation in many cases is a nearly linear temperature dependence that is independent of the hold times after a sufficient duration. Additionally, the growth process may restart from a stagnated state with sufficient temperature increases, where again, the stagnated grain size temperature dependence is linear. For growth laws including size dependent opposing forces, endpoint grain sizes are predicted to be either independent of temperature, or exponentially temperature dependent with thermodynamic reversibility, the latter an impossibility. We derive, heuristically, a stagnation force, phenomenologically incorporating these observations: a near linear temperature dependence of endpoint grain sizes, and irreversible growth. This description reduces to standard laws commonly used for data fitting, and leads to a normal grain size distribution. Other laws are discussed and compared. Fits to size versus time data are successfully made.
Date: August 24, 2000
Creator: Dannenberg, R.; Stach, E.A. & Groza, J.R.
Partner: UNT Libraries Government Documents Department

Activation Energy for Grain Growth in Aluminum Coatings

Description: To produce a specific grain size in metallic coatings requires precise control of the time at temperature during the deposition process. Aluminum coatings are deposited using electron-beam evaporation onto heated substrate surfaces. The grain size of the coating is determined upon examination of the microstructure in plan view and cross-section. Ideal grain growth is observed over the entire experimental range of temperature examined from 413 to 843 K. A transition in the activation energy for grain growth from 0.7 to 3.8 eV {center_dot} atom{sup -1} is observed as the temperature increases from <526 K to >588 K. The transition is indicative of the dominant mechanism for grain growth shifting with increasing temperature from grain boundary to lattice diffusion.
Date: October 14, 2004
Creator: Jankowski, A F; Ferreira, J L & Hayes, J P
Partner: UNT Libraries Government Documents Department

Oxide dispersion strengthening of nickel electrodeposits for microsystem applications.

Description: Oxide dispersion strengthened nickel (ODS-Ni) electrodeposits were fabricated to net shape in a nickel sulfamate bath using the LIGA process. A 20 g/l charge of 10 nm Al{sub 2}O{sub 3} powder was suspended in the bath during electrodeposition to produce specimens containing an approximately 0.001-0.02 volume fraction dispersion of the alumina particulate. Mechanical properties are compared to baseline specimens fabricated using an identical sulfamate bath chemistry without the Al{sub 2}O{sub 3} powder charge. Results reveal that the as-deposited ODS-Ni exhibited significantly higher yield strength and ultimate tensile strength than the baseline material. This increase in as-deposited strength is attributed to Orowan strengthening. The ODS-Ni also showed improved retention of room temperature strength after annealing over a range of temperatures up to 600 C. Microscopy revealed that this resistance to anneal softening was due to an inhibition of grain growth in the presence of the oxide dispersion. Nanoindentation measurements revealed that the properties of the dispersion strengthened deposit were uniform through its thickness, even in narrow, high aspect ratio structures. At elevated temperatures, the strength of the ODS-Ni was approximately three times greater than that of the baseline material although with a significant reduction in hot ductility.
Date: November 1, 2003
Creator: Janek, Richard P.; Kotula, Paul Gabriel; Buchheit, Thomas Edward; Michael, R. P. & Goods, Steven Howard
Partner: UNT Libraries Government Documents Department

Systematic changes of the Nb-Sn reaction with time, temperature and alloying in Restacked-Rod-Process (RRP) Nb3Sn strands

Description: Reaction heat treatments spanning 605 to 750 C and 24 to {approx}400 hours were applied to several sets of RRP strands. Magnetization and transport measurements were used to track the changes of superconducting properties and the amounts of Nb{sub 3}Sn formed. The experiments showed that temperature increases of 15 C produced equivalent properties in half the time within the temperature range 620-680 C. This result was the same whether Ta or Ti was used to alloy the Nb{sub 3}Sn. The bulk pinning force Fp for Ta-alloyed wires displayed a significant drop for temperatures outside the range above, due to tin gradients at low temperatures and grain growth at high temperatures. The F{sub p} drop at high reaction temperatures prevents wire technology from taking advantage of significantly higher Kramer-plot intercept H{sub K} for high-temperature reactions. On the other hand, Ti alloying provides a quick and potent means to increase H{sub K} at reaction temperatures for which F{sub p} remains high.
Date: September 1, 2008
Creator: Ghosh, A.K.; Sperry, E.A.; D'Ambra, J.; /Brookhaven; Cooley, L.D. & /Fermilab
Partner: UNT Libraries Government Documents Department

Soft X-Ray Magnetic Imaging of Focused Ion Beam Lithographically Patterned Fe Thin Films

Description: We illustrate the potential of modifying the magnetic behavior and structural properties of ferromagnetic thin films using focused ion beam 'direct-write' lithography. Patterns inspired by the split-ring resonators often used as components in meta-materials were defined upon 15 nm Fe films using a 30 keV Ga{sup +} focused ion beam at a dose of 2 x 10{sup 16} ions cm{sup -2}. Structural, chemical and magnetic changes to the Fe were studied using transmission soft X-ray microscopy at the ALS, Berkeley CA. X-ray absorption spectra showed a 23% reduction in the thickness of the film in the Ga irradiated areas, but no change to the chemical environment of Fe was evident. X-ray images of the magnetic reversal process show domain wall pinning around the implanted areas, resulting in an overall increase in the coercivity of the film. Transmission electron microscopy showed significant grain growth in the implanted regions.
Date: November 9, 2008
Creator: Cook, Paul J.; Shen, Tichan H.; Grundy, PhilJ.; Im, Mi Young; Fischer, Peter; Morton, Simon A. et al.
Partner: UNT Libraries Government Documents Department

Computer simulation of microstructural dynamics

Description: Since many of the physical properties of materials are determined by their microstructure, it is important to be able to predict and control microstructural development. A number of approaches have been taken to study this problem, but they assume that the grains can be described as spherical or hexagonal and that growth occurs in an average environment. We have developed a new technique to bridge the gap between the atomistic interactions and the macroscopic scale by discretizing the continuum system such that the microstructure retains its topological connectedness, yet is amenable to computer simulations. Using this technique, we have studied grain growth in polycrystalline aggregates. The temporal evolution and grain morphology of our model are in excellent agreement with experimental results for metals and ceramics.
Date: January 1, 1985
Creator: Grest, G.S.; Anderson, M.P. & Srolovitz, D.J.
Partner: UNT Libraries Government Documents Department

A Phase-Field Model for Grain Growth

Description: A phase-field model for grain growth is briefly described. In this model, a poly-crystalline microstructure is represented by multiple structural order parameter fields whose temporal and spatial evolutions follow the time-dependent Ginzburg-Landau (TDGL) equations. Results from phase-field simulations of two-dimensional (2D) grain growth will be summarized and preliminary results on three-dimensional (3D) grain growth will be presented. The physical interpretation of the structural order parameter fields and the efficient and accurate semi-implicit Fourier spectral method for solving the TDGL equations will be briefly discussed.
Date: December 23, 1998
Creator: Chen, L.Q.; Fan, D.N. & Tikare, V.
Partner: UNT Libraries Government Documents Department

Cellular automata modeling of weld solidification structure

Description: The authors explore the use of cellular automata in modeling arc-welding processes. A brief discussion of cellular automata and their previous use in micro-scale solidification simulations is presented. Macro-scale thermal calculations for arc-welding at a thin plate are shown to give good quantitative and qualitative results. Combining the two calculations in a single cellular array provides a realistic simulation of grain growth in a welding process. Results of simulating solidification in a moving melt pool in a poly-crystalline alloy sheet are presented.
Date: December 31, 1993
Creator: Dress, W.B.; Zacharia, T. & Radhakrishnan, B.
Partner: UNT Libraries Government Documents Department

Transport, Growth Mechanisms, and Material Quality in GaN Epitaxial Lateral Overgrowth

Description: Growth kinetics, mechanisms, and material quality in GaN epitaxial lateral over-growth (ELO) were examined using a single mask of systematically varied patterns. A 2-D gas phase reaction/diffusion model describes how transport of the Ga precursor to the growth surface enhances the lateral rate in the early stages of growth. In agreement with SEM studies of truncated growth runs, the model also predicts the dramatic decrease in the lateral rate that occurs as GaN over-growth reduces the exposed area of the mask. At the point of convergence, a step-flow coalescence mechanism is observed to fill in the area between lateral growth-fronts. This alternative growth mode in which a secondary growth of GaN is nucleated along a single convergence line, may be responsible for producing smooth films observed to have uniform cathodoluminescence (CL) when using 1{micro}m nucleation zones. Although emission is comprised of both UV ({approximately}365nm) and yellow ({approximately}550nm) components, the spectra suggest these films have reduced concentrations of threading dislocations normally associated with non-radiative recombination centers and defects known to accompany growth-front convergence lines.
Date: January 11, 1999
Creator: Baca, Albert G.; Bartram, M.E.; Coltrin, M.E.; Crawford, M.H.; Han, J.; Missert, N. et al.
Partner: UNT Libraries Government Documents Department

Influence of orientation pinning on the Goss-texture in Fe-3%Si electrical steel

Description: Despite a large number of investigations on the formation of the Goss-texture in Fe-3%Si electrical steels, the exact mechanisms leading to the preference of this particular orientation are not completely understood so far. As an alternative to the standard explanation of a favored growth of Goss-oriented grains during secondary recrystallization, recently the concept of orientation pinning has been proposed, which considers that the growth of grains with special orientation relationships corresponding to low-angle and twin grain boundaries is disfavored. The present paper present preliminary EBSD-results on the growth of Goss-grains during secondary recrystallization in high-permeability (HiB) transformer steel sheets. A semi-quantitative model to simulate the effect of orientation pinning on the evolution of the Goss-texture is introduced.
Date: December 1, 1998
Creator: Engler, O. & Friedel, F.
Partner: UNT Libraries Government Documents Department

Oxide ceramic alloys and microlaminates. Final report, May 1, 1989--October 31, 1997

Description: Major research conducted in this program falls into the following areas: (a) microstructure and micromechanics of superplastic ceramic composites; (b) solute segregation and grain boundary kinetics of ceramic alloys; (c) sintering, grain growth, and texture development of ceramics and thin films; and (d) in-situ and microlaminate oxide composites. The basic approach to the above research is to utilize the state of the art methods to process colloidal and green ceramics, to employ crystal chemistry and phase equilibria for microalloy and microcomposite design, to comprehensively characterize microstructure development and mechanical performance, to develop models, theories, and simulations to understand the energetics, kinetics and mechanics, and to explore novel microstructures and improved performance for practically important ceramics. Major accomplishments are here.
Date: July 1, 1998
Creator: Chen, I.W.
Partner: UNT Libraries Government Documents Department

Abnormal grain growth -- The origin of recrystallization nuclei?

Description: The origin of recrystallization nuclei is reviewed with particular emphasis on materials in which well-developed cells are present in the deformed state. Nucleation is discussed in terms of coarsening of the subgrain network that develops on annealing and an analogy is made with abnormal grain growth. The results of a theoretical analysis of abnormal growth are summarized. The Monte Carlo model for grain growth is adapted for variable grain boundary energy and mobility in order to investigate the behavior of individual grains with special properties. The simulation results show that both energy and mobility affect abnormal growth as expected from the theoretical analysis. The results are discussed in terms of the stability that subgrain networks may exhibit depending on their mean misorientation.
Date: August 1, 1997
Creator: Rollett, A.D. & Holm, E.A.
Partner: UNT Libraries Government Documents Department

Relationship between nanocrystalline and amorphous microstructures by molecular dynamics simulation

Description: A recent molecular dynamics simulation method for growth of fully dense nanocrystalline materials crystallized from melt was used with the Stillinger-Weber three-body potential to synthesize nanocrystalline Si with a grain size up to 75{Angstrom}. Structures of the highly constrained grain boundaries (GBs), triple lines, and point grain junctions were found to be highly disordered and similar to the structure of amorphous Si. These and earlier results for fcc metals suggest that a nanocrystalline microstructure may be viewed as a two-phase system, namely an ordered crystalline phase in the grain interiors connected by an amorphous, intergranular, glue-like phase. Analysis of the structures of bicrystalline GBs in the same materials reveals the presence of an amorphous intergranular equilibrium phase only in the high-energy but not the low-energy GBs, suggesting that only high-energy boundaries are present in nanocrystalline microstructures.
Date: August 1, 1996
Creator: Keblinski, P.; Phillpot, S.R.; Wolf, D. & Gleiter, H.
Partner: UNT Libraries Government Documents Department

Gas pressure sintering of silicon nitride to optimize fracture toughness

Description: Gas-pressure sintering (GPS) can be used to densify silicon nitride containing a wide variety of sintering additives. Parameters affecting the sintering behavior include densification temperature, densification time, grain growth temperature, grain growth time and heating rates. The Si{sub 3}N{sub 4}-6% Y{sub 2}O{sub 3}-2% A1{sub 2}O{sub 3} samples sintered to high densities at all conditions used in the present study, whereas the Si{sub 3}N{sub 4}-Sr{sub 2}La{sub 4}Yb{sub 4}(SiO{sub 4}){sub 6}O{sub 2} samples required the highest temperatures and longest times to achieve densities {ge}98 % T. D. The main effect on the fracture toughness for Si{sub 3}N{sub 4}-6% Y{sub 2}O{sub 3}-2% A1{sub 2}O{sub 3} samples was the use of a lower densification temperature, which was 1900C in the present study. For the Si{sub 3}N{sub 4}-Sr{sub 2}La{sub 4}Yb{sub 4}SiO4{sub 4}){sub 6}O{sub 2} composition, fracture toughness was sensitive to and improved by a slower heating rate (10c/min), a lower densification temperature (1900`), a higher grain growth temperature (2000C), and a longer grain growth time (2 h).
Date: June 1, 1995
Creator: Tiegs, T.N.; Nunn, S.D.; Beavers, T.M.; Menchhofer, P.A.; Barker, D.L. & Coffey, D.W.
Partner: UNT Libraries Government Documents Department

A Monte Carlo model of Zener pinning which shows f{sup {minus}1} dependence

Description: A novel Monte Carlo (MC) model of Zener pinning has been developed. It differs from previous MC models in that it does not simulate polycrystalline grain growth. Instead a single boundary moving through an array of particles is simulated. The boundary curvature defines the driving force acting on the boundary; this is constant throughout the simulation. By incrementally increasing the volume fraction of particles, the pinning force is gradually increased. The boundary is eventually pinned when driving force equals the pinning force. This defines the Zener criterion and enables the volume fraction dependence of the model to be determined. The value of this approach is that there is no limit imposed on either the volume fraction of particles or their size. Simulations have been carried out over a range of volume fractions, from 0 < f < 0.25 for particles with volumes of 27 sites. The pinning force exerted by particles on a boundary is related to the characteristic shape during bypass, the so called dimple. When the simulation temperature is T{prime} = 0, dimples are not formed, the boundaries experience an artificially strong pinning force and the model exhibits an f{sup {minus}1/2} dependence. When T{prime} is greater than a critical value dimples are formed and the model shows an f{sup {minus}1} volume fraction dependence. The implications of this result for previously MC models of Zener pinning is discussed.
Date: November 1, 1998
Creator: Miodownik, M.
Partner: UNT Libraries Government Documents Department


Description: The primary purpose of this Cooperative Research and Development Agreement (CRADA) between Los Alamos National Laboratory and Molecular Simulations, Inc. (formerly Biosym Technologies, Inc.) was to create a set of computer modeling tools called the Lattice Model System (LAMS) that would accelerate the development of new materials. This set of materials modeling codes would model the atomistic properties of oxides, metals, and semiconductors and would be used to calculate the properties of polycrystalline materials. These models would not only incorporate the atomistic details of actual materials but would also capture the structural complexity of the materials in terms of their grain structures, diffusion, grain growth, and deformation and fracture properties. Principal project tasks were the structuring of microstructure modeling and related methods in a form appropriate for commercial release, the development of user-oriented graphics interface and analysis software, and the optimization of the materials modeling codes to run on a massively parallel computer environment. The grain growth modeling module was in development when the collaborative project was prematurely terminated because of a tiding shortfall. Thus, the complete set of materials modeling codes, the Lattice Model System, was not developed.
Date: September 1, 2000
Partner: UNT Libraries Government Documents Department

Cube-textured nickel substrates for high-temperature superconductors

Description: The biaxial textures created in metals by rolling and annealing make them useful substrates for the growth of long lengths of biaxially textured material. The growth of overlayers such as high-temperature superconductors (HTS) require flat substrates with a single, sharp texture. A sharp cube texture is produced in high-purity Ni by rolling and annealing. The authors report the effect of rolling reduction and annealing conditions on the sharpness of the cube texture, the incidence of other orientations, the grain size, and the surface topography. A combination of high reduction, and high temperature annealing in a reducing atmosphere leads to > 99% cube texture, with mosaic of 9.0{degree} about the rolling direction (RD), 6.5{degree} about the transverse direction (TD), and 5.0{degree} about the normal direction (ND).
Date: February 1, 1998
Creator: Specht, E.D.; Goyal, A.; Lee, D.F.; List, F.A.; Kroeger, D.M.; Paranthaman, M. et al.
Partner: UNT Libraries Government Documents Department

Mirowave annealing of silicon nitride materials

Description: Dense silicon nitride-based ceramics were microwave annealed to determine if microwave heating offers advantages over conventional heating for the enhancement of the high temperature creep resistance. Gas pressure sintered silicon nitride (GPS-SN) and sintered reaction-bonded silicon nitride (SRBSN) were heated in microwave or graphite element furnaces at 1150{degrees}C and 1600{degrees}C. Annealed materials were characterized for the room and high temperature flexural strengths, room temperature fracture toughness values, and high temperature creep properties. In addition, SEM analyses were performed to study grain growth and other microstructural changes. The results of this study showed that both types of furnace anneals at 1150{degrees}C lowered the room temperature strength and toughness values of both SRBSN and GPS-SN materials; however, the anneal treatments at 1600{degrees}C had little effect on the room temperature properties. Both the SRBSN and GPS-SN control and annealed samples had reduced high temperature fast fracture strengths, when compared to the room temperature strengths. Creep tests at 1200{degrees}C indicated that both the SRBSN and the GPS-SN materials that were annealed by microwave heating at I 150{degrees}C for 20 h showed enhanced creep resistance, when compared to unheated controls and conventionally heated materials. No qualitative differences were seen in the microstructures of the SRBSN and the GPS-SN materials which could account for the differences in the creep properties of the annealed materials. Additional experimental work is in progress to further understand the mechanisms for the enhanced creep properties of silicon nitride materials annealed by microwave heating.
Date: August 1, 1997
Creator: Kiggans, J.O. Jr.; Montgomery, F.C. & Tiegs, T.N.
Partner: UNT Libraries Government Documents Department

High-temperature internal oxidation of Ag/1.2at.% Mg and Ag/0.25at.% Mg-0.25at.% Ni.

Description: High-temperature oxygen diffusion and internal oxidation in Ag, Ag/1.2 at.% Mg (Ag-Mg), and Ag/0.25 at.% Mg-0.25 at.% Ni (Ag-Mg-Ni) have been studied, mostly in air and 8% O{sub 2}, at 450-835 C. The focus of the studies was on thermogravimetric analysis, microhardness tests, and optical and electron microscopy observations of grain growth and its inhibition by oxidation. The internal oxidation of both alloys exhibited nearly identical activation energies (0.81 eV for Ag-Mg and 0.83 eV for Ag-Mg-Ni) and rate constants. The maximum O content of both alloys was superstoichiometric (e.g., O/Mg &gt; 1.0) and the maximum O/Mg ratios were higher at lower temperatures than at higher temperatures (e.g., 1.25 at 500 C and 1.05 at 800 C). Diffusion of O in pure Ag was {approx}60 times faster at 825 C and {approx}400 times faster at 500 C than internal oxidation of either of the Ag alloys. Grain growth of both alloys and of the Ag was quantified between 450-800 C and related to internal oxidation.
Date: September 8, 1999
Creator: Balachandran, U.; Goretta, K. C.; McNallan, M. J.; Park, J.-H. & Prorok, B. C.
Partner: UNT Libraries Government Documents Department