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Theoretical Distribution of Slip Angles in an Aggregate of Face-Centered Cubic Crystals

Description: Note presenting an analysis of the relative frequency of occurrence of any given slip-line angle in a plastically deformed polycrystal composed of face-centered cubic crystals for the case of simple tension. The results are compared with those obtained for a polycrystal composed of crystals which have but a single mode of slip and with experimental results.
Date: August 1952
Creator: Hedgepeth, John M.
Partner: UNT Libraries Government Documents Department

LLNL's program on multiscale modeling of polycrystal plasticity

Description: At LLNL a multiscale modeling program based on information-passing has been established for modeling the strength properties of a body-centered-cubic metal (tantalum) ,. under conditions of extreme plastic deformation. The plastic deformation experienced by an explosively-formed shaped-charge jet is an example of �extreme deformation�. The shaped charge liner material undergoes high strain rate deformation at high hydrostatic pressure. The constitutive model for flow stress, which describes the deformation, is highly dependent on pressure, temperature, and strain-rate. Current material models can not be extrapolated to these extreme conditions because the underlying mechanisms of plastic deformation are poorly reflected in the models and laboratory experiments are limited to pressures orders of magnitude less than actual pressures. This disparity between actual deformation conditions and those that can be attained in laboratory experiments is the principle motivation behind the multiscale modeling program. The fundamental elements of LLNL� s multiscale modeling program are distinct models at the atomistic, microscale and mesoscale/continuum length scales. The information that needs to be passed from the lower to higher length scales has been carefully defined to bound the levels of effort required to ''bridge'' length scales. Information that needs to be generated by the different simulations has been specified by a multidisciplinary steering group comprised of physicists, materials scientists and engineers. The ultimate goal of the program is to provide critical information on strength properties to be used in continuum computer code simulations. The technical work-plan involves three principle areas which are highly coupled: 1) simulation development, 2) deformation experiments and 3) characterizations of deformed crystals. The three work areas are presented which provide examples of the progress of LLNL's program.
Date: April 27, 1998
Creator: Diaz De La Rubia, T.; Holmes, N. H.; King, W. E.; Lassila, D. H.; Moriarty, J. A. & Nikkel, D. J.
Partner: UNT Libraries Government Documents Department

Bounds and self-consistent estimates for elastic constants of granular polycrystals composed of orthorhombics or crystal with higher symmetries

Description: Methods for computing Hashin-Shtrikman bounds and related self-consistent estimates of elastic constants for polycrystals composed of crystals having orthorhombic symmetry have been known for about three decades. However, these methods are underutilized, perhaps because of some perceived difficulties with implementing the necessary computational procedures. Several simplifications of these techniques are introduced, thereby reducing the overall computational burden, as well as the complications inherent in mapping out the Hashin-Shtrikman bounding curves. The self-consistent estimates of the effective elastic constants are very robust, involving a quickly converging iteration procedure. Once these self-consistent values are known, they may then be used to speed up the computations of the Hashin-Shtrikman bounds themselves. It is shown furthermore that the resulting orthorhombic polycrystal code can be used as well to compute both bounds and self-consistent estimates for polycrystals of higher-symmetry tetragonal, hexagonal, and cubic (but not trigonal) materials. The self-consistent results found this way are shown to be the same as those obtained using the earlier methods, specifically those methods designed specially for each individual symmetry type. But the Hashin-Shtrikman bounds found using the orthorhombic code are either the same or (more typically) tighter than those found previously for these special cases (i.e., tetragonal, hexagonal, and cubic). The improvement in the Hashin-Shtrikman bounds is presumably due to the additional degrees of freedom introduced into the available search space.
Date: February 1, 2011
Creator: Berryman, J. G.
Partner: UNT Libraries Government Documents Department

Elastic and transport properties in polycrystals of crackedgrains: Cross-property relations and microstructure

Description: Some arguments of Bristow (1960) concerning the effects of cracks on elastic and transport (i.e., electrical or thermal conduction) properties of cold-worked metals are reexamined. The discussion is posed in terms of a modern understanding of bounds and estimates for physical properties of polycrystals--in contrast to Bristow's approach using simple mixture theory. One type of specialized result emphasized here is the cross-property estimates and bounds that can be obtained using the methods presented. Our results ultimately agree with those of Bristow, i.e., confirming that microcracking is not likely to be the main cause of the observed elastic behavior of cold-worked metals. However, it also becomes clear that the mixture theory approach to the analysis is too simple and that crack-crack interactions are necessary for proper quantitative study of Bristow's problem.
Date: October 2, 2007
Creator: Berryman, J.G.
Partner: UNT Libraries Government Documents Department

The evolution of deformation microstructures and local orientations

Description: A brief overview of the evolution of microstructures during deformation is presented within the framework of grain subdivision. Three aspects of the evolving microstructure that are related to recrystallization are emphasized. These include the formation of high angle dislocation boundaries during deformation, the local environment of crystallographic orientations and a new scaling method for modeling detailed microstructural data.
Date: December 31, 1995
Creator: Hughes, D.A.
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

Electrically inactive poly-silicon grain boundaries

Description: Structures, energies, and electronic properties of symmetric [001] tilt grain boundaries in Si have been studied using Stillinger-Weber and Tersoff classical potentials, and semi-empirical (tight-binding) electronic structure methods. The calculated lowest energy (310) grain boundary structure and electronic properties are consistent with previous TEM measurement and calculations. For the controversial (710) grain boundaries, the tight-binding calculations do not show any electronic energy levels in the band gap. This indicates that with every atom fully fourfold coordinated, the (710) grain boundary should be electrically inactive. Some high-energy metastable grain boundaries were found to be electrically active by the presence of the levels introduced in the band gap. Also, the vacancy concentration at the (310) GB was found to be enhanced by many orders of magnitude relative to bulk. The dangling bond states of the vacancies should be electrically active.
Date: May 1, 1996
Creator: Chen, S.P.; Kress, J.D.; Voter, A.F. & Albers, R.C.
Partner: UNT Libraries Government Documents Department

Cratering behavior in single- and poly-crystalline copper irradiated by an intense pulsed ion beam

Description: When treated with intense pulsed ion beams (IPIB), many materials exhibit increased wear resistance, fatigue life, and hardness. However, this treatment often results in cratering and roughening of the surface. In this work, high purity single crystal and polycrystalline copper samples were irradiated with pulses from an IPIB to gain insight into the causes of this cratering behavior. Samples were treated with 1,2,5, and 10 shots at 2 J/cm{sup 2} and 5 J/cm{sup 2} average energy fluence per shot. Shots were about 400 ns in duration and consisted of a mixture of carbon, hydrogen, and oxygen ions at 300 keV. It was found that the single crystal copper cratered far less than the polycrystalline copper at the lower energy fluence. At the higher energy fluence, cratering was replaced by other forms of surface damage, and the single crystal copper sustained less damage at all but the largest number of shots. Molten debris from the Lucite anode (the ion source) was removed and redeposited on the samples with each shot.
Date: December 31, 1998
Creator: Wood, B.P.; Bitteker, L.J.; Waganaar, W.J. & Perry, A.J.
Partner: UNT Libraries Government Documents Department

Uniaxial Compression Experiments on PZT 95/5-2Nb Ceramic: Evidence for an Orientation-Dependent, ''Maximum Compressive Stress'' Criterion for Onset of the F(R1)()A(O) Polymorphic Phase Transformation

Description: Some time ago we presented evidence that, under nonhydrostatic loading, the F{sub R1} {r_arrow} A{sub O} polymorphic phase transformation in unpoled PZT 95/5-2Nb ceramic began when the maximum compressive stress equaled the hydrostatic pressure at which the transformation otherwise took place. More recently, we showed that this simple stress criterion did not apply to nonhydrostatically compressed, poled ceramic. However, unpoled ceramic is isotropic, whereas poled ceramic has a preferred crystallographic orientation and is mechanically anisotropic. If we further assume that the transformation depends not only on the magnitude of the compressive stress, but also its orientation relative to some feature(s) of PZT 95/5-2Nb's crystallography, then these disparate results can be qualitatively resolved. In this report, we first summarize the existing results for unpoled and poled ceramic. Using our orientation-dependent hypothesis and these results, we derive simple arithmetic expressions that accurately describe our previously-observed effects of nonhydrostatic stress on the transformation of unpoled ceramic. We then go on to test new predictions based on the orientation-dependent model. It has long been known that the transformation can be triggered in uniaxial compression: the model specifically requires a steadily increasing axial stress to drive the transformation of a randomly-oriented polycrystal to completion. We show that when the stress is held constant during uniaxial compression experiments, the transformation stops, supporting our hypothesis. We close with a discussion of implications of our model, and ways to test it using poled ceramic.
Date: January 1, 1999
Creator: Carlson, L.W.; Grazier, J.M.; Holcomb, D.J.; Montgomery, S.T. & Zeuch, D.H.
Partner: UNT Libraries Government Documents Department

Reliability Testing of Polysilicon For MEMs Devices

Description: Mission critical applications of MEMS devices require knowledge of the distribution in their material properties and long-term reliability of the small-scale structures. This project reports on a new testing program at Sandia to quantify the strength distribution using samples that reflect the dimensions of critical MEMS components. The strength of polysilicon fabricated with Sandia's SUMMiT 4-layer process was successfully measured using samples with gage sections 2.5 {micro}m thick by 1.7 {micro}m wide and lengths of 15 and 25 {micro}m. These tensile specimens have a freely moving pivot on one end that anchors the sample to the silicon die and prevents off axis loading during testing. Each sample is loaded in uniaxial tension by pulling laterally with a flat tipped diamond in a computer-controlled Nanoindenter. The stress-strain curve is calculated using the specimen cross section and gage length dimensions verified by measuring against a standard in the SEM. The first 48 samples had a means strength of 2.24 {+-} 0.35 GPa. Fracture strength measurements grouped into three strength levels, which matched three failure modes observed in post mortem examinations. The seven samples in the highest strength group failed in the gage section (strength of 2.77 {+-} 0.04 GPa), the moderate strength group failed at the gage section fillet and the lowest strength group failed at a dimple in the hub. With this technique, multiple tests can be programmed at one time and performed without operator assistance at a rate of 20-30 per day allowing the collection of significant populations of data. Since the new test geometry has been proven, the project is moving to test the distributions seen from real geometric features typical to MEMS such as the effect of gage length, fracture toughness, bonding between layers, etch holes, dimples and shear of gear teeth.
Date: April 5, 1999
Creator: LaVan, D.A. & Buchheit, T.E.
Partner: UNT Libraries Government Documents Department

Low-cost metal substrates for films with aligned grain structures

Description: Polycrystalline metal substrates that possess a significant amount of in-plane and out-of-plane crystallographic texture have recently been developed for high-temperature superconducting film applications. These substrates enable the virtual elimination of large angle grain boundaries in subsequent epitaxial films, having been successfully utilized in various oxide thin film architectures. This paper describes the characteristics of these substrates, and briefly discusses their potential applicability in polycrystalline thin-film photovoltaic applications.
Date: June 1, 1996
Creator: Norton, D.P.; Budai, J.D.; Goyal, A.; Lowndes, D.H.; Kroeger, D.M.; Christen, D.K. et al.
Partner: UNT Libraries Government Documents Department

Final Report: Hardening and Strain Localization in Single and Polycrystalline Materials Under Cyclic and Monotonic Deformation, January 11, 1985 - July 31, 1997

Description: The subject program on substructure evolution initially focused on strain localization produced by fatigue cycling and especially how such localization affects the cyclic response of polycrystalline pure metal. The latter stages have dealt with strain localization in the heavy monotonic deformation of alloys, which eventually produces forms of localized deformation that include coarse slip bands (CSB's), which are aligned to slip planes and macroscopic shear bands (MSB's), which are not aligned to slip planes. These forms of strain localization are important in that they limit the usable ductility of the material in forming processes.
Date: March 3, 2000
Creator: Laird, Campbell & Bassani, John L.
Partner: UNT Libraries Government Documents Department

Increased medium-range order in amorphous silicon with increased substrate temperature

Description: Using fluctuation electron microscopy, the authors have measured the medium-range order of magnetron sputtered silicon thin films as a function of substrate temperature from the amorphous to polycrystalline regimes. They find a smooth increase in the medium-range order of the samples, which they interpret in the context of the paracrystalline structural model as an increase in the size of and/or volume fraction occupied by the paracrystalline grains. These data are counter to the long-standing belief that there is a sharp transition between amorphous and polycrystalline structures as a function of substrate temperature.
Date: August 15, 2000
Creator: Voyles, P. M.; Gerbi, J. E.; Treacy, M. M. J.; Gibson, J. M. & Aberlson, J. R.
Partner: UNT Libraries Government Documents Department

Coupled finite element-Monte Carlo simulation of microstructure and texture evolution during thermomechanical processing

Description: A novel simulation technique for predicting the microstructure and texture evolution during thermomechanical processing is presented. The technique involves coupling a finite element microstructural deformation model based on crystal plasticity with a Monte Carlo simulation of recovery and recrystallization. The finite element model captures the stored energy and the crystallographic orientation distributions in the deformed microstructure. The Monte Carlo simulation captures the microstructural evolution associated with recovery and recrystallization. A unique feature of the Monte Carlo simulation is that it treats recrystallization as a heterogeneous subgrain growth process, thus providing the natural link between nucleation and growth phenomena, and quantifying the role of recovery in these phenomena. Different nucleation mechanisms based on heterogeneous subgrain growth as well as strain induced boundary migration are automatically included in the recrystallization simulation. The simulations are shown to account for the extent of prior deformation on the microstructure and kinetics of recrystallization during subsequent annealing. The simulations also capture the influence of the presence of cube orientations in the initial microstructure, and the operation of non-octahedral slip during deformation of fcc polycrystals, on the recrystallization texture.
Date: November 1, 1998
Creator: Radhakrishnan, B.; Sarma, G. & Zacharia, T.
Partner: UNT Libraries Government Documents Department

High-Rate Material Modeling and Validation Using the Taylor Cylinder Impact Test

Description: Taylor Cylinder impact testing is used to validate anisotropic elastoplastic constitutive modeling by comparing polycrystal simulated yield surface shapes (topography) to measured shapes from post-test Taylor impact specimens and quasistatic compression specimens. Measured yield surface shapes are extracted from the experimental post-test geometries using classical r-value definitions modified for arbitrary stress state and specimen orientation. Rolled tantalum (body-centered-cubic metal) plate and clock-rolled zirconium (hexagonal-close-packed metal) plate are both investigated. The results indicate that an assumption of topography invariance with respect to strain-rate is justifiable for tantalum. However, a strong sensitivity of topography with respect to strain-rate for zirconium was observed, implying that some accounting for a deformation mechanism rate-dependence associated with lower-symmetry materials should be included in the constitutive modeling. Discussion of the importance of this topography rate-dependence and texture evolution in formulating constitutive models appropriate for FEM applications is provided.
Date: October 21, 1998
Creator: Maudlin, P.J.; Gray, G.T. III; Cady, C.M. & Kaschner, G.C.
Partner: UNT Libraries Government Documents Department

Thermochemical Analysis for Purification of Polysilicon Melts

Description: Chemical Equilibrium calculations are presented that are relevant to the purification of molten silicon by gas-blowing. The equilibrium distributions of silicon, boron, phosphorus carbon and iron among the solid, liquid and gas phases are reported for a variety of added chemicals, temperatures and total pressures. The identities of the dominant chemical species for each element in each phase are also provided for these conditions. The added gases examined are O(2), air, water, wet air, HCl, Cl(2), Cl(2)/O(2), SiCl(4), NH(3), NH(4)OH, and NH(4)Cl. These calculations suggest possible purification schemes, although kinetic or transport limitations may prove to be significant
Date: May 1, 1999
Creator: Ho, Pauline: Gee, James M.
Partner: UNT Libraries Government Documents Department

An application of multisurface plasticity theory: Yield surfaces of textured materials

Description: Directionally dependent descriptions of the yield behavior of metals as determined by polycrystal plasticity computations are discrete in nature and, in principle, are available for use in large-scale application calculations employing multi-dimensional continuum mechanics codes. However, the practical side of using such detailed yield surfaces in application calculations contains some challenges in terms of algorithm development and computational efficiency. Discrete representations of yield as determined from Taylor-Bishop-Hill polycrystal calculations can be fitted or tessellated into a multi-dimensional piece-wise linear yield surface for subsequent use in constitutive algorithms for codes. Such an algorithm that utilizes an associated flow based multisurface plasticity theory has been implemented in the three dimensional EPIC code and is described in this effort.
Date: September 1, 1995
Creator: Maudlin, P.J.; Wright, S.I. & Kocks, U.F.
Partner: UNT Libraries Government Documents Department

Cast polycrystalline silicon photovoltaic module manufacturing technology improvements. Annual subcontract report, January 1, 1995--December 31, 1995

Description: The objective of this three-year program is to advance Solarex`s cast polycrystalline silicon manufacturing technology, reduce module production cost, increase module performance and expand Solarex`s commercial production capacities. Two specific objectives of this program are to reduce the manufacturing cost for polycrystalline silicon PV modules to less than $1.20/watt and to increase the manufacturing capacity by a factor of three.
Date: June 1, 1996
Creator: Wohlgemuth, J.
Partner: UNT Libraries Government Documents Department

Dislocation processes and deformation twinning in nanocrystalline Al.

Description: Using a recently developed, massively parallel molecular-dynamics (MD) code for the simulation of polycrystal plasticity, we analyze for the case of nanocrystalline Al the complex interplay among various dislocation and grain-boundary processes during low-temperature deformation. A unique aspect of this work, arising from our ability to deform to rather large plastic strains and to consider a rather large grain size, is the observation of deformation under very high grain-boundary and dislocation densities, i.e., in a deformation regime where they compete on an equal footing. We are thus able to identify the intra- and intergranular dislocation and grain-boundary processes responsible for the extensive deformation twinning observed in our simulations. This illustrates the ability of this type of simulations to capture novel atomic-level insights into the underlying deformation mechanisms not presently possible experimentally. smaller grain size, mobile dislocations must be nucleated from other sources, such as the GBs or grain junctions.
Date: January 29, 2002
Creator: Yamakov, V.; Wolf, D.; Phillpot, S. R. & Gleiter, H.
Partner: UNT Libraries Government Documents Department