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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

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

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

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

Random polycrystals of grains containing cracks: Model ofquasistatic elastic behavior for fractured systems

Description: A model study on fractured systems was performed using aconcept that treats isotropic cracked systems as ensembles of crackedgrains by analogy to isotropic polycrystalline elastic media. Theapproach has two advantages: (a) Averaging performed is ensembleaveraging, thus avoiding the criticism legitimately leveled at mosteffective medium theories of quasistatic elastic behavior for crackedmedia based on volume concentrations of inclusions. Since crack effectsare largely independent of the volume they occupy in the composite, sucha non-volume-based method offers an appealingly simple modelingalternative. (b) The second advantage is that both polycrystals andfractured media are stiffer than might otherwise be expected, due tonatural bridging effects of the strong components. These same effectshave also often been interpreted as crack-crack screening inhigh-crack-density fractured media, but there is no inherent conflictbetween these two interpretations of this phenomenon. Results of thestudy are somewhat mixed. The spread in elastic constants observed in aset of numerical experiments is found to be very comparable to the spreadin values contained between the Reuss and Voigt bounds for thepolycrystal model. However, computed Hashin-Shtrikman bounds are much tootight to be in agreement with the numerical data, showing thatpolycrystals of cracked grains tend to violate some implicit assumptionsof the Hashin-Shtrikman bounding approach. However, the self-consistentestimates obtained for the random polycrystal model are nevertheless verygood estimators of the observed average behavior.
Date: July 8, 2006
Creator: Berryman, James G. & Grechka, Vladimir
Partner: UNT Libraries Government Documents Department

A viscoplastic micromechanical model for the yield strength of nanocrystalline materials

Description: In this paper we present a micromechanical approach based on Fast Fourier Transforms to study the role played by dislocation glide and grain boundary (GB) accommodation in the determination of the plastic behavior of nanostructured materials. For this, we construct unit cells representing self-similar polycrystals with different grain sizes in the nanometer range and use local constitutive equations for slip and GB accommodation. We study the effect of grain size, strain rate and pressure on the local and effective behavior of nanostructured fcc materials with parameters obtained from experiments and atomistic simulations. Predictions of a previous qualitative pressure-sensitive model for the effective yield strength behind a shock front are substantially improved by considering strain partition between slip and GB activity. Under quasiestatic conditions, assuming diffusion-controlled mechanisms at GB, the model predicts a strain-rate sensitivity increase in nanocrystalline samples with respect to the same coarse-grained material of the same order as in recently published experiments.
Date: March 14, 2006
Creator: Lebensohn, R; Bringa, E & Caro, A
Partner: UNT Libraries Government Documents Department

Properties of Ni-Al under shock loading

Description: New models for the dynamic response of materials will be based increasingly on better understanding and representation of processes occurring at the microstructural level. These developments require advances in diagnostics and models which can be applied explicitly to microstructural response. Various phenomena occur at the microstructural level which are generally ignored or averaged out in continuum-level models. One example of such 'irregular hydrodynamics' is the roughness imparted to a shock wave as it propagates through a polycrystalline material. We have developed imaging techniques to study spatial variations in shock propagation through polycrystalline materials. In order to interpret spatially-resolved data from polycrystal samples, we need to compare with simulations which represent the microstructure. Here we describe work undertaken to develop a model of the dynamic response of individual grains. The material chosen was Ni-Al alloy, because it exhibits a relatively large degree of elastic anisotropy, and it is relatively easy to manufacture.
Date: January 1, 2002
Creator: Koskelo, A. C. (Aaron C.); McClellan, K. J. (Kenneth J.); Brooks, J. D. (John D.); Paisley, Dennis L. & Swift, D. C. (Damian C.)
Partner: UNT Libraries Government Documents Department

A Study of Microstructural Length Scale Effects on the Behaviour of FCC Polycrystals Using Strain Gradient Concepts

Description: Grain size is a critically important aspect of polycrystalline materials and experimental observations on Cu and Al polycrystals have shown that a Hall-Petchtype phenomenon does exist at the onset of plastic deformation. In this work, a parametric study is conducted to investigate the effect of microstructural and deformation-related length scales on the behavior of such FCC polycrystals. It relies on a recently proposed non-local dislocation-mechanics based crystallographic theory to describe the evolution of dislocation mean spacings within each grain, and on finite element techniques to incorporate explicitly grain interaction effects. Polycrystals are modeled as representative volume elements (RVEs) containing up to 64 randomly oriented grains. Predictions obtained from RVEs of Cu polycrystals with different grain sizes are shown to be consistent with experimental data. Furthermore, mesh sensitivity studies revealed that, when there is a predominance of geometrically necessary dislocations (GNDs) relative to statistically-stored dislocations (SSDs), the polycrystal response becomes increasingly mesh sensitive. This was found to occur specially during the early stages of deformation in polycrystals with small grains.
Date: May 7, 2004
Creator: Cheong, K S; Busso, E P & Arsenlis, A
Partner: UNT Libraries Government Documents Department

Some Geometrical Aspects of Breakdown of Sodium Beta Alumina

Description: The breakdown of single and polycrystalline sodium beta alumina was studied at 300 C and at room temperature. The breakdown in polycrystals involves a propagation of a highly-branched sodium-filled crack network. In single crystals the fracture surface could be studied directly. It was found that the crack network is filled with sodium from the crack faces rather than the crack tip.
Date: November 1, 1979
Creator: DeJonghe, L. C.; Feldman, L. & Millett, P.
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

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