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Characterization of the corrosion environment of the desert near Barstow, California

Description: The corrosion characteristics of the desert atmosphere environment near Barstow, CA have been evaluated for the solar pilot plant receiver panel. Potential degradation mechanisms considered were hot corrosion from molten salts and stress corrosion cracking in aqueous environments. The possibility of degradation from these mechanisms depends on the chemical composition of airborne particulates, aerosols, and gases. These particulates and aerosols were collected near the pilot plant site and analyzed for water and acid soluble Na/sup +/, Mg/sup + +/, Li/sup +/, S/sup =/, SO/sub 4//sup =/, and Cl/sup -/. Comparison and evaluation of the quantities of these ions present with those necessary for corrosion indicate that external corrosion of the receiver due to the desert atmosphere environment is unlikely.
Date: September 1, 1981
Creator: Hughes, D.A.
Partner: UNT Libraries Government Documents Department

The development of high angle deformation boundaries and local orientations in aluminum

Description: High angle boundaries and their local surroundings are important for all aspects of recrystallization including nucleation, growth and texture formation. They are thus important for processing of aluminum alloys. Deformation induced high angle boundaries have been observed and quantitatively characterized using transmission electron microscopy in aluminum following rolling to large reductions. The distribution of local orientations between individual dislocation boundaries and their angle/axis pairs have been measured using convergent beam Kikuchi analysis and are compared to the macroscopic texture. The sequence of near neighbor orientations shows that individual grains subdivide across their thickness into three to four different texture components separated by sharp boundaries. The local orientations surrounding high angle boundaries are much more diverse than suggested by simple models and single crystal studies. The origins for these high angle boundaries are suggested and discussed.
Date: March 1, 1996
Creator: Hughes, D.A.
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

Dislocation structures formed during hot and cold working

Description: During many types of deformation, grains are subdivided into smaller regions by dislocation boundaries. The classification of structures formed by grain subdivision has differed between structures observed following cold deformation and hot deformation. Cold deformed microstructure are characterized by grain subdivision at two size scales. Grain subdivision occurs at the larger scale by geometrically necessary boundaries that delineate differently deforming blocks that contain nearly equiaxed cells. In contrast hot deformed structures have been described with a single level of subdivision by equiaxed subgrains. Infrequently an occasional long boundary provides subdivision at a larger level. However, new findings in warm to hot deformed alloys show that there are closer ties between the cold and hot deformed structures. Specifically, two levels of grain subdivision are observed following some types of high temperature deformation when large areas are surveyed in the TEM. Furthermore, the probability densities of boundary disorientation angles at high temperature show the same universal scaling as those at low temperature.
Date: October 11, 1998
Creator: Hughes, D. A. & Godfrey, A.
Partner: UNT Libraries Government Documents Department

A comparison of the evolution of cold and hot deformation microstructures and textures in fcc metals

Description: Microstructures and textures which develop during cold deformation are compared to those which develop during hot deformation. This comparison is made using the evolutionary framework of grain subdivision and the formation of low energy dislocation structures. During deformation grains are subdivided into differently deforming regions separated by geometrically necessary dislocation boundaries. These boundaries include dense dislocation walls, microbands, lamellar boundaries and subgrains. Grain subdivision occurs as a result of the requirement for strain accommodation balanced by energy considerations. This grain subdivision weakens the texture and increases the texture scatter. The tendency for grain subdivision decreases with increasing temperature of deformation with implications for the microstructural evolution and the texture formation.
Date: December 31, 1993
Creator: Hughes, D. A. & Hansen, N.
Partner: UNT Libraries Government Documents Department

Numerical simulation and experimental observations of initial friction transients

Description: Experiments were performed to better understand the sliding frictional behavior between metals under relatively high shear and normal forces. Microstructural analyses were done to estimate local near-surface stress and strain gradients. The numerical simulation of the observed frictional behavior was based on a constitutive model that uses a state variable approach.
Date: July 1, 1995
Creator: Hughes, D.A.; Weingarten, L.I. & Dawson, D.B.
Partner: UNT Libraries Government Documents Department

On Boundary Misorientation Distribution Functions and How to Incorporate them into 3D Models of Microstructural Evolution

Description: The fundamental difficulties incorporating experimentally obtained-boundary disorientation distributions (BMD) into 3D microstructural models are discussed. An algorithm is described which overcomes these difficulties. The boundary misorientations are treated as a statistical ensemble which is evolved toward the desired BMD using a Monte Carlo method. The application of this algorithm to a number complex arbitrary BMDs shows that the approach is effective for both conserved and non-conserved textures. The algorithm is successfully used to create the BMDs observed in deformation microstructure containing both incidental dislocation boundaries (IDBs) and geometrically necessary boundaries (GNBs).
Date: December 23, 1998
Creator: Godfrey, A.W.; Holm, E.A.; Hughes, D.A. & Miodownik, M.
Partner: UNT Libraries Government Documents Department

Characterization of dislocation wall spacing distributions

Description: While conventionally the length scale of dislocation microstructures has been characterized by only a mean value it is also of interest to know the shape of the wall spacing distribution. In order to investigate the ability to characterize such spacing distributions for near-planar boundaries a model has been developed allowing the construction of a series of dislocation walls that can be sectioned in any plane allowing spacing distributions of the traces to be calculated. Results suggest that distribution shape is not adversely affected by either test section or by realistically low numbers of measurements. Consequently experimental measurements have been made on both low strain ({epsilon} = 0.2 and {epsilon} = 0.3) and high strain ({epsilon} = 2.7) samples. The distributions for all three strains exhibit similar shapes and can be scaled to a single curve for the probability density function.
Date: December 31, 1997
Creator: Godfrey, A. & Hughes, D.A.
Partner: UNT Libraries Government Documents Department

Textural and microstructural evolution during cold-rolling of pure nickel

Description: High purity nickel (99.99%) with a grain size about 100 {mu}m has been deformed by cold-rolling from 37% to 98% reductions. The deformation microstructures and the crystallographic texture have been characterized using transmission electron microscopy and neutron diffraction, respectively. The microstructural evolution has been described within a general framework which consists of a grain subdivision by dislocation boundaries on a finer and finer scale with increasing strain. The influence of this deformation pattern on the texture development is discussed.
Date: January 1, 1994
Creator: Hansen, N.; Jensen, D. J. & Hughes, D. A.
Partner: UNT Libraries Government Documents Department

Multiscale Modeling of Recrystallization

Description: We propose a multi length scale approach to modeling recrystallization which links a dislocation model, a cell growth model and a macroscopic model. Although this methodology and linking framework will be applied to recrystallization, it is also applicable to other types of phase transformations in bulk and layered materials. Critical processes such as the dislocation structure evolution, nucleation, the evolution of crystal orientations into a preferred texture, and grain size evolution all operate at different length scales. In this paper we focus on incorporating experimental measurements of dislocation substructures, rnisorientation measurements of dislocation boundaries, and dislocation simulations into a mesoscopic model of cell growth. In particular, we show how feeding information from the dislocation model into the cell growth model can create realistic initial microstructure.
Date: December 7, 1998
Creator: Godfrey, A.W.; Holm, E.A.; Hughes, D.A.; Lesar, R. & Miodownik, M.A.
Partner: UNT Libraries Government Documents Department

A multi-level code for metallurgical effects in metal-forming processes

Description: The authors present the final report on a Laboratory-Directed Research and Development (LDRD) project, A Multi-level Code for Metallurgical Effects in metal-Forming Processes, performed during the fiscal years 1995 and 1996. The project focused on the development of new modeling capabilities for simulating forging and extrusion processes that typically display phenomenology occurring on two different length scales. In support of model fitting and code validation, ring compression and extrusion experiments were performed on 304L stainless steel, a material of interest in DOE nuclear weapons applications.
Date: August 1, 1997
Creator: Taylor, P.A.; Silling, S.A.; Hughes, D.A.; Bammann, D.J. & Chiesa, M.L.
Partner: UNT Libraries Government Documents Department

Investigation of medium to high strain deformation microstructures using an automated electron back scatter pattern (EBSP) system

Description: The speed of automated EBSP orientation gathering has now increased to a point where it is possible to examine large areas of deformed material at a resolution close to the length scale of the dislocation substructures involved (typically < 1 {micro}m for strains of greater than {epsilon} = 1). Investigation of such deformed samples invariably gives rise to diffraction images of low quality and to many double patterns. To assist in a critical selection of the data, it is usual to store a measure of the indexing confidence for each point. Here the authors describe how deformed samples, which provide the appropriate range of pattern qualities, can be used to calibrate acceptance criteria for unsupervised EBSP measurements on deformation microstructures. An application in the use of such criteria is also described. Two crystals chosen with orientations that are known to develop a well defined texture spread during deformation were channel die deformed to a strain of {epsilon} = 1.5 (78% reduction). The texture spread was determined for each sample from a large number of transmission electron microscope (TEM) orientation measurements taken using a fast semi-automatic technique. EBSP scans were then made on the samples. Orientations failing within the expected (TEM determined) texture spread are assumed to be correct. While those falling outside the expected spread are assumed to be incorrect. Unsupervised acceptance criteria based on an indexing confidence parameter (in their case the fraction of located Kikuchi bands matching the indexing solution) were then tested by comparing the set of orientations selected using a given approval criterion with the set of the orientations falling within the expected spread. Criteria were appraised by determining both the fraction of all orientations that were misclassified (incorrect orientations selected plus correct orientations not selected) or by the fraction of incorrect orientations in the selected ...
Date: March 1, 1998
Creator: Godfrey, A.; Hughes, D.A.; Krieger Lassen, N.C. & Jensen, D.J.
Partner: UNT Libraries Government Documents Department

Capturing recrystallization of metals with a multi-scale materials model

Description: The final report for a Laboratory Directed Research and Development project entitled, ``Capturing Recrystallization of Metals in a Multiscale Materials Model'' is presented. In this project, deformation and recrystallization processes have been followed experimentally and theoretically in order to incorporate essential mechanisms from the defect (dislocation) and grain size length scales. A nonlinear rotational gradient theory has been developed which enables the incorporation of microstructural parameters. The evolution of these parameters during deformation and recrystallization has been characterized qualitatively and quantitatively, applying various electron optic techniques ranging over several length scales. The theoretical and experimental framework developed is general. It has been exemplified by an application to recrystallization in single crystals and bicrystals of aluminum. The recrystallization process has been modeled using a 3-D model for the changes in key structural parameters during recrystallization.
Date: April 1, 2000
Creator: Hughes, D. A.; Bammann, D. J.; Godfrey, A.; Prantil, V. C.; Holm, E. A.; Miodownik, M. A. et al.
Partner: UNT Libraries Government Documents Department