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All sourcesNASA NTRSDOE OSTI
NASA NTRS2009

NASA Tech Briefs, November 2009

Topics covered include: Cryogenic Chamber for Servo-Hydraulic Materials Testing; Apparatus Measures Thermal Conductance Through a Thin Sample from Cryogenic to Room Temperature; Rover Attitude and Pointing System Simulation Testbed; Desktop Application Program to Simulate Cargo-Air-Drop Tests; Multimodal Friction Ignition Tester; Small-Bolt Torque-Tension Tester; Integrated Spacesuit Audio System Enhances Speech Quality and Reduces Noise; Hardware Implementation of a Bilateral Subtraction Filter; Simple Optoelectronic Feedback in Microwave Oscillators; Small X-Band Oscillator Antennas; Free-Space Optical Interconnect Employing VCSEL Diodes; Discrete Fourier Transform Analysis in a Complex Vector Space; Miniature Scroll Pumps Fabricated by LIGA; Self-Assembling, Flexible, Pre-Ceramic Composite Preforms; Flight-speed Integral Image Analysis Toolkit; Work Coordination Engine; Multi-Mission Automated Task Invocation Subsystem; Autonomously Calibrating a Quadrupole Mass Spectrometer; Determining Spacecraft Reaction Wheel Friction Parameters; Composite Silica Aerogels Opacified with Titania; Multiplexed Colorimetric Solid-Phase Extraction; Detecting Airborne Mercury by Use of Polymer/Carbon Films; Lattice-Matched Semiconductor Layers on Single Crystalline Sapphire Substrate; Pressure-Energized Seal Rings to Better Withstand Flows; Rollerjaw Rock Crusher; Microwave Sterilization and Depyrogenation System; Quantifying Therapeutic and Diagnostic Efficacy in 2D Microvascular Images; NiF2/NaF:CaF2/Ca Solid-State High-Temperature Battery Cells; Critical Coupling Between Optical Fibers and WGM Resonators; Microwave Temperature Profiler Mounted in a Standard Airborne Research Canister; Alternative Determination of Density of the Titan Atmosphere; Solar Rejection Filter for Large Telescopes; Automated CFD for Generation of Airfoil Performance Tables; Progressive Classification Using Support Vector Machines; Active Learning with Irrelevant Examples; A Data Matrix Method for Improving the Quantification of Element Percentages of SEM/EDX Analysis; Deployable Shroud for the International X-Ray Observatory; Improved Model of a Mercury Ring Damper; Optoelectronic pH Meter: Further Details; X-38 Advanced Sublimator; and Solar Simulator Represents the Mars Surface Solar Environment.

Source record↗
NASA NTRS1994

Direct measurement of free-energy barrier to nucleation of crystallites in amorphous silicon thin films

A method is introduced to measure the free-energy barrier W(sup *), the activation energy, and activation entropy to nucleation of crystallites in amorphous solids, independent of the energy barrier to growth. The method allows one to determine the temperature dependence of W(sup *), and the effect of the preparation conditions of the initial amorphous phase, the dopants, and the crystallization methds on W(sup *). The method is applied to determine the free-energy barrier to nucleation of crystallites in amorphous silicon (a-Si) thin films. For thermally induced nucleation in a-Si thin films with annealing temperatures in the range of from 824 to 983 K, the free-energy barrier W(sup *) to nucleation of silicon crystals is about 2.0 - 2.1 eV regardless of the preparation conditions of the films. The observation supports the idea that a-Si transforms into an intermediate amorphous state through the structural relaxation prior to the onset of nucleation of crystallites in a-Si. The observation also indicates that the activation entropy may be an insignificant part of the free-energy barrier for the nucleation of crystallites in a-Si. Compared with the free-energy barrier to nucleation of crystallites in undoped a-Si films, a significant reduction is observed in the free-energy barrier to nucleation in Cu-doped a-Si films. For a-Si under irradiation of Xe(2+) at 10(exp 5) eV, the free-energy barrier to ion-induced nucleation of crystallites is shown to be about half of the value associated with thermal-induced nucleation of crystallites in a-Si under the otherwise same conditions, which is much more significant than previously expected. The present method has a general kinetic basis; it thus should be equally applicable to nucleation of crystallites in any amorphous elemental semiconductors and semiconductor alloys, metallic and polymeric glasses, and to nucleation of crystallites in melts and solutions.

Shi, Frank G.↗
NASA NTRS2006

Development and Characterization of Improved NiTiPd High-Temperature Shape-Memory Alloys by Solid-Solution Strengthening and Thermomechanical Processing

The need for compact, solid-state actuation systems for use in the aerospace, automotive, and other transportation industries is currently motivating research in high-temperature shape-memory alloys (HTSMA) with transformation temperatures greater than 100 C. One of the basic high-temperature alloys investigated to fill this need is Ni(19.5)Ti(50.5)Pd30. Initial testing has indicated that this alloy, while having acceptable work characteristics, suffers from significant permanent deformation (or ratcheting) during thermal cycling under load. In an effort to overcome this deficiency, various solid-solution alloying and thermomechanical processing schemes were investigated. Solid-solution strengthening was achieved by substituting 5at% gold or platinum for palladium in Ni(19.5)Ti(50.5)Pd30, the so-called baseline alloy, to strengthen the martensite and austenite phases against slip processes and improve thermomechanical behavior. Tensile properties, work behavior, and dimensional stability during repeated thermal cycling under load for the ternary and quaternary alloys were compared. The relative difference in yield strength between the martensite and austenite phases and the dimensional stability of the alloy were improved by the quaternary additions, while work output was only minimally impacted. The three alloys were also thermomechanically processed by cycling repeatedly through the transformation range under a constant stress. This so-called training process dramatically improved the dimensional stability in these samples and also recovered the slight decrease in work output caused by quaternary alloying. An added benefit of the solid-solution strengthening was maintenance of enhanced dimensional stability of the trained material to higher temperatures compared to the baseline alloy, providing a greater measure of over-temperature capability.

Bigelow, Glen↗
NASA NTRS1991

The role of time and speed in NASA's SUNLITE program

The SUNLITE program of NASA's LaRC aims to demonstrate lower noise and better frequency stability for continuous-wave (CW) solid-state lasers in the microgravity environment of space. The program will utilize laser-diode-pumped nonplanar-ring oscillators regulated by ultra-stable high-finesse Fabry-Perot Spectrometers to produce light beams with phase rate or frequency variations as low as 3 Hz. SUNLITE will use the period-method (P-method) to measure the phase rate and frequency stability of the lasers. The P-method was chosen because it requires less memory space for the raw data, because frequencies can be analyzed on-line in real-time simply by reciprocating the periods (fi = 1/pi), and because the mean and variance of the frequencies can be calculated as fast or faster than they can be with the fastest fast Fourier transformations. Furthermore, for a given signal-to-noise power ratio, the P-method requires less data and less computer time to extract the noise components. Although the P-method does require fast Time Interval Counters, the Fourier transformation method requires comparably fast Sampling Volt meters. For either method, however, time and computer speed play a critical role.

Hafele, Joseph C.↗
NASA NTRS1983

Shock temperatures of SiO2 and their geophysical implications

High pressure shock state temperatures of SiO2 were measured in specimens of single crystal alpha-quartz and fused quartz. Projectile impact and pyrometry techniques were employed to impart pressures in the 60-140 GPa range. Shock-induced phase transformations were observed near 70 and 50 GPa along the alpha- and fused-quartz Hugoniots. It is suggested that the transformation consists of melting of the shock-synthesized stishovite, with melting delayed by the superheating of the crystalline phase. Details of the stishovite-liquid phase boundaries have been determined, including stishovite melting at 4500 K under 70 GPa, with the melting accompanied by a relative volume change of about 2.7% and a latent heat of fusion of about 2.4 MJ/kg. At 100 GPa, an approximate limit of 3500 K for the melting of SiO2 is indicated for solid mantle mineral assemblages, such as the earth's mantle

Lyzenga, G. A.↗
NASA NTRS1984

Containerless drop tube solidification and grain refinement of NiAl3

The possibility of undercooling Ni-Al alloys below the liquidus in order to produce a single phase peritectic structure by containerless drop tube solidification was studied. Containerless process is a technique for both high purity contamination free studies as well as for investigating the undercooling and rapid solidification of alloys by suppression of heterogeneous nucleation on container walls. In order to achieve large undercoolings one must avoid heterogeneous nucleation of crystallization. It was shown that the Marshall Space Flight Center drop tubes ae unique facilities for containerless solidification experiments and large undercoolings are possible with some alloys. The original goal of undercooling the liquid metal well below the liquidus to the peritectic temperature during containerless free to form primarily NiAl3 was achieved. The microstructures were interesting from another point of view. The microstructure from small diameter samples is greatly refined. Small dendrite arm spacings such as these could greatly facilitate the annealing and solid state transformation of the alloy to nearly 10% NiAl3 by reducing the distance over which diffusion needs to occur. This could minimize annealing time and might make it economically feasible to produce NiAl3 alloy.

Ethridge, E. C.↗
NASA NTRS1993

Tetravalent chromium (Cr(4+)) as laser-active ion for tunable solid-state lasers

Major accomplishments under NASA grant NAG-1-1346 are summarized. (1) numerical modeling of the four mirror astigmatically compensated, Z-fold cavity was performed and several design parameters to be used for the construction of a femtosecond forsterite laser were revealed by simulation. (2) femtosecond pulses from a continuous wave mode-locked chromium doped forsterite laser were generated. The forsterite laser was actively mode-locked using an acousto-optic modulator operating at 78 MHz with two Brewster high dispersion glass prisms for intra-cavity chirp compensation. Transform-limited sub-100-fs pulses were routinely generated in the TEM(sub 00) mode with 85 mW of continuous power tunable over 1230-1280 nm. The shortest pulses of 60-fs pulsewidth were measured. (3) Self-mode-locked operation of the Cr:forsterite laser was achieved. Synchronous pumping was used to mode lock the forsterite laser resulting in picosecond pulses, which in turn provided the starting mechanism for self-mode-locking. The pulses generated had an FWHM of 105 fs and were tunable between 1230-1270 nm. (4) Numerical calculations indicated that the pair of SF 14 prisms used in the cavity compensated for quadratic phase but introduced a large cubic phase term. Further calculations of other optical glasses indicated that a pair of SFN 64 prisms can introduce the same amount of quadratic phase as SF 14 prisms but introduce a smaller cubic phase. When the SF 14 prisms were replaced by SFN 64 prisms the pulsewidth was reduced to 50 fs. Great improvement was observed in the stability of the self mode-locked forsterite laser and in the ease of achieving mode locking. Using the same experimental arrangement and a new forsterite crystal with improved FOM the pulse width was reduced to 36 fs.

Seas, A.↗
NASA NTRS2003

The Role of Convection and Growth Competition on Phase Selection in Microgravity

For a wide range of compositions, Fe-Cr-Ni alloys can solidify by a two-step process: a metastable solid forms first, subsequently transforming into the stable solid phase. This two-step solidification produces a profoundly different microstructure, and therefore different properties, in the final solid. The delay time between the two steps of the solidification process varies from microseconds to seconds. This delay time is a strong function of composition and a weak function of the undercooling of the melt below the metastable liquidus. From the results obtained during the MSL-1 mission and in ground based electrostatic levitation testing at NASA MSFC, we know that convection also has a significant effect on the delay time. By examining the effect of convection on delay time over a wide range of convection, for several different alloys, we can gain a better understanding and better control of the microstructural evolution and final structure of stainless steel alloys. MSFC provides ESL for ground-based testing in a low-convection state, and the numerical modeling that enables estimation of the convection levels under the different process conditions.

Source record↗
NASA NTRS2015

Petrology and Cosmochemistry of a Suite of R Chondrites

Chondrites are among the most primitive surviving materials from the early solar system. They are divided into groups based on chemical types defined by mineralogy, bulk composition, and oxygen isotope compositions. Chondrites range in petrographic grade from type 1 to type 7. Type 3 chondrites are the most primitive and are little changed from the nebular solids accreted to form asteroids. They are composed of chondrules, fine-grained matrix, metal and sulfide, plus or minus Ca-Al-rich inclusions. With increasing aqueous alteration at low temperatures, members of some chondrite classes transformed from type 3 towards type 1. With increasing thermal metamorphism and low fluid content, members of other classes changed from type 3 towards type 7. Rumuruti (R) chondrites are a rare group (0.1% of falls) similar to ordinary chondrites in some properties but different in others. They are characterized by low chondrule/matrix modal abundance ratios, high oxidation state, small mean chondrule size, abundant sulfides and low metal contents. R chondrites vary in petrologic type from 3 to 6. They are important objects to study because some of them have undergone metamorphism at high temperatures in the presence of aqueous fluids. In contrast, CM and CI chondrites were heated to low temperatures in the presence of aqueous fluids leading to alteration; they contain low-T hydrous phases (phyllosilicates) and little or no remaining metal. Ordinary chondrites were heated to high temperatures in a low-fluid environment resulting in anhydrous metamorphic rocks. R6 chondrites are highly metamorphosed and some contain the high-T hydrous phases mica and amphibole. R chondrites are thus unique and give us an opportunity to examine whether there are compositional effects caused by high-T, highfluid metamorphism of nebular materials.

Torrano, Z. A.↗
NASA NTRS1991

Heterogeneous processes: Laboratory, field, and modeling studies

The efficiencies of chemical families such as ClO(x) and NO(x) for altering the total abundance and distribution of stratospheric ozone are controlled by a partitioning between reactive (active) and nonreactive (reservoir) compounds within each family. Gas phase thermodynamics, photochemistry, and kinetics would dictate, for example, that only about 1 percent of the chlorine resident in the lower stratosphere would be in the form of active Cl or ClO, the remainder existing in the reservoir compounds HCl and ClONO2. The consistency of this picture was recently challenged by the recognition that important chemical transformations take place on polar regions: the Airborne Antarctic Ozone Experiment (AAOE) and the Airborne Arctic Stratospheric Expedition (AASA). Following the discovery of the Antarctic ozone hole, Solomon et al. suggested that the heterogeneous chemical reaction: ClONO2(g)+HCl(s) yields Cl2(g)+HNO3(s) could play a key role in converting chlorine from inactive forms into a species (Cl2) that would rapidly dissociate in sunlight to liberate atomic chlorine and initiate ozone depletion. The symbols (s) and (g) denote solid phase, or adsorbed onto a solid surface, and gas phase, respectively, and represent the approach by which such a reaction is modeled rather than the microscopic details of the reaction. The reaction was expected to be most important at altitudes where PSC's were most prevalent (10 to 25 km), thereby extending the altitude range over which chlorine compounds can efficiently destroy ozone from the 35 to 45 km region (where concentrations of active chlorine are usually highest) to lower altitudes where the ozone concentration is at its peak. This chapter will briefly review the current state of knowledge of heterogeneous processes in the stratosphere, emphasizing those results obtained since the World Meteorological Organization (WMO) conference. Sections are included on laboratory investigations of heterogeneous reactions, the characteristics and climatology of PSC's, stratospheric sulfate aerosols, and evidence of heterogeneous chemical processing.

Poole, Lamont R.↗
NASA NTRS1994

Diagenetic Microcrystalline Opal Varieties from the Monterey Formation, CA: HRTEM Study of Structures and Phase Transformation Mechanisms

Microcrystalline opal varieties form as intermediary precipitates during the diagenetic transformation of biogenically precipitated non-crystalline opal (opal-A) to microquartz. With regard to the Monterey Formation of California, X-ray powder diffraction studies have shown that a decrease in the primary d-spacing of opal-CT toward that of cristobalite occurs with increasing diagenesis. The initial timing of opal-CT/quartz formation and the value of the primary opal-CT d-spacing, are influenced by the sediment. lithology. Transmission electron microscopy methods (CTEM/HRTEM) were used to investigate the structure of the diagenetic phases and establish transformation mechanisms between the varieties of microcrystalline opals in charts and porcelanites from the Monterey Formation. HRTEM images revealed that the most common fibrous varieties of microcrystalline opals contain varying amounts of structural disorder. Finite lamellar units of cristobalite-and tridymite-type. layer sequences were found to be randomly stacked in a direction perpendicular to the fiber axis. Disordered and ordered fibers were found to have coprecipitated within the same radial fiber bundles that formed within the matrix of the Most siliceous samples. HRTEM images, which reveal that the fibers within radial and lepispheric fiber bundles branch non-crystallographically, support an earlier proposal that microspheres in chert grow via a spherulitic growth mechanism. A less common variety of opal-CT was found to be characterized by non-parallel (low-angle) stacking sequences that often contain twinned lamellae. Tabular-shaped crystals of orthorhombic tridymite (PO-2) were also identified in the porcelanite samples. A shift in the primary d-spacing of opal-CT has been interpreted as an indication of solid-state ordering g toward a predominantly cristobalite structure, (opal-C). Domains of opal-C were identified as topotactically-oriented overgrowths on discrete Sections of opal-CT fibers and as lamellar domains within relict opal-CT fibers. These findings indicate that the type of transformation mechanism depends upon the primary structural characteristics of the authigenic opaline. varieties that are in turn influenced by the sediment lithology.

Cady, Sherry L.↗
NASA NTRS2003

Capabilities of the Materials Contamination Team at Marshall Space Flight Center

The Materials Contamination Team at the Marshall Space Flight Center (MSFC) has been recognized for its contributions supporting the National Aeronautics and Space Administration (NASA) spacecraft development programs. These programs include the Reusable Solid Rocket Motor (RSRM), Chandra X-Ray Observatory, and the International Space Station (ISS). The Environmental Effects Group, with the Materials Contamination Team and the Space Environmental Effects Team has been an integral part of NASA's success by the testing, evaluation, and qualification of materials, hardware, and processes. This paper focuses on the capabilities of the Materials Contamination Team. The Materials Contamination Team's realm of responsibility includes establishing contamination control during all phases of hardware development, including design, manufacturing, assembly, test, transportation, launch site processing, on-orbit exposure, return, and refurbishment. The team continues its mission of reducing the risk of equipment failure due to molecular or particulate contamination. Contamination is a concern in the Space Shuttle with sensitive bond-lines and reactive fluid (liquid oxygen) compatibility as well as for spacecraft with sensitive optics, such as Hubble Space Telescope and Chandra X-ray Observatory. The Materials Contamination Team has a variety of facilities and instrumentation capable of contaminant detection, identification, and monitoring. The team addresses material applications dealing with environments, including production facilities, clean rooms, and on-orbit exposure. The optically stimulated electron emission (OSEE) system, the Ultraviolet (UV) fluorescence (UVF) surface contamination detection, and the Surface Optics Corporation 400 (SOC 400) portable hand-held Fourier Transform Infrared (FTIR) spectrometer are state-of-the-art tools for in-process molecular contamination detection. The team of engineers and technicians also develop contamination calibration standards and evaluate new surface cleanliness inspection technologies. The team utilizes facilities for on-orbit simulation testing of materials for outgassing and molecular film deposition characteristics in the presence of space environmental effects, such as Atomic Oxygen (AO) and UV radiation exposure. The Materials Contamination Team maintains databases for process materials as well as outgassing and optical compatibility test results for specific environments.

Burns, Howard↗
NASA NTRS1985

Volatiles on satellites of the outer solar system

Molecules of cosmochemically abundant elements can act as volatiles and hence play a dominant role in the climatic and surficial evolution of solid bodies. Examples on terrestrial planets are H2O on Earth and H2O and CO2 on Mars. Analogous processes in the outer solar system focussing on CH4, its associated hydrocarbons, and N2 on Titan and Triton, the large moons of Saturn and Neptune were explored. A kilometer-deep C2H6-CH4 ocean was proposed for the surface of Titan to reconcile data on the lower atmosphere with understanding of the photochemical conversion of methane to heavier hydrocarbons. If such ocean exists, then it has dissolved in it an amount of N2 equal to the present atmospheric abundance. Since N2 contributes with CH4 a substantial greenhouse effect, the atmospheric physical and chemical characteristics are strongly coupled to those of the ocean, which change with time as methane is photolyzed in the stratosphere. Some relationship exists to the runaway greenhouse model for primordial Venus and the possible climatic implications of the buffering of Earth's atmospheric CO2 by the oceans. Two important diagnostics, measurable in Titan's atmosphere, of the conditions under which icy satellites formed are the abundances of noble gases and the CHd/CH4 radio. Both of these indicators have been altered during the evolution of Titan's surface-atmosphere system, the former by interaction with the ocean and the latter by progressive photolysis of methane into heavier hydrocarbons. The physical state and composition of volatiles on the surface of Triton is controversial, but plausibly could include CH4 N2 and perhaps CO. If condensed CH4 and N2 are widespread, their transformation to and from the vapor phase dominates the surface energy balance with sunlight. The extreme seasonal modulation of subsolar latitude on Triton is thus primarily expressed by volatile transport rather than large teperature changes, with possibly drastic observational consequences. The presence of two volatile species differing greatly in their vapor pressures make Triton a crude analog of Mars. Triton might be more appropriately regarded as a deep-freeze version of Titan.

Lunine, J. I.↗
NASA NTRS2021

Physics and Chemistry of Te and HgTe-based Ternary Melts

Historically, the theoretical treatment of the liquid phases has always been more difficult and complicated than that for solid and gas phases. A liquid has no lattice structure as crystalline solids and the atoms/molecules in the liquid can migrate through it relatively rapidly. On the other hand, it is also interacting with many other atoms/molecules so that the simplifications of the kinetic theory of gases cannot be employed. For more complicated liquids, such as the liquids of high ionicity and those containing hydrogen bonds and electric dipoles, the understanding is far from complete. At the same time, accurate information on the physics and chemistry of semiconductor melts is needed for the quantitative descriptions of the process of crystal growth from melt. The pre-crystallization phenomena in the liquid phase are critical because the properties of the grown crystals depend on the state and structure of the melt as well as the thermal history of the melt during solidification process. However, the data on the liquid phase, such as thermophysical properties of semiconductor melts are scarce, especially for the HgTe-based II-VI ternary compound semiconductors because of their high vapor pressure and extreme toxicity. Analysis of the thermophysical properties of the melt can provide information about structural transitions of the melt during the solidification process. From a broader point of view, the structure of liquids is much more complicated than the crystalline solids, especially the relaxation behavior through different thermal histories. The theory of hetero-phase fluctuations of liquids is applicable to any many-body systems including condensed-matter physics, field theory, physics of nuclear-matter, cosmology, biology and even sociology. This book summarizes the physics and chemistry from the experimental measurements and theoretical analyses of phase diagram, thermodynamic properties, density, thermal conductivity, viscosity, and electrical conductivity on the binary, pseudo-binary and ternary melts of the most advanced IR-detector material systems of HgCdTe and HgZnTe as well as the analyses of these results. The main objectives of this study are: (1) to provide the phase diagrams and thermodynamic properties of Hg-Cd-Te and Hg-Zn-Te systems through quantitatively fitting the experimental data by assuming an associated solution model for the liquid phase, (2) to experimentally measure the thermophysical properties of the Hg-Cd-Te and Hg-Zn-Te melts, including density, viscosity, electrical conductivity and thermal conductivity as functions of temperature and composition and (3) to enhance the fundamental knowledge of hetero-phase fluctuations and relaxation phenomena in the melts and extend our understanding of the solidification process in order to interpret the experimental results of crystal growth so as to improve the melt growth processes of the compound semiconductor. The physics and chemistry of Te and HgTe-based ternary melts were explored through the studies of the structural transformation during melting, the supercooling during solidification, the relaxation phenomena after rapid cooling of the melts and the metal-semiconductor transition in the melts through the analyses of electrical conductivity and Lorenz number. An in-depth study on the thermophysical properties and their time-dependent structural dynamic processes taking place in the vicinity of the solid-liquid phase transition of the narrow homogeneity range HgTe-based ternary semiconductors as well as the analysis of the homogenization process in the melt will also be presented.

Thermophysical properties↗
NASA NTRS1993

The importance of properties in modeling

Casting and welding of superalloys, stainless steel and titanium alloys are processes which can be improved through modeling of heat flow, fluid flow, residual stress development, and microstructural evolution. These simulations require inputs of thermophysical data, some of which involves the partially or totally liquid state. In particular, these processes involve melting, flow in the liquid, and solidification. Modeling of such processes can lead to an improved understanding of defects such as shrinkage, inclusions, cracks, incomplete filling (or penetration), macrosegregation, improper grain structure, and deviations from dimensional specifications. Effective modeling can shorten process development time and improve quality. An approach to these problems is to develop efficient models; validate through correlations with thermal, distortion, and microstructural data; run parametric studies; extract knowledge based rules; and apply to adaptive closed loop control systems. With the appropriate pre- and post-processing, such analyses can be made 'user friendly'. This would include graphical user interfaces as well as realistic images and color maps. In such form, these models can be used for sensitivity analyses, which are useful in defining appropriate sensors and in the development of control strategies. Such modeling can be done at several levels, e.g., the MARO level, modeling large scale phenomena such as heat and fluid flow or material deformation; the MICRO level, modeling the development of dendrites, grains or precipitates; or at the NANO level, modeling point defects, dislocations, stacking faults, etc. There are many computational issues associated with these simulations, e.g. computational efficiency and accuracy. In addition, there are many materials issues, not the least of which is the availability of accurate high temperature thermophysical data for complex alloys. This would include latent heat of fusion, temperature dependent heat capacity and thermal conductivity (for liquid and solid), viscosity, surface tension, thermal expansion, mechanical properties, etc. Preliminary data is frequently gathered from the literature; however, this is often not available for modern alloys. If additional data are required, measurements can be used; however, these are costly, time consuming and can be erroneous due to a lack of testing standards or impure materials. Microstructural predictors can be extracted from thermal information, e.g. cooling rate and thermal gradient; the prediction of microstructure is dependent on solidus and liquidus temperature, mushy zone permeability, the solidification curve, volume changes, phase transformations, alloying effects (such as surface tension or viscosity), mold/metal reactions, metal/environment reactions, etc. Defect maps may be needed to predict the onset of shrinkage, hot cracking or 'freckling'. Constants may be needed for stress relaxation, dendrite coarsening, vaporization, etc. Visualization was used as a tool to better comprehend complex data sets associated with the analysis of directional solidification (including crystal growth) and welding. Examples include not only isotherms, but also cooling rate, growth rate and thermal gradient. The latter two are not single valued scalars, but rather time and space dependent vector fields. Efficient models were developed for both casting and welding to predict heat flow and the relationship to dendrite and grain growth. These codes include many of the non-linear effects, e.g. radiation, which dominate these processes. The home-built FDM code(s) were designed to be useful not only to the scientist, but also to the process engineer. Special output can be requested to compare directly to experimental data. Visualization procedures were developed to visualize critical results, e.g. fusion zone width at the surface opposite that where the arc is applied ('penetration'). Both elaborate and simplified distortion analyses were carried out. It is clear that extensive mechanical property data are critical in order to accurately predict residual stress patterns. A scheme is currently being developed to integrate these modeling tools into a set of control algorithms; however, the success of this approach is critically dependent on the availability of accurate high temperature thermophysical data.

Giamei, A. F.↗