Analysis of interdiffusion data in multicomponent alloys to extract fundamental diffusion information

This paper reviews new procedures based around the random alloy model that have been established recently for analyzing chemical diffusion data in binary and ternary alloy systems. The authors show how atom-vacancy exchange frequency ratios, individual tracer correlation factors, and vacancy-wind factors can be extracted from the chemical diffusion data. Examples are taken from intrinsic diffusion data in the Ag-Cd and Ag-Cd-Zn alloy systems and from interdiffusion data in the Fe-Ni-Cr and Cu-Fe-Ni alloy systems. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr., of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

Interdiffusion in γ (face-centered cubic) Ni-Cr-X (X=Al, Si, Ge, or Pd) alloys at 900 °C

Interdiffusion in nickel (Ni)-chromium (Cr) (face-centered cubic γ phase) alloys with small additions of aluminum (Al), silicon (Si), germanium (Ge), or palladium (Pd) was investigated using solid-to-solid diffusion couples. Ni-Cr-X alloys having compositions of Ni-22at.% Cr, Ni-21at.%Cr-6.2at.%Al, Ni-22at.%Cr-4.0at.%Si, Ni-22at.%Cr-1.6at.%Ge, and Ni-22at.%Cr-1.6at.%Pd were manufactured by arc casting. The diffusion couples were assembled in an Invar steel jig, encapsulated in Ar after several hydrogen purges, and annealed at 900 °C in a three-zone tube furnace for 168 h. Experimental concentration profiles were determined from polished cross sections of these couples by using electron probe microanalysis with pure element standards. Interdiffusion fluxes of individual components were calculated directly from the experimental concentration profiles, and the moments of interdiffusion fluxes were examined to determine the average ternary interdiffusion coefficients. The effects of ternary alloying additions on the diffusional behavior of Ni-Cr-X alloys are presented in the light of the diffusional interactions and the formation of a protective Cr₂O₃ scale. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, the 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr., of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

Diffusion reaction behaviors of U-Mo/Al dispersion fuel

The uranium (U)-molybdenum (Mo)/aluminum (Al) dispersion fuel that is currently under development for high-performance research reactors has shown complicated diffusion reaction behavior between the U-Mo particles and the Al matrix. Diffusion reactions in U-Mo/Al dispersion fuels were characterized by out-of-pile annealing tests and in-pile irradiation tests in the HANARO research reactor. The effect of the addition of a third element such as silicon (Si), Al, or zirconium (Zr) to U-Mo fuel, and the addition of Si to the Al matrix on the diffusion reaction were also investigated. The growth rate and activation energy for the reaction phases of U-Mo/Al dispersion fuels were obtained. The effect of alloying a small amount of a third element in U-Mo and of Si in the Al matrix on diffusion reaction kinetics was negligible in annealing tests conducted at ∼550 °C. γ phase stability in the U-Mo alloy was enhanced by the addition of 0.1 to 0.2 wt.% Si. The Si accumulated in the interdiffusion layer of U-Mo/Al-Si dispersion fuel annealed at ∼550 °C, whereas Zr migration to the interdiffusion layer of U-Mo-Zr/Al was negligible. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, the 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr., of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

Modeling of kinetics of diffusive phase transformation in binary systems with multiple stoichiometric phases

The thermodynamic extremal principle is used for the treatment of the evolution of a binary system under the assumption that all phases in the system are nearly stoichiometric with no sources and sinks for vacancies in the bulk. The interfaces between the individual phases are assumed to act as ideal sources and sinks for vacancies, and to have an infinite mobility. Furthermore, it is assumed that several phases are nucleated in the contact plane of the diffusion couple at the beginning of the computer experiment. Then, it is shown that the number of newly nucleated phases determines the maximum number of polyfurcations (i.e., branching of a single configuration into several distinct configurations) of the initial contact (Kirkendall) plane. The model is demonstrated on a hypothetical binary system with four stoichiometric phases. The inverse problem, namely, the determination of the tracer diffusion coefficients in newly nucleated phases from the thicknesses of new phases and the positions of polyfurcated Kirkendall planes, is treated too. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, the 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr., of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

Irradiation-enhanced interdiffusion in the diffusion zone of U-Mo dispersion fuel in Al

Uranium-molybdenum (U-Mo) alloy fuel particles dispersed in an aluminum (Al) matrix, designated as U-Mo/Al dispersion fuel, is in the development stage in the worldwide RERTR (Reduced Enrichment for Research and Test Reactors) program. The main issue in developing U-Mo/Al dispersion fuel is the diffusion reaction occurring at the interface between the fuel particles and matrix. To accurately analyze fuel performance, a model to predict the diffusion kinetics is necessary. For this purpose, the authors developed a diffusion layer growth rate correlation for out-of-pile annealing tests and a similar correlation for in-reactor tests. The correlation for in-reactor tests is considerably different from that of out-of-pile tests because it contains factors that amplify diffusion kinetics by fission damage in the diffusion reaction zone. This irradiation enhancement was formulated by a combination of the fission rate in the fuel and fission fragment damage distribution in the diffusion reaction zone. Using a computer code, fission damage factors were obtained as a function of diffusion reaction layer thickness and composition. The model correlation was established and fitted to the in-reactor data. As a result of this data fitting, the interaction layer growth rate is found to be proportional to the square root of the fission fragment damage rate and to have a temperature dependence characterized by the effective activation energy of 46 to 76 kJ/mole, which is smaller by a factor of 4 to 7 than that of out-of-pile tests. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr., of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

Chemical diffusion in the iridium-richA1 andL12 phases in the Ir-Nb system

The diffusion in iridium-rich Ir-Nb alloys has been studied by single-phase interdiffusion experiments. The chemical diffusion coefficient has been measured for the primary fcc solid-solution and theL1₂ ordered compound Ir₃Nb in the temperature range between 1650 and 1950 °C, using Ir/Ir-8Nb and Ir-26Nb/Ir-28Nb diffusion couples, respectively (numbers indicate mol%). While the chemical diffusion coefficient in the solid-solution phase is close to the tracer self-diffusion coefficient of pure iridium, the diffusion in the compound phase is extremely slow: the chemical diffusion coefficient is 1/40 to 1/50 of that in the solid solution. The low diffusion rate in the compound must be beneficial for high-temperature performance of refractory superalloys based on the Ir-Nb system. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr., of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

Carbon diffusion in steels: A numerical analysis based on direct integration of the flux

In the early 1970s, Professor Dayananda developed a technique for the direct integration of fluxes from the concentration profiles in vapor-solid diffusion couples to determine diffusion coefficients and atomic mobilities. As part of a project to control and optimize the industrial carburization process in mild- and low-alloyed steels, a modified integration analysis was applied to determine the mass transfer coefficient in the gas boundary layer and carbon diffusivity in austenite. Because carbon flux and surface carbon content vary with time during single-stage carburizing even with a fixed carbon potential in the atmosphere, a mass balance at the gas-solid interface must serve as a boundary condition. This article discusses the numerical modeling of gas carburizing, and focuses on calculating the mass transfer and carbon diffusivity parameters using the simulated concentration profiles. This approach validates the proposed method by comparing the calculated parameters with those used in simulation. The results were compared with previous determinations and predictions reported in the literature. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, the 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr., of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

Interdiffusion analysis for NiAl versus superalloys diffusion couples

Solid-to-solid diffusion couples, β-NiAl (B2) versus various commercial superalloys (i.e., CM247, GTD-111, IN-939, IN-718, and Waspalloy) were examined to quantify the rate of Al interdiffusion as a function of initial superalloy composition. The diffusion couples were assembled with Invar steel jig encapsulated in Ar by sealing in quartz capsules and annealed at 1050 °C for 96 h. Concentration profiles measured by electron probe microanalysis in the single-phase β-NiAl region were used to determine interdiffusion fluxes and effective interdiffusion coefficients of individual components in the single-phase β-NiAl side of the couple. The values determined using experimental concentration profiles of the single-phase β-NiAl side of the couple were used to predict effective interdiffusion coefficients in multiphase superalloy side of the couple based on mass balance and local continuity of interdiffusion fluxes. Microstructural and compositional stability of protective coatings (e.g., NiCoCrAlY and NiAl) as a function of superalloys composition are discussed based on effective interdiffusion coefficients predicted from diffusion couple studies. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr., of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

Internal carburization and carbide precipitation in Fe-Ni-Cr alloy tubing retired from ethylene pyrolysis service

The events leading to the failure of an alloy grade HP Nb ethylene pyrolysis heater tubing were examined. X-ray maps indicated that a complex oxide coating, which inhibits carbon (C) diffusion, forms on the process side of the tubing during service. Phase equilibria studies predict that even without process C diffusion, metal carbides will precipitate out of the face centered cubic (FCC_Al) matrix. It was estimated that a 6 mm thick tube operating at 1100 °C would completely carburize in two years if the protective coating is damaged. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr. of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

Phase-field investigation of multicomponent diffusion in single-phase and two-phase diffusion couples

Interdiffusion in hypothetical ternary single-phase and two-phase diffusion couples are examined using a phase-field model by numerically solving the nonlinear Cahn-Hilliard and Ginzburg-Landau equations. For diffusion couples assembled with a regular single-phase solution, constant chemical mobilities were used to examine the development of concentration profiles including uphill diffusion and zero-flux plane. Zero-flux plane for a component was observed to develop for a diffusion couple at the composition that corresponds to the activity of that component in one of the terminal alloys. Experimental thermodynamic parameters and composition-dependent chemical mobilities were used to examine the morphological evolution of the interphase boundary in solid-to-solid, two-phase diffusion couples. Instability at the interphase boundary was introduced initially (t=0) by a small compositional fluctuation at the diffuse interface, and its evolution varied largely as a function of terminal alloys and related composition-dependent chemical mobility. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, the 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr., of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

Reactive interdiffusion in the binary system Ni-Si: Morphology of the Ni3Si2 phase

In binary multiphase diffusion, it is generally admitted that interfaces between phases are necessarily plane. However, a few cases exist, as the binary diffusion couples Ni-Si, Mo-Si, and Fe-Al, for which an intermediate phase of each system grows with an irregular needlelike morphology. To characterize the nonplanar growth of Ni₃Si₂ in bulk samples, the authors studied the behavior of intermetallic compound formation by optical microscopy and x-ray microtomography, for different annealing times. They show that both the average height and the tip radius of curvature grow as the square root of time with two diffusion coefficients separated by orders of magnitude. Moreover, x-ray diffraction indicates that the needles are aligned along the crystallographicc-axis. These results could be consistently explained by an anisotropic diffusion model. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr., of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

Diffusional analysis of a multiphase oxide scale formed on a Mo-Mo3Si-Mo5-SiB2 alloy

Diffusional analyses were performed to understand the oxidation at 1300 °C of a multiphase Mo-13.2Si-13.2B (at.%) alloy. During oxidation, a protective glass scale formed with an intermediate layer of (Mo+glass) between the base alloy and external glass scale. Compositional profiles across the (Mo+glass) layer and the external glass scale were determined, and interdiffusion fluxes and effective interdiffusion coefficients for the various components were determined by using “MultiDiFlux” software. The motion of the (alloy/Mo+glass) and (Mo+glass/glass) interphase boundaries after passivation was examined. Additionally, vapor-solid diffusion experiments at 1300 °C were carried out with single-phase Mo₃Si and T2 specimens in addition to a multiphase Mo-10Si-10B (at.%) alloy. These specimens were exposed to vacuum to induce silicon loss resulting in the formation of a Mo layer. An average effective interdiffusion coefficient of Si in Mo at 1300 °C was estimated from the Mo₃Si-vapor couple to be in the order of 8×10⁻¹⁷ m²/s. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr., of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

Copper diffusion into silicon substrates through TaN and Ta/TaN multilayer barriers

A study of copper (Cu) diffusion into silicon substrates through Ta nitride (TaN) and tantalum (Ta/TaN) layers was investigated based on an experimental approach. TaN _x and Ta/TaN _x thin films were deposited by radiofrequency sputtering under argon (Ar) and Ar-nitrogen (N) plasma. The influence of the N₂ partial pressure on the microstructure and the electrical properties is reported. X-ray diffraction patterns showed that the increase of the N₂ partial pressure, from 2 to 10.7%, induces a change in the composition of the δTaN phase, from TaN to TaN_1.13, as well as an evolution of the dominant crystallographic orientation. This composition change is related to a drastic increase of the electrical resistivity over a N₂ partial pressure of 7.3%. The efficiency of TaN layers and Ta/TaN multilayer diffusion barriers was investigated after annealing at temperatures between 600 and 900 °C in vacuum. Secondary ion mass spectrometry profiles showed that Cu diffuses from the surface layer through the TaN barrier from 600 °C. Cu diffusion mechanisms are modified in the presence of a Ta sublayer. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, the 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of the University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr., of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

A transfer-matrix method for analysis of multicomponent diffusion with any number of components

A transfer-matrix method (TMM) is presented for the development of concentration and flux profiles in multicomponent diffusion involving any numbern of components. From interdiffusion fluxes or concentration gradients available at an initial positionx _s, the authors derive expressions for the transfer matrix and its integral so that the concentrations or interdiffusion fluxes of the components can be obtained at any coordinatex. The TMM requires data for interdiffusion coefficients, which are obtained as average values over selected regions by the method of moments developed by Dayananda. Expressions for the concentrations are also obtained from initial conditions on the fluxes or the concentration gradients. The method is also applicable to the case when all the concentrations are known at two ends of a region over which the diffusion coefficients are considered constant. The integration of the fluxes over time, or over the coordinatex, can be evaluated using the transfer-matrix approach, provided the value of the interdiffusion flux is given at a given coordinate. The TMM is applicable to any number of components and can be regarded as a compact generalization of the solutions available for ternary diffusion couples with constant interdiffusion coefficients. An application of the method is illustrated with the experimental data for a ternary Cu-Ni-Zn diffusion couple, and the results are compared with those based on the Fujita-Gosting solution. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr., of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

An examination of a multicomponent diffusion couple

New expressions relating the interdiffusion flux of a component to its own concentration gradient in a multicomponent diffusion couple have been derived and applied to a diffusion couple investigated in the Cu-Ni-Zn system. From these relations, effective interdiffusion coefficients were determined at selected sections in the diffusion zone directly from the locations of the sections relative to the Matano plane. The Cu-Ni-Zn couple was analyzed for interdiffusion fluxes and interdiffusion coefficients with the aid of “MultiDiFlux” program developed for the analysis of interdiffusion in multicomponent systems. The couple was examined for zero-flux plane development, interdiffusion against activity gradients, and diffusion path representation. Diffusion path slopes at selected sections in the diffusion zone were related to the interdiffusion coefficients; slopes at path ends were determined from eigenvectors evaluated from limiting ratios of interdiffusion fluxes. Expressions for internal consistency among the concentration profiles or flux profiles of the individual components were also developed in terms of the terminal alloy compositions and applied to the Cu-Ni-Zn couple in the diffusion zone. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr., of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

The Grain-Boundary Diffusion of Zn in α-Fe

Using semi-infinite amounts of pure Zn and pure Fe as the diffusing media and maintaining constant boundary conditions, the diffusion of Zn into αFe has been experimentally studied at temperatures of 400, 500, and 725 °C. Long diffusion times (4 days) were used to allow the various intermetallic phases to become well developed. It was determined that diffusion of Zn into Fe is not “anomalously rapid,” but follows normal diffusion behavior for polycrystalline metals in that both lattice diffusion and grain-boundary diffusion contribute to mass transport for the temperatures investigated. The grain-boundary diffusion coefficients determined are consistent with those for many other metal systems.     

High-pressure torsion for enhanced atomic diffusion and promoting solid-state reactions in the aluminum–copper system

This study reports that solid-state reactions occur by the application of high-pressure torsion (HPT) to the Al–Cu system even at low homologous temperature. A bulk form of disc consisting of two separate half-discs of pure Al and pure Cu are processed by HPT at ambient temperature under a pressure of 6 GPa. X-ray diffraction analysis and high-resolution transmission electron microscopy confirm the formation of different intermetallic phases such as Al₂Cu, AlCu and Al₄Cu₉, as well as the dissolution and supersaturation of Al and Cu in each matrix. It is shown that the diffusion coefficient is enhanced by 10¹²–10²² times during the HPT processing in comparison with the lattice diffusion and becomes comparable to the surface diffusion. The enhanced diffusion is attributed to the presence of a high density of lattice defects such as vacancies, dislocations and grain boundaries produced by HPT processing.     

Microstructure and Oxidation Behavior of Al and Al/NiCrAlY Coatings on Pure Titanium Alloy

To improve the oxidation resistance of Ti alloys, a NiCrAlY coating was deposited as diffusion barrier between aluminum overlay coating and pure Ti substrate by air plasma spraying method. The microstructure and oxidation behavior of Al coatings with and without NiCrAlY diffusion barrier were investigated in isothermal oxidation tests at 800 °C for 100 h. The results indicate that the weight gain of the Al/NiCrAlY coating was 4.16 × 10^?5 mg² cm^?4 s^?1, whereas that of the single Al coating was 9.52 × 10^?5 mg² cm^?4 s^?1 after 100 h oxidation. As compared with single Al coating, the Al/NiCrAlY coating revealed lower oxidation rate and excellent oxidation resistance by forming thin Al₂O₃ + NiO scales at overlaying coating/diffusion barrier and diffusion barrier/substrate interfaces. Meanwhile, the inward diffusion of Al and the outward diffusion of Ti were inhibited effectively by the NiCrAlY diffusion barrier.     

Modeling of the dynamics of transient liquid films in ternary systems

The presence of a concentration gradient across a thin liquid film leads to the phenomenon of isothermal liquid film migration (LFM). In most cases, the concentration gradient is due to coherency stresses, and the migration of the liquid film results in the relaxation of these stresses. It is also possible to envisage cases in which LFM occurs as a result of chemical solubility differences between stable and metastable phases. Both situations are examined in this contribution with special emphasis on the effect of adding a second solute element to the liquid phase. The kinetics of LFM is examined as a function of the initial liquid concentration and the ratio of the solute diffusion coefficients in a model coherency-driven ternary system. The growth of the Γ₂ inside δ particles in the Zn-Fe-Al system is presented as a possible example of chemically driven LFM. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, the 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr., of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.