R. G. Ballinger

The effect of plastic strain on the evolution of crystallographic texture in Zircaloy-2

Abstract The evolution of crystallographic texture during plastic deformation was investigated in Zircaloy-2 using X-ray and metallographic techniques. Inverse pole figures, the resolved fraction of basal poles, and the volume fraction of twinned material, were determined as a function of plastic strain for several strain paths and initial textures at 298 K and 623 K. Incremental transverse platic strain ratios ( R ) were mesured as a function of plastic strain. Texture rotation occurs early in the deformation process, after as little as 1.5% plastic strain. For compressive plastic strains, the resolved fraction of basal poles increases in the direction parallel to the strain axis. For tensile plastic strains, the resolved fraction of basal poles decreases in the direction parallel to the strain axis. The rate of change of the resolved fraction of basal poles with plastic strain is a function of the initial resolved fraction of basal poles. The texture rotation can be explained by considering the operation of the principal tensile twinning systems, {1012}〈1011〉.

The effect of anisotropy on the mechanical behavior of Zircaloy-2

Abstract The influence of ciystallographic anisotropy on the mechanical properties of Zircaloy-2 was investigated as a function of stress state and plastic strain at 273 and 623 K. Zircaloy-2 plates were tested in the cold worked, stress relieved condition. Crystallographic textures were quantitatively characterized by the Kearns texture number, f , and plotted on direct and inverse pole figures. Uniaxial tension, compression, and plane strain compression tests were performed on 1.27 cm thick plates of Zircaloy-2 having two markedly differential initial Crystallographic textures. The test data was used to construct monotonic yield loci as a function of texture and plastic strain. The complex shapes of the different yield loci indicate a strong anisotropic mechanical behavior in the low plastic strain range at room temperature. The uniaxial flow stresses were found to be a unique function of the texture number, f . The mechanical behavior of Zircaloy-2 is a strong function of the controlling deformation mechanisms and of the texture derived constraint. These deformation mechanisms include prism slip, 〈c + a〉 slip and twinning. Plastic deformation was found to cause significant texture rotation after as little as 1.5% plastic strain. The severest texture rotation occurs with {1012} twinning which gives an 85° rotation of the c axis. Measurements of the transverse plastic strain ratio R as a function of plastic strain were correlated with the texture number, f , and flow stress measurements. A marked strength differential effect was observed when the stress axis is parallel to a direction of high basal pole density.

An Overview of Corrosion Issues for the Design and Operation of High-Temperature Lead- and Lead-Bismuth-Cooled Reactor Systems

Abstract The viability of advanced Pb- or Pb-Bi–cooled fast reactor systems will depend on the development of classes of materials that can operate over the temperature range 650–1200°C. We briefly review the current state of the technology concerning the interaction of Pb and Pb-Bi alloys with structural materials. We then identify the key challenges to successful use of materials in these systems and suggest a path forward to the development of new materials and operating methods to allow higher-temperature operation. Our focus is on the necessary trade-offs that must be considered and how these trade-offs influence R&D choices. Our analysis suggests that three classes of materials will be needed for successful deployment of a lead-alloy–cooled reactor system. A lower-temperature qualified material will be necessary for the pressure boundary. The structural and cladding materials will require 1000°C- and 1200°C-class materials. The 1000°C-class material will be exposed to the 1000°C coolant. The 1200°C-class material will be required for the cladding and structural materials in the core region. The higher-temperature material will be required to accommodate anticipated temperature transients from potential accident scenarios, such as a loss of flow.

Hydrogen transport in nickel-base alloys

The electrochemical permeation technique has been used to characterize hydrogen transport and trapping in pure nickel and in alloys 600, X-750, and 718 at a temperature of 80 °C. The “effective diffusivity” of hydrogen atoms in alloy 600 is reduced by a factor of about 5 compared to pure nickel. This is attributed to both compositional changes and the presence of [(Ti, Nb)C] carbides. Aging of alloy 600, with subsequent M23C6 carbide precipitation, does not significantly influence the measured “effective diffusivity,” which is explained by the dominant effect of preexisting [(Ti, Nb)C] carbides. The “effective diffusivity” of hydrogen atoms in solution-annealed alloy X-750 is reduced by a factor of about 9 compared to that of pure nickel. This is also attributed to compositional changes and [(Ti,Nb)C] carbides. Aging of alloy X-750, which causes precipitation of γ’[Ni3(Al, Ti)], reduces the “effective diffusivity” by an additional factor of 5 or more. Double aging at 885 °C/24 hours, 704 °C/20 hours following hot working yields the greatest reduction in “effective diffusivity.” Analysis of permeation transients using a diffusiontrapping model indicates a binding energy associated with trapping due to the γ’ phase of between-31 and -37 kJ/mol. The “effective diffusivity” of hydrogen in alloy 718 is about 40 pct greater than for alloy X-750 for the same double and direct aging treatments. The average “effective diffusivities” of the double-aged and direct-aged alloy 718 are comparable, but the permeation transients for the double-aged treatment are significantly steeper. The double-aged treatment with predominantly S phase (orthorhombic Ni3Nb) yields a binding energy of about-30 kJ/mol. Analysis of the direct aged-treated 718, which contains predominantly γ′ phase (body-centered tetragonal Ni3Nb) gave a binding energy between -23 and -27 kJ/mol. Segregation of hydrogen atoms to the γ′/matrix interface, combined with a large volume fraction ofγ at grain boundaries, provides the most likely explanation for the enhanced cracking associated with the double-aging treatment in alloy X-750.

Mechanical Properties of Incoloy 908 — An Update

Incoloy 908 is a nickel-iron base superalloy with its coefficient of thermal expansion and mechanical properties optimized for use in Nb3Sn superconducting magnets. Thermoelastic, tensile, fatigue crack growth, fracture toughness,and Charpy impact properties, and the results of conduit pressurization tests are summarized for base and weld metal.A limited number of stress rupture tests were also performed in air. The average yield strength (0.2% offset) for the solution annealed and aged base metal is 1200 MPa at 4.2 K. The fracture toughness,KIC, is greater than 230 MPa√m at 4.2 K. The fatigue behavior at 4.2K is comparable to austenitic stainless steels. Fatigue crack growth rates are a factor of three lower at 4.2 K than 298 K and are independent of heat treatment. At 4.2 K, the 20% cold-work-then-aged material has a 20% higher yield strength and a 10% higher ultimate tensile strength. Gas tungsten arc weld (GTAW) metal with or without Incoloy 908 filler metal exhibited comparable yield and about 10% lower tensile strength when compared with that of the base metal after a 200 hour age at 650°C. Fracture toughness, tensile elongation and Charpy absorbed energy were about 40% of those of the base metal. Leak-before-break behavior was observed in an internal pressurization test at room temperature for a geometry identical to that of the US-Demonstration Poloidal Coil conduit. The stress rupture performance is better than other low COE alloys of a similar type to that of Incoloy 908.

TIMCOAT: An Integrated Fuel Performance Model for Coated Particle Fuel

Abstract An integrated fuel performance model for coated particle fuel has been developed to comprehensively study the behavior of TRISO-coated fuel. Modeling of both pebble-bed and prismatic configurations is possible. In the case of the pebble-bed concept, refueling of pebbles is simulated to account for the nonuniform environment in the reactor core and history-dependent particle behavior. Monte Carlo sampling of particles is employed in fuel failure prediction to capture the statistical features of dimensions; material properties; and, in the case of the pebble-bed concept, the statistical nature of the refueling process. An advanced fuel failure model has been developed based on a probabilistic fracture mechanics approach. The mechanical analysis includes effects of anisotropic irradiation-induced dimensional changes and isotropic irradiation-induced creep, and the fluence-dependent Poisson ratio in irradiation creep. The stress analysis is benchmarked against the calculations of Japanese High Temperature Test Reactor (HTTR) first-loading fuel and finite element result on one case performed by the Idaho National Engineering and Environmental Laboratory. The failure model predictions are compared with NPR1, NPR2, and NPR1A capsule irradiation data. The model results compare very favorably with postirradiation examination results both in terms of failure probability, number of failed particles, and Kr85m R/B evolution during irradiation.

Stress Corrosion Cracking Crack Growth Behavior of Type 316L Stainless Steel Weld Metals in Boiling Water Reactor Environments

Abstract Thermal aging and consequent embrittlement of materials are ongoing issues in cast and duplex stainless steels. Spinodal decomposition is largely responsible for the well-known “475°C” embrittlement that results in drastic reductions in ductility and toughness in cast materials. This process is also operative in welds in cast or wrought stainless steels where delta ferrite is present. While the embrittlement can occur after several hundred hours of aging at 475°C, it can also occur at lower temperatures where ductility reductions have been observed after tens of thousands of hours at 300°C. The effect of thermal aging on mechanical properties, including tensile, toughness, fatigue, and static crack growth, has been investigated at room temperature and in 288°C high-purity water simulating boiling water reactor (BWR) operating conditions. The measurements of tensile, microhardness, and Charpy-impact energy show an increase in strength and a decrease in impact energy after aging for up to 10,000 h ...

Charpy Absorbed Energy and JIc as Measures of Cryogenic Fracture Toughness

In this paper, we present experimental comparisons between the critical energy line integral (JIc) and Charpy absorbed energy (Cv) at 4 and 77 K for materials ranging from fully austenitic steels to ferritic steels. At 4 K the correlation between JIc and Cv is weak, indicating that Cv is a poor indicator of static fracture roughness at this temperature. At 77 K, a good correlation exists between JIc and Cv. A good correlation is also observed between JIc at 77 K and Cv at 4 K. The results are explained by the large temperature rise during the Charpy rest. Further evidence of the temperature rise is the marginal change in Cv between 4 and 77 K and the disparity in fracture modes between Charpy and fracture mechanics specimens. For c stainless steels, Cv changed little from 4 to 77 K whereas JIc increased significantly. For ferritic steels, Cv increased in proportion to JIc from 4 to 77 K. Especially in steels with low nickel contents, fracture surfaces of Charpy specimens revealed higher ductility than those of fracture mechanics test specimens. The results qualitatively support the predicted temperature rises to 130 and 150 K for crack initiation during Charpy tests at 4 and 77 K, respectively. Due to a wide variation in roughness response to temperature rise, the Cv-based regulatory criteria developed for one group of alloys will have no validity when applied to another group. Therefore the Charpy test near absolute zero should not be regarded as a measure of the static fracture resistance. Alternative simplified methods of cryogenic fracture toughness are suggested.

Corrosion Studies in Support of Medium-Power Lead-Alloy-Cooled Reactor

Abstract The performance of structural materials in lead or lead-bismuth eutectic (LBE) systems is evaluated. The materials evaluated included refractory metals (W, Mo, and Ta), several U.S. steels [austenitic steel (316L), carbon steels (F-22, Fe-Si), ferritic/martensitic steels (HT-9 and 410)], and several experimental Fe-Si-Cr alloys that were expected to demonstrate corrosion resistance. The materials were exposed in either an LBE rotating electrode or a dynamic corrosion cell for periods from 100 to 1000 h at temperatures of 400, 500, 600, and 700°C, depending on material and exposure location. Weight change and optical scanning electron microscopy or X-ray analysis of the specimen were used to characterize oxide film thickness, corrosion depth, microstructure, and composition changes. The results of corrosion tests validate the excellent resistance of refractory metals (W, Ta, and Mo) to LBE corrosion. The tests conducted with stainless steels (410, 316L, and HT-9) produced mass transfer of elements (e.g., Ni and Cr) into the LBE, resulting in degradation of the material. With Fe-Si alloys a Si-rich layer (as SiO2) is formed on the surface during exposure to LBE from the selective dissolution of Fe.

Charpy Impact Tests Near Absolute Zero

We review Charpy impact testing at extreme cryogenic temperatures, especially at liquid helium temperature (4 K), considering methods of testing and calibration, thermal behavior during the various stages of testing, and correlations between Charpy absorbed energy and quantitative toughness parameters. Because of the very low specific heats of metals near absolute zero, any surface condensation of gases, convective or conductive heat transfer, or plastic deformation during a test will cause the specimen temperature to rise rapidly. Consequently, valid impact tests of alloys at 4 K can not be performed according to the procedure outlined in ASTM Methods E 23-88. During Charpy tests, the temperature of austenitic steel specimens, initially at or near 4 K, may in fact rise outside the cryogenic regime. Fracture does not occur at the intended temperature, but at an uncontrolled temperature, since materials with different work hardening rates heat differently. In view of the temperature rise variability and scatter in measurements and property correlations, we conclude that it is not possible to accurately estimate the 4 K fracture toughness of ductile steels, or rank them properly, using Charpy tests.