S.A. Maloy

Elastic properties of C40 transition metal disilicides

Abstract Room-temperature and low temperature elastic properties of hexagonal C40 transition metal disilicides, NbSi2 and TaSi2, have been studied using Resonant Ultrasound Spectroscopy (RUS). All five independent elastic stiffness constantscij for NbSi2 and single crystals have been obtained. The temperature dependence of thecij is normal but not large. The orientation dependence of the Youngs and shear moduli was examined in comparison with other transition metal disilicides. The room temperature shear moduli in the {0001} plane, with values of 145.3 and 143.7 GPa for NbSi2 and TaSi2 respectively, are low relative to those in the equivalent pseudo hexagonal {110} close-packed plane for tetragonal C11b MoSi2 and WSi2. The isotropic elastic constants for polycrystalline materials were also calculated. The results show that the various moduli are all much higher than those of the constituent elements. The room temperature Poissons ratios of NbSi2 and TaSi2 are 0.18 and 0.19, respectively, which are smaller than those of the constituent elements and smaller than most materials. The Debye temperatures,θD, were estimated to be 688 K for NbSi2 and 552 K for TaSi2. The elastic properties of C40 VSi2, NbSi2, TaSi2, and CrSi2 and C11b and WSi2 are compared and the possible influence on mechanical behavior discussed.

The mechanical properties of 316L/304L stainless steels, Alloy 718 and Mod 9Cr-1Mo after irradiation in a spallation environment

Abstract The Accelerator Production of Tritium (APT) project proposes to use a 1.0 GeV, 100 mA proton beam to produce neutrons via spallation reactions in a tungsten target. The neutrons are multiplied and moderated in a lead/aluminum/water blanket and then captured in 3 He to form tritium. The materials in the target and blanket region are exposed to protons and neutrons with energies into the GeV range. The effect of irradiation on the tensile and fracture toughness properties of candidate APT materials, 316L and 304L stainless steel (annealed), modified (Mod) 9Cr–1Mo steel, and Alloy 718 (precipitation hardened), was measured on tensile and fracture toughness specimens irradiated at the Los Alamos Neutron Science Center accelerator, which operates at an energy of 800 MeV and a current of 1 mA. The irradiation temperatures ranged from 50°C to 164°C, prototypic of those expected in the APT target/blanket. The maximum achieved proton fluence was 4.5×10 21 p / cm 2 for the materials in the center of the beam. This maximum exposure translates to a dpa of 12 and the generation of 10 000 appm H and 1000 appm He for the Type 304L stainless steel tensile specimens. Specimens were tested at the irradiation temperature of 50–164°C. Less than 1 dpa of exposure reduced the uniform elongation of the Alloy 718 (precipitation hardened) and Mod 9Cr–1Mo to less than 2%. This same dose reduced the fracture toughness by 50%. Approximately 4 dpa of exposure was required to reduce the uniform elongation of the austenitic stainless steels (304L and 316L) to less than 2%. The yield stress of the austenitic steels increased to more than twice its non-irradiated value after less than 1 dpa. The fracture toughness reduced significantly by 4 dpa to ∼100 MPa m1/2. These results are discussed and compared with results of similar materials irradiated in fission reactor environments.

Determination of helium and hydrogen yield from measurements on pure metals and alloys irradiated by mixed high energy proton and spallation neutron spectra in LANSCE

The confident design of accelerator-driven spallation neutron devices will require good estimates of the cross-sections for generation of helium and hydrogen in the mixed spectra of high energy protons and neutrons that will be experienced by the structural materials. Improved estimates of these cross-sections were derived from a series of irradiations that were conducted at relatively low temperatures (<100°C) in the Los Alamos Neutron Science Center (LANSCE) as part of the test program supporting the Accelerator Production of Tritium (APT) Program. In this irradiation campaign, a variety of candidate structural alloys and pure metal dosimeter foils were irradiated in various particle spectra, ranging from 800 MeV protons, to mixed energy distributions of both protons and spallation neutrons, and finally to distributions consisting primarily of high energy neutrons. At proton energies on the order of hundreds of MeV, exceptionally high levels of gas atoms are generated in all elemental constituents of typical iron-based and nickel-based structural alloys, with helium typically on the order of ∼150 appm per dpa and hydrogen at approximately a factor of 3–5 higher. Most of the gas production is due to proton and helium recoils from the proton beam interactions with the specimens, although gas and especially damage production from lower-energy spallation neutrons becomes increasingly significant at locations farther from the beam center. The results show that helium production rate per dpa by protons in elements typically found in structural alloys is relatively insensitive to elemental composition. The measured helium concentrations and the derived cross-sections are larger by about a factor of two, however, than those calculated using the LAHET code which was optimized for prediction of neutron/proton ratios in the target tungsten source rods of the APT test. Unlike helium, the retained hydrogen levels are somewhat sensitive to composition, reflecting primarily different levels of diffusional loss, but hydrogen is still retained at rather high concentrations, allowing a lower bound estimate of the hydrogen generation rates.

Mechanical properties of monocrystalline C11b MoSi2 with small aluminum additions

Molybdenum disilicide (MoSi{sub 2}) is a promising material for high temperature structural applications due to its high oxidation resistance, low density compared to nickel superalloys and high specific stiffness. These advantages are hindered by its poor low temperature fracture toughness, which in polycrystals deteriorates with increasing temperature due to the presence of glassy silica in the grain boundaries that produces intergranular fracture. Given that the effects of small Al additions on the mechanical properties of single crystals have not yet been explored, a study of the Vickers hardness for different crystallographic planes and directions has been carried out in monocrystalline Mo(Si,Al){sub 2} with the tetragonal Cl1{sub b} structure at room temperature and the results compared to measurements carried out in pure MoSi{sub 2} single crystals.

Superior radiation-resistant nanoengineered austenitic 304L stainless steel for applications in extreme radiation environments

Nuclear energy provides more than 10% of electrical power internationally, and the increasing engagement of nuclear energy is essential to meet the rapid worldwide increase in energy demand. A paramount challenge in the development of advanced nuclear reactors is the discovery of advanced structural materials that can endure extreme environments, such as severe neutron irradiation damage at high temperatures. It has been known for decades that high dose radiation can introduce significant void swelling accompanied by precipitation in austenitic stainless steel (SS). Here we report, however, that through nanoengineering, ultra-fine grained (UFG) 304L SS with an average grain size of ~100 nm, can withstand Fe ion irradiation at 500°C to 80 displacements-per-atom (dpa) with moderate grain coarsening. Compared to coarse grained (CG) counterparts, swelling resistance of UFG SS is improved by nearly an order of magnitude and swelling rate is reduced by a factor of 5. M23C6 precipitates, abundant in irradiated CG SS, are largely absent in UFG SS. This study provides a nanoengineering approach to design and discover radiation tolerant metallic materials for applications in extreme radiation environments.

In situ study of defect migration kinetics in nanoporous Ag with enhanced radiation tolerance

Defect sinks, such as grain boundaries and phase boundaries, have been widely accepted to improve the irradiation resistance of metallic materials. However, free surface, an ideal defect sink, has received little attention in bulk materials as surface-to-volume ratio is typically low. Here by using in situ Kr ion irradiation technique in a transmission electron microscope, we show that nanoporous (NP) Ag has enhanced radiation tolerance. Besides direct evidence of free surface induced frequent removal of various types of defect clusters, we determined, for the first time, the global and instantaneous diffusivity of defect clusters in both coarse-grained (CG) and NP Ag. Opposite to conventional wisdom, both types of diffusivities are lower in NP Ag. Such a surprise is largely related to the reduced interaction energy between isolated defect clusters in NP Ag. Determination of kinetics of defect clusters is essential to understand and model their migration and clustering in irradiated materials.

Mechanical properties and microstructure in low-activation martensitic steels F82H and Optimax after 800-MeV proton irradiation

Low-activation martensitic steels, F82H (mod.) and Optimax-A, have been irradiated with 800-MeV protons up to 5.9 dpa. The tensile properties and microstructure have been studied. The results show that radiation hardening increases continuously with irradiation dose. F82H has lesser irradiation hardening as compared to Optimax-A in the present work and DIN1.4926 from a previous study. The irradiation embrittlement effects are evident in the materials since the uniform elongation is reduced sharply to less than 2%. However, all the irradiated samples ruptured in a ductile-fracture mode. Defect clusters have been observed. The size and the density of defect clusters increase with the irradiation dose. Precipitates are amorphous after irradiation.

High strain rate deformation of Ti48Al2Nb2Cr

Abstract The compressive deformation behavior of Ti 48Al 2Nb 2Cr in the duplex microstructural morphology has been studied at strain rates of 0.001 and 2000 s−1 over the temperature range from −196 to 1100°C. The yield stress for the duplex microstructure is strain rate sensitive at 25°C and increases with temperature from 500 to 1100°C at a strain rate of 2000 s−1. TEM investigations revealed that deformation occurs in γ-TiAl by means of {111}〈112〉 twinning, 1/2〈110〉 slip and 〈101〉 superdislocations under all conditions depending on the orientation of the grain with respect to the deformation axis. Optical metallography revealed that twinning increases with increasing strain rate. The flow stress anomaly is discussed in reference to the dislocation substructure in γ-TiAl.

Temperature effects on the mechanical properties of candidate SNS target container materials after proton and neutron irradiation

Abstract This report presents the tensile properties of EC316LN austenitic stainless steel and 9Cr–2WVTa ferritic/martensitic steel after 800 MeV proton and spallation neutron irradiation to doses in the range 0.54–2.53 dpa at 30–100 °C. Tensile testing was performed at room temperature (20 °C) and 164 °C. The EC316LN stainless steel maintained notable strain-hardening capability after irradiation, while the 9Cr–2WVTa ferritic/martensitic steel posted negative hardening in the engineering stress–strain curves. In the EC316LN stainless steel, increasing the test temperature from 20 to 164 °C decreased the strength by 13–18% and the ductility by 8–36%. The effect of test temperature for the 9Cr–2WVTa ferritic/martensitic steel was less significant than for the EC316LN stainless steel. In addition, strain-hardening behaviors were analyzed for EC316LN and 316L stainless steels. The strain-hardening rate of the 316 stainless steels was largely dependent on test temperature. A calculation using reduction of area measurements and stress–strain data predicted positive strain hardening during plastic instability.

High Temperature Tensile Testing of Modified 9Cr-1Mo after Irradiation with High Energy Protons

Abstract This study examines the effect of tensile test temperatures ranging from 50 to 600 °C on the tensile properties of a modified 9Cr–1Mo ferritic steel after high energy proton irradiation at about 35–67 °C to doses from 1 to 3 dpa and 9 dpa. For the specimens irradiated to doses between 1 and 3 dpa, it was observed that the yield strength and ultimate strength decreased monotonically as a function of tensile test temperature, whereas the uniform elongation (UE) remained at approximately 1% for tensile test temperatures up to 250 °C and then increased for tensile test temperatures up to and including 500 °C. At 600 °C, the UE was observed to be less than the values at 400 and 500 °C. UE of the irradiated material tensile tested at 400–600 °C was observed to be greater than the values for the unirradiated material at the same temperatures. Tensile tests on the 9 dpa specimens followed similar trends.