Margaret L. Hamilton

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.

The microstructural origins of yield strength changes in aisi 316 during fission or fusion irradiation

The changes in yield strength of AISI 316 irradiated in breeder reactors have been successfully modeled in terms of concurrent changes in microstructural components. Two new insights involving the strength contributions of voids and Frank loops have been incorporated into the hardening models. Both the radiation-induced microstructure and the yield strength exhibit transients which are then followed by saturation at a level dependent on the irradiation temperature. Extrapolation to anticipated fusion behavior based on microstructural comparisons leads to the conclusion that the primary influence of transmutational differences is only to alter the transient behavior and not the saturation level of yield strength.

Ductility correlations between shear punch and uniaxial tensile test data

The shear punch test was developed to address the need of the fusion reactor structural materials community for small scale mechanical properties tests. It has been demonstrated that effective shear strength data obtained from the shear punch test can be linearly related to uniaxial tensile strength for a wide variety of alloys. The current work explores the existence of a similar relationship between shear punch test data and both the tensile strain hardening exponent and the uniform elongation.

Validation of the shear punch-tensile correlation technique using irradiated materials

In previous studies on a variety of unirradiated materials, a linear relationship was developed between uniaxial tensile strength and effective shear strength, as determined from the shear punch test (SPT). Using the same data, a correlation was also developed to predict tensile uniform elongation from shear punch data. Validation of both correlations using a new database on both irradiated and unirradiated materials has been completed successfully.

The influence of specimen size on charpy impact testing of unirradiated HT-9

Abstract The effect of specimen size on the impact properties of HT-9 has been studied. The data from precracked and notched-only specimens of HT-9 were used along with previously published data on other steels to develop correlations that predict from subsize specimen data the ductile brittle transition temperature (DBTT) and the upper shelf energy (USE) anticipated for full size specimens. Each correlation is based on physical insight and can be applied equally well to both precracked and notched-only specimens. The correlation for USE works best for fracture energy in a range characteristic of irradiated materials, while a previously published correlation works better for more ductile materials. The DBTT correlation, however, works well on all materials for which sufficient data have been published.

Mechanical property changes of low activation ferritic/martensitic steels after neutron irradiation

Mechanical property changes of Fe–XCr–2W–0.2V,Ta (X: 2.25–12) low activation ferritic/martensitic steels including Japanese Low Activation Ferritic/martensitic (JLF) steels and F82H after neutron irradiation were investigated with emphasis on Charpy impact property, tensile property and irradiation creep properties. Dose dependence of ductile-to-brittle transition temperature (DBTT) in JLF-1 (9Cr steel) irradiated at 646–700 K increased with irradiation up to 20 dpa and then decreased with further irradiation showing highest DBTT of 260 K at 20 dpa. F82H showed similar dose dependence in DBTT to JLF-1 with higher transition temperature than that of JLF-1 at the same displacement damage. Yield strength in JLF steels and F82H showed similar dose dependence to that of DBTT. Yield strength increased with irradiation up to 15–20 dpa and then decreased to saturate above about 40 dpa. Irradiation hardening in 7–9%Cr steels (JLF-1, JLF-3, F82H) were observed to be smaller than those in steels with 2.25%Cr (JLF-4) or 12%Cr (JLF-5). Dependences of creep strain on applied hoop stress and neutron fluence were measured to be 1.5 and 1, respectively. Temperature dependence of creep coefficient showed a maximum at about 700 K which was caused by irradiation induced void formation or irradiation enhanced creep deformation. Creep coefficient of F82H was larger than those of JLF steels above 750 K. This was considered to be caused by the differences in N and Ta concentration between F82H and JLF steels.

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.

Biaxial thermal creep of V–4Cr–4Ti at 700°C and 800°C

Abstract A study of the thermal creep properties of V–4Cr–4Ti were performed using pressurized tube specimens. Creep tubes nominally 4.57 mm OD by 0.25 mm wall thickness were pressurized with high-purity helium gas to mid-wall effective stresses below the uniaxial yield strength. Specimens were heated to 700°C and 800°C in an ultra-high vacuum furnace and periodically removed to measure the change in OD with a high-precision laser profilometer. The secondary creep rate was found to be power-law dependent on the applied stress with a stress exponent of 3.7 at 700°C and 2.7 at 800°C. The average activation energy for creep of V–4Cr–4Ti was 299 kJ/mol, which is quite close to the activation energy for self-diffusion in pure vanadium in this temperature regime. The predominant mechanism of creep deformation for the conditions employed in this study is most likely climb-assisted dislocation motion.

Specimen size effects on the tensile properties of JPCA and JFMS

Examinations of specimen size effects on tensile properties were performed with and without neutron irradiation. Specimen thickness was varied to investigate the effect of aspect ratio as well as the thickness itself. Thickness dependence of yield stress, aspect ratio dependence of ultimate stress and aspect ratio dependence of ultimate strain showed good consistency and reproducibility even in heavy neutron irradiation condition. The effect of specimen size on yield stress and ultimate stress was small, but the ductility showed significant dependence on specimen aspect ratio.

Effect of low temperature irradiation on the mechanical properties of ternary V–Cr–Ti alloys as determined by tensile tests and shear punch tests

Abstract Tensile tests and shear punch tests were performed on a variety of vanadium alloys that were irradiated in the Advanced Test Reactor (ATR) at temperatures between 200°C and 300°C to doses between 3 and 5 dpa. Tests were performed at room temperature and the irradiation temperature. The results of both the tensile tests and the shear punch tests show that following low temperature irradiation, the yield strength (YS) increased by a factor of 3–4 while the ultimate strength increased by a factor of approximately 3. Uniform elongation (UE) and tensile reduction in area show that the ductility diminishes following irradiation. The correlation between uniaxial ultimate strength and effective shear maximum strength was in excellent agreement with previous studies on other materials. Using the room temperature test data, the correlation between uniaxial YS and effective shear YS was in excellent agreement with previous studies on other materials.