D.J. Michel

Transient irradiation-induced creep of nickel during deuteron bombardment

Abstract The transient irradiation-induced creep of cold worked, high purity nickel was investigated during bombardment with 22 MeV deuterons at 224°C. The results indicate that the transient creep stage was complete after irradiation for 8 h. Microstructural characterization of specimens irradiated for 2, 4, and 8 h revealed a rapid decrease in dislocation density and a similar increase in defect cluster/loop density with increased irradiation time. The measured creep rates were compared with transient creep rates calculated on the basis of the microstructural data. The comparison shows that a climb-controlled glide mechanism was in agreement with the measured transient creep rates.

Effects of irradiation on the fracture toughness of fbr structural materials

Abstract The elastic-plastic fracture toughness, J Ic , of fast neutron irradiated reactor structural materials was investigated at 427 °C using a single-specimen compliance test method. The materials included Alloy 718, Type 308 weldment, 20% cold worked Type 316 stainless steel and annealed Types 304 and 316 stainless steels. The test results show that irradiation to fluences from 1.6 to 2.2 × 10 22 n/cm 2 (8.2 to 12.0 dpa) produced a degradation of the J Ic initiation toughness level of all materials with the largest reduction observed for the Type 308 weldment. The fractographic features were found to reflect the reduced fracture toughness of the materials in the irradiated condition.

The effect of irradiation on the fatigue and flow behavior of TZM alloy

The pre- and postirradiation fatigue and flow properties of the molybdenum-base alloy TZM have been determined at 427°C. Neutron irradiation to a fluence of 1 × 1022n/cm2 > 0.1 MeV increased the fatigue life of the stress relieved alloy in vacuum by approximately a factor of two in the strain range of 0.11 to 0.18%. The environmental effect of gaseous impurities on the rate of fatigue crack growth in unirradiated TZM was investigated for air pressures from approximately 10−4 to 100 Pa. Compressive now properties were determined at 427°C at a strain rate of 1 × 10−3 sec−1 and at 500°C for a range of strain rates. Irradiation to a fluence of 1.2 × 1022 n/cm2 increased the yield strength of stress relieved TZM by about 50 percent. The ductile to brittle transition temperature appeared to be between 427 and 500°C at a compressive strain rate of 1 × 10−3 sec−1. Irradiation induced changes in microhardness and yield strength were correlated with TEM results using general hardening models.

The shear punch measurement of the mechanical properties of selected unirradiated and irradiated alloys

Abstract A shear punch test technique was employed to determine the mechanical properties of neutron irradiated 9Cr-1Mo and 12Cr-1Mo steels. Comparison of the shear punch results with those from conventional tensile tests show that the shear punch reliably predicts the tensile yield and ultimate strengths of both irradiated and unirradiated 12Cr-1Mo steel and for unirradiated 9Cr-1Mo steel. The method did not reliably estimate the mechanical properties of irradiated 9Cr-1Mo steel and did not provide reliable estimates of the ductilities of these materials.

Neutron irradiation effects on fatigue crack propagation in austenitic stainless steels and nickel-base alloys

Abstract The effects of neutron irradiation on the fatigue crack propagation resistance of nickel-base alloys 718, 625 and Nimonic PE-16 were studied at 427°C. The results show that irradiation produced a small increase in crack propagation resistance of both Alloy 718 and Alloy 625 when compared with unirradiated test results for both alloys. For Nimonic PE-16,the irradiated results show a crack propagation rate significantly higher than for previously reported unirradiated results. The results for Alloy 718 and Alloy 625 also are comparable to those obtained previously for Type 316 stainless steel at 427°C. However, at irradiation and test temperatures of 649°C, the results for annealed Type 316 stainless steel show that neutron irradiation produced a decreased resistance to crack propagation during continuous cycling when compared with the irradiated results at 427°C. The inclusion of tensile hold periods during the fatigue cycle produced a further decrease in the crack propagation resistance of the annealed steel at 649°C. For cold worked Type 316 stainless steel, a decrease in crack propagation resistance was observed during both continuous cycling and hold time tests at 649°C. The results suggest that the austenitic stainless steels and nickel-base alloys may have adequate crack propagation resistance at temperatures up to 500°C for potential consideration as first wall materials. However, the limited data suggest that helium generated by (n, α) reactions may play a significant role in decreasing the crack propagation resistance at temperatures above 500°C in these materials.

Effect of neutron irradiation on fatigue and creep-fatigue crack propagation in alloy 718 at 427°C

Abstract The effects of neutron irradiation on both the fatigue and the creep-fatigue crack propagation performance of alloy 718 were investigated at 427°C at a level of ~7.0 dpa. The experimental results revealed that, for tests conducted with a continuous cyclic load at 0.17 Hz, the rates of crack propagation for both irradiated and unirradiated material conditions were nearly the same and were consistent with the previous results. However, for tests conducted with a one minute tensile hold period at the maximum cyclic load, the crack propagation rate for the irradiated material was found to be an order of magnitude higher than for material tested in the unirradiated condition. Fractographic observations have indicated that the basic mode of crack propagation for the unirradiated material was transgranular, regardless of hold time, as well as for the irradiated material tested with continuous cyclic loading. The crack propagation mode for the irradiated material tested with a one minute hold time was found to be intergranular in character in accordance with the higher crack propagation rates observed. The results suggest that the effects of irradiation exposure may severely reduce the crack propagation performance of alloy 718 during creep-fatigue loading conditions at even moderately elevated temperatures.

Simulation of irradiation-induced creep in nickel

Abstract An experimental technique has been developed to simulate neutron irradiation-induced creep by charged particle bombardment. The experimental apparatus permits on-line computer monitoring of experimental parameters while temperature, stress, and flux are maintained at the desired levels. A typical result obtained with a 0.38 mm (0.015 in.) thick, high-purity nickel specimen bombarded with 22 MeV deuterons at 224°C (435°F) and at a stress of 345 MPa (50.12 ksi) is presented. The result demonstrates that charged particle irradiation can successfully be used to simulate irradiation-induced creep reproducibly in materials whose thickness is typical of nuclear fuel cladding.

Fatigue crack propagation in type 316 stainless steel in vacuum and air environments

Abstract The fatigue crack propagation performance of unirradiated type 316 stainless steel was investigated in vacuum for comparison with results in an air environment under continuous cycling and hold time conditions. The materials included both fast breeder reactor (FBR) and magnetic fusion energy (MFE) reference type 316 steels. The continuous cycling results showed that the crack propagation rates of annealed FBR and MFE materials were essentially identical in a vacuum of 6.7 × 10 −6 Pa at 550° C. However, a significant reduction in crack propagation rates was observed for the FBR material in vacuum at 593°C when compared with the results in the air environment at 593°C. For 20 percent cold worked steel, the results showed that the crack propagation rates for the MFE material at 550°C in vacuum were lower than for the annealed MFE material at this temperature. These results were consistent with data for 20 percent cold worked FBR material at 593° C which showed that cold work reduced the rates of fatigue crack propagation when compared with annealed material. Hold time effects for both steels were significantly larger for evaluations conducted in vacuum or in air after irradiation as compared with previous unirradiated air results. Scanning electron microscope examination of the specimens tested in vacuum showed that the mode of crack propagation was entirely ductile and transgranular in character for continuous cycling tests and typically intergranular for hold time evaluations. The vacuum results obtained in this study were compared with the data for type 316 stainless steel tested in air for both the irradiated and the unirradiated conditions. The relationships developed from these comparisons suggest that the existing data will be of value to fusion design when consideration is given to the effects of environment and irradiation.

Effects of Neutron Irradiation on the Microstructure of Niobium and Niobium-Base Alloys

The microstructure and microhardness of niobium and commercial prototype niobium-base alloys have been investigated following fast neutron irradiation to a fluence of 1.1 X 10 2 2 neutrons (n)/cm 2 ( 0.1 MeV). The observation from the present experiments that molybdenum and zirconium additions were effective in the suppression of void formation suggests that a vacancy trapping mechanism was operative in the present alloys. The smaller hardness increase observed for the ternary alloys suggests that these alloys may maintain good engineering properties in addition to their improved resistance to void formation.

Microstructure and Mechanisms of Ion-Simulated Irradiation-Induced Creep of Nickel

The microstructure of cold-worked, high-purity nickel has been investigated following ion-simulated irradiation-induced creep with 22-MeV deuterons and 70-MeV α-particles. The irradiations were conducted at 224°C (435°F),at stresses between 170 and 345 MPa, and at displacement rates between 13 and 30 x 10 - 8 displacements per atom per second (dpa/s). Transmission electron microscopy (TEM) procedures were used to prepare, observe, and photograph the microstructure of the ion-irradiated uniaxial creep specimens and companion unirradiated specimens. Examination of the ion-irradiated microstructure revealed no substantial differences between the deuteron and α-particle irradiated specimens. In all cases, a heterogeneous distribution of defect clusters or small dislocation loops and network dislocations, or both, were observed. A significant reduction in dislocation density from the unirradiated values was seen for the irradiated specimens. It was found that the small loops and defect clusters provided effective obstacles to dislocation motion as evidenced by the bowing of dislocations between adjacent defects. The microstructural results were evaluated in terms of the theoretical mechanisms proposed for irradiation-induced creep and the previously reported creep simulation results for nickel by Hendrick et al. A model based on the climb-controlled glide of dislocations over dispersed obstacles was found to be consistent with the microstructural results and the experimental creep data.