Johsei Nagakawa

The effect of MC and MN stabilizer additions on the creep rupture properties of helium implanted Fe-25% Ni-15% Cr austenitic alloy

Abstract Helium embrittlement resistance of Fe-25% Ni-15% Cr austenitic alloys with various MX (M = V, Ti, Nb, Zr; X = C, N) stabilizers was compared through post helium implantation creep testing at 923 K. While significant deterioration by helium in terms of creep rupture time and elongation occurred for all materials investigated, the suppression of the deterioration, especially in rupture time, was discerned for the materials in which semi-coherent MC (M = Ti, Ti + Nb, V + Ti) particles were distributed at high density. The material which contains the incoherent M 23 C 6 as predominant precipitates seems to be less degraded by helium than those containing the MXs (M = Zr, V; X = C, N), if compared at the same number density of precipitates. Therefore, it is suggested that the high density dispersion of incoherent M 23 C 6 as well as semi-coherent Ti containing MC particles would be beneficial in reducing the detrimental helium influences on mechanical properties.

Fatigue behavior of 20% cold-worked 316 stainless steel under in situ irradiation with 17 MeV protons at 60 °C

Abstract Load-controlled fatigue tests in the mode of tension–tension were performed for the side-notched 20% cold-worked 316 stainless steel under in situ irradiation and following irradiation with 17 MeV protons at 60 °C. In comparison with the unirradiation tests, fatigue life was substantially prolonged for in situ irradiation tests, while a slight increase of fatigue life was detected in post-irradiation condition. The SEM measurements of spacing of fatigue striations on fracture surface suggested higher resistance to fatigue fracture in the in situ irradiation specimens. Some essential differences in the irradiation effects between in situ and post-irradiation conditions were summarized in this paper.

Effects of helium implantation on creep rupture properties of low activation ferritic steel F82H IEA heat

Thin plate specimens of a low activation ferritic steel, F82H IEA, were cyclotron-implanted with helium at 823 K to concentrations of 100 and 300 appm. Creep rupture properties were subsequently measured at the same temperature and were compared with those from unimplanted controls. No meaningful deterioration by helium was discerned in terms of both creep rupture time and elongation. In addition, the fracture surface remained transgranular and ductile after helium implantation, and no indication of grain boundary failure induced by helium was detected. These results would suggest good resistance of this material toward helium embrittlement.

Calculation of radiation-induced creep and stress relaxation

Abstract Numerical calculation based on a computer simulation of point defect kinetics under stress was performed to predict radiation-induced deformation in an Inconel X-750 bolt in a LWR core and for a 316 stainless steel blanket in experimental fusion reactors with the water-coolant scenario. Although the displacement rate is rather low, modest irradiation creep with nearly linear stress dependence was predicted below 200 MPa at 300°C in the LWR core. This low stress dependence causes significant stress relaxation, which coincides with the experimental data to 2 dpa. An almost equal amount of enhanced irradiation creep strain was predicted at 60°C in both solution annealed and cold worker 316 stainless steel in the water-cooled blanket. The stress relaxation is practically not expected without irradiation in both the cases, but the calculation predicts that it is definitely expected under irradiation.

Calculation of radiation-induced stress relaxation

Abstract A numerical calculation based on point defect kinetics under stress was carried out to evaluate radiation-induced stress relaxation in a solution-annealed 316 stainless steel at low and medium temperatures (60 and 300°C). The calculation shows that the stress relaxation relative to the initial stress is almost independent of the initial stress under irradiation. At 1 × 10−6 dpa/s, stress relaxation at 60°C is greater than that at 300°C and the stress almost disappears in a month of continuous irradiation. Even with a low damage rate of 1 × 10−8 dpa/s, the stress relaxation at 60°C is considerable and exceeding that at 300°C and 1 × 10−6 dpa/s during the first several months of irradiation. The results indicate the importance of the radiation-induced stress relaxation in experimental fusion reactors.

Computer simulation of early-stage irradiation creep

Abstract Irradiation creep at an early stage, where accumulated damage is still well below the initiation of serious microstructural changes and void swelling, has been calculated based on point defect kinetics and dynamically competing mechanisms, namely SIPN, loop growth by SIPA, SIPA climb and glide enabled by SIPA. The result for austenitic stainless steel has revealed that SIPN would produce predominant creep strain in annealed material and, furthermore, respectable creep strain in cold-worked material under certain conditions. It has also implied that transients in point defect kinetics at very low temperatures might induce a creep strain comparable to or larger than that at higher temperatures.

Irradiation creep at 60°C in SUS 316 and its impact on fatigue fracture

Structural materials in fusion reactors will be subjected to irradiation by energetic particles at temperatures widely ranging from liquid He to above 1000°C. Hence, the very large irradiation creep strain at 60°C previously reported in the ORR/ORNL pressurized tube experiment to 8 dpa is important. Computer calculations for the 20% cold-worked SUS 316 demonstrated the transient nature of this radiation-induced creep, caused by the overwhelming flux of excess interstitial atoms lasting nearly one year at 60°C where the diffusivity of vacancies is very low. In order to confirm such a transient nature, continuous creep measurement under irradiation is necessary and was carried out using 17 MeV protons. Development of very significant creep strain, much larger than that at 300°C, and steadily decreasing creep rate were observed at 60°C as the calculation predicted. A significant influence of the dynamic irradiation effect at 60°C on fatigue fracture was also observed.

Irradiation creep transients in Ni-4 at.% Si

Abstract In the course of irradiation creep experiments on Ni-4 at.% Si alloy, two types of creep transients were observed on the termination of irradiation. The short term transient was completed within one minute while the long term transient persisted for nearly ten hours. A change in the temperature distribution was excluded from the possible causes, partly because the stress dependence of the observed transient strains was not linear, and partly because the strain increase expected from the temperature change was much smaller than the observed value. Transient behavior of point defects was examined in conjunction with the climb-glide mechanism and the steady-state irradiation creep data. Calculated creep transient due to excess vacancy flux to dislocations was in good agreement with the observed short term transient. The long term transient appears to be a result of dislocation microstructure change. The present results suggest an enhanced irradiation creep under cyclic irradiation conditions which will be encountered in the early generations of fusion reactors.

Irradiation creep simulation at low proton flux

Abstract Light-ion irradiation creep simulation was carried out for 316 stainless steel at low proton flux ( ~ 2.5×10 −8 dpa/s) and with low fluence ( −3 dpa ) in an attempt to prevent a serious microstructural modification. Both 20% cold worked (CW) and recrystallized and aged (RCA) specimens were examined. A linear stress dependence (41–124 MPa) and a weak temperature dependence (200–350°C) were observed for both specimens in the torsional in-situ creep measurements. The creep rate was only slightly lower in the RCA specimen. A transient with a “negative” irradiation creep was observed only in the RCA specimen as previously reported at higher flux. The SIPA climb mechanism fails to predict the high creep rates of the present study as well as those of the primary in-reactor creep. The CCG model of Henager and Simonen for low fluence appears to be more promising, though it still does not provide a complete explanation.

Microstructural observation of helium implanted and creep ruptured Fe–25%Ni–15%Cr alloys containing various MC and MN formers

Abstract Transmission electron microscopic observations have been carried out on Fe–25%Ni–15%Cr austenitic alloys with various MX (M=V, Ti, Nb, Zr; X=C, N) stabilizers after helium implantation and creep rupture at 923 K. It is shown that suppression of helium embrittlement can be achieved through a higher dispersion density of incoherent precipitates because of their high capability of bubble entrapment. A good agreement between the average distance of grain boundary bubbles exceeding the minimum critical size and the spacing of cavity traces on intergranularly fractured surfaces is obtained. This suggests that the enhancement of grain boundary decohesion by helium is a result of unstable growth of super-critical helium bubbles.