Koichiro Hide

Mechanical response of irradiated thermally-sensitized type 304 stainless steels

Abstract Thermally sensitized type 304 stainless steels, together with solution annealed materials, were irradiated in inert gas environment to neutron fluences up to 5 × 1023 n/m2 (E > 1 MeV) at 563 K, and to 1 × 1024 n/m2 at 613 K, and then examined by a slow strain rate (SSR) tensile test and a microhardness test. The SSR tensile test results showed that 0.2% proof stress of the thermally sensitized materials unirradiated and irradiated to lower neutron fluences of ∼ 2 × 1023 n/m2 was larger than that for the solution annealed one, whereas the former irradiated to the high neutron fluence above ∼ 2 × 1023 n/m2 was smaller than the latter at the same neutron fluences. Microhardness at grain boundary regions was larger than that for grain centers in the irradiated as well as unirradiated thermally sensitized materials. The larger grain boundaries hardening could be attributed to radiation induced segregation of interstitial impurity elements and their interaction with radiation defects. The hardening of the grain boundaries relative to the grain centers increased with neutron fluence. The above neutron fluence dependent 0.2% proof stress of the thermally sensitized and solution annealed materials would primarily be ascribable to the larger hardening of the grain boundary area than that for the grain center. Radiation anneal hardening appeared at 600–650 K and 700–750 K upon post irradiation annealing of thermally sensitized materials.

Influence of tensile stress on cavity growth in nickel under helium irradiation

The influence of tensile stress on cavity behavior in pure nickel under helium irradiation was investigated by in-situ observation using the transmission electron microscope (TEM) in which an ion gun is installed. Specimens were irradiated at 500 °C with 20 keV helium in the TEM. The dose rate was about 1014 He/cm2 s, and the angle between the helium beam and the normal direction of the specimens was about 60 °. The damage rate estimated by the E-DEP-1 code was about 0.6×10−3 dpa/s at its peak position. The main results are as follows: (1) cavity nucleation was accelerated by applying tensile stress, and cavity size in stressed specimens was several times larger than that in stress-free specimens; (2) cavity density in the stressed specimen increased more rapidly than in the stress-free specimen, and then decreased by cavity coalescences; (3) depth of cavity nucleation in the stress-free specimen was about 160 nm, while that in the stressed specimen was about 320 nm; that is, cavities nucleated in deeper regions in the stressed specimen than in the stress-free specimen. This result indicates that helium atoms and vacancies can migrate into the deeper region by applying tensile stress. (4) The experimental results obtained in this study can be explained qualitatively by the mechanism that mobile dislocations drag He-V complexes to the deeper region. This implies that there are similar phenomena in the case of compressive stress.