Shigeki Kasahara

Effects of thermal sensitization on radiation-induced segregation in type 304 stainless steel irradiated with He-ions

Type 304 stainless steels, solution-annealed and thermally sensitized at 923 K for 0.5 to 24 h, were He-ion-irradiated up to about 4 dpa at 723 K and radiation-induced segregation (RIS) at grain boundaries was measured by EDS analysis using a FEG-TEM. Ni, Si and P were enriched, and Cr was depleted at the grain boundaries by irradiation. However, although the irradiation dose was the same for the specimens, the RIS of the elements linearly increased with the logarithm of the thermally sensitizing time, except for Cr in the specimen thermally sensitized for 24 h. The enhancement of RIS was attributed to the radiation-induced point defects having large mobility in thermally sensitized stainless steels, because of an expected decrease in C near the grain boundaries and in the matrix after the sensitization heat treatments. It was clarified by the electrochemical potentiokinetic reactivation test (EPR) that the degree of sensitization increased with the progress of Cr depletion at grain boundaries. The Cr concentration at grain boundaries in the heavily sensitized specimen was not changed, and the width of the depleted area was slightly narrowed by the irradiation. This result could be explained that the diffusion of Cr due to initial Cr concentration gradient near grain boundaries exceeded the inverse Kirkendall effect.

Effect of Mn addition on decrease of Cr depletion at grain boundary in austenitic alloys irradiated with electrons

Abstract Radiation-induced Cr depletion at a grain boundary (GB) is known as one of the major factors to degrade corrosion resistance of austenitic stainless steel. The effect of 10% Mn addition on prevention of the Cr depletion was investigated from a viewpoint of volume size factor (VSF) of Cr in the austenitic alloys irradiated with 1 MeV electrons. VSF of Cr in solution-annealed 316L steel added with 10 wt% Mn was + 0.8%, decreased by 4% compared with 316L. Radiation-induced Cr depletion at GB of 316L + 10%Mn was smaller than that of 316L at 723 and 773 K. Decrease of radiation-induced Cr depletion in 316LF + 10%Mn is thought to be derived mainly from the suppression of vacancy—Cr atom interaction.

Damage structure obtained by cross-sectional observation in silicon carbide irradiated with helium ions

Abstract The microstructural change due to He-ion irradiation has been studied by cross-sectional transmission electron microscope observation in hot-pressed SiC irradiated to 1 × 10 20 ions / m 2 at 300 and 1023 K. Dot and loop-shaped defect clusters are formed between 0.74 and 1.20 μm in depth from the ion-bombarded surface, and the damage peak depth is 1.15μm in the 1023 K irradiation. He bubbles, which are formed between 0.94 and 1.16 μm in depth, are aligned on the c -planes and along radiation-induced dislocations. In the 300 K irradiation, the amorphous zone is formed slightly behind the damage peak depth.

Electrical conductivity changes in silicon carbide under γ-ray irradiation

The effect of γ-ray irradiation on the electrical conductivity change has been studied by the in-situ measurement system at temperatures from 723 to 873 K in hot pressed SiC (HP-SiC) containing 2 wt.% BeO and pressureless sintered SiC (pls-SiC) containing 1 wt.% B and C. Increase of the conductivity is observed due to γ-ray irradiation in SiC, and the conductivity has a tendency to saturate up to a dose of about 2.5×10 4 Gy at each irradiation temperature. The conductivity shows similar values in pre-irradiated HP-SiC and pls-SiC at each temperature. However, the saturated conductivity in γ-ray irradiated HP-SiC is smaller than one in irradiated pls-SiC at the same irradiation temperature. It is considered that BeO existing along grain boundaries is effective to reduce the radiation-induced conductivity (RIC). RIC in HP- and pls-SiC recovers to almost the value in the preirradiation condition by annealing at the irradiation temperature. It is considered that the carriers excited by γ-ray irradiation are captured at the sites with deep energy level.

Effects of Cr2O3 on electrical conductivity during γ-ray irradiation in single crystalline Al2O3

The electrical conductivity change caused by γ-ray irradiation in the temperature range from 450 to 1050°C was studied by measurements using a two-terminal ac method in pure and 1 mol% Cr 2 O 3 -doped single crystal Al 2 O 3 . The conductivity in pure Al 2 O 3 was gradually increased by γ-ray irradiation between 650 and 750°C by about 10% of the conductivity in the unirradiated sample at a dose of 5×10 4 Gy. A conductivity decrease was observed during irradiation at 850°C. Little conductivity change was found during irradiation above 950°C, After irradiation, the increased and decreased conductivity by γ-ray irradiation was recovered by annealing within several hours at the temperatures. In the Cr 2 O 3 -doped Al 2 O 3 , little conductivity change was observed during γ-ray irradiation above 850°C, while slight conductivity decrease was found during 750°C irradiation.

Precise element distribution analysis across grain boundaries of electron-irradiated stainless steels

Abstract Precise element distributions across austenitic stainless steel grain boundaries were investigated by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy. Observed element distributions and TEM electron probe intensity profiles were approximated by one- and two-dimensional Gaussian distributions, respectively. Then, the more precise element distributions should have variances of B — b, and the segregated or depleted element concentrations on grain boundaries should be multiplied by a factor of √B/(B — b) , where B is the variance of the observed element profile and b is the variance of the probe intensity profile. As sample thickness decreased by ion milling, Cr and Ni concentrations were observed to decrease and increase, respectively, on an identical grain boundary. The corrected element concentrations, however, gave constant values on an identical grain boundary of stainless steels.

Effect of additional elements on cavity formation near grain boundary in proton-irradiated austenitic stainless steels

Abstract Using 140 keV H2+ ion irradiation, the segregation of hydrogen along grain boundaries in type 304L, Ti- and Nb-modified austenitic stainless steels was studied. The irradiation was performed at temperatures up to 673 K and doses up to 3 × 1021 H2+/m2. Cavities formed along grain boundaries in type 304L after irradiation. The number density and diameter of the cavities were less in the Ti- and Nb-modified steels compared with that of 304L. It appears that Ti and Nb contribute to a decrease in the amount of hydrogen trapped along grain boundaries.