Kiyotomo Nakata

Solute segregation along non-migrated and migrated grain boundaries during electron irradiation in austenitic stainless steels

Solute distribution and precipitation in the vicinity of the grain boundary in Type 316 steels were studied during electron irradiation up to about 50 dpa at temperatures from room temperature to 873 K. Undersized solute atoms, such as nickel, silicon and phosphorus, segregate toward the grain boundary, and oversized solutes, chromium and molybdenum, segregate away from the grain boundary during irradiation in the temperature range between 623 and 873 K. Enrichment of silicon and phosphorus along the grain boundary occurs after the irradiation at room temperature. The segregation of solute atoms increases with irradiation temperature except for silicon and phosphorus; the concentration of silicon and phosphorus along the grain boundary exhibits a maximum at 773 K. Remarkable depletion of chromium with enrichment of nickel, silicon and phosphorus occurs in the area swept by the migrating grain boundary. Massive M23C6 type carbide precipitates in front of the migrating grain boundary during irradiation in the temperature range from 723 to 873 K in the steels.

Computer simulation of damage depth profiles for 2–7.5 MeV/amu heavy ions incident on pure metals with experimental comparisons

Abstract A computer code EDEP-1 was extended to a high energy region using currently reliable stopping powers. The damage depth profiles calculated for pure metals for Al, Fe, Ni, Cu, Ag, and Ta irradiated with 2–7.5 MeV/amu heavy ions of C, Cl and Cr show that the extended code gives nearly similar profiles and depths of peak damage to those obtained by a Monte Carlo calculation code, TRIM85. The discrepancies from the experimental peak depths are less than 15%. The damage stragglings around the peak are calculated to be much smaller than the experimental values.

Ductilization of TiAl intermetallic alloys by neutron-irradiation

Abstract Post-irradiation tensile properties of Ti-rich TiAl (approximately Ti-47at% Al) intermetallic alloys, produced from the powder product manufactured by a plasma rotating electrode process (PREP), are investigated. The TiAl alloy powders are hot isostatic pressed (HlPed) into a cylindrical large compact, and then consolidated and formed by isothermal hot-forging (IHF). The final duplex structure of the alloys consists of equiaxial grains of the Ti3Al and TiAl(γ) phases with a grain size of approximately 2 μm in diameter. An increase in ductility from 6.0 to 10.3% with a slight increase in yield strength, is observed in specimens tested at 873 K after neutron irradiation at 873 K to a dose of 1 × 1020 n cm−2 (E>1 MeV) in JRR-2. This phenomenon can be understood by a mechanism that involves twins, which are believed to play an important role in the deformation process, nucleating during irradiation and growing during tensile testing after irradiation.

Dislocation loop and cavity formation under He-ion irradiation in a Ti-rich TiAl intermetallic compound

Abstract Damage structure in a Ti-rich TiAl intermetallic compound has been investigated by transmission electron microscopy after He-ion irradiations up to 3 × 10 21 ions/m2 at 623 and 773 K. The irradiations resulted in loop-shaped or dot clusters in the γ-TiAl and α 2 - Ti 3 Al grains. The cluster density in TiAl was over one order of magnitude lower than that in Ti3Al. The clusters in TiAl irradiated to 3 × 10 21 ions/m2 at 773 K were identified as interstitial-type faulted loops lying on {111} planes with the Burgers vector direction of 〈111〉. Cavities were created along the grain boundaries as well as in the matrix in TiAl at 773 K. The EELS analysis indicated that the cavity formation was associated with injected He atoms.

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.

Formation and annealing behavior of defect clusters in electron or He-ion irradiated Ti-rich Ti–Al alloys

In order to clarify the effect of He atoms on the formation and annealing behavior of defect clusters in Ti–Al alloys, a Ti–47 at.% Al intermetallic compound has been irradiated with electrons and He-ions. Helium-ion irradiation enhances the nucleation of defect clusters, especially of interstitial loops, at temperatures from 623 to 773 K in both γ-TiAl and α2-Ti3Al grains of the sample. However, there is little difference between the annealing temperature ranges of defect clusters in TiAl grains formed by He-ion or electron irradiation at 623 K. The dot-shaped clusters and interstitial loops grow scarcely during annealing, but are annihilated by annealing up to 923 K. Cavities are formed after irradiation with He-ions below 10 dpa at 773 K, but no cavities are formed by electron irradiation up to 30 dpa. The cavities in γ-TiAl and α2-Ti3Al grains survive after annealing even at 1053 K for 1.8 ks, keeping their density and diameter to be nearly the same as those in the as-irradiated grains.

Electrical resistivity recovery in Fe-Cr-Ni alloys after neutron irradiation at low temperature

Isochronal and isothermal annealing processes of austenitic Fe-Cr-Ni alloys and Type 316 steel are studied by electrical resistivity after fast neutron irradiation up to 2.9 × 1021n/m2 at 5 K and after quench from 1473 K. Little resistivity change is observed below 80 K in the irradiated specimens and below 280 K in the quenched ones. The resistivity variation of the annealing curves in the irradiated specimens is formed by the resistivity decrease due to defect annihilation and the resistivity increase due to structural change produced by the self-interstitial and vacancy migration. Self-interstitials and vacancies migrate above 100 K and 300 K, respectively. The radiation-induced reesistivity consists of two contributions; the resistivity increase due to defect formation and the resistivity decrease due to disordering of solute atoms by cascade collision during neutron irradiation.

Development and Irradiation Behavior of TiAl-Based Intermetallic Compounds

TiAl-based intermetallic compounds with triple-phase equiaxial microstructure show excellent properties especially in ductility larger than 7% and strength larger than 700 MPa at low to intermediate temperatures. Studies on the irradiation behavior of these alloys have demonstrated good irradiation resistance, though much less information has been available concerning neutron-irradiation experiments. This recent progress gives them enormous potentialities not only for engineering materials, but also for nuclear application.

The effect of solute content on grain boundary segregation in electron-irradiated Fe-Cr-Mn alloys

Solute distribution and microstructural development in the vicinity of grain boundaries in ferritic Fe-10Cr-xMn-3Al (x = 5, 10 or 15) alloys were studied during electron irradiation to 10 dpa at 723 K. In addition, X-ray diffraction analysis was performed for determination of the volume size factor of solutes. The oversized solute atoms, manganese and aluminum, were depleted at grain boundaries, whereas the concentration of the oversized chromium rose sharply at the boundary. The amount of segregation of the solutes decreased with increasing atomic volume depending on manganese content. Segregation of aluminum, which had the greater volume size factor relative to that of manganese, was higher than that of manganese. The amount of radiation-induced segregation of manganese and aluminum at the grain boundary is consistent with arguments based on the atomic size effect, but the enrichment of chromium at the grain boundary is not and seems to related primarily to the formation of chromium-rich precipitates at the boundary.

Damage distribution of heavy-ion irradiation in metals studied by electrical resistivity measurement

Abstract The stacked thin foil samples were irradiated by heavy-ions of C, Cl and Br with energies of 90–160 MeV at a low temperature below liquid nitrogen temperature, and the depth profiles of damage were obtained from the electrical resistivity change by isochronal annealing in the foils of pure Al, Fe, Ni, Cu, Ag and Ta set at various depths from an ion bombarded surface. The difference between the experimental damage peak depth and the theoretical one calculated by the modified EDEP-1 and TRIM codes is studied in terms of atomic number of irradiated metals. The half-value width of the damage distribution obtained from the experiment is larger than the calculated half-value width in the samples irradiated with C, Cl and Br ions.