K. Fukai

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.

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.

Depth dependent damage profile in stainless steel irradiated with He-ions

Type 316 stainless steel was irradiated with 24 MeV He-ions up to 3.2 × 1020 ions/m2 at ambient temperature. A peak in microhardness occurred at a distance of 105 μm from the irradiated surface. Small dislocation loops were seen at depths ranging from 105 to 114 μm with the peak in number density at 110 μm. A pair of parallel lines with a space of 8 μm centered at a distance of 107 μm from the irradiated surface were revealed on the top surface of the specimen by chemical etching after post-irradiation annealing for l h at 1023 K. The parallel lines correspond to arrays of fairly large Frank loops aligned in narrow bands on each side of a region containing helium bubbles. The observed damage profiles agree with those predicted by Littmark and Ziegler.

Role of α2/γ and γ/γ phase boundaries in cavity formation in a TiAl intermetallic compound irradiated with He-ions

Abstract A Ti–48at.%Al intermetallic compound has been irradiated with 200 keV He-ions at 623 and 773 K. The helium cavity density decreases with decreasing α2 and γ lamella width. A plot of the cavity density and lamella width reveals a linear relationship after irradiation to 15 dpa. Cavity density in regions with 300 nm wide lamella is about half of that in large γ grains of the specimen. The α2/γ lamellar boundaries supply a preferential nucleation site for cavities. Although the cavities are also formed on the γ/γ lamellar boundaries, the nucleation is limited to misfit dislocations on the boundaries. Defect-free zones are in the regions of about 50 nm width immediately adjacent to the lamellar boundaries at 773 K. These results suggest that the lamellar boundaries are effective sinks for radiation defects and contribute to the suppression of radiation-induced defect cluster development in TiAl alloys.

Effect of helium on microstructure in type 316 stainless steel

Abstract Solution annealed Type 316 stainless steel samples with 14 and 640 wt ppm boron were irradiated with thermal neutron fluences of 3.9×10 24 and 1.2×10 24 n/m 2 at 340 and 523 K. On post-irradiation annealing for 30 min at 1023 K, Frank loops and helium bubbles formed in the samples containing transmuted helium of 11–490 appm, whereas radiation-produced defect clusters annealed out completely in the sample containing 5 appm helium. The M23C6 type carbide was observed to form in the high-boron steel on annealing for 24 h at 1023 K following thermal neutron irradiation to 1.2×10 24 n/m 2 . The carbide was also revealed in the low-boron steel sample injected with helium to a peak concentration of 1000 appm at ambient temperature and annealed for 24 h at 1023 K. The carbide was observed with dislocations in the region where injected helium produced swelling of the order of 0.5% by the post-irradiation annealing. Helium is considered to stabilize the Frank interstitial loop in Type 316 stainless steel and enhances precipitation of M23C6 type carbide on the loop.

Swelling behavior of welded type 316 stainless steel and its improvement

Abstract Type 316 stainless steel was electron beam welded with titanium foil insertion. The concentration of introduced titanium in the weld metal was 0.1, 0.3 and 0.6 wt% corresponding to the inserted foil thickness of 10, 30 and 60 μm, respectively. All the weld joint showed good mechanical performance. The swelling resistance of the weld metal is effectively improved by the introduced titanium. Although inhomogeneous distribution of titanium makes it difficult to estimate the extent of the improvement quantitatively, the results suggest the applicability of this method to Ti-modified 316 stainless steel, where weld metal is already reported to show reduced swelling resistance.

Swelling susceptibility of electron-beam welded austenitic stainless steel

The void swelling susceptibility of austenitic stainless steel weldments has been examined. Materials used were Type 316 stainless steel containing 0.08% Ti and JPCA. Plates of these steels were electron-beam welded in a vacuum, and disks for irradiation experiments were obtained from the transverse sections, corresponding to the base metal, heat affected zone and weld metal. Irradiation in a High Voltage Electron Microscope (HVEM) was carried out at 773 K up to 15 dpa. In both steels this produced more void swelling in the weld metal than in the base metal, and more void swelling in the heat affected zone than in the base metal for Type 316 stainless steel. Segregation during solidification was detected in the weld metal and this may affect the void swelling susceptibility through the compositional change along with precipitation.

Effects of Mn and Si additions on microstructural development in TiAl intermetallic compounds irradiated with He-ions

A Ti-47 at.% Al intermetallic alloy and three TiAl alloys containing ∼2.0 at.% Mn and/or ∼0.4 at.% Si were prepared by powder metallurgical processing. When the samples were irradiated with He-ions to 3 dpa at 773 K, formation of defect clusters and cavities in TiAl alloys were remarkably suppressed by the addition of Mn. In Mn-added TiAl, although no loops, which were observed in pure TiAl and Si-added samples, were formed, the defect clusters with large strain field were found. It was suggested that the defect clusters were formed by the migration of mixed dumbbell type Mn atom-interstitials. The addition of Si showed no beneficial effects on suppression of radiation damage in TiAl alloys.

Microstructural evolution of He-preinjected austenitic stainless steels under HVEM irradiation

Abstract Two kinds of austenitic stainless steels, 316R (standard type 316 stainless steel) and alloy K (low-C and 0.29 wt% Ti) were irradiated in a high voltage electron microscope (HVEM) after He injection to 2.5 and 10 appm. 45 keV He was injected into the prepared transmission electron microscope (TEM) foils and HVEM irradiation was performed at 773 K to 14 dpa. Without helium preinjection, alloy K shows very few void production and thus the void swelling of alloy K was lower than that of 316R. Injected helium of 2.5 and 10 appm led to the void number densities of 10 22 /m 3 level or more in both steels. Void swelling was largest for 2.5 appm He specimens for both steels and the void growth rate was higher in alloy K than in 316R, and void swelling was higher in alloy K. The order of swelling resistance of these two steels as thus changed after 2.5 appm He injection.

Damage profiles in a stainless steel irradiated with Ar and N ions

Abstract Solution annealed Type 316 stainless steel was irradiated with high-energy Ar and N ions up to about 1 × 10 20 m −2 at ambient temperature. The ion-induced damage profile was analyzed with optical and electron microscopy. The projected range was measured to be 22–40 μm for 57.4–86.2 MeV N 4+ irradiation, which agrees well with the calculation by the extended E-DEP-1 computer code. On the other hand, the measured value for 40.5–51.1 MeV Ar 4+ irradiation was larger by about 20% than the calculated values of 4.6–5.4 μm. Upon post-irradiation annealing for 1 h at 1023 K, chromium nitrides (CrN) precipitated preferentially on large dislocation loops with stacking fault fringes in a 66.6 MeV N-ion irradiated sample, whereas tangled dislocations, M 23 C 6 type carbides and gas bubbles were observed in a 45.7 MeV Ar-ion irradiated sample.