G. Ayrault

Microstructure of single and dual-ion irradiated Fe-20Ni-15Cr and Ti-6Al-4V alloys

Abstract An experimental austenltic alloy, Fe-20 wt. % Ni-15 wt. % Cr, and a commercial titanium alloy, Ti-64 (Ti-6 wt. % Al-4 wt. % V), have been dual-ion irradiated with heavy ions (Ni + or V + ) and 3 He + , and also single-ion irradiated with heavy ions. For the Fe-Ni-Cr alloy, a dose dependence study at 700°C revealed that swelling decreased as the rate of helium injection was increased from 5:1 to 55:1 He:dpa (appm He:dpa); concomitantly, the nature of the cavities changed from void-like (underpressured) at 5:1 He:dpa to near equilibrium (or possibly overpressured) helium bubbles at 55:1 He:dpa. Furthermore, simultaneous injection of helum at 16:1 He:dpa did not cause a significant change in the peak swelling temperature. In the titanium alloy, both single and dual-ion irradiations caused profuse precipitation of β -phase (BCC) platelets in the α -phase (HCP) matrix at all irradiation temperatures (450–650°C). Cavities formed only at the highest irradiation temperature (660°C) during singleion irradiation, but formed at all irradiation temperatures (541–624°C) for dual-ion irradiation at 10:1 He:dpa.

Influence of helium injection rate on the microstructure of dual-ion irradiated type 316 stainless steel☆

Type 316 stainless steel in a solution annealed and aged condition has been dual-ion irradiated with Ni+ and He+ and the resulting microstructure investigated with TEM. Dose dependence studies (3–25 dpa at 625°C and 3 × 10−3 dpa · s−1) were performed at three helium injection rates (5, 15 and 50 appm He/dpa). Bimodal cavity-size distributions formed at all three helium injection rates; both large- and small-cavities in the distributions were preferentially associated with acicular precipitates. The 5 and 15 appm He/dpa irradiations produced similar microstructures and swelling (∼ 1.2 % at 28 dpa), but higher cavity number densities in 50 appm He/dpa samples produced greater swelling (∼ 1.5 % at 20 dpa) despite smaller mean cavity sizes.

Cavity formation during single- and dual-ion irradiation in a 9Cr-1Mo ferritic alloy

Abstract The dose and temperature dependence of cavity formation in a 9Cr-1Mo ferritic alloy irradiated simultaneously with Ni + and He + has been studied with TEM. Comparisons are made with parallel experiments on Ni + -irradiated material that was preinjected with He. For dual-ion irradiation, both intergranular and intragranular cavities formed at all temperatures (450–600°C) and doses (5–25 dpa) investigated. The size of the intergranular cavities increased with increasing temperature, while the size of intragranular cavities decreased. In preinjected samples, cavities formed only at the lowest irradiation temperature (450°C). For 450°C single-ion irradiation and for 450 and 500°C dual-ion irradiation, there was a correlation between subgrain size and maximum cavity size, suggesting that the boundaries of the small (typically ~ 0.5 μm) subgrains act as the primary point-defect sink.

Effect of helium preinjection and prior thermomechanical treatment on the microstructure of Type 316 SS

Abstract Samples of 316 SS were preinjected with 15 appm helium either hot (650°C) or cold (room temperature) and irradiated with 3 MeV Ni+ ions to a dose level of 25 dpa at 625°C in order to test the validity of helium preinjection as a means of simulation of transmutant helium production. Results for preinjected and single-ion irradiated samples were compared to samples irradiated at 625°C to a 25 dpa dose level with 3 MeV Ni+ and simultaneously injected with helium at a rate of 15 appm He/dpa (dual-ion irradiated samples). Preinjected samples exhibited bimodal cavity size distributions. Preinjected samples of solution annealed or solution annealed and aged material showed lower swelling than dual-ion irradiated samples. However, He preinjected 20% cold worked samples showed greater swelling than dual-ion irradiated samples.

Influence of helium injection schedule and prior thermomechanical treatment on the microstructure of type 316 SS

Abstract The influence of different helium injection schedules on microstructure development in Ni + ion-irradiated 316 SS at 625°C is discussed. Injection schedules were chosen to either approximate the MFR condition or mimic the mixed-spectrum reactor condition. Dual-ion irradiation to 25 dpa produced strongly bimodal cavity size distributions in solution annealed and solution annealed and aged samples, whereas single-ion irradiation followed by dual-ion irradiation to the same dose produced a cavity size distribution with a substantial component of intermediate-size cavities. Dual-ion irradiation produced only very small cavities in 20% CW material, while single-ion followed by dual-ion irradiation produced some intermediate size cavities and greater swelling.

Radiation-induced precipitation in single- and dual-ion irradiated Ti-6AI-4V☆

Abstract The crystallographic orientation, morphology, and formation mechanism of precipitates in single-ion (V + ) and dual-ion (V + and He + ) irradiated Ti-6A1–4V has been investigated. The precipitates have the β -phase (bcc) structure, and the orientation of the precipitate crystal lattice relative to the α -phase (hcp) matrix conforms to the Burgers relation, with possibly some small deviations. The precipitates were usually elongated platelets but the shapes of precipitates in dual-ion and single-ion irradiated samples differed somewhat. The platelets had distinct preferred orientations (habit planes), but the habit planes were irrational (i.e., high-index) in both the bcc and hcp crystal systems, and changed with irradiation temperature. Precipitates dissolve on post-irradiation annealing at the irradiation temperature, showing that they are radiation-induced (non-equilibrium) in nature. Available evidence indicates that the precipitation is likely to be due to radiation-induced segregation of vanadium toward defect sinks. This segregation is consistent with a solute size effect.

Segregation in irradiated titanium alloys

Abstract Radiation-induced segregation of V, Al and Mo in α-phase titanium alloys was investigated. Ion irradiation (2.1-MeV Ar + ) at 650°C was used to simulate elevated-temperature neutron irradiation effects. Solute segregation in the near-surface regions of irradiated Ti-6Al-4V, Ti-8Al-1V-1Mo, Ti-3V and Ti-8.7Al was measured with an Auger depth profiling technique. The depth profiles showed evidence of radiation-induced V, Mo and Al segregation toward sample surfaces. This is in accord with a solute size effect.

Microstructural evolution of HT9 under dual-beam charged particle irradiation

Abstract Ferritic steels fabricated for elevated temperature service are potential candidate materials for the structural materials in a fusion reactor. One of the advantages of these steels is that they apparently offer better resistance to void swelling than austenitic steels. In this paper, we describe the microstructural evolution in 12Cr-1Mo-VW (HT9) steel dual or single ion irradiated to 100 dpa. Dual ion irradiations to 25 dpa with 15 appm He/dpa caused significant bias-driven growth at 743 K. The critical radius above which cavities exhibit bias driven growth, increased moderately with irradiation temperature from 683 to 873 K. Irradiations at 723 K to 100 dpa with 15 appm He/dpa showed bimodal cavity distribution and caused 0.14% swelling while with 5 appm He/dpa only small cavities formed, and without helium at 700 K, only dislocation loops and their fragments were observed.

Cavity growth in dual-ion and electron irradiated HT-9 ferritic stainless steels

Abstract In-situ observations of the cavity growth process in dual-ion irradiated HT-9 ferritic stainless steel were performed using HVEMs. The objective of this study is to analyze the cavity growth mechanisms as functions of cavity size and irradiation temperature. Dual-ion irradiations of the HT-9 produced a bi-modal cavity microstructure only at a peak swelling temperature of 743 K. And subsequent electron irradiation at 623 K caused growth of cavities larger than the critical radius. On the other hand after electron irradiation at 573 K a drastic decrease of the critical radius and growth of all the cavities produced by the dual-ion irradiation was observed. These results provide clear evidence of bias-driven and pressure-driven cavity growth mechanisms in ferritic stainless steels, and yield the temperature dependence of the critical radius. A different temperature dependence of swelling between ion irradiation and electron irradiation is found.

Effect of helium on swelling and microstructural evolution in ion-irradiated V-15Cr-5Ti alloy☆

Abstract The effect of helium implantation on the swelling and microstructural evolution in the V-15Cr-5Ti alloy was determined from TEM observations of the alloy after either single-ion irradiation, dual-ion irradiation, or helium implantation followed by single-ion irradiation. In addition to the TEM observations of the irradiated alloy, the effect of helium implantation on the dependence of the RIS of solute atoms was determined by use of the AES, EDXS, and EELS techniques. The swelling of the alloy was negligible (