N.H. Packan

Correlation of neutron and heavy-ion damage: I. The influence of dose rate and injected helium on swelling in pure nickel☆

Abstract Displacement damage structures in pure nickel at the 1 dpa level are compared for two widely disparate damage rates, 10 −7 dpa/s for neutron irradiations and 3 X 10 −3 dpa/s for self-ion bombardments over a range of temperatures spanning those for void formation. Peak swelling at about 0.7% is found at 400° and 600°C, respectively. At equivalent swelling temperatures, voids in the ion-bombarded material are larger and fewer than those from neutron irradiation, especially at temperatures above the peak swelling temperature. Additions of 20 appm He, matching that generated in the neutron irradiations, were made to the ion-bombarded nickel either prior to ion bombardment (preinjection) or during ion bombardment (simultaneous injection). This helium caused increased swelling at the upper and lower temperature extremes. Simultaneously implanted helium did not otherwise significantly affect microstructures, whereas preinjected helium increased the dislocation density and caused more but smaller voids over the full temperature range of swelling.

Triple ion beam irradiation facility

Abstract A unique ion irradiation facility consisting of three accelerators is described. The accelerators can be operated concurrently to deliver three ion beams on one target sample as large as 100 mm2 in area. The energy ranges of the ions are 50 to 400 keV, 200 keV to 2.5 MeV and 1.0 to 5.0 MeV, which allows three different ions in the appropriate mass range to be simultaneously implanted to the same depth in a target specimen. Typical depth ranges are 0.1 to 1.0 μm. The X-Y profiles of all three ion beams are measured by a system of miniature Faraday cups. The ion beam energy of the low voltage accelerator can be ramped periodically during the implantation. Three different types of target chambers are in use at this facility. A triple-beam high vacuum chamber can hold nine TEM specimens at controlled temperatures in the range 400–800 °C during an irradiation by the three simultaneous beams. A second high vacuum chamber on the medium voltage accelerator beam line houses a low- and high-temperature translator and a two-axis goniometer for ion channeling measurements. The third chamber on the high energy beam line can be gas-filled for special stressed specimen irradiations. Applications of this facility to surface modification of materials are described.

Simulation of first wall damage: effects of the method of gas implantation

Cavity formation in an austenitic alloy of similar composition to Type 316 stainless steel has been explored with regard to various methods of gas implantation. Irradiations were conducted at 900 K to doses of 1, 10, and 70 dpa with helium injection levels of 20 appm/dpa. Highest swelling (18%) was exhibited by the unimplanted reference material; a lesser amount by simultaneous helium injection (11%). Greatly reduced swelling due to profuse cavity nucleation was the results of the preinjection of 1400 appm He, either at room temperature (S = 1%) or at 900 K (4%). The dislocation density was not sensitive to helium injection technique. Simultaneous injection of 50 appm H/dpa, along with the He, may have caused a modest increase in the cavity and dislocation concentrations at higher doses. The observations are compared with a theory of void growth kinetics to estimate the relative influence of voids and dislocations as point defect sinks.

Fluence and flux dependence of void formation in pure aluminum

Abstract Void formation in high purity aluminum resulting from irradiation to fluences between 1.5 × 1019 and 1.6×1022 neutrons/cm2 (E ⪢ 0.1 MeV) at a temperature of 55 ± 5°C was studied, primarily by means of transmission electron microscopy. Void size distribution curves were obtained for all fluences, and from these the mean void radius was found to increase in proportion to the irradiation time raised to the one-sixth power. The void concentration displayed a fluence dependence best described by a power law, N ~ (φt)a, in which the exponent decreased from 2.0 at 1019 neutrons/cm2 down to only 0.1 at 1022 neutrons/cm2. Treating the swelling with an analogous power relation, ΔV/V~(φt)b, a similar saturation effect was observed, with the fluence exponent b decreasing roughly from 5 2 to 1 2 over the range effluence studied. Increases in the microhardness of irradiated specimens were found to be consistent with an expression based upon void contributions to the impeding of dislocation motions. Irradiation at a factor-of-ten lower flux produced effects upon the void morphology (e.g., half as many but larger and more elongated voids) much like those resulting from irradiation at a higher temperature. In each case, the lowering of the vacancy supersaturation during irradiation is a consequence. When material degassed prior to irradiation was compared with similarly irradiated as-received material (containing 15–20 atomic ppm hydrogen), no difference in the void concentrations or sizes was evident. The experimental observations are compared with the current models for void formation. Two models most consistent with the experimental evidence both involve transmutation-produced helium playing a crucial role in void nucleation — one involving heliumstabilized spikes and the other small helium bubbles as the nuclei of voids.

Effects of pulsed dual-ion irradiation on phase transformations and microstructure in Ti-modified austenitic alloy

Abstract The influence of pulsed 4 MeV Ni ion bombardment, with and without simultaneous helium injection, has been explored in a low swelling, Ti-modified austenitic stainless steel. Irradiations were carried out to 70 dpa at 950 K; the pulsing frequencies were either 60 s on/off or 1 s on/off. Compared to continuous irradiation, pulsing caused a decrease in the interstitial loop diameter at 1 dpa, although at higher doses the overall dislocation density was not affected. Pulsing and helium both promoted the stability of MC precipitates and retarded the subsequent G phase formation; in some cases G-phase was suppressed and η phase formed instead. Small bubble-like cavities were observed to grow into large voids after steady dual beam irradiation to 70 dpa. However, this conversion was suppressed by pulsed irradiation to 70 dpa and furthermore the sizes of the small cavities were somewhat reduced. The results are explained in terms of current mechanistic understanding of mean point defect kinetics and the evolution of microstructure and microcomposition during irradiation with superimposed annealing periods.

A helium-induced shift in the temperature dependence of swelling☆

Abstract In an annealed austenitic alloy undergoing bombardment with 4 MeV Ni ions to doses between 1 and 70 dpa at 840, 900, 950, 1025, and 1100 K, the introduction of simultaneously-implanted helium at a rate of 20 appm He/dpa moves the swelling versus temperature curve up the temperature scale by 40 to 70 K. Co-implantation of hydrogen (deuterium) at a rate of 50 appm D/dpa simultaneously with the helium causes little or no additional systematic effects. The major change in microstructure caused by the gases is an enhancement of cavity nucleation by factors of 2 to 5 at 840 to 950 K, increasing to factors of thousands at 1100 K. Concurrently there is a reduction in the size of cavities and in swelling at all temperatures below 1025 K, and an increase in cavity size and in swelling at 1100 K, where the cavities are gas stabilized. At 1025 K the increase in nucleation of cavities outpaces the reduction in size and causes increased swelling. The primary effects of the gases are decided at low doses, below 1 dpa, where cavity nucleation is completed and where the conditions governing cavity growth are established; at higher doses swelling is determined by cavity growth, which is dependent on dose only. The gases cause copious formation of cavities on grain boundaries, boding ill for mechanical properties.

Electron microscope in situ annealing study of voids induced by irradiation in aluminum

Abstract The shrinkage of irradiation-induced voids in 8001 aluminum alloy and pure aluminum (Cominco Grade 69) was investigated by annealing thin-foil specimens in a high-vacuum electron microscope. The general shrinkage characteristics of both materials were quite similar, but the shrinkage curves for individual voids showed considerable variation. Most of the voids exhibited an initial high rate of shrinkage. Both effects may reflect the influence of unresolvable local vacancy sinks.

Pulsed flux effects on radiation damage

Abstract Pulsed irradiation fluxes can cause alteration in the development of irradiation microstructures when compared to steady irradiation microstructures. Theoretical analysis of irradiation damage development and examination of pulsed ion irradiation microstructures have indicated conditions for expected pulsing effects on fusion reactor materials. In pure metals, high instantaneous damage rates and pulse annealing periods comparable to defect relaxation times can cause significant pulsing effects. In addition, irradiation affected phases in alloys are altered by pulsed irradiation compared to steady irradiation.

Effects of rapidly pulsed ion bombardment on microstructure and phase stability in a Ti-modified stainless steel

Abstract A low-swelling Ti-modified austenitic stainless steel has been bombarded with pulsed 4 MeV Ni 2+ and in some cases simultaneously with 0.2–0.4 MeV He + ion beams at 948 K over a dose range from 1 to 70 dpa. The interruption periods studied were 60, 1, 10 −3 ,or 10 −5 s on time with equal off time. Continuous irradiation was also employed as a comparison. At 1 dpa, pulsed irradiation caused interstitial dislocation loops (and subsequent MC precipitates) to be more refined with decreasing pulse periods down to the realm of the vacancy lifetime (10 −1 to 10 −3 s); still faster pulsing at 10 −5 s yielded little difference from steady irradiation. At higher doses, 40–70 dpa, pulsing reduced the amount of radiation-induced G phase. Long pulse periods promoted the thermally-stable MC phase, while shorter periods like 10 −3 s aided M 6 C and M 23 C 6 development. In helium-implanted material bubbles were present for all pulsing conditions but bias-driven voids only developed in continuously-irradiated specimens. The results are interpreted with regard to the inherent fluctuations in point defect concentrations that result from cascade formation. The sensitivity of damage microstructure to pulsed bombardment is seen as a possible new tool for the control of microstructure in ion implantation experiments.

Effects of pulsed and/or dual ion irradiation on microstructural evolution in a Ti and Si modified austenitic alloy

Abstract The influence of pulsed 4 MeV Ni-ion bombardment, with and without simultaneous helium injection, at 958 K and damage levels from 1 to 50 dpa has been studied in a low swelling, Ti- and Si-modified austenitic stainless steel. Compared to continuous irradiation, pulsing caused an increase in the number density of interstitial loops formed during irradiation. Helium also increased the nucleation of interstitial loops. The main precipitates formed were a large number of small TiC particles uniformly distributed in the matrix, and a small number of relatively large eta and 6 precipitates. These coarse precipitates were somewhat larger in the pulsed specimens. Pulsing appeared to produce no significant change in swelling compared to continuous irradiation. However, for one specimen irradiated to 54 dpa, pulsing concurrent with substantial temperature fluctuations caused by beam heating may have been responsible for a larger swelling compared to continuous irradiation.