Dual-ion Irradiation Research Articles

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

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 (<0.1%) for dual-ion irradiations at temperatures to 725°C and irradiation damage levels to 200 dpa and for single-ion irradiations at temperatures to 750°C and irradiation damage levels to 300 dpa. The morphology and distribution of irradiation-produced precipitates were distinctly different for the single- and dual-ion irradiated alloy. A helium implantation rate in the range of 5 to 9 appm He/dpa caused the accelerated growth of some helium-filled cavities in the alloy to give a bi-modal distribution of cavity sizes.

New dual-ion irradiation station at the University of Tokyo

The High-fluence Irradiation Test (HIT) facility of the University of Tokyo, under construction since 1983, was completed in the fall of 1985 with the assembly of the target station for dual-ion irradiation. The HIT facility was composed of two types of accelerators, viz., a Van de Graaff accelerator for light ions and a Tandem accelerator (Tandetron) for heavy ions. The target stations has some new and advantageous features for the measurement of irradiation parameters, e.g., ion-beam profile monitoring. The constitution of the dual-ion irradiation system of HIT is described together with a brief indication of the test results of a dual-ion experiment.

Evidence for a mechanism of swelling variation with composition in irradiated Fe-Cr-Ni alloys

Abstract Irradiations with 4 MeV Ni ions and 200–400 keV He ions were carried out on two alloys, Fe-15Cr-15Ni and Fe-15Cr-35Ni, at 675°C and doses up to 84dpa. Both dual-ion irradiation experiments and sequenced He injection-anneal-Ni irradiations were used. The dual-ion experiment showed that the two alloys exhibited large differences in microstructural development, with the low nickel alloy having significantly greater swelling. The injection-anneal-irradiation experiment was designed to test the hypothesis, suggested by our earlier work, that the lower swelling of the high nickel alloy may result from a larger critical radius/critical number of gas atoms required to achieve bias driven swelling. This experiment provided a direct measurement of these critical quantities by the induction of a bimodal cavity size distribution. The measurement gave minimum critical radii of about 5nm for the high nickel alloy and < 0·5nm for the low nickel alloy, values consistent with the hypothesized mechanism. The basis...

Influence of helium-generated interstitials in dual ion irradiations

Simulation of neutron-induced swelling by dual ion irradiation is influenced very strongly by the presence of injected interstitials due to self ions. Co-injected helium also generates extra interstitials, with its influence on void nucleation magnified by a synergism with those due to self ions. Low energy experiments are particularly affected since measurements have to be made where most ions come to rest. When the high helium/dpa ratios typical of fusion devices are employed the effect of helium-generated interstitials may dominate that of helium-as-gas. Co-injected helium therefore often appears to decrease swelling at a given damage level. The suppression of void nucleation by helium-generated interstitials decreases with increasing sink strength and is dominant at both low temperatures when recombination is high and at high temperatures when vacancy re-emission from vacancy clusters is significant. It is proposed that this behavior is an artifact of the simulation process and is atypical of the fusion reactor environment.

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

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.

Irradiation response in titanium modified austenitic stainless steels prepared by rapid solidification processing. Part II: Dual ion irradiations

Dual ion irradiations have been performed on Path A PCA and other alloys with increased titanium and carbon content, prepared by rapid solidification processing. Comparison with the neutron response shows many points of similarity. The effect of the amount of Ti and C included, and its initial distribution, is investigated, as is the difference between fully processed material, including consolidation by hot-extrusion, and the as-rapidly-solidified alloy. It is shown that increased TiC content can be effective in controlling swelling both directly, and by increasing the microstructural stability.

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

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.

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

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.

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

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.

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

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.

Effects of pulsed dual-ion irradiation on microstructural development

The effect of pulsed irradiation on the development of microstructure during Ni ion bombardment has been investigated in a simple austenitic alloy similar to type 316 stainless steel. Bombardment conditions were 10 dpa, 940 K, pulsing with equal on/off times of either 0.5 or 60 s, and the addition of 20 appm He/dpa to some specimens either by room temperature preimplantation or by dual-beam coimplantation. Particular care was taken to minimize thermal pulses from beam heating (to < 2°C). The results show that pulsing has a subtle influence, and the effects on specific cavity parameters are complex. Pulsing produced a small increase in swelling in the helium-free case, but a slight decrease for helium-implanted specimens, and it seems to have counteracted the usual stimulative effects of helium on cavity nucleation. An assessment is made of the often-conflicting observations in the literature; there is a need for further, well-controlled experiments.

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

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 3He +, 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.

Microstructures developed in ‘simulated’ fusion irradiations

Microstructural information is reviewed for irradiation conditions which ‘simulate’ one or more salient features of the irradiation environment anticipated at first wall and blanket positions in a fusion reactor. The effects of high energy neutron damage from T[d,n], Be[d,n] and 16 MeV proton irradiations, and the effects of simultaneous atom displacement damage and helium and hydrogen production from mixed-spectrum fission reactor and dual-ion irradiation experiments are summarized for a variety of metals. It is shown that observations of microstructural sensitivity to damage rate, irradiation temperature and helium are providing important clues for developing damage resistant structural alloys for fusion reactor appiications.

The Study of Fusion Reactor Materials Behavior Using Dual-Ion Irradiation

Small concentrations of transmutant helium (e.g., 10-7 He·disp-1 in Type 316 stainless steel) are produced by neutrons during irradiation in fast-reactors and have been simulated in charged-particle irradiations by He preinjection. In a D-T magnetic fusion reactor (MFR), the 14-MeV plasma neutrons produce higher and more significant concentrations of transmutant helium (e.g., 1.5 × 10-5 He·disp-1 in Type 316 stainless steel). These levels of helium are sufficient to impact heavily on the irradiation microstructure development, and hence, on the mechanical properties of candidate MFR structural materials. We have used simultaneous dual-ion (heavy ions + 3He+) irradiations to simulate the radiation environment in an MFR for our studies of microstructural evolution during irradiation of candidate MFR alloys. Single and dual-ion irradiations of an Fe-20Ni-15Cr alloy, a V-15Cr binary alloy, and selected Ti alloys (Ti-64, Ti-811, and Ti 15333) have been performed over a range of doses, helium to dpa ratios, and temperatures. For the Fe-20Ni-15Cr alloys, a dose dependence study at 700°C revealed that swelling decreased as the rate of helium injection was increased from 2.3:1 to 21:1 (appm He:dpa). Furthermore, the simultaneous injection of helium did not cause a signficant change in the peak swelling temperature of this alloy but did tend to inhibit cavity growth at higher temperatures. A dose dependence study (5-55 dpa) on the V-15Cr alloy at 650° revealed ~20% void swelling at 55 dpa with void diameters approaching 900 Å.