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.     

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.     

The influence of silicon and helium on the microstructural stability of ion-irradiated incoloy DS alloys

In order to test their void swelling behaviour under irradiation, several alloys based on the solid-solution nickel alloy Incoloy DS (18Cr-38Ni-Fe) with additions of 0.05, 0.43, 0.92 and 2.24 wt% Si have been studied using 4 MeV helium and 46 MeV nickel ion irradiation in the Harwell VEC. For irradiations of 60 dpa with 10 appm He the void swelling decreased from ˜ 0.9% to negligible levels with increasing silicon content. After irradiation to 90 dpa following injection with 10 appm He the 2.24% Si alloy showed <1.0% swelling at an apparent peak swelling temperature of 625° C. This alloy was subsequently irradiated to check the swelling response with concentrations of helium and hydrogen appropriate to fusion-reactor conditions. Following irradiation to 60 dpa after 1000 appm He injection the swelling peak was shifted to 575° C where a swelling maximum of 4% was observed. At 625° C with 1000 appm He alone, swelling was 2.0% compared with 1.2% in samples injected with 1000 appm He +1000 appm H. This small reduction in swelling was associated with a higher cavity (bubble) concentration in the hydrogen implanted sample. Fine-scale precipitation of Ni₃Si(γ'), η-carbide and G-phase was observed after irradiation together with helium bubbles attached to the η- and G-phase precipitates. The precipitation and void swelling was significantly greater in irradiated samples containing 1000 appm He than in those with 10 appm and irradiated to 90 dpa. It is concluded that although the Incoloy DS alloy possibly has a potential for fission-reactor core applications it has little to commend it for fusion-reactor use where the high swelling response, microstructural instability and likely long-term induced activation arising from the higher nickel content are clearly undesirable factors.     

Void formation in solution-treated aisi 316 and 321 stainless steels under 46.5 mev ni6+ irradiation

The swelling and radiation damage structure developed in solution-treated 316 and 321 stainless steels bombarded by 46.5 MeV Ni⁶⁺ ions in the Variable Energy Cyclotron (VEC) have been determined. Foils were pre-injected with 10^?5 a/a He at room temperature and subsequently bombarded by Ni⁶⁺ ions in the temperature range 450–750°C at a damage rate of 1–3 × 10^?3 dpa per second to doses up to 300 dpa and specimens from the foils were examined by transmission electron microscopy. The data obtained were compared with data from other experiments aimed at simulating the fast-neutron irradiation of 316 and 321 steels, in particular previous work with 20 MeV C²⁺ ions and with data on fast-reactor bombarded material. The swelling rates in Ni-ion bombarded specimens were about a factor two less than those in C-ion bombarded specimens and in good agreement with swelling rates in 5 MeV Ni⁺- and neutron-bombarded material. The peak swelling temperature after a dose of 40 dpa was 650°C in 316 steel and 625°C in 321 steel where the swelling was about 5.8% and 4.6% respectively.     

The effects of large concentrations of helium on the mechanical properties of neutron-irradiated stainless steel

Tensile and creep properties have been determined on specimens of type 316 stainless steel irradiated in the High Flux Isotope Reactor in the range 380 to 785°C. Irradiation of type 316 in this reactor partially simulates fusion reactor irradiation, with displacement damage levels up to 120 dpa and helium contents up to 6000 appm achieved in two years. Samples irradiated in the annealed condition to about 100 dpa and 4000 appm helium showed an increased yield strength between 350 and 600°C and, except at 350°C, a reduced ultimate tensile strength compared with values for the unirradiated material. Samples irradiated in the 20%-cold-worked condition showed decreases in both yield and ultimate tensile strengths at all test temperatures. The irradiated samples of both annealed and cold-worked material exhibited little strain hardening, and total elongations were small and became zero,for tests at 650° C. Tensile tests at 575°C and creep-rupture tests at 550°C showed strong effects of fluence on strength and ductility for helium contents above about 30 appm. Optical metallography showed extensive carbide precipitation at all temperatures and precipitation of a second phase, believed to be sigma, at the higher temperatures.     

Proton irradiation emulation of PWR neutron damage microstructures in solution annealed 304 and cold-worked 316 stainless steels

Solution annealed (SA) 304 and cold-worked (CW) 316 austenitic stainless steels were pre-implanted with helium and were irradiated with protons in order to study the potential effects of helium, irradiation dose, and irradiation temperature on microstructural evolution, especially void swelling, with relevance to the behavior of austenitic core internals in pressurized water reactors (PWRs). These steels were irradiated with 1 MeV protons to doses between 1 and 10 dpa at 300 °C both with or without 15 appm helium pre-implanted at ∼100 °C. They were also irradiated at 340 °C, but only after 15 appm helium pre-implantation. Small heterogeneously distributed voids were observed in both alloys irradiated at 300 °C, but only after helium pre-implantation. The pre-implanted steels irradiated at 340 °C exhibited homogenous void formation, suggesting effects of both helium and irradiation temperature on void nucleation. Voids developed sooner in the SA304 alloy than CW316 alloy at 300 and 340 °C, consistent with the behavior observed at higher temperatures (>370 °C) for similar steels irradiated in the EBR-II fast reactor. The development of the Frank loop microstructure was similar in both alloys, and was only marginally affected by pre-implanted helium. Loop densities were insensitive to dose and irradiation temperature, and were decreased by helium; loop sizes increased with dose up to about 5.5 dpa and were not affected by the pre-implanted helium. Comparison with microstructures produced by neutron irradiation suggests that this method of helium pre-implantation and proton irradiation emulates neutron irradiation under PWR conditions.     

Void Swelling of Solution-Annealed Type 316 Stainless Steel under Long Time Proton Irradiation

Solution-annealed type 316 stainless steel was irradiated by 150 keV proton to a dose of about 6 dpa at the irradiation temperature ranging 450–700°C. To examine the effect of aging during irradiation, the present proton irradiation was carried out for about 25 h at a low dose rate of 7×10–^?5dpa/s. The specimens without He preinjection showed much smaller void swelling than those preinjected with He to the content of 10 at.ppm. Similarly to the case of neutron irradiations, the void swelling in the He preinjected specimens showed the temperature dependence with double peaks, and the peak swelling temperatures were about 550 and 650°C. In these specimens with He preinjection. void number density decreased and average void diameter increased with the increase of irradiation temperature in the range of 450–600°C, but these trends were reversed between 600 and 650°C. The volume of the grain boudary M₂₃C6 precipitates increased with the increase of irradiation temperature from 600 to 700°C, and it was concluded that the decrease of soluble carbon due to the precipitation of M₂₃C6 caused the second swelling peak at 650°C.     

The temperature dependence of nickel-ion damage in nickel

The temperature dependence of void and dislocation structures was studied in high-purity nickel irradiated with 2.8 MeV ⁵⁸Ni⁺ ions to a displacement density of 13 displacements per atom (dpa) at a displacement rate of $\text{7 × 10}{}^{\mathrm{?2}}\text{dpa/sec}$ over the temperature range 325 to 625°C. Dislocation loops, with no significant concentrations of voids, were observed in specimens irradiated at 475°C and below. Specimens irradiated between 525 and 725°C contained both voids and dislocations. The maximum swelling was measured as 1.2% at 625°C. Analysis of the data by theoretical models for void nucleation and growth indicated that the swelling in the present experiment was principally limited by void growth at low temperatures and by void nucleation at high temperatures. The data were also compared with previously reported neutron and nickel-ion irradiation results.     

Momentum transfer and damage energy gradient effects on helium distributions in type 304 stainless steel

The elastic scattering of 18 MeV α particles and transmission electron microscopy were used to study the effects of atomic displacement damage on preinjected helium distributions in type 304 stainless steel at 500°C. Helium implantation was conducted at 25°C or 500°C. The target foils were bombarded in either the solution annealed or 60% cold worked condition. Damage energy profiles were varied by changing the bombarding ion species (oxygen or hydrogen) or incident ion energy. Theoretical calculations of helium transport by atomic collisions were made for comparison with the experiment. For all of the experimental conditions, helium was shown to be very effectively trapped. No significant changes in helium concentration profiles or long-range migration (&> 100 nm) were induced by the superposition of highly nonuniform atomic displacement profiles at damage levels up to ~20 dpa. Most of the helium could be accounted for by the appearance of small near-equilibrium bubbles.     

Effects of temperature and helium on void formation in self-ion irradiated aluminum

High-purity aluminum was irradiated with 9 MeV aluminum ions at several temperatures in the range 25–125°C, both with and without pre-injected helium. The aluminum ion irradiation doses ranged from 0.2 to 20 dpa. Void swelling in the temperature range studied was found to occur only when the aluminum contains helium gas, and as little as 0.1 appm helium is sufficient to nucleate a substantial void population. In the absence of helium, even doses of 20 dpa do not result in void formation. The void number density and size follow the general trends in conventional nucleation theories, and are similar to trends observed in nickel and stainless steels. The microstructure quite often contained heterogeneous distributions of void stringers, suggesting nucleation of voids on dislocations.     

Precipitation and void-swelling in nickel-manganese austenitic stainless steels

The precipitation and void-swelling characteristics of austenitic stainless steels in which nickel is partially replaced by manganese have been investigated. Alloy compositions were chosen on the basis of manganese being half as effective as nickel in stabilizing austenite, and steels with “nickel equivalent” contents of 25–37% were examined. The steels were irradiated with 46 MeV Ni⁶⁺ ions to 60 dpa at 625°C and also aged for 1000 h at 600°C. The high-Mn alloys (20–30% Mn) were very susceptible to the formation of intermetallic phases during thermal ageing but less so in the shorter-duration irradiation experiment. Irradiation promoted the formation of Ni- and Si-rich phases—the suicide G phase (in which Mn can replace Ti) and in one instance M₆C. The Cr-rich carbide M₂₃C₆ formed in both the aged and irradiated steels. Among the high-Mn alloys, void-swelling decreased with increasing Ni and (Ni+Mn) contents, although a 25Ni-1Mn steel showed no swelling at 625°C.     

Quantifying helium distribution in dual-ion beam irradiated SiCf/SiC composites by electron energy loss spectroscopy

In this study, we report a method to quantify the helium distribution in the SiC_f/SiC composites, which are used as the first-wall materials of fusion reactor. The helium-bubble formation in Hi-Nicalon Type-S (HNS) was observed in the irradiated SiC_f/SiC composites at a level of 100 dpa and at 800 °C and 1000 °C, respectively. We applied transmission electron microscopy and electron energy loss spectroscopy to investigate the helium-gas-bubbles-formation mechanisms. To simulate the practical first-wall environment of Deuterium–Tritium (D–T) fusion reactor, a dual-ion beam (6 MeV Si³⁺ and 1.13 MeV He⁺) was performed to irradiate the SiC_f/SiC composites. The relationship between the energy shift of He K-edge and the radius of the bubble of the SiC composites was estimated by electron energy loss spectroscopy analysis. The results show that all of the helium atoms irradiated at 1000 °C and formed the bubbles. On the other hand, at 800 °C, only 25.5% of the helium atoms form the helium bubbles. A clear thermal-dependent formation mechanism is found.     

Observations of the microstructure of a 8% manganese stainless steel (ICL 016) before and after irradiation with 46 MeV nickel ions

Results are given of a preliminary investigation of the microstructure of a commercial Mn 8%/Cr 19%/Ni 7% austenitic steel (ICL 016) before and after irradiation with 46 MeV nickel ions. Pre-irradiation phases observed were Cu-rich precipitates (d ~ 10 nm) and α-MnS phase. A surface-localised ferromagnetism observed after annealing or irradiation was found to be due to α'-martensite formed as a result of an increase in the $\text{γ/ga'}$ transformation temperature due to evaporation of austenising elements such as Mn.Ion irradiation to 60 dpa at 625°C resulted in void-swelling of ~ 7% in solution-treated alloy containing 10 appm He. whereas swelling of ~ 1.8% occurred in the absence of helium. Irradiation also resulted in the formation of thin lath-like precipitates and the coarsening of the Cu precipitates. The results indicate that this manganese-containing alloy has an average swelling response when helium is present, with an indication that swelling can be reduced by pre-ageing at 700°C. In the ST or STA condition the alloy does not seem to offer any advantage in terms of void-swelling over other Fe-Cr-Ni austenitic steels currently favoured for LMFBR applications. The swelling sensitivity of the alloy to helium and the tendency to induced surface ferromagnetism indicate the need for further study before selecting this type of alloy for use in fusion reactors.     

The influence of He/dpa ratio and displacement rate on microstructural evolution: a comparison of theory and experiment

A kinetic model was developed to investigate the influence of the displacement rate and helium generation rate on microstructural evolution in austenitic stainless steels. The model integrates the rate equations describing the evolution of point defects, small point defect clusters, helium-vacancy clusters, and the larger cavity size distribution that is responsible for observable swelling. Cavity (bubble) nucleation is accounted for by the helium-vacancy cluster evolution, while void formation occurs when bubbles grow beyond a critical size in the larger cavity distribution.

A series of ion irradiation experiments were used to both calibrate the model and to provide a comparison between model predictions and experimental observations. The experiments involved single and dual-beam irradiations of solution annealed AISI-316 stainless steel at 873 K. The displacement rates were in the range of 2 × 10⁻³ to 1 × 10⁻² dpa/s and the helium-to-dpa ratios were in the range of 0 to 50 appm He/dpa. The maximum displacement dose was 25 dpa. The experiments revealed a significant effect of helium on both the dislocation structure and the cavity distribution. The model predictions of helium effects over a broad range of He/dpa ratios and displacement rates were consistent with experimental observations.     

Saturation of proton-induced swelling in AISI 316

There is some evidence which suggests that void swelling may not increase continuously with increasing irradiation but may saturate at a level dependent on irradiation conditions, helium/dpa level or artifacts of the simulation such as the presence of injected interstitials. An experiment was therefore performed to determine the saturation level of swelling of annealed AISI 316 at 625°C in the absence of helium and injected interstitials. Using 140 keV protons and step-height measurements it was found that saturation did not occur until 260% swelling was achieved at ~ 600 dpa. Prior to saturation the swelling curve exhibited the anticipated bilinear form with a steady state swelling rate of 0.64%/dpa based on a 20 eV threshold energy.     

Irradiation damage in proton irradiated palladium-iron solid solutions

Transmission electron microscopy was used to investigate the irradiation damage, in particular irradiation induced precipitation (IIP), in Pd-base alloys containing 2, 8, 12 and 18 at % Fe. The specimens were irradiated mainly using 400 keV protons at a current density of 0.16 μA/mm² over the temperature range 110 to 750°C. A few samples containing 2 and 8% Fe were also irradiated using 3 MeV NiP⁺ ions. The irradiation microstructure of the proton irradiated alloys consists of dislocation loops over the temperature range 110 to 550°C and voids up to 650°C in all the alloys. IIP of Pd₃Fe was observed only in the Pd-18% Fe alloy between 110 and 500°C, irradiated to a dose of 0.9 dpa. Pd₃Fe was associated with dislocation loops, voids and grain boundaries. IIP was not observed in the Pd-2,8 and 12% Fe alloys proton irradiated to the same dose, nor to a higher dose of 1.5 dpa. It was also not observed in the 2 and 8% Fe alloys irradiated at 600 and 700°C by 3 MeV Ni⁺ ions.The absence of IIP in the more dilute alloys is attributed to the fast back diffusion of Fe atoms, which is due to the high mobility of vacancies in these alloys. This causes the Fe concentration at the sinks to remain below the solubility limit. Therefore, even though Fe is an undersized solute, the size effect alone is not sufficient for the production of IIP at point defect sinks in most Pd-Fe alloys. It is proposed that IIP can occur only when the alloy concentration is high enough to minimize the rate of back diffusion, which depends not only on the vacancy mobility but also on the concentration gradient near point defect sinks.     

Nickel ion bombardment of type 304 stainless steel: Comparison with fast reactor swelling data

Annealed Type 304 stainless steel containing 15 atomic ppm of helium has been bombarded with 5 MeV nickel ions at 525°C to 700°C. A pronounced swelling peak occurs at 625°C, compared to a swelling peak temperature of about 475°C in reactor. TEM measurements of void swelling at 625°C as a function of ion dose show a swelling of almost 40% at 124 dpa without evidence of saturation. Measurements of gross swelling of the ion-bombarded material by a new step-height method provide information that is in good agreement with TEM data, and can be extended to larger swellings. The step-height results indicate a swelling of over 90% at 290 dpa at 625°C. The ion-produced swelling agrees well with in-reactor data when the two are compared at the respective peak swelling temperatures, and the void concentrations and average void diameters are comparable for the two cases. The high ion dose results are used to guide extrapolation of reactor data to higher fluences, leading to the following predictions for swellings at the peak swelling temperature in reactor: 18% swelling at $\text{1× 10}{}^{23}\text{n/cm}{}^2$ (fast), 50% at $\text{2 × 10}{}^{23}$, and 80% at $\text{3 × 10}{}^{23}$.     

Void swelling of an oxide dispersion strengthened ferritic alloy in a high voltage electron microscope

An oxide dispersion strengthened ferritic alloy with nominal composition Fe-13Cr-3.5Ti-1.5Mo-2TiO₂ and a cast alloy with a composition close to that of the matrix of the oxide dispersion strengthened alloy are irradiated in a high voltage electron microscope in the temperature range 380–550°C. The alloys are doped with 0–30 ppm helium. For alloys containing 10 ppm He a peak swelling temperature at 450°C is found. A maximum swelling of 1.1% is found at an irradiation dose of 20 dpa. In the absence of He no swelling is found in the temperature range 430–470°C. The swelling rate is highest at the onset of swelling. The results obtained here are quite similar to those for some ferritic steels such as FV607, EM 12 and HT9, except for the influence of He and for the dose dependence.     

Nickel-ion bombardment of annealed and cold-worked type 316 stainless steel

Bombardment with high doses of 5 MeV nickel ions has produced swellings as high as 90% and 60%, respectively, in annealed and 20% cold-rolled Type 316 steels. The steels contained 15 ppm of cyclotron-injected helium. Swellings were determined by both transmission electron microscopy and by a step-height method that measures the total swelling integrated along the ion path. The swelling in annealed Type 316 has a pronounced peak in the vicinity of 625°C, which is about 155°C higher than the peak swelling temperature in-reactor. The magnitudes of the swelling, void densities and void sizes produced in annealed Type 316 by nickel ions and in-reactor at the respective peak swelling temperatures are similar and it is concluded that the nickel ion bombardments provide an acceptable simulation of in-reactor behavior. Using the high dose ion results to guide extrapolation of presently available EBR-II data to higher fluences leads to the prediction that the swelling of annealed Type 316 steel at the peak swelling temperature will reach 40% at $\text{2 × 10}{}^{\mathrm{p23}}\text{n/cm}{}^2\text{(E > 0.1 MeV)}$ in EBR-II core, and 70% at $\text{3 × 10}{}^{23}\text{n/cm}{}^2$. These fluences in EBR-II correspond to 155 and 230 dpa respectively. Twenty percent reduction by cold-rolling reduces the ion produced swelling by 35% at 625°C and by 50% at 575°C.     

The void-swelling behaviour of solution-treated FV548 stainless steel irradiated with 22 MeV C2+ and 46.5 MeV Ni6+ ions and the influence of heat-treatment

A study has been made of the void-swelling behaviour of 1150°C solution-treated FV548 steel irradiated with 22 MeV C²⁺ and 46.5 MeV Ni⁶⁺ ions in the Harwell Variable Energy Cyclotron (VEC) after pre-injection with 10 ppm helium. The temperature dependence of void-swelling in the range 400–700°C, and the dose dependence of void-swelling at 600°C, have been established. In addition, the effects of ageing at 650°C, solution-treatment at 1300°C, and irradiation without helium pre-injection, on the void-swelling at 600°C have been investigated.Differences between void-swelling values obtained using C²⁺ ions, Ni⁶⁺ ions, and previously reported values obtained from 1 MeV electron irradiation of 1150°C solution-treated FV548 steel are discussed, as are the marked differences between the void-swelling behaviour reported here for VEC-irradiated FV548 steel and the established behaviour of type 316 steel irradiated under the same conditions. Finally, the present results are compared with the published data on reactor-irradiated solution-treated FV548 steel.