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.     

Elevated-temperature heavy-ion damage in (Co0.78Fe0.22)3V ordered alloy

Irradiation-induced changes in microstructure were studied in a (Co_0.78Fe_0.22)₃V long-range-ordered alloy after bombardment with 4 MeV nickel ions at temperatures from 843 to 1023 K and damage levels from 10 to 70 dpa. Small cavities and Frank loops were developed, and particles of pre-existing VC phase were redistributed by the irradiation. Long-range order was retained under all irradiation conditions, but domain size was effectively reduced by the Frank loops that served as antiphase boundaries. Simultaneous injection of helium at a rate of 3 at.ppm/dpa and deuterium at a rate of 12 at.ppm/dpa during irradiation produced gas bubbles in the grain boundaries, increased the dislocation and cavity densities, reduced the mean cavity size, but had little effect on the ordered domain size or the redistribution of VC particles. Even with injected gases, swelling remained below 0.25%. It is deduced that swelling did not reach the rapid bias-driven state because the critical cavity size was large.     

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

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.     

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

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.     

Effect of helium on void swelling in molybdenum

Simultaneous irradiation of molybdenum with helium and heavy ions (Ta³⁺) using a dual beam facility resulted in continued void nucleation in molybdenum to high dose levels, but the added helium had no measurable effect on the void swelling or swelling rate when compared with results for heavy ion irradiation without helium. Pretreatment by neutron irradiation or preinjection with helium resulted in no significant microstructural changes compared to no pretreatment. Also the temperature dependence of swelling was essentially unchanged when helium was added to the irradiation. The lack of a strong helium effect was attributed to the high inherent void nucleation rate in molybdenum. The overall swelling rate was similar to that observed for neutron irradiation and correlated well with the microstructural features that were observed. At the highest temperature and dose (1475 K and 40 dpa), simultaneous helium and heavy ion irradiation did result in a very nonuniform void distribution; thus, helium may have a greater effect on the microstructure at temperatures above those reported here.     

Very high swelling and embrittlement observed in a Fe–18Cr–10Ni–Ti hexagonal fuel wrapper irradiated in the BOR-60 fast reactor

The highest void swelling level ever observed in an operating fast reactor component has been found after irradiation in BOR-60 with swelling in Kh18H10T (Fe–18Cr–10Ni–Ti) austenitic steel exceeding 50%. At such high swelling levels the steel has reached a terminal swelling rate of 1%/dpa after a transient that depends on both dpa rate and irradiation temperature. The transient duration at the higher irradiation temperatures is as small as 10–13 dpa depending on which face was examined. When irradiated in a fast reactor such as BOR-60 with a rather low inlet temperature, most of the swelling occurs above the core center-plane and produces a highly asymmetric swelling loop when plotted vs. dpa. Voids initially harden the alloy but as the swelling level becomes significant the elastic moduli of the alloy decreases strongly with swelling, leading to the consequence that the steel actually softens with increasing swelling. This softening occurs even as the elongation decreases as a result of void linkage during deformation. Finally, the elongation decreases to zero with further increases of swelling. This very brittle failure is known to arise from segregation of nickel to void surfaces which induces a martensitic instability leading to a zero tearing modulus and zero deformation.     

Microstructural and microchemical comparisons of AISI 316 irradiated in HFIR and EBR-II

Two alternative interpretations of a unique but limited set of swelling data have been advanced in an attempt to predict the influence of helium on the swelling of 20% cold-worked AISI 316. In this paper it is shown that swelling data for annealed AISI 316 provide substantial insight concerning the role of helium on swelling. Additional insight has been obtained by a series of microstructural and microchemical examinations conducted on specimens of both annealed and cold-worked steel after irradiation in EBR-II and HFIR. These reactors differ greatly in neutron spectra and in AISI 316 generate quite different helium/dpa ratios and solid transmutant levels.At least in the range 500–750°C, the results of these studies show that helium's influence on swelling is manifested in the cavity density but not in the dislocation density or the major features of the microchemical evolution of the alloy matrix. The results also suggest that the influence of helium at high displacement levels is not strong on either the total swelling or the steady-state swelling rate.     

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.     

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.     

Irradiation effect on mechanical properties in structural materials of fast breeder reactor plant

The effects of displacement per atom (dpa) level, helium content, and the ratio of helium content to dpa level on the tensile and creep properties have been investigated in the assumed irradiation damage range of FBR structural materials. The assumed irradiation damage range is up to about 1 dpa and about 30 appm for helium content. Austenitic stainless steel and high-chromium martensitic steel are considered as FBR structural materials. As a result, it is shown that the dpa level is a promising index for evaluating neutron irradiation damage.     

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.     

Neutron-induced swelling and embrittlement of pure iron and pure nickel irradiated in the BN-350 and BOR-60 fast reactors

Pure iron and nickel were irradiated in the range of 2-15 × 10⁻⁷ dpa/s at 345-650 °C to very high neutron exposures in two fast reactors, BOR-60 and BN-350, to study void swelling and changes in mechanical properties of these two metals. Both nickel and iron swell in this temperature range with the maximum swelling rate at ∼500 °C in nickel, but possibly at ?350 °C for iron. It also appears that the swelling rate in nickel and possibly in iron may be dependent on the dpa rate, increasing with decreasing dpa rate. The evolution of mechanical properties of the two metals is quite different. The differences reflect the fact that b.c.c. iron is subject to a low-temperature embrittlement arising from a shift in ductile-brittle transition temperature, while f.c.c. nickel is not. Nickel, however, exhibits high temperature embrittlement, thought to arise from the collection of transmutant helium gas at the grain boundaries. Iron is not strongly affected by transmutation since it generates much less helium during equivalent irradiation.     

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.     

Void swelling and precipitation in a titanium-modified austenitic stainless steel under proton irradiation

The correlation between void swelling and precipitation behavior in a 10% cold worked Fe-16.2Ni-14.6Cr-2.37Mo-1.79Mn-0.53Si-0.24Ti-0.06C alloy was examined with 200 keV proton irradiation. Swelling peak temperature after the proton irradiation to 10 dpa was about 823 K, and void swelling decreased steeply with increase in irradiation temperature from 823 to 923 K. Void swelling increased rapidly from 1.9 to 12.1% with increase in irradiation dose from 20 to 45 dpa at 873 K. Fine intragranular TiC precipitates, which were formed during initial stage of irradiation, dissolved gradually with increase in irradiation dose from 10 to 45 dpa at 873 K, while the amount of precipitation of needle-shaped Fe₂P phase containing titanium increased with increasing dose. The reduction of sink strength of the TiC precipitates due to the dissolution during irradiation was thought to cause the increase of swelling rate with increase in irradiation dose from 20 to 45 dpa at 873 K.     

硅对低活化马氏体钢电子辐照行为的影响

利用超高压透射电子显微镜研究了两种成分的低活化马氏体钢(CLAM钢)的辐照损伤行为。结果表明：电子辐照能在未添加硅的CLAM钢中产生辐照空洞;在450℃下辐照至14dpa时,空洞数密度约为8.7×10²¹m^－3,辐照肿胀率约为0.26％;在450℃下的辐照肿胀率明显比500℃下的高;当损伤率为2×10^－3dpa/s时,添加合金元素硅能显著提高CLAM钢的抗辐照肿胀能力,未在添加硅的CLAM钢中实验观察到辐照空洞的形成。在450℃下进行辐照时,添加硅的CLAM钢出现明显的辐照共格析出现象。     

纳米压痕技术研究堆内构件材料304NG不锈钢的三束辐照效应

在中国原子能科学研究院的三束辐照实验平台上,对国产堆内构件材料核级控氮304NG不锈钢进行重离子、氢和氦同时辐照,并采用连续刚度纳米压痕技术测量了辐照前后样品的硬度和弹性模量的变化。结果表明,6dpa下,随着辐照温度的升高,辐照硬化减弱;300℃下,随着辐照剂量的增加,辐照硬化增大,高剂量情况下,氢、氦注入区的辐照硬化更显著,表明存在氢、氦增强硬化效应。     

Swelling and microstructure of austenitic stainless steel ChS-68 CW after high dose neutron irradiation

Austenitic stainless steel ChS-68 serving as fuel pin cladding was irradiated in the 20% cold-worked condition in the BN-600 fast reactor in the range 56-84 dpa. This steel was developed to replace EI-847 which was limited by its insufficient resistance to void swelling. Comparison of swelling between EI-847 and ChS-68 under similar irradiation conditions showed improvement of the latter steel by an extended transient regime of an additional ∼10 dpa. Concurrent with swelling was the development of a variety of phases. In the temperature range 430-460 °С where the temperature peak of swelling was located, the principal type of phase generated during irradiation was G-phase, with volume fraction increasing linearly with dose to ∼0.5% at 84 dpa. While the onset of swelling is concurrent with formation of G-phase, the action of G-phase cannot be confidently ascribed to significant removal from solution of swelling-suppressive elements such as silicon. A plausible mechanism for the higher resistance to void swelling of ChS-68 as compared with EI-847 may be related to an observed higher stability of faulted dislocation loops in ChS-68 that impedes the formation of a glissile dislocation network. The higher level of boron in ChS-68 is thought to be one contributor that might play this role.     

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.     

Void formation in 10Cr-2Mo ferritic steel irradiated in HFIR

Microstructural evolution of a 10Cr-2Mo ferritic steel was investigated after irradiation in the High Flux Isotope Reactor (HFIR) at 500°C and doses up to 57 dpa. About 300 appm of helium was also produced after 57 dpa because the steel contained 1 wt% Ni. Many tiny helium bubbles were observed in a specimen irradiated to 34 dpa, either inside of or adjacent to particles of radiation-produced x- and Laves-phase precipitates. By contrast, after 57 dpa many of the tiny bubbles had developed into larger voids. Most of the voids were associated with radiation-induced x-phase particles.     

Cavity nucleation calculations for irradiated metals

A theory of helium-assisted cavity nucleation in irradiated metals is modified and applied to conditions of continuous helium generation. The theory considers the nucleation and growth of cavities by coprecipitation of vacancies, interstitials, and inert gas atoms. Calculations are performed for type 304 stainless steel for comparison with ion irradiation experiments at $\text{~ 2 × 10}{}^{\mathrm{?4}}\text{dpa/s}$, with helium implantation at the rate of ~10^?2 appm/s, to a total damage of ~ 5 dpa, over the temperatures 773–973 K. Total cavity number density calculated ranges from 10²³ m^?3 at 773 K to 10²⁰ m^?3 at 973 K. The calculated incubation time for cavity appearance is 1000–3000 s (0.2–0.6 dpa). The calculated plot of cavity density versus time approximately reproduces the experimental data. Predicted cavity size distributions are roughly bell-shaped, but skewed in favor of larger cavity sizes. Calculated and experimental mean sizes agree within a factor of 3. The predictions of the model are found to change very little when most parameters are varied within reasonable limits. The model is, however, found to be strongly sensitive to cavity : matrix surface energy, as well as the rate that helium atoms are displaced from dislocations.