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.     

Swelling as a consequence of gamma prime (γ') and M23(C,Si)6 formation in neutron irradiated 316 stainless steel

An unexpected ordered phase, γ', found to be nominally Ni₃Si, was identified during the investigation of 20% CW 316 stainless steel irradiated to high neutron fluences. The γ' forms at temperatures below ~525°C and increases in volume function tion with fluence. While the emergence of this phase is coincident with the onset of substantial swelling, there is no direct association of voids and γ' particles. At temperatures higher than the γ' regime, other precipitates, including M₂₃(C, Si)₆ predominate. Void formation is also coincident with the emergence of the M₂₃(C, Si)₆ phase and usually involves a void-precipitate association. Formation of these precipitates depletes the austenite matrix of silicon and possibly other eleme which exert a strong influence on void nucleation. A model is presented which relates the high relative partial diffusivity silicon to a depression of the void nucleation rate. Removal of silicon from the matrix then leads to an enhanced nucleat rate.     

Void Swelling in Solution Treated, 20% Cold-Worked and 20% Cold-Worked+Aged Ti-Modified 316 Steel

Void swelling in 0.045%Ti-modified 316 stainless steel was investigated by 200-keV C⁺ ion bombardment. Three metallurgical conditions, i.e. solution treated, 20% cold-worked and 20% cold-worked + aged at 898 K for 1,000 h, were compared. Solution treated materials showed considerably low swelling, while the other two conditioned materials showed much higher swelling. The swelling for 20% cold-worked + aged materials was the highest. In solution treated condition, a number of fine-scale TiC precipitates appeared uniformly in the irradiated matrix. The precipitates and also solute Ti and C are considered to suppress void nucleation by vacancy trapping mechanism. In 20% cold-worked condition, high density dislocation networks were observed being heavily decorated dy TiC precipitates, leading to void swelling increase as a consequence of reduced dislocation sink strength and of depletion of solute Ti and C in matrix. In 20% cold-worked + aged condition, as a result of solute depletion by precipitation of various type precipitates and partial recovery of cold-worked dislocation structure, an enhancement of void swelling occurred. The 0.045% Ti-modified 316 steel was found to be more swelling resistant than type 316 steel.     

Void-swelling in solution-treated FV548 steel irradiated in a high-voltage electron microscope

A study has been made of void formation and growth in 1150°C solution-treated FV548 steel irradiated with 1 MeV electrons in the Harwell AEI EM7 high-voltage microscope (HVM). Voids are observed to form in the temperature range 200–650°C and measurements have been made of the void numbers and sizes and associated swelling as a function of temperature. Over most of the temperature range the void numbers increase rapidly to a maximum value while the voids grow continuously throughout irradiation. A particularly interesting feature of the results is that the variation in void numbers with irradiation temperature is slight up to 550°C and then the numbers decrease markedly at higher temperatures. The associated swelling increases linearly with dose and the swelling rate is a function of temperature.The present results are compared and contrasted with those from other simulation experiments on stainless steels. The most prominent distinguishing features in terms of the influence exerted by gas and by the damage process occurring during irradiation on void nucleation and growth are identified and discussed.     

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.     

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.     

Quantitative modelling of swelling in M316 steel

The chemical rate theory is widely used to describe the microstructural evolution in a material during irradiation. We describe recent improvements to the theory that increase its predictive capability. The incubation dose prior to the onset of void swelling is modelled by allowing partition of the gas between the various sinks in the microstructure. New dislocation and void sink strengths have been derived incorporating the field effects. These improvements to the theory have been incorporated into a new FACSIMILE computer code designated VS5. The new code has been successfully employed to model void swelling during HVM, VEC and fast reactor irradiation of 316 steel.     

The effects of cold working and pre-irradiation heat treatment on void formation in neutron-irradiated type 316 stainless steel

The effect of fast-neutron irradiation on void formation in Type 316 stainless steel having undergone specific thermalmechanical treatments was investigated by transmission electron microscopy. The study showed that, for irradiation at the three lower temperatures (420, 475 and 580°C): (1) the void volume decreased with increasing cold work; (2) the reduction in swelling was due to a decrease in both void-number density and void size; (3) the decrease in void size with increasing cold-work level was enhanced at higher irradiation temperatures; (4) cold working from 0 to 10% decreased the voidnumber density, and void volume, more than in the range from 10 to 20%; (5) void formation in the 20% cw steel which had been heat treated 100 h at 650°C before irradiation was similar to that of the solution-treated steel. The temperature dependence of swelling of the cold-worked material was different from that of the solution-treated steel. Irradiation at 650°C resulted in a larger void volume in the cold-worked material than for irradiation at 475 or 580°C. The effects of cold work and irradiation temperature on void growth are consistent with the predictions of a diffusion-controlled model.     

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。     

Void growth and its relation to intrinsic point defect properties

Some of the many factors that can influence the formation and growth of voids in irradiated materials are reviewed. The interdependence of the void growth and the total evolving microstructure is emphasized and the appropriate rate theory outlined. The importance of saturable traps (solute atoms, coherent precipitates and sessile dislocations) for the intrinsic point defects is highlighted. Finally, the overall state and relevance of our theoretical understanding of the void swelling phenomenon is discussed and useful avenues for future research are indicated.     

双束同时辐照低活性Fe-Cr-Mn(W、V)合金微观组织损伤的影响

研究了时效热处理低活性Fe Cr Mn(W、V)钢双束同时辐照损伤行为 ,结果表明 :92 3K/ 3 0 0 0h时效合金 ,经单独电子辐照 (1 0a- 1)出现低密度空洞 ,而经双束同时辐照的时效合金 ,在辐照初期就形成间隙型位错环和微小空洞。与无时效合金相比 ,随时效温度增加 ,空洞尺寸、空洞密度和空洞肿胀量增大。随时效温度的提高碳化物析出数量增多 ,奥氏体中合金元素Cr、Mn、W、V降低 ,He的存在有效地促进空洞肿胀量增大。     

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.     

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.     

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.     

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.     

Void swelling in a 9Cr ferritic/martensitic steel irradiated with energetic Ne-ions at elevated temperatures

In this work the void swelling behavior of a 9Cr ferritic/martensitic steel irradiated with energetic Ne-ions is studied. Specimens of Grade 92 steel (a 9%Cr ferritic/martensitic steel) were subjected to an irradiation of ²⁰Ne-ions (with 122 MeV) to successively increasing damage levels of 1, 5 and 10 dpa at a damage peak at 440 and 570 °C, respectively. And another specimen was irradiated at a temperature ramp condition (high flux condition) with the temperature increasing from 440 up to 630 °C during the irradiation. Cross-sectional microstructures were investigated with a transmission electron microscopy (TEM). A high concentration of cavities was observed in the peak damage region in the Grade 92 steel irradiated to 5 dpa, and higher doses. The concentration and mean size of the cavities showed a strong dependence on the dose and irradiation temperature. Enhanced growth of the cavities at the grain boundaries, especially at the grain boundary junctions, was observed. The void swelling behavior in similar 9Cr steels irradiated at different conditions are discussed by using a classic void formation theory.     

Radiation-induced formation, annealing and ordering of voids in crystals: Theory and experiment

Void ordering has been observed in very different radiation environments ranging from metals to ionic crystals bombarded with energetic particles. The void ordering is often accompanied by a saturation of the void swelling with increasing irradiation dose, which makes an understanding of the underlying mechanisms to be both of scientific significance and of practical importance for nuclear engineering. We show that both phenomena can be explained by the original mechanism based on the anisotropic energy transfer provided by self-focusing discrete breathers or quodons (energetic, mobile, highly localized lattice solitons that propagate great distances along close-packed crystal directions). The interaction of quodons with voids can result in radiation-induced “annealing” of selected voids, which results in the void ordering under special irradiation conditions. We observe experimentally radiation-induced void annealing by lowering the irradiation temperature of nickel and copper samples pre-irradiated to produce voids or gas bubbles. The bulk recombination of Frenkel pairs increases with decreasing temperature resulting in suppression of the production of freely migrating vacancies (the driving force of the void growth). On the other hand, the rate of radiation-induced vacancy emission from voids due to the void interaction with quodons remains essentially unchanged, which results in void dissolution. The experimental data on the void shrinkage and void lattice formation obtained for different metals and irradiating particles are explained by the present model assuming the quodon propagation length to be in the micron range, which is consistent with independent data on the irradiation-induced diffusion of interstitial ions in austenitic stainless steel.     

Effect of helium on void formation in nickel

This study examines the influence of helium on void formation in self-ion irradiated nickel. Helium was injected either simultaneously with, or prior to, the self-ion bombardment. The void microstructure was characterized as a function of helium deposition rate and the total heavy-ion dose. In particular, at 575°C and 5 × 10^?3 displacements per atom per second the void density is found to be proportional to the helium deposition rate. The dose dependence of swelling is initially dominated by helium driven nucleation. The void density rapidly saturates after which swelling continues with increasing dose only from void growth. We conclude that helium promotes void nucleation in nickel with either helium implantation technique, pre-injection or simultaneous injection. Qualitative differences, however, are recognized.     

Microstructural stability of an HT-9 fuel assembly duct irradiated in FFTF

To explore whether the known resistance of fully tempered HT-9 to neutron-induced phase instability and void swelling are maintained under realistic time-dependent reactor operating conditions, the radiation-induced microstructure of an HT-9 ferritic/martensitic hexagonal duct was examined following a 6-year irradiation campaign of a fuel assembly in the Fast Flux Test Reactor Facility (FFTF). Microscopy examination was conducted on specimens irradiated to 4 dpa at 505 °C, 28 dpa at 384 °C and 155 dpa at 443 °C where quoted temperatures are the average operating temperatures over the lifetime of the duct.The dislocation and phase microstructure were observed to remain relatively unchanged at 4 dpa at 505 °C, but significant microstructural changes were observed to have occurred at 28 and 155 dpa and 384 and 443 °C respectively. At these doses the microstructures have experienced precipitation and formation of interstitial loops. In addition, void swelling had occurred at 155 dpa with an average swelling of ∼0.3%, although some local areas swelled as much as 1.2%. In general it appears that this alloy retains its swelling resistance under typical reactor operation conditions up to 155 dpa.