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Heterogeneous void formation in 14 MeV nickel-ion irradiated 316 SS
Abstract
The microstructure of type 316 austenitic stainless steel has been investigated following 14 MeV Ni-ion irradiation to 3.3× 1016 ions/cm2 (40 dpa peak damage) at temperatures from 450°C to 650°C. Specimens were prepared in cross section to allow analysis over the entire damage range, and the void parameters, dislocation structure, and precipitation response are reported. Heterogeneous void formation was observed in the absence of gas. The swelling curve from this study has been compared to that derived from helium preinjection work. It was found that helium shifts the peak swelling temperature upwards in excess of 100°C for similar damage rates. The precipitation response shows a marked change from small block/rod morphology at 450-550°C to Fe2P thin lathe precipitates at 600-650°C. Void formation was detected only for the lower temperature range and was associated with the small block/rod precipitates.
... The distribution becomes bimodal, which explains the decrease in mean size. As small cavities are not present at 40 dpa and precipitation starts close to 40 dpa, it is assumed that cavities have nucleated on precipitates after the nucleation of the latter [54]. ...
The effect of injected interstitials on loop and cavity microstructures is investigated experimentally and numerically for 304L austenitic stainless steel irradiated at 450°C with 10 MeV Fe5+ ions up to about 100 dpa. A cluster dynamics model is parametrized on experimental results obtained by transmission electron microscopy (TEM) in a region where injected interstitials can be safely neglected. It is then used to model the damage profile and study the impact of self-ion injection. Results are compared to TEM observations on cross-sections of specimens. It is shown that injected interstitials have a significant effect on cavity density and mean size, even in the sink-dominated regime. To quantitatively match the experimental data in the self-ions injected area, a variation of some parameters is necessary. We propose that the fraction of freely migrating species may vary as a function of depth. Finally, we show that simple rate theory considerations do not seem to be valid for these experimental conditions.
... The work of Sindelar et al. used 14MeV nickel ion beams to examine void formation in 316L steel. [8,9] Based in part on this work, we performed our implantations at 400, 500, and 600˚C using a 20MeV nickel ion beam with a flux of (400-500nAcurrent/3mm2) The choice of the energy of the ion beam affects the current available for the implantation and the depth of the implantation. An implantation depth of 3-5μm is sufficient for our experiments. ...
The goal of this LDRD project is to develop a rapid first-order experimental procedure for the testing of advanced cladding materials that may be considered for generation IV nuclear reactors. In order to investigate this, a technique was developed to expose the coupons of potential materials to high displacement damage at elevated temperatures to simulate the neutron environment expected in Generation IV reactors. This was completed through a high temperature high-energy heavy-ion implantation. The mechanical properties of the ion irradiated region were tested by either micropillar compression or nanoindentation to determine the local properties, as a function of the implantation dose and exposure temperature. In order to directly compare the microstructural evolution and property degradation from the accelerated testing and classical neutron testing, 316L, 409, and 420 stainless steels were tested. In addition, two sets of diffusion couples from 316L and HT9 stainless steels with various refractory metals. This study has shown that if the ion irradiation size scale is taken into consideration when developing and analyzing the mechanical property data, significant insight into the structural properties of the potential cladding materials can be gained in about a week.
Description
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This two-volume set is a collection of research results of radiation effects in structural materials and nuclear fuels. Complex topics are covered in detail with supporting evidence.
Volume I — 50 papers on: microstructures; gas effects; radiation-induced segregation or phase changes; micro structural modeling; fundamental defect behavior; and special measurement techniques.
Volume II — 39 papers on: pressure vessel steels; mechanical properties; irradiation creep and swelling; damage facilities and dosimetry; breeder core materials; and fuels and ceramics.
Radiation induced segregation (RIS) on lattice defects and phase stability under irradiation have been investigated in an optimized solution annealed 316L(N) with an increased content of N and a significant amount of Nb. Primary Z-phase nitrides were observed within the as-received microstructure. 5 MeV Fe³⁺ ion irradiation was conducted at 450°C up to 1dpa. Post-ion irradiation characterization was performed via nanoscale techniques coupling and correlation to associate radiation induced defects such as cavities and dislocation loops to the local chemistry. Cr, Fe and Ni observed RIS trends were expected, but P was solely measured at the vicinity of cavities. Void-precipitate association has been detected thanks to transmission electron microscopy (TEM) and it is believed to explain the low reduced atomic density measured at the location of a feature within an atom probe tomography (APT) volume. Nb-rich novel features were observed after irradiation and first results are brought to establish a link between these features and the primary Z-phase.
Ion irradiations have been performed at 450 °C on 304 and 304L austenitic stainless steels representative of PWR internals structure up to high doses (close to 100 dpa). TEM and APT have been carried out on irradiated samples. Depending on the dose, Frank loops, dislocation network, cavities, precipitates and segregations have been observed in both 304 steels grades. Their evolution with dose, and in particular for cavities and corresponding swelling, is described and potential microstructural differences between materials are highlighted. It appears that swelling is limited even up to high doses in both materials due to only slight evolution of cavity density and size from intermediate to high doses. As the irradiation temperature has been chosen to simulate PWR microstructure taking into account a temperature/flux shift effect, the microstructural results are discussed and compared with those of PWR.
Thin foils of 316L were irradiated in situ in a Transmission Electron Microscope with 4 MeV Au ions at 450 degrees C and 550 degrees C. Similar irradiations were performed at 450 degrees C in FeNiCr. The void and dislocation microstructure of 316L is found to depend strongly on temperature. At 450 degrees C, a dense network of dislocation lines is observed in situ to grow from black dot defects by absorption of other black dots and interstitial clusters whilst no Frank loops are detected. At 550 degrees C, no such network is observed but large Frank loops and perfect loops whose sudden appearance is concomitant with a strong increase in void density as a result of a strong coupling between voids and dislocations. Moreover, differences in both alloys microstructure show the major role played by the minor constituents of 316L, increasing the stacking fault formation energy, and possibly leading to significant differences in swelling behaviour.
Thin foils of 316L were irradiated in situ in a Transmission Electron Microscope with 4 MeV Au ions at 450 °C and 550 °C. Similar irradiations were performed at 450 °C in FeNiCr. The void and dislocation microstructure of 316L is found to depend strongly on temperature. At 450 °C, a dense network of dislocation lines is observed in situ to grow from black dot defects by absorption of other black dots and interstitial clusters whilst no Frank loops are detected. At 550 °C, no such network is observed but large Frank loops and perfect loops whose sudden appearance is concomitant with a strong increase in void density as a result of a strong coupling between voids and dislocations. Moreover, differences in both alloys microstructure show the major role played by the minor constituents of 316L, increasing the stacking fault formation energy, and possibly leading to significant differences in swelling behaviour.
Helium produced in materials by (n,..cap alpha..) transmutation reactions during neutron irradiations or subjected in ion bombardment experiments causes substantial changes in the response to displacement damage. In particular, swelling, phase transformations and embrittlement are strongly affected. Present understanding of the mechanisms underlying these effects is reviewed. Key theoretical relationships describing helium effects on swelling and helium diffusion are described. Experimental data in the areas of helium effects on swelling and precipitation is reviewed with emphasis on critical experiments that have been designed and evaluated in conjunction with theory. Confirmed principles for alloy design to control irradiation performance are described.
A model is presented that determines the effect of oxygen and helium on the energies of the common vacancy-cluster configurations, namely the void, dislocation loop and stacking-fault tetrahedron. Representative calculations are given for the case of irradiated copper. The presence of oxygen tends to enhance the stability of the void during nucleation compared to the other vacancy-cluster morphologies by decreasing the void surface energy through a chemisorption process. Helium also tends to stabilize void formation, because of the high binding energy of helium to a vacancy (or cluster of vacancies) compared to a dislocation. Gas concentrations as low as 0. 01 a. p. p. m. He and 5 a. p. p. m. O are predicted to stabilize void formation in copper at certain temperatures. The predictions of the model are in good agreement with the available experimental results.
Mechanisms by which oxygen affects swelling during Fe-ion irradiation are investigated in a series of Fe-15Ni-13Cr-base alloys containing minor additions of titanium, silicon and carbon. The irradiations are carried out at 948 and 1000 K to doses up to 100 dpa. Effects of both residual oxygen content and accelerator-injected oxygen are studied. It is found that pre-injected oxygen promotes cavity swelling and, in some cases, leads to a fine distribution of stable bubbles for oxygen injection levels as high as 1000 at.p.p.m. The results are interpreted in terms of the chemical activity of oxygen in forming precipitates with alloying elements, and in terms of radiation dissolution of oxygen leading to oxygen-pressurized bubbles. Accounting for the interaction with alloying elements, we conclude that oxygen exhibits a direct cavity-pressurization effect on swelling similar to that previously reported for helium. The results are analyzed in terms of the theory of the critical radius and critical number of gas atoms, together with Gibbs free energies or precipitate formation.
The extensive literature on oxygen chemisorption and solubility in metals is briefly reviewed, with special emphasis on the reduction of surface tension associated with oxygen adsorption. A thermodynamic model based on the adsorption equations of Gibbs and Langmuir is developed to determine the relative stability in the presence of oxygen of the void compared to the dislocation loop and stacking fault tetrahedron. Representative calculations are performed for copper, nickel, and austenitic stainless steel. Atomistic and elastic continuum calculations predict that void formation should not occur in most pure face-centered cubic metals during quenching or irradiation. However, the thermodynamic model predicts that oxygen concentrations of 30 to 1000 appm will stabilize void formation in copper, nickel, and stainless steel. Foils of copper and several Fe-Cr-Ni stainless steels containing various amounts of oxygen have been examined with electron microscopy following ion bombardment. The presence of 30 to 1000 appm O resulted in significant amounts of void formation, whereas no voids were observed in low-oxygen specimens, in agreement with the model predictions. Oxygen introduced by ion implantation was more effective in promoting void formation than residual oxygen. Solutes such as phosphorus in stainless steel reduced the effectiveness of oxygen as a void-stabilizing agent.
It is a well-known fact that the total dislocation density that evolves in irradiated metals is a strong function of irradiation temperature. The dislocation density comprises two components, however, and only one of these (Frank loops) retains its temperature dependence at high fluence. The network dislocation density approaches a saturation level which is relatively insensitive to starting microstructure, stress, irradiation temperature, displacement rate and helium level. The latter statement is supported in this paper by a review of published microstructural data. A model has been developed to explain the insensitivity to many variables of the saturation network dislocation density in irradiated metals. This model also explains how the rate of approach to saturation can be sensitive to displacement rate and temperature while the saturation level itself is not dependent on temperature.
A theory of void nucleation in metals has been modified to incorporate the effects of non-ideality in the inert gas. The mechanisms of nucleation are unchanged, but the critical void size has increased and critical gas content for spontaneous nucleation has decreased considerably, and the process aided. Gas-assisted nucleation becomes easier, and homogeneous nucleation is unaffected.
Fusion reactor helium generation rates in stainless steels are intermediate to those found in EBR-II and HFIR, and swelling in fusion reactors may differ from the fission swelling behavior. Advanced titanium-modified austenitic stainless steels exhibit much better void swelling resistance than AISI 316 under both EBR-II (up to ~120 dpa) and HFIR (up to ~44 dpa) irradiations. The stability of fine titanium carbide (MC) precipitates plays an important role in void swelling resistance for the cold-worked titanium-modified steels irradiated in EBR-II. Furthermore, increased helium generation in these steels can (a) suppress void conversion, (b) suppress radiation-induced solute segregation (RIS), and (c) stabilize fine MC particles, if sufficient bubble nucleation occurs early in the irradiation. The combined effects of helium-enhanced MC stability and helium-suppressed RIS suggest better void swelling resistance in these steels for fusion service than under EBR-II irradiation.
The rate theory of irradiated metals, as developed by Bullough and Brailsford, is used to characterize the microstructure of a material in which gas-filled cavities exist. The growth dynamics of one such cavity is then carefully analyzed.The critical radius concept discussed by Hayns and Russell has been refined in terms of the “blocking point” of Lifschitz and Slyozov; this permits a detailed tracing of the transition from equilibrium bubble growth to the displacement-limited case. Use of Van der Waals equation of state for helium gas in the cavity has a surprisingly large effect on the dynamics of smaller cavities. Comparison is made between the theoretical blocking point radius and the experimental values of critical radius in stainless steel based on the helium inventory method. Good agreement is found below 650°C, but high temperatures lead to serious disagreements.
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 hydorgen (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 cacity growth are established; at higher doses swelling is determined by cacity growth, which is dependent on dose only. The gases cause copious formation of cavities on grain boundaries, boding ill for mechanical properties.
Cavity (void) formation and swelling in non-fissile materials during neutron irradiation and charged particle bombardments are reviewed. Helium is the most important inert gas and is primarily active as a cavity nucleant. It also enhances formation of dislocation structure. Preimplantation of helium overstimulates cavity nucleation and gives a different temperature response of swelling than when helium is coimplanted during the damage process. Helium affects, and is affected by, radiation-induced phase instability. Many of these effects are explainable in terms of cavity nucleation on submicroscopic critical size gas bubbles, and on the influence of the neutral sink strength of such bubbles. Titanium and zirconium resist cavity formation when vacancy loops are present.
The theory of swelling is reviewed in terms of basic concepts and simulation and impurity effects. The basic theory employs the formalism of chemical reaction rates. Efficiencies of voids, dislocations, and other extended defects for absorption of vacancies and interstitials have been derived. Phenomena such as void coalescence due to void growth and the effects of gas entrapped in voids have been modeled. Important questions, such as the dose dependence of swelling in various possible regimes, have been answered. The theory has been further developed to describe the effects associated with simulation of reactor swelling by charged particle bombardment. These include the temperature shift of swelling with changes in dose rate and the changes in swelling rate due to ion injection, the presence of nearby surfaces, and different modes of point defect generation. Impurities may have dramatic effects on swelling. Impurity trapping of point defects and some aspects of impurity segregation are understood theoretically. Improvements in the theory are possible, particularly in conjunction with experimental work. The more important areas are: additional mechanisms of impurity action, evolution of dislocation density, capture efficiencies of voids and other sinks, and the effects of gas other than in simply pressurizing cavities. From the theory, quantitative predictions of swelling have been made utilizing parameters obtained from microstructural measurements on the same material at lower doses.
A cross-section technique was developed and employed to observe 14 MeV nickel-ion damage to Type 316 stainless steels. Solution-quenched 316 SS and the P-7 alloy were irradiated at a flux of to fluences up to . The heavy-ion irradiation of the P-7 at 650°C produced a unique damage state. A near-surface void-free zone was observed. In addition, a bi-modal void distribution was produced at the calculated peak of the damage distribution.
Helium has been shown to cause major changes in the radiation effects response of metals and alloys. Examples are described. Physical mechanisms underlying these effects are discussed in terms of the theory of radiation effects. An extended treatment of the critical cavity radius concept is developed. Several new analytical results are presented. Applications are reviewed covering the temperature extension of swelling by gas, the evolution of bimodal cavity size distributions and the necessity for gas as a prerequisite to swelling under some conditions. The effect of helium on the dose dependence of swelling is described in terms of its effects on the balance of point defect sink strengths. Mechanisms underlying the enhanced formation and growth of cavities on precipitates when helium is present are discussed. A proposal is described to explain recently observed large effects of helium on phase stability.
The development of microstructure as a function of the irradiation dose has been studied for thick foils of copper irradiated by 500 keV copper ions. A comparison between the results obtained with degassed samples and those for non-degas-sed samples has allowed us to show that the gases dissolved in the metal aided the nucleation of interstitial loops. The relationships between the interstitial loops and voids were studied, and some conclusions drawn concerning the growth of voids and the effect of free surfaces.ZusammenfassungDie Veränderung des Gefüges in Abhängigkeit von der Bestrahlungsdosis wurde an dicken Kupferfolien durchgeführt, die mit 500 keV-Kupferionen bestrahlt worden waren. Durch Vergleich der Ergebnisse an entgasten und nicht entgasten Proben konnte gezeigt werden, dass die im Material gelösten Gase die Keimbildung von Zwischengitterversetzungsringen fördern. Der Zusammenhang zwischen diesen Versetzungsringen und den Poren wird untersucht; einige Folgerungen, die sich auf das Porenwachstum und den Einfluss freier Oberflächen beziehen, werden gezogen.
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 extensive experimental data base on irradiated austenitic alloys reveals that swelling as a function of dose can be divided into an initial transient period of low swelling rate, followed eventually by a rate of about 1%/dpa. Whereas the transient depends strongly on microstructure, temperature, and composition, the final rate of swelling is nearly independent of these variables. Models of void nucleation and growth are reviewed to demonstrate that they provide theoretical results which are in general agreement with the basic features of the observed swelling behavior. According to these models, the transient period comprises two regimes, one period of nucleation to obtain the void number density at a given irradiation temperature plus a period to reach parity between the dislocation and the void sink strength. The universal swelling rate eventually achieved is characterized by a state of sink parity.
Radiation-Induced Voids in Metals
- Nelson