Peak Swelling Temperature Research Articles

Irradiation hardening and void swelling behaviors of F82H IEA by using dual-ion irradiation

ABSTRACT The irradiation hardening behavior and the void swelling behavior of the Japanese reduced-activation ferritic/martensitic steel, i.e. F82H IEA heat, were investigated by using single- and dual-ion irradiation experiments in this study. For irradiation hardening of F82H, a map of the hardness ratio before/after ion irradiation was obtained for 270–500 °C up to 80 dpa. The peak temperature of irradiation hardening was at about 350°C. On the other hand, it was shown that an extra hardening occurred in helium co-implanted F82H for 400–430 °C except for the temperatures less than 400 °C and more than 450 °C. For the temperature dependence of void swelling in helium co-implanted F82H, it was confirmed that the peak swelling temperature was identified at about 470 °C. That amount of swelling (average value) was 0.4% at 20 dpa.

Effect of dpa rate on the temperature regime of void swelling in ion-irradiated pure chromium

Pure chromium is a promising candidate for coating of Zircalloy fuel rods in light water-cooled power reactors to avoid or delay hydrogen generation in accident scenarios. Void swelling of chromium is one possible contributor to coating-interface failure and needs to be studied. The effect of dpa rate on void swelling of Cr was studied using 5 MeV Fe ion irradiation to 15 peak dpa at peak dpa rates of 3.5 × 10−5, 3.5 × 10−4, and 3.5 × 10−3 dpa/s, at six temperatures between 350 °C and 650 °C. The post-transient (steady-state) swelling rate of pure Cr is ∼0.05%/dpa. Swelling in Cr also appeared to start at a much higher swelling rate at very low doses. The observed dependence of peak swelling temperature on dpa rate agrees well with earlier theoretical models and clearly demonstrated the well-known “temperature shift” phenomenon. After determination of the peak swelling temperatures under ion irradiation at three dpa rates we extrapolated downwards to dpa rates characteristic of both sodium-cooled fast reactors and light water-cooled reactors.

Temperature-Dependent Cavity Swelling in Dual-Ion Irradiated Fe and Fe-Cr Ferritic Alloys

Fe-Cr ferritic-martensitic (FM) steels are promising structural material candidates for fusion and advanced fission reactors due to their attractive mechanical properties and volumetric swelling resistance. However, significant discrepancies exist regarding the effect of solutes and irradiation temperature on cavity swelling under ion versus neutron irradiation conditions. In this study, simultaneous dual ion irradiations (8 MeV Ni3+ ions and energy-degraded 3.5 MeV He2+ ions) were used to quantify the cavity swelling behavior in ultra-high purity Fe and Fe-Cr alloys (3-14 wt.% Cr), Fe-10 wt.% Cr-780 wt.ppm C, and Eurofer97 FM steel. The irradiations were conducted over a wide temperature range (400-550 °C) with a mid-range dose of ~30 displacements per atom (dpa) and 0.1 appm/dpa He implantation rate. Using state-of-the-art transmission electron microscopy (TEM), we reveal that pure Fe has a ~50 °C lower peak swelling temperature than Fe-Cr alloys, which is attributed to higher vacancy mobility in pure Fe. Chromium solute appears to strongly suppress cavity swelling in Fe-Cr alloys for temperatures below ~470 °C, but seems to have little effect or slightly enhances swelling above ~470 °C. Cavities were observed in all the irradiated samples between 400-550 °C. This indicates that the narrow temperature range of observable cavities reported in prior ion irradiated Fe-Cr ferritic alloy studies is likely an artefact associated with the use of low ion energies (<5 MeV), which leads to pronounced near-surface and implanted ion effects that suppress cavity swelling even at midrange depths (particularly at high temperatures).

Temperature-dependent cavity swelling in dual-ion irradiated Fe and Fe-Cr ferritic alloys

Fe-Cr ferritic-martensitic (FM) steels are promising structural material candidates for fusion and advanced fission reactors due to their attractive mechanical properties and volumetric swelling resistance. However, significant discrepancies exist regarding the effect of solutes and irradiation temperature on cavity swelling under ion versus neutron irradiation conditions. In this study, simultaneous dual ion irradiations (8 MeV Ni3+ ions and energy-degraded 3.5 MeV He2+ ions) were used to quantify the cavity swelling behavior in ultra-high purity Fe and Fe-Cr alloys (3-14 wt.% Cr), Fe-10 wt.% Cr-780 wt.ppm C, and Eurofer97 FM steel. The irradiations were conducted over a wide temperature range (400-550°C) with a mid-range dose of ~30 displacements per atom (dpa) and 0.1 appm/dpa He implantation rate. Using state-of-the-art transmission electron microscopy (TEM), we reveal that pure Fe has a ~50°C lower peak swelling temperature difference than Fe-Cr alloys, which is attributed to higher vacancy mobility in pure Fe. Chromium solute appears to strongly suppress cavity swelling in Fe-Cr alloys for temperatures below ~470°C, but seems to have little effect or slightly enhances swelling above ~470°C. Cavities were observed in all the irradiated samples between 400-550°C. This indicates that the narrow temperature range of observable cavities reported in prior ion irradiated Fe-Cr ferritic alloy studies is likely an artifact associated with the use of low ion energies (<5 MeV), which leads to pronounced near-surface and implanted ion effects that suppress cavity swelling even at midrange depths (particularly at high temperatures).

Irradiation-induced swelling of pure chromium with 5 MeV Fe ions in the temperature range 450–650 °C

A surface coating using pure chromium has been proposed to increase the accident tolerance of Zircaloy cladding in pressurized water reactors. However, there is not much irradiation experience with Cr alloys and especially pure Cr. In the present study, pure chromium was irradiated with 5 MeV Fe ions to 50 peak dpa (displacements per atom) at temperatures of 450, 500, 550, 600 and 650 °C. Then irradiation at the peak swelling temperature of 550 °C was conducted to 50, 100, 150 peak dpa. Swelling at 50 dpa was observed over the entire temperature range studied, 450–650 °C, but appeared to be decreasing strongly at the temperature boundaries of the experiment. After an initial transient of rapid swelling, chromium was observed to swell at a rate of ~0.03–0.04%/dpa (up to 120 local dpa), which is much lower than pure Fe at 0.2%/dpa. This low swelling rate was found to be relatively insensitive to dpa rate, which varied by a factor of ~2 over the depth of data collection. Swelling was observed to begin quickly with an incubation period less than 10 dpa. Self-organization in the form of void ordering was observed to be developing at 50 dpa, becoming better defined with increasing dose. The void alignment direction is determined to be the 〈111〉 axial direction.

Ion irradiation studies on the void swelling behavior of a titanium modified D9 alloy

The sensitivity of Positron Annihilation Spectroscopy (PAS) for probing vacancy defects and their environment is well known. Its applicability in determination of swelling and the peak swelling temperature was put to test in our earlier work on ion irradiated D9 alloys [1]. Upon comparison with the peak swelling temperature determined by conventional step height measurements it was found that the peak swelling temperature determined using PAS was 50 K higher. It was conjectured that the positrons trapping in the irradiation induced TiC precipitation could have caused the shift. In the present work, D9 alloys have been implanted with 100 appm helium ions and subsequently implanted with 2.5 MeV Ni ions up to peak damage of 100 dpa. The nickel implantations have been carried out through a range of temperatures between 450 °C and 650 °C. The evolution of cavities and TiC precipitates at various temperatures has been followed by TEM and this report provides an experimental verification of the conjecture.

Void swelling in ion irradiated (15Ni–14Cr), Ti-modified stainless steel: A study using positron annihilation and step height measurements

The void swelling behaviour of (15Ni–14Cr) Ti-modified steels simulated by heavy ion irradiation, has been investigated using step height measurements and positron annihilation spectroscopy. 18% cold-worked samples were pre-implanted with helium at 170 keV and 275 keV energies so as to create a uniform helium concentration of 100 appm around the 588 nm depth region. This was followed by a 2.5 MeV Ni ion irradiation to create a peak damage of ~ 84 dpa (displacement per atom) at various irradiation temperatures between 700 and 970 K. The gross swelling which has occurred throughout the ion range is determined by step height measurements. Defect-sensitive positron annihilation lineshape S-parameter measurements show clear changes consequently to void formation as a function of sample depth. From the variation of the average S-parameter as a function of irradiation temperature, the peak swelling temperature has been deduced. The changes observed in step height measurements are compared with positron annihilation measurements and the results are discussed.

Cavity swelling and dislocation evolution in SiC at very high temperatures

The temperature and fluence dependence of cavity swelling and dislocation development in CVD SiC irradiated with fast neutrons at high temperatures (1050–1460 °C, up to 9.6 dpa) were evaluated using transmission electron microscopy. The cavity swelling was very limited below 1300 °C (<0.01% at 1300 °C, 9.3 dpa). Temperature and fluence dependent swelling became visible above ∼1400 °C. The maximum value of the cavity swelling was 0.25% at 1460 °C, 9.6 dpa, but this appeared to be below the peak swelling temperature. Frank loops were the dominant dislocation structure in this temperature regime, and the number density decreased and the size increased with increasing irradiation temperature. The loop microstructures depended less significantly on both the irradiation temperature and fluence below 1200 °C. A significant decrease in the number density and increase in the size were observed at 1300–1460 °C.

Positron beam studies of void swelling in ion irradiated Ti-modified stainless steel

The void swelling behavior of heavy ion irradiated D9 steel has been investigated using variable low energy positron beam. The normalized defect-sensitive S-parameter shows up a large increase in the depth region corresponding to the maximum radiation damage as a function of irradiation temperature. From the variation of S-parameter as a function of irradiation temperature, the peak swelling temperature has been deduced and the results are discussed.

Irradiation creep of low-activation ferritic steels in FFTF/MOTA

Irradiation creep behavior of low-activation steels, developed as structural materials for fusion reactors (JLF series steels), was investigated to obtain a fundamental understanding of these alloys under fast neutron irradiation in FFTF. (2.25–8)Cr(1–2)W bainitic steels and 12Cr-2W ferritic steels showed superior creep resistance to type-316 stainless steels under fast neutron irradiation up to 520°C. At temperatures below 460°C the creep strain increased with increasing Cr content up to 7 Cr, and further increments of Cr content up to 12% reduced the creep strain. At temperatures between 460 and 600°C, 7–8 Cr ferritic steels showed the largest creep strain. Swelling-enhanced creep, near the peak swelling temperature of 410°C, was also observed. The 9Cr-2W ferritic steel JLF-1 presented excellent properties, suggesting it as a leading candidate alloy for structural components of fusion reactors.

Dislocation climb and interstitial loop growth under cascade damage irradiation

The effects of intracascade clustering and recombination in radiation damage have been considered previously in semiquantitative calculations involving vacancy accumulation at voids, within the concept of production bias. To model void swelling and microstructural evolution quantitatively, similar effects on dislocation climb and interstitial loop growth have to be considered. In this regard, at elevated temperatures (such as in the peak-swelling temperature regime), the concentration of freely migrating vacancies is much higher than that of the interstitials, owing to the evaporation from the primary vacancy clusters (i.e. those produced by intracascade clustering). It is not immediately obvious how the dislocations can be net interstitial sinks, and hence that the observed nucleation and growth of the interstitial loops at elevated temperatures can be correctly predicted as in the conventional theory.

Production bias and void swelling in the transient regime under cascade damage conditions

Abstract Molecular dynamics (MD) studies of collision cascades have firmly established that, in addition to clusters of vacancies, clusters of self-interstitial atoms (SIAs) are formed within the cascade volume during the thermal spike phase of a cascade. These clusters are formed in a segregated fashion such that the vacancy-rich core is surrounded by SIA clusters. At temperatures above stage V the vacancies evaporate from the vacancy cluster and diffuse into the medium whereas the SIA clusters remain thermally stable at temperatures even above the peak swelling temperature. This asymmetry in the production of free and mobile vacancies and SIAs gives rise to a production bias. Some of the vacancies evaporating from the vacancy-rich core annihilate at the SIA clusters surrounding it and others escape into the medium. It is this escaping fraction of vacancies which determines the strength of the production bias. Diffusion calculations have been performed to estimate the magnitude of this escaping fraction ...

Microstructural evolution of titanium modified austenitic stainless steel after FFTF irradiation

Abstract The titanium-modified austenitic stainless steel (JPCA) and the standard type 316 steel were neutron irradiated in the FFTF/MOTA up to about 40 dpa at 370°C to 600°C. Microstructural examination proved the superior resistance to void swelling of the titanium-modified steel. Comparison with the results of HFIR irradiation experiments from the JAERI/ORNL collaboration showed the temperature dependence of void swelling to be significantly different for JPCA irradiated in FFTF. The peak swelling temperature in FFTF irradiation was below 420°C, which was about 100°C lower than that found during HFIR irradiation. Such a difference in swelling behavior would appear to be due to the effects of the significant difference in helium production rates on cavity nucleation and bubble to void conversion process.

Heterogeneous void formation in 14 MeV nickel-ion irradiated 316 SS

The microstructure of type 316 austenitic stainless steel has been investigated following 14 MeV Ni-ion irradiation to 3.3× 10 16 ions/cm 2 (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 Fe 2P 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.

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

In-situ observations of the cavity growth process in dual-ion irradiated HT-9 ferritic stainless steel were performed using HVEMs. The objective of this study is to analyze the cavity growth mechanisms as functions of cavity size and irradiation temperature. Dual-ion irradiations of the HT-9 produced a bi-modal cavity microstructure only at a peak swelling temperature of 743 K. And subsequent electron irradiation at 623 K caused growth of cavities larger than the critical radius. On the other hand after electron irradiation at 573 K a drastic decrease of the critical radius and growth of all the cavities produced by the dual-ion irradiation was observed. These results provide clear evidence of bias-driven and pressure-driven cavity growth mechanisms in ferritic stainless steels, and yield the temperature dependence of the critical radius. A different temperature dependence of swelling between ion irradiation and electron irradiation is found.

Temperature aspects of pulsed ion bombardment in an austenitic alloy

The influence of pulsed ion irradiation to a damage level of 10 dpa at temperatures from 843 to 1023 K has been studied in a quaternary Fe-17% Cr-17% Ni-2.5% Mo austenitic alloy. Pulsing periods of either 60 s on/60 s off or 1 s on/1 s off were used, together with continuous bombardment as a control. Helium (200 appm) was supplied either by simultaneous or room temperature preimplantation, or omitted. While most specimens experienced only 2 to 3 K temperature fluctuations with the beam pulses, certain specimens were deliberately allowed to undergo 15 to 50 K variations. Pulsed irradiation effects were found to range from little effect at 843 K, through a notable modification in the relationship between helium and cavity parameters at 938 K (near the peak swelling temperature), to a substantial reduction of swelling at 1023 K. The influence of the simultaneous temperature excursions compared with constant-temperature pulsed ion bombardment (mostly compared at 843 K) was small. These results are interpreted in terms of the effects of pulsing and helium on the stability of cavities at high temperatures through the critical size and number of gas atoms needed for a small cavity to begin bias-driven void growth.

Anomalous void growth and swelling in materials at low temperatures for cyclic irradiation and background temperature

Peak swelling in irradiated materials occurs in a temperature range of 0.35 – 0.5 T M. Analytic studies using the fully dynamic rate theory for Al and Mo have shown that in materials undergoing temperature oscillations, whether externally applied or induced by cyclic irradiation, swelling is enhanced below the peak, swelling temperature and is retarded above it. Anomalous results seen in those studies for aluminum at low temperatures have Indicated that quite different trends relate swelling seen in steady vs cyclic irradiation far from the peak swelling region. The same behavior has now been seen in calculations done for molybdenum and in an expanded series of calculations done for aluminum. Although well below the peak swelling temperature ( 0.2 T M < T < 0.35 T M ) pulsing is found to either slow void growth or induce void shrinkage. The different physics governing the two regimes has been identified — around the peak swelling temperature vacancies and interstitials are both mobile, responding faster in the higher-temperature perturbations and thus acting as though the material sees a raised ambient temperature; for low temperatures the vacancies are Immobile and the increased absorption of still-mobile interstitials by voids during the higher-temperature excursions dominates.

Effects of C and Nb additions to Fe16Ni15Cr alloy on void swelling

A simulation study has been carried out to Investigate systematically the influence of 0.6 wt.% Nb additions to Fe16Ni15Cr alloys with 0, 0.075 and 0.15 wt.% C on void swelling over temperature ranges from 723 to 873 K by heavy ion irradiations. TEM observations reveal that the addition of Nb and/or C reduces swelling with a downward shift of the peak swelling temperature. It is also shown that Nb in solution is quite effective for swelling suppression.

Proton irradiation damage in cyclically stressed aluminum

High purity aluminum specimens have been irradiated at 50°C with 800 MeV protons using the Los Alamos Meson Physics Facility (LAMPF). Specimens cyclically stressed during irradiation produced only a third as many voids as unstressed control specimens. This is consistent with a model of Green and Weertman which predicts that moving dislocations should be better vacancy sinks than stationary ones. The reduced swelling was observed under the severe conditions of peak swelling temperature and simultaneous He implanatation by transmutation. The detailed mode of dislocation motion responsible for the reduction is still under investigation.

Void growth and swelling for cyclic pulsed radiation

The analysis of Ghoniem and Kulcinski of a single radiation pulse has been extended to include the effects of temperature oscillations and multiple pulses by coupling six simultaneous nonlinear ordinary differential equations for point defect concentrations and sink strengths with a heat equation governing radiation-produced temperature fluctuations. The pulsed irradiation considered is that of the 800 MeV proton beam at the Clinton P. Anderson Meson Physics Facility (LAMPF), and numerical calculations have been done for aluminum and molybdenum. The model material is assumed to have been irradiated to a specific microstructure, and the calculation is begun after the material has returned to thermal equilibrium conditions. Void growth is seen to proceed at a nearly linear rate after initial transients (caused by the assumption of initial thermal equilibrium) in both vacancy concentration and vacancy loop size and number density. The temperature pulses associated with cyclic rather than steady irradiation generally enhance void growth if the ambient temperature is below the material peak swelling temperature, and conversely decrease void growth when the ambient temperature is above peak swelling temperature. The exception seen occurs when the temperature pulse is great enough that thermal emission of vacancies, rather than radiation production, is dominant.