4.04.2.1 Compositional Dependence of Void Swelling

Nimonic PE16 was first identified as a low-swelling alloy in the early 1970s. Void swelling data derived from density measurements on fuel pin cladding mate­rials from the Dounreay Fast Reactor (DFR) were reported by Bramman et al.1 and were complemented by electron microscope examinations described by Cawthorne etal.8 Swelling in STA PE16 was found to be lower than in heat-treated austenitic steels and comparable to cold-worked steels. Comparison of data for PE16 and FV548 (a Nb-stabilized austenitic steel) irradiated under identical conditions in DFR to a peak neutron fluence of ^6 x 1026nm~2 indicated that the lower swelling of PE16 was due to smaller void concentrations at irradiation temperatures up to ^550 °C and reduced void sizes at higher

temperatures. At around the same time, Hudson et al9 compared the swelling behavior of PE16, type 316 steel, and pure nickel, using 20MeV C2 ion irradia­tions in the Harwell VEC (variable energy cyclotron). The materials were implanted with 10 appm (atomic parts per million) ofhelium prior to ion bombardment to peak displacement doses >200 dpa (N/2) at 525 °C. Void swelling in 316 steel and nickel exceeded 10% at the highest doses examined, compared to ^0.5% in PE16. Void nucleation appeared to occur earlier in nickel (at ^0.1 dpa) than in PE16 or type 316 (~2 dpa), but the peak void concentration was higher by a factor of about 10 in the austenitic steel than in nickel or PE16.

Hudson et al.9 originally attributed the swelling resistance of PE16 to the presence of the coherent, ordered face-centered cubic, Ni3(Al, Ti) g0 precipi­tates, which were thought either to trap vacancies and interstitials at their surface, thereby enhancing point — defect recombination, or to inhibit the climb of dis­locations, thereby preventing them from acting as preferential sinks for interstitial atoms. In support of the first ofthese two suggested mechanisms, Bullough and Perrin10 argued that the surface of a coherent precipitate would be a more effective trapping site than an incoherent one where the identity ofthe point defects would immediately be lost (and where, as a consequence, void nucleation was likely to occur). The efficiency of point defect trapping would be expected to be greater the higher the total surface area of the g0 precipitates, that is, to be inversely proportional to the precipitate size at constant volume fraction. On the other hand, the second mechanism proposed by Hudson etal. should be most effective at an intermedi­ate particle size where dislocation pinning is strongest. Support for the latter process was provided by Williams and Fisher11 from HVEM (high-voltage electron microscope) irradiations of PE16 at a damage rate of about 10~2dpas_1 at 600 °C, where the swelling rate was higher at small (3 nm) and large (70 nm) g0 particle diameters than at intermediate sizes of about 20 nm.

However, it is now considered that any effect that the g0 precipitates may have on the swelling resistance of Nimonic PE16 is secondary to that of the matrix composition. The generally low-swelling behavior of Ni-based alloys compared to austenitic steels was shown by Johnston etal.12 following bom­bardment with 5 MeV Ni2+ ions at 625 °C. The dam­age rate in these experiments was 10~2dpas_1 and the displacement dose was originally estimated as 140 dpa but this was subsequently revised by Bates and Johnston13 to 116 dpa (based on displacement
energy Ed = 40 eV). In addition to precipitation- hardened alloys, including PE16 and Inconel 706, this experiment included nonhardenable high-Ni alloys, such as Inconel 600 and Hastelloy X, a range of commercial steels, and Fe-Cr-Ni ternary alloys containing 15% Cr and 15-35% Ni. The alloys were preimplanted with 15 appm helium prior to ion bom­bardment, and the irradiation temperature was chosen as being close to the peak swelling temperature for ion — irradiated austenitic steels. The extent of void swelling was determined by electron microscope examinations in low-swelling alloys, but was estimated from step — height measurements (comparing the surfaces of irra­diated and nonirradiated regions) in high-swelling materials. As illustrated in Figure 1, the results showed negligible swelling (<0.1%) in PE16, Inconel 706, Hastelloy X, and the Fe-15Cr-35Ni ternary alloy, low swelling (<1%) in other high-Ni alloys, but high swelling (generally >20%) in austenitic steels. In com­mercial alloys containing ~18% Cr, minimum swelling occurred at Ni contents of about 40-45%. Although void diameters generally appeared to be smaller in the Ni-based alloys than in austenitic steels, the main factor accounting for reduced swelling was a much lower void concentration. In the ternary alloys, reducing the Ni content from 35% to 30% resulted in

Figure 1 Swelling versus nickel content of commercial alloys and ternary Fe-15Cr-Ni alloys bombarded with Ni2+ ions to a damage level of 116 dpa at 625 °C. Reproduced from Johnston, W. G.; Rosolowski, J. H.; Turkalo, A. M.; Lauritzen, T. J. Nucl. Mater.1974, 54, 24-40.

an increase in overall swelling from <0.1% to ~12%, although it was noted that the 35% Ni alloy showed a localized swelling of ^5% in a region close to a grain boundary. Additional experiments reported by Johnston et a/.12 indicated that the peak swelling tem­perature for PE 16 irradiated with 5 MeV Ni2 ions was 675 °C, but even then, swelling at 116 dpa remained below 0.2%.

Swelling data for a wider range of pure Fe-Cr-Ni austenitic alloys, with Cr contents up to 30% and Ni up to 100%, following Ni ion bombardment to 116 dpa at 675 °C, were reported by Bates and Johnston.1 These results showed a strong depen­dence on both Cr and Ni, with the swelling increas­ing with increasing levels of Cr but being minimized at Ni contents of about 45-60%. Examination of the dose dependence of swelling in ternary alloys con­taining 15% Cr and 20-45% Ni showed that the incubation dose required for the onset of swelling increased with increasing Ni content. Furthermore, although high-swelling rates of the order of 1% per dpa were attained in 20-35% Ni alloys, the swelling rate of the 45% Ni alloy remained low even at doses above 250 dpa.

Following their earlier C2+ ion irradiation experi­ments, Hudson and coworkers moved to the use of

46.5 MeV Ni6+ ions to investigate void swelling behavior. This was considered preferable because the recoil spectra of high-energy Ni ions provided a better simulation of fast neutron damage, and because carbon implantation encouraged the forma­tion of carbides which acted as void nucleation sites. A summary of some of the Ni ion irradiation work carried out by the Harwell group was given by Makin et a/.14 No significant differences in the swelling behavior of Nimonic PE16 were evident between ST or aged conditions. Peak swelling in Ni6+ ion- irradiated PE16 (preimplanted with 10appm He) occurred at 625 °C, where a swelling of ~1.5% was recorded at 120dpa(N/2). Void concentrations in PE16 were reported to be lower by a factor of about 5 than in similarly irradiated type 316 and 321 aus­tenitic steels.

A drawback of charged particle irradiation experi­ments for evaluating void swelling is that the evolu­tion of other microstructural features may differ significantly from that during neutron irradiation (see also Chapter 1.07, Radiation Damage Using Ion Beams). In the case of Nimonic PE16, for exam­ple, the precipitation and/or redistribution of the g0 phase during long-term neutron exposure might be expected to influence swelling behavior. In order to simulate swelling in a more appropriate micro­structure, Bajaj eta/.15 examined the effect of4MeV Ni2 ion irradiation on PE16, which had been pre­conditioned by exposure to neutrons in Experimental Breeder Reactor-II (EBR-II). Reactor-conditioned samples had been exposed to neutron fluences in the range of 3-6 x 1026nm~2 (E > 0.1 MeV) at tem­peratures from 454 to 593 °C. Swelling rates during Ni ion irradiations at 675 °C were higher by a factor ofabout five in reactor-conditioned material than in a nonconditioned sample. The increased swelling rate was attributed to changes in the matrix composition resulting from an increased volume fraction of g0 in the reactor-conditioned material.

Early attempts to account for the effects of matrix composition on void swelling focused on the stability of the austenite phase. Harries16 suggested that the swelling behavior of austenitic steels and nickel — based alloys could be rationalized in terms of their Ni and Cr equivalent contents (i. e., the relative austenite and ferrite stabilizing effects of their con­stituent elements), with the composition of high — swelling alloys then falling into the (g + s) phase field in the Fe-Cr-Ni ternary phase diagram. Harries postulated that the partitioning of solute elements into the sigma phase would have a detrimental effect on the swelling resistance of austenite. Watkin17 took a similar approach, but found that an improved correlation could be obtained using the concept of electron vacancy numbers rather than Ni and Cr equivalents. The average electron vacancy number, Nv of the matrix is calculated from the atomic frac­tions of its constituents, with allowance being made for the precipitation of carbides and g0 (or g00, etc.), and has been widely used to predict the susceptibility of nickel-based alloys to the formation of intermetal­lic phases.18Nv was calculated from:

Nv = 0.66Ni + 1.70Co + 2.66Fe + 3.66Mn + 4.66(Cr + Mo)

Watkin found that void swelling in a range of alloys with Ni contents up to ^43%, which were irradiated in DFR to a peak dose of 30 dpa at 600 °C, remained low for Nv below about 2.5 (corresponding to low susceptibility to s phase formation), but increased approximately linearly at higher Nv. However, as was clearly argued by Bates and Johnston,13 correlations based on sigma-forming tendency could not account for the minimum in swelling observed at about 45% Ni, since higher Ni contents should continue to be beneficial.

A better understanding of the swelling behavior of Fe — and Ni-based alloys resulted from a series of fast neutron irradiation experiments which were carried out in EBR-II in the early 1980s. Irradiation tem­peratures in these experiments ranged from about 400 to 650 °C. Initial data for a range of commercial alloys, including ferritic and austenitic steels, as well as nickel-based alloys, were reported by Bates and Powell19 and Powell eta/.,20 with higher dose data (up to a peak fluence (E > 0.1 MeV) of ^25 x 1026nm~2, corresponding to 125 dpa) being reported by Gelles21 and Garner and Gelles.22 Swelling data for Fe-Cr-Ni ternary alloys, irradiated in EBR-II to a peak fluence of 22 x 1026nm~2 (^110dpa), were presented by Garner and Brager23 The extent of void swelling in these experiments was determined by density change measurements. In general, alloys with nickel contents in the range of 40-50% exhib­ited the lowest swelling. Swelling in commercial nickel-based alloys was generally lower in ST than in aged conditions, this being attributed to the bene­ficial (though temporary) effect of minor elements remaining in solution and being able to interact with point defects19; subsequent precipitation during irra­diation would be expected to reduce this benefit and the resulting densification, though small, would also effectively reduce the measured swelling. Swelling data for a number of ST alloys, which were irradiated in the AA-1 rig in EBR-II, are shown in Figure 2; data are shown for two withdrawals, at peak fluences of 14.7 x 1026nm~2 and 25.3 x 1026nm~2, with mea­surements for Inconel 600 and Inconel 625 reported at both fluence levels, data for Nimonic PE16 and Inconel 706 at the lower level, and data for Incoloy 800 and Hastelloy X at the higher level. The nickel contents of the alloys range from about 34% in Inco- loy 800 to 75% in Inconel 600. Swelling remained relatively low in the three Inconel alloys and in PE16. However, both Incoloy 800 and Hastelloy X exhibited high swelling at some temperatures, with swelling in the latter reaching ^80% at 593 °C. The reason for such high swelling in neutron-irradiated Hastelloy X (nickel content ^48%) is unclear, but it was noted that densification up to 3% occurred at the lower irradiation temperatures — indicating microstructural instability and possibly signaling changes in the com­position of the matrix which may have affected the swelling behavior. (Note that Hastelloy X was identi­fied as a low-swelling alloy in the Ni2 ion irradiation experiments described by Johnston eta/.12)

Some data for different heat-treated conditions of PE16 at the higher fluence level were reported by

Garner and Gelles,22 and are compared for irradia­tions at 538 °C (more or less corresponding to the peak swelling temperature for PE16 in the AA-1 experiment) with lower fluence data from Bates and Powell19 in Figure 3. The heat-treated condi­tions indicated in Figure 3 are ST (ST 4h at 1080 °C), A1 (ST and aged 16 h at 705 °C), A2 (ST and aged 1 h at 890 °C plus 8h at 750 °C), and OA (ST and aged 24 h at 840 °C). Note that the silicon content of the PE16 used in these experiments was much lower at 0.01% than the level of ^0.2% typi­cally found in UK heats of the alloy. Overall, the data appear to show little effect of initial heat treatment on the swelling of PE16, except that the OA condi­tion exhibited the most swelling (5.2%) at the higher fluence.

Although it is clear that the swelling behavior of austenitic alloys is largely dependent on nickel con­tent, there is ample evidence to show that minor solute additions can have significant effects. Much of the work on minor solutes has focused on steels similar to type 316, but some data are available for higher nickel alloys. For example, Mazey and Hanks24 used

46.5 MeV Ni6+ ion irradiations to examine the effects of Si, Ti, and Al additions on the swelling response of model alloys with base compositions approximating that of the matrix phase in PE16. Solute additions of ^0.25% Si or 1.2% Ti reduced swelling, but the addition of ~1.2% Al (in the absence of Si or Ti) markedly increased it. The beneficial effect of Si was believed to arise from its high diffusivity in solution (this is discussed further in Section 4.04.2.2), whereas that of Ti appeared to be related to the formation of Z phase (hexagonal-structured Ni3Ti). The addition of Al resulted in an increase in the concentration of voids, the surfaces of which were coated in a thin layer of the g0 phase (Ni3Al). A beneficial effect of Si on the swelling response of modified Incoloy DS alloys under Ni6+ ion irradiation was also reported by Mazey et a/.25 However, it should be noted that high Si con­tents can give rise to the formation of radiation — induced phases which are enriched with Ni and Si, such as the Ni3Si form of g0 and the silicide G-phase (M6Ni16Si7, where M is usually Ti, Nb, or Mn). G-phase particles are generally found in association with large voids and their formation may therefore give rise to an increase in the swelling rate.26,27

Swelling data derived from density measurements for neutron irradiated, modified Incoloy DS alloys, with Si contents ranging from 0.19 to 2.05% (com­pared to a specified level of 1.9—2.6% in the commer­cial alloy), are compared with data for a ‘PE16 matrix

alloy’ and Nimonic PE16 in Figure 4. The materials were all in ST condition apart from PE16 which was in an STA condition (aged 4h at 750 °C). The alloys were irradiated in the UK-1 rig in EBR-II to fluences in the range of 9-16 x 1026nm~2 (E> 0.1 MeV) at temperatures of ^390-640 °C. These data are pre­viously unpublished except those for STA PE16 (heat DAA 766) which were reported by Boothby.28 Swelling in the modified Incoloy DS alloys generally decreased with increasing Si content. The 0.19% Si

alloy exhibited high swelling at all temperatures with indications of swelling peaks at about 440 and 640 °C. Increased Si levels tended to suppress the high tem­perature swelling peak and reduce the magnitude of swelling at lower temperatures. The PE16 matrix alloy containing 0.24% Si exhibited a high tempera­ture swelling peak but moderate swelling below ^550 °C, suggesting a beneficial effect of Mo (this being the main compositional difference between the PE16 matrix alloy and the modified Incoloy DS alloys)

Figure 4 Void swelling data derived from density measurements for Nimonic PE16, a PE16 matrix alloy, and modified Incoloy DS alloys, irradiated in the UK-1 rig in Experimental Breeder Reactor-II. Unpublished data from Boothby, R. M.; Cattle, G. C. Void Swelling in EBR-2 Irradiated Nimonic PE16 and Incoloy DS; FPSG/P(90)10, with permission from AEA Technology Plc.

at lower temperatures. However, swelling in the PE16 matrix alloy remained significantly higher at all tem­peratures than in STA Nimonic PE16 (containing 0.15% Si), indicating a significant benefit of the g0 forming elements Al and Ti.