Chemical composition of different as-produced Al alloys in wt.%.
A literature review on highly irradiated 5xxx and 6xxx series Al alloys is conducted to understand the expected changes in mechanical properties of high flux reactor (HFR) vessel material in relation with microstructural aspects beyond the current surveillance data to support the HFR Surveillance Program (SURP). It was found that the irradiation swelling in 5xxx series alloys is not a crucial degradation mechanism. Dislocation damage is expected to reach a saturation limit in both 5xxx and 6xxx series alloys at relatively low fast-fluence values (<2 × 1026 n/m2). The damage caused by precipitation of transmutation Si is found to be the dominant mechanism affecting the fracture toughness properties of irradiated 5xxx and 6xxx series Al alloys at high thermal fluence values. Tensile and fracture toughness data collected from the literature up to very high thermal fluences are analyzed in comparison with the available HFR surveillance data to predict the behavior of the HFR vessel material beyond current surveillance data. The observed changes in mechanical properties are classified into four different regimes. The contribution of various irradiation damage mechanisms, namely the displacement damage and transmutation damage, to the evolution of microstructure and mechanical properties is discussed in all four regimes for 5xxx and 6xxx series alloys.
- irradiation defects in aluminum alloys
- displacement damage
- transmutation damage
- Mg2Si precipitates
- fracture toughness
- radiation hardening
Aluminum alloys have a higher tolerance to radiation effects than most other metals when irradiated at ambient temperatures due to its low melting point (
In particular, 5
The components of these reactors can experience a large amount of neutron fluences, up to several 1027 n/m2, during their operational life. For the HFR hotspot, Hotspot is the location on vessel wall where highest neutron fluence is received. Assuming that the irradiation conditions at the HFR hotspot are kept unchanged as they are in 2015.
Hotspot is the location on vessel wall where highest neutron fluence is received.
Assuming that the irradiation conditions at the HFR hotspot are kept unchanged as they are in 2015.
This article is organized as follows. First, a brief review of various irradiation-induced damage mechanisms in Al alloys is presented. Next, the tensile data collected from the literature is analyzed to understand the contributions of various irradiation-induced damage mechanisms to the changes in the mechanical properties of these materials up to high irradiation fluences. Finally, the fracture toughness data from HFR SURP is compared with that of the literature, and the underlying damage mechanisms influencing fracture toughness properties are discussed to explain the suitability of literature data for the prediction of HFR SURP data beyond the current surveillance data.
2. Literature on irradiation effects in Al alloys
A substantial amount of literature is published on the irradiation behavior of Al alloys [2–11]. The available dataset on 6
To help the discussion on differences in irradiation damage mechanisms in different Al alloys, a brief review on the differences in chemical composition and microstructure of 5
|5154 (HFR vessel)||Balance||3.10–3.50||≤0.25||≤0.05||0.15–0.35||≤0.40||≤0.10||0.10–0.20||≤0.20||–|
3. Irradiation-induced damage mechanisms in Al alloys
The damage caused by neutron irradiation is the major degradation mechanism leading to irradiation hardening and embrittlement of Al alloys used in Materials Test Reactors (MTRs). Both thermal and fast neutrons cause damage in Al alloys. Displacement damage by fast neutrons and transmutation damage by both thermal and fast neutrons are the two major damage mechanisms in irradiated Al alloys [3, 9, 10]. The relative contribution of these different damage mechanisms and the resulting impact on the mechanical properties depend on the alloy composition, thermal-to-fast fluence ratio (TFR), irradiation temperature, and other irradiation conditions.
3.1. Displacement damage
As in other metals, displacement damage is initiated by the production of primary knock-on atoms (PKAs) through elastic collision of fast (high energy) neutrons with the Al matrix. The resulting PKAs trigger displacement cascades leading to the formation of lattice vacancies, self-interstitial atoms, and dislocation loops. With increasing irradiation dose, dislocation loops grow and encounter the other loops or dislocation network. When the loops interact with each other, they coalesce and contribute to the increase in network dislocation density. Interaction between individual dislocations and loops also contribute to the network.
The irradiation-induced dislocation density determines the extent of irradiation hardening and embrittlement resulting from displacement damage. It is known from literature that the dislocation density in irradiated metals evolves toward a saturation value with increasing dose . This occurs when the dislocation annihilation rate reaches the value of the production rate. The resulting contribution of displacement damage to irradiation hardening and embrittlement remains nearly constant above the irradiation dose levels at which dislocation density reaches a saturation value. From that point onward, transmutation-produced Si plays a dominant role in contributing to irradiation hardening of Al alloys as discussed further in the next section. A detailed discussion on the evolution of displacement damage in Al alloys can be found in Refs. [9, 15].
3.2. Transmutation damage
Transmutation damage in aluminum can be caused by both fast and thermal neutrons. Fast neutrons produce gaseous products like He and H through (
leading to an increase in Si content with increasing thermal neutron fluence. In most metals, the gaseous transmutation products play a larger role in the development of radiation damage microstructure than nongaseous transmutants. However, Al alloys used in MTRs are different in this respect. Depending upon the thermalization of the neutron spectrum, the solid transmutation product Si can have a stronger effect on radiation damage structure than gaseous transmutation products, as discussed in more detail in the following subsections.
3.2.1. Gaseous transmutation damage
Gaseous transmutation products can have a substantial influence on the radiation damage structure by promoting cavity formation and swelling. Gaseous transmutation products favor cavity nucleation by bubble formation at locations such as grain boundaries and stable particle–matrix interfaces, which otherwise are not suitable for nucleation of pure vacancy clusters.
It should be noted that the resistance to cavity formation and swelling differ between different types of Al alloys even in the presence of similar amounts of gaseous transmutation products. Alloys that promote trapping and recombination of point defects reduce vacancy supersaturation and hence exhibit increased resistance to cavity formation and swelling . For instance, 5052-O and 6061 alloys have an excellent resistance to cavity formation and swelling, when compared to pure Al and grade 1100 alloys. Literature reports  show that the incubation dose for cavity formation of 5052-O alloys, ~ 5 × 1026 n/m2, is about 1000 times that of pure Al. Such strong resistance to cavity formation is imparted to the solute Mg present in the solid solution, which can act as trapping and recombination sites for vacancies and interstitials to reduce vacancy supersaturation . Once the Mg is drawn from solution to form Mg2Si precipitates, the trapping and recombination sites are presumably shifted to these Mg2Si precipitates, whose high spatial density might provide overlapping point-defect capture zones. High concentrations of precipitates are expected to contribute to reduced swelling by trapping gases, making these gases not available for cavity nucleation. Farrell et al.  reported that the radiation swelling in 5052-O is only about 1% at a fast fluence of ~18 × 1026 n/m2. The corresponding thermal fluence value is 31 × 1026 n/m2 with about 7% of transmutation-produced Si. Only sparsely distributed voids are found in 5052-O microstructure at these high fluence values . The contribution of voids to the increase in strength and decrease in ductility of this alloy is found to be negligible at this small amount of swelling . No swelling data was published for 5154-O alloy in these conditions. However, due to the similarity in microstructures of both 5052-O and 5154-O alloys and matching irradiation conditions, a comparable swelling behavior can be predicted in 5154-O alloy at HFR vessel hotspot. Using the swelling data of 5052-O from Farrell et al. , the estimated swelling in 5154-O alloy will be ~0.3% for the projected HFR hotspot fluence values by the end of 2025. From these arguments, it can be concluded that the creation of voids and bubbles in 5
3.2.2. Solid transmutation damage
Transmutation-produced Si by thermal neutrons causes substantial radiation damage in Al alloys. Kapusta et al.  confirmed that the Si-content is a major indicator for the neutron irradiation effects on the basis of postirradiation testing of Al alloys containing 2.12% Si from transmutation. A quick estimate of the production rate of transmutation-produced Si (~0.084 wt.%/year of 270 effective full power days at HFR hotspot) can be obtained by multiplying the thermal fluence with the standard thermal neutron absorption cross section for Al (=230 milli barn (mb)) . The solubility of Si in the Al matrix below 373 K is negligible. Hence, the transmutation-produced Si will either precipitate in elemental form as in pure Al, grade 1100 and 6061 alloys or forms Mg2Si precipitates as in 5
The structure, size, and distribution of these precipitates (Si and Mg2Si) in the microstructure will determine the resulting mechanical properties of irradiated alloys. For a given volume fraction of precipitates in the microstructure, finer precipitates result in higher strength, but lower ductility and fracture toughness properties. The structure of the Mg2Si precipitates in 5052 alloy irradiated to 9.7 × 1026 n/m2 thermal fluence is found to be similar to the thermally aged Mg2Si precipitates in 6
The location of this transmutation-produced Si precipitates in the microstructure will have substantial impact on the mechanical properties of the alloys. In 1100 and 6061 alloys, it was identified that the transmutation-produced Si will precipitate as elemental Si particles, which are uniformly distributed in the matrix and associated with voids . Farrell et al.  reported a noncrystalline Si-coating inside the voids of 1100-O Al alloy at a high thermal fluence (
From the above discussion, it can be concluded that the transmutation-produced Si is the dominant irradiation damage mechanism in 5
4. Discussion on irradiation-induced damage effects on mechanical properties of 5
xxx and 6 xxx series Al alloys
Although fracture toughness data on irradiated Al alloys is scarce, significant data on tensile properties is available in the literature. In this section, tensile data on irradiated Al alloys collected from literature is plotted as a function of thermal fluence to understand the changes in tensile properties with the evolution of irradiation-induced microstructural damage (or transmutation-produced Si content). Once this relation is established, then one can make a bridge to correlate these changes to corresponding changes in fracture toughness properties, where only limited data is published in the literature.
Farrell et al.  published data on tensile behavior of 5052-O aluminum alloy (Al–2.2% Mg) heavily irradiated in HFIR to fluences greater than 1027 n/m2 in contact with cooling water at 328 K (see Figure 1). HFIR is predominantly a thermal reactor with a strong fast neutron component. The thermal neutron fluence (
Comparison of yield and tensile strength properties of all these alloys including HFR-SURP tensile data, shown with added trend lines in Figure 1 (a, b), reveals specific trends in irradiation hardening and embrittlement behavior. Each of these alloys showed a rapid hardening regime (and corresponding drop in ductility) at the beginning, followed by a transition regime toward a relatively slow hardening (and stable ductility) regime. A brittle regime is observed in some alloys at the end, as shown schematically in Figure 2. Depending on whether an alloy is of 5
4.1. Tensile behavior of 6
xxx series alloys
In case of 6
In the transition regime (regime 2), precipitation of transmutation-produced Si takes over as the major contributing mechanism, while the dislocation density reaches a saturation limit. It is known from literature that the transmutation Si in 6
Assuming a saturation density of ~ 6 × 1014 m−2 in Al alloys (same as in steel), a rough estimate of the total contribution of dislocation hardening can be made using the following equation :
A low hardening rate observed in regime 3 of these alloys can be solely attributed to the growth of existing precipitates. No further increase in the precipitate density occurs in this regime leading to a stable ductility. The final brittle regime (regime 4) with an increasing hardening rate and a decreasing ductility is observed only in 6061-T6 alloy (from CRD A-2 tubes of HFBR) at very high fluences. Although this alloy is the same as 6061-T6 alloy and irradiated at similar temperatures, a difference in behavior is observed due to irradiation at very high TFR, as explained in Section 4.4.
4.2. Tensile behavior of 5
xxx series alloys
The differences in irradiation hardening trends in all four regimes of 5
The contribution of both mechanisms continues in the transition regime until dislocation damage reaches a saturation value (at <2 × 1026 n/m2 of fast fluence or <4 × 1026 n/m2 of thermal fluence). Simultaneously, a saturation in the density of precipitates is expected to occur in this regime, leading to the formation of no new Mg2Si precipitates.
With further irradiation, the hardening continues with a decreasing rate as Mg2Si precipitates continue to grow until all the Mg is pulled out from the Al solid solution in the final slow hardening regime. Based on the stoichiometric analysis, production of 0.58 wt.% transmutation Si will consume 1% Mg in the alloy. That means all the Mg in 5154-0 alloy is consumed at ~1.85% transmutation Si (~8.66 × 1026 n/m2 of thermal fluence) and in 5052-O alloy at ~1.27% transmutation Si (~5.95 × 1026 n/m2 of thermal fluence).
With continued irradiation, the newly formed transmutation-produced Si either decorates existing precipitates (like in 6
Similar to 6
4.3. Fracture toughness behavior of 5
xxx and 6 xxx series alloys
In this section, first the fracture toughness data from HFR SURP is plotted against the available literature data on highly irradiated Al alloys. The evolution of fracture toughness behavior of 5
4.3.1. Literature fracture toughness data of 5
xxx series Al alloys
No additional data on fracture toughness properties of 5
4.3.2. Literature fracture toughness data of 6
xxx series Al alloys in comparison with HFR SURP data
Only limited data was published on fracture toughness properties of irradiated Al alloys [5, 8, 10]. The most relevant data for the HFR (irradiation temperatures < 373 K) is plotted in Figure 4 in comparison with HFR SURP data. Data from 6061-T6 alloy irradiated at < 373 K in the High Flux Isotope Reactor (HFIR) in Oak Ridge National Laboratory, USA matches quite well with the HFR SURP data. As it can be seen from Figure 4, there is one high fluence data point published by Weeks et al.  beyond the current surveillance data of the HFR vessel. This data is from the CRDF A-2 tubes of the HFBR in Brookhaven National Laboratory, USA, produced from 6061-T6 alloy, irradiated at 338 K up to a thermal fluence of 42 × 1026 n/m2. The corresponding fast fluence of this data point is 2 × 1026 n/m2, which gives a high TFR of 21, compared to the HFR hotspot TFR value of maximum 1.4. The total measured Si at this fluence was found to be ~8 wt.%, including 0.6% of initial Si content. The reported thermal fluence and Si content of this data point are approximately two times the estimated thermal fluence (~20 × 1026 n/m2) and Si (~4.3 %) content of the HFR hotspot by the end of 2025. Note that this data is from the same material and at the same irradiation conditions for which the tensile data at very high thermal fluences (~42 × 1026 n/m2) is also available (see Figure 1).
4.3.3. Fracture toughness behavior of 5
xxx and 6 xxx series alloys
In the rapid hardening regime (regime 1), the fracture toughness value drops rapidly in line with the observed hardening and ductility behavior of 5
As the irradiation continues, both the dislocation density and the Mg2Si precipitate density evolve toward a saturation limit describing the slow decrease of fracture toughness toward a plateau in the transition regime. Transmission electron microscopy results of precipitate microstructure reported in  are shown in Figures 5 and 6. From these results it can be seen that the saturation density is achieved at ~3 × 1026 n/m2 of thermal fluence for 5154-O alloy of HFR vessel. Note that these pictures were taken using a “JEOL JEM-1200ex STEM/TEM” machine operating at 120 keV, located in JGL laboratory at NRG.
After that, the fracture toughness of 5154-O reaches a plateau at a thermal fluence of ~ 4 × 1026 n/m2 after which no further increase in dislocation and precipitate density is expected (Figure 3). In fact, a small decrease in particle density may occur later in this regime due to particle coalescence during their growth. The opposite effects of a small decrease in particle density and a slow hardening due to precipitate growth on embrittlement could be compensating each other leading to a plateau in the fracture toughness behavior (similar to ductility) in regime 3. The behavior of 5154-O alloy is expected to be similar to 6
It is important to understand how long the plateau in the fracture toughness (or regime 3) will continue. This depends on the location of the precipitation of the transmutation Si. As already mentioned in Section 3.2.2, further increase in Si production to high values can lead to Si precipitation at grain boundaries. Fracture toughness value drops when the precipitation of Si at the grain boundaries cumulates to an extent that the dominant deformation and fracture mechanisms shift from the bulk microstructure to the grain boundaries. A heavy discontinuous precipitation observed at the grain boundary in 5052-O alloy at a thermal fluence of 31 × 1026 n/m2  has resulted in no substantial effects on ductility of this alloy. This suggests that the nature of fracture at these high fluence is still controlled by bulk deformation mechanisms (instead of mechanisms controlled by grain boundaries). Due to the similarity in 5154-O and 5052-O alloys (and irradiation conditions), the ductility and fracture toughness properties of the 5154-O alloy are also expected to show a plateau until such high fluences. Indeed, the observation of significant amount of micron-scale dimples on the fracture surface of 5154-O alloy irradiated to a thermal fluence of 9.81× 1026 n/m2 (Figure 7) proves that similar behavior can be expected from the 5154-O alloy .
CRDF A-2 tubes of HFBR produced from 6061-T6 alloy have shown a fracture toughness value of ~8 (MPa)·m1/2 after irradiation to a much higher thermal neutron fluence of ~42 × 1026 n/m2 at 338 K (see Figure 4). The decrease in fracture toughness from an unirradiated value of 21.75 (MPa)·m1/2 for this alloy is primarily attributed to the following: (i) formation of very fine (~8 nm) Si-rich precipitates in the grains due to high TFR of 21 (as explained in Section 4.4) and (ii) large silicon flakes occupying about one-fifth of the grain boundary area at this high transmutation-produced Si content of 8 wt.% . Fracture surface of this alloy revealed substantial intergranular separation with some residual ductility indicating that the contribution of grain boundary fracture mechanisms is increased at such high fluence values to enter into the brittle regime (regime 4).
From the above discussion, no differences in the evolution of irradiation damage at high fluences (in regime 3 and 4) are expected between 5
4.4. Effect of thermal-to-fast flux ratio (TFR)
It is known from the literature that a high difference in TFR can have substantial effect on irradiation hardening and embrittlement behavior of the same material . It was highlighted in  that both the thermal and fast neutrons play independent and important roles leading to microstructural damage and corresponding property changes. A very high TFR, ranging from 80 to 500, could explain the observed craze-cracking in AG3-NET alloy (Al–3% Mg) beam tubes in the Reactor Haut Flux (RHF) at Grenoble . Lijbrink et al.  pointed out that fast neutron flux reduces the effectiveness of the Si precipitation hardening process. A possible explanation for this behavior (as given in [5, 12]) is as follows. Fast flux has two opposite effects on precipitation:
The kinetic energy supplied by fast flux temporarily increases the solubility limit of Si in the matrix and opposes the condensation requirements for the precipitation.
Local energy needed for jumping the nucleation barrier can be readily supplied by the fast flux.
However, the fast flux can be destructive when a freshly formed nucleus is hit by fast neutron collision. That means, at equal thermal fluence values, Si precipitation hardening is more effective at higher TFR. This leads to finer precipitate distribution, causing higher irradiation hardening, lower ductility, and eventually lower fracture toughness values at higher TFR. Indeed, the higher hardening rate observed in 6061-T6 alloy from CRDF A-2 tubes of HFBR irradiated at TFR of 21 compared to the similar 6061-T6 alloy irradiated in HFIR at TFR of 1.7 explains this behavior (Figure 1 (b)). Consequently, the high fluence data point from CRDF A-2 of HFBR at TFR = 21 (>>0.8–1.4 for HFR hotspot), shown in Figure 4, is likely to give a conservative estimation of the fracture toughness value under HFR conditions.
5. Summary and conclusions
A literature review on highly irradiated 5
The contribution of various irradiation damage mechanisms to the evolution of microstructure and mechanical properties is discussed in all four regimes for 5
For the 5154-O alloy at the hotspot irradiation conditions, regime 1 ends at ~2 × 1026 n/m2. Regime 2 is observed between ~2 × 1026 and ~4 × 1026 n/m2. Finally, the plateau in regime 3 starts at ~4 × 1026 n/m2 and is expected to continue up to very high thermal fluences, that is, greater than the estimated hotspot thermal fluence by the end of 2025 (~20 × 1026 n/m2). This is because for a 5052-O alloy, which was irradiated at similar conditions as HFR hotspot and resembles the alloy microstructure and composition of 5154-O, a plateau in ductility was observed from a thermal fluence of ~4 × 1026 n/m2 until ~31 × 1026 n/m2. It should be noted that the estimated HFR hotspot thermal fluence by the end of 2025 (~20 × 1026 n/m2) is only two-thirds of the studied 5052-O alloy.
Additionally, high fluence fracture toughness data is found from the CRDF A-2 tubes of the HFBR in Brookhaven National Laboratory, USA, produced from 6061-T6 alloy, irradiated at 338 K, up to 42 × 1026 n/m2. The corresponding fast fluence of this data point is 2 × 1026 n/m2, which gives a high TFR of 21 compared to the HFR hotspot TFR value of maximum 1.4. The reported thermal fluence and Si content of this data point are approximately two times the estimated thermal fluence (~20 × 1026 n/m2) and Si (~4.3%) content of the HFR hotspot by the end of 2025. Knowing that the transmutation-produced Si induces major damage to the microstructure of irradiated Al alloys, this high fluence data point from CRDF A-2 of HFBR is likely to give a conservative estimation of the fracture toughness value under HFR conditions due to irradiation of this alloy at much higher TFR (leading to high embrittlement) and negligible differences in the embrittlement behavior of 5
From the above observations of literature tensile and fracture toughness data on irradiated Al alloys, one can conclude that the probability of the fracture toughness of HFR hotspot to fall below the design limit is negligible up until the currently estimated hotspot thermal fluence at the end of 2025. Assuming that the irradiation conditions at the HFR hotspot are kept unchanged as they are in 2015.
Assuming that the irradiation conditions at the HFR hotspot are kept unchanged as they are in 2015.
The work presented in this article is performed as a part of SURveillance Program (SURP) of HFR vessel with the financial support of NRG. The author thanks Dr. O. Wouters and Ir. T.O. van Staveren for useful discussion and critical review of this work. The author also acknowledges Dr. C. Li for helping in the extraction of data from the literature.
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- Hotspot is the location on vessel wall where highest neutron fluence is received.
- Assuming that the irradiation conditions at the HFR hotspot are kept unchanged as they are in 2015.
- Assuming that the irradiation conditions at the HFR hotspot are kept unchanged as they are in 2015.