**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**

temperature, the homologous temperature of Al alloys is around 0.32*Tm*, when compared to, forinstance,~0.175*Tm* foraustenitic steel,~0.17*Tm* forferritic steel,and~0.14*Tm* for*α*-Zr.Inmetals, it is known that noticeable thermal diffusion of vacancies occurs at homologous temperatures above0.3*Tm*.Thisthermallyinducedmovementofvacanciesatroomtemperature(RT)promotes mutual recombination of vacancies and interstitials, resulting in a lower density of point defect

In particular, 5*xxx* and 6*xxx* series Al alloys exhibit a good combination of mechanical, thermal, corrosion resistance, and irradiation swelling resistance properties in a research reactor environment, which make these alloys a suitable choice for in-core structures and reactor vessel components of research reactors. The reactor vessel of the high flux reactor (HFR) in Petten has been fabricated from the aluminum alloy ASTM B209 [1], specification Al 5154–O with a

The components of these reactors can experience a large amount of neutron fluences, up to

microstructure and mechanical properties can occur at these high fluence conditions. To this end, a dedicated SURveillance Program (SURP) is executed to understand, predict, and measure the influence of neutron radiation damage on the mechanical properties of the vessel material. As a part of SURP, a literature survey on irradiated Al alloys that are relevant for HFR vessel material is conducted to obtain fundamental understanding on expected mechan‐ ical property changes in relation with microstructural damage mechanisms, which forms the

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

A substantial amount of literature is published on the irradiation behavior of Al alloys [2–11]. The available dataset on 6*xxx* series alloys is considerably larger due to their widespread use in several research reactors and cold-neutron sources [2, 4–10]. On the other hand, only limited data were found on 5*xxx* series alloys [3, 9, 11, 12]. The published data from the SURP of the HFR vessel are also included in this review [13]. Although 5*xxx* and 6*xxx* series alloys are

a maximum thermal

Substantial damage to the material's

, during their operational life. For the HFR hotspot,a

clusters, which are seeds for the damage microstructure.

restriction on Mg content to a maximum of 3.5 wt.%.

fluence of ~20 × 1026 n/m2 is expected by the end of 2025.b

several 1027 n/m2

394 Radiation Effects in Materials

goal of this work.

a

b

beyond the current surveillance data.

**2. Literature on irradiation effects in Al alloys**

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.

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 [15]. This occurs when the dislocation annihilation rate reaches the value of the production rate. The resulting contribution of displacement damage to irradiation hardening and embrit‐ tlement 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 (*n, α*) and (*n, p*) transmutation reactions [9]. On the other hand, thermal neutrons cause transmutation of Al into Si through the following sequential reactions,

$$^{27}\text{Al}(n,\gamma)^{28}\text{Al}, \, ^{28}\text{Al} \to \, ^{28}\text{Si} + \beta,\tag{1}$$

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 trans‐ mutation 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 supersatu‐ ration and hence exhibit increased resistance to cavity formation and swelling [3]. 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 [16] 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 [3]. 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. [9] 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 transmutationproduced Si. Only sparsely distributed voids are found in 5052-O microstructure at these high fluence values [9]. 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 [3]. No swelling data was published for 5154-O alloy in these conditions. However, due to the similarity in micro‐ structures 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. [9], 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*xxx* series alloys is not a crucial degra‐ dation mechanism for the expected hotspot thermal fluence values of HFR vessel by the end of 2025.

#### *3.2.2. Solid transmutation damage*

(1)

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.

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 [15]. This occurs when the dislocation annihilation rate reaches the value of the production rate. The resulting contribution of displacement damage to irradiation hardening and embrit‐ tlement 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

Transmutation damage in aluminum can be caused by both fast and thermal neutrons. Fast neutrons produce gaseous products like He and H through (*n, α*) and (*n, p*) transmutation reactions [9]. On the other hand, thermal neutrons cause transmutation of Al into Si 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 trans‐ mutation product Si can have a stronger effect on radiation damage structure than gaseous

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

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 supersatu‐ ration and hence exhibit increased resistance to cavity formation and swelling [3]. 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 [16] show that the incubation

transmutation products, as discussed in more detail in the following subsections.

Interaction between individual dislocations and loops also contribute to the network.

be found in Refs. [9, 15].

396 Radiation Effects in Materials

**3.2. Transmutation damage**

the following sequential reactions,

*3.2.1. Gaseous transmutation damage*

clusters.

Transmutation-produced Si by thermal neutrons causes substantial radiation damage in Al alloys. Kapusta et al. [11] 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)) [9]. 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*xxx* series alloys until all the Mg in solid solution is consumed.

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*xxx* alloys [3]. However, irradiation-assisted Mg2Si precipitates in 5*xxx* alloys are observed to be fine compared to precipitates in thermally aged 6061 alloy [9], probably because irradiation-assisted precipitation occurs at temperatures much lower than the thermal aging temperature of 433 K, thereby favoring a large number of nucleation sites. In case of control rod drive follower (CRDF) A-2 tubes of the High Flux Beam Reactor (HFBR) in Brookhaven National Laboratory, USA, produced from 6061-T6 alloy, irradiated at 338 K up to a very high thermal fluence of 42 × 1026 n/m2 , a high concentration of very fine (8 nm) amorphous Si-rich particles are observed in the microstructure in place of original Mg2Si precipitates [5]. The corresponding fast fluence 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 the initial Si content.

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 [9]. Farrell et al. [17] reported a noncrystalline Si-coating inside the voids of 1100-O Al alloy at a high thermal fluence (*E* < 0.025 eV) of ~2.3 × 1027 n/m2 . The 6061 alloy irradiated to ~1027 n/m2 at ~328 K has shown a decoration of original Mg2Si precipitates with transmutation-produced Si in addition to the association of Si particles with voids [9]. Precipitation of this Si along the grain boundary can lower the fracture toughness. For example, CRDF A-2 tubes of HFBR produced from 6061-T6 alloy have shown a drop in fracture toughness to ~8 (MPa)·m1/2 from an unirradiated value of 21.75 (MPa)·m1/2 after irradiation to a thermal neutron fluence of ~42 × 1026 n/m2 at 338 K (see **Figure 4**). The microstructure of this alloy, with a very high transmutation-produced Si content of 8 wt.%, has shown large silicon flakes occupying less than one-fifth of the grain boundary area [5]. Similarly, heavy discontinuous precipitation at grain boundaries is observed in 5052 alloy irradiated up to a thermal fluence of ~31 × 1026 n/m2 [3].

From the above discussion, it can be concluded that the transmutation-produced Si is the dominant irradiation damage mechanism in 5*xxx* and 6*xxx* series Al alloys irradiated at temperatures <373 K. Consequently, transmutation-produced Si is taken as the measure of the irradiation damage in HFR vessel wall. There are differences in how this transmutationproduced Si will influence the mechanical properties of 5*xxx* and 6*xxx* Al alloys, which will be discussed in the next section.
