Main characteristics of radiation embrittlement of materials

Август 20, 2022

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Main characteristics of radiation embrittlement of materials

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2. One of the main effects of irradiation on the mechanical properties of materials is a significant
3. Radiation embrittlement of materials can be classified as follows: Radiation embrittlement of ductile materials (austenitic steels,
4. The methodologies for investigating these effects differ accordingly. Radiation embrittlement of plastic materials is studied by
5. Radiation embrittlement of brittle-ductile materials is assessed by examining the fracture toughness, the change in fracture
6. On fig.1 the most typical example of temperature dependence of the basic mechanical characteristics of irradiated
7. occurrence, there are the following In addition to the temperature ranges of fundamental differences between these
8. CONTRIBUTION OF DIFFERENT TYPES OF RADIATION DEFECTS IN THE STRESS OF FLOW OF IRRADIATED MATERIALS
9. Depending on the irradiation temperature the microstructure of materials may undergo strong changes. Thus, at temperatures
10. An example of a defect structure formed in this temperature range is the tiny clusters shown
11. RELATION BETWEEN RADIATION HARDENING AND DEFECT STRUCTURE PARAMETERS OF IRRADIATED MATERIALS
13. The magnitude of radiation hardening due to radiation defects is not a constant value and varies
14. MECHANISMS OF RADIATION EMBRITTLEMENT MATERIALS
15. It is currently believed [1-3] that among structural reactor materials, materials with a BCC lattice type
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One of the main effects of irradiation on the mechanical properties

of materials is a significant reduction in their plastic properties and fracture toughness. This phenomenon is called radiation embrittlement.

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Radiation embrittlement of materials can be classified as follows:
Radiation embrittlement of

ductile materials (austenitic steels, nickel and other HCC materials without a brittle-ductile transition) at test temperatures not exceeding 0.4 Tp is called low-temperature radiation embrittlement.
Low-temperature radiation embrittlement of ductile-brittle materials (ferritic and ferritic-pearlitic case steels, most bcc materials, etc.) at test temperatures that do not exceed 0.4 Tp.
High-temperature radiation embrittlement observed in all irradiated polycrystalline materials at test temperatures exceeding 0.45...0.5 Tpl.

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The methodologies for investigating these effects differ accordingly.
Radiation embrittlement of plastic materials is studied by changing of tensile curves, in

particular, uniform and total elongation.
As a rule, the value ,

where δi. and δir. - is the relative elongation, respectively, of the original and irradiated materials. %

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$Radiation embrittlement of brittle-ductile materials is assessed by examining the fracture$

Radiation embrittlement of brittle-ductile materials is assessed by examining the fracture

toughness, the change in fracture energy and by determining the value of ΔTx, which characterises the temperature shift of the brittle- ductile transition.

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On fig.1 the most typical example of temperature dependence of the

basic mechanical characteristics of irradiated materials on an example of austenitic steel 0Х16Н15М3Б
[4] is resulted. As it is visible from figure, there are 2 temperature intervals of plasticity reduction. The region of low-temperature radiation embrittlement (LTRO) corresponds to test temperatures ≤ 600°С, and the region of high-temperature radiation embrittlement (HTRO) corresponds to test temperatures ≥ 700°С (Fig.1, b).

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occurrence, there are the following
In addition to the temperature ranges of fundamental differences between these

effects. In contrast to BTRO the effect of HTRO:

is largely eliminated by post-radiation annealing ;
is associated with radiation hardening of the materials Δσ = σobl. - where σobl and σisx are stresses of flow of materials in irradiated and initial states respectively (see fig.1 a);
is associated mainly with a decrease in the uniform elongation of the materials and is not accompanied by a significant change in the lateral contraction.
Accordingly, the BTRO effect is not eliminated by post-radiation annealing and is not normally associated with radiation hardening [5].
Irradiation of metallic materials produces an essential change in the shape and parameters of the hardening curve. The yield strength after high doses of neutron irradiation, for example, in stainless steels increases several times (see, e.g., Fig. 1, a) and in pure annealed metals the flow stresses can increase by more than 10 times [4]. The tensile strength increases to a lesser extent, as can also be seen in Fig. 1, a.

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CONTRIBUTION OF DIFFERENT TYPES OF RADIATION DEFECTS IN THE STRESS OF

FLOW OF IRRADIATED MATERIALS

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Depending on the irradiation temperature the microstructure of materials may undergo

strong changes. Thus, at temperatures not exceeding 300°C the microstructure of irradiated materials has the following features [7]:
high density of the smallest clusters of radiation defects with sizes not exceeding 5 nm;
depending on the temperature and the degree of preliminary cold deformation, the Frank loops of 9...30 nm in size can be observed in the structure of the material;
no visible pores or bubbles;
no radiation induced segregation effects are observable;
very little change in all structural components at fluences above 1 nm.

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An example of a defect structure formed in this temperature range

is the tiny clusters shown in Figure 3 [8]. A similar defect structure has been observed in other materials irradiated under the same conditions

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RELATION BETWEEN RADIATION HARDENING AND DEFECT STRUCTURE PARAMETERS OF IRRADIATED MATERIALS

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The magnitude of radiation hardening due to radiation defects is not

a constant value and varies with increasing irradiation fluence (Fig.4) [11].
Analysis of the dose dependences of the components composing the defect structure allows us to draw several important conclusions:
- the dominant tendency of evolution of practically all components of microstructure of irradiated materials is the tendency to saturation. This leads to a corresponding dependence of mechanical properties. The only difference is that at low irradiation temperatures (T ≤ 0.25 Tpl) the saturation occurs at fluences not exceeding 0.1 masl.

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MECHANISMS OF RADIATION EMBRITTLEMENT MATERIALS

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It is currently believed [1-3] that among structural reactor materials, materials

with a BCC lattice type - both pure metals and ferrite and ferrite-perlite steels - have the greatest propensity for low-temperature radiation embrittlement. Two of the most accepted models of radiation embrittlement can be distinguished for these materials: Fischer [15] and Odette et al [16,17]. Largely similar to Odette, the model proposed by Williams et al [18]. A similarity of the models is that they both consider (postulate) as causes of embrittlement two sources of hardening (i.e. Δδ/ δ = kΔσ): matrix hardening caused by radiation defects (clusters, loops) and radiation hardening caused by radiation-induced (stimulated) excretions, for example, copper-bearing excretions in hull steels.
However, they also have significant differences in approach: the Odette model is based on the theoretical concepts of the "velocity theory" of defect structure evolution, while the Fischer model, substantially simpler, describes empirical dependencies of matrix hardening obtained from numerous experiments.

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