Main characteristics of radiation embrittlement of materials

1.

Main characteristics of radiation
embrittlement of materials

2.

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.

3.

Radiation embrittlement of materials can be classified as follows:
1.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.
2.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.
3.High-temperature radiation embrittlement observed in all irradiated
polycrystalline materials at test temperatures exceeding 0.45...0.5 Tpl.

4.

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. %

5.

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 brittleductile transition.

6.

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).

7.

In addition to the temperature ranges of occurrence, there are the following
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.

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 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.

10.

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

11.

RELATION
BETWEEN
RADIATION
HARDENING AND DEFECT STRUCTURE
PARAMETERS
OF
IRRADIATED
MATERIALS

12.

13.

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.

14.

MECHANISMS
OF
RADIATION
EMBRITTLEMENT MATERIALS

15.

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.

16.

17.

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