Following the primary damage induced during a few picoseconds, the irradiated material goes through several stages of evolution, as described in Chapter 3, over a

An Introduction to Nuclear Materials: Fundamentals and Applicatioins, First Edition.

K. Linga Murty and Indrajit Charit.

© 2013 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2013 by Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 6.1 A flowchart showing various causes of radiation damage and their consequent effects. Courtesy: US Department ofEnergy.

long period of time. We need to now discuss how the higher order defects are formed from the primary damage defects in the irradiated materials. For this, let us revisit the specific characteristics of two primary damage defects: vacancies and self-interstitial atoms summarized in Table 6.1.

volume assume lattice as an elastic contin­uum. The relaxation volume for SIA is consid­ered positive (because ofthe volume increase), while the relaxation volume for vacancies is considered negative for monovacancies (because of the volume shrinkage).

The high relaxation volume due to SIAs causes large lattice distortions, which create a strong interaction with other SIAs and other lattice defects (dislocations, impurity atoms, etc.). As we noted, the classical picture of self-interstitial like the interstitial impurity atoms is untenable energetically. Thus, the single SIAs become stable only in a dumbbell or split interstitial configuration around a single lattice site, as shown in Figure 6.2. The dumbbell axis is generally found to be along (100) in FCC metals, (110) direction in BCC metals, and (0001) in HCP metals. Multiple interstitials (interstitial clusters) are created by the aggregation of mobile SIAs at higher temperatures. Multiple interstitials have a high binding energy (~1 eV). Figure 6.3(a) and (b) show two di-interstitial configurations in FCC and BCC lat­tices, respectively.

Impurity atoms can act as effective traps for SIAs. Stable interstitial-impurity complexes having undersized impurity atoms (with respect to the host lattice atom) do not dissociate thermally under a certain temperature range where vacan­cies become mobile. Binding energy of the interstitial-impurity complexes vary typically between 0.5 and 1.0 eV. However, weaker trapping is generally observed with oversized impurity atoms. Figure 6.4 shows an interstitial-undersized atom complex. It has got a mixed dumbbell configuration that is stable. This configura­tion however can reorient itself through jumping of the undersize impurity atom across the vertices of the octahedron (forming a cage-like structure) shown in Figure 6.4. The activation energy associated with this type of motion, known as cage motion, is quite small, on the order of 0.01 eV.

Smaller binding energies (~0.1eV) are associated with multiple vacancies com­pared to multiple interstitials, often observed in irradiated metals. Various configu­rations of vacancies are shown in Figure 6.5. The migration energy of divacancies is

less than that for two monovacancies (0.9 versus 1.32 eV for Ni). We have com­mented on the energetics of divacancy formation in Section 2.2. Tetravacancies can only migrate by dissociation. Nonetheless, it can act as the first stable nucleus for further clustering.

Vacancies can bind with oversize solute/impurity atoms in order to lower the overall free energy of the solid. Estimates of the binding energy of a vacancy to an oversize solute in an FCC lattice range from ~0.2-1.0eV. These solutes can act as efficient traps for vacancies in the lattice.

6.1.1