26.08.2015   An Introduction to Nuclear Materials

Radiation Anneal Hardening (RAH)

An additional hardening effect occurs upon annealing of BCC metals after irradiation. This phenomenon is known as RAH. Note that hardening starts at 120 °C and increases to a maximum at 180 °C before decreasing. A second

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Figure 6.28 Effect of neutron fluence on friction and source hardening in mild steel [25].

hardening peak appears at 300 °C before the yield strength drops due to recovery effect. The first peak is due to migration of oxygen to defect clusters and the second peak is due to the migration of carbon (Figure 6.29). Formation of respective inter­stitial impurity and defect cluster complexes at the corresponding temperature leads to enhanced obstruction from the dislocation movement and manifests in the form of additional hardening peaks. This phenomenon has also been observed in molybdenum, vanadium, iron alloys where the interstitial impurities like car­bon, nitrogen and oxygen are responsible. Irradiated FCC metals/alloys generally do not show this kind of effect upon annealing.

6.2.1.1 Channeling: Plastic Instability

In some highly irradiated metals, the onset of necking coincides with yielding with no uniform deformation. This kind of behavior has been shown in Figure 6.23a in the stress-strain curve of A533B irradiated to 0.89 dpa as well as in Figure 6.24 for

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Figure 6.29 RAH in niobium containing 35 wppm C, 41 wppm O, and 5 wppm N following irradiation to 2x 1018 n cm~2 and annealing for 2 h [24, 26].

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Figure 6.30 Channeling in 316SS tensile tested to 5% strain at room temperature (neutron irradiation: 0.78dpa and 80°C) [21].

mild steel at the highest fluence. This unusual macroscopic behavior is due to the microscopic phenomenon of dislocation channeling. However, this effect has noth­ing to do with the PKA channeling that occurs due to the crystallinity of materials — so avoid getting confused between the two terms. In dislocation channeling, an avalanche of dislocations can be released to move on particular slip planes along planar channels that have been cleared of obstacles. As dislocations see these paths as the path of least resistance, the dislocations generated move through these chan­nels. Thus, the strain remains highly localized. In this way, eventually stress con­centration sites are created where these dislocation channels intersect the grain boundaries. Figure 6.30 shows microstructural evidence of dislocation channels in a 316-type stainless steel irradiated to 0.78 dpa at 80 °C.

6.2.2