Radiation Damage - Environmental Degradation of Metals - page 482

Figure 9.11 Transition curves for unirradiated and irradiated impact specimens of
mild steel, Ref. 23.

Figure 9.11 shows the results obtained in impact tests before and after irradiation
with 18.6 MeV deuterons for an average total exposure of 6.7 10 17 particles/

cm 2 [26]. The transition temperature has changed from 1 ° C to about 8 ° C. Simi-

larly, in molybdenum, an increase from 30 ° Cto70 ° C has been reported [27].

9.6.1 Mechanism
Irradiation strengthening of metals is a result of the barriers to dislocation motion
that are produced from precipitation of radiation-induced defects, i.e., vacancies
and interstitials. The barriers formed are voids and dislocation loops. In some
alloy systems, such as nickel alloys and stainless steels, irradiation-induced pre-
cipitation of second phases such as carbides may also contribute to the observed
strength increase. While the pinning of dislocations directly contributes to the
increase in yield stress, the critical shear stress also increases because it depends
on the breakaway of pinned dislocations.
The controlling factor in the effects of irradiation on deformation behavior is
the irradiation-produced microstructure, which is temperature-dependent. It has
been demonstrated that [28]

1. At low irradiation temperatures (below 0.35 T m ), irradiation-produced vacan-

cies and interstitials collect into small dislocation loops.