There have also been a number of studies of the effect of alloying on loop formation. These studies have not examined all the alloying elements of interest, but Mn and P have been shown to have an important influence on the cluster distributions observed by TEM in Fe binary or ternary alloys. For example, Ebraimi and coworkers76,77 examined the effects of adding Ni (and P). They found that a higher density of smaller loops was observed in a Fe-Mn alloy (as compared to pure iron irradiated under similar con­ditions), whereas P added to an Fe-Ni alloy caused an increase in loop size.

Phosphorus dissolves substitutionally in iron and the solid solubility is 0.5 at.% (0.27 wt%) at 400 °C.87 Jones and Buswell,88 in reviewing the available micro­structural evidence, concluded that the hardening observed in low Cu steels could be attributable to precipitation hardening by M3P particles produced by the irradiation-induced segregation of phosphorus to defect sinks and the depletion of phosphorus in solid solution, as detected by TEM and AP methods.

Nagai et a/.89 have reported results from a CDB study of Fe-0.3wt% Cu, Fe-0.15wt% Cu, and Fe-0.05 wt% Cu alloys irradiated at 8.3 x 1018 n cm~2, E > 1 MeVat ^300 °C (the irradiation time was 144 h). As a result of CDB and positron lifetime measure­ments on irradiated and annealed samples, the authors reported the formation of microvoids (~10 vacancies), dislocation loops, and Cu-mono-vacancy-Cu com­plexes. They considered that the microvoids were decorated with Cu in all the alloys studied, and that in all cases the microvoids were almost completely coated with Cu. After electron irradiation,90 vacancy clusters and single vacancies surrounded by Cu (v-Cun, where n > 6) were observed in electron — irradiated Fe-Cu, and vacancy clusters were observed Fe-Ni and Fe-P, but no vacancy clustering in Fe-C, Fe-Si, or Fe-Mn was observed.

A recent development of some importance is the observation (primarily using the LEAP) of MnNiSi clusters in irradiated low Cu steels. For example, Miller et a/.91 characterized the irradiation-induced microstructure of low copper (0.05 wt%) high nickel (1.26 and 1.78 wt% Ni) VVER-1000 forging and weld materials that were neutron irradiated to a total flu — ence of 1.38 x 1023 nm~2 (E > 1 MeV). Atom probe tomography revealed ultrafine Ni-Mn-Si-enriched clusters but no CECs. The number density of clusters in the VVER-1000 weld was estimated to be ~1.5 x 1023 m~ , while the number density of clusters in the forging was estimated to be slightly lower at 1 x 1023 m~ . These ultrafine clusters may, or may not, be associated with vacancies. The observa­tions of such clusters may be interpreted as evidence of a mechanism not encompassed by the framework set out in this section. This is further discussed in the next section.

There is strong evidence that interstitial solutes (ISs) such as C and N are attracted to the point defects produced by irradiation. ISs may well add to preexisting SIA clusters, and may even inhibit their growth. Conversely, they appear to encourage the formation of multiple-vacancy complexes. Little and Harries92 further demonstrated that the amount of free nitrogen, indicated by the height of the Snoek internal friction peaks, decreased with increasing irradiation fluence, such that it was zero with fluences of about 2 x 1018ncm~2. This was attributed to trapping of free nitrogen or precipitation of nitrides at point defects or defect clusters.

4.05.4.5.3 MD and hardening

Various scientists have attempted to determine the nature of the defects which result in hardening. Soneda65 quoted evidence from Ortner93 showing that AHv (the change in Vickers hardness) and AS (related to the volume fraction of open-volume defects) increase after irradiation of a low Cu steel EP2, indicating that vacancy-type defects are formed by irradiation. During the postirradiation annealing, AS starts to recover at a lower temperature than AHv. This clearly indicates that the change in AS is unre­lated to the change in AHv, and thus, vacancy-type defects are not solely responsible for the observed irradiation-induced hardening.