Void swelling is only one component of microstruc­tural and microchemical evolutions that take place in alloys under irradiation. In addition to loops and network dislocations, other coevolutions include sol­ute segregation and irradiation-enhanced-induced — altered precipitation. In the mid-1980s, CBM and RT models of dislocation loop and network evolution were self-consistently integrated in the computer code MicroEv, which also included a parametric treatment of precipitate bubble-void nucleation sites.133,144 Later work in the 1990s further developed and refined this code.163 A major objective of much of this research was to develop models to make quanti­tative predictions of the effect of the He/dpa ratios on void swelling for fusion reactor conditions.

CBMs have been used to parametrically evaluate the effects of many irradiation variables and ma­terial parameters15,114,118,128,129,140,149,150 as well as

to model swelling as a function of temperature, dpa and dpa rates, and the He/dpa ratio (see both Stoller and Odette references). The CBMs have also been both informed by and compared with data from experiments in both fast and mixed thermal-fast spectrum test reactors, including EBR-II (fast), FFTF (fast), and HFIR (mixed),16,119 complemented by exten-

26,124,125,128,129,157,164a,164-171

sive dual ion CPI results.

The semiempirical CBM models and concepts ratio­nalize a wide range of seemingly complex and some­times disparate observations, including the following:

• Void nucleation on bubbles

• The general trends in the temperature, dpa, and He/dpa dependence of the number densities of bubbles and voids

• Incubation dpa and postincubation swelling rates, including the effects of temperature and stress

• The occurrence of bimodal cavity size distribu­tions of small He bubbles and larger voids

• Bubble nucleation on dislocations and precipitate interfaces

• Swelling that is increased, decreased, or unaltered by increasing GHe, depending on the combination of other irradiation and material variables

• Suppression of void swelling by a very high num­ber of densities of bubbles

• Highly coupled concurrent evolutions of all the microstructural features, resulting in weaker trend toward refinement of precipitate and loop struc­tures at higher GHe and, in the limit of very high Nb, suppression of loops and precipitation

• Strong effects of the schedule and temperature history of He implantation in CPI

• Effects of alloying elements on swelling incubation associated with corresponding influence on pre­cipitation, solute segregation, and the self-diffusion coefficient

• Swelling resistance of AuSS that have stable fine — scale precipitates that trap He in small interface bubbles

• The much higher swelling resistance of bcc FMS compared with fcc AuSS

The concept of trapping He in a high number den­sity of bubbles to enhance the swelling and HTHE resistance (and creep properties in general) was imple­mented in the development of AuSS containing fine-scale carbide and phosphide phases. Figure 19 shows the compared cavity microstructures resulting in «6% void swelling in a conventional AuSS (Figure 19(a)) to an alloy modified with Ti and heat treated to produce a high density of fine-scale TiC (Figure 19(b)) phases with less than 0.2% bubble swelling following irradiation to 45 dpa and 2500appm He at 600°C.172 There are many other examples of swelling-resistant AuSS that were suc­cessful in delaying the onset of swelling to much

higher dpa than in conventional AuSS. However, as illustrated in Figure 7, these steels also eventually swell. This has largely been attributed to thermal — irradiation instability and coarsening of the fine-scale precipitates that provide the swelling resistance.172

FMS are much more resistant to swelling than advanced AuSS.15102,104116128,129,162,169,174,175 The swelling resistance of FMS, compared with AuSS, can be attributed to a combination of their (a) lower dislocation bias; (b) higher sink densities for parti­tioning He into a finer distribution of bubbles, thus increasing m*; (c) low void to dislocation sink ratios; (d) a higher self-diffusion coefficient that increases m*; and (e) lower He/dpa ratios.15,176 However, void swelling does occur in FMS, as well as in unalloyed Fe,177 and is clearly promoted by higher He/dpa ratios. Higher He can decrease incubation times for void formation and increase Zv/Zd ratios closer to 1, resulting in higher swelling rates.52,157,168-171 Recent models predict significant swelling in FMS,178 and the potential for high postincubation swelling rates in these alloys remains to be assessed. Swelling in FMS clearly poses a significant life-limiting chal­lenge in fusion first wall environments in the temper­ature range between 400 and 600 °C.

NFA, which are dispersion strengthened by a high density of nanometer-scale Y-Ti-O-enriched features, are even more resistant to swelling and other mani­festations of radiation damage than FMS.22,23,51,179,180 Irradiation-tolerant alloys will be discussed in Section 1.06.6.