The effects of irradiation on the microstructure and mechanical properties of nickel-based alloys are com­plex and, although the main factors affecting their behavior have been identified, a full understanding of radiation-induced effects remains elusive. This is par­ticularly true of the precipitation-hardened alloys, typified by Nimonic PE16 and Inconel 706, where the role of the hardening phases — which confer high ther­mal creep strength, but are redistributed during irradi­ation and may possibly influence swelling behavior and contribute to intergranular embrittlement — is unclear. The radiation-induced effects considered in this chapter-void swelling, irradiation creep, the evolution ofprecipitate and dislocation structures, and irradiation embrittlement — are interrelated in several ways, but particularly through the effects of point defect fluxes and the consequent redistribution of solute atoms.

The beneficial effect of nickel on the swelling resistance of austenitic alloys is well known, but a

clear explanation for the minimum in swelling that is found in alloys containing about 40-45% Ni has not been forthcoming. There is general agreement that the major influence of alloy composition on swelling arises through its effects on the effective vacancy diffusivity and on segregation via the inverse Kirkendall effect. However, on what appears to be the mistaken assumption that the swelling resis­tance of nickel-based alloys derives from an extended void nucleation period, swelling models have largely focused on factors affecting the nucleation rather than the growth of voids. Data for neutron-irradiated Nimo — nic PE16, for example, indicate that its swelling resis­tance is due to a combination of a comparatively low saturation void concentration, which is reached at a relatively low displacement dose, and a low void growth rate. The minimum critical void radius concept appears to offer the most plausible explanation for the minimum in swelling found at intermediate nickel contents, although experimental data comparing the behavior of PE16 and a nonprecipitation hardenable alloy with a similar matrix composition indicate that, in addition to the Ni content of the alloy, the presence of Si and/or the y’ forming solutes Al plus Ti may also be important. The dependence of the void growth rate on Ni may be related to the effects of radiation-induced segregation on the bias terms for the absorption of point defects at sinks, though again there is evidence that minor solutes, including Si, B, and Mo, as well as the y’-forming elements, have a beneficial effect on the overall swelling behavior of nickel-based alloys.

The irradiation creep behavior of nickel-based alloys generally appears to be similar to that of austenitic steels, though the higher thermal creep
strength of precipitation-hardened alloys is an advantage at higher operating temperatures. The main drawback of precipitation-hardened nickel — based alloys for reactor core applications is per­ceived to be a high susceptibility to irradiation embrittlement. Although it has been suggested that a combination of matrix hardening and of grain boundary weakening due to the formation of brittle intergranular layers of g0 (e. g., in the case of Nimonic PE16) or Z phase (in the case of Inconel 706) is responsible for the irradiation embrittlement of these alloys, there is strong evidence, at least for g0- hardened materials, that helium is the primary cause of low ductility failures. Experimental data have shown that the implantation or generation of rela­tively small amounts of helium can give rise to low tensile ductility, with intergranular failures initiated by either the growth and linkage of cavities or by wedge cracking depending on test conditions and helium distribution, under conditions where grain boundary g0 layers are not formed. However, irre­spective of the details of the embrittlement mecha­nism, it is evident that this aspect of radiation damage does not preclude the in-core application of nickel-based alloys, as has clearly been demon­strated by the successful use ofPE16 for fuel element cladding irradiated to high burn-ups in PFR. The long-term integrity of PE16 cladding is attributable to the dimensional stability of the alloy, arising from a combination of good swelling resistance and high creep strength, and relatively low operating stresses which allay irradiation embrittlement concerns.