The effects of fast neutron irradiation on the tensile properties of several precipitation-hardened nickel — based alloys were investigated in the 1970s and 1980s.

Figure 11 Dark field, transmission electron micrograph, illustrating the distribution of g precipitates in solution treated and aged Nimonic PE16 irradiated in Experimental Breeder Reactor-II to 69 dpa at 570 °C. Unpublished data from Boothby, R. M. The Microstructure of EBR-II Irradiated Nimonic PE16; AEA TRS 2002 (FPSG/P(90)23), with permission from AEA Technology Plc.

The materials examined included a number of g/g’ — hardened alloys, such as the Inconel alloys 706 and 718 and the developmental alloys D68 and 7818, as well as g’-hardened alloys similar to Nimonic PE16. Earlier work by Broomfield eta/.82 on thermal reactor irradiated materials indicated that PE16 was more susceptible to irradiation embrittlement at elevated test temperatures than austenitic steels. Broomfield83 found that thermal neutron irradiated PE16 was most severely embrittled in low strain tests at ^550-650 °C, and attributed this to an increased tendency for intergranular failure arising from the effects of helium generated from the 10B(n, a)7Li reaction. Boron itself is considered to have a benefi­cial effect on (unirradiated) creep rupture life, as it segregates to grain boundaries and inhibits intergran­ular cracking, and additions of a few 10s of ppm are therefore, generally made to nickel-based alloys, including PE16.84 Nickel is also a major source of helium in neutron-irradiated alloys, with the two — stage 58Ni(n, g)59Ni(n, a)56Fe reaction becoming the dominant source at high thermal neutron fluences, and nickel threshold reactions accounting for the greater part of helium production in fast neutron spectra.85 For example, the rate of helium generation in fast reactor irradiated PE16 was estimated by Boothby28 to be ~1.2appm per dpa, with about 85% of the helium being generated from nickel threshold reactions (see also Chapter 1.06, The Effects of Helium in Irradiated Structural Alloys). Nevertheless, other factors, including irradiation — induced strengthening and grain boundary segrega­tion and precipitation effects, have been implicated in the embrittlement of fast neutron irradiated nickel-based alloys.