The primary motivation for the earliest research was the observation that even a small concentration of bulk He, in some cases in the range of one appm or less, generated in fission reactor irradiations of AuSS, could lead to HTHE, manifested as significant re­ductions in tensile and creep ductility and creep rupture times. The degradation of these properties coincided with an increasing transition from trans­granular to intergranular rupture.10,85-89 HTHE is attributed to stress-driven nucleation, growth, and coalescence of grain boundary cavities formed on the He bubbles. The early studies included mixed spec­trum neutron irradiations that produce large amounts of He in alloys containing Ni and B. Figure 6 shows one extreme example of the dramatic effect of HTHE on creep rupture times for a 20% cold-worked (CW) 316 stainless steel tested at 550°C and 310MPa following irradiations between 535 and 605 °C in the mixed spectrum HFIR that produced up to 3190 appm He and 85 dpa.88 At the highest He concentration, the creep rupture time is reduced by over four orders of magnitude, from several thousand to less than 0.1 h. A comprehensive review of the large early body of research on He effects on mechanical properties of AuSS can be found in Mansur and Grossbeck.11

The early fission reactor irradiations research on HTHE was later complemented by extensive accelerator-based He ion implantation experiments, primarily carried out in the 1980s (see Schroeder and Batfalsky90 and Schroeder, Kesternich and Ullmaier91 as examples) but that have continued to recent times.92 HTHE models were developed during this period, primarily in conjunction with the He ion implanta­tion experiments.93-100 The He implantation studies and models are discussed further in Sections 1.06.3.6 and 1.06.3.7. A more general review of He effects, again primarily in AuSS, can be found in Ullmaier99 and a comprehensive model-based description of the behavior of He in metals in Trinkaus.96

Research on He effects was also greatly stimulated by the discovery of large growing voids in irradiated AuSS.1 As an example, Figure 7(a) shows swelling curves for a variety of alloys used in reactor applica — tions.102-104 Figure 7(b) illustrates macroscopic conse­quences of this phenomenon in an AuSS.105 Figure 8 shows a classical micrograph of a solution annealed (SA) AuSS with dislocation loops and line segments, preci­pitates, precipitate-associated and matrix voids, and possibly He bubbles (the small cavities). RT-based modeling studies of void swelling began in the early 1970s,106,107 peaking in the 1980s, and continuing up to recent times.108 Most of the earliest models emphasized the complex effects of He on void swelling. ,

As discussed in more detail below, these and later models rationalized many observed swelling trends and also suggested approaches to developing more swelling-resistant AuSS, largely based on trapping

He in small bubbles at the interfaces of fine-scale precipitates. Reviews summarizing mechanisms and modeling of swelling carried out during this period, including the role of He, can be found in Odette,1 Odette, Maziasz and Spitznagel,1 Mansur,11 Mansur and Coghlan,114 Freeman,115 and Mansur.116

Reviews of experimental studies of void swelling can be found in later studies by Maziasz16 and Zinkle, Maziasz and Stoller.117

Further motivation for understanding He effects was stimulated by a growing interest in the effects of the very high transmutation levels produced in fusion reactor spectra (see Section 1.06.1).89,99,111,112,118 Experimental studies comparing microstructural evolutions in AuSS irradiated in fast (lower He) and mixed spectrum (high He) reactors provided key insight into the effects of He.16,119,120 Helium effects were also systematically studied using dual-beam He-heavy ion CPI.26,121-129

Beginning in the mid-1970s, a series of studies specifically addressed the critical question of how to use fission reactor data to predict irradiation effects in fusion reactors,15,109-112,118,130-133 and this topic remains one of intense interest to this day. An indica­tion of the complexity of He effects is illustrated in Figure 9, showing microstructures in a dual-beam He-heavy ion irradiation of a SA AuSS to 70 dpa and 625 °C at different He/dpa.1 3 In this case, voids do not form in the single heavy ion irradiation without He. At intermediate levels, of 0.2 appm/dpa, large voids are observed, resulting in a net swelling of 3.5%. At even higher levels of 20appm/dpa, the voids are more numerous, but smaller, resulting in less net swelling of 1.8%. These observations show that some He promotes the formation of voids, but that higher amounts can reduce swelling. Figure 10 shows the effect of various conditions for

introducing 1400 appm He coupled with a 4MeV Ni ion irradiation of a swelling-prone model SA AuSS to 70 dpa at 625 °C.125 In this case, the swelling is largest («18% due to voids) with no implanted He and smallest («1%) with He preimplanted at ambient temperature due to the very high density of bub­bles. These results also show that voids can form

at sufficiently high CPI damage rates without He, probably assisted by the presence of impurities like oxygen and hydrogen. Most notably, however, the swelling decreases with increasing bubble num­ber densities.

The emphasis of more recent experimental work has been on SPNI that generate large amounts

of He, compared with fission reactors, as well as dis­placement damage (see Section 1.06.4). The SPNI studies have focused on mechanical properties and microstructures, primarily at lower irradiation tem­peratures, nominally below the HTHE regime. In addition, as discussed in Section 1.06.2, a previously proposed in situ He injection technique31,49 has recently been developed and implemented to study He-displacement damage interactions in mixed spec­trum reactor irradiations (e. g., HFIR) at reactor­relevant dpa rates.23,51-53 As discussed in Section 1.06.5, recent modeling studies have emphasized electronic and atomistic evaluations of the energy parameters that describe the behavior of He in solids, including interactions with point and extended defects134-136 (and see Section 1.06.5). The refined parameters are being used in improved RT and Monte Carlo models of He diffusion and clustering to form bubbles on dislocations, precipitates, and GBs, as well as in the matrix, as discussed in Section 1.06.6.

It is again important to emphasize that the broad framework for predicting He effects is an under­standing and modeling of its generation, transport, and fate, as well as the multifaceted consequences of this fate. We begin with a discussion of the role of He in void swelling and other microstructural evo­lution processes. We then return to the issue of HTHE.