High temperature helium embrittlement occurs at elevated temperatures (typically near or above 0.5 TM) when sufficient levels of helium are produced by nuclear transmutation reactions and mechanical stress is applied during irradiation. Intergranular fracture is induced by the transformation of grain boundary bubbles to voids, leading to breakaway growth, cavity coalescence, and rupture in the presence of mechanical stress.120,152,153,274-277 The application of tensile stress during high temperature irradiation induces migration of the helium to the grain bound­aries, where large cavities can be formed.120 In the absence of applied stress, the helium bubbles are distributed throughout the material. The observed tensile ductility due to helium embrittlement decreases with decreasing strain rate120,278 and decreasing

stress120 (opposite of the behavior observed in many unirradiated metals and alloys), pointing out the importance of exposure time at elevated temper­ature on helium embrittlement. Figure 30 shows examples of the grain boundary microstructures of an Fe-Cr-Ni ternary alloy preimplanted with 160appm He during annealing at 750 °C with and without applied tensile stress.279 Cavity formation along the grain boundary is very limited in the absence of applied stress for annealing times up to 60 h, whereas pronounced grain boundary cavity swelling occurs for annealing times as short as 8h when ^20 MPa stress is applied. Evidence for high temperature helium embrittlement has been observed during tensile and creep testing of austen­itic stainless steel at temperatures above 550 °C (^0.45-0.5 TM) when the helium concentration exceeds ^30appm.255,265,277,280,281 Austenitic stainless steels containing fine dispersions of precipitates exhibit better resistance to helium embrittlement than simple Fe-Cr-Ni alloys, and microstructural investigations suggest that helium trapping at grain interior locations (thereby impeding the flow of helium to grain bound­aries) is an important factor.152,277,282-284 It has been observed that ferritic/martensitic steels exhibit better resistance to grain boundary helium cavity formation and growth compared to austenitic stain­less steels.274,285-287 This has been attributed to

several potential factors, including efficient trapping

Triple grain junction

Figure 30 Effect of exposure time and applied stress during annealing at 750 °C on the formation of grain boundary cavities in Fe-17Cr-17Ni austenitic alloy preimplanted with 160 appm helium. Reproduced from Braski, D. N.; Schroeder, H.; Ullmaier, H. J. Nucl. Mater. 1979, 83, 265-277.

of helium in the ferritic steel grain interior by pre­cipitates and other features, a potentially larger criti­cal radius for conversion of helium bubbles to voids in ferritic steel, and lower matrix strength for ferritic steel compared to austenitic steel.119,274,286,288 The helium bubble densities observed in model Fe-Cr ferritic alloys and commercial ferritic steels following high temperature implantation are comparable to that observed in austenitic steels.11 Relatively good resistance to helium embrittlement compared to austenitic stainless steel has been observed in other bcc metals such as Nb and Nb-1Zr (no severe embrittlement observed for He concentrations up to 100-500 appm),289-291 whereas simple fcc metals such as pure copper are readily susceptible to helium embrittlement even at relatively high (tensile) strain rates at temperatures near 0.5 TM for He concentra­tions of 100-330 appm.292,293