A Comparison of Cavity Formation in Neutron Irradiated Nanostructured Ferritic Alloys and Tempered Martensitic Steels at High He/dpa Ratio

摘要

Fusion neutron spectra produce ≈ 2000 appm He at 200 dpa, and the He/dpa ratio is even higher in spallation proton targets. Helium precipitates gas bubbles in the matrix and on dislocations, precipitate interfaces and grain boundaries [1]. Helium bubbles can act as formation sites for growing voids at lower irradiation temperature and creep cavities on stressed grain boundaries at high temperatures. Both 14Cr nanostructured ferritic alloys (NFA) and 9Cr tempered martensitic steels (TMS) are resistant, but not immune, to irradiation effects. For example, TMS are embrittled by irradiation hardening and grain boundary He at lower irradiation temperatures [2], and undergo significant void swelling at high He/dpa ratios in dual ion irradiations [3]. NFA are more resistant to irradiation damage at all temperatures. The irradiation damage resistance of NFA derives from their high sink densities, including stabilized dislocation structures, and large number of nm-scale Y-Ti-0 enriched nanofeatures (NF) which trap He in small bubbles [1]. However, their overriding advantage is that NFA can operate at temperatures, up to 800℃, above the displacement damage-swelling regime [1]. At T ＞ 0.5T_m, the primary irradiation damage issue is creep embrittlement due to accumulation of He on grain boundaries. The objective of this research is to characterize the transport, fate and consequences of He at spallation proton and fusion neutron relevant He/dpa ratios and dpa rates.