The use of refractory metal alloys in radiation envir­onments can offer high-temperature capabilities not matched in other alloy categories. Refractory metal alloys also offer exceptional compatibility with liquid metal coolants. As described in some detail in this

Figure 26 Comparison of the increase in Vickers Hardness for tungsten and tungsten alloys for similar dose and irradiation temperatures. Reproduced from He, J. C.; Hasegawa, A.; Abe, K. J. Nucl. Mater. 2008, 377, 348-351; Kurishita, H.; Kobayashi, S.; Nakai, K.; etal. J. Nucl. Mater. 2008, 377, 34-40.

chapter through mechanical property comparisons, these materials are sensitive to impurity contami­nation during metallurgical processing as well as in-service exposures that can lead to grain bound­ary embrittlement issues. The inherent irradiation response of bcc-structured materials also limits refractory metal use at temperatures >0.3 Tm, with significant degradation in material properties with displacive irradiation doses as low as 0.03 dpa.3

Improvements in the irradiated mechanical prop­erties of refractory metal alloys have been observed in recent experimental work, even at low irradia­tion temperatures. This is in part through improved control over impurity levels and also through ther­momechanical processing techniques that result in microstructures with reduced sensitivity to radi­ation embrittlement. This was discussed with refer­ence to LCAC molybdenum,1 where samples

irradiated in the stress-relieved condition showed improvement over material in the recrystallized condition up to the recrystallization temperature. Further development of HP-LCAC molybdenum has resulted in higher aspect ratio grain morpholo­gies that led to plain strain conditions in the grain lamellae during deformation.82 In addition, reduced
grain sizes or higher aspect ratios decrease dis­tances to defect sinks, further reducing irradia­tion sensitivity. While Mo has traditionally been used to study the behavior of W, the microstruc­tural changes and purity control that have been employed for irradiation studies of Mo have not been incorporated into W.

The control over precipitate formation in the pre­irradiated condition appears to result in changes to some physical material properties, specifically, swelling and densification in Nb-lZr,25,27 that may lead to variations in mechanical properties. An understanding of the effect of preirradiation thermo­mechanical processing or in-service microstructural changes that occur during irradiation may lead to improved properties or the ability to avoid dangerous embrittlement issues that can occur through precipi­tate development. This may be of particular interest in Nb and Ta-base alloys that incorporate Zr or Hf additions that react with impurity elements and pro­duce precipitates.

Alloying Mo and W with Re results in improved mechanical properties of unirradiated alloys, in­creased radiation hardening, and radiation-induced embrittlement.62,120 However, much of this work is
on recrystallized, high Re concentration material, the purity of which may not be ideal. The effect that RIS has on the degradation of properties of Mo-Re alloys is a matter of concern. Further work is needed on higher purity, lower Re (5-20 wt% Re) concentration material with reduced grain size, or that with a tai­lored aspect ratio similar to that of LCAC-Mo.

Initial results show improvements to the irradiated properties of Mo and W through the incorporation of either rare earth oxide124 or TiC additions.112,143 These additions aid in restricting grain growth, pro­vide sinks for radiation-induced defects, and act as obstacles to or deflection points for crack propagation. Though these results are preliminary, they outline the need for further examination of incorporating stable dispersion strengthening particles to refractory metal alloys.