Like all other refractory metal alloys, tungsten is sensitive to embrittlement issues following irradi­ation, the causes of which are point defect genera­tion, impurity segregation to grain boundaries, and radiation-enhanced precipitation. The strengthening ofthe metal matrix can raise the deformation stress to levels greater than the cleavage strength of the alloy, resulting in brittle failure. While only a limited num­ber of mechanical property tests have been performed on tungsten and its alloys, the actual composition of the materials reported is not well characterized and therefore may have a significant range of impurity levels that can affect grain boundary cohesion and the mechanical strength of the material. Similarly, preirradiation heat treatments have also shown minor improvements in the postirradiation mechanical
properties,59 likely through the development and coarsening of interstitial impurities into precipitate formations that reduce grain boundary sensitivities.

For the irradiated tensile properties of tungsten, two works are typically referenced that make up the bulk of the data available. In the work of Steichen,1 the properties of wrought tungsten, stress-relieved at 1273 K, irradiated at 658 K to fluences between 0.4 x 1022 and 0.9 x 1022ncm~2 (E > 0.1 MeV), were examined, the results of which are shown in Figure 24. The irradiated yield strength of the material increased to approximately twice that of the unirradiated values,
while significant reduction in ductility was observed. Only in tensile tests well above the irradiation temper­ature did ductility values approach that of the unirra­diated material.

In the work by Gorynin et al.,59 pure W consoli­dated through powder metallurgy was irradiated at temperatures of up to 1073 K in both a mixed and fast reactor up to 2 x 1022ncm~2 (E > 0.1 MeV). Samples irradiated and tested at temperatures near 573 K showed brittle failures at low stress levels, while some ductility and appreciable hardening were observed for samples tested and irradiated at

1073 K. Limited recovery of strength and ductility was observed in postirradiated material annealed at 1473 K for 1 h.

Embrittlement following irradiation due to radia­tion hardening and loss of grain boundary strength due to impurities resulted in increased DBTT for the aforementioned work. The DBTT is dependent on the test conditions in addition to material conditions prior to irradiation and should be used with caution. The DBTT in samples examined by Steichen111 increases from ~-333 K in the unirradiated condition to 503 K, following 1-2 dpa irradiation at ~-653 K, while DBTT values increased from 673 Kunirradiated to 873 K after 1 dpa at 373 K for sintered W.59

Increased DBTT values with irradiation were also reported by Krautwasser et at}42 in powder metallurgy to form W, W-10Re, and Densimet 18 (W-3.4Ni-1.6Fe) bend-test bars irradiated between 525 and 575 Kup to 5.6 x 1021 ncm~2 (E> 0.1 MeV) (see Figure 25). While the addition of Re to W results in improved nonirradiated mechanical properties,1 2 the increased DBTT in the irradiated W-10Re is more severe than in pure W. In the case of the former, the possible development of the w-phase may be responsible for the higher DBTT values and the general increased sensitivity to radiation hardening. The w-phase observed in W-26Re irradiated from 2 to 9.5 dpa at temperatures between 373 and 800 °C141 is reported as precipitating as plate-like particles on the {110} planes of the W matrix, therefore, restricting slip in the material. [2]

It should be noted that due to the limited mechan­ical property data available for W and W-Re alloys, particularly the lack of irradiated data at ele­vated temperatures, accurate determination of the DBTT cannot be made. Nonetheless, increases in DBTT between 200 and 500 K for ~1 dpa of damage reported for the various grades of pure tungsten create limitations on its use, particularly at low irradiation temperatures.59,109,142 Based on irradia­tion data for Mo alloys, the minimum irradiation temperature which avoids severe radiation embrit­tlement is >0.3 Tm or ^1300K for tungsten at neutron fluences >0.03 dpa or 1 x 1021ncm~ (E > 0.1 MeV),3 which correlates with the irradia­tion defect recovery data on tungsten compiled by Keys et a/.127

Recent work in the development of ultra-fine grained tungsten incorporating TiC additions has shown promising results in reducing the sensitivity to radiation-induced degradation of properties.143,144 The grain size refinement, in the range of 50-200 nm, depending on TiC additions and process, theoreti­cally reduces the effective size of weak grain bound­aries that can act as crack initiators. In addition, significant reductions are observed in the density of void formation in the materials relative to pure W at irradiations conducted at 873 K and 2 x 1020n cm~ , though interstitial loop densities are unchanged. While unirradiated room temperature tensile prop­erties still show brittle fracture behavior, the fracture stress is up to four times higher in the W-TiC sam­ples than in pure W in addition to showing 100 K lower DBTT in impact testing. In microhardness measurements following irradiation, the W-TiC samples exhibited no radiation hardening compared with pure W. The change in Vickers hardness follow­ing irradiation for the W-TiC material of Kurishita et a/.143 compared to neutron — and proton-irradiated W and W-Re alloys135 irradiated to similar tempera­tures and doses is shown in Figure 26. The reduced sensitivity of the W-TiC alloy to radiation hardening offers the potential for further development of these alloys for nuclear applications.