Two earlier reviews of the irradiation-induced prop­erties of Mo and TZM have been presented as part of the UWMAK-III fusion reactor study86 and for the SP-100 space nuclear power program.19 Much of the known swelling data on irradiated Mo is contained in these reviews, with the majority of data for irradia­tions <10dpa and temperatures below 1073 K. The swelling data available are considerably scattered, with little coherence to examinations on the swelling as a function of temperature or dose.

Swelling in Mo is expected to begin around 573­673 K and continue to temperatures near 1573 K.19 Maximum swelling in pure Mo remains below 4% for fluences up to 1 x 1023 ncm~2 (E > 0.1 MeV), ^50dpa, with peak swelling at irradiation tempera­tures near 900 K. Attempts at consolidating the reported swelling data as a function of irradiation temperature through normalizing the fluences proved
to be inaccurate in determining the upper bound limit for maximum swelling. 9 The swelling data col­lected from numerous sources,53,87-89 including those contained in the review work of Brimhall et a/.90 for irradiated Mo as a function of dose and irradia­tion temperature, are provided in Figure 12. Void swelling was found to be < 1% in Mo irradiated to 8 x 10 n cm, E > 0.1 MeV at temperatures between 673 and 1173 K by Evans.53 Void swelling studied by Stubbins et a/.88 in 3.1 MeV 51V+ ion-irradiated Mo between 1173 and 1393 K up to 50dpa remained below 4%, while irradiations between 1523 and 1780 K were near 10%.

Void ordering has been observed in both neutron-

irradiated28,89,91 and ion-irradiated Mo88 at tempera­tures between 700 and 1373 K. Garner and Stubbins89 examined the irradiation and material conditions that contribute to void ordering. Irradiation tempera­tures near 700 K delineate the lower boundary tem­perature for void lattice formation at irradiations above 20 dpa. At lower doses, void lattice formation was not observed. The void superlattice constant, mea­sured as the distance between void centers along the <100> direction in the material, is found to increase with temperature from ^2.4 nm at 700 K to 4.5 nm
at 1176 K.91 Swelling is expected to reach a maximum of 3-4% on the development of the void lattice struc­ture, based on an attainment of an equilibrium ratio of void diameter to void superlattice parameter.92 At temperatures >1423 K, void lattice formation is no longer observed, leading to the high values of swelling observed in the material ion irradiated to high doses.88

The onset of void growth in neutron-irradiated material appears to be accelerated in cold-worked materials compared to annealed materials, reaching a maximum in swelling at doses near 40 dpa for tem­peratures below 873 K and 20 dpa at higher tempera — tures.89 At higher doses, swelling decreases through void shrinkage, with swelling values approaching those of annealed materials. Void shrinkage has also been reported by Bentley eta/.93 and Evans53 to occur because of changes in the void sink bias89 presum­ably due to the segregation of transmuted species at the void surfaces, making them more attractive for interstitials.

Irradiation-induced swelling in TZM has been reported53,94-97 and generally shows similar temper­ature dependence as the pure metal. The fluence and temperature dependence of swelling of TZM was examined by Powell et a/.95 and Gelles et a/.,94 with

results from the latter shown in Figure 13. Peak swelling in TZM following irradiation to 1.78 x 1023 ncm~2 and 873 K remained below 4%, though the data are limited to irradiation temperatures below 923 K. Only limited data are available on direct com­parisons between TZM and pure Mo, with Bentley and Wiffen96 reporting 1% swelling in Mo-0.5%Ti and TZM alloys and 0.6% swelling in pure Mo under the same irradiation conditions. Similarly, 4% swelling was observed in TZM and 3% in pure Mo following irradiation to 5.4 x 1022ncm~2 at 923 K.97 In examining Mo and TZM of different preirra­diated material conditions, Evans53 observed equal or greater swelling in TZM compared to Mo follow­ing irradiation to 3.5 x 1022ncm~2 (E> 0.1 MeV) at 823 and 873 K. However, in the materials irradiated at 723 K for the same fluence, the TZM alloy showed lower swelling, except in the carburized condition. The Ti and Zr atoms not tied up as carbides are assumed to have played a role in reducing void size in the material at the lower temperature.

There is little information on the swelling behav­ior of Mo-Re alloys. Measured swelling of 0.44% in Mo-50Re irradiated to 5.3 x 1022ncm~2 (E > 0.1 MeV) at temperatures which rose during irradiation from 1128 to 1329 K was reported.26 For irradiated Mo-Re alloys, radiation-induced segregation (RIS) and transmutation can lead to precipitation of
equilibrium or nonequilibrium phases, which can be detrimental to mechanical properties. This is examined in the next section.

Electrical resistivity changes to 5.4 dpa irradiated Mo at 733 K were examined by Zakharova et a/.98 using single crystal samples. Increases in resistivity of 10-14% and 92-110% were measured at postirra­diation test temperatures of 298 and 77 K, respec­tively. The largest resistivity changes were measured in the [100] direction. A residual 10% increase in resistivity was measured following annealing above 0.6 Tm associated with the accumulation of trans­muted radionuclides.

The changes in electrical resistivity of LCAC-Mo over a 353-1373K irradiation temperature range up to 3.3 dpa were examined by Li eta/.99 and Cockeram et a/, with the latter examining the recovery of

resistivity following isochronal anneals. The room temperature resistivity for 353 K irradiated LCAC — Mo rapidly increases between 0.01 and 0.1 dpa saturating near 0.2 dpa for an ^42% increase over the unirradiated value.99 Increases in room tempera­ture resistivity of 10-12% were reported following 0.5-1.2 dpa irradiation at 543 K, and 3.3-5.3% after 1.4-2.4dpa at 878 K. At irradiation temperatures >1208 K, little (<3%) to no net increase in resistivity was observed for irradiations up to 3.3 dpa. This is reflected in the higher mobility of vacancies and

interstitials formed during irradiation to diffuse to sinks where annihilation occurs, reducing the electri­cal scattering effects that these defects have at lower irradiation temperatures. The small increases mea­sured at the higher irradiation temperatures were pri­marily due to transmutation products. As is shown in the next section, the changes in electrical resistivity with increasing irradiation temperatures also correlate with changes in measured hardness, though at a greater level of sensitivity. This is controlled by microstruc­tural changes, as the small dislocation loops and voids of high distribution density appearing at the lower irradiation temperatures coarsen into larger and fewer defects that have less interaction with deforma­tion dislocations.