Little coverage of the changes in mechanical proper­ties following irradiation has been given to Nb and Nb alloys, with the majority of the data for tempera­tures below 800 K. Some preliminary experimental work on the irradiated mechanical properties of Nb alloys Cb-752 (Nb-10W-2.5Zr)30 and FS-85 (Nb — 10W-28Ta-1Zr)31 is available. However, these alloys are not commercially produced and have shown indications of thermal aging instabilities, leading to grain boundary embrittlement.12,32,33 The irradiated mechanical properties of these alloys show similar radiation hardening as in the pure metal, but with mechanical properties more sensitive to thermal aging conditions. The bulk of the irradiated mechan­ical properties data is for the Nb-1Zr alloy as well as the pure metal, and is covered in this review.

The irradiated mechanical properties of Nb and Nb-1Zr are strongly governed by irradiation temper­ature, determining whether the mechanical properties are controlled by dislocation loops or a combination of loops and voids in the microstructure. As cavity for­mation can be delayed or suppressed by higher irradi­ation temperatures in Nb-1Zr, mechanical property comparisons between the alloy and the base metal will reflect their irradiated microstructure. For Nb and Nb-1Zr irradiated to 3 x 1022ncm~2 at ^728 K, the pure metal contains both dislocation loops and voids, while the alloy exhibits no void formation.19 A compar­ison of the tensile properties of Nb and Nb-1Zr irra­diated under similar conditions is shown in Figure 3. The irradiated strength of both materials shows an increase in tensile strength above the unirradiated

condition, with Nb-1Zr showing a greater sensitivity to irradiation. As the mechanical properties of Nb-1Zr are dominated by the dislocation loop structures, yield instability is observed in the material, leading to the early onset of necking. This results in <0.2% uniform elongation, though total elongation near 10% is still achieved. The yield instability is associated with dislo­cation channeling, in which deformation dislocations will create defect-free channels along their slip plane, following the annihilation of the loop structures. This occurs only after enough applied stress is achieved to overcome the obstacles, but the net effect is a nonuni­form plastic deformation through channels that allow for the movement of deformation dislocations at reduced stress.

The irradiated Nb samples whose properties are shown in Figure 3 contain, in addition to dislocation loops, voids that limit dislocation channeling by providing added obstacles to deformation, resulting in some measure of uniform elongation and work hardening upon yielding. The microstructure depen­dence on the tensile properties can best be illustrated by the comparison shown in Figure 4 of Nb irradiated at 328 and 733 K. The higher irradiation temperature results in the development of microstructural voids and thus the significant differences in the tensile curves. The lower irradiation temperature results in dislocation channeling following yield and the

associated work softening during necking to failure at around 11% total elongation. While the higher irradi­ation temperature sample was irradiated to a higher total fluence, the effect of dose is observed only on the
relative strength increase over the unirradiated condi­tion. The higher irradiation temperature produced voids in the microstructure, providing additional obsta­cles to deformation and higher uniform elongations and modest work hardening.

Little is known with regard to the aging properties of Nb-1Zr or the combined thermal and radiation effects. The addition of 1 wt% Zr to Nb creates a dispersion-strengthened alloy, in which the Zr com­bines with interstitial impurities creating fine preci­pitates throughout the material. The development of these fine precipitates on aging at 1098 K can increase the tensile strength between 50 and 100 MPa over the annealed condition and provide an effective strength­ening greater than that observed through modest irradiation31 (Table 1).

Irradiation of Nb-1Zr to 0.9 dpa at 1098 K showed a modest increase in yield and ultimate tensile strength to 135 and 192 MPa, respectively, over the annealed condition. This increase in tensile strength either through aging or irradiation results in a cor­responding decrease in uniform elongation from 15% to 3.5% and total elongation from 25% to 15%. Aging at temperatures above 1098 K produces little effective hardening as the precipitates coarsen in the microstructure.33 Irradiation to 0.9 dpa at 1248 and 1398 K of Nb-1Zr showed only a modest increase in the yield strength over the aged and annealed specimens, though ultimate tensile strength and elon­gation were unchanged or less. Irradiation to 1.88 dpa at 1223 K resulted in weaker tensile properties
compared to the 0.9 dpa sample, believed to be due to further precipitate coarsening. The time under irradiation conditions for the 1.88 dpa sample was near 1100 h and produced similar tensile properties as that of the aged-only material.

As discussed in the preceding paragraphs, the irradiated properties of Nb and Nb-1Zr are governed by their microstructure and are influenced by temper­ature, displacement damage rate, and neutron spec­trum. The tensile properties of neutron-irradiated Nb-1Zr for damage levels between 0.1 and 5 dpa (Horak et al.34 and Wiffen35) summarized by Zinkle and Wiffen3 are shown in Figure 5. At temperatures below 800 K, a large increase in the tensile strength from irradiation is observed with the corresponding low uniform elongations. At higher temperatures, uniform elongation increases because of the presence of voids in the microstructure. However, the data plotted in Figure 5 show uniform elongations remain­ing low up to 1100 K, while radiation hardening is relatively moderate, suggesting that impurities are the source of the reduced elongation values.

No irradiated fracture toughness data exist for Nb or Nb-1Zr, though comparisons can be made from the larger irradiated vanadium alloy database, in which fracture toughness embrittlement becomes a concern when tensile strength exceeds 600-700 MPa and there­fore at temperatures below 400 K for Nb-1Zr.36 How­ever, if a conservative value is assigned to the critical stress to induce cleavage fracture of ^400 MPa (40% lower than that observed in vanadium alloys),

fracture toughness becomes a concern at tempera­tures below 800 K for Nb-1Zr.3 While irradiated ten­sile strength above 800 K is close to the unirradiated values, uniform elongation values remain low until irradiation temperatures >1000K. Therefore, a con­servative approach towards engineering design needs to be taken with this alloy.

The mechanical properties ofirradiated refractory alloys can be influenced by the formation of He developed through the (n, a) reactions, leading to the grain boundary formation of bubbles and the eventual embrittlement of the material. Some scoping investiga­tions on the effect of He on the irradiated mechanical properties of Nb-1Zr have been performed. Wiffen37 investigated the high-temperature mechanical proper­ties of 50MeV a-irradiated Nb—1Zr. In tensile tests conducted at 1273 and 1473 K, no significant effect of He on the strength or ductility of Nb-1Zr was observed for samples containing 2-20 appm He. Later analysis of the creep ductility reductions was found to be dependent on the observed precipitate phase develop­ment through the pick-up of oxygen during implan­tation.38 He-implanted Nb-1Zr through 100 MeV a-irradiations at 323 and 873 K by Sauges and Auer39 found no significant effect on ductility up to 80 appm He. Wiffen19 observed that uniform elongations stayed around 1% between test temperatures of 723 and 1073 K on 130 appm 10B doped Nb-1Zr irradiated in a fast reactor between 723 and 1223 K up to 6 x 1022 n cm~2. These were slightly higher than those observed in undoped material; this is believed to be due to the formation of He bubbles in the grains of the material
acting similar to voids in generating obstacles to dislocation channeling. In general, no detrimental effects on mechanical properties were reported for accelerator-injected He between 1273 and 1473 K for He concentrations <200 appm.37,40