Like all group V transition metals, the affinity of Nb for, and its ability to dissolve, high concentrations of interstitial atoms such as hydrogen, oxygen, nitrogen, and to a lesser extent, carbon can strongly influence the properties of the metal through defect-impurity interactions. Hydrogen, carbon, and oxygen impurities have a strong effect on the tensile Ductile-brittle transition temperature (DBTT) of pure Nb (reviewed by Hahn et a/.13), with hydrogen levels near 10 ppm increasing the DBTT to 173 K and over 273 K at levels >100 ppm (DBTT of high purity Nb and Nb alloys is near 73 K14,15). The effects of oxygen and carbon were less severe, but influential at levels of 100 ppm and greater. The effect of nitrogen on embrittlement also appears to be as severe as that of oxygen, though some uncertainty exists as to whether solubility limits have been exceeded in the data.1 The effect of interstitial impurities on the irradiated properties of Nb and Nb-base alloys is significant and has been examined, though the overall database for irradiated properties is limited.

The interplay between the radiation-created defects and the interstitial impurity elements was investi­gated by Igata et a/.16 for pure Nb (70 wppm oxygen and 30 wppm nitrogen) irradiated to 3.4 x 1020 n cm-2 (E > 1 MeV) at temperatures below 413 K and post­irradiation annealed up to 973 K. Increases in yield strength over the as-irradiated values following annealing were measured at 473 and 673 K, attrib­uted to the interplay of the defect clusters trapping oxygen and nitrogen atoms, respectively. Above 773 K, no difference between the annealed and as-irradiated yield stress was observed.

Hautojarvi eta/.17 and Naidu eta/.18 examined the interaction between vacancies and interstitial impurities in irradiated Nb through positron annihi­lation studies. In high-purity Nb, vacancy clustering within the collision cascades is observed, starting as low as 160 K, with vacancy migration peaking around 250 K, but in materials with higher hydrogen content, the vacancy migration stage shifts to temperatures close to 400 K.17 The irradiation exposure at inter­mediate temperatures (0.3—0.6 Tm) can lead to void swelling, irradiation creep, and helium embrittlement through processes involved in (n, a) reactions or impurity gas atoms. Naidu eta/.18 examined the effect of He and its interaction with vacancies in pure Nb, leading to the development of bubbles through a-irradiated specimens. At temperatures between 623 and 1023 K, bubble growth occurs through the addition of He atoms and vacancies, followed by migration and coalescence at higher temperatures, eventually leading to the annealing out of the He bubbles and vacancy complexes above 1173 K.18

The irradiation-induced swelling of pure Nb gen­erally appears at temperatures between 673 and 1323 K with peak swelling near 873 K (0.32 Tm), though these limits are not clearly defined and are based on the very limited data available, compiled by Wiffen19 and Pionke and Davis.1 A maximum swelling of 4.8% fol­lowing irradiation to 2.5 x 1022ncm~2 at 858 K was reported.1 However, the magnitude of swelling shows considerable scatter in the literature, possibly reflect­ing the influence of impurity concentrations and dif­ferences in irradiation conditions and microstructural interpretation of the materials.19 Fischer20 reported that void concentration increased four to seven times for a fourfold increase in flux for the same total fluence. This produced a reduction in void size with flux and therefore a reduction in the total swelling.

Loomis and Gerber21-23 examined the influence of oxygen and substitutional binary alloy additions on the swelling of 3 MeV 58Ni+ ion-irradiated Nb up to ^50 dpa. Void formation and characteristics in size and morphology were found to be dependent on temperature, oxygen concentration, and the type of substitutional alloy addition. The average void diam­eter was found to increase with temperature as well as oxygen up to 0.02 at.%. Higher oxygen concentra­tions resulted in a decrease in void diameter to 0.1 at.% O, above which void diameters showed no significant changes. The number density of voids was found to decrease with temperature, but increase with oxygen concentration to ^0.06 at.%, above which the number density showed no significant change. As the volume fraction of swelling (А V/ V) is propor­tional to both the void number and the cube of the void diameter, the volume fraction is observed to increase with temperature and oxygen concentration to ^0.04 at.%, followed by a decrease and plateau of the volume fraction above 0.1 at.%. The dependence of А V/ V on temperature and oxygen concentration is illustrated in Figure 1. Microstructural examina­tion revealed an ordering of the voids into a lattice — type structure in the material irradiated at 1050 K to ^40 dpa and oxygen concentration >0.039 at.% oxygen. The higher temperature of the maximum swelling as compared to the neutron irradiation data is believed to be associated with the higher displace­ment damage rate of the ion-bombarded material,19 though the higher impurity levels may also provide an influence.

The effect of dilute (^2.4 at.%) substitutional alloy addition on the swelling of 0.06 at.% oxygen — doped Nb was also examined for 3 MeV 58Ni+ ion irradiation at 1225 K. The AV/V was determined to increase through the addition of Ta, but decreased with increasing effectiveness by the addition of Ti, Zr, V, and Hf. The addition of the reactive alloying elements to Nb suppresses void formation through the gettering of interstitial impurities that act as void nucleation sites. The AV/V was determined to be unaffected by the addition of Ni or Fe. The depen­dence of AV/V on temperature, oxygen, and substitu­tional addition is also shown in Figure 1.

Figure 1 The dependence of void volume fraction (AV/V) in 3 MeV 58Ni+ ion-irradiated Nb on the concentration of oxygen and dilute solute additions. Reproduced from Loomis, B. A.; Gerber, S. B. J. Nucl. Mater. 1983, 17, 224-233.

Swelling in Nb-1Zr has been examined, though only scattered data are available in the examination of temperature and flux dependence. The available swelling data on Nb—1Zr, compiled by Powell eta/.24 and Watanabe eta/.25 presented in Figure 2, show the lack of data on the temperature range in which peak swelling appears. The swelling data shown in the figure were measured through electron microscopy, with the exception of the data by Powell et a/.24 and Wiffen26 Alloy impurity chemistry, in addition to interpretation and measurement error, may account for the scatter associated with the lower tempera­tures. The work of Watanabe et a/.25 and Garner et a/.27 indicates that irradiation-induced swelling is dependent on the thermomechanical history of the material. In that material, cold-working followed by solution anneal and aging exhibited swelling, while material not given the preirradiated cold-working showed some densification. The changes in density of the material are dependent on the phase-related transformations involving precipitation.

Swelling in Nb-1Zr appears to be centered over a more narrow temperature range than in Nb, with a peak near 1073 K that is higher than that of the pure metal. While the addition ofZr to Nb appears to delay nucleation of voids to higher temperatures, the voids that form are of larger size than those appearing in pure Nb under comparable conditions. For example,

Figure 2 Swelling as a function of irradiation temperature and dose for neutron-irradiated Nb-1Zrfrom available literature compiled by Powell etal.24 and Watanabe etal.25

following irradiation to 2.5 x 1022ncm~2 (E > 0.1 MeV) at 1063 K, the diameter, concentration, and volume fraction of voids in Nb-1Zr was 57.5 nm, 1.8 x 102°m~3, and 2.2%, respectively,1 whereas under similar conditions, the same void parameters in pure Nb were 18.6 nm, 2.8 x 1021 m~3, and 1.04%.

While void formation and swelling in Nb and Nb-1Zr occurs, the total swelling is generally <5% and within engineering limits, even for high neutron exposures >10 dpa.3 The addition of Ti to Nb was found to increase void resistance and has been found to suppress void formation in V at concentrations as low as 3%.29 The combination of reactive alloy ele­ments and Nb in the C-103 alloy may suggest a greater void formation resistance than in pure Nb and Nb—1Zr.