The overall mechanical property data for irradiated Ta and Ta-base alloys are very limited, with most studies involving irradiation at temperatures <1073 K. In general, the behavior of Ta and its alloys is similar to that of other bcc materials in that radiation harden­ing is observed with significant reductions in elonga­tion at temperatures <0.3 Tm (Tm = 3290 K, pure Ta). As is discussed in this section, the addition of solute strengthening elements creates an increased sensitivity to radiation hardening of the material. In addition to the lack of high-temperature irradiation behavior, impact and fracture toughness data for irradiated Ta and Ta alloys are also limited.

As with all refractory metals, the mechanical be­havior of pure Ta is highly dependent on the impu­rity levels in the material. This may explain the observed differences between the work of Brown et a/.54 and Chen et a/.,55 of 800 MeV proton irradia­tions up to 11 dpa at temperatures <673 K (Figure 7). While chemical analysis quantifying the purity of Ta was not reported in the former, irradiation to 0.26 dpa resulted in a yield strength increase from 350 to 525 MPa over the unirradiated value with a corresponding drop in ductility below 2%. Flow insta­bility following yield was characteristic of samples irradiated to 0.26 and 2.9 dpa.54 Tensile properties of high-purity Ta irradiated to 0.6-11 dpa tested at room temperature and 523 K showed similar increases in tensile strength, while the uniform elongation re­mained near 8% following irradiation to 0.6 dpa or

higher.55

The tensile properties of neutron-irradiated Ta were reported by Claudson and Pessl,30 Wiffen,19
and more recently by Byun and Maloy.56 In the first, irradiation to 0.13 dpa (where irradiation to 0.76 x 1022ncm~2, E > 0.1 MeV is ~1.0 dpa in pure Ta57) at 673 K resulted in increased yield strength, though no significant loss in ductility occurred over the unirradiated control. However, work softening following the yield drop was observed.

Irradiation to higher displacement doses in pure Ta by Wiffen19 showed the potential lower operating temperature limitation of Ta. Following irradiation to 1.97 dpa at 663 K, yield and ultimate tensile strengths increased to near 600 MPa with a corresponding drop in ductility to <0.3% uniform but with total elonga­tion near 10%. The observed plastic instability, at­tributed to the lack of uniform elongation following yielding, resulted from dislocation channeling. Some recovery of ductility is observed following irradia­tion to 913 K, which correlates with temperatures approximating the maximum swelling temperature (Figure 6) and a change in the dominating microstruc­tural features influencing deformation behavior in the metal. The tensile data are presented in Figure 8, along with the irradiated properties of T-111, which are discussed later.

The recent work of Byun and Maloy56 investi­gated tensile behavior as a function of fluence for pure Ta, Ta-1W, and Ta-10W, establishing deforma­tion mode maps for pure Ta and Ta-1W that outline the conditions in which brittle failure and uniform and unstable plastic deformation occur. Following fast-reactor exposures at temperatures <373 K, a progressive hardening and gradual loss in ductility are observed in the tensile properties of pure Ta, leading to a near doubling of the yield stress by 0.14 dpa over the unirradiated value (Figure 9(a)).

An early onset of necking or plastic instability was observed in Ta at doses above 0.0004 dpa. The lower elongation strains in the pure Ta compared with the work by Chen et al.55 is believed to be due to the higher oxygen content in the material.56

The introduction of 1 wt% W resulted in a near — 30% increase in unirradiated strength over pure Ta (Figure 9(b)). The Ta—1W alloy showed greater sen­sitivity to radiation hardening than the pure metal. The tensile properties as a function of dose were similar to those of the pure Ta. However, above 0.004 dpa, plastic instability becomes more predomi­nant in the Ta—1W alloy and occurs immediately following yielding. For Ta—1W irradiated from 0.7 to 7.5 dpa in a mixed proton and neutron irradiation from the same study, hardening was saturated with little change in ductility (insert shown in Figure 9(b)).

Macroscopic deformation mode maps produced for Ta and Ta—1W by Byun and Maloy56 are a graphi­cal way of predicting the performance of a material in an irradiation environment. The deformation mode map for pure Ta is shown in Figure 10(a), while that
of Ta—1W is shown in Figure 10(b). The yield and plastic instability stress were directly obtained from tensile data, while the fracture stress was calculated through a linear strain hardening model for necking deformation, assuming that during instable deforma­tion, the strain hardening rate remains unchanged and is approximately the plastic instability stress. The frac­ture and plastic instability stresses are independent of dose, with a ratio between the stresses of ^2 for the materials studied. The fracture strength decreases with dose if the material becomes embrittled, for example, through interstitial segregation or secondary phase precipitation at grain boundaries, though this was not observed in their work. The yield strength is highly dose dependent, though the yield stress was signifi­cantly lower than the fracture strength in Ta—1W, suggesting that the material may show limited ductil­ity to even higher displacement doses. The effect of increasing test temperature for each material further increases the boundaries for uniform deformation behavior. This increase was found to be greater in pure Ta.

The room temperature unirradiated tensile strength of Ta-10W is nearly double the value of the Ta—1W and triple that of pure Ta in the material investigated by Byun and Maloy,56 and also shows an increased sensitivity in radiation hardening over the pure metal (Figure 9(c)). This sensitivity is also clearly apparent at higher irradiation temperatures near 673 K, as shown in the comparison of tensile curves that were compiled by Ullmaier and Carsughi58 of earlier work (Figure 11). Near room temperature irradiation of Ta-10W to the mixed proton and spallation neutron exposure by Byun and Maloy56 to doses between 2 and 25.2 dpa showed prompt necking following yielding. Total elongation values of <3% were observed for doses between 2 and 7.5 dpa, with near-zero ductility observed at 25.2 dpa. Fast neutron irradiation studies of Ta-10W by Gorynin et a/.59 observed brittle failure after 0.13 dpa in materials irradiated and tested near 600 K. Less than 5% total elongation was measured following 1.97 dpa irradiation at 700 K, despite a near

doubling of the yield stress over the unirradiated mate­rial. Limited ductility was also observed following 2.63 dpa exposure in materials irradiated and tested at 1073 K, with a yield strength increase from 240 to 315 MPa over the unirradiated control. While low- temperature embrittlement following exposure to 0.13 dpa was reported in the neutron-irradiated mate — rials59 and limited ductility following mixed proton and neutron exposure,56 the interstitial concentra­tions on the behavior of these materials may be more influential than the irradiation spectrum.

Similar to Ta and Ta—10W, very limited data exist on the irradiated properties of T-111. The most referenced base-line study is that by Wiffen,1 shown as in Figure 8. Large increases in yield and ultimate tensile strengths are observed following irra­diations to 1.9 x 1022ncm~2 (E> 0.1 MeV), 2.5 dpa, at 688 and 913 K. The increase in radiation hardening is substantially greater than that observed in pure Ta irradiated under similar conditions. Yield and

Figure 10 Deformation mode map of (a) pure Ta and (b) Ta-1W for room temperature irradiations, illustrating fracture, plastic instability, uniform plasticity, and elastic regions as a function of stress and displacement dose. The increases in the uniform plasticity region for temperatures of 523 K are superimposed. Reproduced from Byun, T. S.; Maloy, S. A. J. Nucl. Mater. 2008, 377, 72-79.

ultimate tensile strengths of around 1250 MPa are reported for irradiation at 688 K, with uniform and total elongation of <0.3% and 4.5%, respectively. Irradiation at 913 K improves uniform and total elon­gation values only slightly to ^2.5% and 8%. These values represent more than a 50% reduction in ductil­ity over the unirradiated values. No known irradiated property data for T-111 exist for temperatures above
913 K. As tensile strengths ofboth Ta-10W and T-111 exceed 1000 MPa at temperatures below 1000 K19,56,59 and are well above the stresses that produce brittle behavior in vanadium alloys for which more data are available, it is likely that these Ta alloys are embrittled under these conditions.3 Further expansion of the irradiated materials database including fracture tough­ness data for Ta and Ta alloys irradiated near and above 1000 K is much needed to ascertain the upper temperature limitations. However, based on this pre­liminary data, temperatures below1000 K may need to be avoided for Ta and Ta-base alloys.

Low-fluence irradiations to 1.2 x 1015ncm~2 at room temperature and 623 K have been performed to evaluate the performance of T-111 and Ta-10W for use in radioisotope power applications.60 These low-dose irradiations produced little change in the tensile properties of the two alloys. Some variations in the total elongation were observed in T-111, which may be related to the distribution and make-up ofthe Hf-rich compounds in the material as well as the effects of radiation. Thermal stability of T-111 can be an issue, as a brittle behavior following 1100 h aging at 1398 K has been reported,41 due to precipi­tation of Hf-rich compounds along grain boundaries. It is not known how the combination of long-term thermal aging under irradiation affects the structure- property relationships or how the detrimental pre­cipitation of the interstitial elements with Hf can be controlled.