1.04.8.1 Tensile Behavior

The refractory metals are the metals in groups V and VI of the periodic table: vanadium, niobium, and tantalum in group V and chromium, molybdenum, and tungsten in group VI. All have the characteristic of a high melting point, hence the term refractory. The group VI metals are typically brittle, even with­out irradiation. For example, chromium is almost never used pure or as a major alloy element, although it is invaluable as a minor alloying element. Molyb­denum and tungsten are both brittle in nature but can be made into useful structural alloys by controlling interstitial impurities and by the addition of minor elements. In contrast to the brittle behavior of the group VI metals, the group V metals are inherently ductile. Structural alloys based upon this group have been developed, primarily for very high temperature and space applications. The primary disadvantage of the refractory metals is their formation of volatile oxides as opposed to protective oxide layers. Vana­dium and molybdenum oxides have melting points below metal working temperatures so that the metals become wet and can have liquid oxide drip off them.

Unlike the tensile behavior of fcc metals, where there is a smooth increase in strength as plastic defor­mation proceeds and work hardening progresses, bcc metals typically exhibit a load drop, or yield point, almost immediately following the onset of plastic deformation, as discussed in Section 1.04.7.2.

In the case of refractory metals, mechanical prop­erties are largely determined by interstitial solutes. High purity refractory metals do not exhibit a yield point but behave more like fcc metals. Niobium alloys irradiated in Li at 1200°C for over three months in EBR-II had total elongations of about 60%. Despite any irradiation hardening, the near absence of oxygen resulted in a very soft material at these high temperatures.50 However, since intersti­tials are nearly always present, tensile behavior is more typically characteristic of bcc metals.

Irradiation-produced defects interact with inter­stitial elements, resulting, in some cases, in severe embrittlement. The tantalum alloy, Westinghouse T — 111 (Ta—8W—2Hf) is used in Figure 25 to illustrate a commonly observed phenomenon of plastic instabil — ity.51 Plastic deformation becomes local, with high levels of slip on closely spaced planes where disloca­tions sweep out the irradiation-generated defects giving rise to local channels of very high deformation. This phenomenon, called channel deformation, is very common in irradiated metals. The result is a sudden and severe load drop with the fracture surface showing what appears to be a completely ductile chisel point fracture.52 Addition of 405 wt ppm oxygen to T-111 results in a cleavage fracture with no measurable plastic deformation, as shown in Figure 26.51 In both Figures 25 and 26, corresponding unirradiated alloys are shown demonstrating ductile behavior. In the unirra­diated condition, the addition of 400 wt ppm oxygen has only minor effects on strength and ductility, as can be concluded by a comparison of Figures 25 and 26. However, irradiation hardening superimposed upon the oxygen interstitial hardening appears to raise the yield stress above the cleavage stress for the alloy. It is suggested that interstitial solutes such as oxygen dif­fuse to irradiation-produced defect clusters, enhanc­ing their hardening effect.53,54 All three behaviors are observed in irradiated refractory metals: ductile with hardening, plastic instability, and cleavage fracture in the elastic range.55

The synergism between interstitial hardening and irradiation hardening does not necessarily lead to immediate catastrophic embrittlement. This behavior is shown in Figure 27 for vanadium containing a very high level of oxygen, 2100 wt ppm. Irradiation to a fluence level of 1.5 x 1019 (E > 1 MeV) leads to the familiar plastic instability but with several per cent plastic strain.53

Interstitial solutes, especially oxygen, may be controlled by the addition of gettering elements. In the vanadium system, titanium has been successful. Alloys in the V-Cr-Ti system have been studied for application to fusion reactors. In refractory metal
alloys, it is the oxygen in solution that is detrimen­tal, so that the oxygen must be combined with the titanium.56 This usually requires a heat treatment of sufficiently long times and high temperatures to pre­cipitate the oxygen. In the vanadium-titanium system,

a heat treatment of two hours at 950 °C has been found sufficient to achieve ductility, whether or not it is to be irradiated.57,58 Confusion over oxygen pres­ent in solution and oxygen combined in precipitates is believed to be one reason for the disparity in tensile data for this class of alloys and perhaps accounts for the relatively high level of ductility observed in Figure 27.

Upon irradiation of alloys in the range of V—3— 5Cr-3-5Ti in the HFIR, no cleavage fracture with­out plastic deformation was observed.59,60 However, plastic instability was commonly observed at irradia­tion and test temperatures below 400 °C. Irradiations in the range of 4-6 dpa in the HFIR produced uniform elongations from 0.2 to 0.6% and total elon­gations below 4%. Corresponding irradiations at 500 °C did not reveal plastic instability and produced uniform elongations in the range of 2-5%.59,60 Irra­diations to 3-5 dpa in the advanced test reactor (ATR) demonstrated plastic instability for irradiation and test temperatures of about 200 °C, with uniform elongations below 0.5%.61 Irradiations conducted in the high flux beam reactor (HFBR) at exposures of only 0.1 and 0.5 dpa corroborated these results and demonstrated a transition in the fracture mechanism between 300 and 400 °C, resulting in a significant increase in ductility at temperatures above 400 °C, Figure 28 62