Tensile ductility is a more vulnerable parameter than strength to radiation effects since it tends to be very high in unirradiated austenitic stainless steels and is often reduced to quite low levels by irradiation. It is also of more concern since strengthening, although not reliable due to its slow initiation, is usually a beneficial change. In contrast, embrittlement is always detrimental. Like strength, ductility exhibits saturation with increasing fluence, although the behavior is significantly more complex than that of strength. The general trends in type 316 stainless steel are shown in Figure 7 for material irradiated in the EBR-II. These data are for the same specimens for which the yield strength was shown in Figure 2.9 Fast reactor data are used here to avoid the compli­cation of helium effects. Once stabilization of the dislocation microstructure is achieved, a smooth curve approaching an apparent saturation is observed.

More information can be gleaned from ductility data if they are viewed in terms of irradiation and test temperature. Figure 822 shows total tensile elon­gation for a series of irradiated austenitic alloys at a displacement level of 30 dpa in both annealed and cold-worked conditions. The room temperature ductility exceeds 10%, but it decreases rapidly with increasing temperature up to approximately 300 °C and then exhibits the expected increase with tem­perature observed for unirradiated alloys. Beyond 500 °C, ductility again decreases with an onset of intergranular embrittlement resulting from helium introduced through transmutations in the thermal flux of the HFIR.

Energy (MeV) Figure 6 Cross-section for (n, a) reactions as a function of neutron energy.

Uniform elongation, the elongation at the onset of plastic instability, or necking, appears to be most sensi­tive to the effects of irradiation and, in general, is less dependent on specimen geometry than other para­meters such as total tensile elongation. The low values of uniform elongation are often cause for great con­cern, which is usually justified. However, it should be borne in mind that if stresses remain below the yield
stress of a metal, elongation becomes a secondary con­cern. As long as limited plastic deformation relieves the stress that produced it, a structure remains intact.

The high level of irradiation strengthening observed at temperatures below 300 °C, which is due to black dot defect clusters and small loops, also results in low ductility throughout this tempera­ture range. Small helium bubbles and helium-defect

Figure 9 Uniform elongation as a function of irradiation and test temperature at a displacement level of 10dpa.

The trend curves are for type 316 stainless steel and PCA. Reproduced from Grossbeck, M. L.; Ehrlich, K.; Wassilew, C. J. Nucl. Mater. 1990, 174, 264.

clusters also contribute to hardening and reduction in ductility, but this form of helium embrittlement is not related to the severe intergranular embrit­tlement that is observed above 500 °C. Both these effects are apparent in Figure 9 where uniform elongation for an extensive set of austenitic alloys irradiated in thermal and fast spectrum reactors is shown.1 The specimens irradiated in the fast spec­trum (<5 appm He) exhibit consistently higher duc­tility than the mixed-spectrum reactor specimens (500-1000 appm He) even at this low displacement level, especially above 600 °C, where helium embrit­tlement is certain to control.

A similar pattern is exhibited at 30 dpa where a very limited uniform elongation characteristic of lower temperatures is apparent. After a restoration of ductility above 400 ° C, ductility again decreases above 500 °C due to the onset of intergranular helium embrittlement. Differences in alloy behavior, especially in the case of titanium-modified alloys somewhat clouds the understanding of helium
embrittlement observed in Figure 10.11 However, at 50 dpa, where helium levels exceed 4000 appm, the trend becomes clear with the fast reactor specimens showing uniform elongations several times larger than those observed in mixed-spectrum reactors (Figure 11).11 What is less expected is the recovery of ductility at 50 °C at 50 dpa compared to the results at 30 dpa. This irradiation annealing effect has also been observed at 230 °C by Ehrlich, where strength of the alloy 1.4988 decreased continuously from 10 to 30 dpa.20 Results from an experiment in the Oak Ridge Research Reactor (ORR), where the spectrum was tailored to produce a ratio of He per dpa characteristic of a fusion reactor, show similar low levels of uniform elongation for cold-worked alloys at low temperatures, but high uniform elonga­tions were observed in annealed type 316 stainless steel at 60 °C. This high ductility was drastically reduced between 200 and 330°C before the micro­structure characteristic ofhigher temperatures became

effective.21

20

700

Figure 10 Uniform elongation of austenitic stainless steels irradiated in fast and thermal reactors to a displacement level of 30dpa. Severe helium embrittlement is shown at 600°C. Reproduced from Grossbeck, M. L.; Ehrlich, K.; Wassilew, C.

J. Nuct. Mater. 1990, 174, 264.