Tensile behavior is determined by the irradiation — induced defect structure previously discussed. Aus­tenitic stainless steels will again be used for the example since they are typical of fcc alloys and in
many respects to other alloys (see Chapter 2.09, Properties of Austenitic Steels for Nuclear Reac­tor Applications and Chapter 4.02, Radiation Damage in Austenitic Steels). The behavior of other example classes of alloys will be discussed in later sections of this chapter. The tensile behavior characteristic of austenitic stainless steels is shown in Figure 2, where yield strength is plotted as a function of fluence and displacement level.9 Saturation in strength is clear with the saturation time becoming shorter as irradiation temperature is increased. At temperatures above about 500 °C, saturation is evi­dent, but in this case, strength decreases.

This decrease is a result of recovery of the cold — worked microstructure of the 20% cold-worked type 316 stainless steel presented in Figure 2. Figure 3

shows yield strength resulting from the recovery of a cold-worked dislocation structure and the generation of a radiation-induced microstructure, resulting in a saturation strength independent of the initial condi­tion of the alloy. 0 Again, it is seen that the approach to saturation is faster with increasing temperature, with saturation achieved between 5 and 10 dpa at 538 and 650 °C, but 15-20 dpa is necessary to achieve saturation at 427 °C.

Saturation is observed in yield strength curves for fluences as high as 9 x 1022ncm~2 in a fast reactor (45 dpa), but more recent data show a hint of soft­ening above 50 dpa,11,12 and other fast reactor data have shown a reduction in strength even for dis­placements below 50 dpa, as shown in Figure 4.1 This could result from coarsening of the microstruc­ture or depletion of interstitial elements from the matrix due to precipitation. This effect is also observed in martensitic steels irradiated to high dpa levels in the FFTF, but in this class of alloys, recovery of the martensitic lath structure is also a factor.12 However, even in austenitic steels, it is difficult to attribute such softening with certainty to an irradiation effect because of the strong influence
of irradiation temperature on strength.14 Indeed, uncertainties in irradiation temperature are an inher­ent difficulty in neutron irradiation experiments.