Irradiation temperature does have an effect on microstructure — for instance higher irradiation temperature results in larger <a> loops, <c> loops do not

form at 77°C (350K), and Zr2(Fe, Ni) SPPs do not become amorphous above an irradiation temperature of about 100°C (453K) (see e. g. Griffiths et al, 1996). In addition, post-irradiation temperatures cause effects that give insight to the microstructure stability.

Damage in the form of <a> loops appears to be stable in post-irradia­tion annealing conditions to about 400°C (673K). Figure 4.18 (Adamson & Bell, 1986) shows that 1 hour at 400°C is a threshold condition for damage in size and density of <a> loops. Above that temperature, or quite likely longer times at that temperature, results in a marked increase in loop size and decrease in loop density. A temperature of 550°C (823K) for 1 h is suf­ficient to reduce the loop density to zero. This is accompanied by a dramatic decrease in hardness, as discussed below. Complementary data (Cheng et al, 1994) indicate no changes in <a> loops after 200 days at 316°C (588K).

On the other hand, <c> component dislocations are quite resistant to change over the whole temperature range where <a> loops disappear. Yang (1989) and Kruger (1990) have shown that 1 h at 560°C (833K) or 575°C (848K) causes little or no change in <c> loop density or size. One hour at 675°C results in a 50% reduction in <c> loop density, while 1 h at 750°C (1023K) results in removal of all loops.

Figure 4.18 indicates hardness decreases in concert with changes in the <a> loop size and density. This is an indication that <c> loops do not have influence on the hardness. A summary is given by Adamson (2006). An addi­tional study (Ribis et al, 2007), confirms the results of Adamson and Bell

4.18 Post-irradiation microstructure (<a> loop density and size) and hardness of Zircaloy-2 irradiated to a fluence of 6.5 x 1024 n/m2 (E > 1 MeV). Upper: TEM after annealing at indicated temperatures. Lower: density, size and hardness as functions of annealing temperature (Adamson & Bell, 1986).

(1986), and add modelling equations for the recovery process. Bourdiliau et al. (2010) go a step further and show that there is a direct relation between recovery of hardness and recovery of ultimate tensile stress (UTS) for both SRA Zircaloy-4 and Zr1Nb. However the recovery for Zr1Nb is more slug­gish than for Zircaloy-4, as shown in Fig. 4.19. Zr1Nb does not fully recover the irradiation-induced hardening, primarily due the effects of the thermally stable, irradiation-induced phase which forms in that alloy.

Post-irradiation annealing also has effects on irradiation-affected SPPs. The observed phenomena give important insights into, for instance,

corrosion mechanisms. For Zircaloy Yang (1989), Kruger (1990) and Cheng et al. (1994) report that post-irradiation annealing causes SPPs to recrystal­lize, to regain Fe and Ni, and to form under specific conditions of time and temperature. Minimal effects are observed for 316°C (589K) for 30 days, but for 200 days significant amounts of Fe diffuse back to the precipitates. At 400°C (673K) Fe diffuses back to precipitates in less than 10 days, and Fe-rich precipitates form at grain boundaries. At higher temperatures >560°C (833K) amorphized SPPs recrystallize, Fe and Cr diffuse back to SPPs, and re-precipitation occurs in the matrix and grain boundaries. Recent studies by Vizcaino et al. (2010) tend to confirm the earlier results.