There are relatively few data in the literature concerning the postirradiation creep behavior of zir­conium alloys as pointed out by Peehs and Fleisch.149 Even in the thorough review by Franklin et a/.,134 very few results concerning the postirradiation creep are given. In the case of the SRA zirconium alloys142,150-155 or RXA Zy-2,142,156 several authors have shown that irradiation leads to a strong decrease of the creep rate (Figure 17). This phenomenon is attributed to the presence of a high density of irradi­ation defects that harden the material. However, according to Ito eta/.142 and Schaffler eta/.,1 2 irradi­ation does not seem to affect strongly the stress sensitivity coefficient of SRA Zy-4 (Zircaloy-4), at least for the high stress range. However, for low applied stress, Ito et a/.142 have shown that the stress sensitivity coefficient is lower after irradiation than before irradiation. They have also shown that irradi­ation has a weak effect on the creep activation energy of SRA Zy-4 for temperatures from 330 to 600 °C and for stresses from 77 to 384 MPa. Murty and Mahmood157 have suggested that the creep anisot­ropy of RXA Zy-2 is decreased by irradiation. According to these authors, this phenomenon is due to the activation of other slip systems than the pris­matic slip system after irradiation, such as the basal and the pyramidal slip systems.

Cappelaere et a/.154 and Limon and Lehmann15 have shown that for low applied stress, a ‘tertiary

creep’ occurs for SRA Zy-4, even though the creep strain level remains low. This phenomenon cannot be explained by the increase of the stress due to the thinning of the wall of the tube. This phenomenon is therefore interpreted as a result of the recovery of the irradiation defects during the creep test and also due to the beginning of the recrystallization that can occur for high-temperature creep tests. Tsai and Billone15 have come to the same conclusions by analyzing their own long-term creep tests. The recovery of irradiation loops during creep tests has been observed, using TEM, by several authors on SRA Zy-4(154) or RXA Zr-1% Nb-O alloy,124 but it is the recent work by Ribis etal.105 that gives the most detailed study of the microstructure evolution during creep tests of the above alloy. The microstructure is compared to that observed after postirradiation heat treatment or after creep of the nonirradiated mate­rial. In this study, it is clearly shown that in RXA zirconium alloys, the irradiation loops are progres­sively annealed during the creep test, as for a heat treatment without an applied stress, the magnitude of the recovery being similar in both cases. Moreover, these authors show that other mechanisms associated with the deformation occur. Indeed, it is noticed that for tests performed at 400 °C and for low applied stress (130 MPa), in addition to the recovery of loops, the microstructure observed after creep tests exhibits a high dislocation density, much higher than the dislocation density observed in the nonirradiated material deformed up to the same plastic strain. According to these authors, this phenomenon results

from the irradiation loops that act as obstacle to dislocation motion, especially in the prismatic planes, and limit their mean free path. This leads to an important multiplication of dislocations in order to accommodate the plastic strain. This high dislocation density can then lead to a significant hardening in addition to the hardening due to loops. This could explain that for long-term creep test performed at 400 °C under an applied stress of 130 MPa, although a significant recovery of the irradiation damage occurs, the creep strain remains limited. Additional hardening due to the high density of small p-Nb needles can also occur in the case of Zr-Nb alloys. For higher applied stress, higher than 200 MPa, these authors suggest that a sweeping of loops probably occurs. This mechanism is believed to be similar to the dislocation channeling mechanism that is observed for burst tests and tensile tests.113,124 This mechanism therefore allows the deformation of the material for high applied stress, despite the high loop density.