A heat treatment performed at a temperature higher than the irradiation temperature on vari­ous zirconium alloys results in a recovery of the radiation-induced hardening90,138 (Figure 16). This recovery can also be measured using microhardness tests.101,102,105,139-142 The recovery of the hardening is always associated with the recovery of the ductility and the fracture properties.138

Howe and Thomas90 have shown that in a cold — worked zirconium alloy most of the recovery occur­ring between 280 and 450 °C appears to be the annealing out of radiation damage rather than cold work. In the case of strongly cold-worked zirconium alloys such as SRA Zy-4, radiation hardening recov­ery is also observed. The hardness of the material can even become lower than the initial hardness of the SRA Zy-4(105) owing to the recovery of the disloca­tions, in addition to the recovery of the loops.

Some authors,101,140,143 on the basis of various experimental results, have suggested that there is an interaction between oxygen and irradiation — produced dislocation loops, which increases the dislocation-defect barrier interaction. During the recovery, this phenomenon can lead to an additional hardening, as shown by Snowden and Veevers.140

Several authors48,101,105,141,144,145 have shown that during a heat treatment performed on a RXA zirco­nium alloy, the (a) loop density strongly decreases and the loop size increases. This decrease of the obstacle density to dislocation motion has been clearly correlated to the decrease of the radiation — induced hardening.101,105

Concerning the nature of the loops, Kelly and Blake48 have studied 240 loops in a zirconium alloy sample heat-treated at 490 °C during 1 h after irradi­ation up to a fluence of 1.4 x 1024nm~2. These authors show that, although the initial microstructure is composed of both interstitial and vacancy loops in equal amount, after the heat treatment, two-thirds of the analyzed loops are vacancy loops and only one — third are interstitial loops. This implies that the interstitial loops undergo a more rapid recovery than the vacancy loops. These observations have been recently confirmed by Ribis et al.,105 who stud­ied the evolution of the proportion of the vacancy loops and interstitial loops with heat treatment for various temperatures. These authors have shown that after 960 h at 450 °C, only large vacancy loops in low density are observed.

In the literature, several mechanisms are proposed in order to explain the irradiation damage recovery. The most commonly agreed mechanism is based on bulk diffusion of vacancies during the recovery and their exchange between loops of various size.105,146-148 Indeed, the smaller vacancy loops emit vacancies that diffuse toward larger vacancy loops, which absorb more vacancies than they emit, leading to a growth of the larger loops at the expense of the smaller loops. On the other hand, interstitial loops always absorb vacancies whatever their size, since the vacancies are in supersaturation during the heat treatment, exp­laining the rapid disappearance of the interstitial

105,146

loops.