Several authors96,112,113,119-121,123-125 have studied

the deformation mechanisms using TEM by taking thin foils out of the specimens after testing. They have observed that, as for many other irradiated metals, after testing, numerous cleared bands free of irradiation defects are present in the material (Figure 15). These cleared bands are the consequence of the dislocation channeling mechanism reviewed in detail by Hirsch,110 Wechsler,126 and Luft.127 According

to several authors,128-130 the irradiation-induced loops, which are obstacles to dislocation glide, can be overcome by dislocations when a sufficient stress is applied, the loops being subsequently annihilated or dragged by dislocations following different pos­sible mechanisms.108-110,131,132 This process of

removal of irradiation loops by moving dislocations produces a cleared zone free of defects inside the grain. These obstacle-free channels or swaths will therefore constitute preferred areas for further dis­location gliding, leading to plastic strain localiza­tion at the grain scale with regions of very high local plastic strain surrounded by regions of almost zero plastic strain. According to Williams et at}1 and Adamson et at., 9 the local plastic strain could reach up to 100% inside these bands. Some dis­agreement on the activated slip systems seems to remain in the case of zirconium alloys. Indeed, some authors have observed channels along the prismatic planes101,119 for tests performed at 250 and 327 °C on a Zircaloy-2 containing 1500 ppm oxygen, whereas

more recently other authors113,124,125 have observed

channels along the basal plane as well as along the prismatic plane depending on the loading conditions. This discrepancy could probably be explained by the differences in the texture or test temperature used by the different authors. Nevertheless, it is now clearly proved113,124 that for materials with texture characteris­tic of RXA tubing or rolled sheets, with (c) axes ori­ented in the (r, в) plane with an angle between 20° to 45° to the radial (r) direction, and for internal pressure tests or transverse tensile test performed at 350 °C, only basal channels are observed for low plastic strain level. Therefore, most of the plastic strain is believed to occur by basal slip inside the channels. However, it is shown that, for an axial tensile test, basal slip is not active because of its very poor orientation and only prismatic and maybe pyramidal channels can be observed.

The fact that the basal slip becomes the easy glide slip system at 350 °C after irradiation constitutes a major change in the deformation mechanisms since, before irradiation, for the same test temperature it is the prismatic slip system that is the easy glide slip system. This change in the deformation mechanisms can be explained by the difference in the interaction between the irradiation-induced loops and the dis­locations gliding either in the basal plane or in the prismatic plane, as pointed out previously. Indeed, the junction created between a dislocation gliding in the basal plane and a loop is always glissile, whereas it is sessile when the dislocation is gliding in the pris­matic plane. Therefore, when the dislocation glides in the basal plane and encounters a loop, the loop can be dragged along the slip plane, leading to a progres­sive clearing of the basal channel.

Since the loops are cleared by gliding dislocations inside the channels, it is usually assumed133 that within the channels a strain softening occurs. This phenomenon is believed to be the cause of the decrease of the strain-hardening rate with irradiation and thus to the early localization of the deformation at the specimen scale, explaining the dramatic decrease of the uniform elongation after irradia — tion.96,133 According to several authors,119,127 the strong texture of the rolled sheets or tubing leads to an even stronger localization of the plastic strain. Indeed, due to the texture, the (c) axis of the hcp grains is along the (r в) plane in the case of a tube. Since for internal pressure test or transverse tensile tests the channels are along the basal plane, the basal channels can easily propagate from grain to grain, as has been shown by Onimus et a/.113,124 When the entire section of the specimen is crossed by disloca­tion channels, a strong necking is observed on the specimen. As was pointed out by Franklin eta/.,134 the RXA alloys are more susceptible to the plastic insta­bility since the dislocation tangles that remain in SRA alloys are believed to inhibit the easy glide and the plastic flow localization.

As discussed by Onimus and Bechade,135 the polycrystalline nature of the material is also believed to play an important role in the overall macroscopic response of irradiated zirconium alloys after irradiation. Indeed, the intergranular stresses that develop because of strain incompatibilities between grains can balance the local microscopic softening occurring in the dislocation channels up to the UTS.

Based on various mechanical data such as Knoop hardness test136 or plane strain and plane stress tensile tests, several authors93, 2 have shown that the irradia­tion decreases the plastic anisotropy of the RXA zirconium alloys. Concerning the SRA zirconium alloys, the mechanical behavior is already more iso­tropic before irradiation than RXA zirconium alloys137 and the relative decrease of the anisotropy is therefore lower.122 According to these authors,122,136 this decrease of the anisotropy of RXA zirconium alloys is due to the fact that the basal slip is more activated after irradiation than before irradiation.