4.01.2.1 Mechanical Behavior During Tensile Testing

4.01.2.1.1 Irradiation hardening: Macroscopic behavior

As for many other metals, zirconium alloys exhibit strong hardening after neutron irradiation. It is indeed observed by numerous authors90-99 and reviewed21,77,100 that the yield stress (YS), as well as the ultimate tensile strength (UTS), of both recrystallization-annealed (RXA) and stress-relieved annealed (SRA) zirconium alloys is strongly increased by neutron irradiation (Figures 12 and 13). Micro­hardness tests also prove this phenomenon.101-10 The irradiation-induced hardening increases rapidly for fluences below 1 x 1024nm~2 (E > 1 MeV), at irra­diation temperatures between 320 and 360 °C, but saturates above 1 x 1024n e

change occurs from 1 x 1024 up to 1.5 x 1025nm~2 (E > 1 MeV).92 It is however to be noticed that some authors do not find a clear saturation of the irradiation-induced hardening for fluences up to 1.5 x 1025n m~2 and irradiation temperatures between 320 and 360 °C.9297 Although the YS (and UTS) of SRA Zr alloys is significantly higher than the YS of

RXA Zr alloys before irradiation, the YS of both alloys, measured after high irradiation doses, at saturation,

21,90,100

become close.

According to Higgy and Hammad,92 and reviewed by Douglass,21 as the irradiation temperature increases from temperatures below 100 °C up to temperatures between 320 and 360 °C, the irradiation-induced hard­ening decreases. According to these authors, this shows that the accumulation of damage decreases as the irra­diation temperature increases, presumably due to recovery during irradiation.

The chemical composition seems to play a second­ary role in the irradiation-induced hardening com­pared to the effect of the metallurgical state (SRA vs. RXA). The oxygen content is nevertheless shown to have a slight effect on the irradiation-induced harden­ing. Indeed, Adamson and Bell101 have shown using microhardness tests that the irradiation-induced hard­ening is higher for RXA Zy-2 alloy with high oxygen content (1800 ppm) than in the case of an RXA Zy-2 alloy with low oxygen content (180 ppm).

It can also be noticed that the test temperature seems to have only a small influence on the irradiation — induced hardening, for a given irradiation temperature, up to a test temperature of 400 °C. Indeed, as reported by Onchi eta/.96 (Figure 14), the YS of both irradiated and unirradiated RXA Zy-2 decreases with the test temperature, the decrease being only slightly lower for the irradiated specimens between 20 and 300 °C. However, beyond a test temperature of 400 °C, a strong decrease of the irradiation hardening occurs due to the recovery of the irradiation damage.