Participation à la conférence ECCC2017

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ASTRID is a fast-reactor prototype for the 4^th generation of nuclear power plants. The material to be used for fuel cladding is a cold-worked austenitic stainless steel called AIM1 (15Cr-15Ni, Ti-stabilized, optimized in Si and P). This grade was developed to limit irradiation-induced swelling and improve mechanical properties in normal operating conditions. In the case of incidental situations, the cladding can rapidly reach higher temperatures (700-950°C) and its microstructural and mechanical behaviour in this temperature range is experimentally addressed in this paper.

 Isothermal creep tests up to 1000°C under a wide range of stress levels were performed to study viscoplastic flow, microstructural evolution under stress and damage/failure processes. Microstructural characterizations (precipitation, recovery, and recrystallization states) on stress-free thermally aged samples were also performed and compared with post-mortem examinations of creep specimens.

The viscoplastic behaviour and the microstructural evolution considerably depend on temperature. Up to 750°C, AIM1 shows better creep strength than more conventional 15-15Ti grades. Beyond 750°C, dislocation mobility increases which promotes recovery and recrystallization processes. As a consequence, competition between work hardening due to viscoplastic deformation and softening due to dynamic recovery takes place. At 950°C, viscoplastic flow strongly depends on recrystallization during the creep test, in particular during the tertiary stage. Softening due to gradual recrystallization leads to longer tertiary stages and higher ductility during creep tests under lower stress levels. Ductile fracture predominates at any temperature. The recrystallization kinetics and mechanisms are not significantly affected by slow viscoplastic strain at any temperature.

Characterizations on thin foils and carbon extractive replicas showed a large variety of precipitates, such as Cr-rich borides, phosphides, and Cr- and Ti-rich carbides. Precipitates are expected to affect creep resistance, recovery and recrystallization kinetics by pinning dislocations and grain boundaries.