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International audience; ASTRID is a fast-reactor prototype for the 4th generation of nuclear power plants. The material used for fuel cladding is a cold-worked austenitic stainless steel called AIM1. This grade was developed to limit irradiation-induced swelling and improve microstructural stability and mechanical properties in normal operating conditions. In case of incidental situations, the cladding might rapidly reach higher temperatures (700-950DC) where its stability might be affected. The microstructural and mechanical behaviour in this temperature range is experimentally addressed in this paper.Isothermal creep tests up to 1000DC under a wide range of stress levels enable to study viscoplastic flow, microstructural evolution under stress and damage/failure processes. In order to evaluate the effect of loading, 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.Up to 750DC, AIM1 shows better creep strength than previous generations of 15-15Ti grades. Beyond 750DC, 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 950DC, viscoplastic flow is strongly affected by recrystallization during creep test, especially in the tertiary stage. Softening due to gradual recrystallization leads to longer tertiary stages and higher ductility during tests under lower stress levels (about 40 MPa applied stress). Ductile fracture predominates 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.

International audience

This study deals with acicular ferrite, bainite, and martensite microstructures observed in three low alloy steels. Electron backscattering diffraction (EBSD) was used to assess crystallographic features of these microstructures. In each area studied by EBSD mapping, ‘crystallographic packets’ defined as clusters of points sharing the same crystallographic orientation were compared with ‘morphological packets’ observed in the corresponding light micrograph. Microtexture studies suggested that acicular ferrite and upper bainite grow with Nishiyama– Wassermann relationships with the parent austenite phase, whereas lower bainite and martensite consist of highly intricate packets having Kurdjumov–Sachs relationships with the parent phase. In all cases three highly misoriented texture components were found within each former austenite grain. Electron backscattering diffraction also gave information about the cleavage and intergranular reverse temper embrittlement fracture mechanisms of these steels. In...