Microstructural evolutions and creep behaviour at high temperature of an austenitic stainless steel

The ASTRID project aims at designing a fast-reactor prototype for the 4. generation of nuclear power plants. The material to be used for fuel cladding is a cold-worked austenitic stainless steel stabilized with titanium (15Cr-15Ni Ti type) and optimized in minor elements. This material was developed to limit recovery and irradiation-induced swelling and to improve microstructural stability and mechanical properties in normal operating conditions. In case of incidental situations (irradiation temperature ≥ 650 °C), the cladding might rapidly reach higher temperatures up to 950 °C where its stability could be affected. The present work aims at improving knowledge and understanding of the microstructural evolution and creep behaviour of this steel at these temperatures (650 C-950 °C). Microstructural characterizations of thermally-aged samples have been performed in order to study the effect of temperature on metallurgical evolutions (precipitation, recovery and recrystallization). A phenomenological model including recovery and recrystallization processes was set up to reproduce hardness measurements versus ageing time and temperatures. Isothermal creep tests up to 950 °C under a wide range of stress levels allowed investigation of viscoplastic flow, microstructural evolution under stress and damage/failure processes. In order to evaluate the effect of high-temperature loading, microstructural characteristics of stress-free thermally-aged samples were compared with post-mortem examinations of creep specimens. At 650 °C and 750 °C the value of stress exponent is higher than 7. The main deformation mechanism during creep test is power-low creep, which is consistent with the results found in the literature. Beyond 850 °C, the increase in dislocation mobility promotes recovery and recrystallization processes. As a consequence, a competition between work hardening due to viscoplastic deformation and softening due to dynamic recovery takes place. At 950 °C, viscoplastic flow is strongly affected by recrystallization during creep test, especially in the tertiary stage. The comparison between microstructures of crept specimens and stress-free, thermally-aged samples leads to the conclusion that the recrystallization kinetics is accelerated by application of a mechanical loading. As for the fracture behaviour, creep tests under air environment at lower temperatures (650 °C-750 °C), led to predominating ductile fracture but some intergranular zones were observed on fracture surfaces. Creep tests under high vacuum at higher temperatures (850 °C-950 °C) lead to a high fracture elongation with a reduction of area up to 100%. (author)