Study of the microstructure and small punch behavior of a 9Cr ODS tube

Abstract

The microstructure of a 9Cr ODS tube (supplied by CEA) obtained by a powder metallurgy route followed by hot extrusion and cold pilgering of the final cladding, has been investigated by means of scanning and transmission electron microscopy. The alloy presents a weak fiber texture and a bimodal grain size distribution. Three different families of precipitates have been identified: isolated and large Ti-Al oxides, Cr rich precipitates placed preferentially at prior austenite grain boundaries and martensitic lath boundaries and a fine distribution of Y-Ti nano-oxides within grains. In order to correlate this microstructure with the mechanical properties, 3 mm diameter discs drilled from the tube wall were tested by small punch technique at room temperature, 300 °C and 500 °C to reproduce the load system to which the claddings will be subjected under service conditions. Changes in the slip character and failure mode as temperature increases were observed. At low temperatures the alloy exhibited a wavy slip character and a quasi-ductile fracture mode. At high temperature, 500 °C, the fracture tended to be more brittle, what may be attributed to an intergranular deformation mechanism leading to the weakening of grain boundaries. Keywords: ODS steels, Small punch, Deformation mechanisms, Electron microscopy