The influence of microstructure on the fracture mechanisms of ferritic ODS steels
Oxide dispersion strengthened (ODS) steels are candidate materials for fuel cladding tubes in Gen-IV nuclear fission reactors, manufactured using cold pilgering process, and as structural material for fusion reactors. Fracture toughness is an important design parameter required for handling and application of a material. Despite having good high temperature strength and irradiation swelling resistance, ODS steel are known to possess slightly lower fracture toughness than non-ODS ferritic martensitic steels at room temperature and even lower at higher temperatures.
Cold pilgering produces similar anisotropic microstructure as hot extrusion and hot rolling, which are commonly used post processing techniques for manufacturing ODS steels. Anisotropic microstructure results in anisotropic fracture behavior. The fracture mechanisms that lead to anisotropic fracture behavior in ODS steels are interesting and have not been studied in great detail. These fracture mechanisms are directly related to the microstructure (alignment of ultrafine grains, alignment and morphology of coarse grains, particle concentration/alignment towards rolling/extrusion direction, crystallographic texture) which in turn is dependent on the composition and the manufacturing method. Understanding the factors leading to anisotropic fracture behavior could lead to mitigation or beneficial use of fracture anisotropy.
Investigations are also done on understanding secondary cracking, which is often observed in ODS steels. Secondary cracking absorbs energy but can lead to design problems. In this work, the reasons why secondary cracking favors certain orientations or occurs more often in samples made through a certain manufacturing technique are discussed. Knowledge about factors affecting secondary cracking would help to avoid them, or to utilize them in certain orientations where they facilitate high fracture toughness without secondary cracking.
In this work, fracture toughness testing on three ODS steels are performed in order to make a connection between microstructure and fracture properties. Miniature fracture mechanics C(T) specimens are made (0.25T and 0.16T) and quasi-static testing is performed using unloading compliance and normalization method. The basic microstructure is investigated using SEM and TEM which throws light on the nano-oxide clusters and sub-micron particles. EBSD is done to obtain information about grain size, morphology and crystallographic orientation. This microstructural information is then linked to the fracture behaviour by fracture surface and crack propagation investigation using SEM and EBSD respectively.