The influence of microstructure on the fracture behaviour of ferritic ODS steels

Oxide dispersion strengthened (ODS) steels are candidate materials for cladding tube and structural components in Generation IV nuclear fission reactors and as candidate materials for structural components in fusion devices. Fracture toughness is an important parameter required for the structural integrity and workability of a material. Despite having high strength at high temperatures and high irradiation swelling resistance, ODS steels have been known to possess lower fracture toughness than non-ODS ferritic martensitic steels, their immediate competitor. They also exhibit anisotropic fracture behaviour, especially for the hot-rolled and hot-extruded variants. In addition, ODS steels tend to form secondary cracks, which absorb energy but can lead to design problems.
In the present work, the microstructural features which cause low fracture toughness, anisotropic fracture behaviour and secondary cracking are investigated. This information can help manufacturers develop ODS steels with better fracture properties. Fracture toughness testing on three different batches of ODS steels are performed using miniature fracture toughness C(T) specimens using the unloading compliance method. The basic microstructure, fracture surfaces and crack propagation are investigated using techniques such as SEM, TEM and EBSD and compared with the fracture behaviour. A quantitative assessment of the microstructural parameters affecting fracture toughness is made using a critical strain based fracture toughness expression.
It was observed that the low fracture toughness of ODS steels is predominantly affected by the bond strength between the void initiating particle and the matrix. The size and inter-particle spacing of void initiating particles along with the yield stress did not dominantly affect the fracture toughness. The anisotropic fracture behaviour in ODS steels was found to be predominantly affected by the anisotropic grain morphology. Crystallographic anisotropy and anisotropy in void initiating particle distribution did not dominantly affect the fracture anisotropy. Secondary cracking favoured hot-rolled over hot-extruded specimens due to higher degree of microstructural anisotropy. Secondary cracks could stabilize primary crack propagation as well as prevent cleavage fracture at low temperatures. However, the drawback with secondary cracks was that they initiated earlier or at lower loads than the primary crack.