Deformation and damage mechanisms of oxide dispersion strengthened steel under high temperature cyclic loading

Oxide dispersion strengthened (ODS) steels are promising structural material candidates for both generation four (GEN-IV) fission reactors and breeding blanket material in future fusion power plants. Within the framework of FP7 project MatISSE (Material's innovations for safe and sustainable nuclear energy in Europe), characterization and mechanical testing of several Fe-Cr alloys was undertaken. One of the alloys under investigation is a tempered martensitic 9Cr-ODS steel. The microstructure typically consists of a high density of hierarchically organized internal interfaces such as prior austenitic grain, packet or block boundaries, lath boundaries and sub-grains with high dislocation density. Coarse non-regular M23C6 carbides decorate different boundaries and are rich in Fe, Cr, and W. The complex nano-sized Y-Ti-O particles were observed embedded in the matrix. Tensile tests at various temperatures showed a good compromise between strength and ductility. However, for application, the behavior of ODS steels under cyclic loading is of decisive importance.The fully reversed strain-controlled low-cycle fatigue tests at elevated temperatures revealed a distinctive cyclic response. Apart from the higher cyclic stress levels, the steel manifests complex cyclic softening which is significantly lower in comparison to that observed for similar non-ODS steels. The presence of nano-sized particles was found beneficial in terms of mitigating the well-known cyclic softening in non-ODS steels. Testing includes creep-fatigue investigations by introducing a hold-time at the peak tensile strain. Such a hold period simulates the loading of a component under stationary operation. Introducing creep into the fatigue loading results in lower fatigue life. The tests are accompanied by microstructural investigations for the identification of main deformation and damage mechanisms.