Highlights

Phase Stability under Irradiation of Precipitates in Oxide-Dispersion-Strengthened (ODS) Steels Relevant for Generation-IV Nuclear Reactors

Scientific Achievement

In this program, an investigation of the stability under irradiation of nano-sized oxide precipitates in ODS steels is carried out.  Several ODS Ferritic/Martensitic steels produced by mechanical alloying with Y₂O₃ particles are considered for advanced nuclear power applications.  Since these alloys derive their good properties from their special microstructure (especially the oxide dispersion), it is necessary to assess the stability under irradiation of their microstructure in general and the nano-sized particles in particular.  Prior to irradiation these alloys contain carbides, and/or intermetallics, and oxide particles depending on the alloy composition.  Characterization of the initial precipitate population reveals (within the limitations of the technique) two populations of nano-particles in term of size: small nano-clusters of size less than 10 nm, and larger nano-particles up to few hundred nms.  The alloys were irradiated with Fe and Kr ions in the IVEM at 25°C and 500°C.  The use of 1 MeV Kr ions allowed high doses (more than 100 dpa) to be achieved.  Various effects of ion irradiation were observed as they occurred, including amorphization of large particles at 25°C, dislocation loop formation, voids, precipitate dissolution and formation.  At 500°C amorphization does not occur, and the fine particles are still present at the end of the irradiations.  Another important observation is the fact that the alloys retain their microstructural grain morphology even after reaching doses as high as 100 dpa, which is very important to assess whether the properties relying on grain microstructure can be retained under high doses.

The IVEM facility allows the irradiation of the materials with ions to induce radiation damage to levels which cannot be easily attained otherwise, and to follow the damage as it occurs.  Such work can be compared with neutron irradiation induced damage for Gen-IV nuclear reactors for which these alloys are considered for fuel cladding applications.

Significance

The investigation of the irradiation-altered phase stability of oxide precipitates in ODS steels is necessary to determine whether the favorable mechanical properties of these steels are maintained under irradiation.  This study thus addresses one of the main materials research issues for this class of steels as identified by the Gen-IV working groups.  The research program could also create fundamental understanding of the irradiation precipitation/dissolution problem by studying a “model” system in which the variables (dose, dose rate, temperature etc) can be controlled and their effects understood individually, which is made possible with IVEM unique facility.

Performers