Identification of new phases in annealed Fe–18CrWTi ODS powders

Fe–18CrWTi ODS steels are developed by CEA for the SFR generation IV program. The powders of a Fe–18CrWTi alloy reinforced by Y2O3 by mechanical milling have been annealed at 850 °C, 1100 °C and 1300 °C and their microstructure has been studied by SEM and TEM in order to better understand the precipitation mechanisms. The powders are constituted of large micrometric grains surrounded by nanometric ferritic grains. At 850 °C, no nano-oxide could be detected. At 1100 °C, some Y2Ti2O7 precipitates of several hundreds of nanometres are detected at the grains boundaries, but again no nano-oxide could be observed. At 1300 °C, the precipitation has greatly changed. The sample contains hexagonal plate-shaped micrometric Y2TiO5 precipitates. A high density of nano-oxides of size around 20–30 nm is also revealed by EFTEM and HRTEM. They are of different types such as Y2O3, Y2Ti2O7 and a new phase containing Y, Ti and O that we called PyrOrtho (orthorhombic with a = 7.24 Å, b = 12.4 Å and c = 18.2 Å). This study shows that addition of Ti makes the precipitation sequence very complex.