Nano oxide particles in 18Cr oxide dispersion strengthened (ODS) steels with high yttria contents

Abstract

Oxide dispersion strengthened (ODS) steels containing yttria are candidate materials for fuel cladding tubes in future nuclear reactors. Their performance is determined by the size, distribution and volume fraction of the oxide dispersoids. The addition of yttria leads to a significant loss in ductility of steel limiting the yttria content in most ODS steels to 0.35%. This limitation may be overcome by powder forging of ODS steel powders. Two steels having nominal compositions (in wt%) of Fe-18Cr-2W-0.285Ti-0.5Y 2 O 3 and Fe-18Cr-2W-0.571Ti-1Y 2 O 3 were prepared by mechanical alloying of elemental powders with yttria powder in a Simoloyer attritor. The alloy powders were consolidated by powder forging at 1473 K in a flowing hydrogen atmosphere. The microstructure of powder forged ODS steels was characterized by electron microscopy. Both the steels exhibited a fine-grained recrystallized microstructure after forging. Fine (<10 nm) cuboidal oxide particles were found to be present in a ferrite matrix. A few coarse oxide particles were also observed. These coarse (>10 nm) oxide particles were present mainly along the ferrite grain boundaries. The crystal structure of oxide particles was determined by applying fast fourier transformation (FFT) on the fringes observed by high resolution transmission electron microscopy. The extracted FFTs were indexed to identify the crystal structure. The coarse particles were found to be Y 2 O 3 , TiO 2 and Cr 2 O 3 . The fine particles were identified as Y 2 TiO 5 and Y 2 Ti 2 O 7 . The face centre cubic Y 2 Ti 2 O 7 oxide particles were semi coherent whereas the orthorhombic Y 2 TiO 5 oxide particles were found to be incoherent with the ferrite matrix. The number fraction, average size and size distribution of these oxide particles was determined for both the alloys. Cr rich shells were observed around most of the particles which may prevent the coarsening of oxide particles at elevated temperatures. Reasons for this are discussed. • High yttria 18Cr ODS steels were successfully developed through novel hot powder forging route. • Recrystallized equiaxed grains with a uniform distribution of nano oxides particles were observed in the forged alloys. • A majority of oxides present were Y 2 TiO 5 or Y 2 Ti 2 O 7 oxides. Cr-rich shells were associated with these complex oxides. • The cuboidal Y 2 TiO 5 oxides were incoherent while the spherical Y 2 Ti 2 O 7 oxides were semi-coherent with the ferrite matrix. • HR-TEM also confirmed the formation of nano oxide particles such as Y 2 O 3 , TiO 2 and Cr 2 O 3 . These too were analysed.