On the structure and chemistry of complex oxide nanofeatures in nanostructured ferritic alloy U14YWT

Abstract

The remarkable radiation damage resistance of nanostructured ferritic alloys (NFAs) is attributed to the large numbers of matrix nanofeatures (NFs) of various types, which can enhance the recombination of displacement defects and trap transmutant helium in fine scale bubbles. Characterizing the chemistry, crystallographic structure and orientation relationships of the NFs is critical to understanding how they enhance the radiation damage resistance of NFAs. Conventional and high-resolution transmission electron microscopy and energy-dispersive spectroscopy were used to characterize the various types of NF and larger oxide phases in a model 14Cr–3 W–0.4Ti–0.25Y₂O₃ NFA (14YWT) hot isostatic pressed (HIP-ed) at 1150°C. Large CrTiO₃ precipitates (50–300 nm) and small diffracting NFs (<5 nm) were found in this alloy. One major new result is the observation of an additional type of nanofeature (10–50 nm), orthorhombic in structure, with a square center cross-section, which constitutes a new kind of Y–Ti-oxide phase with lattice parameters different from those of known Y and Ti complex oxides. The interfaces of these particles seem to be semicoherent, while manifesting a possible orientation relationship with the BCC matrix. The ratio of Y to Ti varies between <1 and 2 for these larger NFs.

Keywords:

• oxide dispersion strengthened steels
• nanostructured ferritic alloys
• TEM
• yttrium–titanium oxide precipitates

Acknowledgements

The authors would like to thank Dr Longzhou Ma at UNLV for his kind permission to use the TEM at the Harry Reid Environmental Center when needed. The many helpful discussions with Professor J.P. Hirth are also gratefully acknowledged. This TEM work at LANL was funded by Los Alamos National Laboratory LDRD. The alloy development and characterization studies carried out at UCSB were previously supported by the DOE Offices of Fusion Energy Sciences and Nuclear Energy.