Characterisation of nanosized oxides in ODM401 oxide dispersion strengthened steel

Changes to the microstructure of oxide dispersion strengthened (ODS) alloy ODM401 in response to high temperature heat treatments have been observed. Extensive analysis of thin foil and extraction replica specimens has been performed using transmission electron microscopy (TEM), convergent beam electron diffraction (CBED) and energy dispersive X-ray spectroscopy (EDS). A distribution of 2 nm (mean diameter) particles, with a number density of 2 × 1023 m−3, was observed in the extruded material. Following a heat treatment of 1 h at 1300 °C the number density of the dispersoids was diminished and their mean diameter increased to an average size of 5 nm. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and chemically sensitive, high resolution, phase contrast, transmission electron microscopy has been used to positively identify nanometer scale pyrochlore oxides. Identification and a complete characterisation of the A2B2O7 structures, including the lattice positions of trivalent (A) and tetravalent (B) species is presented. The composition of the oxide particles was shown to be complex and the role of cations other than yttrium and titanium, in particular aluminium, must be considered in future analysis of the oxide phases and their nucleation and growth behaviour. The majority of the cube shaped oxide particles displayed one of two orientation relationships (OR) with the ferritic matrix. Cube on cube and the Baker and Nutting, cube edge on cube, relationships were identified.