A comparison of two high spatial resolution imaging techniques for determining carbide precipitate type and size in ferritic 9Cr-1Mo steel

Abstract

Two high spatial resolution imaging techniques, focused gallium ion beam imaging in conjunction with XeF2 gas (FIB/XeF2) and high-speed atomic force microscopy (HS-AFM), were used to analyse 9Cr-1Mo ferritic steel samples, which had been exposed for extended periods to hot CO2 gas containing traces of CO, H2, H2O and CH4. The carbide precipitates embedded in the metal matrix were observed and their morphology, size and spatial distribution were quantified using these two techniques. The lower resolution of the FIB/XeF2 imaging technique suggested that small carbide precipitates (<50 nm) may be missed, while the existence of a limited flow layer introduced by sample preparation may influence the HS-AFM results. The gallium ion beam was used to remove a thin oxide layer of approximately 50 nm from sample surfaces prior to FIB/XeF2 imaging, avoiding the influence of surface contamination. HS-AFM provided higher resolution (∼5 nm) than FIB/XeF2 imaging. A quantitative comparison of the experimental data confirmed the value of both FIB/XeF2 and HS-AFM for imaging carbide precipitates, while clarifying their strengths and limitations.