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

Two high spatial resolution imaging techniques, focused gallium ion beam imaging in conjunction with XeF gas (FIB/XeF) 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 CO gas containing traces of CO, H, HO and CH. 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/XeF 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/XeF imaging, avoiding the influence of surface contamination. HS-AFM provided higher resolution (∼5 nm) than FIB/XeF imaging. A quantitative comparison of the experimental data confirmed the value of both FIB/XeF and HS-AFM for imaging carbide precipitates, while clarifying their strengths and limitations.