Quantitative atom probe analysis of carbides

Compared to atom probe analysis of metallic materials, the analysis of carbide phases results in an enhanced formation of molecular ions and multiple events. In addition, many multiple events appear to consist of two or more ions originating from adjacent sites in the material. Due to limitations of the ion detectors measurements generally underestimate the carbon concentration. Analyses using laser-pulsed atom probe tomography have been performed on SiC, WC, Ti(C,N) and Ti2AlC grains in different materials as well as on large M23C6 precipitates in steel. Using standard evaluation methods, the obtained carbon concentration was 6–24% lower than expected from the known stoichiometry. The results improved remarkably by using only the 13C isotope, and calculating the concentration of 12C from the natural isotope abundance. This confirms that the main reason for obtaining a too low carbon concentration is the dead time of the detector, mainly affecting carbon since it is more frequently evaporated as multiple ions. In the case of Ti(C,N) and Ti2AlC an additional difficulty arises from the overlap between C2+, C42+ and Ti2+ at the mass-to-charge 24 Da.

Research highlights► Laser-pulsed APT has been performed on five different carbide materials. ► Quantification using standard methods gives too low carbon concentration. ► Detector dead time and multiple events lead to loss of carbon. ► A method based on 13C is shown to improve quantification of carbon.