Particle encapsulation techniques for atom probe tomography of precipitates in microalloyed steels

Atom probe tomography (APT) provides sub-nm resolution in the analysis of complex industrial steels. It can resolve the carbonitride precipitates in Nb-Ti microalloyed high-strength low-alloy (HSLA) steels that strongly affect material performance and illuminate the complex precipitation sequence before and during the thermo-mechanical controlled process (TMCP). However, the precipitate concentration is low in HSLA steels during austenite conditioning, especially at temperatures > 850 °C, so that the probability of detecting precipitates via APT is below 5%. Here, we demonstrate two encapsulation-based approaches that increase the precipitate concentration in the APT sample volume sufficiently to enable the analysis of sparse precipitates. The first method is based on metallographic etching and direct targeting of precipitates in the steel. A focused ion beam was used to mark precipitation sites. Encapsulation with nickel-phosphorus (Ni-P) enabled localized APT and increased the yield by a factor of 10. The second method relies on the chemical extraction of precipitates and subsequent encapsulation in a silicon oxide (SiO_x) network at a very high particle density. Analysis of tips cut from the encapsulated particles increased the yield by a factor of >15. We discuss and compare the spatial and chemical accuracy obtained in the analysis of pure Nb-, Ti- and mixed Nb-Ti carbonitrides.