Quantification of APT physical limitations on chemical composition of precipitates in Fe–Cr alloys

Abstract

Among the different nano-features at the origin of embrittlement of Fe–Cr alloys and steels, α′ precipitates play a major role in alloys with Cr content higher than about 10at.%. If Atom Probe Tomography (APT) is recognized as an efficient technique for characterizing α′ precipitates, discussions remain on its ability to measure the actual composition of small particles. Two APT limitations are at the origin of these discussions: its lateral resolution (and on a smaller scale the depth resolution) and local magnification effects due to the difference in field evaporations between the matrix and the particles. In this study, the impacts of these two limitations are quantified for the first time using numerical approaches and an analytical model. This will provide an overview of the interpretation for the α’ particles chemical composition experimentally measured by APT.The results show that: (i) the major effect of local magnification is ion focussing with no mixing in the core of the particles for particle radius larger than 1 nm, (ii) lateral resolution is the main contributor to the composition bias. Depending on the lateral resolution, the core of the small particles may be diluted by matrix atoms but the dilution does not exceed 5.2 at.%. The extent of the decrease in measured Cr concentration of the particles depends on the Cr concentration difference between particles and the surrounding matrix.