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Plasma focused ion beam microscopy (PFIB) is a recent nanofabrication technique that is suitable for site-specific atom probe sample preparation. Higher milling rates and fewer artifacts make it superior to Ga+ FIBs for the preparation of samples where large volumes of material must be removed, for example, when trying to avoid lift-out techniques. Transmission Kikuchi diffraction (TKD) is a method that has facilitated phase identification and crystallographic measurements in such electron transparent samples. We propose a procedure for preparing atom probe tomography (APT) tips from mechanically prepared ribbons by using PFIB. This is highly suitable for the preparation of atom probe tips of interfaces such as interphase boundaries from challenging materials where lift-out tips easily fracture. Our method, in combination with TKD, allows the positioning of regions of interest such as interfaces close to the apex of the tip. We showcase the efficacy of the proposed method in a case study on Alloy 718, where the interface between γ-matrix and δ-phase has not been yet extensively explored through APT due to preparation challenges. Results show depletion of γ″-precipitates near the γ/δ interface. A quantitative evaluation of the composition of phases in the bulk versus near the interface is achieved.
Polycrystalline Ni-based superalloys for aerospace and power generation applications are often precipitation hardened to achieve strengthening at elevated temperatures. Here, atom probe microscopy has become an essential tool to study the complex morphology of nanoscale precipitates. This study focuses on Alloy 718, which is hardened by semi-coherent, ordered γ′ (Ni3(Al, Ti)) and γ″ (Ni3(Nb)) particles. According to previous research, these particles often occur as duplets or triplets with a stacking sequence dependent on prior processing. This creates various interfaces with a strong impact on the mechanical properties, highlighting the importance of quantitative studies which are challenging with electron microscopy. We present atom probe data reconstruction and analysis approaches particularly suited for precipitation hardened superalloys. While voltage atom probe allows for an accurate reconstruction, the acquired data volume is often limited. Laser-assisted atom probe provides statistically significant data, but the loss of crystallographic information requires correlation with voltage-mode datasets. We further describe an advanced iso-surface method where initially arbitrarily chosen concentration thresholds of Al + Ti for γ′ and Nb for γ″ particles are optimized. Recognizing the importance of the precipitate stacking order, the different types of precipitate interfaces are quantified, and these methods may be applicable to other engineering alloys.
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