TOPICAL COLLECTION: 3D MATERIALS SCIENCE Quantification of Solute Topology in Atom Probe Tomography Data: Application to the Microstructure of a Proton-Irradiated Alloy 625 IMAN GHAMARIAN, LI-JEN YU, and EMMANUELLE A. MARQUIS The analysis of solute clustering in atom probe tomography (APT) has almost exclusively relied on a simple algorithm based on the simple friend-of-friend analysis where a threshold distance or maximum separation defines whether atoms are part of a cluster or part of the matrix. This method is however limited to very specific microstructures and is very sensitive to parameter selection. To expand the range and applicability of current APT analysis tools, we introduce new quantitative data analysis methods based on density-based hierarchical clustering algorithms and relevant to solute clustering and segregation. We demonstrate the methods’ performance on the complex microstructure developing in a proton-irradiated Alloy 625, specifically focusing on the analyses of nanoscale Al clusters, Si clusters, and Si-decorated dislocation loops. https://doi.org/10.1007/s11661-019-05520-6 Ó The Minerals, Metals & Materials Society and ASM International 2019 I. INTRODUCTION MATERIALS characterization finds its purpose in the understanding and quantification of the relation- ships between processing parameters, microstructure, and properties. Standardized quantitative methods and algorithms have been developed for many characteriza- tion techniques, such as extracting grain size informa- tion from metallography data, or precipitate size and shape from electron microscopy imaging. The technique of atom probe tomography (APT), on the other hand, is still waiting for standardized, reproducible, and quan- titative methods to be developed and adopted by the APT community. APT generates three-dimensional reconstructions containing information on the position and identity of a fraction of the atoms (typically between 50 and 80 pct) contained in the sampled volume of material. For a detailed background on the technique, please refer to published monographs. [1,2] In brief, the volume of interest is prepared in the form of a sharp needle with typical dimensions of 50 to 150 nm apex radius. The needle is placed at cryogenic temperature ( < 50 K) and subjected to a high electric field (few to tens of V/nm). Thermal or voltage pulses are then used to trigger field evaporation of surface atoms. The atomic identity of the evaporating ions is inferred from time- of-flight measurements. The ions¢ original locations on the specimen surface are estimated using idealized specimen geometry and ion trajectories. Depth infor- mation is calculated from the evaporation sequence. A reconstructed volume is therefore constituted of discrete points where atoms are inferred to be located. In reality, spatial positioning accuracy varies for each element present in the specimen and within the position in the reconstructed volume; it depends on a number of factors that include the element itself, its local chemistry, local crystallographic orientation, and proximity to structural defects. Plane spacings smaller than 1 A ˚ can be resolved in some cases, [3] while solute atoms can be grossly mispositioned in other cases. [4] Current development efforts are focusing on addressing and improving the accuracy of the reconstruction algorithm and represen- tation of the data, and forward modeling has already provided significant insights into some of the evapora- tion behavior of complex systems. [4–6] In addition to improving the reconstruction algo- rithms, developing reproducible quantification methods for microstructural features is required to increase the impact of APT in the field of materials science. Despite uncertainty on the locations of some of the detected atoms, numerous examples have shown that the tech- nique is capable of resolving nanometer-scale features, such as grain boundaries, precipitates, and solute clusters. Solute clusters are generally defined as a region of space in which solutes are denser than in the surrounding matrix. Such regions confer special IMAN GHAMARIAN, LI-JEN YU, and EMMANUELLE A. MARQUIS are with Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI, 48104. Contact e-mail: emarq@umich.edu. Manuscript submitted May 21, 2019. METALLURGICAL AND MATERIALS TRANSACTIONS A