Comparison of Compositional and Morphological Atom-Probe Tomography Analyses for a Multicomponent Fe-Cu Steel R. Prakash Kolli 1, * and David N. Seidman 1,2 1 Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208-3108, USA 2 Northwestern University Center for Atom-Probe Tomography (NUCAPT), Northwestern University, Evanston, IL 60208-3108, USA Abstract: A multicomponent Fe-Cu based steel is studied using atom-probe tomography. The precipitates are identified using two different methodologies and subsequent morphological and compositional results are compared. The precipitates are first identified using a maximum separation distance algorithm, the envelope method, and then by a concentration threshold method, an isoconcentration surface. We discuss in detail the proper selection of the parameters needed to delineate precipitates utilizing both methods. The results of the two methods exhibit a difference of 44 identified precipitates, which can be attributed to differences in the basis of both methods and the sensitivity of our results to user-prescribed parameters. The morphology of the precipitates, characterized by four different precipitate radii and precipitate size distribution functions ~PSDs!, are compared and evaluated. A variation of less than ;8% is found between the different radii. Two types of concentration profiles are compared, giving qualitatively similar results. Both profiles show Cu-rich precipitates containing Fe with elevated concentrations of Ni, Al, and Mn near the heterophase interfaces. There are, however, quantitative disagreements due to differences in the basic foundations of the two analysis methods. Key words: precipitate morphology, concentration profiles, atom-probe tomography, maximum separation distance, envelope method, proxigram, Fu-Cu steel, high-strength low-carbon ~HSLC! steel I NTRODUCTION Analysis of atom-probe tomographic ~APT! data has histor- ically been time intensive and often involves customized software that provides visual reconstructions, concentra- tions, morphologies, and positional analyses of internal features, such as clusters or precipitates. Although the de- sired results are the same, the applied methodologies are often quite different. Commonly used methods to study precipitation in APT data sets include volumetric searches, compositional thresholds, searches for clusters of atoms based on a separation distance ~s!, and radial distribution functions ~RDFs!, which each have their own advantages and disadvantages and provide different quantitative results ~Hellman et al., 2000, 2002, 2003; Hyde & English, 2000; Miller, 2000a; Al-Kassab, 2002; Heinrich et al., 2003; Vau- mousse et al., 2003; Miller & Kenik, 2004; Vurpillot et al., 2004; de Geuser et al., 2006; Sudbrack et al., 2006a!. The terms cluster and precipitate possess distinct phys- ical meanings within the discipline of physical metallurgy in terms of nucleation, growth, and coarsening models in a multiphase multicomponent system ~Christian, 2002!.A cluster refers to an agglomeration of atoms forming as a result of random solid-state concentration fluctuations. At a critical dimension, which is a function of the net revers- ible work to make a nucleus, the cluster becomes stable and it is denoted a nucleus and this is a precipitate. An unstable cluster is denoted an embryo. A nucleus ~precipitate! will then undergo growth and coarsening ~Ratke & Voorhees, 2002!. We also note that in terms of identification of precip- itates for atom-probe data, the term cluster also denotes a grouping of atoms that can either be a cluster or a precipi- tate in terms of nucleation, growth, and coarsening models. In this investigation two different precipitate identifica- tion methodologies, the maximum separation distance en- velope method ~known as the envelope method!~Hyde, 1993; Miller, 2000a, 2000b; Miller & Kenik, 2004!, and a Received March 20, 2006; accepted April 20, 2007. *Corresponding author. E-mail: p-kolli@northwestern.edu Microsc. Microanal. 13, 272–284, 2007 DOI: 10.1017/S1431927607070675 Microscopy AND Microanalysis © MICROSCOPY SOCIETY OF AMERICA 2007