Field evaporation events as Markov chains: A time-of-flight atom-probe study of iridium, Pt-Rh alloys, and metallic glasses

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Abstract
Field evaporation events in time-of-flight atom-probe analyses were treated as Markov chains. Both the random chain and the chain with a given distribution of cluster sizes were considered. The validity of the analysis was checked with a set of atom-probe data of iridium. The data agree with a random distribution of the two isotopes. Two sets of atom-probe data were collected from Pt-Rh alloys, and were analyzed and compared with these models. It is found that the experimental distributions fit best with the distributions including slight degrees of clustering in the field evaporation events. Thus, microclustering of Rh and Pt atoms occurs in Pt-Rh alloys, which are generally believed to be complete random solid solutions over the entire composition range. From the atom-probe data, however, we are not yet able to estimate the cluster size distribution of Rh and Pt atoms in the alloys. Our atom-probe data of a metallic glass agree well within the statistical uncertainty of the data with a random distribution of the constituents. If microclustering occurs, it is still beyond the sensitivity of the method as constrained by the amount of data available.