On the formation of nano-sized precipitates during cooling of NiAl- strengthened ferritic alloys

Abstract We present an experimental study on NiAl- type precipitates in ferritic superalloys that aim for application as structural materials at high temperature. The focus of this study is on the characterization of hyperfine precipitates that form during sample cooling after the aging treatment and which have a strong influence on the alloys' room temperature hardness. Size and crystallographic information of the precipitates were obtained by electron microscopy. The chemical nature of different phases was analyzed by atom probe tomography (APT). In order to identify the hyperfine precipitates in APT reconstructions, a modified version of the maximum separation method for cluster selection is proposed. Instead of a separate erosion step, the proposed method makes use of a Delaunay tessellation which does not require any user defined input parameters and further, gives a direct access to the morphology of the clusters. The modified algorithm was tested on simulated datasets and then successfully applied to experimental datasets containing precipitates with radii down to one nanometer. The result of the chemical analysis shows that the formation of precipitates during sample cooling is a kinetically controlled process which is dominated by two different mechanisms in dependence on the temperature.