Determining the composition of small features in atom probe: bcc Cu-rich precipitates in an Fe-rich matrix

Analysis of Radiation Damage in Light Water Reactors: Comparison of Cluster Analysis Methods for the Analysis of Atom Probe Data

Irradiation of reactor pressure vessel (RPV) steels causes the formation of nanoscale microstructural features (termed radiation damage), which affect the mechanical properties of the vessel. A key tool for characterizing these nanoscale features is atom probe tomography (APT), due to its high spatial resolution and the ability to identify different chemical species in three dimensions. Microstructural observations using APT can underpin development of a mechanistic understanding of defect formation. However, with atom probe analyses there are currently multiple methods for analyzing the data. This can result in inconsistencies between results obtained from different researchers and unnecessary scatter when combining data from multiple sources. This makes interpretation of results more complex and calibration of radiation damage models challenging. In this work simulations of a range of different microstructures are used to directly compare different cluster analysis algorithms and identify their strengths and weaknesses.

Atom Probe Tomographic Characterization of Nanoscale Cu-Rich Precipitates in 17-4 Precipitate Hardened Stainless Steel Tempered at Different Temperatures

The formation of copper-rich precipitates of 17-4 precipitate hardened stainless steel has been investigated, after tempering at 350-570°C for 4 h, by atom probe tomography (APT). The results reveal that the clusters, enriched only with Cu, were observed after tempering at 420°C. Segregation of Ni, Mn to the Cu-rich clusters took place at 450°C, contributing to the increased hardening. After tempering at 510°C, Ni and Mn were rejected from Cu-rich precipitates and accumulated at the precipitate/matrix interfaces. Al and Si were present and uniformly distributed in the precipitates that were <1.5 nm in radius, but Ni, Mn, Al, and Si were enriched at the interfaces of larger precipitates/matrix. The proxigram profiles of the Cu-rich precipitates formed at 570°C indicated that Ni, Mn, Al, and Si segregated to the precipitate/matrix interfaces to form a Ni(Fe, Mn, Si, Al) shell, which significantly reduced the interfacial energy as the precipitates grew into an elongated shape. In addition, the number density of Cu-rich precipitates was increased with the temperature elevated from 350 up to 450°C and subsequently decreased at higher temperatures. Also, the composition of the matrix and the precipitates were measured and found to vary with temperature.

The maximum separation cluster analysis algorithm for atom-probe tomography: parameter determination and accuracy

Atom-probe tomography is a materials characterization method ideally suited for the investigation of clustering and precipitation phenomena. To distinguish the clusters from the surrounding matrix, the maximum separation algorithm is widely employed. However, the results of the cluster analysis strongly depend on the parameters used in the algorithm and hence, a wrong choice of parameters leads to erroneous results, e.g., for the cluster number density, concentration, and size. Here, a new method to determine the optimum value of the parameter dmax is proposed, which relies only on information contained in the measured atom-probe data set. Atom-probe simulations are employed to verify the method and to determine the sensitivity of the maximum separation algorithm to other input parameters. In addition, simulations are used to assess the accuracy of cluster analysis in the presence of trajectory aberrations caused by the local magnification effect. In the case of Cu-rich precipitates (Cu concentration 40-60 at% and radius 0.25-1.0 nm) in a bcc Fe-Si-Cu matrix, it is shown that the error in concentration is below 10 at% and the error in radius is <0.15 nm for all simulated conditions, provided that the correct value for dmax, as determined with the newly proposed method, is employed.

Critical consideration of precipitate analysis of Fe-1 at.% Cu using atom probe and small-angle neutron scattering

An Fe-1 at.% Cu model alloy was examined by atom probe (3DAP) and small-angle neutron scattering (SANS) to verify the accordance of the gained results. The Fe-Cu alloy was heat-treated for various times at 500°C, forming Cu-rich precipitates within the Fe matrix. The chemical compositions of the precipitates and matrix found by 3DAP were used to calculate the magnetic scattering contrast. Additionally, a magnetic moment of the precipitates that contain a significant amount of Fe was taken into account for the calculation of magnetic scattering contrast. This in turn is used for the evaluation of the magnetic scattering curves gained by SANS. Both the 3DAP data as well as the scattering curves were analyzed with regard to radius, number density, and volume fraction of the precipitates as a function of aging time. The results yielded by both techniques are in good agreement and correspond to the development of the hardness of the alloy. Minor differences can be related to the cluster search algorithm used for the analysis of the 3DAP data as well as Fe overestimation based on different field phases.

A comparison of the structure of solute clusters formed during thermal ageing and irradiation

Nanometre scale clusters form in Cu-containing reactor pressure vessel (RPV) steels during neutron irradiation. These clusters have a deleterious effect on mechanical properties, which can result in embrittlement and limit the reactor operating life. Thermal ageing of RPV steels can also induce the formation of solute clusters but it is not clear how similar these are to those formed during irradiation. In this work atom probe tomography, combined with detailed structural assessments of the structure of solute clusters, is used to address this issue. A series of thermal ageing heat treatments has been performed on several high- and low-Ni RPV welds to produce 1-4 nm diameter solute clusters. The same materials have also been neutron irradiated. The results show that CuMnNiSi enriched clusters formed during thermal ageing have, on average, higher Cu contents and lower Mn, Ni and Si contents than those found in irradiation-induced clusters. The effect of increasing bulk Ni is to encourage the formation of clusters with significantly higher Ni content, slightly higher Mn and Si contents and significantly lower Cu contents. At very high doses and dose rates MnNiSi enriched clusters can form even in high-Cu welds. Despite differences in the compositions of individual clusters formed during irradiation and during thermal ageing, clusters in both exhibit similar structure. In particular, well developed clusters in both materials have Cu-enriched cores whose peripheries are enriched in Ni, Mn and, in most cases, Si.