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Duplex stainless steels are desirable for use in power generation systems due to their attractive combination of strength, corrosion resistance, and cost. However, thermal embrittlement at intermediate homologous temperatures of ~475°C and below, limits upper service temperatures for many applications. New lean grade duplex alloys have improved thermal stability over standard grades and potentially increase the upper service temperature or the lifetime at a given temperature for this class of material. The present work compares the thermal stability of lean grade, alloy 2003 to standard grade, alloy 2205, through a series of isothermal agings between 260°C and 482°C for times between 1 and 10,000 hours. Aged samples were characterized by changes in microhardness and impact toughness. Additionally, atom probe tomography was performed to illustrate the evolution of the α-α′ phase separation in both alloys at select conditions. Atom probe tomography confirmed that phase separation occurs via spinodal decomposition for both alloys and identified the presence of Ni-Cu-Si-Mn-P clusters in alloy 2205 that may contribute to embrittlement of this alloy. The impact toughness model predictions for upper service temperature show that alloy 2003 may be viable for use in 288°C applications for 80 year service lifetimes based on a Charpy V-notch criteria of 47 J at room temperature. In comparison, Alloy 2205 should be limited to 260°C applications for the same room temperature toughness of 47 J. © 2015. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/ 2 Keywords: duplex stainless steels, 475°C embrittlement, spinodal decomposition, atom probe tomography 1.0 Introduction Duplex stainless steels (DSS) comprise a unique class of materials that possess desirable properties of both the face-centered cubic (austenitic) and body-centered cubic (ferritic) phases within their microstructures. The ferrite and austenite phases are present in roughly equal volume fractions,typically ranging from 30-70% ferrite. Relative to their austenitic counterparts, DSS tend to have higher strength, higher toughness, improved corrosion resistance (especially to localized corrosion), and exceptional resistance to halide stress corrosion cracking [1,2]. Additionally, their relatively low nickel content lowers the cost of these alloys and helps ensure price stability.DSS are widely used in chemical processing, desalination, pulp and paper, storage, and transportation industries due to their high strength and good corrosion resistance [1]. Components commonly manufactured from DSS include storage tanks, pipes, pressure vessels, heat exchangers, seawater systems, rotors, and structural members. However, DSS have had little application in power generation industries, in part due to concerns with thermal embrittlement. The thermal embrittlement that limits broader applications of DSS generally occurs at temperatures between 204°C and 538°C with a peak embrittlement rate near 475°C. ...

The atom probe has provided a technique for nanoscale characterization for more than 30 years. The three-dimensional atom probe (3DAP) determines the spatial coordinates and the elemental identities of the atoms within a small volume of a metallic or semiconducting specimen [1]. Fundamentally new analytical techniques have been developed recently to estimate the composition and parameters such as the radius of gyration and number density of features as small as 1 nm [1]. These techniques, which may be applied to nanoscale features such as embryos, clusters, ultrafine precipitates and the core regions of dislocations, are based on the principle that the solute atoms in a solute-enriched feature are closer together than in the surrounding matrix and permits the atoms associated with these features to be separated from those in the matrix (maximum separation method) [2]. The chemical short range ordering tendencies of solute atoms in each phase and their nearest neighbor configurations may also be statistically estimated. These techniques permit the earliest (pre-precipitation and kinetically controlled) stages of phase separation and decomposition to be investigated. Examples are given for an oxide-dispersion strengthened (ODS) ferritic steel.Nanoscale clusters and precipitates may be visualized with atom maps in which the position of each solute atom is displayed, Fig. 1a. The solute atoms associated with each cluster may be identified with the maximum separation method described above, Fig. 1b. The size of each cluster may be determined in terms of the radius of gyration or Guinier radius from either the positions of all the solute atoms associated with each cluster or the positions of the atoms of an individual element. Information on the behavior of the different solutes present in the cluster may then be evaluated, Fig. 2. A radial concentration profile with its origin at the center of mass may be constructed for spherical clusters and precipitates to evaluate the tendency for interfacial segregation and to estimate the sharpness of the precipitate-matrix interface, Fig. 3. The composition of individual clusters may be determined by the envelope method: a fine three-dimensional grid is superimposed upon the data; each cluster is defined by all cells of the grid containing the solute atoms identified by the maximum separation method, Fig. 1c; the composition of each cluster is the relative proportions of all atoms within the cells defining the cluster. The outermost cells may be eroded to minimize the influence of cells at the cluster-matrix interface.