Nanostructure evolution during low temperature aging of three binary Fe-Cr alloys has been investigated by atom probe tomography. A new method based on radial distribution function (RDF) analysis to quantify the composition wavelength and amplitude of spinodal decomposition is proposed. Wavelengths estimated from RDF have a power-law type evolution and are in reasonable agreement with wavelengths estimated using other more conventional methods. The main advantages of the proposed method are the following: (1) Selecting a box size to generate the frequency diagram, which is known to generate bias in the evaluation of amplitude, is avoided. (2) The determination of amplitude is systematic and utilizes the wavelength evaluated first to subsequently evaluate the amplitude. (3) The RDF is capable of representing very subtle decomposition, which is not possible using frequency diagrams, and thus a proposed theoretical treatment of the experimental RDF creates the possibility to determine amplitude at very early stages of spinodal decomposition.

Atom-probe tomography was used to investigate phase separation and copper (Cu) clustering in the ferrite phase of a 25Cr-7Ni super duplex stainless steel. The steel was subjected to a tensile load during aging at 325 degrees C for 5800 h. The degree of phase separation into alpha (Fe-rich) and alpha' (Cr-rich) was small, but still, it was the highest in the steel subjected to the highest load. Cu was found to cluster, and the number density of clusters increased with increasing load. In the material subjected to the highest load, Cu was enriched in regions that were neither Fe-rich nor Cr-rich. These regions also had the highest number density of Cu clusters.

The concurrent phase separation and clustering of alloying elements in the ferrite phase of duplex stainless steel weldments after stress aging at 325 degrees C have been investigated by atom probe tomography analysis. Both phase separation, into Fe-rich and Cr-rich ferrite, and solute clustering were observed. Phase separation in the heat-affected zone (HAZ) is most pronounced in the high alloyed SAF 2507, followed by SAF 2205 and SAF 2304. Moreover Cu clustering was observed in the HAZ of SAF 2507. However, decomposition in the weld bead (25.10.4L) was more pronounced than in the HAZs, with both phase separation and clustering of Ni-Mn-Si-Cu. The observed differences in the decomposition behaviors in the HAZ and weld bead can be attributed to the high Ni content and the characteristic microstructure of the weld bead with high internal strains. In addition, an applied tensile stress during aging of weldments has been found to further promote the kinetics of phase separation and clustering.

Clustering of alloying elements in solution-treated Fe-Cr-based alloys is of considerable importance for their microstructure stability upon aging. The clustering of Cr after solution treatment in three stainless steel alloy categories has been studied by atom probe tomography. Furthermore, phase-field simulations are applied to examine the effect of initial clustering on phase separation evolution. It is concluded that the clustering of Cr found in solution-treated ferritic and duplex alloys plays a critical role in the nanostructure evolution during aging.

In the present work the 475 degrees C embrittlement in binary Fe-Cr and ternary Fe-Cr-X (X=Ni, Cu and Mn) alloys have been investigated. The mechanical properties were evaluated using microhardness and impact testing, and the structural evolution was evaluated using atom probe tomography (APT). The APT results after aging at 500 degrees C for 10 h clearly showed that both Ni and Mn accelerate the ferrite decomposition. No evident phase separation of either the Fe-20Cr or Fe-20Cr-1.5Cu samples was detected after 10 h of aging and thus no conclusions on the effect of Cu can be drawn. Cu clustering was however found in the Fe-20Cr-1.5Cu sample after 10 h aging at 500 degrees C. The mechanical property evolution was consistent with the structural evolution found from APT. Samples aged at 450 and 500 degrees C all showed increasing hardness and decreasing impact energy. The embrittlement was observed to take place mainly during the first 10 h of aging and it could primarily be attributed to phase separation, but also substitutional solute clustering and possibly carbon and nitrogen segregation may contribute in a negative way.