COPPER PRECIPITATION IN FECU, FECUMN, AND FECUNI DILUTE ALLOYS FOLLOWED BY X-RAY-ABSORPTION SPECTROSCOPY

Samples of FeCu, FeCuMn, FeCuNi, and FeCuCr alloys containing 1.5 wt% of each solute have been electron irradiated around 290-degrees-C or t hermally aged at 500-degrees-C for various times. It is known that suc h treatments induce Cu precipitation; the Cu depletion of the matrix i s measured by the resistivity decrease of the samples. The crystallogr aphic environment of the solute atoms in the irradiated or aged sample s has been studied by XAS (x-ray absorption spectroscopy). The data sh ow that Cr, Mn, and Ni atoms mainly remain in BCC solid solution durin g the Cu precipitation. The first Cu precipitates are found to be of B CC structure, i.e. coherent with the matrix. At the longest ageing tim es, they have become of FCC structure. In the electron-irradiated samp les, the data show that, up to a fluence of 5 C cm-2, most of the Cu p recipitates (> 80%) are still of BCC structure. The fraction of BCC pr ecipitates has been estimated from both the x-ray-absorption near-edge structure (XANES) and extended x-ray-absorption fine-structure (EXAFS ) data. It appears that, for both FeCu and FeCuMn samples, similarly a ged, the Fcc fraction is larger in cold-rolled samples than in pre-ann ealed and quenched ones. The results obtained are consistent, in the F eCu case, with a simple linear correspondence between fluence and time , 1 C cm-2 at 300-degrees-C being equivalent to approximately 10 h at 500-degrees-C. The Cu precipitation is found to be accelerated, at lea st in its first stages, by the presence of Mn in the alloy.