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 thermally aged at 500-degrees-C for various times. It is known that such treatments induce Cu precipitation; the Cu depletion of the matrix is measured by the resistivity decrease of the samples. The crystallographic environment of the solute atoms in the irradiated or aged samples has been studied by XAS (x-ray absorption spectroscopy). The data show that Cr, Mn, and Ni atoms mainly remain in BCC solid solution during the Cu precipitation. The first Cu precipitates are found to be of BCC structure, i.e. coherent with the matrix. At the longest ageing times, they have become of FCC structure. In the electron-irradiated samples, the data show that, up to a fluence of 5 C cm-2, most of the Cu precipitates (> 80%) are still of BCC structure. The fraction of BCC precipitates 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 aged, the Fcc fraction is larger in cold-rolled samples than in pre-annealed and quenched ones. The results obtained are consistent, in the FeCu 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 least in its first stages, by the presence of Mn in the alloy.