Structural evolution of NiAg heterogeneous alloys upon annealing

NiAg heterogeneous alloys were studied by x-ray diffraction and x-ray absorption spectroscopy at the Ni K-edge using a total electron yield detection. In the as-deposited NixAg1-x alloys of 0.10 and 0.15 Ni atomic fraction, most of the Ni atoms are in substitutional sites in the Ag matrix. At higher Ni concentration, the Ni atoms outside the Ag-rich phase become numerous enough to group together in small clusters. An important disorder in the neighbourhood of Ni atoms is demonstrated. At low annealing temperature (up to 250 degrees C), in Ni0.20Ag0.80 and Ni0.35Ag0.65, some Ni atoms are still present in substitutional sites in the Ag matrix and the small Ni particles are under strain. A very shea-range order exists in this state. After a 250 degrees C annealing, the Ni particles grow, and the Ag-rich phase remains in a steady structural state. After a higher annealing (400 degrees C), the local Ni atomic environment becomes well ordered and typical of the pure Ni FCC phase. The Ag-rich crystallites are impoverished in Ni atoms and grow with elimination of defects. Ni grains are generally smaller than 1 nm for as-deposited alloys and reach several nanometres after a 400 degrees C annealing for 10 min.