SAXS and XAFS characterization of nano-scale precipitates in copper-base alloys

Analyses of small-angle X-ray scattering (SAXS) and extended X-ray absorption fine structure (EXAFS) were performed for characterizing precipitates formed in a, Cu-Ni-Si alloy without and with a, small amount of Fe, the strength and electrical conductivity of which were improved by aging process. These alloy samples were aged at 720 K after a solution treatment. The SAXS profiles of the samples were measured to investigate the size of precipitates. The results of SAXS measurements showed that nanometer-size precipitates formed in the alloy samples during isothermal aging. The precipitates in the Cu-alloy sample without Fe appeared to be coarsened in a multi-modal size distribution by the aging process. In contrast, the precipitates with homogeneous size were formed for the Cu-alloy sample with Fe. The environmental structure of Ni and Fe of these alloys evaluated by EXAFS measurements revealed that the precipitates of these alloys have the structure of δ-Ni₂Si, and Fe substitutes Ni in δ-Ni₂Si. It is presumed that the precipitates of homogeneous size in the Fe-added Cu alloy were formed because highly dispersed Fe atoms serve as nucleation sites of Ni₂Si.