Small-angle X-ray scattering characterization of precipitates in Cu-Ti alloys

In order to understand the aging characteristics of Cu-Ti alloys, extended X-ray absorption fine structure (EXAFS) and small-angle X-ray scattering (SAXS) measurements were performed for characterizing changes in the local structure around titanium and the precipitate size in the Cu-Ti alloys by aging at approximately 720 K after solution treatment. Cu-1.3, 2.6, and 4.5 at.% Ti alloys were prepared using high-purity base metals. The amount of titanium dissolved in the copper matrix during aging was estimated from the electrical resistivity of the alloys. The aging characteristics showed that the precipitation rate of Cu-Ti intermetallic compounds in the copper matrix increased with increasing bulk titanium amount. The local structures of titanium in precipitates, presumably Cu₄Ti, are indistinguishable in the present alloys by EXAFS analysis because of the similarity of EXAFS functions for a Cu₄Ti intermetallic compound and a substitutional solid solution in copper. However, the EXAFS analysis revealed that the atomic distance of a Cu-Ti pair was significantly smaller than that estimated from the atomic radii of copper and titanium, implying that there is a strong atomic correlation in the pair. SAXS measurements were carried out for analyzing the growth behavior of precipitates in alloys during aging. The SAXS results revealed that a small amount of precipitates was formed in Cu-1.3 at.% Ti in the initial aging stage, whereas precipitates with approximately 2 nm radius were formed in Cu-2.6 and 4.5 at.% Ti alloys. By aging up to 2000 s, the precipitates in Cu-1.3, 2.6 and 4.5 at.% Ti alloys grew to approximately 3.5 nm in radius. These aging characteristics were discussed on the basis of the number density of precipitates calculated from the size of precipitates and the amount of super-saturated titanium in the copper matrix.