SAXS and XAFS Characterization of Precipitates in a High-Performance Cu–Ni–Si Alloy

Abstract

Analyses of small-angle X-ray scattering (SAXS) and X-ray absorption fine structure (XAFS) were performed for characterizing precipitates formed in a Cu-3.1 at %Ni-1.4 at%Si alloy, the strength and electrical conductivity of which were improved by aging. SAXS profiles and XAFS spectra of samples aged at 720 K for different periods of time after a solution treatment were measured. SAXS profiles of samples, which were aged after the solution treatment and subsequently cold rolled, were also measured to investigate the effect of dislocations on precipitation. The results of SAXS measurements showed that nanometer-size precipitates formed in the alloy samples during isothermal aging at 720 K. The precipitates in the samples without cold rolling were coarsened in a single modal size distribution with increasing aging time. In contrast, the precipitates formed in the cold-rolled samples appeared to be coarsened in a multi-modal size distribution with increasing aging time. This aging characteristic of the cold-rolled samples is presumably attributable to their good electrical conductivity. The results of XAFS measurements at the Ni K-edge showed that nickel was substituted for copper in the face-centered cubic (fcc) copper matrix and that the local structure around nickel was changed by isothermal aging. With increasing aging time, extended X-ray absorption fine structure (EXAFS) functions at the Ni K-edge of the samples were found to be changed, which implies that nickel atoms were precipitated as nickel-silicon clusters or intermediate compounds in the fcc copper matrix. In addition, X-ray absorption near edge structure (XANES) spectra at the Ni K-edge indicated that the electronic structure of nickel in the samples was influenced by silicon during aging.

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

“…In addition, this method can also evaluate the characteristic features of atomic correlations. In fact, EXAFS was used for analyzing the local structure of nickel in Cu-Ni-Si alloys, where precipitation occurred by aging [7]. The results showed that as nickel dissolved in the copper matrix precipitated to form nickel silicide, the local structure of nickel changed by aging.…”

“…However, the SAXS signal from copper alloys generally contains an intense background scattering from surface imperfections and concentration fluctuations [5,6], resulting in difficulty in analyzing SAXS profiles. In order to overcome this difficulty, the indirect Fourier transform (IFT) method has been applied to analyze SAXS profiles, so that the information of precipitates is successfully extracted from SAXS profiles that have intense background scattering [7]. With regard to the local structure of precipitates, extended X-ray absorption fine structure (EXAFS) analysis can be a powerful tool for characterizing the local structure around a specific solute element.…”

An X-ray absorption spectroscopy investigation of the local atomic structure in Cu–Ni–Si alloy after severe plastic deformation and ageing

“…Takahashi et al [7,8] characterized the effects of ageing and Fe element addition on precipitates in a Cu-3.1Ni-1.4Si (at.%) alloy using small-angle X-ray scattering and extend X-ray absorption fine structure (EXAFS) techniques. They observed a change in the EXAFS signal at the Ni K-edge in the aged alloy samples.…”

Discontinuous precipitates in age-hardening CuNiSi alloys

“…To date, the age-hardening precipitates in the alloys have been widely accepted as being δ-Ni 2 Si intermetallics with an orthorhombic structure [6][7][8][9][10][11][12][13][14][15][16][17], although other types of precipitates, such as β-Ni 3 Si, (Cu,Ni) 3 Si, and other metastable phases, might coexist [8,[12][13][14][15]. In their most recent study, Hu et al investigated the subsequent age-hardening of δ-Ni 2 Si precipitates (i.e., continuous precipitates (CPs)) that form during isothermal aging, and explained their behavior using classical theories of phase transformation in association with energy calculations [14]; the CPs in Cu-2.6 Ni-1.6 Si-0.5 Cr (in at.%) alloys did not exhibit any changes in their δ-Ni 2 Si crystal structure, but underwent changes in their morphology and other crystallographic features.…”

“…The precipitating phase responsible for the age-hardening has been investigated by a number of researchers, especially regarding the early stages of aging [6][7][8][9][10][11][12][13][14][15]. To date, the age-hardening precipitates in the alloys have been widely accepted as being δ-Ni 2 Si intermetallics with an orthorhombic structure [6][7][8][9][10][11][12][13][14][15][16][17], although other types of precipitates, such as β-Ni 3 Si, (Cu,Ni) 3 Si, and other metastable phases, might coexist [8,[12][13][14][15].…”