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Hirokazu Sasaki, Syunta Akiya, Kuniteru Mihara, Yojiro Oba, Masato Onuma, [Jun Uzuhashi](https://orcid.org/0000-0003-2023-8158), [Tadakatsu Ohkubo](https://orcid.org/0000-0003-3548-1951)

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© 2024  Journal  of  Japan  Institute  of  Copper[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Characterization of δNi<sub>2</sub>Si Precipitates in Cu-Ni-Si Alloy by Small-Angle X-ray Scattering, Small-Angle Neutron Scattering, and Atom Probe Tomography](https://mdr.nims.go.jp/datasets/171f592b-d905-4703-8d65-bb5e0a27b884)

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Characterization of δNi2Si Precipitates in Cu-Ni-Si Alloy by Small-Angle X-ray Scattering, Small-Angle Neutron Scattering, and Atom Probe TomographyCharacterization of δNi2Si Precipitates in Cu-Ni-Si Alloy by Small-Angle X-rayScattering, Small-Angle Neutron Scattering, and Atom Probe Tomography+1Hirokazu Sasaki1,+2, Syunta Akiya2, Kuniteru Mihara3, Yojiro Oba4, Masato Onuma5, Jun Uzuhashi6 andTadakatsu Ohkubo61Analysis Technology Center, Furukawa Electric Co., Ltd., Yokohama 220-0073, Japan2Material Laboratory, Furukawa Electric Co., Ltd., Nikko 321-1493, Japan3Electronics Component Material Division, Furukawa Electric Co., Ltd., Tokyo 100-8322, Japan4Department of Mechanical Engineering, Toyohashi University of Technology, Toyohashi 441-8580, Japan5Applied Quantum Science and Engineering, Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan6Research Center for Magnetic and Spintronic Materials, National Institute for Materials Science, Tsukuba 305-0047, JapanThe strength of a Cu-Ni-Si alloy can be improved by finely dispersing Ni-Si-based compounds as precipitates into the Cu matrix throughheat treatment. This requires quantitatively evaluating the size distribution and dispersion state to investigate the strengthening effect of theprecipitate. In this work, we utilized transmission electron microscopy, small-angle X-ray scattering (SAXS), small-angle neutron scattering(SANS), and atom probe tomography (APT) to analyze these Ni-Si precipitates. The APT results showed two types of diffusion layers at theinterface between the Cu matrix and precipitates. The alloy contrast variation method was used to examine the difference in SAXS and SANSintensity in absolute units, which indicated that the δNi2Si precipitates are distorted. [doi:10.2320/matertrans.MT-D2024005](Received September 2, 2024; Accepted October 22, 2024; Published December 25, 2024)Keywords: transmission electron microscopy, small-angle x-ray scattering, small-angle neutron scattering, atom probe tomography, copper-nickel-silicon alloy, ¤Ni2Si precipitate1. IntroductionAs electronic devices such as smartphones have becomesmaller, lighter, and more powerful, their electroniccomponents have also become smaller and increased inperformance. The requirements for copper alloy strips usedin conductive parts such as leads and connectors of theseelectronic components are also increasing, requiring materialswith both higher strength and conductivity than that ofconventional materials. Cu-Ni-Si alloys have been knownto exhibit high strength and conductivity after heat treatment,where Ni-Si compounds are dispersed finely within thecopper matrix.Transmission electron microscopy (TEM) is a key methodfor analyzing Ni-Si precipitates dispersed in the coppermatrix, and makes it possible to evaluate the shape, size,density, and crystal structure of the precipitates [1–6]. Thismethod is widely used for initial analysis.In addition to TEM, the present study used atom probetomography (APT) to analyze the size, shape, andcomposition of the precipitates. In the APT method, the 2Dposition-sensitive detector detects the ions that haveevaporated from the surface of the needle-shaped sample’sapex, and the obtained data are reconstructed as 3D atommaps at the nanometer scale. At the same time, the type ofatoms can be identified by the time of flight in which theevaporated atoms reach the detector. The objective of APTinvestigation is to analyze the size, shape, and compositionof precipitates on the order of several nanometers to tensof nanometers in Cu-Ni-Si alloys. The feature of APT thatenables 3D visualization on the nanometer order can beeffectively utilized to analyze structures with 3D geometries,in contrast to TEM, which is mainly a 2D analysis method.Small-angle X-ray scattering (SAXS) is effective forquantitatively evaluating the number density, shape, etc. ofprecipitates in metals [7–12]. The measurement volume ofSAXS is determined by the diameter of the irradiated X-raybeam and the thickness of the sample, and under typicalmetal measurement conditions, it is several million timeslarger than the observation volume of TEM, making itsuitable for quantifying statistical values such as the averagesize of precipitates.In this study, small-angle neutron scattering (SANS) wasused in addition to SAXS to evaluate precipitates in Cu-Ni-Sialloys. SANS uses neutrons as the irradiation beam, whichoffers various advantages over SAXS. First, its strongmaterial permeability makes it possible to measure thicksamples. For copper measurements, neutrons can passthrough a few millimeters of thickness, eliminating the needto polish the sample to the thickness required for SAXSmeasurements. As a result, the measurement volume to be100,000 to 1,000,000 times larger than that of SAXS, whichis important for gaining a more accurate understanding ofthe correlation between the electrical and mechanicalproperties of the metal products and precipitates. In addition,the scattering length of neutrons is not proportional to theatomic number, unlike that of X-rays, and neutron contrastalso differs from that of X-rays. By using the contrastdifference between neutrons and X-rays, information such asthe composition and density of precipitates can be obtained[9–12]. Lastly, neutrons have spin, which enables analysisof magnetic materials. However, this feature is not generallyused in the analysis of copper alloys.+1This Paper was Originally Published in Japanese in J. Japan Inst. Copper62 (2023) 85–89. The abstract and captions of Fig. 3 and 4 have beenslightly modified.+2Corresponding author, E-mail: hirokazu.sasaki@furukawaelectric.comMaterials Transactions, Vol. 66, No. 1 (2025) pp. 44 to 49©2024 Journal of Japan Institute of Copperhttps://doi.org/10.2320/matertrans.MT-D2024005In this study, the structure of precipitates in copper alloyswas analyzed in detail by utilizing these various analyticalmethods.2. Experimental Procedure2.1 SpecimensThe specimens were Cu-Ni-Si Corson alloy containing2.5mass% Ni and 0.6mass% Si. These were melted, cast,heat-treated, rolled, annealed, and solution heat-treated.Aging precipitation heat treatment was then conducted.SAXS and SANS measurements were conducted on theCu-Ni-Si alloy samples at aging temperatures of 425, 450,500, and 550°C, in addition to solution annealing onlysamples. The aging time was 2 h.2.2 TEM observationTEM samples were prepared using a focused ion beam(FIB) method. SIINT–3050TB was used for processing, andthe Ga ion beam acceleration voltage was 30 kV. After FIBthin film processing, Ar ion milling at 2 kV was conducted.TEM observation was conducted using a JEOL JEM-2100Plus, and a JEOL JEM-ARM200F was used for scanningTEM (STEM) observation. The electron beam accelerationvoltage was 200 kV. High-angle annular dark field (HAADF)STEM images were taken in the STEM observations.2.3 APT observationSamples for APT measurements were prepared using anFIB. A Ga ion beam with an acceleration voltage of 30 kVwas used to prepare a needle-shaped sample, and a 5-kV ionbeam was used for final cleaning. A FEI Helios G4UX wasused as the FIB. In this measurement, a LEAP5000XS wasused, which has higher spatial resolution and detectionefficiency than those used in previous studies [13, 14]. Themeasurement temperature was 30K. An ultraviolet light witha wavelength of 355 nm was used as the pulsed laser to assistevaporation [15].2.4 SAXS measurementThe samples were polished to a thickness of approximately20 µm. A Rigaku NANO-Viewer was used as the laboratorysystem. A Mo–Kα X-ray source was used, and the energyof the incident X-ray was 17.47 keV. The sample-to-detectordistance was 0.5m. Ultra-SAXS (USAXS) measurementswere also conducted at the beamline BL08B2 of SPring-8.The energy of the incident X-ray was 18 keV, and the sample-to-detector distance was 16m. The laboratory SAXS profilewas converted to the absolute value of the intensity usingglassy carbon [16]. A single SAXS profile was created byconnecting the USAXS profile in accordance with theintensity of the laboratory SAXS profile.2.5 SANS measurementThe sample was approximately 2.1mm thick. Themeasurements with camera lengths of 0.8m and 9m took100 minutes and 40 minutes, respectively. The SANS profilesmeasured with the two camera lengths were combined toform a single SANS profile. The measurements wereconducted at room temperature. The irradiated neutron beamwas about 15mm in diameter. SANS measurements wereconducted using the SANS-J beamline installed in the JRR3research reactor at the Japan Atomic Energy Agency.Absolute intensities were measured using Al standards forneutron irradiation. Compared with the measurementsconducted in the previous studies [13, 14], this measurementdiffers in that the q-range is wider and absolute valueprocessing was conducted. Therefore, the fitting processcarried out and the profile intensity can be discussed.3. Results and Discussions3.1 TEM observation resultsThe bright-field TEM image is shown in Fig. 1. Numerousprecipitates about 10 nm in size were observed in the leftregion of the TEM image. Contrast due to strain wasobserved in the copper matrix surrounding the precipitates.Furthermore, precipitates nearly 100 nm in size were alsoobserved, as seen in the right area of the TEM image.Approximately one precipitate 50 nm to 100 nm in size wasobserved in every 1 µm square of the TEM field of view. Thiswas also observed in the samples that had only been solutiontreated, indicating a phase that existed before the agingprecipitation heat treatment. An enlarged HAADF-STEMimage of one of the precipitates is shown in Fig. 2. Analysisof this image revealed that the precipitated phase was δNi2Si,as previously reported [13, 14].Fig. 1 Bright-field TEM image of 550°C heat-treated copper alloy.Fig. 2 HAADF-STEM image of δNi2Si precipitate.Characterization of δNi2Si by SAXS, SANS and APT 453.2 APT observation resultsFigure 3 shows the analysis results of a sample heat-treated at 550°C using an APT. The use of ultraviolet laserpulses made it possible to stably acquire data from a widearea, as shown in Fig. 3 [17]. In this figure, the iso-concentration surface of 14 atomic% Ni and 7 atomic% Si areshown. Within this field of view, the distributions of Ni andSi are almost the same, and the precipitated phase is notspherical but ellipsoidal, close to a disk.To analyze the compositional details within the precip-itates, the APT result of one of the precipitates is shown inFig. 4. Figure 4(a) shows the iso-concentration surface of 7atomic% Si. Figure 4(b) shows the composition profilewithin the precipitate created in the direction indicated bythe arrow in the figure. As shown in the figure, theprecipitating phase is Ni2Si since the ratio of Ni to Si is2:1 in the center. Interdiffusion of Ni, Si, and Cu at theinterface between the precipitated phase and the Cu matrixwas also observed. A closer look at the diffusion area revealstwo characteristics as indicated by arrows 1 and 2. In theregion indicated by arrow 1, the Si composition is constantand the Ni composition decreases toward the Cu matrix. Thisregion is considered to be δ(Ni1¹y, Cuy)2Si as suggested byYi et al. [18]. In the region indicated by arrow 2, Ni and Sidiffuse into the copper matrix at less than 10%. Figure 5shows a model diagram of the precipitated phase estimatedfrom these APT results.3.3 SAXS and SANS measurement resultsThe measurement results of SAXS and SANS are shown inFigs. 6 and 7. Each figure shows the small-angle scatteringprofiles of the solution-treated and heat-treated samples ataging temperatures of 425, 450, 500, and 550°C,respectively. Compared with the solution-treated Corsonalloy, the 425°C aging sample shows a shoulder indicatingthe formation of nanoparticles in the region of q = 0.4 to2 nm¹1. As the aging heat treatment temperature increases,the shoulder indicating scattering moves toward the low-qNi: 14.0at%Si: 7.0at%Z / nmY / nmX / nm50-20050 -50-50 0 0-100-300Z / nmY / nmX / nm50-20050 -50-50 0 0-100-300Fig. 3 3D atom map obtained by APT (iso-concentration-surfaces of Ni at 14 at% and Si at 7 at%). (online color)(b)100806040200Composition (atomic %)20151050Distance (nm) Cu Ni Si12210 nm(a)Fig. 4 Si 3D atom map of single precipitate (iso-concentration-surfaces of Si at 7 at%) and composition profiles obtained by APT.(online color)H. Sasaki et al.46side. These results indicate that the Ni-Si precipitatesgradually become coarser.The Ni-Si precipitates in the 425 and 450°C aging samplescould be fitted as spherical particles, with an average particlesize of 2.2 nm for the 425°C aging sample and 2.6 nm for the450°C aging sample. The precipitates of the 500 and 550°Caging samples are disk-like ellipsoids, as shown by theresults of the APT, so it is appropriate to fit them with thisshape.3.4 SAXS and SANS profiles analysisThe SAXS and SANS profiles were analyzed on thebasis of the APT results of the precipitates. Although theprecipitates of the 500 and 550°C aging samples differ insize, their profile shapes are similar, so the analysis wasconducted on the 550°C aging sample. From the results of theAPT, the precipitates of the 550°C aging sample are assumedto be disk-like ellipsoids. Therefore, SAXS and SANSprofiles were fitted on the basis of this assumption. In thiscase, the axis ratio of the ellipsoid was assumed to be 0.3.Phases larger than 50 nm observed in the TEM image inFig. 1 are assumed to be spherical for fitting. In this fitting,the background caused by incoherent scattering, whichgenerally appears in the high-q region above q = 2 nm¹1, isset as a constant value, and the background resulting fromthe coarse structure, which is more prominent in the low-qregion, is taken as q¹4.The fitting results are shown by the solid line in Fig. 8.The shoulder near q = 0.3 nm¹1, indicated by the arrow in thefigure, corresponds to the precipitates. The shoulder aroundq = 0.05 nm¹1, indicated by the dashed arrow, corresponds tocoarse phases. Assuming a spherical model and fitting usinga SANS profile, the average radius was 38 nm.The precipitates were fitted with SAXS profiles, and theaverage major axis radius of the ellipsoid was 8.9 nm.Conversely, the ellipsoid fitted with SANS profiles had anaverage major axis radius of 6.6 nm. The difference in theresults between SAXS and SANS is due to the difference inthe scattering contrast of the δ(Ni1¹y, Cuy)2Si diffusion layer,where Cu diffuses into the precipitated phase. As mentionedin previous studies [13, 14], the scattering contrast of theShell 2: Diffusion layer in Cu parent phaseShell 1: (Ni,Cu)2SiNi2SiFig. 5 Core-shell model of precipitate in copper alloy. (online color)10-210-1100101102103104105106 Intensity, I / cm-12 4 6 810-12 4 6 81002 4 6 8 Scattering vector, q  / nm-1 as 425 450 500 550 425 fitting 450 fittingFig. 6 SAXS profiles of copper alloys. (online color)10-210-1100101102 Intensity, I / cm-14 5 610-12 3 4 5 61002 3 4 Scattering vector, q  / nm-1 as 425 450 500 550Fig. 7 SANS profiles of copper alloys. (online color)10-210-1100101102103104105106Intensity, I /  cm-12 4 6 80.12 4 6 812 4 6 8Scattering vector, q / nm-1 SANS SANS fitting SAXS SAXS fittingFig. 8 SAXS and SANS profiles of 550°C heat-treated copper alloys.(online color)Characterization of δNi2Si by SAXS, SANS and APT 47δ(Ni1¹y, Cuy)2Si diffusion layer in the copper matrix is largefor X-rays and small for neutrons. Therefore, the precipitateswere measured to be larger in SAXS and smaller in SANS.Subsequently, analyses were conducted using the alloycontrast variation method, which estimates the compositionand density of the precipitates from the intensity ratio of theabsolute SAXS and SANS profiles [9–12]. The scatteringlengths corresponding to the elements differ between X-raysand neutrons, and this difference is reflected in the intensityratios of SAXS and SANS profiles. The scattering lengthdensity difference ¦ρ for the δNi2Si precipitates in the coppermatrix is different between X-rays and neutrons. Thisdifference is between the scattering length density of thematrix phase and that of the precipitated phase [9]. If thedifference in scattering length density of X-rays is ¦ρx andthat of neutrons is ¦ρn, the intensity ratio of SAXS andSANS is ¦ρx2/¦ρn2 when the precipitate is a single phase.When the precipitate phase in the copper matrix is δNi2Si,¦ρx2/¦ρn2 is theoretically 80. In this calculation, the latticeconstants of δNi2Si are assumed to be a = 0.706 nm, b =0.499 nm, and c = 0.372 nm [19], and the density is 7.37g/cm3. Conversely, the intensity ratio calculated from theSAXS and SANS profiles of the 550°C aging sample was345. One reason for this difference is the effect of thediffusion layer observed in the APT analysis. Since thecomposition in this region is different from that of δNi2Si, thescattering length-density difference is also different [13, 14].In addition, it was previous reported that the δNi2Siprecipitate phase and the copper matrix phase are lattice-matched, i.e., the lattice parameter of the δNi2Si precipitatephase is larger and its density is smaller due to lattice misfit[18], which can cause the difference in the intensity ratio. Ifthe density of δNi2Si is 7.01 g/cm3, ¦ρx2/¦ρn2 is 345, whichcan be interpreted as the intensity ratio of the SAXS andSANS profiles. If we assume that the lattice constant of theprecipitate phase is uniformly large, this corresponds to a1.8% increase in the lattice constant, but in reality, the latticeconstant is not uniform within the precipitate phase, and thestrain of each crystal axis is also considered to be non-uniform. From the aforementioned experimental results, theintensity ratio of the SAXS and SANS profiles should beinterpreted taking into account the effects of both the latticestrain and diffusion layer. However, detailed modeling is asubject for future work.4. Conclusion(1) The APT results suggest the existence of two types ofdiffusion layers at the interface between the precipitatephase and copper matrix.(2) In the SAXS and SANS profiles, the precipitates of the550°C aging specimen can be fitted as ellipsoids. Themajor axis radius of the precipitates was 8.9 nm inSAXS and 6.6 nm in SANS, which may be due to thedifference in X-ray and neutron contrast of the diffusionlayer.(3) The intensity ratios of absolute SAXS and SANSprofiles were analyzed using the alloy contrast variationmethod, the findings of which suggest that theprecipitate phase is not a simple δNi2Si. This may bedue to the density change of the δNi2Si phase caused bythe lattice strain between the precipitated phase and thecopper matrix and the diffusion layer observed in theAPT, which may affect the intensity of the SAXS andSANS profiles. Detailed modeling will be conducted inthe future, taking these findings into account.AcknowledgmentsThe STEM observations were supported by the Micro-structure Analysis Platform of the University of Tokyo,which is part of the Nanotechnology Platform funded bythe Ministry of Education, Culture, Sports, Science andTechnology of Japan. Laboratory SAXS measurements wereconducted in collaboration with the Institute for IntegratedRadiation and Nuclear Science, Kyoto University. SPring-8SAXS measurements were conducted at the HyogoPrefecture Beamline BL08B2 (Proposal No. 2019A3337).SANS measurements were conducted at the SANS-J of JRR3in JAEA (Proposal No. 2021A-A43). 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