Okajima  Toshihiro

Sanada  Takashi

Sato  Shigeo

Shinoda  Kozo

Suzuki  Shigeru

Takahashi  Yohei

MATERIALS TRANSACTIONS

English

Takahashi, Yohei

Sanada, Takashi

Sato, Shigeo

Okajima, Toshihiro

Shinoda, Kozo

Suzuki, Shigeru

Institutional Repositories DataBase (IRDB)

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

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

SAXS and XAFS characterization of Precipitatesin a High-Performance Cu-Ni-Si Alloy著者 Takahashi  Yohei, Sanada  Takashi, Sato Shigeo, Okajima  Toshihiro, Shinoda  Kozo,Suzuki  Shigerujournal orpublication titleMaterials Transactionsvolume 48number 2page range 101-104year 2007URL http://hdl.handle.net/10097/52239SAXS and XAFS Characterization of Precipitatesin a High-Performance Cu–Ni–Si AlloyYohei Takahashi1, Takashi Sanada1, Shigeo Sato1, Toshihiro Okajima2,Kozo Shinoda3 and Shigeru Suzuki31NISSAN ARC, LTD., Yokosuka 237-0061, Japan2Kyushu Synchrotron Light Research Center, Tosu 841-0005, Japan3Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, JapanAnalyses of small-angle X-ray scattering (SAXS) and X-ray absorption ﬁne structure (XAFS) were performed for characterizingprecipitates 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 proﬁlesand XAFS spectra of samples aged at 720K 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 onprecipitation. The results of SAXS measurements showed that nanometer-size precipitates formed in the alloy samples during isothermal agingat 720K. The precipitates in the samples without cold rolling were coarsened in a single modal size distribution with increasing aging time. Incontrast, the precipitates formed in the cold-rolled samples appeared to be coarsened in a multi-modal size distribution with increasing agingtime. This aging characteristic of the cold-rolled samples is presumably attributable to their good electrical conductivity. The results of XAFSmeasurements at the Ni K-edge showed that nickel was substituted for copper in the face-centered cubic (fcc) copper matrix and that the localstructure 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 clustersor intermediate compounds in the fcc copper matrix. In addition, X-ray absorption near edge structure (XANES) spectra at the Ni K-edgeindicated that the electronic structure of nickel in the samples was inﬂuenced by silicon during aging. [doi:10.2320/matertrans.48.101](Received October 13, 2006; Accepted November 13, 2006; Published January 25, 2007)Keywords: copper base alloy, precipitation, small angle X-ray scattering, X-ray absorption ﬁne structure1. IntroductionCopper-base alloys are extensively utilized for electricalparts such as lead frames and connectors, owing to their highelectrical conductivity. Since high strength is required ofcopper-base alloys used for small microelectronic devices,copper–beryllium alloys are sometimes applied owing totheir high electrical conductivity and high strength. However,because of the toxicity of beryllium, novel metallic materialsfree of hazardous substances are needed as an alternative tocopper–beryllium alloys. For that reason, various copper-base alloys, such as copper–titanium (Cu–Ti) binary al-loys1–5) and copper–nickel–silicon (Cu–Ni–Si) alloys,6–12)have been developed. They show excellent properties as aresult of controlling the processing conditions. The highperformance of copper-base alloys is achieved by the ﬁneprecipitates that form in the copper matrix during aging. Thealloy composition, aging conditions and pretreatment tech-niques are important parameters for obtaining high perform-ance in copper-base alloys. For example, the aging character-istics of Cu–Ti alloys have been examined systematically,5)and the eﬀect of cold rolling before aging on the properties ofCu–Ni–Si alloys has been also investigated.12)In order to control precipitates, it is essential to character-ize their state accurately. Precipitates formed by aging havebeen studied using transmission electron microscopy, but itis diﬃcult to evaluate systematic changes in the size of suchprecipitates because only local areas are observed. There-fore, the size and atomic-scale structure of precipitatesshould be characterized with other methods, in order toexamine the eﬀects of precipitates on the properties ofcopper-base alloys.Small-angle X-ray scattering (SAXS) analysis is a poten-tial method for characterizing compositional ﬂuctuations inalloys.13,14) Since SAXS has been used for analyzing thenano-structure in high-content alloys,15,16) it should beextendable to nano-structural analysis of low-content alloysas well. Analysis of X-ray absorption ﬁne structure (XAFS)may also be useful for analyzing the local atomic structurearound a speciﬁc element in alloys.17) This method isapplicable to analysis of precipitation in alloys, in whichthe content of alloying elements dissolved in the matrix ischanged by aging. The objective of this work is to character-ize the eﬀects of aging and cold rolling on precipitates in acopper alloy, Cu-3.1 at%Ni-1.4 at%Si, using SAXS andXAFS methods.2. Experimental Procedure2.1 Sample preparationThe alloy evaluated in this work was a high-purity Cu-3.1 at%Ni-1.4 at%Si alloy, which showed high strength andlow electrical resistivity. The detailed preparation method ofthe alloy is described elsewhere.12) Alloy samples wereprepared from 6N (99.9999%) copper, 4 N nickel, and 6Nsilicon using a hydrogen plasma arc melting method. Thesamples were annealed at about 1173K, and this process iscalled the solution treatment. Some of the samples weresubsequently cold rolled by 50%. In the following discussion,the samples that were solution treated will be denoted as STand those that were cold rolled after the solution treatmentwill be denoted as STCR. The alloy samples were isother-mally aged at 720K for diﬀerent periods of time.A previous investigation of aging characteristics revealedMaterials Transactions, Vol. 48, No. 2 (2007) pp. 101 to 104#2007 The Japan Institute of Metalsinteresting results.12) Although both ST and STCR samplesshowed fundamentally similar property changes with alonger aging time, some diﬀerences were observed betweenthem. After aging for 1000 s, the hardness of ST and STCRsamples increased, while their resistivity decreased. Both STand STCR samples showed nearly their maximum hardnessafter aging for 10000 s, but the resistivity of the STCRsamples was signiﬁcantly reduced. This suggests that coldrolling may have an additional eﬀect on precipitation in thisalloy. Thus, samples aged for 0, 1000 and 10000 s wereselected for SAXS and XAFS analyses in the present work.2.2 MeasurementsSAXS measurements were carried out using the NanoStarapparatus (Bruker AXS). A high-ﬂux Cu K X-ray beamobtained with cross-coupled Go¨bel mirrors and a subsequentthree pinhole collimation system was introduced into thesample chamber. The sample chamber and the beam path tothe two-dimensional position-sensitive proportional counterwere evacuated to eliminate air scattering. The scatteringproﬁles were obtained by averaging the images in a circulardirection. The proﬁles were measured in the scattering vectorq (q ¼ 4 sinð2=2Þ=) range from 0.2 to 3.0 nm1, where is the X-ray wavelength and 2 is the scattering angle. Thedata acquisition time was 3 h for each proﬁle.Measurements of Ni K-edge XAFS were carried out usingthe BL15 beam line at the Saga Light Source (SAGA-LS) inJapan. The XAFS spectra at the Ni K-edge were measuredusing a Si (1 1 1) double crystal monochromator in thetransmission mode at room temperature. The XAFS spectrawere analyzed using REX2000 XAFS analysis software(Rigaku Co.). In addition, a simulation of the extended X-rayabsorption ﬁne structure (EXAFS) function k3ðkÞ wasperformed using the FEFF 8.2 code.18)3. Results and Discussion3.1 SAXS analysisFigure 1(a) shows SAXS proﬁles of ST samples aged at720K for 0, 1000 and 10000 s. For the ST samples aged for0 s, i.e., the solution-treated sample, the scattering intensitydecreases linearly in the log-log plot, which presumablyresults from the inhomogeneity of the atomic distribution inthe alloy and/or from surface roughness. Such scattering maybe regarded as background scattering, although it remains inthe scattering proﬁles for the aged samples. The SAXSproﬁles of the ST sample aged for 1000 s shows a hump atabout 1 nm1. This hump indicates the appearance of smallsize precipitates. The position of the hump shifts to the lowerscattering vector side in the ST sample aged for 10000 s,which implies that the size of the precipitates became larger.SAXS proﬁles of STCR samples aged at 720K for 0, 1000and 10000 s are shown in Fig. 1(b). The proﬁles of thesamples aged for 0 s and 1000 s are similar to those of the STsamples. However, the hump in the SAXS proﬁle of thesample aged for 10000 s shows lower contrast than the proﬁleof the ST samples. This result indicates that precipitates werenot simply coarsened in this sample. It is interesting to notethat the aging characteristics of the alloy were signiﬁcantlyinﬂuenced by cold rolling after the solution treatment, whichimplies that precipitation was strongly aﬀected by disloca-tions formed by cold rolling.In order to investigate the size distribution of precipitatesformed in the samples by aging, the SAXS proﬁles wereanalyzed using the indirect Fourier transform method.19)Figures 2(a)–(c) show the distance-distribution functions(DDFs) for ST samples aged at 720K for 0, 1000 and10000 s, respectively. The DDFs for STCR samples aged at720K for 0, 1000 and 10000 s are shown in Figs. 3(d)–(f),0.1 1101102103104105106Intensity (a.u.)q / nm-10.1 1101102103104105106Intensity (a.u.)q / nm-1(a)0 s1000 s10000 s(b)0 s1000 s10000 sFig. 1 (a) SAXS proﬁles of ST samples aged for 0, 1000 and 10000 s and (b) SAXS proﬁles of STCR samples aged for 0, 1000 and10000 s.102 Y. Takahashi et al.respectively. As the DDFs were calculated from scatteringproﬁles that included background scattering, the functions forthe samples before aging do not necessarily represent the realsize distribution of precipitates. Nevertheless, aging charac-teristics are observed in these DDFs for the samples aged for1000 and 10000 s. In the ST and STCR samples aged for1000 s, precipitates of 2 nm in radius appear to have formed,although there may have been some diﬀerence in the sizedistribution of precipitates between the two types of samples.The DDF for the ST sample aged for 10000 s shows thatprecipitates of about 3 nm in radius formed as a result ofaging, indicating that the precipitates were coarsened innearly a single modal distribution. On the other hand, theDDF for the STCR sample aged for 10000 s does not show aclear peak. This indicates that precipitates formed in thissample were coarsened not in a single modal size distribu-tion, but in a multi-modal size distribution. Dislocationsformed by cold rolling are thought to suppress coarsening ofprecipitates in the single modal distribution through prefer-ential precipitation on dislocations. Such preferential precip-itation in the cold-rolled samples might have signiﬁcantlydecreased the amount of nickel and silicon dissolved in thematrix by aging, resulting in improvement of the electricconductivity of the STCR samples.3.2 XAFS analysisX-ray absorption near edge structure (XANES) spectra atthe Ni K-edge for the alloy samples without cold rolling (STsamples) aged for 0, 1000 and 10000 s are shown in Fig. 3.The XANES spectra of the ST samples are similar to that ofNi foil with a face-centered cubic (fcc) structure, whichimplies that nickel was substituted for copper in the fcccopper matrix. The XANES spectrum of the ST sample agedfor 1000 s was diﬀerent from that of the solution-treatedsample. This indicates that the electronic structure of nickelwas changed by aging, which is thought to result from theinteraction of nickel with another element, particularlysilicon, during aging. However, no clear diﬀerence wasobserved in XANES spectra between the samples aged for1000 and 10000 s. Therefore, the average electronic structureof nickel precipitated in the copper matrix was apparentlysimilar in the samples aged over 1000 s, although the amountof nickel dissolved in the copper matrix decreased duringaging. These results imply that most of the nickel had alreadystarted to interact with silicon after aging for 1000 s.Figure 4 shows k3-weighted EXAFS functions at the Ni K-edge for ST samples aged for 0, 1000 and 10000 s. Theamplitude of the EXAFS functions decreased in the agedsamples, especially in the high-k region. Characteristicshapes located at about 55 nm1 and 70 nm1 in the functionsfor the ST sample aged for 0 s were unclear in the agedsamples. The changes in resistivity12) due to aging indicatedthat the amount of nickel dissolved in the matrix of thepresent alloy samples aged at 720K for 1000 s and 10000 swas about 70% and 30%, respectively. Nevertheless, thechanges in the EXAFS spectra due to aging imply that the8300 8320 8340 8360 8380 8400Normalized Absorption  (a.u.)  Ni 0 s   1000 s  10000 s Energy / eVFig. 3 XANES spectra at the Ni K-edge of ST samples aged for 0, 1000and 10000 s. XANES spectrum of nickel foil is also shown for reference. r / nm r / nm(a)(b)(c)(d)(e)(f)0 4 8 12 0 4 8 12p(r) (a.u.)p(r) (a.u.)p(r) (a.u.)Fig. 2 (a)–(c) DDFs for ST samples aged for 0, 1000 and 10000 s and (d)–(f) DDFs for STCR samples aged for 0, 1000 and 10000 s.0 20 40 60 80 100 120-100102030405010000 s1000 sk3χ(k) (a.u.)k / nm-10 sFig. 4 EXAFS functions k3ðkÞ at the Ni K-edge for ST samples aged for0, 1000 and 10000 s.SAXS and XAFS Characterization of Precipitates in a High-Performance Cu–Ni–Si Alloy 103formation of nickel-rich precipitates was accompanied by achange in local structure around nickel in the alloy.The EXAFS function of a model structure calculated usingthe FEFF 8.2 code,18) in which nickel atoms are substitutedfor copper in the fcc copper matrix, is shown in Fig. 5. Thecalculated function reproduces fairly well the shape andphase of the experimental EXAFS function for the ST sampleaged for 0 s, as shown in Fig. 5. This is because nickel wasdissolved in the copper matrix in the solution-treated sample.It has been reported that a possible precipitate compound inthe Cu–Ni–Si system might be nickel silicide, such as Ni3Siand -Ni2Si.6,8) In order to characterize the local structure ofprecipitates formed in the aged samples, EXAFS functionsfor Ni3Si and -Ni2Si were calculated as shown in Fig. 4. Thecalculated functions were dissimilar to the experimentalEXAFS functions for the samples aged for 1000 s and10000 s. This implies that most of the nickel atoms were notprecipitated as such ideal compounds in the copper matrix. Itis likely that they were precipitated as nickel-rich clusters orintermediate compounds, perhaps containing silicon, al-though further structural analysis must be carried out toclarify this point.4. Concluding RemarksSAXS and XAFS analyses were carried out to characterizeprecipitates in the Cu-3.1 at%Ni-1.4 at%Si alloy. The mainﬁndings obtained in this work are summarized below.(1) SAXS proﬁles showed that nanometer-size precipitateswere formed during isothermal aging at 720K. Theprecipitates were coarsened in nearly a single modalsize distribution with increasing aging time in thesamples without cold rolling before aging. On the otherhand, the coarsening of the precipitates in the cold-rolled alloy samples seemed to occur in a multi-modalsize distribution. This phenomenon is thought to becaused by preferential precipitation on dislocationsformed by cold rolling.(2) XANES and EXAFS spectra showed that nickel wassubstituted in the fcc copper matrix in the solution-treated samples. The XANES spectra also indicated thatthe electronic structure of nickel was changed duringaging, suggesting that nickel interacted with silicon toform precipitates. The EXAFS spectra at the Ni K-edgechanged in the aged alloy samples. This spectral changewould correspond to an aging-induced change in thelocal structure around nickel in the samples, whichsuggests the formation of nickel-rich precipitates.AcknowledgementsThe authors wish to express their thanks to Professor K.Mimura, Professor M. Isshiki and Professor Y. Waseda fortheir valuable discussions. XAFS measurements were per-formed with the permission of the SAGA-LS. This work wassupported by a grant-in-aid for scientiﬁc research from theJapan Society for the Promotion of Science (No. 17310041).REFERENCES1) S. Nagarjuna, M. Srinivas, K. Balasubramanian and D. S. Sarma: Scr.Metall. Mater. 33 (1995) 1455–1460.2) S. Nagarjuna, M. Srinivas, K. Balasubramanian and D. S. Sarma: Acta.Mater. 44 (1996) 2285–2293.3) S. Nagarjuna, M. Srinivas, K. Balasubramanian and D. S. Sarma:Mater. Sci. Eng. A 259 (1999) 34–42.4) S. Nagarjuna, K. Balasubramanian and D. S. Sarma: Mater. Sci. Eng. A225 (2001) 118–124.5) S. Suzuki, K. Hirabayashi, H. Shibata, K. Mimura, M. 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SAXS and XAFS characterization of Precipitates in a High-Performance Cu-Ni-Si Alloy

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