- Email: [email protected]
Surface precipitation of chromium in rapidly solidified Cu–Cr alloys
Applied Surface Science 277 (2013) 83–87
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Surface precipitation of chromium in rapidly solidified Cu–Cr alloys Milan Bizjak a , Blaˇz Karpe a , Gregor Jakˇsa b , Janez Kovaˇc a,b,∗ a b
Faculty of Natural Science and Engineering, University of Ljubljana, Ljubljana, Slovenia Joˇzef Stefan Institute, Jamova 39, Ljubljana, Slovenia
a r t i c l e
i n f o
Article history: Received 20 December 2012 Received in revised form 29 March 2013 Accepted 2 April 2013 Available online 11 April 2013 Keywords: Rapid solidification Cu–Cr alloy Kinetics of precipitation In situ measurements of electrical resistivity X-ray photoelectron spectroscopy
a b s t r a c t Rapidly solidified ribbons of Cu–Cr alloys with 2.27 and 4.20 at.% of chromium were produced using the melt-spinning method. Alloys were analyzed by electron microscopy for complete solubility of Cr in copper matrix. To avoid disturbing effects of Cr phase particles, the kinetics and the sequence of microstructural transformations during heating were analyzed only the sample with 2.27 at.% of chromium with complete Cr solubility in the copper matrix. We then investigated the precipitation process for this alloy that was subsequently heated at a constant rate. The increased solid solubility obtained allowed the extensive precipitation of a Cr-rich phase. The kinetics and the sequence of microstructural changes that occurred during the heating were analyzed using an in situ measurement of the electrical resistance. The quenched microstructure was analyzed at transition points using scanning and transmission electron microscopy. X-ray photoelectron spectroscopy, as a very surface-sensitive method, was applied to study the changes in the chemical composition of the surface for the Cu–Cr alloy ribbons in the temperature range 400–700 ◦ C during an in situ heat treatment in an ultra-high vacuum. The results show a relatively rapid precipitation of chromium to the surface, which starts at 400 ◦ C and is correlated with a change in the microstructure and the electrical resistance. The Cr-precipitation is faster at higher temperatures and follows the parabolic law. The resistivity results for the supersaturated binary alloy were analyzed using the Ozawa method to give an activation energy for the precipitation of 196 ± 10 kJ mol−1 . © 2013 Elsevier B.V. All rights reserved.
1. Introduction Copper alloys containing chromium are technologically important materials due to their good electrical conductivity and their hardness [1]. Alloys with a good electrical conductivity produced by a conventional metallurgical process contain less than 1 at.% of Cr. The maximum solubility of Cr in the Cu matrix is 0.8 at.% at the eutectic temperature of 1076 ◦ C [2]. From the binary phase diagram of Cu–Cr it follows that the solubility is reduced to 0.008 at.% Cr at 600 ◦ C. Precipitation hardening (solution treatment, quenching and aging) leads to a high level of strength and a good electrical conductivity, which can amount to more than 80% of the electrical conductivity of pure copper. With the rapid-solidification technique a higher solubility of the alloyed elements in solid solutions can be achieved, which is especially important for the production of copper-based alloys with elements that have a low solubility in the equilibrium state. Some previous studies showed that rapid solidification of a solid solution of Cu–Cr alloys led to supersaturated
compounds with up to 5 at.% of Cr [2,3]. However, after rapid solidification the microstructure is unstable and is transformed to a more stable state during the heat treatment. The properties of the final product depend on the evolution of the microstructure during the consolidation of the rapidly solidified product. The goal of the present work was to study the precipitation process in supersaturated Cu–Cr alloys produced by rapid solidification (RS). We describe the main features of the microstructure of the supersaturated Cu–Cr alloys. Measurements of the electrical resistivity were used to investigate the kinetics of the precipitation process in the rapidly solidified Cu–Cr alloys. We paid special attention to the Cr-precipitation phenomena from the supersaturated compound to its surface. For this purpose a surface-sensitive method for the characterization of the surface and the sub-surface compositions, i.e. X-ray photoelectron spectroscopy (XPS), was applied.
2. Experimental ∗ Corresponding author at: Joˇzef Stefan Institute, Jamova 39, Ljubljana, Slovenia. Tel.: +386 1 477 3403; fax: +386 1 477 3440. E-mail address: [email protected] (J. Kovaˇc). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.04.005
Melt spinning was used to produce the rapidly solidified ribbons. The initial Cu–Cr alloys were made by vacuum induction melting of electrolytic copper with a purity of 99.9% and chromium
84
M. Bizjak et al. / Applied Surface Science 277 (2013) 83–87
Fig. 1. SEM micrographs of the cross-section of the slowly cooled ingot of Cu–2.27 at.% Cr alloy (a), of the rapidly solidified ribbons of Cu–2.27 at.% Cr alloy (b) and rapidly solidified ribbons of Cu–4.2 at.% Cr alloy (c).
with a purity of 99.5% in a graphite crucible, followed by casting into a steel mold. The alloy ingots were then placed in a graphite crucible and inductively heated above the melting temperature of the alloy. The melt was ejected through the nozzle of the crucible with an orifice diameter of 0.9 mm onto the rotating wheel at a speed of about 25 m/s in an Ar atmosphere. The thickness of the rapidly solidified Cu–Cr ribbons was 30–50 m and each ribbon was about 2.5 mm wide. Two different Cu–Cr alloys with compositions of Cu–2.27 at.% Cr and Cu–4.20 at.% Cr were prepared using this method. In order to follow the changes in the RS microstructure an in situ measurement of the electrical resistivity was applied during the heating of the alloy [4]. The four-point method was used, with the contacts and wires made from platinum. The temperature and the electrical resistivity were measured simultaneously on 12-cm-long segments of the RS ribbons during heating in a tube furnace, which was purged with argon. After the analyses of the phase-transition temperatures we characterized the ribbons with three techniques: scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffractometry (XRD). The surface composition of selected samples was analyzed during different annealing treatments by X-ray photoelectron spectroscopy (XPS) in a PHI-TFA XPS spectrometer produced by Physical Electronics, Inc. Surface-sensitive methods like Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy are often used to follow the evolution of the surface composition in studies of surface-segregation phenomena [5,6]. In our case the samples were mounted on a Mo sample holder, which was annealed by indirect resistive heating. The XPS spectra were excited by X-ray radiation from an Al-standard source. The base pressure in the spectrometer was 2 × 10−8 Pa. The analyzed area was 0.4 mm in diameter and the analyzed depth was about 3–5 nm. The peaks of Cu 2p, Cr 2p, O 1s and C 1s were present in the measured XPS spectra. Relative sensitivity factors, provided by the instrument producer, were used to calculate the surface concentrations [7]. Prior to the heating the surfaces of the samples were cleaned by ion sputtering in order to remove the surface contamination and the adsorbed species. The samples of solidified ribbons were heated in two ways: in the first way the samples were heated to 700 ◦ C at a constant heating rate of ∼10 K/min and in the second way the samples were isothermally heated at temperatures of 400 ◦ C and 600 ◦ C for 60–100 min. A similar investigation of Cu segregation in Ag–Cu samples was carried out by some of the authors [6]. After the heat treatments the samples were cooled and XPS depthprofile analyses were performed to follow the subsurface in-depth distribution of the elements. For this purpose ion sputtering was performed with a 1 keV Ar+ beam rastered over a 4 mm × 4 mm area. For these parameters a sputtering rate of about 1.5 nm/min was measured on a Ni/Cr reference multilayer structure. We estimate that the relative uncertainty for the calculated concentrations is about 20%.
3. Results and discussion In the first step a slowly cooled alloy of composition Cu–2.27 at.% Cr was metallographically examined and compared to the rapidly solidified alloy. The alloy obtained by slow cooling was later used for the production of rapidly solidified ribbons. Fig. 1a shows the microstructure of the Cu–2.27 at.% Cr ingot. The microstructure of the alloy consists of chromium spheroids, 1–5 m in diameter, surrounded by a copper matrix as well as a fine eutectic. The crystal grains are relatively large. After the process of rapid solidification a fine-grained structure is obtained. The typical microstructure of a transverse cross-section of a ribbon is shown in Fig. 1b. For the Cu–2.27 at.% Cr alloy, the microstructure consists of a zone with columnar grains extending from the wheel surface to the free surface of the ribbon, with a very narrow, equiaxed zone near the wheel surface (Fig. 1b). The microstructure of the rapid solidified alloy showed no signs of precipitation when examined in an electron microscope. The rapidly solidified ribbons with a higher Cr concentration (Cu–4.20 at.% Cr alloy) consist of a two zones. The zone with the columnar grains is on the wheel side and the zone with globular grains extends to the upper free surface. Some particles enriched with Cr were found in this case at boundaries as well as inside the crystal grains. The density of such particles is the highest for the globular zone (Fig. 1c). Fig. 2 shows a part of the XRD spectrum (1 1 1) from the rapidly solidified Cu–2.27 at.% Cr alloy. For comparison the XRD spectra from slowly (Cu) and rapidly solidified (RS Cu) pure electrolytic copper samples are added. From Fig. 2 it follows that the diffraction peak (1 1 1) from the supersaturated alloy is shifted toward a lower diffraction angle, which is related to the larger distance between the crystalline planes. The lattice parameter of the rapidly solidified Cu–Cr alloy is larger than that of the rapidly
Fig. 2. A part of the XRD spectrum (1 1 1) from the rapidly solidified Cu alloy with 2.27 at.% Cr and pure electrolytic copper, before and after rapid solidification.
M. Bizjak et al. / Applied Surface Science 277 (2013) 83–87
85
Concentration (at.%)
100
Cu Cr
80 60 40 20 0
0
200
400
600
800
Temperature (o C)
Fig. 3. Electrical resistivity and the first temperature derivative of resistance vs. temperature for the rapidly solidified Cu–Cr ribbon alloy with 2.27 at.% Cr heated at a rate of 3 K/min.
Fig. 4. Electrical first temperature derivative for the rapidly solidified Cu–Cr ribbon alloy with 2.27 at.% Cr at different heating rates.
solidified pure electrolytic copper. This parameter decreases with the precipitation of chromium from the supersaturated Cu–Cr solid solution. The calculated value for the chromium in the solid solution using the equation [Cr] = a/0.00026 (a is a difference between the lattice parameters of the rapidly solidified alloy and the reference sample from pure copper) confirms that the rapid solidification of the Cu–2.27 at.% Cr alloy occurred without segregation [8]. For subsequent investigation of the precipitation kinetics and microstructural changes in supersaturated solid solution during heating, we have selected an alloy with 2.27 at.% Cr, where no
Fig. 6. Change of the surface composition for a ribbon of rapidly solidified Cu–2.27 at.% Cr alloy during annealing from room temperature to 700 ◦ C at a constant heating rate of ∼10 K/min.
evidence of Cr precipitation in the as cast state was found in order to avoid disturbing effects of Cr phase particles. After the rapid solidification the Cu–Cr ribbons were heat treated. The Cu–Cr alloy first undergoes a precipitation process due to its supersaturation. This phenomenon is proved by the change in the electrical resistivity. Characteristic results of the electrical resistivity measured during the heating of the rapidly solidified Cu–Cr ribbons of the Cu–2.27 at.% Cr alloy are given in Fig. 3. At lower temperatures the resistivity increases linearly with the increasing temperature because of the linear temperature coefficient of the resistivity. At higher temperatures the change in the resistivity becomes nonlinear and yields more information about the precipitates. The growth of the precipitates reduces the contents of the solute atoms in the matrix and results in a continuous decrease in the electrical resistivity. The measured curve clearly shows the temperature interval that is related to a change in the electrical resistivity, which is further related to the precipitation of dissolved atoms from a supersaturated solid solution. This can be seen more precisely in the curve of the first temperature derivative of the electrical resistivity. The minima of the first-derivative curves show a broad temperature interval for the microstructural changes. The temperature expected for the beginning of the solid-solution decay is 310 ◦ C. The decay takes a long time and it occurs at around 450 ◦ C. The most intensive precipitation process occurs at Tm , the minimum of the first-derivative curve. These minima are shifted to higher temperatures by increasing the heating rate (Fig. 4) from 3 to 15 K/min. The shift is a consequence of the shorter time available for the reaction inside the temperature interval. The shifts were used for the calculation of the activation energies according to Ozawa’s method [9]. From the shift of Tm the activation energy for the precipitation was calculated and found to be 196 ± 10 kJ mol−1 . The activation energy for the precipitation
Fig. 5. Microstructure of rapidly solidified Cu alloy with 2.27 at.% Cr. TEM micrograph of the precipitates in the grains and at the grain boundaries from the Cu-matrix after heating to 400 ◦ C (a), SEM micrographs of the precipitates at the grain boundaries after heating to 900 ◦ C (b) and the Cr-depleted region in the subsurface area of the ribbon after heating (c).
86
M. Bizjak et al. / Applied Surface Science 277 (2013) 83–87 40
400 oC
90 80
Cu
400 C
70
Surface Cr Concentration (at.%)
Surface Concentration (at.%)
100
60 50 40
Cr
30 20
a
10 0 0
10
20
30
40
50
60
70
80
90
100
35
o
600 C
30 25 20
o
400 C
15 10
b
5 0 0,0
2,0
Annealing Time (min)
4,0
6,0
8,0
10,0
Annealing Time (min1/2 )
Fig. 7. Change of the surface composition for a ribbon of rapidly solidified Cu–2.27 at.% Cr alloy during isothermal annealing at 400 ◦ C (a) and the chromium surface concentration as a function of the square root of time during heating at 400 ◦ C and 600 ◦ C (b).
100 90
Concentration (at.%)
occurring in this temperature interval is in good agreement with the literature data for a 0.6% Cr alloy [10,11]. At the beginning of the microstructural changes in the crystalline grain the phase rich in chromium was precipitated. The changes that occurred during heating to 760 ◦ C are related to the diffusion of chromium toward the grain boundaries and with the growth of particles rich in chromium. After the initial growth the amount of precipitated phase approached a volume that is in accordance with the equilibrium phase diagram at 760 ◦ C. Further heat treatment yielded a continuous growth of particles, but not due to the Cr precipitation from the saturated solid solution. The reason is related to the growth of smaller particles into larger ones. Fig. 5 shows the changes in the microstructure on the cross-section of the ribbon of the rapidly solidified Cu–2.27 at.% Cr alloy after heating at different temperatures. In addition to the bulk-related microstructural changes we were also interested in the surface phenomena on the supersaturated Cu–Cr alloy. Therefore, we characterized the surface of the rapidly solidified Cu–Cr ribbons with the surface-sensitive method XPS. Fig. 6 shows the time evolution of the surface concentrations of the elements Cu and Cr obtained with the XPS method during the annealing of the rapidly solidified Cu–Cr ribbon of the Cu–2.27 at.% Cr alloy from room temperature to 700 ◦ C at a constant heating rate of ∼10 K/min. Only the Cr and Cu concentrations are shown, since other elements like C and O are less relevant and were present in very low concentrations. The Cr surface concentration remains low and nearly constant up to a temperature of 400 ◦ C. After this temperature the surface concentration of the Cr starts to increase and at 600 ◦ C it reaches a constant value. The value of the Cr surface concentration is about 40 at.% and it is much higher than the nominal bulk Cr concentration. This is clear evidence for Cr precipitation to the surface of the supersaturated alloy. The kinetics of the chromium surface enrichment was followed in other measurements where rapidly solidified ribbons were isothermally annealed at different temperatures. Fig. 7a shows the change in the surface concentration during isothermal annealing of the rapidly solidified Cu–Cr ribbon at 400 ◦ C. At this temperature the Cr precipitates toward the surface, which is consequently enriched with Cr and depleted with Cu. In order to compare the kinetics of the Cr surface precipitation we plotted, in Fig. 7b, the time evolution of the Cr-enrichment as a function of the square root of annealing time at two different temperatures. From Fig. 6b it is clear that the Cr-concentration is faster at higher temperatures and that it follows the parabolic low. After the annealing treatment of the rapidly solidified ribbon we performed XPS depth profile analyses to determine the depth distribution of the elements in the subsurface region. Fig. 8 shows the XPS depth profile of the concentrations of the elements Cu and
Cu
80 70 60 50 40 30 20
Cr
10 0
0
5
10
15
Sputtering time (min)
20
25
Fig. 8. XPS depth profile of the concentration of elements measured by the XPS method on a ribbon of the rapidly solidified Cu–2.27 at.% Cr alloy after annealing at 600 ◦ C.
Cr after the annealing of the rapidly solidified Cu–Cr ribbon with Cu–2.27 at.% Cr at 600 ◦ C. The surface region of the annealed sample is enriched with Cr. From the depth profile in Fig. 7 we can estimate that the Cr-enriched layer has a thickness of about 5 nm, taking into account a sputtering rate of about 1.5 nm/min. This confirms that the Cr precipitated at the surface. The Cr-depleted region is expected to be formed beneath the Cr-enriched surface layer due to the migration of the solute from the supersaturated matrix. This is in fact confirmed by the result of a metallographic analysis, which showed a Cr-depleted (precipitation-free) region beneath the sample surface (Fig. 5c). 4. Conclusions The results of the analysis of the kinetics of the microstructural changes in the rapidly solidified Cu–Cr alloys can be summarized as follows: 1. In the rapidly solidified ribbons of the Cu–Cr alloy with 2.27 at.% Cr with a thickness of 35 m we obtained supersaturation over the whole thickness of the ribbon. For the other alloy with the higher Cr concentration (4.20 at.% of Cr) the precipitation of Crenriched particles occurred. These particles are located in the grains as well as at the grain boundaries. The number of such particles is the largest for the globular zone, which is present on the top free surface of the ribbons. 2. In situ measurements of the electrical resistivity with the fourpoint method are appropriate for following the changes in the microstructure. The dependence of the electrical resistivity on
M. Bizjak et al. / Applied Surface Science 277 (2013) 83–87
3. 4.
5.
6.
the heating time and temperature shows that the microstructure changed in the interval between 310 and 760 ◦ C. In this temperature interval the precipitation of chromium occurred from the supersaturated solid solution ˛Cu . The activation energy for precipitation was determined to be 196 ± 10 kJ mol−1 . The decay of the supersaturated Cu–Cr alloy is relatively slow. During annealing in the temperature interval 400 ◦ C to 700 ◦ C the surface concentration of Cr increases, indicating a strong Cr surface precipitation. At the same time the Cu at the surface is depleted. Our results show that the Cr-enriched surface layer has a thickness of about 5 nm and beneath this layer a Cr-depleted region is found. The Cr-related precipitation is related to the observed change in the surface resistivity and a change in the microstructure. The rate of Cr-surface precipitation increases with the annealing temperature during the isothermal annealing. The Crprecipitation kinetics follows the parabolic law. We suppose that the precipitation mechanism for the chromium is a migration of excess Cr-atoms from the supersaturated matrix toward the grain boundaries or to the free surface, and in this way the surface or grain-boundary energies are lowered.
87
Acknowledgments This work was supported by Slovenian Research Agency (ARRS), Project J2-4287 and Research Program P2-0082. The authors wish to thank Tatjana Filipiˇc for her assistance with the measurements. References [1] J.B. Correia, H.A. Davies, C.M. Sellars, J. Sainz, I. Gutierrz, F. Castro, M. Fuentes, J. Shohoji, H. Carvalhinhos, Materials Science and Engineering A 133 (1991) 265. [2] A. Bell, H.A. Davies, Materials Science and Engineering A 226–228 (1997) 1039. [3] M.A. Morris, D.G. Morris, Acta Metallurgica 35 (1987) 2511. [4] M. Bizjak, L. Kosec, A.C. Kneissl, B. Kosec, International Journal of Materials Research 99 (2008) 101. [5] P. Lejˇcek, S. Hofmann, Critical Reviews in Solid State and Materials Sciences 33 (2008) 133. [6] J. Vaníˇcková, J. Kovaˇc, J. Ded, P. Lejˇcek, Z. Samardˇzija, A. Zalar, International Journal of Materials Research 100 (3) (2009) 311. [7] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-Ray Photoelectron Spectroscopy, Physical Electronics Inc., Eden Prairie, MN, USA, 1995. [8] J.B. Correia, H.A. Davies, C.M. Sellars, Acta Materialia 45 (1997) 177. [9] T. Ozawa, Bulletin of the Chemical Society of Japan 38 (1965) 1881. [10] R.W. Knights, P. Wilkes, Metallurgical Transactions 4 (1973) 2398. [11] J. Szablewski, B. Kuˇznicka, Physica Status Solidi A 108 (K5) (1988).
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Surface precipitation of chromium in rapidly solidified Cu–Cr alloys Milan Bizjak a , Blaˇz Karpe a , Gregor Jakˇsa b , Janez Kovaˇc a,b,∗ a b
Faculty of Natural Science and Engineering, University of Ljubljana, Ljubljana, Slovenia Joˇzef Stefan Institute, Jamova 39, Ljubljana, Slovenia
a r t i c l e
i n f o
Article history: Received 20 December 2012 Received in revised form 29 March 2013 Accepted 2 April 2013 Available online 11 April 2013 Keywords: Rapid solidification Cu–Cr alloy Kinetics of precipitation In situ measurements of electrical resistivity X-ray photoelectron spectroscopy
a b s t r a c t Rapidly solidified ribbons of Cu–Cr alloys with 2.27 and 4.20 at.% of chromium were produced using the melt-spinning method. Alloys were analyzed by electron microscopy for complete solubility of Cr in copper matrix. To avoid disturbing effects of Cr phase particles, the kinetics and the sequence of microstructural transformations during heating were analyzed only the sample with 2.27 at.% of chromium with complete Cr solubility in the copper matrix. We then investigated the precipitation process for this alloy that was subsequently heated at a constant rate. The increased solid solubility obtained allowed the extensive precipitation of a Cr-rich phase. The kinetics and the sequence of microstructural changes that occurred during the heating were analyzed using an in situ measurement of the electrical resistance. The quenched microstructure was analyzed at transition points using scanning and transmission electron microscopy. X-ray photoelectron spectroscopy, as a very surface-sensitive method, was applied to study the changes in the chemical composition of the surface for the Cu–Cr alloy ribbons in the temperature range 400–700 ◦ C during an in situ heat treatment in an ultra-high vacuum. The results show a relatively rapid precipitation of chromium to the surface, which starts at 400 ◦ C and is correlated with a change in the microstructure and the electrical resistance. The Cr-precipitation is faster at higher temperatures and follows the parabolic law. The resistivity results for the supersaturated binary alloy were analyzed using the Ozawa method to give an activation energy for the precipitation of 196 ± 10 kJ mol−1 . © 2013 Elsevier B.V. All rights reserved.
1. Introduction Copper alloys containing chromium are technologically important materials due to their good electrical conductivity and their hardness [1]. Alloys with a good electrical conductivity produced by a conventional metallurgical process contain less than 1 at.% of Cr. The maximum solubility of Cr in the Cu matrix is 0.8 at.% at the eutectic temperature of 1076 ◦ C [2]. From the binary phase diagram of Cu–Cr it follows that the solubility is reduced to 0.008 at.% Cr at 600 ◦ C. Precipitation hardening (solution treatment, quenching and aging) leads to a high level of strength and a good electrical conductivity, which can amount to more than 80% of the electrical conductivity of pure copper. With the rapid-solidification technique a higher solubility of the alloyed elements in solid solutions can be achieved, which is especially important for the production of copper-based alloys with elements that have a low solubility in the equilibrium state. Some previous studies showed that rapid solidification of a solid solution of Cu–Cr alloys led to supersaturated
compounds with up to 5 at.% of Cr [2,3]. However, after rapid solidification the microstructure is unstable and is transformed to a more stable state during the heat treatment. The properties of the final product depend on the evolution of the microstructure during the consolidation of the rapidly solidified product. The goal of the present work was to study the precipitation process in supersaturated Cu–Cr alloys produced by rapid solidification (RS). We describe the main features of the microstructure of the supersaturated Cu–Cr alloys. Measurements of the electrical resistivity were used to investigate the kinetics of the precipitation process in the rapidly solidified Cu–Cr alloys. We paid special attention to the Cr-precipitation phenomena from the supersaturated compound to its surface. For this purpose a surface-sensitive method for the characterization of the surface and the sub-surface compositions, i.e. X-ray photoelectron spectroscopy (XPS), was applied.
2. Experimental ∗ Corresponding author at: Joˇzef Stefan Institute, Jamova 39, Ljubljana, Slovenia. Tel.: +386 1 477 3403; fax: +386 1 477 3440. E-mail address: [email protected] (J. Kovaˇc). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.04.005
Melt spinning was used to produce the rapidly solidified ribbons. The initial Cu–Cr alloys were made by vacuum induction melting of electrolytic copper with a purity of 99.9% and chromium
84
M. Bizjak et al. / Applied Surface Science 277 (2013) 83–87
Fig. 1. SEM micrographs of the cross-section of the slowly cooled ingot of Cu–2.27 at.% Cr alloy (a), of the rapidly solidified ribbons of Cu–2.27 at.% Cr alloy (b) and rapidly solidified ribbons of Cu–4.2 at.% Cr alloy (c).
with a purity of 99.5% in a graphite crucible, followed by casting into a steel mold. The alloy ingots were then placed in a graphite crucible and inductively heated above the melting temperature of the alloy. The melt was ejected through the nozzle of the crucible with an orifice diameter of 0.9 mm onto the rotating wheel at a speed of about 25 m/s in an Ar atmosphere. The thickness of the rapidly solidified Cu–Cr ribbons was 30–50 m and each ribbon was about 2.5 mm wide. Two different Cu–Cr alloys with compositions of Cu–2.27 at.% Cr and Cu–4.20 at.% Cr were prepared using this method. In order to follow the changes in the RS microstructure an in situ measurement of the electrical resistivity was applied during the heating of the alloy [4]. The four-point method was used, with the contacts and wires made from platinum. The temperature and the electrical resistivity were measured simultaneously on 12-cm-long segments of the RS ribbons during heating in a tube furnace, which was purged with argon. After the analyses of the phase-transition temperatures we characterized the ribbons with three techniques: scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffractometry (XRD). The surface composition of selected samples was analyzed during different annealing treatments by X-ray photoelectron spectroscopy (XPS) in a PHI-TFA XPS spectrometer produced by Physical Electronics, Inc. Surface-sensitive methods like Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy are often used to follow the evolution of the surface composition in studies of surface-segregation phenomena [5,6]. In our case the samples were mounted on a Mo sample holder, which was annealed by indirect resistive heating. The XPS spectra were excited by X-ray radiation from an Al-standard source. The base pressure in the spectrometer was 2 × 10−8 Pa. The analyzed area was 0.4 mm in diameter and the analyzed depth was about 3–5 nm. The peaks of Cu 2p, Cr 2p, O 1s and C 1s were present in the measured XPS spectra. Relative sensitivity factors, provided by the instrument producer, were used to calculate the surface concentrations [7]. Prior to the heating the surfaces of the samples were cleaned by ion sputtering in order to remove the surface contamination and the adsorbed species. The samples of solidified ribbons were heated in two ways: in the first way the samples were heated to 700 ◦ C at a constant heating rate of ∼10 K/min and in the second way the samples were isothermally heated at temperatures of 400 ◦ C and 600 ◦ C for 60–100 min. A similar investigation of Cu segregation in Ag–Cu samples was carried out by some of the authors [6]. After the heat treatments the samples were cooled and XPS depthprofile analyses were performed to follow the subsurface in-depth distribution of the elements. For this purpose ion sputtering was performed with a 1 keV Ar+ beam rastered over a 4 mm × 4 mm area. For these parameters a sputtering rate of about 1.5 nm/min was measured on a Ni/Cr reference multilayer structure. We estimate that the relative uncertainty for the calculated concentrations is about 20%.
3. Results and discussion In the first step a slowly cooled alloy of composition Cu–2.27 at.% Cr was metallographically examined and compared to the rapidly solidified alloy. The alloy obtained by slow cooling was later used for the production of rapidly solidified ribbons. Fig. 1a shows the microstructure of the Cu–2.27 at.% Cr ingot. The microstructure of the alloy consists of chromium spheroids, 1–5 m in diameter, surrounded by a copper matrix as well as a fine eutectic. The crystal grains are relatively large. After the process of rapid solidification a fine-grained structure is obtained. The typical microstructure of a transverse cross-section of a ribbon is shown in Fig. 1b. For the Cu–2.27 at.% Cr alloy, the microstructure consists of a zone with columnar grains extending from the wheel surface to the free surface of the ribbon, with a very narrow, equiaxed zone near the wheel surface (Fig. 1b). The microstructure of the rapid solidified alloy showed no signs of precipitation when examined in an electron microscope. The rapidly solidified ribbons with a higher Cr concentration (Cu–4.20 at.% Cr alloy) consist of a two zones. The zone with the columnar grains is on the wheel side and the zone with globular grains extends to the upper free surface. Some particles enriched with Cr were found in this case at boundaries as well as inside the crystal grains. The density of such particles is the highest for the globular zone (Fig. 1c). Fig. 2 shows a part of the XRD spectrum (1 1 1) from the rapidly solidified Cu–2.27 at.% Cr alloy. For comparison the XRD spectra from slowly (Cu) and rapidly solidified (RS Cu) pure electrolytic copper samples are added. From Fig. 2 it follows that the diffraction peak (1 1 1) from the supersaturated alloy is shifted toward a lower diffraction angle, which is related to the larger distance between the crystalline planes. The lattice parameter of the rapidly solidified Cu–Cr alloy is larger than that of the rapidly
Fig. 2. A part of the XRD spectrum (1 1 1) from the rapidly solidified Cu alloy with 2.27 at.% Cr and pure electrolytic copper, before and after rapid solidification.
M. Bizjak et al. / Applied Surface Science 277 (2013) 83–87
85
Concentration (at.%)
100
Cu Cr
80 60 40 20 0
0
200
400
600
800
Temperature (o C)
Fig. 3. Electrical resistivity and the first temperature derivative of resistance vs. temperature for the rapidly solidified Cu–Cr ribbon alloy with 2.27 at.% Cr heated at a rate of 3 K/min.
Fig. 4. Electrical first temperature derivative for the rapidly solidified Cu–Cr ribbon alloy with 2.27 at.% Cr at different heating rates.
solidified pure electrolytic copper. This parameter decreases with the precipitation of chromium from the supersaturated Cu–Cr solid solution. The calculated value for the chromium in the solid solution using the equation [Cr] = a/0.00026 (a is a difference between the lattice parameters of the rapidly solidified alloy and the reference sample from pure copper) confirms that the rapid solidification of the Cu–2.27 at.% Cr alloy occurred without segregation [8]. For subsequent investigation of the precipitation kinetics and microstructural changes in supersaturated solid solution during heating, we have selected an alloy with 2.27 at.% Cr, where no
Fig. 6. Change of the surface composition for a ribbon of rapidly solidified Cu–2.27 at.% Cr alloy during annealing from room temperature to 700 ◦ C at a constant heating rate of ∼10 K/min.
evidence of Cr precipitation in the as cast state was found in order to avoid disturbing effects of Cr phase particles. After the rapid solidification the Cu–Cr ribbons were heat treated. The Cu–Cr alloy first undergoes a precipitation process due to its supersaturation. This phenomenon is proved by the change in the electrical resistivity. Characteristic results of the electrical resistivity measured during the heating of the rapidly solidified Cu–Cr ribbons of the Cu–2.27 at.% Cr alloy are given in Fig. 3. At lower temperatures the resistivity increases linearly with the increasing temperature because of the linear temperature coefficient of the resistivity. At higher temperatures the change in the resistivity becomes nonlinear and yields more information about the precipitates. The growth of the precipitates reduces the contents of the solute atoms in the matrix and results in a continuous decrease in the electrical resistivity. The measured curve clearly shows the temperature interval that is related to a change in the electrical resistivity, which is further related to the precipitation of dissolved atoms from a supersaturated solid solution. This can be seen more precisely in the curve of the first temperature derivative of the electrical resistivity. The minima of the first-derivative curves show a broad temperature interval for the microstructural changes. The temperature expected for the beginning of the solid-solution decay is 310 ◦ C. The decay takes a long time and it occurs at around 450 ◦ C. The most intensive precipitation process occurs at Tm , the minimum of the first-derivative curve. These minima are shifted to higher temperatures by increasing the heating rate (Fig. 4) from 3 to 15 K/min. The shift is a consequence of the shorter time available for the reaction inside the temperature interval. The shifts were used for the calculation of the activation energies according to Ozawa’s method [9]. From the shift of Tm the activation energy for the precipitation was calculated and found to be 196 ± 10 kJ mol−1 . The activation energy for the precipitation
Fig. 5. Microstructure of rapidly solidified Cu alloy with 2.27 at.% Cr. TEM micrograph of the precipitates in the grains and at the grain boundaries from the Cu-matrix after heating to 400 ◦ C (a), SEM micrographs of the precipitates at the grain boundaries after heating to 900 ◦ C (b) and the Cr-depleted region in the subsurface area of the ribbon after heating (c).
86
M. Bizjak et al. / Applied Surface Science 277 (2013) 83–87 40
400 oC
90 80
Cu
400 C
70
Surface Cr Concentration (at.%)
Surface Concentration (at.%)
100
60 50 40
Cr
30 20
a
10 0 0
10
20
30
40
50
60
70
80
90
100
35
o
600 C
30 25 20
o
400 C
15 10
b
5 0 0,0
2,0
Annealing Time (min)
4,0
6,0
8,0
10,0
Annealing Time (min1/2 )
Fig. 7. Change of the surface composition for a ribbon of rapidly solidified Cu–2.27 at.% Cr alloy during isothermal annealing at 400 ◦ C (a) and the chromium surface concentration as a function of the square root of time during heating at 400 ◦ C and 600 ◦ C (b).
100 90
Concentration (at.%)
occurring in this temperature interval is in good agreement with the literature data for a 0.6% Cr alloy [10,11]. At the beginning of the microstructural changes in the crystalline grain the phase rich in chromium was precipitated. The changes that occurred during heating to 760 ◦ C are related to the diffusion of chromium toward the grain boundaries and with the growth of particles rich in chromium. After the initial growth the amount of precipitated phase approached a volume that is in accordance with the equilibrium phase diagram at 760 ◦ C. Further heat treatment yielded a continuous growth of particles, but not due to the Cr precipitation from the saturated solid solution. The reason is related to the growth of smaller particles into larger ones. Fig. 5 shows the changes in the microstructure on the cross-section of the ribbon of the rapidly solidified Cu–2.27 at.% Cr alloy after heating at different temperatures. In addition to the bulk-related microstructural changes we were also interested in the surface phenomena on the supersaturated Cu–Cr alloy. Therefore, we characterized the surface of the rapidly solidified Cu–Cr ribbons with the surface-sensitive method XPS. Fig. 6 shows the time evolution of the surface concentrations of the elements Cu and Cr obtained with the XPS method during the annealing of the rapidly solidified Cu–Cr ribbon of the Cu–2.27 at.% Cr alloy from room temperature to 700 ◦ C at a constant heating rate of ∼10 K/min. Only the Cr and Cu concentrations are shown, since other elements like C and O are less relevant and were present in very low concentrations. The Cr surface concentration remains low and nearly constant up to a temperature of 400 ◦ C. After this temperature the surface concentration of the Cr starts to increase and at 600 ◦ C it reaches a constant value. The value of the Cr surface concentration is about 40 at.% and it is much higher than the nominal bulk Cr concentration. This is clear evidence for Cr precipitation to the surface of the supersaturated alloy. The kinetics of the chromium surface enrichment was followed in other measurements where rapidly solidified ribbons were isothermally annealed at different temperatures. Fig. 7a shows the change in the surface concentration during isothermal annealing of the rapidly solidified Cu–Cr ribbon at 400 ◦ C. At this temperature the Cr precipitates toward the surface, which is consequently enriched with Cr and depleted with Cu. In order to compare the kinetics of the Cr surface precipitation we plotted, in Fig. 7b, the time evolution of the Cr-enrichment as a function of the square root of annealing time at two different temperatures. From Fig. 6b it is clear that the Cr-concentration is faster at higher temperatures and that it follows the parabolic low. After the annealing treatment of the rapidly solidified ribbon we performed XPS depth profile analyses to determine the depth distribution of the elements in the subsurface region. Fig. 8 shows the XPS depth profile of the concentrations of the elements Cu and
Cu
80 70 60 50 40 30 20
Cr
10 0
0
5
10
15
Sputtering time (min)
20
25
Fig. 8. XPS depth profile of the concentration of elements measured by the XPS method on a ribbon of the rapidly solidified Cu–2.27 at.% Cr alloy after annealing at 600 ◦ C.
Cr after the annealing of the rapidly solidified Cu–Cr ribbon with Cu–2.27 at.% Cr at 600 ◦ C. The surface region of the annealed sample is enriched with Cr. From the depth profile in Fig. 7 we can estimate that the Cr-enriched layer has a thickness of about 5 nm, taking into account a sputtering rate of about 1.5 nm/min. This confirms that the Cr precipitated at the surface. The Cr-depleted region is expected to be formed beneath the Cr-enriched surface layer due to the migration of the solute from the supersaturated matrix. This is in fact confirmed by the result of a metallographic analysis, which showed a Cr-depleted (precipitation-free) region beneath the sample surface (Fig. 5c). 4. Conclusions The results of the analysis of the kinetics of the microstructural changes in the rapidly solidified Cu–Cr alloys can be summarized as follows: 1. In the rapidly solidified ribbons of the Cu–Cr alloy with 2.27 at.% Cr with a thickness of 35 m we obtained supersaturation over the whole thickness of the ribbon. For the other alloy with the higher Cr concentration (4.20 at.% of Cr) the precipitation of Crenriched particles occurred. These particles are located in the grains as well as at the grain boundaries. The number of such particles is the largest for the globular zone, which is present on the top free surface of the ribbons. 2. In situ measurements of the electrical resistivity with the fourpoint method are appropriate for following the changes in the microstructure. The dependence of the electrical resistivity on
M. Bizjak et al. / Applied Surface Science 277 (2013) 83–87
3. 4.
5.
6.
the heating time and temperature shows that the microstructure changed in the interval between 310 and 760 ◦ C. In this temperature interval the precipitation of chromium occurred from the supersaturated solid solution ˛Cu . The activation energy for precipitation was determined to be 196 ± 10 kJ mol−1 . The decay of the supersaturated Cu–Cr alloy is relatively slow. During annealing in the temperature interval 400 ◦ C to 700 ◦ C the surface concentration of Cr increases, indicating a strong Cr surface precipitation. At the same time the Cu at the surface is depleted. Our results show that the Cr-enriched surface layer has a thickness of about 5 nm and beneath this layer a Cr-depleted region is found. The Cr-related precipitation is related to the observed change in the surface resistivity and a change in the microstructure. The rate of Cr-surface precipitation increases with the annealing temperature during the isothermal annealing. The Crprecipitation kinetics follows the parabolic law. We suppose that the precipitation mechanism for the chromium is a migration of excess Cr-atoms from the supersaturated matrix toward the grain boundaries or to the free surface, and in this way the surface or grain-boundary energies are lowered.
87
Acknowledgments This work was supported by Slovenian Research Agency (ARRS), Project J2-4287 and Research Program P2-0082. The authors wish to thank Tatjana Filipiˇc for her assistance with the measurements. References [1] J.B. Correia, H.A. Davies, C.M. Sellars, J. Sainz, I. Gutierrz, F. Castro, M. Fuentes, J. Shohoji, H. Carvalhinhos, Materials Science and Engineering A 133 (1991) 265. [2] A. Bell, H.A. Davies, Materials Science and Engineering A 226–228 (1997) 1039. [3] M.A. Morris, D.G. Morris, Acta Metallurgica 35 (1987) 2511. [4] M. Bizjak, L. Kosec, A.C. Kneissl, B. Kosec, International Journal of Materials Research 99 (2008) 101. [5] P. Lejˇcek, S. Hofmann, Critical Reviews in Solid State and Materials Sciences 33 (2008) 133. [6] J. Vaníˇcková, J. Kovaˇc, J. Ded, P. Lejˇcek, Z. Samardˇzija, A. Zalar, International Journal of Materials Research 100 (3) (2009) 311. [7] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-Ray Photoelectron Spectroscopy, Physical Electronics Inc., Eden Prairie, MN, USA, 1995. [8] J.B. Correia, H.A. Davies, C.M. Sellars, Acta Materialia 45 (1997) 177. [9] T. Ozawa, Bulletin of the Chemical Society of Japan 38 (1965) 1881. [10] R.W. Knights, P. Wilkes, Metallurgical Transactions 4 (1973) 2398. [11] J. Szablewski, B. Kuˇznicka, Physica Status Solidi A 108 (K5) (1988).