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Determination of solidus and liquidus in the system Cu–Ni by the spot technique
Journal of Alloys and Compounds 468 (2009) 275–279
Determination of solidus and liquidus in the system Cu–Ni by the spot technique K. Ananthasivan, S. Balakrishnan, I. Kaliappan, S. Anthonysamy ∗ , R. Pankajavalli, P.R. Vasudeva Rao Materials Chemistry Division, Chemistry Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India Received 2 August 2007; received in revised form 19 December 2007; accepted 20 December 2007 Available online 3 January 2008
Abstract The solidus and liquidus in the system Cu–Ni over the entire composition range were determined by using the spot-technique. The solidus temperatures of some nickel rich alloys were also investigated using differential thermal analysis (DTA). The experimental data obtained were shown to be in agreement with the assessed values. The experimental set up used in this study was an improvised version of the apparatus used in earlier studies. It was demonstrated that by using this set up the temperatures of melting transitions could be determined within ±2 K. Temperature calibration was carried out by determining the melting points of pure metals viz., Au, Ni and Cu. The salient features and the utility of the ‘spot-technique’ were also demonstrated. © 2008 Elsevier B.V. All rights reserved. Keywords: Spot technique; Cu–Ni alloys; Solidus; Liquidus; Pyrometry; DTA
1. Introduction Copper nickel alloys are of interest both from a theoretical stand point and practical application. The system Cu–Ni is of relevance to cupronickel alloys. These alloys have potential industrial applications that demand superior mechanical and corrosion resistance properties [1]. The subsolidus equilibria in this system have been studied extensively [1]. The solidus and liquidus in this system have been experimentally determined by Feest and Doherty [2], Schurmann and Shulz [3] and Predel and Mohs [4]. Tarby et al. [5] calculated the Cu–Ni phase diagram with the help of the experimental data published until 1971. Massalski et al. [6] proposed an optimized phase diagram based on the data reported in Ref. [2]. Mey [7] assessed the phase diagram based on the investigations made by Feest and Doherty [2], Schurmann and Schulz [3] and Predel and Mohs [4]. They discarded the experimental values reported in earlier investigations that had larger uncertainties caused by the undercooling of the alloys dur-
∗
Corresponding author. Tel.: +91 44 27480098; fax: +91 44 27480065. E-mail address: [email protected] (S. Anthonysamy).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.12.075
ing measurement. The phase boundaries given in Ref. [2] included only a couple of data points above 1573 K. Further, the solidus and liquidus proposed by Tarby et al. [5] differs substantially from that suggested in Refs. [6,7]. Hence, we undertook to re-determine the solidus and liquidus in the system Cu–Ni. The spot technique – an innovative method developed by Ackermann and Raugh [8] based on optical pyrometry – was used to determine these equilibria. These experiments were carried out in an experimental facility that was recently established in our laboratory for carrying out high temperature investigations on alloys and ceramics [9]. In the conventional spot experiments reported so far [8,10–16], the image of the spot was manually observed. In the studies carried out in our laboratory we recorded the images of the spot by using an image capture system and demonstrated the utility of this technique in identifying steep liquidus boundaries [9,17]. Since the system Cu–Ni is characterized by closely spaced solidus and liquidus boundaries, an investigation of this system would help us to establish the accuracy achievable with this apparatus and technique. The solidus temperatures obtained in this study were also compared with the results obtained by using
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differential thermal analysis (DTA). The results obtained are discussed and the magnitude of the errors are analysed. 2. Experimental 2.1. Starting materials High purity copper (99.99%) was obtained from M/s. Goodfellow Metals, U.K., in the form of granules. Nickel metal granules (99.99%) were procured from M/s. Leco Corporation Inc., USA. Analar grade nitric acid was supplied by M/s. E Merk (India) Ltd., Mumbai, India. Dimethyl glyoxime (AR grade) was procured from M/s. S.D. Finechem Ltd., Mumbai.
2.2. Synthesis of alloys Some of the alloy samples employed in this study were prepared by arc melting the elements in a triarc furnace supplied by M/s. Centorr Associates Inc., USA. Mixtures of Cu and Ni of desired stoichiometry were melted by using a dc arc generated between a nonconsumable, thoriated tungsten cathode and a water cooled, copper anode. The melting was done under argon containing about 4 ppm each of oxygen and moisture (IOLAR-2 grade argon gas, supplied by M/s. Indian Oxygen Limited) at a pressure of 1–10 Pa. To remove the residual oxygen in the arcing chamber, zirconium metal sponge was melted before melting the alloy components. The button was homogenized by flipping and remelting for about six times. In the rest of the experiments the alloys were made in situ during the measurement.
2.3. Principle of the spot technique The basic principle behind the spot technique is described in detail elsewhere [9]. In this technique, a Knudsen cell containing the sample is heated by either RF induction or electron bombardment [8]. Upon melting, the reflectivity as well as emissivity [15] of the sample (solid alloy) changes due to liquefaction. This liquid globule acts as a mirror and reflects the image of the orifice of the Knudsen cell, as a black spot. When the temperature (and composition) of the alloy falls in a two-phase field, where solid and liquid coexist, instead of a complete black spot, several small black spots appear. These small spots coalesce to form a complete spot, once the sample is heated to its liquidus temperature. Recent work carried out in our laboratory [18] showed that the spot technique could be extended to temperatures lower than 1073 K by using an external illumination procedure. This technique is essentially the same as the conventional spot technique, but for the reversal in the direction of illumination.
2.4. Experimental assembly A schematic diagram of the experimental assembly used in the present study is shown in Fig. 1. The experimental assembly consisted of a vacuum chamber, which housed the sample held in a crucible kept inside a Knudsen cell, a sample support assembly, a vacuum system (diffusion pumping system M/s. Balzers GmbH, Switzerland), a radiofrequency (RF) generator, a pyrometer (M78, M/s. Micron, USA) and an image capture system. The image capture system consisted of a long distance microscope (QM1, M/s. Questar, Inc. USA), a charge coupled device (CCD) camera (WAT 207 A, M/s. Watec Co. Ltd., Japan), and a video monitor. The details of construction of this experimental assembly are described in detail elsewhere [9,17].
2.5. Experimental procedure 2.5.1. Spot experiments In a typical “high temperature experiment” about 500 mg of the sample was taken in a cup made out of alumina or graphite. This cup together with the Knudsen cell was degassed prior to the experiment in vacuum for about 2 h by heating at a temperature, which was at least 100 K higher than the desired temperature of investigation. In order to minimize the residual concentration of the gaseous impurities like oxygen and moisture, the vacuum chamber was flushed with helium (containing less than 4 ppm each of moisture and oxygen).
Fig. 1. Schematic showing the experimental system used for the spot experiments: 1, IR two color pyrometer; 2, fixture for pyrometer; 3, optical rail; 4, quartz optical window; 5, bottom flange; 6, vacuum chamber; 7, sample support assembly; 8, molybdenum table; 9 radiation shield; 9a, top radiation shield; 10, RF coil; 11, Knudsen cell; 12, quartz prism; 13, tungsten legs; 14, crucible; 15, sample. Subsequently, it was evacuated to about 1 × 10−3 Pa and the Knudsen cell was heated by RF induction. The image of the Knudsen cell orifice was viewed using the image capture system, while the temperature was measured by focusing the pyrometer on to the black body hole. The temperatures corresponding to the appearance of the broken spot (solidus), as well as the complete spot (liquidus) were recorded. These measurements were made during both the heating and the cooling cycles. A heating rate of 10 K min−1 was employed to reach a temperature that was about 100 K less than the solidus temperature. Subsequently the sample was heated at a rate of 1 K min−1 up to its liquidus temperature. 2.5.2. DTA experiments A differential thermal analyser (Model – Seiko 320, Japan) was used in the DTA experiments. Both the sample temperature and the differential temperature were measured within ±0.5 K by using a Pt–13% Rh/Pt (Type-R) thermocouple. The accuracy in the temperature measurement was testified by determining the freezing points of pure metals viz., indium, tin, lead, aluminum and gold (Table 1). This calibration was found to conform to the ITS-90 [19] scale. In a typical experiment about 100 mg of the alloy was loaded in an alumina crucible and was heated to about 100 K above the solidus temperature at a heating rate of 5 K min−1 . The onset of melting was identified by extrapolating the limbs of the DTA curve.
2.6. Chemical assay of the alloys used in the present investigation 2.6.1. Estimation of Cu and Ni The Cu–Ni alloys employed in the present investigation were assayed for their constituents by using the procedures described by Vogel [20]. The entire
K. Ananthasivan et al. / Journal of Alloys and Compounds 468 (2009) 275–279 Table 1 Comparison of the recommended values (ITS-90) of the melting points of pure metals with those obtained in this study Sample In Sn Pb Al Au
ITS-90 recommended Tm [19] (K)
Values obtained in this work (K)
429.6 504.88 600.42 933 1337.4
430.2 505.2 599 933.2 1336.7
Tm : melting point.
alloy sample (about 500 mg) was dissolved in an appropriate quantity of 8 M nitric acid. The copper present in this solution was quantitatively determined by electrodepositing it on a platinum gauze electrode. The nickel present in a known volume of the remaining solution was estimated gravimetrically, by precipitating it as Ni-dimethylglyoxime (DMG) complex. The residual carbon present in some of the alloy beads was estimated by quantitatively measuring the carbon dioxide evolved, when these alloys were fused in presence of oxygen.
3. Results and discussion 3.1. Temperature measurement The pyrometer was calibrated against a standard black body source (M/s. Mikron Inc. USA, Model No. M 390). In order to ascertain the accuracy of the temperature measurement, the melting points of the pure metals, Au, Cu, and Ni were determined. The values obtained are indicted in Table 2. From the measured values of the melting points of these metals, it is evi-
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dent that the melting temperatures in the range 1350–1730 K could be measured within an uncertainty of 2 K, using the present experimental technique and apparatus. 3.2. Observations on the spot effect The photographs showing the spots pertaining to the solidus temperature, liquidus temperature and an intermediate temperature are shown in Fig. 2.The onset of the solidus could be recognized by the appearance of small broken black spots on a bright background of the solid alloy. The completion of the liquefaction could be visually confirmed by the dissolution of the last spec of solid upon which a perfectly circular black spot was formed. The black spot turned into a bright spot upon illuminating the orifice with an external light source. However, the intensity of this external light source was not high enough to cause the reversal of the spot effect at temperatures above 1600 K. In spite of the fact that only a small quantity of the alloy (500 mg) was used in the spot experiments, the dendrites that were formed on the surface of the solidifying sample could be observed because of the good magnification of the long distance microscope. Thus it was possible to detect the presence of specs of oxide particle floating on the melt and discard the data corresponding to such specimens. 3.3. Solidus and liquidus in the system Cu–Ni The values of the solidus and liquidus temperatures obtained in the present investigation are given in Table 2. These results are also depicted in Fig. 3.
Table 2 Solidus and liquidus temperatures in the system Cu–Ni at.% Ni
Solidus (K)
Liquidus (K)
at.% Ni
Solidus (K)
Liquidus (K)
0 5.20 10.21 10.21 10.70 14.16 14.16 17.45 17.45 17.45 20.58 20.58 19.87 19.87 27.91 27.91 29.92 29.92 30.27 32.13$ 37.21$ 40.23 40.23 45.29 49.81 49.81
1356*, 1358*, 1360* – 1393 – 1393 1400 1404 1437 1439 – 1434 1434 1433 1433 – – 1474 – – 1469 1498 1511 1513 – – –
– 1383 1413 1411 1420 1423 – 1446 1447 1446 1454 1456 1455 1458 1498 1498 1509 1510 1521 – – – 1547 1575 1615 1615
50.00 54.85 54.85 60.29 60.29 64.53 69.11 69.11 69.29$ 73.39$ 74.82 77.19$ 80.56 80.56 83.23$ 85.07 85.07 89.44$ 89.66 89.66 89.66 91.9$ 95.81 96.84$ 100 Au
1543 1566 1567 1593 1603 1613 1623 1620 1615 1628 1652 1648 1652 1650 1665 1684 1686 1675 1686 1688 1686 1689 – 1693 1726*, 1726*, 1729* 1334*, 1336*, 1336*
1603 1626 – 1643 – 1654 1648 1644 – – 1678 – 1677 1679 – 1711 1716 – 1705 1707 1707 – 1721 – –
Melting point of pure metal, $: DTA.
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Fig. 2. (A) Cu melting conventional black spot (A1) illuminated spot. (B) Cu–Ni alloy (80 at.% Cu) liquidus, conventional (B1) partially illuminated. (C) Cu–Ni alloy (80 at.% Cu) above solidus, conventional (C1) partially illuminated.
Tarby et al. [5] presented the values obtained by interpolation from the solidus and liquidus reported by Feest and Doherty [2]. The latter were obtained using only two experimental measurements made above 1573 K. Massalski et al. [6] proposed an optimized phase diagram for this system which was based on the values reported in Ref. [2]. Feest and Doherty [2] attributed the deviation found in the experimental values reported by Hansen and Anderko [21] to the inaccuracies inherent in the thermal analysis technique. More over, this technique might pose difficulties in the determination of the phase boundaries in the system Cu–Ni due to non-equilibrium solidification [2]. However, such uncertainties in the estimation of the solidus are less pronounced in the nickel rich region. Hence, in this study we limited our DTA investigations of the solidus only to the nickel rich region. From Fig. 3 it is evident that the experimental data obtained in the present investigation are in agreement with the values pro-
posed by Massalski et al. [6], within ±10 K. In the nickel rich region our liquidus values are closer to those proposed in Ref. [6] than the values recommended by Mey [7]. The latter author used experimental data obtained from different sources. Feest and Doherty [2] obtained the values of solidus and liquidus temperatures by using a near equilibrium method and opined that the use of dynamic methods for the determination phase equilibria could lead to larger uncertainties. Even though the method adopted in the present investigation is in principle a dynamic technique, due to the very low heating rates employed near the phase transition temperatures, the values obtained using the spot technique are accurate and are close to the equilibrium values. The above fact is further substantiated by the good agreement between our data and those reported by Feest and Doherty [2]. The liquidus temperatures obtained for alloys in the composition range 50–65 at.% nickel exceed the values predicted by Massalski et al. [6] and Mey [7] by 10–15 K. Mey [7] arrived at optimized solidus liquidus boundaries by using experimental data that had a spread of about 25 K. The scatter observed in this study is not very different from the variation in the experimental results used by Mey [7]. The sensitivity of a DTA measurement could suffer because of the diminished magnitude of the heat effect in the vicinity of the liquidus. The spot technique, coupled with an image capture system with a good magnification, helps reduce such uncertainties. The solidus temperatures of the nickel rich alloys measured using DTA, appear to be lower than the proposed values by about 25 K. Thus, it appears that the data determined using the spot technique are more accurate. 3.4. Plausible uncertainties in composition and temperature
Fig. 3. Partial phase diagram of the system Cu–Ni showing the solidus and liquidus lines.
The present experimental investigations were performed at 1 × 10−3 Pa. Due to the higher vapour pressure of copper [22],
K. Ananthasivan et al. / Journal of Alloys and Compounds 468 (2009) 275–279 Table 3 Typical data showing the dependence of carbon pick up as a function of the Ni content Initial composition from weight (at.% Ni)
Carbon content after the experiment (ppm)
5.6 10.75 16.20 21.88 27.24 31.95 36.65 47.29
45.6 100.3 94.9 157.1 204.7 152.0 195.8 180.5
in the temperature range of the present investigation, some loss of copper was inevitable. However, chemical assay of the alloys after the completion of the experiments showed that the loss of copper was less than 0.2 at.%. Graphite crucibles were used for containing the alloys that were rich in copper (60–100 at.% Cu). Physical examination of these alloys did not evidence the reaction of these alloys with the container. However, alloys which contained greater than 40 at.% Ni, were found to wet the crucibles. Chemical analysis revealed that the carbon contamination in these alloys increased with the nickel content (Table 3). The highest value of residual carbon was found to be about 1000 ppm. The data pertaining to those alloys, which were contaminated with more than 100 ppm carbon were rejected. Crucibles made out of alumina were used instead of graphite crucibles, to contain those alloys which had nickel content greater than 40 at.%, in order to avoid contamination of these specimens with carbon. The chemical analysis of the alloys revealed that their composition did not deviate from the intended value by more than 0.5 at.%. The chemical compositions used for constructing the solidus and liquidus boundaries were the mean of the values before and after a given measurement. In view of the above, it can be concluded that the experimental technique employed in the present investigation can measure the solidus and liquidus temperatures of alloys within ±10 K. The uncertainty in the chemical composition of the alloys, however, would depend on factors such as the loss due to preferential evaporation of a component or the contamination of the alloy due to reaction with the container, that are specific to the system under investigation. For the system Cu–Ni, this uncertainty was very much within the experimental uncertainties in the chemical assay (±0.5 wt%).
279
4. Conclusions The solidus and liquidus boundaries in the system Cu–Ni were determined using the spot technique. The technique was validated by measuring the melting points of pure metals. The experimentally determined solidus and liquidus temperatures were in reasonable agreement with those proposed by Massalski et al. [6]. It was demonstrated that the use of spot technique could eliminate errors arising due to the oxidation and contamination of the liquid alloy with insoluble impurities. The uncertainties in the measured values of composition and temperature were estimated to be 0.5% and 10 K, respectively. The DTA measurement appeared to underestimate the solidus temperature of some nickel rich alloys. References [1] [2] [3] [4] [5] [6]
[7] [8] [9] [10] [11] [12] [13] [14] [15]
[16] [17] [18] [19] [20]
[21] [22]
S. Srikanth, K.T. Jacob, Mater. Sci. Technol. 5 (1989) 427. E.A. Feest, R.D. Doherty, J. Inst. Met. 99 (1971) 102. E. Schurmann, E. Shulz, A. Metallkde. 62 (1971) 758. B. Predel, R. Mohs, Arch. Eisenhuttenw 42 (1971) 575. S.K. Tarby, J.C. Bowker, W.L. Stockdate, J. Inst. Met. 100 (1972) 374. T.B. Massalski, L.H. Bennett, J.l. Murray, H. Baker (Eds.), Binary Alloy Phase Diagrams, American Society for Metals International, Metals Park, OH, 1986. S. Mey, Z. Metallkunde 78 (1987) 502. R.J. Ackermann, E.G. Raugh, High Temp. Sci. 21 (1972) 161. K. Ananthasivan, PhD dissertation, University of Madras, Chennai, India, 2002. S.P. Garg, R.J. Ackermann, Metall. Trans. A 8 (1977) 239. S.P. Garg, R.J. Ackermann, J. Nucl. Mater. 64 (1977) 265. S.P. Garg, R.J. Ackermann, Trans. IIM 32 (1979) 62. S.P. Garg, R.J. Ackermann, Scripta Metall. 13 (1979) 611. S.P. Garg, Y.J. Bhatt, R. Venkataramani, Mater. Sci. Forum 3 (1985)419. M.S. Chandrasekharaiah, J.L. Margrave, D. Das, in: J.F. Schooley (Ed.), Temperature—Its Measurement and Control in Science and Industry, vol. 6, Part 1, American Institute of Physics, New York, 1992, p. 373. S.P. Garg, PhD thesis, University of Bombay, Bombay, India, 1980. K. Ananthasivan, I. Kaliappan, P.R. Vasudeva Rao, C. Sudha, A.L.E. Terrance, J. Nucl. Mater. 305 (2) (2002) 95. K. Ananthasivan, I. Kaliappan, P.R. Vasudeva Rao, J. Alloys Compd. 352 (2003) 148. H. Preston-Thomas, Metrologia 27 (1) (1990) 3–10. A.I. Vogel, A Text Book of Quantitative Inorganic Analysis Including Elementary Instrumental Analysis, 3rd ed., The ELBS and Longman, London, UK, 1975. M. Hansen, K. Aderko (Eds.), Constitution of Binary Alloys, 2nd ed., McGraw-Hill, New York/London, 1958, p. 601. O. Knacke, O. Kubaschewski, H. Hesselmann (Eds.), Thermochemical Properties of Inorganic Substances, vols. I and II, 2nd ed., ASM International, Materials Park, OH, 1991.
Determination of solidus and liquidus in the system Cu–Ni by the spot technique K. Ananthasivan, S. Balakrishnan, I. Kaliappan, S. Anthonysamy ∗ , R. Pankajavalli, P.R. Vasudeva Rao Materials Chemistry Division, Chemistry Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India Received 2 August 2007; received in revised form 19 December 2007; accepted 20 December 2007 Available online 3 January 2008
Abstract The solidus and liquidus in the system Cu–Ni over the entire composition range were determined by using the spot-technique. The solidus temperatures of some nickel rich alloys were also investigated using differential thermal analysis (DTA). The experimental data obtained were shown to be in agreement with the assessed values. The experimental set up used in this study was an improvised version of the apparatus used in earlier studies. It was demonstrated that by using this set up the temperatures of melting transitions could be determined within ±2 K. Temperature calibration was carried out by determining the melting points of pure metals viz., Au, Ni and Cu. The salient features and the utility of the ‘spot-technique’ were also demonstrated. © 2008 Elsevier B.V. All rights reserved. Keywords: Spot technique; Cu–Ni alloys; Solidus; Liquidus; Pyrometry; DTA
1. Introduction Copper nickel alloys are of interest both from a theoretical stand point and practical application. The system Cu–Ni is of relevance to cupronickel alloys. These alloys have potential industrial applications that demand superior mechanical and corrosion resistance properties [1]. The subsolidus equilibria in this system have been studied extensively [1]. The solidus and liquidus in this system have been experimentally determined by Feest and Doherty [2], Schurmann and Shulz [3] and Predel and Mohs [4]. Tarby et al. [5] calculated the Cu–Ni phase diagram with the help of the experimental data published until 1971. Massalski et al. [6] proposed an optimized phase diagram based on the data reported in Ref. [2]. Mey [7] assessed the phase diagram based on the investigations made by Feest and Doherty [2], Schurmann and Schulz [3] and Predel and Mohs [4]. They discarded the experimental values reported in earlier investigations that had larger uncertainties caused by the undercooling of the alloys dur-
∗
Corresponding author. Tel.: +91 44 27480098; fax: +91 44 27480065. E-mail address: [email protected] (S. Anthonysamy).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.12.075
ing measurement. The phase boundaries given in Ref. [2] included only a couple of data points above 1573 K. Further, the solidus and liquidus proposed by Tarby et al. [5] differs substantially from that suggested in Refs. [6,7]. Hence, we undertook to re-determine the solidus and liquidus in the system Cu–Ni. The spot technique – an innovative method developed by Ackermann and Raugh [8] based on optical pyrometry – was used to determine these equilibria. These experiments were carried out in an experimental facility that was recently established in our laboratory for carrying out high temperature investigations on alloys and ceramics [9]. In the conventional spot experiments reported so far [8,10–16], the image of the spot was manually observed. In the studies carried out in our laboratory we recorded the images of the spot by using an image capture system and demonstrated the utility of this technique in identifying steep liquidus boundaries [9,17]. Since the system Cu–Ni is characterized by closely spaced solidus and liquidus boundaries, an investigation of this system would help us to establish the accuracy achievable with this apparatus and technique. The solidus temperatures obtained in this study were also compared with the results obtained by using
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K. Ananthasivan et al. / Journal of Alloys and Compounds 468 (2009) 275–279
differential thermal analysis (DTA). The results obtained are discussed and the magnitude of the errors are analysed. 2. Experimental 2.1. Starting materials High purity copper (99.99%) was obtained from M/s. Goodfellow Metals, U.K., in the form of granules. Nickel metal granules (99.99%) were procured from M/s. Leco Corporation Inc., USA. Analar grade nitric acid was supplied by M/s. E Merk (India) Ltd., Mumbai, India. Dimethyl glyoxime (AR grade) was procured from M/s. S.D. Finechem Ltd., Mumbai.
2.2. Synthesis of alloys Some of the alloy samples employed in this study were prepared by arc melting the elements in a triarc furnace supplied by M/s. Centorr Associates Inc., USA. Mixtures of Cu and Ni of desired stoichiometry were melted by using a dc arc generated between a nonconsumable, thoriated tungsten cathode and a water cooled, copper anode. The melting was done under argon containing about 4 ppm each of oxygen and moisture (IOLAR-2 grade argon gas, supplied by M/s. Indian Oxygen Limited) at a pressure of 1–10 Pa. To remove the residual oxygen in the arcing chamber, zirconium metal sponge was melted before melting the alloy components. The button was homogenized by flipping and remelting for about six times. In the rest of the experiments the alloys were made in situ during the measurement.
2.3. Principle of the spot technique The basic principle behind the spot technique is described in detail elsewhere [9]. In this technique, a Knudsen cell containing the sample is heated by either RF induction or electron bombardment [8]. Upon melting, the reflectivity as well as emissivity [15] of the sample (solid alloy) changes due to liquefaction. This liquid globule acts as a mirror and reflects the image of the orifice of the Knudsen cell, as a black spot. When the temperature (and composition) of the alloy falls in a two-phase field, where solid and liquid coexist, instead of a complete black spot, several small black spots appear. These small spots coalesce to form a complete spot, once the sample is heated to its liquidus temperature. Recent work carried out in our laboratory [18] showed that the spot technique could be extended to temperatures lower than 1073 K by using an external illumination procedure. This technique is essentially the same as the conventional spot technique, but for the reversal in the direction of illumination.
2.4. Experimental assembly A schematic diagram of the experimental assembly used in the present study is shown in Fig. 1. The experimental assembly consisted of a vacuum chamber, which housed the sample held in a crucible kept inside a Knudsen cell, a sample support assembly, a vacuum system (diffusion pumping system M/s. Balzers GmbH, Switzerland), a radiofrequency (RF) generator, a pyrometer (M78, M/s. Micron, USA) and an image capture system. The image capture system consisted of a long distance microscope (QM1, M/s. Questar, Inc. USA), a charge coupled device (CCD) camera (WAT 207 A, M/s. Watec Co. Ltd., Japan), and a video monitor. The details of construction of this experimental assembly are described in detail elsewhere [9,17].
2.5. Experimental procedure 2.5.1. Spot experiments In a typical “high temperature experiment” about 500 mg of the sample was taken in a cup made out of alumina or graphite. This cup together with the Knudsen cell was degassed prior to the experiment in vacuum for about 2 h by heating at a temperature, which was at least 100 K higher than the desired temperature of investigation. In order to minimize the residual concentration of the gaseous impurities like oxygen and moisture, the vacuum chamber was flushed with helium (containing less than 4 ppm each of moisture and oxygen).
Fig. 1. Schematic showing the experimental system used for the spot experiments: 1, IR two color pyrometer; 2, fixture for pyrometer; 3, optical rail; 4, quartz optical window; 5, bottom flange; 6, vacuum chamber; 7, sample support assembly; 8, molybdenum table; 9 radiation shield; 9a, top radiation shield; 10, RF coil; 11, Knudsen cell; 12, quartz prism; 13, tungsten legs; 14, crucible; 15, sample. Subsequently, it was evacuated to about 1 × 10−3 Pa and the Knudsen cell was heated by RF induction. The image of the Knudsen cell orifice was viewed using the image capture system, while the temperature was measured by focusing the pyrometer on to the black body hole. The temperatures corresponding to the appearance of the broken spot (solidus), as well as the complete spot (liquidus) were recorded. These measurements were made during both the heating and the cooling cycles. A heating rate of 10 K min−1 was employed to reach a temperature that was about 100 K less than the solidus temperature. Subsequently the sample was heated at a rate of 1 K min−1 up to its liquidus temperature. 2.5.2. DTA experiments A differential thermal analyser (Model – Seiko 320, Japan) was used in the DTA experiments. Both the sample temperature and the differential temperature were measured within ±0.5 K by using a Pt–13% Rh/Pt (Type-R) thermocouple. The accuracy in the temperature measurement was testified by determining the freezing points of pure metals viz., indium, tin, lead, aluminum and gold (Table 1). This calibration was found to conform to the ITS-90 [19] scale. In a typical experiment about 100 mg of the alloy was loaded in an alumina crucible and was heated to about 100 K above the solidus temperature at a heating rate of 5 K min−1 . The onset of melting was identified by extrapolating the limbs of the DTA curve.
2.6. Chemical assay of the alloys used in the present investigation 2.6.1. Estimation of Cu and Ni The Cu–Ni alloys employed in the present investigation were assayed for their constituents by using the procedures described by Vogel [20]. The entire
K. Ananthasivan et al. / Journal of Alloys and Compounds 468 (2009) 275–279 Table 1 Comparison of the recommended values (ITS-90) of the melting points of pure metals with those obtained in this study Sample In Sn Pb Al Au
ITS-90 recommended Tm [19] (K)
Values obtained in this work (K)
429.6 504.88 600.42 933 1337.4
430.2 505.2 599 933.2 1336.7
Tm : melting point.
alloy sample (about 500 mg) was dissolved in an appropriate quantity of 8 M nitric acid. The copper present in this solution was quantitatively determined by electrodepositing it on a platinum gauze electrode. The nickel present in a known volume of the remaining solution was estimated gravimetrically, by precipitating it as Ni-dimethylglyoxime (DMG) complex. The residual carbon present in some of the alloy beads was estimated by quantitatively measuring the carbon dioxide evolved, when these alloys were fused in presence of oxygen.
3. Results and discussion 3.1. Temperature measurement The pyrometer was calibrated against a standard black body source (M/s. Mikron Inc. USA, Model No. M 390). In order to ascertain the accuracy of the temperature measurement, the melting points of the pure metals, Au, Cu, and Ni were determined. The values obtained are indicted in Table 2. From the measured values of the melting points of these metals, it is evi-
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dent that the melting temperatures in the range 1350–1730 K could be measured within an uncertainty of 2 K, using the present experimental technique and apparatus. 3.2. Observations on the spot effect The photographs showing the spots pertaining to the solidus temperature, liquidus temperature and an intermediate temperature are shown in Fig. 2.The onset of the solidus could be recognized by the appearance of small broken black spots on a bright background of the solid alloy. The completion of the liquefaction could be visually confirmed by the dissolution of the last spec of solid upon which a perfectly circular black spot was formed. The black spot turned into a bright spot upon illuminating the orifice with an external light source. However, the intensity of this external light source was not high enough to cause the reversal of the spot effect at temperatures above 1600 K. In spite of the fact that only a small quantity of the alloy (500 mg) was used in the spot experiments, the dendrites that were formed on the surface of the solidifying sample could be observed because of the good magnification of the long distance microscope. Thus it was possible to detect the presence of specs of oxide particle floating on the melt and discard the data corresponding to such specimens. 3.3. Solidus and liquidus in the system Cu–Ni The values of the solidus and liquidus temperatures obtained in the present investigation are given in Table 2. These results are also depicted in Fig. 3.
Table 2 Solidus and liquidus temperatures in the system Cu–Ni at.% Ni
Solidus (K)
Liquidus (K)
at.% Ni
Solidus (K)
Liquidus (K)
0 5.20 10.21 10.21 10.70 14.16 14.16 17.45 17.45 17.45 20.58 20.58 19.87 19.87 27.91 27.91 29.92 29.92 30.27 32.13$ 37.21$ 40.23 40.23 45.29 49.81 49.81
1356*, 1358*, 1360* – 1393 – 1393 1400 1404 1437 1439 – 1434 1434 1433 1433 – – 1474 – – 1469 1498 1511 1513 – – –
– 1383 1413 1411 1420 1423 – 1446 1447 1446 1454 1456 1455 1458 1498 1498 1509 1510 1521 – – – 1547 1575 1615 1615
50.00 54.85 54.85 60.29 60.29 64.53 69.11 69.11 69.29$ 73.39$ 74.82 77.19$ 80.56 80.56 83.23$ 85.07 85.07 89.44$ 89.66 89.66 89.66 91.9$ 95.81 96.84$ 100 Au
1543 1566 1567 1593 1603 1613 1623 1620 1615 1628 1652 1648 1652 1650 1665 1684 1686 1675 1686 1688 1686 1689 – 1693 1726*, 1726*, 1729* 1334*, 1336*, 1336*
1603 1626 – 1643 – 1654 1648 1644 – – 1678 – 1677 1679 – 1711 1716 – 1705 1707 1707 – 1721 – –
Melting point of pure metal, $: DTA.
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Fig. 2. (A) Cu melting conventional black spot (A1) illuminated spot. (B) Cu–Ni alloy (80 at.% Cu) liquidus, conventional (B1) partially illuminated. (C) Cu–Ni alloy (80 at.% Cu) above solidus, conventional (C1) partially illuminated.
Tarby et al. [5] presented the values obtained by interpolation from the solidus and liquidus reported by Feest and Doherty [2]. The latter were obtained using only two experimental measurements made above 1573 K. Massalski et al. [6] proposed an optimized phase diagram for this system which was based on the values reported in Ref. [2]. Feest and Doherty [2] attributed the deviation found in the experimental values reported by Hansen and Anderko [21] to the inaccuracies inherent in the thermal analysis technique. More over, this technique might pose difficulties in the determination of the phase boundaries in the system Cu–Ni due to non-equilibrium solidification [2]. However, such uncertainties in the estimation of the solidus are less pronounced in the nickel rich region. Hence, in this study we limited our DTA investigations of the solidus only to the nickel rich region. From Fig. 3 it is evident that the experimental data obtained in the present investigation are in agreement with the values pro-
posed by Massalski et al. [6], within ±10 K. In the nickel rich region our liquidus values are closer to those proposed in Ref. [6] than the values recommended by Mey [7]. The latter author used experimental data obtained from different sources. Feest and Doherty [2] obtained the values of solidus and liquidus temperatures by using a near equilibrium method and opined that the use of dynamic methods for the determination phase equilibria could lead to larger uncertainties. Even though the method adopted in the present investigation is in principle a dynamic technique, due to the very low heating rates employed near the phase transition temperatures, the values obtained using the spot technique are accurate and are close to the equilibrium values. The above fact is further substantiated by the good agreement between our data and those reported by Feest and Doherty [2]. The liquidus temperatures obtained for alloys in the composition range 50–65 at.% nickel exceed the values predicted by Massalski et al. [6] and Mey [7] by 10–15 K. Mey [7] arrived at optimized solidus liquidus boundaries by using experimental data that had a spread of about 25 K. The scatter observed in this study is not very different from the variation in the experimental results used by Mey [7]. The sensitivity of a DTA measurement could suffer because of the diminished magnitude of the heat effect in the vicinity of the liquidus. The spot technique, coupled with an image capture system with a good magnification, helps reduce such uncertainties. The solidus temperatures of the nickel rich alloys measured using DTA, appear to be lower than the proposed values by about 25 K. Thus, it appears that the data determined using the spot technique are more accurate. 3.4. Plausible uncertainties in composition and temperature
Fig. 3. Partial phase diagram of the system Cu–Ni showing the solidus and liquidus lines.
The present experimental investigations were performed at 1 × 10−3 Pa. Due to the higher vapour pressure of copper [22],
K. Ananthasivan et al. / Journal of Alloys and Compounds 468 (2009) 275–279 Table 3 Typical data showing the dependence of carbon pick up as a function of the Ni content Initial composition from weight (at.% Ni)
Carbon content after the experiment (ppm)
5.6 10.75 16.20 21.88 27.24 31.95 36.65 47.29
45.6 100.3 94.9 157.1 204.7 152.0 195.8 180.5
in the temperature range of the present investigation, some loss of copper was inevitable. However, chemical assay of the alloys after the completion of the experiments showed that the loss of copper was less than 0.2 at.%. Graphite crucibles were used for containing the alloys that were rich in copper (60–100 at.% Cu). Physical examination of these alloys did not evidence the reaction of these alloys with the container. However, alloys which contained greater than 40 at.% Ni, were found to wet the crucibles. Chemical analysis revealed that the carbon contamination in these alloys increased with the nickel content (Table 3). The highest value of residual carbon was found to be about 1000 ppm. The data pertaining to those alloys, which were contaminated with more than 100 ppm carbon were rejected. Crucibles made out of alumina were used instead of graphite crucibles, to contain those alloys which had nickel content greater than 40 at.%, in order to avoid contamination of these specimens with carbon. The chemical analysis of the alloys revealed that their composition did not deviate from the intended value by more than 0.5 at.%. The chemical compositions used for constructing the solidus and liquidus boundaries were the mean of the values before and after a given measurement. In view of the above, it can be concluded that the experimental technique employed in the present investigation can measure the solidus and liquidus temperatures of alloys within ±10 K. The uncertainty in the chemical composition of the alloys, however, would depend on factors such as the loss due to preferential evaporation of a component or the contamination of the alloy due to reaction with the container, that are specific to the system under investigation. For the system Cu–Ni, this uncertainty was very much within the experimental uncertainties in the chemical assay (±0.5 wt%).
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4. Conclusions The solidus and liquidus boundaries in the system Cu–Ni were determined using the spot technique. The technique was validated by measuring the melting points of pure metals. The experimentally determined solidus and liquidus temperatures were in reasonable agreement with those proposed by Massalski et al. [6]. It was demonstrated that the use of spot technique could eliminate errors arising due to the oxidation and contamination of the liquid alloy with insoluble impurities. The uncertainties in the measured values of composition and temperature were estimated to be 0.5% and 10 K, respectively. The DTA measurement appeared to underestimate the solidus temperature of some nickel rich alloys. References [1] [2] [3] [4] [5] [6]
[7] [8] [9] [10] [11] [12] [13] [14] [15]
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