Experimental investigation and thermodynamic modeling of the Nd–Ni system

Journal of Alloys and Compounds 398 (2005) 127–132

Experimental investigation and thermodynamic modeling of the Nd–Ni system Mianliang Huang a,∗ , R. William McCallum a,b , Thomas A. Lograsso a a

Materials and Engineering Physics Program, Ames Laboratory, Iowa State University, Ames, IA 50011-3020, USA b Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011-3114, USA Received 10 February 2005; accepted 21 February 2005 Available online 25 March 2005

Abstract The Nd–Ni system has been investigated via experiments and thermodynamic modeling. In the experimental part, alloys across the entire composition range were prepared by arc melting pure Nd and Ni slugs and annealing the alloys at 500 ◦ C (<70 at.% Ni) and 800 ◦ C (>70 at.% Ni) for 2 weeks. The annealed alloys were then subjected to differential scanning calorimetry and differential thermal analysis (DTA) measurements to determine the invariant reaction temperatures. Except for the peritectic reaction liquid + Nd7 Ni3 ↔ Nd3 Ni and the polymorphous phase transformation of Nd2 Ni7 found in this study, all the other invariant reactions are confirmed to be the same types but with significant different temperatures as reported results. In the modeling part, the available phase equilibrium and thermodynamic data in the Nd–Ni system were analyzed by using thermodynamic models for the Gibbs energies of individual phases. An optimal set of thermodynamic parameters were obtained using WinPhad software. The calculated phase equilibria and thermodynamic properties from the model parameters were compared to the corresponding experimental data and good agreement was obtained. © 2005 Elsevier B.V. All rights reserved. Keywords: Nd–Ni phase diagram; Thermal analysis; Thermodynamic modeling

1. Introduction AB5-type hydrogen storage alloys made of rare earth (Ce, La, Nd and Pr) with Ni are crucial electrode materials for high-energy density and environmentally friendly Ni-MH batteries [1,2]. The phase diagrams and thermodynamic properties of the alloy systems have significant impact on the alloy design and developments. The assessed Nd–Ni system [3] was based primarily on Pan and Zheng [4] using X-ray diffraction (XRD) and differential thermal analysis (DTA); eight previous reported compounds, Nd3 Ni [5], Nd7 Ni3 [6], NdNi [7,8], NdNi2 [9,10], NdNi3 [11–13], Nd2 Ni7 [12,13], NdNi5 [12,14,15] and Nd2 Ni17 [12], were confirmed. Qi et al. [16] determined six liquidus data points in the composition range 64.3–72.1 at.% Ni using optical microscopy, electron probe ∗

Corresponding author. Tel.: +1 515 294 8685. E-mail address: [email protected] (M. Huang).

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microanalysis and inductively coupled plasma chemical analysis. Based on these phase equilibria data [4,16] and the reported thermodynamic properties [17], Du and Clavaguera [18] modeled the Nd–Ni system. However, the previously reported data [4] of invariant reaction and congruent melting temperatures were significantly different from those determined in our preliminary study. In addition, in our ongoing research of physical properties of the Nd–Ni–Si ternary phases, considerable difficulties have been encountered in the preparation of single-phase/single crystal samples because of the conflicting phase diagram information. Therefore, a thermodynamic description for the Nd–Ni system is desired within our research program aiming at establishing the phase diagram for the Nd–Ni–Si ternary system by using both the experiment and thermodynamic modeling. In this study, experiments were carried out to determine the invariant reaction temperatures and melting points of Nd–Ni compounds. The thermodynamic description for the whole

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system is presented by considering the present experimental results and the reliable literature data. 2. Experimental procedure

their magnetic state with decreasing temperature. The reader is referred to the article by Du and Clavaguera [18] for more ␾ details. xs Gm is the excess Gibbs energy with interaction parameters expressed in the Redlich–Kister polynomial [20]: ␾

Nine alloys, Nd–x at.% Ni (where x = 4, 10, 24, 28, 30, 40, 60, 75, 83.3 and 90) were arc-melted in a Zr gettered Ar atmosphere using high purity elements of Ni (99.99%) and Nd. The Nd was prepared by the Materials Preparation Center at the Ames Laboratory and contained the following major impurities in ppm (atomic) O ∼ 469, C ∼ 444, H ∼ 71, N ∼ 216, Si ∼ 80, Fe ∼ 20 and Cr ∼ 17. Relative mass losses during melting were less than 0.5%. The ingots were wrapped with tantalum foil, sealed in quartz tubes filled with 99.998% argon and homogenized in a furnace at 500 ◦ C (<70 at.% Ni) and 800 ◦ C (>70 at.% Ni) for 2 weeks and furnace cooled. The differential thermal analysis investigations were carried out in a Perkin-Elmer DTA 7 allowing a maximum temperature of 1500 ◦ C. The thermocouples were calibrated at a transition temperature of high purity K2 SO4 and the melting point of Au and the temperature measurements were accurate to within ±5 ◦ C. For the measurements, samples weighing 20–60 mg were cycled at heating and cooling rates of 10 ◦ C/min in 50 cc/min of Zr gettered Ar in tantalum crucibles which were sealed by arc welding to prevent Nd loss by oxidation and vaporization. MgO crucibles were used for Ni-rich alloys due to the reaction between nickel and tantalum at high temperatures. Some of the alloys on Nd-rich side were analyzed in a differential scanning calorimeter (DSC). The thermocouples used in the DSC were calibrated against the melting points of pure In and Al and the accuracy of the temperature was estimated to be within ±2 ◦ C. DSC measurements for the alloys wrapped with tantalum foils were carried out in copper pans.

3. Thermodynamic modeling

xs Gm = xNd xNi [0 L + 1 L(xNd − xNi ) + 2 L(xNd − xNi )2 ] (2) where 0 L, 1 Land 2 L are the interaction parameters which could be temperature dependent. 3.2. Intermetallic compounds There are eight intermetallic compounds in the Nd–Ni binary system [3,4], i.e. Nd3 Ni, Nd7 Ni3 , NdNi, NdNi2 , NdNi3 , Nd2 Ni7 , NdNi5 and Nd2 Ni17 . They are treated by a twosublattice model with Nd on one sublattice and Ni on the other one. Their Gibbs energies are expressed by the following equation in terms of one mole of atoms referring to the pure elements in their nonmagnetic reference states: a Prb = a0 Gref + b0 Gref + A + BT GNi m Nd Ni

where 0 Gref i is the molar Gibbs energy of the pure element i (i = Nd, Ni) with its defined reference structure in a nonmagnetic state from the compilation by Dinsdale [19]. A and B are the parameters to be evaluated during the course of optimization.

4. Results and discussion The invariant temperatures were determined by DSC or DTA for the whole system and large differences were observed between this study and Pan and Zheng [4]. Fig. 1 shows the DTA trace for Nd–4 at.% Ni alloy, indicating three thermal events at 611, 853 and 960 ◦ C, which correspond to the eutectic reaction liquid ↔ Nd3 Ni + dhcp (Nd), dhcp (Nd) ↔ bcc (Nd) and liquidus curve, respectively. The

3.1. Liquid, bcc, dhcp and fcc phases The liquid, bcc (Nd), dhcp (Nd) and fcc (Ni) phases are treated by a one sublattice model with the Gibbs energy expressed as following equation in terms of one mole of atoms: ␾

␾

␾

Gm = xNd o GNd + xNi o GNi + RT (xNd ln xNd + xNi ln xNi ) ␾

␾

+xs Gm + mg Gm

(1)

in which xi is the mole fraction of element i (i = Nd, Ni), R the ␾ gas constant. Gi the molar Gibbs energy of element i with a structure of ‘␾’ (liquid, bcc, fcc and dhcp) in the nonmagnetic state. In the present work, the Gibbs energy functions for Ni and Nd are taken from the SGTE-compilation by Dinsdale ␾ [19]. mg Gm represents the change in the Gibbs energy of the phases in transforming from their paramagnetic state to

(3)

Fig. 1. DTA results of alloy Nd–4 at.% Ni.

M. Huang et al. / Journal of Alloys and Compounds 398 (2005) 127–132

decrease in decomposition temperature at 853 ◦ C of dhcp (Nd) to bcc (Nd) in Nd–4 at.% Ni alloy compared to pure Nd of 863 ◦ C [21] indicates that the nature of this invariant reaction is metatectic, liquid + dhcp (Nd) ↔ bcc (Nd). The temperature of 611 ◦ C for eutectic reaction between Nd3 Ni and dhcp (Nd) is 41 ◦ C higher than that previously reported [4]. The determination of the congruent melting points of the compounds Nd3 Ni (618 ± 2 ◦ C) is challenging because of its proximity with the invariant reactions. Due to the oxidation of Nd during heating, very small changes in stoichiometry result in the observation of more than one thermal event. After several heating cycles, the peaks shifted which made it difficult to determine what reactions corresponded to which peaks. In order to obtain repeatable and accurate results, the composition for the starting materials should always be Ndrich (slightly <25 at.% Ni) for Nd3 Ni so that only the two peaks (one for eutectic and one for melting) can be observed during the measurements. In this case, an alloy of Nd–24 at.% Ni was used to determine the congruent melting point of Nd3 Ni, as shown in Fig. 2. During the first heating, the largest peak is associated with the eutectic reaction between dhcp (Nd) and Nd3 Ni with an onset at 611 ◦ C, the second peak is associated with the melting of remaining Nd3 Ni. Due to the oxidation of Nd during subsequent thermal cycles, the overall composition is depleted in Nd and the fraction of Nd3 Ni is increased. With the increased fraction of Nd3 Ni, the second thermal event, the congruent melting, is thereby enhanced, as shown in the second and third heating. Conversely, the signal for the eutectic reaction correspondingly decreases, broadening and becoming less distinct. The congruent melting nature of Nd7 Ni3 was not confirmed in the present work. Nd7 Ni3 was found to decompose via a peritectic reaction at 597 ◦ C, which is in contrast

Fig. 2. DSC traces of alloy Nd–26 at.% Ni with a heating rate of 1 ◦ C/min. The big peak in the first heating curve is associated with the eutectic reaction between dhcp (Nd) and Nd3 Ni and the small peak is associated with the melting of Nd3 Ni. It can be seen that the peak for the eutectic reaction decreased and that for the melting of Nd3 Ni increased due to the oxidation of Nd with cycles.

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Fig. 3. DSC traces of alloy Nd–28 at.% Ni with a heating rate of 1 ◦ C/min. The sharp peak is associated with the peritectic reaction between Nd3 Ni and Nd7 Ni3 and the flat hump is associated with the dissolving of the remaining Nd3 Ni. The hump became smaller due to the oxidation of Nd with cycles and disappeared eventually when the composition shifted to Ni-rich side (>30 at.% Ni) in the third heating, where the peak for eutectic reaction between Nd7 Ni3 and NdNi was observed as indicated in the inset.

to Refs. [3,4], where a congruent melting point at 616 ◦ C was reported. This difference in melting behavior was elucidated through multiple heating/cooling cycles. Fig. 3 shows the DSC traces of alloy with a starting composition of Nd–28 at.% Ni. In Fig. 3, during the initial heating cycle, a large thermal peak (597 ◦ C) is associated with an invariant reaction between Nd7 Ni3 and Nd3 Ni, while the second event at approximately 608 ◦ C, could be associated with either the dissolving of Nd3 Ni or Nd7 Ni3 into the liquid. During subsequent cycles, the overall composition shifts toward Nd7 Ni3 due to the oxidation of Nd and the relative fraction of Nd7 Ni3 is increased which should cause the second event to shift to higher temperatures and increase in magnitude if Nd7 Ni3 is a congruently melting compound. However, during the second heating cycle, this thermal event shifts to lower temperatures and is diminished in magnitude while the invariant temperature remains unchanged. Such behavior is only consistent with the peritectic formation of Nd7 Ni3 . Furthermore, continued depletion of Nd should lead to the disappearance of the second event as the overall composition surpasses Nd–30 at.% Ni, as is observed during the third heating cycle. During the third heating, the second thermal event has disappeared completely and a tiny peak with an onset at 585 ◦ C has appeared (which is the eutectic reaction between Nd7 Ni3 and NdNi, see Fig. 4). The onset temperature (597 ◦ C) for the sharp peak is the same for the alloys with a composition on both sides of the stoichiometric compound Nd7 Ni3 , indicating the nature of the invariant reaction between Nd7 Ni3 and Nd3 Ni is peritectic not the eutectic reaction at 565 ◦ C as reported in Ref. [4]. For higher Ni content alloys, DSC or DTA measurements generally found higher temperatures for almost all invariant and congruent melting temperatures, as shown in Figs. 4–7.

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Fig. 4. DSC curves for alloy Nd40 Ni60 .

Fig. 4 shows the DSC traces for Nd–40 at.% Ni alloy. A eutectic reaction between Nd7 Ni3 and NdNi at 585 ◦ C was observed which is 45 ◦ C higher than the result reported in Ref. [4]. Fig. 5 shows the DTA results for Nd–50 at.% Ni and Nd–60 at.% Ni alloys. The first tiny peak observed for Nd–50 at.% Ni is associated with the eutectic reaction between NdNi and NdNi2 at 773 ◦ C and the large peak with an onset at 819 ◦ C is for the melting of NdNi. Both the temperatures are higher than those reported in Ref. [4] with values at 720 and 780 ◦ C, respectively. Fig. 6 shows the DTA curves for Nd–75 at.% Ni alloy, indicating four thermal events. The three temperatures at 964, 1068 and 1193 ◦ C are higher than those reported by Pan and Zheng [4] for the decomposition temperature of NdNi2 (940 ◦ C), NdNi3 (1030 ◦ C) and Nd2 Ni7 (1134 ◦ C), respectively; the temperature at 1172 ◦ C, which lies between the decomposition temperatures of NdNi3 and Nd2 Ni7 , is likely the reported phase transformation temperature of Nd2 Ni7 from rhombohedral to hexagonal structure [13], which is similar to our previous

Fig. 5. DTA curves for alloys NdNi and Nd–60 at.% Ni. The small peak in the curve for NdNi is associated with the eutectic reaction between NdNi and NdNi2 and the second large peak is for the melting of NdNi.

Fig. 6. DTA result of alloy NdNi3 showing four peaks with onset temperatures at 964, 1068, 1172 and 1193 ◦ C, respectively.

study in the Ni–Pr system [22]. Fig. 7 shows DTA heating curves for Nd–83.3 at.% Ni (NdNi5 ) and Nd–90 at.% Ni with a heating rate of 10 ◦ C/min. The melting point of NdNi5 is 1405 ◦ C which is slightly lower than that (1420 ◦ C) reported in Ref. [4]. The inset in Fig. 7 is the heating curve for Nd–90 at.% Ni alloy with a heating rate of 1 ◦ C/min, indicating two overlapping peaks with onsets at 1282 and 1285 ◦ C, respectively. Virkar and Raman [12] and Pan and Zheng [4] observed that Nd2 Ni17 is stable in the temperature range 1250–1290 ◦ C. Our result suggests that Nd2 Ni17 is stable in a much more narrow temperature range 1282–1285 ◦ C. Optimization of model parameters using selected experimental data was carried out using the WinPhad software, a computer program developed by CompuTherm (Madison, USA) for the calculation and optimization of binary phase diagrams. This program can accept different types of data, such as thermodynamic quantities (enthalpy of formation and entropy of formation) and phase equilibrium data (invariant

Fig. 7. DTA results of alloy NdNi5 and Nd–90 at.% Ni. The inset with 1 ◦ C/min heating rate shows two peaks.

M. Huang et al. / Journal of Alloys and Compounds 398 (2005) 127–132

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Table 1 Optimized thermodynamic model parameters of Nd–Ni binary system, in SI units liquid

Liquid: 0 LNd,Ni = −166540 + 38.798 T ; 1 Lliquid = 34987 − 5.354 T ; 2 Lliquid = 24991 Nd,Ni Nd,Ni fcc (Ni): 0 Lfcc Nd,Ni = 100000 bcc (Nd): 0 Lbcc Nd,Ni = −8500 dhcp 0 dhcp (Nd): LNd,Ni = −10000 dhcp Nd3 Ni Nd3 Ni: Gm − 0.750 GNd − 0.250 Gfcc Ni = −28093 + 7.967 T dhcp Nd7 Ni3 − 0.70 GNd − 0.30 Gfcc Nd7 Ni3 : Gm Ni = −33789 + 10.595 T dhcp fcc NdNi 0 0 NdNi: Gm − 0.5 GNd − 0.5 GNi = −46879 + 12.065 T dhcp NdNi NdNi2 : Gm 2 − 0.33330 GNd − 0.67770 Gfcc Ni = −36598 + 4.329 T dhcp NdNi3 − 0.250 GNd − 0.750 Gfcc = −31601 + 3.128 T NdNi3 : Gm Ni dhcp Nd2 Ni7 0 0 ␣Nd2 Ni7 : Gm − 0.2222 GNd − 0.7778 Gfcc Ni = −29901 + 2.9435 T dhcp Nd2 Ni7 ␤Nd2 Ni7 : Gm − 0.22220 GNd − 0.77780 Gfcc Ni = −28895 + 2.247 T dhcp NdNi NdNi5 : Gm 5 − 0.16670 GNd − 0.83330 Gfcc = −25860 + 2.6645 T Nd dhcp Nd2 Ni17 fcc 0 0 Nd2 Ni17 : Gm − 0.1053 GNd − 0.8947 GNi

Fig. 9. Enlarged part of the calculated Nd–Ni phase diagram on Nd-rich side together with experimental data. The inset indicates Nd7 Ni3 decomposition with a peritectic reaction.

= −16245 + 1.62603 T

temperatures and compositions and phase boundaries), in the same operation. In this optimization, all of the parameters are in the form of A + BT. The parameter A can be optimized from the enthalpies of formation and B from the entropies of formation of the phases. During optimization, the A and B parameters are optimized to satisfy both the thermodynamic data and phase diagram information within the uncertainty. The obtained optimized model parameters are given in Table 1. Fig. 8 shows the calculated phase diagram using the optimized parameters listed in Table 1. It can be seen that the experimental data are well reproduced by the modeling. Fig. 9 is the enlarged part on Nd-rich side of the calculated phase diagram with the experimental results; the inset indicates that the stoichiometric compound Nd7 Ni3 decomposes with a peritectic reaction. Fig. 10 is the enlarged part on Ni-rich side. Table 2 lists the experimental and calculated invariant reactions in the Nd–Ni system.

Fig. 10. Enlarged part of the calculated Nd–Ni phase diagram on Ni-rich side together with experimental data. Nd2 Ni17 is only stable in the temperature range 1282–1285 ◦ C. Table 2 Invariant reactions in the Nd–Ni system Temperature (◦ C) (at.% Ni in liquid)

Reaction

Liquid + bcc (Nd) ↔ dhcp (Nd) Liquid ↔ Nd3 Ni + dhcp (Nd) Liquid ↔ Nd3 Ni Liquid + Nd7 Ni3 ↔ Nd3 Ni Liquid ↔ Nd7 Ni3 + NdNi Liquid ↔ NdNi Liquid ↔ NdNi + NdNi2 Liquid + NdNi2 ↔ NdNi3 Liquid + ␣Nd2 Ni7 ↔ NdNi3 Liquid + NdNi5 ↔ ␣Nd2 Ni7 ␣Nd2 Ni7 ↔ ␤Nd2 Ni7 Liquid + NdNi5 ↔ ␤Nd2 Ni7 Liquid ↔ NdNi5 Liquid + NdNi5 ↔ Nd2 Ni17 Liquid ↔ Nd2 Ni17 + fcc (Ni) Nd2 Ni17 ↔ NdNi5 + fcc (Ni) Fig. 8. Calculated Nd–Ni binary phase diagram together with the experimental data.

a b

Modeling

Experimental

853 (12.99) 611 (21.84) 618 (0.25) 596 (30.05) 584 (33.81) 819 (50.0) 774 (56.54) 964 (64.90) 1068 (67.97) 1171 (71.30) 1171 1191 (72.13) 1405 (83.33)a 1286 (91.74) 1285 (91.75) 1277 (89.47)

853a , 863b 611a , 570 (∼19)b 618a , 590b 598a , 565b 585a , 540 (∼35)b 819a , 780b 773a , 720b 964a , 940 (63)b 1068a , 1030 (64)b 1172a 1172a 1193a , 1134 (66)b 1420b 1300 (∼91)b 1290 (91)b 1282a , 1250b

This work. Experimental, references [3,4].

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sion. The research was performed at Ames Laboratory. Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract Number W-7405ENG-82.

References

Fig. 11. The enthalpy formation of the solid phases and the experimental data. Reference state is solid Nd and Ni.

The enthalpies of formation of the solid phases calculated from the thermodynamic parameters at 25 ◦ C and the experimental data are shown in Fig. 11. The calculated results are in good agreement with the data of Guo and Kleppa [17].

5. Conclusions Nd7 Ni3 was found to decompose into Nd3 Ni and liquid with a peritectic reaction, not the reported eutectic reaction. DSC and DTA indicate significant difference from the reported data of the temperatures for all the invariant reactions and melting points in the Nd–Ni system. A self-consistent thermodynamic description of the Nd–Ni binary system is obtained by using the CALPHAD technique and WinPhad software. The comparison shows that the computed phase diagram and thermodynamic properties are in good agreement with the experimental data.

Acknowledgements This work was supported by the U.S. Department of Energy, Office of Basic Energy Science, Materials Science Divi-

[1] J. Yin, K. Tanaka, N. Tanaka, Mater. Trans. 43 (2002) 417– 420. [2] K. Kadir, T. Sakai, I. Uehara, J. Alloys Compd. 257 (1997) 115– 121. [3] Y.Y. Pan, P. Nash, in: P. Nash (Ed.), Phase Diagrams of Binary Nickel Alloys, ASM International, Materials Park, OH, 1991, pp. 225–229. [4] Y.Y. Pan, J. Zheng, Wuli Xuebao 34 (1985) 384–389. [5] R. Lemaire, D. Paccard, Bulletin de la Societe Francaise de Mineralogie et de Cristallographie 90 (1967) 311–315. [6] G.L. Olcese, J. Less-Common Met. 33 (1973) 71–81. [7] S.C. Abrahams, J.L. Bernstein, R.C. Sherwood, J.H. Wernick, H.J. Williams, Phys. Chem. Solids 25 (1964) 1069–1080. [8] A.E. Dwight, R.A. Conner Jr., J.W. Downey, Acta Cryst. 18 (1965) 835–839. [9] J.H. Wernick, S. Geller, Trans. Metall. Soc. AIME 218 (1960) 866–868. [10] R.C. Mansey, G.V. Raynor, I.R. Harris, J. Less-Common Met. 14 (1968) 329–336. [11] D. Paccard, R. Pauthenet, C. R. Acad. Sci., Ser. A B 264B (1967) 1056–1059. [12] A.V. Virkar, A. Raman, J. Less-Common Met. 18 (1969) 59– 66. [13] K.H.J. Buschow, A.S. Van der Goot, J. Less-Common Met. 22 (1970) 419–428. [14] J.H. Wernick, S. Geller, Acta Cryst. 12 (1959) 662–665. [15] A.E. Dwight, Trans. Am. Soc. Met. 53 (1961) 479–500. [16] G. Qi, Z. Li, K. Itagaki, A. Yazawa, Mater. Trans., JIM 30 (1989) 583–593. [17] Q. Guo, O.J. Kleppa, Metall. Mater. Trans. B: Process Metall. Mater. Process. Sci. 26B (1995) 275–279. [18] Y. Du, N. Clavaguera, Calphad 20 (1996) 289–296. [19] A.T. Dinsdale, Calphad 15 (1991) 317–425. [20] O. Redlich, A. Kister, Ind. Eng. Chem. 40 (1948) 345. [21] K.A. Gschneidner Jr., F.W. Calderwood, in: K.A. Gschneidner Jr., L. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare Earths, North Holland Physics Publishing, Amsterdam, 1986, pp. 1–161. [22] M. Huang, T.A. Lograsso, J. Alloys Compd. 395 (2005) 75– 79.