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Experimental investigation of phase equilibria in the Ce-Fe-Ni system at 950 and 750 °C
Calphad 64 (2019) 284–291
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Experimental investigation of phase equilibria in the Ce-Fe-Ni system at 950 and 750 °C
T
I. Fartushnaa,b, , M. Mardania, A. Khvana, V. Cheverikina, M. Gorshenkovc, A. Dinsdaled ⁎
a
Thermochemistry of Materials Scientific Research Centre, NUST MISIS, Moscow, Russia Department of Certification and Analytical Control, NUST MISIS, Moscow, Russia c Physical materials science department, NUST MISIS, Moscow, Russia d Hampton Thermodynamics Ltd, Hampton, UK b
ARTICLE INFO
ABSTRACT
Keywords: Ce-Fe-Ni system Experimental Phase diagram
The Ce-Fe-Ni system has been experimentally studied by microstructural analysis, electron microprobe analysis and X-ray diffraction over the whole concentration range. Isothermal sections at 950 and 750 °C for this system were constructed for the first time. The phase relations at 750 °C measured in this work are different from those published for 700 °C in particular because of the identification of a new binary phase Ce5Ni19. A continuous solid solution Ce(Fe,Ni)2 (MgCu2-type structure, cF24-Fd-3m), was found at 750 °C, with mutual substitution of Fe and Ni atoms. The liquid phase is present in the system both at 950 and 750 °C. Almost all binary-based phases at these temperatures show wide homogeneity regions and possess a constant Ce content.
1. Introduction Ce-based compounds with transitional metals (TM = Fe, Co, Ni), crystallizing with an MgCu2 phase structure, form materials with interesting properties. They have received much attention not only owing to their outstanding soft magnetic properties including high Curie temperature, high saturation magnetization, high permeability, and low coercivity, but also due to their various applications in electromagnetic microwave absorption, magnetic refrigeration systems, magnetic recording devices, magnetic resonance imaging and sensors [1]. Rare earth elements have been regarded as providing the basis for valuable new materials, due to their unique electrical and magnetic properties. The alloys of the R-Ni systems are widely used as hydrogen storage materials in engines and automobiles due to their high hydrogen absorption ability [2,3]. Doping rare earth elements into Fe-Ni alloys can significantly improve their magnetic properties. In order to obtain high quality samples during synthesis or heat treatment, a detailed knowledge of the underlying phase relationships is needed. Phase diagrams can serve as a road map for the purpose of process optimization. However, there is only a limited amount of data on phase equilibria in the Ce-Fe-Ni system, so there is a need to understand the existence and stability of compounds in this system to provide a basis for further research into the potential applications of R-Fe-TM alloys. The present investigation is part of a series of papers on phase equilibria in the Ce-Fe-TM (TM = Mn, Co, Ni) systems [4–8]. Phase equilibria associated with ⁎
crystallization in the Ce-Fe-Ni system were studied by us earlier [8] and liquidus and solidus projections and a Scheil diagram were constructed. A new binary compound Ce5Ni19 was found [8]. Phase equilibria below the solidus have been reported in works [9,10] where a partial isothermal section of this system at 700 °С had been presented. However, the isothermal section at 700 °С [9,10] does not include the new binary compound Ce5Ni19. In this paper our results on phase equilibria in the Ce-Fe-Ni system for whole concentration range at 950 and 750 °C are presented. The determined phase relations provide necessary data for the critical thermodynamic assessment of the Ce-Fe-Ni ternary system. The boundary binary systems accepted in this work were discussed in [8]. 2. Experimental methods The preparation and control of the sample compositions as well as the methods of microstructural analysis (SEM and electron probe microanalysis (EPMA)) and X-ray diffraction (XRD) are described in detail in our previous work [8]. The starting materials, used in the present work were: Ce – purity 99.8 wt%, Fe – purity 99.99 wt% and Ni – purity 99.9 wt%. The samples were prepared by arc-melting using a nonconsumable tungsten electrode under a high-purity argon atmosphere (99.998%). Before arc-melting, pure Ti was first melted to provide addition cleaning. To determine the exact composition of the samples energy dispersive X-ray spectroscopy (EDX) data was used.
Corresponding author. E-mail address: [email protected] (I. Fartushna).
https://doi.org/10.1016/j.calphad.2019.01.002 Received 30 October 2018; Received in revised form 19 December 2018; Accepted 12 January 2019 0364-5916/ © 2019 Elsevier Ltd. All rights reserved.
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Table 1 Phase composition of the annealed Ce-Fe-Ni alloys at 950 and 750 °C. #
Alloy Nominal
Measured
Ce
Fe
Ni
Ce
Fe
Ni
1
15
20
65
2
15
30
55
3
15
50
35
4 5 6 7 8 10
15 30 30 30 33 10
70 20 40 50 37 85
15 50 30 20 30 5
11 13 16 17 18
25 30 15 30 30
40 30 60 60 10
35 40 25 10 60
19
20
30
50
24
15
35
50
27
15
40
45
28
15
45
40
29 32 33 34
15 30 25 20
65 35 35 40
20 35 40 40
35 37
30 20
25 20
45 60
38
20
25
55
– 15.0 13.7 – 13.4 15.4 14.5 – 31.1 – – – 9.8 26.0 – – – – – 19.8 20.4 34.8 – 14.9 15.2 15.2 15.0 15.2 – – 18.2 20.5 – 20.6 20.3 20.9 20.7
– 21.2 29.3 – 52.0 49.6 71.0 – 38.8 – – – 85.3 39.2 – – – – – 30.1 30.4 50.9 – 40.4 40.6 44.9 45.5 64.8 – – 42.6 39.5 – 20.6 20.5 25.3 25.3
– 63.8 57.0 – 34.7 35.1 14.5 – 30.1 – – – 5.0 34.8 – – – – – 50.1 49.2 14.3 – 44.8 44.2 39.8 39.5 20.0 – – 39.2 40.0 – 58.8 59.2 53.8 54.0
The samples were annealed at 950 and 750 °C during 90–185 h in a tube furnace (Nabertherm RHTV 120/300/1700) with an accuracy in temperature of ± 3 °С. Annealing was achieved by holding the samples in an argon atmosphere (99.998%), which was additionally purified by titanium plate. The samples were placed in an Al2O3 crucible. Each sample was further wrapped in tantalum foil. After annealing the samples were quenched to retain the equilibrium microstructures. The samples for microstructure analysis were prepared with a (Struers Labopol-5) machine for grinding and polishing. The grinding procedure was carried out with SiC-paper and the polishing using a diamond suspension with, successively, grain sizes of 9, 3 and 1 µm. Chemical etching was not used. The prepared samples were examined with a scanning electron microscopy (SEM) using a TESCAN VEGA LMH microscope with a LaB6 cathode and an energy dispersive X-ray microanalysis system – Oxford Instruments Advanced AZtecEnergy. The measurement error in determining the concentration of elements using X-ray analysis was 0.1 wt%. X-ray powder diffraction (XRD) was carried out with a diffractometer Rigaku Ultima 4 using CoKα-filtered radiation. The fine powder was prepared by grinding in an agate mortar. The phases were determined by comparing diffraction patterns with the literature or a calculated pattern using the PowderCell [11] and WINXPOW [12] software packages. Rietveld analysis was performed and the mass fraction of each phase was determined using the PowderCell [11] software package. The lattice parameters were calculated by leastsquares refinement.
Heattreatment
Phase composition
950 °C, 750 °C 950 °C, 750 °C, 950 °C, 750 °C, 750 °C, 750 °C, 750 °C, 750 °C, 750 °C, 950 °C, 750 °C, 750 °C, 750 °C, 750 °C, 750 °C, 950 °C, 750 °C, 950 °C, 750 °C, 950 °C, 750 °C, 950 °C, 750 °C, 950 °C, 750 °C, 750 °C, 750 °C, 750 °C, 950 °C, 750 °C, 750 °C, 950 °C, 750 °C, 950 °C, 750 °C,
(γFe,Ni) + CeNi5 (γFe,Ni) + CeNi5 (γFe,Ni) + CeNi5 (γFe,Ni) + CeNi5 + Ce5Ni19 (γFe,Ni) + Ce2Ni7 + CeNi3 (αFe) + Ce2Ni7 + CeNi3 (αFe) + CeNi3 + Ce(Fe,Ni)2 CeNi3 + Ce(Fe,Ni)2 (αFe) + Ce(Fe,Ni)2 (αFe) + Ce2Fe17 + Ce(Fe,Ni)2 (αFe) + Ce(Fe,Ni)2 L + (γFe,Ni) (αFe) + Ce2Fe17 + Ce(Fe,Ni)2 CeNi3 + Ce(Fe,Ni)2 CeNi3 + Ce(Fe,Ni)2 (αFe) + CeNi3 Ce2Fe17 + Ce(Fe,Ni)2 L + CeNi3 CeNi3 + Ce(Fe,Ni)2 (γFe,Ni) + Ce2Ni7 + Ce5Ni19 (αFe) + Ce2Ni7 (γFe,Ni) + CeNi5 (αFe) + Ce5Ni19 (γFe,Ni) + CeNi5 + Ce5Ni19 (αFe) + Ce2Ni7 (γFe,Ni) + Ce2Ni7 (αFe) + Ce2Ni7 (αFe) + CeNi3 CeNi3 + Ce(Fe,Ni)2 CeNi3 + Ce(Fe,Ni)2 (γFe,Ni) + Ce2Ni7 + CeNi3 (αFe) + Ce2Ni7 + CeNi3 CeNi3 + Ce(Fe,Ni)2 CeNi5 + Ce5Ni19 (αFe) + Ce5Ni19 + Ce2Ni7 (γFe,Ni) + Ce2Ni7 + Ce5Ni19 (αFe) + Ce2Ni7
90 h 90 h 185 h 90 h 95 h 95 h 95 h 95 h 95 h 95 h 90 h 95 h 95 h 95 h 95 h 95 h 90 h 95 h 90 h 185 h 90 h 95 h 90 h 185 h 90 h 185 h 95 h 95 h 95 h 90 h 95 h 95 h 90 h 95 h 90 h 95 h
3. Results and discussion The phase equilibria in the Ce-Fe-Ni system were studied, by means of SEM, EPMA and X-ray analysis of the 38 ternary alloys annealed at 950 and 750 °C for 90–185 h. The phase compositions of the studied alloys and the microprobe results are given in Tables 1 and 2, respectively. A few measurements for each phase were performed using the microprobe method. On the basis of all the available alloys, the isothermal sections of the Ce-Fe-Ni system at 950 and 750 °C, covering entire composition range, were constructed. These are shown in Fig. 1. The composition of each alloy is marked in Fig. 1. Note: in Table 2 the average values of a number of measurements for each phase are given with the mean squares deviations, while in Fig. 1 all the measured values are shown. The microstructures of some annealed samples from different phase regions are presented in Fig. 2. The XRD patterns of some annealed alloys are shown in Fig. 3. 3.1. Solid phases Significant amounts of the third component dissolves into almost all of the binary-based intermetallic phases. The phases CeNi5, Ce5Ni19, Ce2Ni7 and CeNi3 extend deeply into the ternary system, forming wide solid-solutions with significant variations in their lattice parameters. The homogeneity regions of these phases possess a constant Ce content. The extension of these solid solutions and their homogeneity ranges are shown in the isothermal sections of the Ce-Fe-Ni system. 285
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Table 2 The composition of the Ce-Fe-Ni phases according to the EPMA examination. #
Alloy
Heat-treatment
Nominal
1 2
3
Measured
Ce
Fe
Ni
Ce
Fe
Ni
15 15
20 30
65 55
15.0 13.7
21.2 29.3
63.8 57.0
750 °C, 95 h 950 °C, 90 h
-
-
-
750 °C, 185 h
13.4
52.0
34.7
950 °C, 90 h
15.4
49.6
35.1
750 °C, 95 h
15
50
35
4
15
70
15
14.5
71.0
14.5
750 °C, 95 h
6
30
40
30
31.1
38.8
30.1
750 °C, 95 h
10
10
85
5
9.8
85.3
5.0
750 °C, 95 h
11
25
40
35
26.0
39.2
34.8
750 °C, 95 h
19
20
30
50
19.8
30.1
50.1
950 °C, 90 h
20.3
30.5
49.2
750 °C, 185 h
24
15
35
50
34.8
50.9
14.3
950 °C, 90 h
27
15
40
45
14.9
40.3
44.8
750 °C, 95 h 950 °C, 90 h
28
15
45
40
15.3 15.2 15.0
40.1 44.9 45.5
44.6 39.8 39.5
750 °C, 185 h 950 °C, 90 h 750 °C, 185 h
29
15
65
20
15.2
64.8
20.0
750 °C, 95 h
34
20
40
40
18.2
42.6
39.2
950 °C, 90 h
20.5
39.5
40.0
750 °C, 95 h
20.6
20.6
58.8
950 °C, 90 h
20.3
20.5
59.2
750 °C, 95 h
37
38
20
20
20
25
60
55
20.9
25.3
53.8
20.7
25.3
54.0
950 °C, 90 h 750 °C, 95 h
The Laves phase compounds CeFe2 and CeNi2 form a continuous solid solution Ce(Fe,Ni)2 (MgCu2-type structure, cF24-Fd 3 m) at 750 °C. The homogeneity region of the phase follows the isoconcentrate 33.3 at % Ce due to Ni/Fe mutual substitution, while it is almost a linear phase in terms of the concentration of Ce, where its width does not exceed 1 at % (Table 2). The continuous solid solution between CeNi2 and CeFe2 was reported in [9,10,13,14]. At a temperature of 950 °C this continuous solid solution Ce(Fe,Ni)2 no longer exists. The crystal structures and the lattice parameters of the phases in the Ce-Fe-Ni system are summarized in Table 3.
Microprobe results, at.% Phase
Ce
Fe
CeNi5 (γFe,Ni) CeNi5 (γFe,Ni) CeNi5 Ce5Ni19 (γFe,Ni) Ce2Ni7 CeNi3 (αFe) Ce2Ni7 CeNi3 (αFe) Ce(Fe,Ni)2 CeNi3 (αFe) Ce(Fe,Ni)2 (αFe) Ce2Fe17 Ce(Fe,Ni)2 CeNi3 Ce(Fe,Ni)2 (γFe,Ni) + Ce2Ni7 Ce5Ni19 (αFe) Ce2Ni7 (γFe,Ni) CeNi5 Ce5Ni19 (γFe,Ni) CeNi5 Ce2Ni7 Ce2Ni7 (γFe,Ni) (αFe) Ce2Ni7 (αFe) CeNi3 (γFe,Ni) Ce2Ni7 (αFe) Ce2Ni7 CeNi3 CeNi5 Ce5Ni19 (αFe) + Ce5Ni19
17.2 ± 0.1 0.7 ± 0.1 17.0 ± 0.1 0.8 ± 0.1 16.9 ± 0.3 21.1 ± 0.1 0.5 ± 0.2 22.6 ± 0.1 25.4 ± 0.2 0.9 ± 0.1 22.5 ± 0.3 25.3 ± 0.2 0.9 ± 0.1 33.0 ± 0.3 25.5 0.9 33.4 ± 0.5 0.5 ± 0.1 10.4 ± 0.4 33.2 25.9 33.4 0.9 ± 0.2 22.7 ± 0.1 21.3 ± 0.1 0.9 ± 0.1 22.7 ± 0.1 0.7 ± 0.1 17.1 ± 0.1 21.1 0.7 16.9 ± 0.1 21.2 ± 0.2 22.5 ± 0.1 0.8 ± 0.1 0.8 ± 0.1 22.7 ± 0.1 0.9 ± 0.2 25.2 ± 0.1 0.1 22.7 ± 0.1 0.9 ± 0.1 22.6 25.4 ± 0.1 17.2 ± 0.1 21.2 ± 0.1 5.2 5.86 6.7 22.7 ± 0.2 21.4 ± 0.2 0.9 22.7 ± 0.1 21.3 ± 0.1 2.9 22.9 ± 0.1
14.0 ± 74.1 ± 16.9 ± 70.7 ± 20.6 ± 14.8 ± 89.3 ± 25.6 ± 28.0 ± 96.1 ± 27.6 ± 23.7 ± 97.9 ± 35.9 ± 44.4 97.9 34.3 ± 99.0 ± 86.0 ± 44.7 38.4 28.8 86.0 ± 21.2 ± 22.2 ± 95.1 ± 22.9 ± 82.8 ± 25.7 ± 16.5 83.8 27.7 ± 21.4 ± 17.7 ± 87.2 ± 95.9 ± 20.8 ± 97.6 ± 41.5 ± 90.3 26.5 ± 96.2 ± 28.8 25.4 ± 24.9 ± 19.0 ± 75.0 71.14 71.8 14.0 ± 15.8 ± 84.3 20.2 ± 22.0 ± 84.05 17.7 ±
(αFe) + Ce2Ni7 Ce2Ni7 Ce5Ni19 (γFe,Ni) Ce2Ni7 Ce5Ni19 (αFe) + Ce2Ni7 Ce2Ni7
Ni
0.1 0.1 0.3 0.9 0.5 0.2 0.3 0.2 0.2 0.1 0.6 0.3 0.1 0.1 0.6 0.1 0.6
0.2 0.5 0.4 0.2 0.9 0.1 0.2 0.1 0.1 0.4 0.1 0.4 0.4 0.3 0.2 0.2 0.5 0.2 0.1 0.2
0.1 0.2 0.2 0.2 0.1
68.8 ± 0.2 25.2 ± 0.1 66.1 ± 0.1 28.5 ± 0.9 62.5 ± 0.2 64.2 ± 0.2 10.2 ± 0.1 51.8 ± 0.1 46.6 ± 0.1 3.0 ± 0.1 49.9 ± 0.3 51.0 ± 0.1 1.2 ± 0.1 31.1 ± 0.3 30.1 1.2 32.3 ± 0.1 0.5 ± 0.1 3.6 ± 0.2 22.1 35.7 37.8 13.0 ± 0.1 56.1 ± 0.5 56.5 ± 0.3 4.0 ± 0.2 55.5 ± 0.9 16.5 ± 0.1 57.2 ± 0.2 62.4 15.5 55.4 ± 0.2 57.5 ± 0.4 59.8 ± 0.3 12 ± 0.1 3.4 ± 0.3 56.5 ± 0.4 1.5 ± 0.1 33.3 ± 0.2 9.6 50.8 ± 0.4 2.9 ± 0.4 48.6 49.2 ± 0.3 57.9 ± 0.1 59.8 ± 0.2 19.8 23 21.5 63.2 ± 0.1 62.8 ± 0.2 14.7 57.0 ± 0.1 56.7 ± 0.1 13.1 59.4 ± 0.1
Among the binary compounds, CeNi3 (CeNi3-type structure, hP24P63/mmc) has the widest homogeneity region at 750 °C and according to the EPMA data of alloy #4 (Table 2, Fig. 1 b, 2 a) dissolves up to 44.4 at% Fe. According to [9,10] the solubility of Fe in CeNi3 at 700 °C already is 41 at%. This phase is located along the isoconcentration line of 25 at% Ce and extends widely along this isoconcentrate, reflecting significant mutual substitution of Ni and Fe atoms. The single-phase microstructures of alloys # 11 and #33, together with the microprobe results, indicate the constant Ce content in the phase. Traces of the Laves phase Ce(Fe,Ni)2 are present along the grain boundaries. The 286
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Fig. 1. The isothermal sections of the Ce-Fe-Ni system at 950 °C (a) and 750 °C (b): ◐ – two-phase samples, • – three-phase samples, △ – EPMA data.
solubility of Fe in this phase at 950 °C is less and does not exceed 30 at %. According to EPMA data on alloys ## 3, 34 the maximum solubility of Fe in Ce2Ni7 (Ce2Ni7-type structure, hP36-P63/mmc) is 26 and 28 at% at 950 and 750 °C, respectively (Table 2, Figs. 1, 2 b). In the works [9,10] the solubility of Fe in Ce2Ni7 at 700 °C was measured as 16 at% Fe. The chemical compositions of Ce2Ni7 (see Table 2) indicated that this phase was a line compound with a constant Ce content. The newly identified binary phase Ce5Ni19 (Ce5Co19-type structure, hR72-R 3 m), found by us recently, was detected both at 950 and 750 °C. The maximum solubility of Fe in this phase at 950 °C reaches 22 at% according to EPMA data of the alloys # 19 and # 38 (Table 2, Fig. 1 a, 2 c). At a temperature of 750 °C the solubility of Fe in Ce5Ni19 is smaller and according to EPMA data of alloy # 37 is 16 at% (Table 2, Fig. 1 b). In [28,29] it was reported that the ternary solid solution of Fe in CeNi5 (CaCu5-type structure, hP6-P6/mmm) extends up to the CeNi4Fe composition. However, our results show that the phase extends beyond the CeNi4Fe composition. In [30] the homogeneity range in CeFexNi5-x was reported up to x = 1.75. According to our data the solubility of Fe in CeNi5 reaches 27.7 and 20.6 at% at 950 and 750 °C, respectively, according to the EPMA data of alloys #27 and #2 (Table 2, Figs. 1, 2 d, e and 3 a). The solubility of Fe in CeNi5 at 700 °C in [9,10] was measured as 19 at% Fe. The homogeneity range of the Ce2Fe17 (Zn17Th2-type structure, hR57-R 3 m) is significantly smaller, the solubility of Ni in Ce2Fe17 according to EPMA data of alloy # 10 was measured as 3.6 at% at 750 °C (Table 2, Figs. 1, 2 f). At a temperature of 950 °C the solubility of Ni in Ce2Fe17 does not exceed 2.5 at%. It should be noted that we have not observed the low temperature modification of the Ce2Fe17 phase (αCe2Fe17) with a hexagonal Th2Ni17-type structure and observed only the high temperature modification (βCe2Fe17) with a rhombohedral Th2Zn17-type structure. The maximum solubility of cerium in the (γFe,Ni)-phase and the (αFe)-phase was determined to be no more than 1 at% (see Table 2). No ternary compound was found.
phase L + (γFe,Ni) + Ce2Fe17 and L + (γFe,Ni) + CeNi3 appear. Moreover, at this temperature the Laves phase Ce(Fe,Ni)2 does not exist, it forms at a lower temperature. The isothermal section at 950 °C is characterized by the presence of narrow three-phase regions and wide two-phase regions. Near the Fe-Ni boundary, a continuous single phase region of (γFe,Ni) was obtained. The (γFe,Ni)-phase defines the character of phase equilibria at this temperature and co-exists with all phases, namely, with Liquid (L), CeNi5, Ce5Ni19, Ce2Ni7, CeNi3, Ce2Fe17 and forms five three-phase fields (γFe,Ni) + CeNi5 + Ce5Ni19, (γFe,Ni) + Ce5Ni19 + Ce2Ni7, (γFe,Ni) + Ce2Ni7 + CeNi3, L + (γFe,Ni) + CeNi3, L + (γFe,Ni) + Ce2Fe17 and the corresponding two-phase regions. The existence of the three-phase regions (γFe,Ni) + CeNi5 + Ce5Ni19 and (γFe,Ni) + Ce2Ni7 + CeNi3 as well as their locations, were established from the SEM, EPMA and XRD data of alloys #27 and #3 which had been annealed at 950 °C for 90 h (Tables 1 and 2, Fig. 1 a, 2 d and 3 a, b). The BSE images of annealed alloy #27, as shown in Fig. 2 d, reveal it to be located in the three-phase region which consists of a dark (γFe,Ni)-phase, a grey Ce5Ni19 and a dark grey CeNi5 phase. It should be noted, that according to the XRD data of alloy #27, the (γFe,Ni)-phase was not observed in this alloy, instead the (αFe)-phase was clearly identified, because the (γFe,Ni)-phase transformed into (αFe)-phase on cooling (Fig. 3 a). In the alloy #3, which is located in the three-phase region (γFe,Ni) + Ce2Ni7 + CeNi3, the amount of the CeNi3 phase is very low (Fig. 3 b). This gives us reason to believe that the composition of this alloy is almost on the boundary tie-line (γFe,Ni) + Ce2Ni7. The composition of these phases is well established by the microprobe method (Table 2). The XRD pattern (Fig. 3 b) confirmed this three-phase equilibrium. It should be noted, that according to the X-ray data, in addition to the (γFe,Ni)-phase, the (αFe)-phase also can be clearly identified in this alloy, as it was formed during cooling (Fig. 3 b). According to the SEM, EPMA and XRD results alloys # 19 and # 38, annealed at 950 °C at 90 h, are located in the three-phase region (γFe,Ni) + Ce5Ni19 + Ce2Ni7 (Tables 1 and 2, Fig. 1 a, 2 c). The microstructure of alloy #19 is illustrated in Fig. 2 c and clearly shows three phases: dark, grey and light-grey. These correspond to (γFe,Ni), Ce5Ni19 and Ce2Ni7, respectively. The BSE images and X-ray diffraction patterns of alloys ## 1, 2, 24 are nearly identical. It illustrates that those alloys are located in the two-phase region between (γFe,Ni) + CeNi5. On increasing the Ni content of the alloys, the Ni content of both phases increases progressively, as we can see in the Table 2.
3.2. Isothermal section at 950 °C Based on the experimental results obtained in this work and the information of relevant binary systems in the literature, the isothermal section of the Ce-Fe-Ni system at 950 °C was constructed and is shown in Fig. 1 a. The isothermal section at 950 °C differs from the solidus surface [8]. In the Ce-rich region at 950 °C there is a wide liquid phase domain. In addition, at 950 °C two three-phase fields with the liquid 287
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Fig. 2. Microstructures of annealed alloys of the Ce-Fe-Ni system: a – 15Ce-70Fe-15Ni (#4), 750 °C, ×2000, (αFe) + CeNi3 + Ce(Fe,Ni)2; b – 15Ce-50Fe-35Ni (#3), 750 °C, ×2000, (αFe) + Ce2Ni7 + CeNi3; c – 20Ce-30Fe-50Ni (#19), 950 °C, ×2000, (γFe,Ni) + Ce5Ni19 + Ce2Ni7; d – 15Ce-40Fe-45Ni (#27), 950 °C, ×2000, (γFe,Ni) + Ce5Ni19 + CeNi5; e – 15Ce-30Fe-55Ni (#2), 750 °C, ×2000, (γFe,Ni) + Ce5Ni19 + CeNi5; f – 10Ce-85Fe-5Ni (#10), 750 °C, ×2000, (αFe) + Ce2Fe17 + Ce(Fe,Ni)2.
The location of the liquid phase boundary was determined by the construction and processing of a series of intersecting isopleths, so that each alloy was located in at least three of them. The isopleths were
constructed based on DTA results [8]. The vertices of the respective tieline triangles involving the liquid phase have been indicated by dashed lines in Fig. 1 a. 288
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Fig. 3. X-ray diffraction patterns of annealed Ce-Fe-Ni alloys: a – 15Ce-40Fe-45Ni (#27), 950 °C, (αFe) + Ce5Ni19 + CeNi5; b – 15Ce-50Fe-35Ni (#3), 950 °C, (γFe,Ni) + (αFe) + Ce2Ni7 + CeNi3.
3.3. Isothermal section at 750 °C
+ Ce2Ni7, involving the Ce5Ni19 phase, exists at 750 °C (Tables 1 and 2, Fig. 1 b). It was determined by SEM and EMPA on alloy # 37 annealed at 750 °C/95 h, which contains three phases: (αFe) + Ce5N19 + Ce2Ni7. The location of the corners of this three-phase field was established by microprobe measurements on the solid phases (Table 2). Thus, based on phase composition in the alloys above, the threephase regions (γFe,Ni) + CeNi5 + Ce5Ni19 and (αFe) + Ce5Ni19 + Ce2Ni7 were obtained, instead of the three-phase region (Fe) + CeNi5 + Ce2Ni7, given in Refs. [9,10] at 700 °C. According to the SEM and EPMA results of alloys # 3 and #34 annealed at 750 °C/95 h, these are located in the three-phase region (αFe) + Ce2Ni7 + CeNi3 (Tables 1, 2, Fig. 1 b, 2 b). The microstructure of these samples (Fig. 2 b) was composed of the dark, grey and light phases. These three phases were identified as (αFe), Ce2Ni7 and CeNi3, respectively. SEM and EPMA analysis revealed that alloy #4, annealed at 750 °C for 95 h, is in the (αFe) + CeNi3 + Ce(Fe,Ni)2 three-phase region. In the SEM micrograph of this sample, Fig. 2 a, all three phases are well distinguished. They were identified by microprobe analysis as follows: the dark grains correspond to the (αFe)-phase, while the grey and the light-grey correspond to CeNi3 and Ce(Fe,Ni)2, respectively. The location of the corners of the three-phase field was established by microprobe measurements on the solid phases (Table 2). Fig. 2 f represents the micrograph of sample #10. Evidently, this alloy was in a three-phase equilibrium (αFe) + Ce2Fe17 + Ce(Fe,Ni)2. The dark grains and grey grains were identified as (αFe) and Ce2Fe17, respectively, based on the SEM and EPMA results. The light-grey grains are Ce(Fe,Ni)2.
Based on the experimental results obtained in this work and the information of relevant binary systems in the literature, the isothermal section of the Ce-Fe-Ni system at 750 °C was constructed and is shown in Fig. 1 b. In the concentration range up to 33.3 at% Ce the isothermal section at 750 °C is very similar in character to the solidus surface [8]. The differences involve a change in the solubility of the third component in each of the phases and, thus, in a shift of the corners of the three-phase fields. As the majority of the phases stable at 750 °C have significant homogeneity ranges, these shifts are significant. In addition, at 750 °C the three-phase field (αFe) + (γFe,Ni) + Ce2Ni7 appears. In the Ce-rich region at 750 °C there is a wide liquid phase domain. The (αFe) and (γFe,Ni) phases define the character of the phase equilibria at 750 °C and co-exists with all intermetallic phases CeNi5, Ce5Ni19, Ce2Ni7, CeNi3, Ce(Fe,Ni)2, Ce2Fe17 and form six three-phase fields (γFe,Ni) + CeNi5 + Ce5Ni19, (αFe) + (γFe,Ni) + Ce5Ni19, (αFe) + Ce5Ni19 + Ce2Ni7, (αFe) + Ce2Ni7 + CeNi3, (αFe) + CeNi3 + Ce(Fe,Ni)2 and (αFe) + Ce(Fe,Ni)2 + Ce2Fe17 as well as the corresponding two-phase regions. The existence of the three-phase region (γFe,Ni) + CeNi5 + Ce5Ni19 was established based on the results of SEM and EPMA on sample # 2 annealed at 750 °C/185 h (Tables 1 and 2, Fig. 1 b, 2 e). The microstructure of this sample (Fig. 2 e) confirms that it consists of three phases, namely (γFe,Ni) (dark grains), CeNi5 (grey grains) and Ce5Ni19 (light-grey grains). Moreover, another very narrow three-phase region (αFe) + Ce5Ni19 289
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Table 3 The crystal structures and lattice parameters of the Ce-Fe-Ni phases.
1
Phase
Crystal structure
Lattice parameters, Å
Alloy
Refs.
δFe γFe αFe
W, cI2-Im3m Cu, cF4-Fm-3m W, cI2-Im3m
(Ni) (γFe,Ni)
Cu, cF4-Fm-3m Cu, cF4-Fm-3m
δCe γCe Ce2Fe17 ht
W, cI2-Im3m Cu, cF4-Fm3m Zn17Th2, hR57-R-3m
Ce2Fe17 rt CeNi5
Th2Ni17, hP38-P63/mmc CaCu5, hP6-P6/mmm
Ce5Ni19
Ce5Co19, hR72-R-3m
Ce2Ni7
Ce2Ni7 hP36-P63/mmc
CeNi3
`CeNi3 hP24-P63/mmc
CeFe2
MgCu2, cF24-Fd-3m
CeNi2
MgCu2, cF24-Fd-3m
Ce(Fe,Ni)2 CeNi Ce7Ni3
MgCu2, cF24-Fd-3m TlI, oS8-Cmcm Th7Fe3, hP20-P63mc
a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a
at 915 °C at 25 °C 15Ce-50Fe-35Ni (#3), 950 °C 15Ce-40Fe-45Ni (#27), 950 °C 50 at.% Ni 15Ce-50Fe-35Ni (#3), 950 °C 89Fe-11Ce, annealed at 1050 °C,40 hours 20.8Ce-79.2Ni (#41), as-cast 15Ce-40Fe-45Ni (#27), 950 °C 20.8Ce-79.2Ni (#41), as-cast 15Ce-40Fe-45Ni (#27), 950 °C 20.8Ce-79.2Ni (#41), as-cast 15Ce-50Fe-35Ni (#3), 950 °C 20.8Ce-79.2Ni (#41), as-cast 15Ce-50Fe-35Ni (#3), 950 °C 66.6Fe-33.4Ce, annealed at 900 °C, 40 hours 20.8Ce-70.2Ni (41), as-cast -
[15] [15] [15] Th. w. Th. w. [16] [17] Th. w. [15] [15] [18] [19] [20] [21] [22] [23] [8] Th. w. Th. w. Th. w. [24] [25] [8] Th. w. [25] [8] Th. w. [18] [20] [14] [8] [26] [27]
= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
2.9315 3.6467 2.8665 2.870(1) 2.875(1) 3.52 3.5750 3.590(1) 4.12 5.1610 8.485, c = 12.433 8.496, c = 12.414 8.482(2), c = 12.410(5) 8.49, c = 8.281 4.860, c = 3.996 4.882, c = 4.004 4.877(1), c = 3.993(1) 4.935(1), c = 4.050(1) 4.924(1), c = 48.418(5) 4.972(1), c = 49.010(5) 4.905, c = 24.38 4.927, c = 24.45 4.928(1), c = 24.46(1) 4.994(1), c = 24.514(5) 4.960, c = 16.56 4.948, c = 16.48 5.028(2), c = 16.59(2) 7.296 7.295 7.2100 7.215(1)
= 3.788, b = 10.556, c = 4.366 = 9.926, c = 6.311
Th. w. − results of this work
Our studies of phase equilibria at the solidus temperatures had shown that there is a continuous solid solution (γFe,Ni) between Ni and Fe. In contrast, however, these results for 750 °С show no continuous solid solution (γFe,Ni) because this phase in the Fe-rich region of the boundary binary Fe-Ni system transforms into the (αFe)-phase. Therefore, at 750 °C the two phase region (αFe) + (γFe,Ni) and the very narrow three-phase region (αFe) + (γFe,Ni) + Ce5Ni19 should be present. In conclusion, comparing the phase relations at 950 and 750 °C, similar topology of the isothermal sections at these two temperatures can be noted within the Ni-rich region, as can see in Fig. 1 a and b. For example, at both temperatures, the (γFe,Ni)-phase and four binary phases CeNi3, Ce2Ni7, Ce5Ni19, CeNi5 were found and all were line compounds. At both temperatures a wide liquid phase domain in the Ce-rich region is present. A total of six three-phase equilibria were observed in the isothermal section at 750 °C, in contrast to five at 950 °C. The continuous solid solution Ce(Fe,Ni)2, between Laves phases CeFe2 and CeNi2, forms at 750 °C, while at 950 °C Ce(Fe,Ni)2 does not exists. Differences between these two sections also existed in the Fe-rich region. For example, at 950 °C two three-phase fields with the liquid phase appear, while at 750 °C the (αFe)-phase appears. The isothermal section at 750 °C constructed in this work was further compared with the reported one at 700 °C. Some differences could be found in the phase relations between the current work and the studies [9,10]. Most evidently, the new binary phase Ce5Ni19, found by us previously, was absent in works [9,10]. Moreover, in contrast to the current section at 750 °C, a much lower solubility of Fe in Ce2Ni7 was observed in the section at 700 °C [9,10].
4. Conclusions 1. Phase equilibria in the Ce-Fe-Ni system at 950 and 750 °C covering the entire concentration range were studied. Isothermal sections at these temperatures were constructed. The phase relations at 750 °C measured in this work are different from those which had been obtained at 700 °C [9,10] by the inclusion of the new binary phase Ce5Ni19, found by us previously. 2. A continuous solid solution Ce(Fe,Ni)2 (MgCu2-type structure, cF24Fd 3 m) forms at 750 °C and contains 33.3 at% Ce due to mutual substitution of Fe and Ni atoms. A continuous (γFe,Ni) phase field at 750 °C near the Fe-Ni boundary was observed to cover a composition region from the Fe-rich corner to the Ni-rich corner. 3. The isothermal section at 950 °C is characterized by the presence of five three-phase fields (γFe,Ni) + CeNi5 + Ce5Ni19, (γFe,Ni) + Ce5Ni19 + Ce2Ni7, (γFe,Ni) + Ce2Ni7 + CeNi3, L + (γFe,Ni) + CeNi3, L + (γFe,Ni) + Ce2Fe17 and the corresponding two-phase regions. 4. The isothermal section at 750 °C is characterized by the presence of six three-phase fields (γFe,Ni) + CeNi5 + Ce5Ni19, (αFe) + (γFe,Ni) + Ce5Ni19, (αFe) +Ce5Ni19 + Ce2Ni7, (αFe) + Ce2Ni7 + CeNi3, (αFe) + CeNi3 + Ce(Fe,Ni)2 and (αFe) + Ce(Fe,Ni)2 + Ce2Fe17 as well as the corresponding two-phase regions. 5. Almost all of the binary-based phases, with the exception of Ce2Fe17, have substantial solubility of the third component. The maximum solubility of Fe in CeNi3 is 44.4 and 30 at% at 750 and 950 °С, respectively. The solubility of Fe in Ce2Ni7 is 26 and 28 at% at 950 and 750 °C, respectively. The maximum solubility of Fe in Ce5Ni19 at 950
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and 750 °C reaches 22 and 16 at%, respectively. The solubility of Fe in CeNi5 is 27.7 and 20.6 at% at 950 and 750 °C, respectively. The solubility of Ni in Ce2Fe17 is ~2.5 and 3.6 at 950 and 750 °C, respectively. All binary-based phases are line compounds.
[12] WINXPOW Software, Stoe & Cie GmbH Darmstadt, 1998. [13] I.R. Harris, G. Longworth, X-ray and Mössbauer studies of the pseudo-binary system Ce(Fe1−xNix)2, J. Less Common Met. 45 (1) (1976) 63–77. [14] R.C. Mansey, G.V. Raynor, I.R. Harris, Rare-earth intermediate phases VI. Pseudobinary systems between cubic laves phases formed by rare-earth metals with iron, cobalt, nickel, aluminum and rhodium, J. Less Common Met. 14 (3) (1968) 337–347. [15] T.B. Massalski (Ed.), Binary Alloy Phase Diagrams, 2nd edition, ASM International, Metals Park, OH, 1990. [16] H.L. Luo, P.E. Duwez, Solid solutions of rhodium with copper and nickel, J. LessCommon Met. 6 (1964) 248–249. [17] G. Cacciamani, J. De Keyzer, R. Ferro, U.E. Klotz, J. Lacaze, P. Wollants, Critical evaluation of the Fe–Ni, Fe–Ti and Fe–Ni–Ti alloy systems, Intermetallics 14 (10–11) (2006) 1312–1325. [18] Y.C. Chuang, C.H. Wu, Z.B. Shao, Investigation of the Ce-Fe binary system, J. Less Common Met. 136 (1) (1987) 147–153. [19] S. Kramp, M. Febri, P. l'Héritier, P. mEzekwenna, J.C. Joubert, Synthesis of magnetic intermetallics and their interstitial hydrides, J. Magn. Magn. Mater. 157–158 (1996) 73–75. [20] A.V. Khvan, M. Mardani, I.V. Fartushna, E.A. Syutkin, V.V. Cheverikin, A.T. Dinsdale, An experimental investigation of thermodynamic properties of βFe17Ce2, Fe2Ce, and ternary Fe13.1-11.0Mn3.9-6.0Ce2 (τ1) phase, Thermochim. Acta 672 (2019) 1–8. [21] K.H.J. Buschow, J.S. Van Wieringen, Crystal structure and magnetic properties of cerium‐iron compounds, Physica Status Solidi (b) 42 (1) (1970) 231–239. [22] F. Pourarian, W.E. Wallace, Hydrogen storage in CeNi5−xCux, J. Less-Common Met. 87 (1982) 275–281. [23] F. Meyer Liautaud, A. Pasturel, C.H. Allibert, C. Colinet, Thermodynamic study of the valence state of cerium and hydrogen storage in Ce(Ni1−xCux)5 compounds, J. Less-Common Met. 110 (1985) 119–126. [24] A.V. Virkar, A. Raman, Crystal structures of AB3 and A2B7 rare earth-nickel phases, J. Less-Common Met. 18 (1969) 59–66. [25] K.H.J. Buschow, A.S. Van Der Goot, The crystal structure of rare-earth nickel compounds of the type R2Ni7, J. Less-Common Met. 22 (4) (1970) 419–428. [26] A.E. Dwight, R.A. Conner Jr., J.W. Downey, Equiatomic compounds of the transition and lanthanide elements with Rh, Ir, Ni and Pt, Acta Crystallogr. 18 (1965) 835–839. [27] G.L. Olcese, Crystal structure and magnetic properties of some 7:3 binary phases between lanthanides and metals of the 8th group, J. Less-Common Met. 33 (1973) 71–81. [28] H. Oesterreicher, F.T. Parker, M. Misroch, Giant intrinsic magnetic hardness in RFe5−xNix (R= rare earth, x = 4 to 5), Appl. Phys. 16 (2) (1978) 185–189. [29] F. Pourarian, A. Pedziwiatr, W.E. Wallace, Magnetic properties of RNi4Fe alloys (R = La, Ce, or Y), J. Appl. Phys. 55 (6) (1984) 1981–1983. [30] F. Pourarian, M.Z. Liu, B.Z. Lu, M.Q. Huang, W.E. Wallace, Magnetic and crystallographic characteristics of CeNi5-xMx (M = Fe, Mn) alloys and their hydrides, J. Solid State Chem. 65 (1) (1986) 111–117.
Acknowledgments The work was carried out with financial support from the Ministry of Science and Higher Education of the Russian Federation in the framework of Increase Competitiveness Program of NUST “MISiS” (No. К2-2018-010), implemented by a governmental decree dated 16th of March 2013, N 211. References [1] P. Scoboria, A. Harrus, B. Andraka, T. Mihalisin, S. Raaen, R.D. Parks, L III absorption, resistivity, and susceptibility studies of the Ce(Ni1−xCox)2 and Y (Ni1−xCox)2 systems, J. Appl. Phys. 55 (1984) 1969–1971. [2] R.A. Guidotti, G.B. Atkinson, M.M. Wong, Hydrogen absorption by rare earthtransition metal alloys, J. Less Common Met. 52 (1) (1977) 13–28. [3] G. Sandrock, A panoramic overview of hydrogen storage alloys from a gas reaction point of view, J. Alloy. Compd. 293 (1999) 877–888. [4] I. Fartushna, A. Khvan, A. Dinsdale, V. Cheverikin, D. Ivanov, A. Kondratiev, An experimental investigation of liquidus and solidus projections for the Fe-Mn-Ce system, J. Alloy. Compd. 654 (2016) 424–434. [5] I. Fartushna, A. Khvan, A. Dinsdale, V. Cheverikin, D. Ivanov, A. Kondratiev, Phase equilibria in the Fe-Mn-Ce system at 900 °C, J. Alloy. Compd. 658 (2016) 331–336. [6] I. Fartushna, M. Mardani, A. Khvan, E. Donkor, V. Cheverikin, A. Kondratiev, A. Dinsdale, Investigation of phase equilibria in the Ce-Co-Fe system during solidification, J. Alloy. Compd. 735 (2018) 1682–1693. [7] I. Fartushna, M. Mardani, A. Khvan, V. Cheverikin, D. Ivanov, A. Kondratiev, A. Dinsdale, Phase equilibria in the Ce-Co-Fe system at 900 °C, J. Alloy. Compd. 765 (2018) 644–649. [8] M. Mardani, I. Fartushna, A. Khvan, V. Cheverikin, A. Dinsdale, Experimental investigation of phase transformation in Ce-Fe-Ni system, J. Alloy. Compd. 781 (2019) 524–540. [9] K. Orimoloye, D. Kevorkov, M. Medraj, Phase equilibria and magnetic phases in the Fe-rich regions of the Ce-Fe-{Ni, Si, Al}-B quaternary systems, J. Alloy. Compd. 763 (2018) 289–295. [10] K. Orimoloye, Experimental investigation of the Fe-rich regions in the Ce-Fe{Ni,Si,Al}-B quaternary systems: high-through put screening of magnetic phases, Spectr. Res. Repos. (2016). [11] W. Kraus, G. Nolze, POWDERCELL, Rev. 1.8a, Federal Institute for Materials Research and Testing (BAM), Berlin, 1996.
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Experimental investigation of phase equilibria in the Ce-Fe-Ni system at 950 and 750 °C
T
I. Fartushnaa,b, , M. Mardania, A. Khvana, V. Cheverikina, M. Gorshenkovc, A. Dinsdaled ⁎
a
Thermochemistry of Materials Scientific Research Centre, NUST MISIS, Moscow, Russia Department of Certification and Analytical Control, NUST MISIS, Moscow, Russia c Physical materials science department, NUST MISIS, Moscow, Russia d Hampton Thermodynamics Ltd, Hampton, UK b
ARTICLE INFO
ABSTRACT
Keywords: Ce-Fe-Ni system Experimental Phase diagram
The Ce-Fe-Ni system has been experimentally studied by microstructural analysis, electron microprobe analysis and X-ray diffraction over the whole concentration range. Isothermal sections at 950 and 750 °C for this system were constructed for the first time. The phase relations at 750 °C measured in this work are different from those published for 700 °C in particular because of the identification of a new binary phase Ce5Ni19. A continuous solid solution Ce(Fe,Ni)2 (MgCu2-type structure, cF24-Fd-3m), was found at 750 °C, with mutual substitution of Fe and Ni atoms. The liquid phase is present in the system both at 950 and 750 °C. Almost all binary-based phases at these temperatures show wide homogeneity regions and possess a constant Ce content.
1. Introduction Ce-based compounds with transitional metals (TM = Fe, Co, Ni), crystallizing with an MgCu2 phase structure, form materials with interesting properties. They have received much attention not only owing to their outstanding soft magnetic properties including high Curie temperature, high saturation magnetization, high permeability, and low coercivity, but also due to their various applications in electromagnetic microwave absorption, magnetic refrigeration systems, magnetic recording devices, magnetic resonance imaging and sensors [1]. Rare earth elements have been regarded as providing the basis for valuable new materials, due to their unique electrical and magnetic properties. The alloys of the R-Ni systems are widely used as hydrogen storage materials in engines and automobiles due to their high hydrogen absorption ability [2,3]. Doping rare earth elements into Fe-Ni alloys can significantly improve their magnetic properties. In order to obtain high quality samples during synthesis or heat treatment, a detailed knowledge of the underlying phase relationships is needed. Phase diagrams can serve as a road map for the purpose of process optimization. However, there is only a limited amount of data on phase equilibria in the Ce-Fe-Ni system, so there is a need to understand the existence and stability of compounds in this system to provide a basis for further research into the potential applications of R-Fe-TM alloys. The present investigation is part of a series of papers on phase equilibria in the Ce-Fe-TM (TM = Mn, Co, Ni) systems [4–8]. Phase equilibria associated with ⁎
crystallization in the Ce-Fe-Ni system were studied by us earlier [8] and liquidus and solidus projections and a Scheil diagram were constructed. A new binary compound Ce5Ni19 was found [8]. Phase equilibria below the solidus have been reported in works [9,10] where a partial isothermal section of this system at 700 °С had been presented. However, the isothermal section at 700 °С [9,10] does not include the new binary compound Ce5Ni19. In this paper our results on phase equilibria in the Ce-Fe-Ni system for whole concentration range at 950 and 750 °C are presented. The determined phase relations provide necessary data for the critical thermodynamic assessment of the Ce-Fe-Ni ternary system. The boundary binary systems accepted in this work were discussed in [8]. 2. Experimental methods The preparation and control of the sample compositions as well as the methods of microstructural analysis (SEM and electron probe microanalysis (EPMA)) and X-ray diffraction (XRD) are described in detail in our previous work [8]. The starting materials, used in the present work were: Ce – purity 99.8 wt%, Fe – purity 99.99 wt% and Ni – purity 99.9 wt%. The samples were prepared by arc-melting using a nonconsumable tungsten electrode under a high-purity argon atmosphere (99.998%). Before arc-melting, pure Ti was first melted to provide addition cleaning. To determine the exact composition of the samples energy dispersive X-ray spectroscopy (EDX) data was used.
Corresponding author. E-mail address: [email protected] (I. Fartushna).
https://doi.org/10.1016/j.calphad.2019.01.002 Received 30 October 2018; Received in revised form 19 December 2018; Accepted 12 January 2019 0364-5916/ © 2019 Elsevier Ltd. All rights reserved.
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Table 1 Phase composition of the annealed Ce-Fe-Ni alloys at 950 and 750 °C. #
Alloy Nominal
Measured
Ce
Fe
Ni
Ce
Fe
Ni
1
15
20
65
2
15
30
55
3
15
50
35
4 5 6 7 8 10
15 30 30 30 33 10
70 20 40 50 37 85
15 50 30 20 30 5
11 13 16 17 18
25 30 15 30 30
40 30 60 60 10
35 40 25 10 60
19
20
30
50
24
15
35
50
27
15
40
45
28
15
45
40
29 32 33 34
15 30 25 20
65 35 35 40
20 35 40 40
35 37
30 20
25 20
45 60
38
20
25
55
– 15.0 13.7 – 13.4 15.4 14.5 – 31.1 – – – 9.8 26.0 – – – – – 19.8 20.4 34.8 – 14.9 15.2 15.2 15.0 15.2 – – 18.2 20.5 – 20.6 20.3 20.9 20.7
– 21.2 29.3 – 52.0 49.6 71.0 – 38.8 – – – 85.3 39.2 – – – – – 30.1 30.4 50.9 – 40.4 40.6 44.9 45.5 64.8 – – 42.6 39.5 – 20.6 20.5 25.3 25.3
– 63.8 57.0 – 34.7 35.1 14.5 – 30.1 – – – 5.0 34.8 – – – – – 50.1 49.2 14.3 – 44.8 44.2 39.8 39.5 20.0 – – 39.2 40.0 – 58.8 59.2 53.8 54.0
The samples were annealed at 950 and 750 °C during 90–185 h in a tube furnace (Nabertherm RHTV 120/300/1700) with an accuracy in temperature of ± 3 °С. Annealing was achieved by holding the samples in an argon atmosphere (99.998%), which was additionally purified by titanium plate. The samples were placed in an Al2O3 crucible. Each sample was further wrapped in tantalum foil. After annealing the samples were quenched to retain the equilibrium microstructures. The samples for microstructure analysis were prepared with a (Struers Labopol-5) machine for grinding and polishing. The grinding procedure was carried out with SiC-paper and the polishing using a diamond suspension with, successively, grain sizes of 9, 3 and 1 µm. Chemical etching was not used. The prepared samples were examined with a scanning electron microscopy (SEM) using a TESCAN VEGA LMH microscope with a LaB6 cathode and an energy dispersive X-ray microanalysis system – Oxford Instruments Advanced AZtecEnergy. The measurement error in determining the concentration of elements using X-ray analysis was 0.1 wt%. X-ray powder diffraction (XRD) was carried out with a diffractometer Rigaku Ultima 4 using CoKα-filtered radiation. The fine powder was prepared by grinding in an agate mortar. The phases were determined by comparing diffraction patterns with the literature or a calculated pattern using the PowderCell [11] and WINXPOW [12] software packages. Rietveld analysis was performed and the mass fraction of each phase was determined using the PowderCell [11] software package. The lattice parameters were calculated by leastsquares refinement.
Heattreatment
Phase composition
950 °C, 750 °C 950 °C, 750 °C, 950 °C, 750 °C, 750 °C, 750 °C, 750 °C, 750 °C, 750 °C, 950 °C, 750 °C, 750 °C, 750 °C, 750 °C, 750 °C, 950 °C, 750 °C, 950 °C, 750 °C, 950 °C, 750 °C, 950 °C, 750 °C, 950 °C, 750 °C, 750 °C, 750 °C, 750 °C, 950 °C, 750 °C, 750 °C, 950 °C, 750 °C, 950 °C, 750 °C,
(γFe,Ni) + CeNi5 (γFe,Ni) + CeNi5 (γFe,Ni) + CeNi5 (γFe,Ni) + CeNi5 + Ce5Ni19 (γFe,Ni) + Ce2Ni7 + CeNi3 (αFe) + Ce2Ni7 + CeNi3 (αFe) + CeNi3 + Ce(Fe,Ni)2 CeNi3 + Ce(Fe,Ni)2 (αFe) + Ce(Fe,Ni)2 (αFe) + Ce2Fe17 + Ce(Fe,Ni)2 (αFe) + Ce(Fe,Ni)2 L + (γFe,Ni) (αFe) + Ce2Fe17 + Ce(Fe,Ni)2 CeNi3 + Ce(Fe,Ni)2 CeNi3 + Ce(Fe,Ni)2 (αFe) + CeNi3 Ce2Fe17 + Ce(Fe,Ni)2 L + CeNi3 CeNi3 + Ce(Fe,Ni)2 (γFe,Ni) + Ce2Ni7 + Ce5Ni19 (αFe) + Ce2Ni7 (γFe,Ni) + CeNi5 (αFe) + Ce5Ni19 (γFe,Ni) + CeNi5 + Ce5Ni19 (αFe) + Ce2Ni7 (γFe,Ni) + Ce2Ni7 (αFe) + Ce2Ni7 (αFe) + CeNi3 CeNi3 + Ce(Fe,Ni)2 CeNi3 + Ce(Fe,Ni)2 (γFe,Ni) + Ce2Ni7 + CeNi3 (αFe) + Ce2Ni7 + CeNi3 CeNi3 + Ce(Fe,Ni)2 CeNi5 + Ce5Ni19 (αFe) + Ce5Ni19 + Ce2Ni7 (γFe,Ni) + Ce2Ni7 + Ce5Ni19 (αFe) + Ce2Ni7
90 h 90 h 185 h 90 h 95 h 95 h 95 h 95 h 95 h 95 h 90 h 95 h 95 h 95 h 95 h 95 h 90 h 95 h 90 h 185 h 90 h 95 h 90 h 185 h 90 h 185 h 95 h 95 h 95 h 90 h 95 h 95 h 90 h 95 h 90 h 95 h
3. Results and discussion The phase equilibria in the Ce-Fe-Ni system were studied, by means of SEM, EPMA and X-ray analysis of the 38 ternary alloys annealed at 950 and 750 °C for 90–185 h. The phase compositions of the studied alloys and the microprobe results are given in Tables 1 and 2, respectively. A few measurements for each phase were performed using the microprobe method. On the basis of all the available alloys, the isothermal sections of the Ce-Fe-Ni system at 950 and 750 °C, covering entire composition range, were constructed. These are shown in Fig. 1. The composition of each alloy is marked in Fig. 1. Note: in Table 2 the average values of a number of measurements for each phase are given with the mean squares deviations, while in Fig. 1 all the measured values are shown. The microstructures of some annealed samples from different phase regions are presented in Fig. 2. The XRD patterns of some annealed alloys are shown in Fig. 3. 3.1. Solid phases Significant amounts of the third component dissolves into almost all of the binary-based intermetallic phases. The phases CeNi5, Ce5Ni19, Ce2Ni7 and CeNi3 extend deeply into the ternary system, forming wide solid-solutions with significant variations in their lattice parameters. The homogeneity regions of these phases possess a constant Ce content. The extension of these solid solutions and their homogeneity ranges are shown in the isothermal sections of the Ce-Fe-Ni system. 285
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Table 2 The composition of the Ce-Fe-Ni phases according to the EPMA examination. #
Alloy
Heat-treatment
Nominal
1 2
3
Measured
Ce
Fe
Ni
Ce
Fe
Ni
15 15
20 30
65 55
15.0 13.7
21.2 29.3
63.8 57.0
750 °C, 95 h 950 °C, 90 h
-
-
-
750 °C, 185 h
13.4
52.0
34.7
950 °C, 90 h
15.4
49.6
35.1
750 °C, 95 h
15
50
35
4
15
70
15
14.5
71.0
14.5
750 °C, 95 h
6
30
40
30
31.1
38.8
30.1
750 °C, 95 h
10
10
85
5
9.8
85.3
5.0
750 °C, 95 h
11
25
40
35
26.0
39.2
34.8
750 °C, 95 h
19
20
30
50
19.8
30.1
50.1
950 °C, 90 h
20.3
30.5
49.2
750 °C, 185 h
24
15
35
50
34.8
50.9
14.3
950 °C, 90 h
27
15
40
45
14.9
40.3
44.8
750 °C, 95 h 950 °C, 90 h
28
15
45
40
15.3 15.2 15.0
40.1 44.9 45.5
44.6 39.8 39.5
750 °C, 185 h 950 °C, 90 h 750 °C, 185 h
29
15
65
20
15.2
64.8
20.0
750 °C, 95 h
34
20
40
40
18.2
42.6
39.2
950 °C, 90 h
20.5
39.5
40.0
750 °C, 95 h
20.6
20.6
58.8
950 °C, 90 h
20.3
20.5
59.2
750 °C, 95 h
37
38
20
20
20
25
60
55
20.9
25.3
53.8
20.7
25.3
54.0
950 °C, 90 h 750 °C, 95 h
The Laves phase compounds CeFe2 and CeNi2 form a continuous solid solution Ce(Fe,Ni)2 (MgCu2-type structure, cF24-Fd 3 m) at 750 °C. The homogeneity region of the phase follows the isoconcentrate 33.3 at % Ce due to Ni/Fe mutual substitution, while it is almost a linear phase in terms of the concentration of Ce, where its width does not exceed 1 at % (Table 2). The continuous solid solution between CeNi2 and CeFe2 was reported in [9,10,13,14]. At a temperature of 950 °C this continuous solid solution Ce(Fe,Ni)2 no longer exists. The crystal structures and the lattice parameters of the phases in the Ce-Fe-Ni system are summarized in Table 3.
Microprobe results, at.% Phase
Ce
Fe
CeNi5 (γFe,Ni) CeNi5 (γFe,Ni) CeNi5 Ce5Ni19 (γFe,Ni) Ce2Ni7 CeNi3 (αFe) Ce2Ni7 CeNi3 (αFe) Ce(Fe,Ni)2 CeNi3 (αFe) Ce(Fe,Ni)2 (αFe) Ce2Fe17 Ce(Fe,Ni)2 CeNi3 Ce(Fe,Ni)2 (γFe,Ni) + Ce2Ni7 Ce5Ni19 (αFe) Ce2Ni7 (γFe,Ni) CeNi5 Ce5Ni19 (γFe,Ni) CeNi5 Ce2Ni7 Ce2Ni7 (γFe,Ni) (αFe) Ce2Ni7 (αFe) CeNi3 (γFe,Ni) Ce2Ni7 (αFe) Ce2Ni7 CeNi3 CeNi5 Ce5Ni19 (αFe) + Ce5Ni19
17.2 ± 0.1 0.7 ± 0.1 17.0 ± 0.1 0.8 ± 0.1 16.9 ± 0.3 21.1 ± 0.1 0.5 ± 0.2 22.6 ± 0.1 25.4 ± 0.2 0.9 ± 0.1 22.5 ± 0.3 25.3 ± 0.2 0.9 ± 0.1 33.0 ± 0.3 25.5 0.9 33.4 ± 0.5 0.5 ± 0.1 10.4 ± 0.4 33.2 25.9 33.4 0.9 ± 0.2 22.7 ± 0.1 21.3 ± 0.1 0.9 ± 0.1 22.7 ± 0.1 0.7 ± 0.1 17.1 ± 0.1 21.1 0.7 16.9 ± 0.1 21.2 ± 0.2 22.5 ± 0.1 0.8 ± 0.1 0.8 ± 0.1 22.7 ± 0.1 0.9 ± 0.2 25.2 ± 0.1 0.1 22.7 ± 0.1 0.9 ± 0.1 22.6 25.4 ± 0.1 17.2 ± 0.1 21.2 ± 0.1 5.2 5.86 6.7 22.7 ± 0.2 21.4 ± 0.2 0.9 22.7 ± 0.1 21.3 ± 0.1 2.9 22.9 ± 0.1
14.0 ± 74.1 ± 16.9 ± 70.7 ± 20.6 ± 14.8 ± 89.3 ± 25.6 ± 28.0 ± 96.1 ± 27.6 ± 23.7 ± 97.9 ± 35.9 ± 44.4 97.9 34.3 ± 99.0 ± 86.0 ± 44.7 38.4 28.8 86.0 ± 21.2 ± 22.2 ± 95.1 ± 22.9 ± 82.8 ± 25.7 ± 16.5 83.8 27.7 ± 21.4 ± 17.7 ± 87.2 ± 95.9 ± 20.8 ± 97.6 ± 41.5 ± 90.3 26.5 ± 96.2 ± 28.8 25.4 ± 24.9 ± 19.0 ± 75.0 71.14 71.8 14.0 ± 15.8 ± 84.3 20.2 ± 22.0 ± 84.05 17.7 ±
(αFe) + Ce2Ni7 Ce2Ni7 Ce5Ni19 (γFe,Ni) Ce2Ni7 Ce5Ni19 (αFe) + Ce2Ni7 Ce2Ni7
Ni
0.1 0.1 0.3 0.9 0.5 0.2 0.3 0.2 0.2 0.1 0.6 0.3 0.1 0.1 0.6 0.1 0.6
0.2 0.5 0.4 0.2 0.9 0.1 0.2 0.1 0.1 0.4 0.1 0.4 0.4 0.3 0.2 0.2 0.5 0.2 0.1 0.2
0.1 0.2 0.2 0.2 0.1
68.8 ± 0.2 25.2 ± 0.1 66.1 ± 0.1 28.5 ± 0.9 62.5 ± 0.2 64.2 ± 0.2 10.2 ± 0.1 51.8 ± 0.1 46.6 ± 0.1 3.0 ± 0.1 49.9 ± 0.3 51.0 ± 0.1 1.2 ± 0.1 31.1 ± 0.3 30.1 1.2 32.3 ± 0.1 0.5 ± 0.1 3.6 ± 0.2 22.1 35.7 37.8 13.0 ± 0.1 56.1 ± 0.5 56.5 ± 0.3 4.0 ± 0.2 55.5 ± 0.9 16.5 ± 0.1 57.2 ± 0.2 62.4 15.5 55.4 ± 0.2 57.5 ± 0.4 59.8 ± 0.3 12 ± 0.1 3.4 ± 0.3 56.5 ± 0.4 1.5 ± 0.1 33.3 ± 0.2 9.6 50.8 ± 0.4 2.9 ± 0.4 48.6 49.2 ± 0.3 57.9 ± 0.1 59.8 ± 0.2 19.8 23 21.5 63.2 ± 0.1 62.8 ± 0.2 14.7 57.0 ± 0.1 56.7 ± 0.1 13.1 59.4 ± 0.1
Among the binary compounds, CeNi3 (CeNi3-type structure, hP24P63/mmc) has the widest homogeneity region at 750 °C and according to the EPMA data of alloy #4 (Table 2, Fig. 1 b, 2 a) dissolves up to 44.4 at% Fe. According to [9,10] the solubility of Fe in CeNi3 at 700 °C already is 41 at%. This phase is located along the isoconcentration line of 25 at% Ce and extends widely along this isoconcentrate, reflecting significant mutual substitution of Ni and Fe atoms. The single-phase microstructures of alloys # 11 and #33, together with the microprobe results, indicate the constant Ce content in the phase. Traces of the Laves phase Ce(Fe,Ni)2 are present along the grain boundaries. The 286
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Fig. 1. The isothermal sections of the Ce-Fe-Ni system at 950 °C (a) and 750 °C (b): ◐ – two-phase samples, • – three-phase samples, △ – EPMA data.
solubility of Fe in this phase at 950 °C is less and does not exceed 30 at %. According to EPMA data on alloys ## 3, 34 the maximum solubility of Fe in Ce2Ni7 (Ce2Ni7-type structure, hP36-P63/mmc) is 26 and 28 at% at 950 and 750 °C, respectively (Table 2, Figs. 1, 2 b). In the works [9,10] the solubility of Fe in Ce2Ni7 at 700 °C was measured as 16 at% Fe. The chemical compositions of Ce2Ni7 (see Table 2) indicated that this phase was a line compound with a constant Ce content. The newly identified binary phase Ce5Ni19 (Ce5Co19-type structure, hR72-R 3 m), found by us recently, was detected both at 950 and 750 °C. The maximum solubility of Fe in this phase at 950 °C reaches 22 at% according to EPMA data of the alloys # 19 and # 38 (Table 2, Fig. 1 a, 2 c). At a temperature of 750 °C the solubility of Fe in Ce5Ni19 is smaller and according to EPMA data of alloy # 37 is 16 at% (Table 2, Fig. 1 b). In [28,29] it was reported that the ternary solid solution of Fe in CeNi5 (CaCu5-type structure, hP6-P6/mmm) extends up to the CeNi4Fe composition. However, our results show that the phase extends beyond the CeNi4Fe composition. In [30] the homogeneity range in CeFexNi5-x was reported up to x = 1.75. According to our data the solubility of Fe in CeNi5 reaches 27.7 and 20.6 at% at 950 and 750 °C, respectively, according to the EPMA data of alloys #27 and #2 (Table 2, Figs. 1, 2 d, e and 3 a). The solubility of Fe in CeNi5 at 700 °C in [9,10] was measured as 19 at% Fe. The homogeneity range of the Ce2Fe17 (Zn17Th2-type structure, hR57-R 3 m) is significantly smaller, the solubility of Ni in Ce2Fe17 according to EPMA data of alloy # 10 was measured as 3.6 at% at 750 °C (Table 2, Figs. 1, 2 f). At a temperature of 950 °C the solubility of Ni in Ce2Fe17 does not exceed 2.5 at%. It should be noted that we have not observed the low temperature modification of the Ce2Fe17 phase (αCe2Fe17) with a hexagonal Th2Ni17-type structure and observed only the high temperature modification (βCe2Fe17) with a rhombohedral Th2Zn17-type structure. The maximum solubility of cerium in the (γFe,Ni)-phase and the (αFe)-phase was determined to be no more than 1 at% (see Table 2). No ternary compound was found.
phase L + (γFe,Ni) + Ce2Fe17 and L + (γFe,Ni) + CeNi3 appear. Moreover, at this temperature the Laves phase Ce(Fe,Ni)2 does not exist, it forms at a lower temperature. The isothermal section at 950 °C is characterized by the presence of narrow three-phase regions and wide two-phase regions. Near the Fe-Ni boundary, a continuous single phase region of (γFe,Ni) was obtained. The (γFe,Ni)-phase defines the character of phase equilibria at this temperature and co-exists with all phases, namely, with Liquid (L), CeNi5, Ce5Ni19, Ce2Ni7, CeNi3, Ce2Fe17 and forms five three-phase fields (γFe,Ni) + CeNi5 + Ce5Ni19, (γFe,Ni) + Ce5Ni19 + Ce2Ni7, (γFe,Ni) + Ce2Ni7 + CeNi3, L + (γFe,Ni) + CeNi3, L + (γFe,Ni) + Ce2Fe17 and the corresponding two-phase regions. The existence of the three-phase regions (γFe,Ni) + CeNi5 + Ce5Ni19 and (γFe,Ni) + Ce2Ni7 + CeNi3 as well as their locations, were established from the SEM, EPMA and XRD data of alloys #27 and #3 which had been annealed at 950 °C for 90 h (Tables 1 and 2, Fig. 1 a, 2 d and 3 a, b). The BSE images of annealed alloy #27, as shown in Fig. 2 d, reveal it to be located in the three-phase region which consists of a dark (γFe,Ni)-phase, a grey Ce5Ni19 and a dark grey CeNi5 phase. It should be noted, that according to the XRD data of alloy #27, the (γFe,Ni)-phase was not observed in this alloy, instead the (αFe)-phase was clearly identified, because the (γFe,Ni)-phase transformed into (αFe)-phase on cooling (Fig. 3 a). In the alloy #3, which is located in the three-phase region (γFe,Ni) + Ce2Ni7 + CeNi3, the amount of the CeNi3 phase is very low (Fig. 3 b). This gives us reason to believe that the composition of this alloy is almost on the boundary tie-line (γFe,Ni) + Ce2Ni7. The composition of these phases is well established by the microprobe method (Table 2). The XRD pattern (Fig. 3 b) confirmed this three-phase equilibrium. It should be noted, that according to the X-ray data, in addition to the (γFe,Ni)-phase, the (αFe)-phase also can be clearly identified in this alloy, as it was formed during cooling (Fig. 3 b). According to the SEM, EPMA and XRD results alloys # 19 and # 38, annealed at 950 °C at 90 h, are located in the three-phase region (γFe,Ni) + Ce5Ni19 + Ce2Ni7 (Tables 1 and 2, Fig. 1 a, 2 c). The microstructure of alloy #19 is illustrated in Fig. 2 c and clearly shows three phases: dark, grey and light-grey. These correspond to (γFe,Ni), Ce5Ni19 and Ce2Ni7, respectively. The BSE images and X-ray diffraction patterns of alloys ## 1, 2, 24 are nearly identical. It illustrates that those alloys are located in the two-phase region between (γFe,Ni) + CeNi5. On increasing the Ni content of the alloys, the Ni content of both phases increases progressively, as we can see in the Table 2.
3.2. Isothermal section at 950 °C Based on the experimental results obtained in this work and the information of relevant binary systems in the literature, the isothermal section of the Ce-Fe-Ni system at 950 °C was constructed and is shown in Fig. 1 a. The isothermal section at 950 °C differs from the solidus surface [8]. In the Ce-rich region at 950 °C there is a wide liquid phase domain. In addition, at 950 °C two three-phase fields with the liquid 287
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Fig. 2. Microstructures of annealed alloys of the Ce-Fe-Ni system: a – 15Ce-70Fe-15Ni (#4), 750 °C, ×2000, (αFe) + CeNi3 + Ce(Fe,Ni)2; b – 15Ce-50Fe-35Ni (#3), 750 °C, ×2000, (αFe) + Ce2Ni7 + CeNi3; c – 20Ce-30Fe-50Ni (#19), 950 °C, ×2000, (γFe,Ni) + Ce5Ni19 + Ce2Ni7; d – 15Ce-40Fe-45Ni (#27), 950 °C, ×2000, (γFe,Ni) + Ce5Ni19 + CeNi5; e – 15Ce-30Fe-55Ni (#2), 750 °C, ×2000, (γFe,Ni) + Ce5Ni19 + CeNi5; f – 10Ce-85Fe-5Ni (#10), 750 °C, ×2000, (αFe) + Ce2Fe17 + Ce(Fe,Ni)2.
The location of the liquid phase boundary was determined by the construction and processing of a series of intersecting isopleths, so that each alloy was located in at least three of them. The isopleths were
constructed based on DTA results [8]. The vertices of the respective tieline triangles involving the liquid phase have been indicated by dashed lines in Fig. 1 a. 288
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Fig. 3. X-ray diffraction patterns of annealed Ce-Fe-Ni alloys: a – 15Ce-40Fe-45Ni (#27), 950 °C, (αFe) + Ce5Ni19 + CeNi5; b – 15Ce-50Fe-35Ni (#3), 950 °C, (γFe,Ni) + (αFe) + Ce2Ni7 + CeNi3.
3.3. Isothermal section at 750 °C
+ Ce2Ni7, involving the Ce5Ni19 phase, exists at 750 °C (Tables 1 and 2, Fig. 1 b). It was determined by SEM and EMPA on alloy # 37 annealed at 750 °C/95 h, which contains three phases: (αFe) + Ce5N19 + Ce2Ni7. The location of the corners of this three-phase field was established by microprobe measurements on the solid phases (Table 2). Thus, based on phase composition in the alloys above, the threephase regions (γFe,Ni) + CeNi5 + Ce5Ni19 and (αFe) + Ce5Ni19 + Ce2Ni7 were obtained, instead of the three-phase region (Fe) + CeNi5 + Ce2Ni7, given in Refs. [9,10] at 700 °C. According to the SEM and EPMA results of alloys # 3 and #34 annealed at 750 °C/95 h, these are located in the three-phase region (αFe) + Ce2Ni7 + CeNi3 (Tables 1, 2, Fig. 1 b, 2 b). The microstructure of these samples (Fig. 2 b) was composed of the dark, grey and light phases. These three phases were identified as (αFe), Ce2Ni7 and CeNi3, respectively. SEM and EPMA analysis revealed that alloy #4, annealed at 750 °C for 95 h, is in the (αFe) + CeNi3 + Ce(Fe,Ni)2 three-phase region. In the SEM micrograph of this sample, Fig. 2 a, all three phases are well distinguished. They were identified by microprobe analysis as follows: the dark grains correspond to the (αFe)-phase, while the grey and the light-grey correspond to CeNi3 and Ce(Fe,Ni)2, respectively. The location of the corners of the three-phase field was established by microprobe measurements on the solid phases (Table 2). Fig. 2 f represents the micrograph of sample #10. Evidently, this alloy was in a three-phase equilibrium (αFe) + Ce2Fe17 + Ce(Fe,Ni)2. The dark grains and grey grains were identified as (αFe) and Ce2Fe17, respectively, based on the SEM and EPMA results. The light-grey grains are Ce(Fe,Ni)2.
Based on the experimental results obtained in this work and the information of relevant binary systems in the literature, the isothermal section of the Ce-Fe-Ni system at 750 °C was constructed and is shown in Fig. 1 b. In the concentration range up to 33.3 at% Ce the isothermal section at 750 °C is very similar in character to the solidus surface [8]. The differences involve a change in the solubility of the third component in each of the phases and, thus, in a shift of the corners of the three-phase fields. As the majority of the phases stable at 750 °C have significant homogeneity ranges, these shifts are significant. In addition, at 750 °C the three-phase field (αFe) + (γFe,Ni) + Ce2Ni7 appears. In the Ce-rich region at 750 °C there is a wide liquid phase domain. The (αFe) and (γFe,Ni) phases define the character of the phase equilibria at 750 °C and co-exists with all intermetallic phases CeNi5, Ce5Ni19, Ce2Ni7, CeNi3, Ce(Fe,Ni)2, Ce2Fe17 and form six three-phase fields (γFe,Ni) + CeNi5 + Ce5Ni19, (αFe) + (γFe,Ni) + Ce5Ni19, (αFe) + Ce5Ni19 + Ce2Ni7, (αFe) + Ce2Ni7 + CeNi3, (αFe) + CeNi3 + Ce(Fe,Ni)2 and (αFe) + Ce(Fe,Ni)2 + Ce2Fe17 as well as the corresponding two-phase regions. The existence of the three-phase region (γFe,Ni) + CeNi5 + Ce5Ni19 was established based on the results of SEM and EPMA on sample # 2 annealed at 750 °C/185 h (Tables 1 and 2, Fig. 1 b, 2 e). The microstructure of this sample (Fig. 2 e) confirms that it consists of three phases, namely (γFe,Ni) (dark grains), CeNi5 (grey grains) and Ce5Ni19 (light-grey grains). Moreover, another very narrow three-phase region (αFe) + Ce5Ni19 289
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Table 3 The crystal structures and lattice parameters of the Ce-Fe-Ni phases.
1
Phase
Crystal structure
Lattice parameters, Å
Alloy
Refs.
δFe γFe αFe
W, cI2-Im3m Cu, cF4-Fm-3m W, cI2-Im3m
(Ni) (γFe,Ni)
Cu, cF4-Fm-3m Cu, cF4-Fm-3m
δCe γCe Ce2Fe17 ht
W, cI2-Im3m Cu, cF4-Fm3m Zn17Th2, hR57-R-3m
Ce2Fe17 rt CeNi5
Th2Ni17, hP38-P63/mmc CaCu5, hP6-P6/mmm
Ce5Ni19
Ce5Co19, hR72-R-3m
Ce2Ni7
Ce2Ni7 hP36-P63/mmc
CeNi3
`CeNi3 hP24-P63/mmc
CeFe2
MgCu2, cF24-Fd-3m
CeNi2
MgCu2, cF24-Fd-3m
Ce(Fe,Ni)2 CeNi Ce7Ni3
MgCu2, cF24-Fd-3m TlI, oS8-Cmcm Th7Fe3, hP20-P63mc
a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a
at 915 °C at 25 °C 15Ce-50Fe-35Ni (#3), 950 °C 15Ce-40Fe-45Ni (#27), 950 °C 50 at.% Ni 15Ce-50Fe-35Ni (#3), 950 °C 89Fe-11Ce, annealed at 1050 °C,40 hours 20.8Ce-79.2Ni (#41), as-cast 15Ce-40Fe-45Ni (#27), 950 °C 20.8Ce-79.2Ni (#41), as-cast 15Ce-40Fe-45Ni (#27), 950 °C 20.8Ce-79.2Ni (#41), as-cast 15Ce-50Fe-35Ni (#3), 950 °C 20.8Ce-79.2Ni (#41), as-cast 15Ce-50Fe-35Ni (#3), 950 °C 66.6Fe-33.4Ce, annealed at 900 °C, 40 hours 20.8Ce-70.2Ni (41), as-cast -
[15] [15] [15] Th. w. Th. w. [16] [17] Th. w. [15] [15] [18] [19] [20] [21] [22] [23] [8] Th. w. Th. w. Th. w. [24] [25] [8] Th. w. [25] [8] Th. w. [18] [20] [14] [8] [26] [27]
= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
2.9315 3.6467 2.8665 2.870(1) 2.875(1) 3.52 3.5750 3.590(1) 4.12 5.1610 8.485, c = 12.433 8.496, c = 12.414 8.482(2), c = 12.410(5) 8.49, c = 8.281 4.860, c = 3.996 4.882, c = 4.004 4.877(1), c = 3.993(1) 4.935(1), c = 4.050(1) 4.924(1), c = 48.418(5) 4.972(1), c = 49.010(5) 4.905, c = 24.38 4.927, c = 24.45 4.928(1), c = 24.46(1) 4.994(1), c = 24.514(5) 4.960, c = 16.56 4.948, c = 16.48 5.028(2), c = 16.59(2) 7.296 7.295 7.2100 7.215(1)
= 3.788, b = 10.556, c = 4.366 = 9.926, c = 6.311
Th. w. − results of this work
Our studies of phase equilibria at the solidus temperatures had shown that there is a continuous solid solution (γFe,Ni) between Ni and Fe. In contrast, however, these results for 750 °С show no continuous solid solution (γFe,Ni) because this phase in the Fe-rich region of the boundary binary Fe-Ni system transforms into the (αFe)-phase. Therefore, at 750 °C the two phase region (αFe) + (γFe,Ni) and the very narrow three-phase region (αFe) + (γFe,Ni) + Ce5Ni19 should be present. In conclusion, comparing the phase relations at 950 and 750 °C, similar topology of the isothermal sections at these two temperatures can be noted within the Ni-rich region, as can see in Fig. 1 a and b. For example, at both temperatures, the (γFe,Ni)-phase and four binary phases CeNi3, Ce2Ni7, Ce5Ni19, CeNi5 were found and all were line compounds. At both temperatures a wide liquid phase domain in the Ce-rich region is present. A total of six three-phase equilibria were observed in the isothermal section at 750 °C, in contrast to five at 950 °C. The continuous solid solution Ce(Fe,Ni)2, between Laves phases CeFe2 and CeNi2, forms at 750 °C, while at 950 °C Ce(Fe,Ni)2 does not exists. Differences between these two sections also existed in the Fe-rich region. For example, at 950 °C two three-phase fields with the liquid phase appear, while at 750 °C the (αFe)-phase appears. The isothermal section at 750 °C constructed in this work was further compared with the reported one at 700 °C. Some differences could be found in the phase relations between the current work and the studies [9,10]. Most evidently, the new binary phase Ce5Ni19, found by us previously, was absent in works [9,10]. Moreover, in contrast to the current section at 750 °C, a much lower solubility of Fe in Ce2Ni7 was observed in the section at 700 °C [9,10].
4. Conclusions 1. Phase equilibria in the Ce-Fe-Ni system at 950 and 750 °C covering the entire concentration range were studied. Isothermal sections at these temperatures were constructed. The phase relations at 750 °C measured in this work are different from those which had been obtained at 700 °C [9,10] by the inclusion of the new binary phase Ce5Ni19, found by us previously. 2. A continuous solid solution Ce(Fe,Ni)2 (MgCu2-type structure, cF24Fd 3 m) forms at 750 °C and contains 33.3 at% Ce due to mutual substitution of Fe and Ni atoms. A continuous (γFe,Ni) phase field at 750 °C near the Fe-Ni boundary was observed to cover a composition region from the Fe-rich corner to the Ni-rich corner. 3. The isothermal section at 950 °C is characterized by the presence of five three-phase fields (γFe,Ni) + CeNi5 + Ce5Ni19, (γFe,Ni) + Ce5Ni19 + Ce2Ni7, (γFe,Ni) + Ce2Ni7 + CeNi3, L + (γFe,Ni) + CeNi3, L + (γFe,Ni) + Ce2Fe17 and the corresponding two-phase regions. 4. The isothermal section at 750 °C is characterized by the presence of six three-phase fields (γFe,Ni) + CeNi5 + Ce5Ni19, (αFe) + (γFe,Ni) + Ce5Ni19, (αFe) +Ce5Ni19 + Ce2Ni7, (αFe) + Ce2Ni7 + CeNi3, (αFe) + CeNi3 + Ce(Fe,Ni)2 and (αFe) + Ce(Fe,Ni)2 + Ce2Fe17 as well as the corresponding two-phase regions. 5. Almost all of the binary-based phases, with the exception of Ce2Fe17, have substantial solubility of the third component. The maximum solubility of Fe in CeNi3 is 44.4 and 30 at% at 750 and 950 °С, respectively. The solubility of Fe in Ce2Ni7 is 26 and 28 at% at 950 and 750 °C, respectively. The maximum solubility of Fe in Ce5Ni19 at 950
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and 750 °C reaches 22 and 16 at%, respectively. The solubility of Fe in CeNi5 is 27.7 and 20.6 at% at 950 and 750 °C, respectively. The solubility of Ni in Ce2Fe17 is ~2.5 and 3.6 at 950 and 750 °C, respectively. All binary-based phases are line compounds.
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