An experimental investigation of the liquidus projection in the Fe–Ce–C system

Journal of Alloys and Compounds 651 (2015) 350e356

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An experimental investigation of the liquidus projection in the FeeCeeC system A.V. Khvan*, I.V. Fartushna, M. Mardani, A.T. Dinsdale, V.V. Cheverikin Thermochemistry of Materials Scientific Research Centre, NUST MISIS, Leninsky Prosp. 4, 119049 Moscow, Russia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 July 2015 Received in revised form 6 August 2015 Accepted 10 August 2015 Available online 13 August 2015

The liquidus projection and invariant reactions involving the liquid phase in the FeeCeeC system were analyzed using microstructure investigation, X-ray diffraction (XRD) and differential thermal analysis (DTA). In contrast to a previous analysis of the phase diagram for the system three ternary eutectic reactions were observed. It was also observed that the region of primary solidification of the Ce2C3 phase has a deep extension towards FeeC side of the system. © 2015 Elsevier B.V. All rights reserved.

Keywords: Phase equilibrium Liquidus Invariant reactions Phase diagram Solidus FeeCeeC Steels Lanthanoids

1. Introduction

2. Literature survey

The phase diagram for the FeeCeeC system is of some interest for the processing of steels as lanthanoids are frequently used in the refinement of steel melts. Ce is also used as an inoculant in steels to form vermicular or nodular graphite. However, there is only rather limited published information about the kind of phase equilibria and phase transformations that may take place. The only experimental investigation of the system in the composition range up to 40 at%Ce and up to 60 at%C was carried out by Park et al. [1], which was accepted in the subsequent review of Raghavan [2]. On the basis of light microscopy investigations and X-ray analysis of ascast and annealed samples Park et al. [1] proposed a liquidus projection (Fig 1) and isothermal sections at 800
C and 950
C. However, the temperatures of the proposed invariant reactions were not measured. In the present work experimental investigations using SEM, EPMA, EDX and DTA were carried out over the whole concentration range in order to determine the invariant reactions in the system and phase transformations during solidification.

2.1. Binaries

* Corresponding author. E-mail address: [email protected] (A.V. Khvan). http://dx.doi.org/10.1016/j.jallcom.2015.08.074 0925-8388/© 2015 Elsevier B.V. All rights reserved.

2.1.1. FeeCe There are two stable intermetallic compounds in this system, Ce2Fe17 and CeFe2 (C15 Laves phase). The existing experimental phase diagram data have been critically assessed by Su and Tedenac [3]. No experimental data on the thermodynamic properties of phases in this system have been reported. The phase diagram from the assessment of Su and Tedenac [3] (Fig. 2) was accepted for the present work as it is in good agreement with known experimental data [4e6].

2.1.2. CeeC Two carbides form in this system, Ce2C3 and CeC2. The CeC2 carbide has two modifications, a high temperature b form with a CaF2 structure, and a low temperature a form with a CaC2 structure. Peng et al. [7] carried out a critical assessment based on the experimental phase diagram information for the system and measured enthalpies of formation of the carbides. The thermodynamic description and phase diagram (Fig. 3) they obtained for the system was accepted for the present work.

A.V. Khvan et al. / Journal of Alloys and Compounds 651 (2015) 350e356

351

Fig. 1. Liquidus projection suggested by Park et al. [1].

Fig. 4. Phase diagram for the FeeC system from the assessment of Gustafson [8].

Fig. 2. Phase diagram for the FeeCe system from the assessment of Su and Tedenac [3].

2.1.3. FeeC One of the first critical assessments of data for the FeeC system was made by Gustafson [8] and this is still widely used. The phase diagram from Gustafson's work is shown in Fig. 4. There are later descriptions of data for the system, Hallstedt et al. [9] and Naraghi et al. [10], although the phase equilibria above room temperature are not appreciably different from those proposed by Gustafson. 2.2. Ternary intermetallic compounds The possibility of the substantial solubility of C in the Ce2Fe17Cx phase has been reported by Zhong et al. [11] and Liao et al. [12]. Carbon occupies the 9e positions in the rhombohedral crystal structure (hR66), so the maximum possible solubility of carbon corresponds to x ¼ 3. However, the maximum carbon solubility in

the compound in practice is not clear. Park et al. [1] observed two intermetallic compounds Fe4Ce4C7 with a primitive tetragonal structure tP* and Fe2Ce2C3 with a hexagonal structure. Both compounds were reported to form through solid state reactions and no equilibrium with the liquid phase has been observed. The Fe4Ce4C7 compound was also later analyzed by Zhang and Li [13] who reported that the Ce content is slightly higher than stoichiometric and this has been explained in terms of Ce occupation on some Fe sites. The crystallographic data on Fe2Ce2C3 is rather doubtful. In the later work of Witte and Jeitschko [14] it was reported that the Fe2Ce2C3 compound observed by Park [1] has an identical structure to the carbide Ce3.67FeC6 on the basis of powder diffraction patterns. This has a prototype La3.67[Fe(C2)3] with a structure P63/m. However, the stoichiometric composition of the compound is rather different from that observed by Park possibly implying a rather large homogeneity range for this ternary compound. Fuerst and Herbst [15] synthesized the compound Ce2Fe14C by using spin melting with high substrate velocity to obtain ribbons with a disordered or amorphous structure. The Ce2Fe14C compound was then formed by heat treatment. This phase was reported to have a P42/mnm structure with the prototype Nd2Fe14B. However, since this compound was observed only under rather metastable conditions and was not reported by Park [1] it is considered to be metastable in the FeeCeeC system in contrast to the FeeLaeC system where the equivalent compound has been reported to be stable. 3. Experimental 3.1. Sample preparation The alloys investigated in the present work are listed in Table 1 and their compositions displayed graphically in Fig. 5. They were prepared from starting materials with purity Fee99.99%, Cee99.9% and Ce99.8%. The alloys were melted in an arc-furnace with tungsten electrodes on a water-cooled copper hearth in an Ar atmosphere purified by a Tiemelt. To achieve homogeneity, the buttons were turned over and remelted six times. The ingot weights were typically 5 g. The weight losses did not exceed 0.5%. The samples were studied using differential thermal analysis (DTA), X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM), and electron microprobe (EPMA). 3.2. Microscopy

Fig. 3. Phase diagram for the CeeC system from the assessment of Peng et al. [7].

3.2.1. Mechanical preparation of metallographic sample Metallographic specimens for microscopic examination were

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Table 1 Compositions of the alloys investigated during this present work expressed in mole fractions. These are also shown graphically in Fig. 5. The thermal arrests obtained by DTA are also given in this table with the suggested solidification paths.





Sample composition

Heating, C

Cooling, C

Reactions

1

Fee0.8Cee0.1C

2

Fee0.75Cee0.1C

3

Fee0.6Cee0.25C

636 e 598 e e 595 e 767 595

4

Fee0.6Cee0.075C

5

Fee0.425Cee0.075C e 896 e 598 e e

630 e 583 614 e 590 e 768 586 861 e 593 1038 e 907 e 594 986 e

893

881

e e e 599 1024 e 890 1339 1265 1144 1276 1257 1131 1709 1143 1121 1017 e e e 881 e 1150 1127 e 1145 1126 1297 1120 1316 e 1120

e e e 595 995 e 883 1336

1110 e e 983 e 880 e 1134 1111 e 1130 1123 1304 1120 e e 1093

1145

1146

L/ðCeÞ L/ðCeÞ þ Ce2 C3 L/ðCeÞ þ Ce2 C3 þ C15 L/Ce2 C3 L/C15 þ Ce2 C3 L/C15 þ Ce2 C3 þ ðCeÞ L/Ce2 C3 L/Ce2 C3 þ C15 L/Ce2 C3 þ C15 þ ðCeÞ L/C15 L/Ce2 C3 þ C15 L/Ce2 C3 þ C15 þ ðCeÞ L/Ce2 Fe17 CX L þ Ce2 Fe17 CX /C15 L þ Ce2 Fe17 CX /Ce2 C3 þ C15 L/Ce2 C3 þ C15 L/Ce2 C3 þ C15 þ ðCeÞ L/Ce2 Fe17 CX L/Ce2 Fe17 CX þ Ce2 C3 ðpath 1Þ L/Ce2 Fe17 CX þ Ce2 C3 þ C15 (path 2) L þ Ce2 Fe17 CX /Ce2 C3 þ C15 L/Ce2 C3 þ C15 L/Ce2 C3 þ C15 þ ðCeÞ L/Ce2 C3 þ C15 þ ðCeÞ L/Ce2 C3 L/Ce2 Fe17 CX þ Ce2 C3 L/Ce2 Fe17 CX þ Ce2 C3 þ C15 L/Ce2 C3 L/ðFCCÞ þ Ce2 C3 L/ðFCCÞ þ Ce2 C3 þ CeC2 . L/CeC2 L þ CeC2 /Ce2 C3 L/ðFCCÞ þ Ce2 C3 þ CeC2 L/CeC2 L þ CeC2 /Ce2 C3 L/Ce2 C3 þ CeC2 þ FCC L/FCC L þ FCC/Ce2 Fe17 CX L þ FCC/Ce2 Fe17 CX þ Ce2 C3 L/Ce2 Fe17 CX þ Ce2 C3 L/Ce2 Fe17 CX þ Ce2 C3 þ C15 L/CeC2 L þ CeC2 /Ce2 C3 L/Ce2 C3 þ CeC2 þ FCC L/CeC2 L þ CeC2 /Ce2 C3 L/Ce2 C3 þ CeC2 þ FCC L/ðFCCÞ þ Ce2 C3 L/ðFCCÞ þ Ce2 C3 þ CeC2 L/FCC L/ðFCCÞ þ Ce2 C3 L/ðFCCÞ þ Ce2 C3 þ CeC2 L/FCC L/FCC þ graphite L þ graphite/FCC þ CeC2

592

6

Fee0.4Cee0.22C

7

Fee0.35Cee0.27C

8

Fee0.29Cee0.43C

9

Fee0.27Cee0.47C

10

Fee0.25Cee0.6C

11

Fee0.25Cee0.15C

12

Fee0.15Cee0.4C

13

Fee0.1Cee0.3C

14

Fee0.05Cee0.15C

15

Fee0.04Cee0.11C

16

Fee0.02Cee0.11C

prepared by a grinding-and-polishing machine (Struers Labopol-5). Firstly, the grinding procedure was carried out using SiC-paper and then diamond discs with grain sizes of 9 and 6 microns respectively. Diamond suspension or spray was applied at regular intervals during the preparation. In the final stage, the samples were subjected to oxide polishing. The estimation of the quality of the samples and preliminary metallographic analysis was carried out using light microscope Olympus GX71F-5. 3.2.2. SEM Scanning electron microscopy was carried out using a TESCAN

1131 e 1120 1710

VEGA LMH microscope with a LaB6 cathode and an energy dispersive X-ray microanalysis system e Oxford Instruments Advanced AZtecEnergy. Both backscattered electron and secondary electron imaging were used in the analysis. An electron microprobe analysis (EMPA) of the phases was carried out using a four-crystal wave spectrometer (the analyzed particle size was larger than 10 mm). The acceleration voltage used for the EMPA was 20 kV. The measurements were carried out with a simultaneous check against standard samples. The statistical error of the detector for the determination of element concentrations using X-ray analysis is 0.2% wt. The detector was calibrated using standard samples of Co

A.V. Khvan et al. / Journal of Alloys and Compounds 651 (2015) 350e356

Fig. 5. Compositions of the alloys investigated during this present work (see also Table 1).

(99.99%), Cu (99.999%), Ni (99.99%), Mn (99.99%), Fe (99.999%), CeO2(99.99%), C(99.99%) and Cr (99.99%) from Oxford Instruments. In order to minimize errors, which are inevitable during measurement of carbide compositions, further calibrations using the carbides Fe3C, Ce2C3 and CeC2 were carried out. The accuracy of each measurement is also dependent on the sample itself, eg. the size of the particle which is measured, the quality of the surface etc. In order to increase the precision of the measurement, 15e20 points were analyzed for each phase. 3.3. X-ray diffraction analysis X-ray diffraction (XRD) analysis was applied to determine the phase composition of the alloys, using CuKa radiation operated at a voltage of 40 kV, a current of 40 mA and filtered with a Ni-crystal monochromator. XRD measurements were performed using a multipurpose X-ray diffractometer (Bruker-AXS D8 Discover). A parallel beam with a divergence of 0.03
is formed using the mirror of Gobel. The reflected intensity of the beam was measured using LYNXEYE position sensitive detector (angular resolution of 0.015
). Phase determination was carried out by consulting the EVA database supplied by Bruker. 3.4. DTA Phase transition temperatures were measured with the DTA Setsys Evo Setaram. The pure metal standards, Cu (99.999%), Au (99.999%) and Ni (99.999%) were used for the calibration. DTA experiments were carried out on as-cast samples contained within a closed crucible made of either Al2O3 and ZrO2 (for the samples with liquidus temperatures above 1600
C) under a continuous flow of argon (99.998% purity) which was additionally purified through filters. The heating and cooling rate was 5
C/min. The temperatures of the invariant reactions were determined from the onset. Data for the solid state phase transition temperatures were determined from the heating curves while the liquidus temperatures were taken from the cooling curves. The analysis was carried out two or three times for each sample in order to obtain accurate values. The results of the DTA analysis are presented in Table 1. 4. Results and discussion The analysis of the microstructure for the alloys 1 and 2 showed the presence of a fine ternary eutectic. Dendrites of primary solidified Ce can clearly be seen in alloy 1 (Fig 6a). The dark crystals

353

correspond to the formation of Ce2C3 through the monovariant reaction L/ðCeÞ þ Ce2 C3 . The solidification of the alloy 1 finishes with formation of the ternary eutectic E1: L/ðCeÞ þ Ce2 C3 þ C15 at 590
C (see Table 1). Dendrite cells of Ce were not observed in the alloy 2, however, but crystals of the Ce2C3 phase were observed (Fig. 6b). This indicates that the position of the composition of the ternary eutectic is situated in between these two alloys. Alloys 3 and 4 also complete their solidification with the formation of the ternary eutectic reaction E1: L/ðCeÞ þ Ce2 C3 þ C15, which was confirmed in the DTA analysis (Table 1). The primary Ce2C3 carbides have a globular form in the microstructure of alloy 3 (Fig. 6c). The regions of the grey phase correspond to Laves C15 phase. The light regions correspond to the formation of the ternary eutectic. In contrast to alloy 3 the solidification of alloy 4 (Fig. 6d) starts with formation of the Laves C15 phase, the spherical particles of the Ce2C3 phase form during monovariant reaction around the crystals of the C15 phase and solidification then finishes with ternary eutectic E1. Solidification of alloy 5 begins with the formation of the Ce2Fe17Cx phase, the C15 phase forms later around the primary crystals as a result of the monovariant peritectic reaction, and the invariant transition reaction U3: L þ Ce2 Fe17 CX /Ce2 C3 þ C15 (Fig. 6e) at ~895
C. The solidification finishes with the formation of the eutectic E1: L/ðCeÞ þ Ce2 C3 þ C15. The results of the microstructure analysis of alloys 6 and 7 were rather surprising. Both of these alloys show a ternary eutectic in their microstructures which was not observed by Park [1]. This consists of the Laves C15, Ce2C3 and Ce2Fe17CX phases, as confirmed by the X-ray analysis (Fig. 7a). Alloy 6 also contains another ternary eutectic E1: L /ðCeÞ þ Ce2 C3 þ C15 what probably means that this alloy is very close to a maximum on the liquidus valley, presumably between Ce2C3 and CeFe2 (Fig. 6f). The amount of ternary eutectic E2: L/Ce2 Fe17 CX þ Ce2 C3 þ C15 in the alloy 7 is slightly higher than in the alloy 6 (Fig. 6g). The temperature of this ternary eutectic is estimated to be 880
C as a result of DTA analysis on alloys 6, 7, and 11 (Table 1). The existence of this E2 eutectic is possible only if the low temperature modification of the Ce2Fe17CX is in equilibrium with the liquid phase in the ternary system, and this also indicates the existence of the P1 reaction. Alloy 11 starts to solidify with the formation of the iron based FCC solid solution (Figs. 6h and 7b), which is followed by the formation of the Ce2Fe17Cx phase through the monovariant reaction L þ FCC/Ce2 Fe17 CX and the U1 reaction: L þ FCC/Ce2 Fe17 CX þ Ce2 C3 . The solidification finishes with the ternary E2 eutectic reaction. The solidification of the alloy 8 starts with the formation of the Ce2C3 carbide (Fig. 6i) which indicates that the monovariant line between primary solidification fields of CeC2 and Ce2C3 lies closer to FeeC side than proposed by Park [1]. The solidification of this alloy continues with formation of the monovariant eutectic L/ðFCCÞ þ Ce2 C3 and finishes with formation of ternary eutectic E3: L /ðFCCÞ þ Ce2 C3 þ CeC2 . Alloy 9 is situated in the primary solidification field of the CeC2 carbide, which is followed by the L þ CeC2 /Ce2 C3 monovariant reaction and the eutectic reaction E3: L/ðFCCÞ þ Ce2 C3 þ CeC2 . In the version of the diagram proposed by Park graphite take part in this eutectic reaction instead of the Ce2C3 carbide. However, this was not confirmed in our investigations of alloys 14e16. Alloys 14 and 15 contain evidence of a ternary eutectic in the microstructure, the only difference between two alloys is that alloy 14 is closer to the eutectic point and contains small primary particles of the Ce2C3 carbide (Fig. 6j), while solidification of the alloy 15 starts with formation of austenite dendrites (Fig. 6k). In contrast to the two previous alloys the ternary eutectic reaction does not take place for alloy 16. The microstructure of alloy 16 consists only of austenite primary dendrites and particles of the carbide CeC2 (Fig. 6l). The absence of the ternary eutectic in the

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Fig. 6. Microstructure of the as-cast samples of the FeeCeeC system: a) Fee0.8Cee0.1C; b) Fee0.75Cee0.1%C; c) Fee0.6Cee0.25C; d) Fee0.6Cee0.075C; e) Fee0.425Cee0.075C; f) Fee0.4Cee0.22C; g) Fee0.35Cee0.27C; h) Fee0.25Cee0.15C; i) Fee0.29Cee0.43C; j) Fee0.05Cee0.15C; k) Fee0.04Cee0.11C; l) Fee0.02Cee0.11C; m) Fee0.15Cee0.4C.

A.V. Khvan et al. / Journal of Alloys and Compounds 651 (2015) 350e356

Fig. 7. XRD of the analyzed samples: a) Fee0.35Cee0.27C; b) Fee0.25Cee0.15C; c) Fee0.05Cee0.15C.

355

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5. Conclusions Three ternary eutectics were identified in the system: E1: L 4 FCC þ Ce2C3þC15, E2: L 4 Ce2C3 þ Ce2Fe17C3 þ C15 and E3: L 4 Ce2C þ Ce2C3 þ FCC. The liquidus projection of the system was constructed on the basis of the experimental investigations reported here. No equilibrium between ternary carbides and the liquid phase has been observed. It was observed that the region of primary solidification of the Ce2C3 phase has a deep extension towards FeeC side of the system.

Acknowledgments Fig. 8. Liquidus projection based on the results of the experimental work described in this paper. The dotted lines show the position of the suggested solidus solid phase concentration triangular.

Table 2 Table of invariant reactions.

The work was carried out with financial support from the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST«MISiS» (No. К2-2014-014).

References


Reaction

T, C

E1 : L/ðCeÞ þ Ce2 C3 þ C15 E2 : L/Ce2 Fe17 CX þ Ce2 C3 þ C15 U3 : L þ Ce2 Fe17 CX /Ce2 C3 þ C15 U1 : L þ FCC/Ce2 Fe17 CX þ Ce2 C3 P1 : L þ FCC þ Ce2 Fe17 Cx ðhtÞ/Ce2 Fe17 Cx ðrtÞ E3 : L/ðFCCÞ þ Ce2 C3 þ CeC2 U2 : L þ graphite/FCC þ CeC2

590 880 895 983 893  T  1063 1120 1145

microstructure of alloy 16 can be explained only if the position of the alloy was outside the eutectic triangular CeC2eCe2C3eFCC. This would not be possible if graphite took part in the reaction instead of the Ce2C3 carbide. The X-ray investigation of the alloy 14 (Fig. 7c) also confirmed the presence of the Ce2C3 phase in the sample, which also supports the suggestion that graphite does not take part in the ternary eutectic reaction. Graphite was not also observed in the XRD patterns of the alloys 8 and 9. Alloys 10, 12 and 13 have very similar microstructures, with the only difference being in the amount of the carbide phase. All three alloys are situated in the field of primary solidification of the CeC2 carbide, which is followed by the formation of the Ce2C3 carbide on the surface of CeC2 grains (Fig. 6m). This is followed in turn by the ternary eutectic reaction corresponding to a thermal arrest at 1120
C as determined through DTA analysis. On the basis of the investigations discussed above a new liquidus projection for the FeeCeeC is proposed as shown in Fig. 8. The dotted lines show the position of the suggested solidus solid phase concentration triangular. Ternary invariant reactions are listed in Table 2.

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