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Experimental Investigation of Fe–Co–La System: Liquidus and Solidus Projections

Abstract

Phase equilibria in the Co–Fe–La system were studied using differential thermal analysis (DTA), X-ray diffraction analysis, scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Liquidus and solidus projections and a melting diagram for this system over the whole concentration range and a Scheil diagram for solidification were constructed. The ternary compound (Co,Fe)17La2 (τ) (Th2Zn17-type structure) forms by peritectic reaction L + (γCo,Fe) + Co13La ⇄ τ at 978 °C. This ternary compound is located along the isoconcentrate of 11 at.% La and extends from 45.8 to 79.3 at.% Co. The Co13La phase has the widest homogeneity region and dissolves up to 43.3 at.% Fe. The solubilities of Fe in Co5La, Co7La2, Co19La5, and Co3La2 were established to be 10.3, 9.2, 4.7, and 3.2 at.%, respectively. The solubility of Fe in Co1.7La2 and CoLa3 according to EPMA does not exceed 1 at.%. The maximum solubility of La in (γCo,Fe) and (αCo,Fe) is determined to be less than 1 at.%.

Introduction

Rare-earth (R) and transition-metal (M) compounds have many potential applications such as the production of permanent magnets[1,2] as well as practical applications in a number of important high-technology branches of modern industry including solid-state electronics, aviation and space technology, and nuclear energy, among others. Permanent magnets based on rare-earth metals such as RCo5, R2Co7, and R2Co17 have very high magnetic crystallographic anisotropy and are characterized by record values of coercive force.[1,3,4,5,6,7]

R-TM alloys are well established as hydrogen-storage materials due to their excellent performance, being applied as such in engines and automobiles due to their good ability to absorb hydrogen.[8,9] However, data on the phase equilibria and types of transformations in the R–Fe–TM and R–Fe–C systems remain limited, and information on the phase equilibria in these systems during crystallization is practically absent. Thus, obtaining systematic information on the phase diagrams of the R–Fe–TM and R–Fe–C ternary systems (M = Mn, Co, Ni; R = La, Ce) over the whole concentration and temperature ranges is relevant. Recently, the phase equilibria in the {La, Ce}–Fe–C and Ce–Fe–{Mn, Co, Ni} systems were investigated.[10,11,12,13,14,15,16,17,18,19] We present herein the results of a study on the phase equilibria in the La–Fe–Co system.

Literature Survey

Binary La–Fe System

Only limited experimental information is available for the phase diagram of the La–Fe system. An abnormal shape of the liquidus was observed between 8 and 19 at.% La[20] and has been discussed in a number of papers[21,22,23,24] in terms of the possible existence of a stable or metastable miscibility gap in the liquid.

Recently, an experimental investigation of the binary La–Fe system was carried out by Mardani et al.[10] Our experimental observations did not confirm the presence of a stable miscibility gap in the liquid phase of the La–Fe system. The La–Fe system is of simple eutectic type with eutectic reaction l ⇄ (αFe) + (βLa) at 788 °C and 88 at.% La. In addition, there is a metatectic reaction (δFe) ⇄ L + (γFe) at 1383 °C due to (δFe) ⇄ (γFe). The liquid phase in the metatectic reaction contains ~ 50 at.% La.

Binary Fe–Co System

The phase diagram of the Fe–Co system adopted in the current work is according to Ohnuma et al.[25] The following crystalline phases are formed in the system: (δFe)-phase (high-temperature bcc solution based on iron), (αCo,γFe)-phase (fcc solution based on pure components), (αFe)-phase (low-temperature bcc solution based on iron), ordered cubic phase FeCo (α’) (B2 prototype of CsCl), and (εCo)-phase (hcp-solution based on cobalt). On the liquidus and solidus lines of the (αCo,γFe) phase, a minimum is present.

The temperature of the γ (fcc) → α (bcc) transition increases with increasing Co concentration and reaches a maximum at 979 °C and 44 at.% Co. At ~ 50 at.% Co and 730 °C, the bcc αFe phase is transformed into an ordered cubic structure B2 with CsCl-type structure (α’). The solid solution (αCo,γFe) at a temperature of 239 °C and 94.7 at.% Co decomposes by eutectoid reaction into α’(CoFe) + (εCo).

Binary La–Co System

The phase diagram of the La–Co system adopted in this work is according to thermodynamic modeling by Wang et al.,[26] which generally agrees well with the experimental data of Buschow and Velge.[27] The system is characterized by the presence of seven intermetallic compounds: CoLa3, Co1.7La2, Co3La2, Co7La2, Co19La5, Co5La, and Co13La. Only the CoLa3 compound melts congruently, at a temperature of 543 °C, while the remaining six compounds are formed by peritectic reactions at 570 °C, 695 °C, 800 °C, 868 °C, 1090 °C, and 1185 °C, respectively. The Co7La2 compound exists in two polymorphic modifications; however, the transformation temperature is unknown.

The La–Co system also contains two eutectics: L ⇄ Co1.7La2 + CoLa3 with coordinates 523 °C and 31.5 at.% Co, and L ⇄ Co1.7La2 + CoLa3 with coordinates 521 °C and 18.7 at.% Co.[26] However, note that the temperatures of these eutectics according to different authors differ significantly. Therefore, in this work, the temperature and composition of these eutectics were verified.

Ternary La–Fe–Co System

There is a lack of experimental data for the La–Fe–Co system. Kharchenko et al.[28] investigated phase equilibria in the system at 800 °C (in the range up to 20 at.% La) and 400 °C (in the range from 20 to 100 at.% La) using SEM and X-ray diffraction. The existence of the ternary compound (Co,Fe)17−xLa2 (Th2Zn17-type structure, R-3m, a = 8.524, c = 12.37 Å) had been reported by Kharchenko et al.[28] This compound has a homogeneity region of 17–22 at.% Fe. It was reported that the solubility of Fe in Co13La extends up to 30 at.%[28] However, data about phase equilibria in the La–Fe–Co system during crystallization are completely absent, as well as information on the temperature and nature of the formation of the ternary compound.

Experimental Methods

Sample Preparation

The samples were melted from starting materials with purity of Fe–99.99%, Co–99.9%, and La–99.9% in an arc furnace with an inconsumable tungsten electrode on a water-cooled copper hearth in an Ar atmosphere purified by a Ti melt. The samples were remelted four or five times to ensure their chemical homogeneity. The weight loss was no more than 0.1%. The ingot weight was 3 g. The composition of each sample was analyzed by microprobe analysis, revealing good agreement with the composition of the initial alloy mixture.

To study equilibria at solidus temperature, after preparation, some of the samples were annealed at subsolidus temperatures. The annealing was performed in a tube furnace (Nabertherm RHTV 120/300/1700) with temperature accuracy of ± 3 °C in Ar atmosphere. The samples were placed in an Al2O3 crucible and additionally wrapped by a titanium lid to avoid oxidation. After annealing, some samples were quenched in oil to retain the equilibrium microstructures. On the whole, a few hours was sufficient to achieve equilibrium during annealing at subsolidus temperature. However prolonged annealing time was necessary for some samples. Annealing temperatures were selected to be 5–10 °C below [Tann = Tsol – (5–10 °C)] solidus temperatures determined from the DTA curves of the as-cast and annealed alloys. The annealing temperatures and times are summarized in Table 4.

Microstructure Analysis

Samples for microstructure analysis were prepared using a grinder and polisher (Struers Labopol-5) machine. Grinding was carried out with SiC paper and continued with polishing using diamond discs with grain size of 9, 3, and 1 microns, respectively. Diamond suspension was applied at regular intervals during the preparation.

The prepared samples were examined by optical microscopy (OM, Olympus-GX71F-5) and 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). For analysis, both backscattered electron and secondary-electron imaging were used. A four-crystal wave spectrometer was used during the electron probe microanalysis (EPMA) of all the phases (analyzed particle size larger than 2 μm). The EPMA acceleration voltage was set at 20 kV. The measurement error in determining the concentration of elements using X-ray analysis was 0.1 wt.%.

Differential Thermal Analysis

A DSC LABSYS evo Setaram was used to measure the phase-transition temperatures of the alloys. For calibration, pure metal standards Sn (99.9995%), Al (99.995%), Ag (99.99%), Cu (99.999%), and Ni (99.99%) were used. DTA samples were placed in an Al2O3 crucible, and experiments were carried out under flow of argon with 99.998% purity on as-cast and annealed samples. The heating and cooling rate was 5 and 10 °C/min. The temperatures of the invariant reactions were determined from the onset. Data for phase-transition temperatures were taken from heating curves. The liquidus temperatures on heating were evaluated from the peak maximum, and those on cooling from the corresponding onset. Pronounced supercooling effects were not observed for the investigated alloys, and the liquidus temperatures for some alloys were therefore taken from the corresponding cooling curve since thermal effects are shown more clearly on cooling. At the end of the heating and cooling process, Calisto Processing software was used to obtain the DTA curves, which were later calibrated based on the equipment’s calibration factor.

X-Ray Diffraction Analysis

X-ray diffraction (XRD) analysis was carried out to determine the phases present in the alloys, using Cu Kα filtered radiation on fine powder samples prepared by grinding in an agate mortar. XRD measurements were performed on a multipurpose X-ray diffractometer (Bruker-AXS D8 Discover) and DRON-3.0 diffractometer (Bourevestnik, Inc., St. Petersburg, Russia). The lattice parameters were refined by the least-squares method. The phases were determined by comparing diffraction patterns with literature or patterns calculated using the PowderCell[29] and WINXPOW[30] software packages. Both the PowderCell and WINXPOW software packages were used to calculate lattice parameters based on the least-squares method.

Experimental Investigation

La–Co System

The temperature of the peritectic reaction for the formation of compound La5Co19, L + LaCo5 ⇄ La5Co19, was reported to be 868 °C by Wang et al.[26] and Ray and Strnat,[31] while this temperature was not observed by Buschow and Velge.[27] Therefore, to verify the temperature of this reaction, 30La-70Co alloy was prepared. The microstructure of this as-cast sample is shown in Fig. 1(a) and (b). The solidification path of this alloy is rather complex: after primary crystallization of the LaCo5 phase (dark grains), the phases La5Co19 (dark-gray grains), La2Co7 (gray grains), La2Co3 (light-gray grains), and La2Co1.7 (white) form through a sequence of peritectic reactions. The DTA curve of this alloy upon heating up to 1150 °C is shown in Fig. 2, indicating five phase transformations. The cooling curve also shows these transformations with slight overcooling. The thermal effect at 854 °C corresponds to the peritectic reaction for the formation of La5Co19 phase (L + LaCo5 ⇄ La5Co19), being somewhat lower than proposed in literature.[26,31] The thermal effect at 840 °C corresponds to the peritectic reaction L + La5Co19 ⇄ La2Co7, which is significantly higher than proposed in literature.[26,27] The thermal effect for the peritectic reaction L + La2Co7 ⇄ La2Co3 was measured at 694 °C, in good agreement with literature.[26,27] The effect at 710 °C, which has not been previously observed, probably corresponds to the polymorphic transformation of the phase La2Co7.

Fig. 1
figure 1

Microstructure of as-cast alloys of La–Co system: a 30La-70Co, ×1000, LaCo5 + La5Co19 + La2Co7 + La2Co3 + La2Co1.7; b 30La-70Co, ×2000, LaCo5 + La5Co19 + La2Co7 + La2Co3 + La2Co1.7; c 69La-31Co, ×2000, eutectic (La2Co1.7 + La3Co); d 80La-20Co, ×5000, (βLa) + La3Co + eutectic ((βLa) + La3Co)

Fig. 2
figure 2

Heating curve for as-cast alloy 30La-70Co

There is some controversy concerning the composition of the eutectic reaction L ⇄ La3Co + La2Co1.7. Verification of the temperature and composition of this eutectic was performed on an as-cast sample with composition 69La-31Co. The obtained results are presented in Table 1 and 2 and Fig. 1(c). The microstructure of this alloy (Fig. 2c) is completely eutectic (La3Co + La2Co1.7). The eutectic temperature was measured to be 514 °C, higher than proposed in Ref. [27,32] but lower than proposed by Wang et al.[26] The eutectic composition according to EPMA data was 30.5 at.% Co, in good agreement with Buschow and Velge.[27]

Table 1 Phase composition of alloys of La–Co system and temperature of phase transformations during crystallization
Table 2 Chemical composition of phases of as-cast alloys of La–Co system according to EMPA

Verification of the temperature and composition for the eutectic L ⇄ (βLa) + La3Co was performed on as-cast samples 10La-90Co and 20La-80Co. The results obtained are presented in Table 1 and Fig. 1(d). The microstructure of this alloy (Fig. 1d) shows the primary (βLa)-phase and the eutectic ((βLa) + La3Co). The composition of this eutectic was established by the microprobe method as 19.8 at.% Co, in good agreement with Ref. [27,32]. The eutectic temperature was measured to be 530 °C, slightly higher than proposed in Ref. [26,27,32]. The revised phase diagram of the La–Co system is shown in Fig. 3.

Fig. 3
figure 3

Revised phase diagram of La–Co system: open triangles—DTA data

La–Fe–Co System

The phase equilibria in the La–Fe–Co system were investigated during solidification. The resulting liquidus (Fig. 4a) and solidus projections (Fig. 4b) and the melting diagram (Fig. 4c) of the La–Fe–Co system have been constructed over the whole concentration range. The phase compositions of the studied alloys and chemical compositions of the phases of the La–Fe–Co system according to the EPMA results are reported in Table 3 and 4, respectively. Table 3 also presents the liquidus and solidus temperatures of the investigated alloys. The compositions of the investigated alloys in Fig. 4 correspond to measured compositions of alloys, whereas in Table 3 nominal compositions are given, and in Table 4 both nominal and measured compositions are given. The microstructures of some as-cast and annealed samples are shown in Fig. 5 and 6, respectively. The XRD patterns of some as-cast alloys are shown in Fig. 7. The crystal structure and lattice parameters of the phases of the Fe–Co–La system are presented in Table 5.

Fig. 4
figure 4figure 4

Liquidus (a) and solidus (b) projections and the melting diagram (c) of the La–Co–Fe system: open circle—composition of sample, dotted circle—two-phase sample, filled circle—three-phase sample, open and filled triangle—EPMA data for as-cast and annealed alloys, respectively

Table 3 Phase composition of studied alloys of Co–Fe–La system and temperature of phase transformations at crystallization
Table 4 Composition of Co–Fe–La phases according to EPMA data
Fig. 5
figure 5

Microstructure of as-cast alloys of La–Co–Fe system: a 40Ce-50Co-10Fe (#1), ×1000, (αCo,Fe) + τ + La2Co1.7 + eutectic (La2Co1.7 + La3Co); b 20Fe-50Co-30La (#8), ×2000, (γCo,Fe) + τ + La2Co3 + La2Co1.7 + eutectic (La2Co1.7 + La3Co); c 15Fe-75Co-10La (#10), ×2000, (γCo,Fe) + LaCo13 + LaCo5 + La2Co7 + La2Co3; d 10Fe-10Co-80La (#36), ×2000, (αCo,Fe) + (βLa) + La3Co; e 2Fe-33Co-65La (#38), ×1000, (αCo,Fe) + La2Co1.7 + eutectic (La2Co1.7 + La3Co); f 10Fe-75Co-15La (#44), ×2000, LaCo13 + LaCo5 + La2Co7 + La2Co3; g 10Fe-65Co-25La (#45), ×2000, LaCo13 + τ + La2Co7 + La2Co3 + La2Co1.7 + eutectic (La2Co1.7 + La3Co); h 20Fe-55Co-25La (#46), ×2000, LaCo13 + La2Co3 + La2Co1.7 + eutectic (La2Co1.7 + La3Co); i 5Fe-75Co-20La (#7), ×1000, LaCo13 + LaCo5 + La5Co19 + La2Co7 + La2Co3 + La2Co1.7; j15Fe-45Co-40La (#61), ×2000, τ + La2Co7 + La2Co3 + eutectic (La2Co1.7 + La3Co); k 2Fe-53Co-45La (#65), ×2000, LaCo5 + La2Co7 + La2Co3 + La2Co1.7; l 2Fe-20Co-78La (#67), ×2000, eutectic ((αCo,Fe) + La3Co + (βLa))

Fig. 6
figure 6

Microstructure of alloys of La–Co–Fe system annealed at subsolidus temperatures: a 5Fe-75Co-20La (#7), 810 °C, ×2000, La2Co7 + LaCo5 +La5Co19; b 10Fe-62Co-25La (#54), 660 °C, ×2000, La2Co3 + τ + La2Co7; c 5Fe-50Co-45La (#62), 560 °C, ×500, (αCo,Fe) + La2Co3 + La2Co1.7; d 10Fe-30Co-60La (#34), 510 °C, ×2000, (αCo,Fe) + La3Co + La2Co1.7; e 10Fe-10Co-80La (#36), 520 °C, ×2000, (αCo,Fe) + La3Co + (βLa)

Fig. 7
figure 7

X-ray diffraction patterns of as-cast alloys in La–Co–Fe system: a 5Fe-75Co-20La (#7), as-cast, LaCo13 + LaCo5 + La5Co19 + La2Co7 + La2Co3; b 15Fe-60Co-25La (#50), as-cast, LaCo13 + τ + LaCo5 + La2Co3 + La2Co1.7

Table 5 Crystal structure and lattice parameters of La–Fe–Co phases

Ternary Compound (Co,Fe)17La2 (τ)

The existence of the ternary compound (Co,Fe)17La2 (τ) (structure type Th2Zn17, R-3m), which was previously reported,[28,33,34] was confirmed in our study. It was shown that ternary compound τ forms by peritectic reaction LP1 + LaCo13 + LaCo5 ⇄ τ at 978 °C and has a wide homogeneity range from 45.8 to 79.5 at.% Co located along the isoconcentrate of ~ 11 at.% La. This somewhat contradicts the data of Kharchenko et al.,[28] who reported that the homogeneity region of the τ phase is located along the isoconcentrate of 15 at.% La. The iron-rich boundary of the region of homogeneity of the τ phase is defined as 45.8 at.% Co according to the EPMA data of alloy #1 (Fig. 5a, Table 4). The homogeneity region of the cobalt-rich side τ phase is defined as 79.5 at.% Co according to the EPMA data of alloy # 2 (Table 4).

Solid Phases

In addition to the ternary compound (Co,Fe)17La2 (τ) in the La–Co–Fe system, other phases based on the binary compounds Co13La, Co5La, Co19La5, Co7La2, Co3La2, Co1.7La2, and CoLa3 and components (δFe), (γCo,Fe), (αCo,Fe), (γLa), and (βLa) take part in phase equilibria.

Among the binary compounds, Co13La (structural type NaZn13, cF112-Fm-3c) has the widest homogeneity region at solidus temperature and, according to EPMA data of as-cast alloys #3 and #4 (Table 4), dissolves up to 43.3 at.% Fe. The homogeneity region of this phase is located along the isoconcentrate of 7 at.% La and extends widely into a ternary system, which reflects significant mutual substitution of Co and Fe atoms. The lattice parameter a of Co13La phase gradually increases with increasing Fe content (Table 5).

According to the EPMA data of as-cast alloy #5, the solubility of Fe in Co5La is 10.3 at.% (Table 4). The solubility of Fe in Co7La2 at solidus temperature reaches 9.2 at.% according to the EPMA data of cast alloy #6. The homogeneity regions of the remaining phases are smaller. The solubility of Fe in the Co19La5 phase at solidus temperature is 4.7 at.% according to the EPMA data of alloy #7 annealed at subsolidus temperature (810 °C, 4 h). The solubility of Fe in Co3La2 is even less and amounts to 3.2 at.% at solidus temperature. The solubility of Fe in Co1.7La2 and CoLa3 according to EPMA does not exceed 1 at.%. The maximum solubility of La in (γCo,Fe) and (αCo,Fe) phases is determined to be less than 1 at.% (Table 4).

Liquidus Projection

The liquidus surface of the Fe–La–Co system (Fig. 4a) is characterized by the 13 primary solidification fields of the ternary compound (Co,Fe)17La2 (τ), solid solutions based on binary phases Co13La, Co5La, Co19La5, Co7La2, Co3La2, Co1.7La2, and CoLa3, and component-based solid solutions (δFe), (γCo,Fe), (αCo,Fe), (γLa), and (βLa), separated by appropriate monovariant curves and participating in ten four-phase invariant equilibria. One four-phase invariant equilibrium is of eutectic type, two are of peritectic type, the the rest are of transitional type.

The primary solidification field of the (γCo,Fe)-phase occupies the major part of the liquidus (Fig. 4a). Analysis of the microstructures of the as-cast samples showed that the compositions of most of the investigated alloys (#3, 4, 6, 8–33) are located in the primary solidification field of the (γCo,Fe) phase. Typical microstructures of alloys from this region are shown in Fig. 5(b) and (c). The solidification of alloy #8 begins with formation of the (γCo,Fe)-phase (black grains), continues with the formation of τ phase (gray grains) around the primary grains, then phases Co3La2 (light-gray grains) and Co1.7La2 (light grains) are formed, and the solidification finishes with eutectic reaction (Co1.7La2 + CoLa3) (Fig. 5b). For alloy #10, the crystallization path is also rather complex: after the primary crystallization of the (γCo,Fe) phase (black grains), the phases Co13La (dark-gray grains), Co5La (gray grains), Co7La2 (light-gray grains), and Co3La2 (light grains) crystallize by peritectic reactions (Fig. 5c).

Alloys #1, 34–43 are located in the field of primary crystallization of the (αCo,Fe) phase (Fig. 5a, d, e). The border of this field is limited by the composition of alloys #1, 36–39, in which only a small amount of primary grains of (αCo,Fe) phase are observed (Fig. 5a, d, e).

Alloys #2, 7, 44–53 are located in the field of primary crystallization of the Co13La phase (Fig. 5f–i). The observation of primary (γCo,Fe) in sample #8 (Fig. 5b) in contrast to primary Co13La in samples #44 and #46 (Fig. 5f, g) indicates that the monovariant curve L + (γCo,Fe) ⇄ Co13La of joint crystallization of the phases (γCo,Fe) and Co13La is located between the compositions of these alloys. Note that the monovariant curve L + (γCo,Fe) ⇄ Co13La has a saddle point corresponding to the maximum temperature on the solidus surface (γCo,Fe) + Co13La at ~ 1300 °C.

The location of the monovariant curve P1U1, separating the fields of primary solidification of Co13La and τ, is evident from the observation of the primary Co13La in samples #45 and #50 (Fig. 5g) in contrast to τ-phase in samples #54–56. Some attention should be focused on the microstructure of samples #45 and #50 (Fig. 5g), where questions arise regarding whether the primary solidified phase is Co13La phase (black grains) or τ phase (dark-gray grains). Both Co13La and τ appear primary. Possibly, the primary phase in these alloys is the Co13La phase, then after primary crystallization of Co13La, the liquid becomes depleted of lanthanum, and the τ phase crystallizes as primary, as well. This indicates that these alloys are located almost on a monovariant curve L + Co13La ⇄ τ. In alloy #45, after crystallization of the Co13La phase (black grains) and τ phase (dark-gray grains), the phases LaCo5, La2Co7, and La2Co3 crystallize. Solidification finishes with the formation of the eutectic (La2Co1.7 + CoLa3) (Fig. 5g).

Alloys #54–62 are located in the field of primary crystallization of the τ-phase (Fig. 5j). The relative position of both monovariant curves and the composition of the ternary compound allow us to conclude that the compound melts incongruently. The incongruent formation of the τ phase is also confirmed by the nature of crystallization and liquidus temperature of the alloys in this composition range.

In alloy #1, which is shown in Fig. 5(a), the primary grains of the (αCo,Fe)-phase with a typical cubic shape (black) are clearly visible, then the τ phases (dark gray) crystallize in this alloy and also appear primary. This indicates that this alloy is located very close to the monovariant curve of joint crystallization of the phases τ and (αCo,Fe). The solidification in this alloy continues with the formation of the Co1.7La2 phase (gray grains) and finishes with the formation of the eutectic (Co1.7La2 + CoLa3) (Fig. 5a).

In the La-rich corner and along the Co–La side, the fields of primary crystallization of phases based on the components (βLa) and (γLa) and phases based on binary compounds Co19La5, Co7La2, Co3La2, Co1.7La2, and CoLa3 are present. They are all very narrow. The field (βLa) is plotted by observation of the primary (βLa) phase in alloys #63 and #64 in contrast to the primary (αCo,Fe)-phase in alloy #36. The field of (γLa), as well as (δFe), is shown tentatively.

The boundaries of the fields of primary crystallization of phases based on cobalt lanthanides Co19La5, Co7La2, Co3La2, Co1.7La2, and CoLa3 are determined mainly from the microstructure of the alloys located along the isoconcentrate 2 at.% Fe. The field of primary crystallization of the Co5La phase is determined from the microstructure of alloys #5 and #65 (Fig. 5k), which shows that these alloys are located in the field of primary crystallization of the Co5La phase. Moreover, the microstructure of alloy #65 contains a small amount of primary grains of the Co5La (black grains). After the primary crystallization of Co5La (black grains), the Co7La2 phase (dark-gray grains) forms around the primary grains, then the Co7La2, Co3La2, and Co1.7La2 phases crystallize (Fig. 5k). The location of the monovariant curve P1U3, separating the fields of primary solidification of τ and Co5La, is evident from the observation of the primary τ in samples #54, 57, and 62 in contrast to Co5La in samples #5 and #65 (Fig. 5k).

No alloys were investigated in the primary crystallization field of the La5Co19 phase, but note that this field is the narrowest and is limited by the composition point of alloy #65. The monovariant curves L + LaCo5 ⇄ La5Co19 (p5U1) and L + La5Co19 ⇄ La2Co7 (p6U1) do not extend appreciably into the ternary system. The intersections of these curves give the composition of the liquid U1. The composition of the liquid at point U1 cannot contain more than 1 at.% Fe.

The primary solidification field of the La2Co7 phase is also very small. There were no alloys inside this field, but the border of this field is limited by the composition of alloys #62, 65, and 66, in which the primary phase is τ, LaCo5, and La2Co3, respectively (Fig. 5k).

Only alloy #66 is located in the field of primary solidification of the phase La2Co3. The solidification of this alloy continues with the formation of the La2Co1.7 phase, and finishes with the formation of the eutectic (Co1.7La2 + CoLa3).

The primary solidification field of the Co1.7La2 and CoLa3 phases are very narrow and limited by the composition of alloys #37, 38, and 39, respectively, in which only a small amount of primary grains of (αCo,Fe) phase is observed (Fig. 5e). In alloys #37 and #38, after primary crystallization of the (αCo,Fe) phase (black grains), the La2Co1.7 phase crystallizes, and solidification finishes with the formation of the eutectic (La2Co1.7 + CoLa3) (Fig. 5e). The very small amount of primary (αCo,Fe) grains in these alloys indicates that the primary crystallization field of the Co1.7La2 phase does not extend into the ternary system more than up to 2 at.% Fe (Fig. 5e). In alloy #39, the (αCo,Fe) phase solidifies as a primary phase, then the CoLa3 phase crystallizes, and solidification finishes with the formation of the ternary eutectic ((αCo,Fe) + (βLa) + CoLa3). Note that the monovariant curve L ⇄ (αCo,Fe) + CoLa3 has a saddle point corresponding to the maximum temperature on the solidus surface (αCo,Fe) + CoLa3 at 540 °C.

Analysis of the microstructure of alloy #67 showed the presence of a ternary eutectic ((βLa) + (αFe,Co) + La3Co). This eutectic is very fine. Therefore, this alloy was heated and cooled in DTA equipment at the rate of 5 °C/min in order to obtain the solidification morphologies of this alloy. Only the microstructure of this alloy after DTA (Fig. 5l) allowed the identification of three phases in the eutectic: (βLa) (white grains), La3Co (gray matrix), and (αFe,Co) (black grains). This ternary eutectic was also observed in alloys #39 and #64; moreover, in the first alloy the (αFe,Co) phase is primary, and in the second alloy (βLa). The composition of the ternary eutectic ((βLa) + (αFe,Co) + La3Co) according to the EPMA data of alloys #64 and 67 is 1.4Fe-20.3Co-78.3La (Table 4, Fig. 5l).

The isotherms of the liquidus surface (Fig. 4a) are constructed based on the DTA results (Table 3), and additionally refined via the constructed vertical sections, so that each alloy is involved in at least three sections.

Solidus Projection

Figure 4b shows the solidus surface projection of the Fe–Co–La system in the whole composition range resulting from this research. The solidus surface of this system is characterized by the presence of the following three-phase regions: τ + LaCo13 + LaCo5, La5Co19 + LaCo5 + La2Co7, La2Co7 + τ + La2Co3, LaCo5 + La2Co7 + τ, τ + (αCo,Fe) + La2Co3, LaCo13 + τ + (αCo,Fe), La2Co3 + La2Co1.7 + (αCo,Fe), (αCo,Fe) + La3Co + La2Co1.7, (αCo,Fe) + La3Co + (βLa), and (αCo,Fe) + Co13La + (γCo,Fe) and the corresponding two-phase regions. The corners of these triangles are plotted using the microprobe results for individual phases (Table 4). The solidus temperatures of the three-phase regions, as measured by DTA (Table 3), are shown in the solidus projection in Fig. 4b.

The existence and position of the very narrow three-phase region La5Co19 + LaCo5 + La2Co7 with the participation of the La5Co19 phase is established based on the SEM and EPMA results for alloy #7 annealed at the solidus temperature (810 °C, 4 h) (Table 3, 4, Fig. 6a). The microstructure of this alloy clearly shows three phases (Fig. 6a): dark, gray, and light gray. These correspond to La5Co19, LaCo5, and La2Co7, respectively. The heating curve of this alloy shows that the temperature of the corresponding isothermal plane is 818 °C. Therefore, this three-phase region La5Co19 + LaCo5 + La2Co7 is formed via U-type reaction L + La5Co19 ⇄ La2Co7 + LaCo5 taking place at 818 °C.

The SEM and EMPA results of alloys #45 and 54 annealed at the solidus temperature (660 °C, 2 h) clearly show the three phases La2Co7 (dark-gray grains) + τ (gray grains) + La2Co3 (light-gray grains) (Fig. 6b). The composition of the solid phases in this equilibrium was measured by the microprobe method (Table 4). The three-phase structure (La2Co7 + τ + La2Co3) of these alloys (Fig. 6b) and invariant effects at 673 °C on their heating curves (Table 3) (corresponding to U-type reaction) determine the position and temperature of the relevant three-phase region.

Two very narrow three-phase regions with the participation of the τ phase are also formed on the solidus surface in the Co-rich corner: τ + LaCo13 + LaCo5 and LaCo5 + La2Co7 + τ. The boundaries of the three-phase region τ + LaCo13 + LaCo5 are established based on the SEM and EPMA data for the two-phase LaCo13 + LaCo5 alloy #22, annealed at solidus temperature (970 °C, 3 h) and the EPMA data of alloy #2 containing the τ-phase (Table 4). There were no samples corresponding to this three-phase region; however, it should be pointed out that this region is very narrow. Even though there is no experimental evidence concerning the exact position of this region, it cannot differ substantially from that shown in Fig. 4(b). The high content of Co in the τ-phase (79.3 at.% according to EPMA data of alloy #2) leads to the conclusion that alloy #22 lies very close to the associated boundary tie-lines of the τ + LaCo13 + LaCo5 three-phase region. The heating curve of this alloy shows that the temperature of the respective isothermal plane is 978 °C (Table 3). The existence of the LaCo5 + La2Co7 + τ three-phase region is obvious in terms of phase equilibrium rules. Since there are no alloys in this composition range, the vertices of the respective tie-line triangle are shown tentatively. The temperature of this isothermal plane (687 °C) is determined from the heating curve of some as-cast alloys (#45, 47, 49) in which the respective phase region is not in equilibrium.

The τ-phase in addition to the LaCo13, LaCo5, La2Co7, and La2Co3 phases is in equilibrium with the (αCo,Fe) phase, forming two more three-phase regions: τ + (αCo,Fe) + La2Co3 and LaCo13 + τ + (αCo,Fe). There were no samples corresponding to these three-phase regions; however, it should be pointed out that these regions are very narrow. Even though there is no experimental evidence concerning the exact position of these regions, it cannot differ substantially from that shown in Fig. 4(b). The position of the vertices of the respective tie-line triangles is determined from the data of neighboring two-phase alloys and the mutual arrangement of the regions of phase homogeneity.

The existence of the three-phase region (αCo,Fe) + La2Co3 + La2Co1.7 and its location were established directly, on the basis of the investigation of three-phase samples. The SEM and EMPA results of alloys #59 and #62 annealed at the subsolidus temperature (560 °C, 2 h) clearly show the three phases (Fig. 6c). The dark and gray regions are the (αCo,Fe) and La2Co3 phases, respectively. The light-gray regions correspond to La2Co1.7. The compositions of the solid phases in these equilibria were measured by the microprobe method (Table 4). According to the DTA data of these alloys and alloys #8, 31, 58, 65, and 66, the temperature of the respective isothermal plane is 569 °C (Table 3). This three-phase region is formed by the transition-type reaction L + La2Co3 ⇄ (αCo, Fe) + La2Co1.7 at 569 °C.

According to the SEM and EPMA results, the alloy #34, annealed at subsolidus temperature (510 °C), is located in the three-phase region La3Co + La2Co1.7 + (αCo,Fe). Three phases can be well distinguished in the microstructure of this alloy (Fig. 6d). They were identified as follows: black grains corresponds to (αCo,Fe), gray and light-gray grains correspond to La2Co1.7 and La3Co, respectively. The heating curves of this alloys, as well as as-cast alloys #1, 15, 16, 18, 20, 29, 34, 35, 37, and 38, show an invariant effect at 515 °C (Table 3). The types of equilibria are confirmed both by the relative positions of the points of liquid and solid phases in equilibria, and by their temperature in comparison with those of incoming and outgoing ones. A three-phase region of the solidus surface La3Co + La2Co1.7 + (αCo,Fe) results from a four-phase invariant transition-type reaction L + (αCo,Fe) ⇄ La3Co + La2Co1.7 at 515 °C, rather than a eutectic type. The arguments for this conclusion are as follows: (1) the solidus temperature of the field La3Co + La2Co1.7 + (αCo,Fe) is higher than the temperature of the eutectic L ⇄ La3Co + La2Co1.7 in the Co–La binary system (DTA analysis of the binary eutectic alloy 69La-31Co indicates the temperature of the eutectic reaction L ⇄ La3Co + La2Co1.7 to be 514 °C); (2) in most alloys, the eutectic (La3Co + La2Co1.7) was observed. This could not be observed in the alloys if the invariant equilibrium were of the eutectic type L ⇄ (αCo, Fe) + La3Co + La2Co1.7. To confirm this experimentally, a comparative thermal analysis, at a very low heating rate of 5 °C/min, was carried out on the ternary alloy #38 and the binary eutectic alloy 69La-31Co. It was observed that the binary alloy began to melt earlier than the ternary alloy. Further DTA tests at a heating rate of 5 K/min revealed that the temperatures of the ternary alloy #38 and binary alloy 69La-31Co were 515 °C and 514 °C, respectively.

There is one additional isothermal plane, (αCo,Fe) + La3Co + (βLa), involving the (αCo,Fe)-phase, which forms on the solidus surface. The boundaries of this three-phase region are established based on the SEM and EPMA data for the three-phase region (αCo,Fe) (black grains) + La3Co (gray grains) + (βLa) (light-gray grains) for alloy #36 annealed at the subsolidus temperature (520 °C, 2 h) (Table 4, Fig. 6e). The heating curve of this alloy, as well as as-cast alloys #39, 63, 64, and 67, shows an invariant effect at 527 °C (Table 3). This three-phase region forms via eutectic-type reaction L ⇄ (αCo,Fe) + La3Co + (βLa) at 527 °C. The eutectic type of this equilibrium is confirmed by the ternary eutectic microstructure (αCo,Fe) + La3Co + (βLa) of alloy #37 (Fig. 5l).

Invariant Equilibria

The superposition of the liquidus and solidus surfaces of the Fe–Co–La system in the form of a melting diagram is shown in Fig. 4(c). The three-phase regions of the solidus surface are the result of invariant four-phase reactions. Three-phase fields at the solidus surface result from one eutectic LE1 ⇄ La3Co + (βLa) + (αCo,Fe), two peritectic LP1 +LaCo13 + LaCo5 ⇄ τ and LP2 + (γCo,Fe) + Co13La ⇄ (αCo,Fe), and seven U-type four-phase invariant equilibria LU1 + LaCo13 ⇄ (αCo,Fe) + τ, LU2 + La5Co19 ⇄ LaCo5 + La2Co7, LU3 + LaCo5 ⇄ τ + La2Co7, LU4 + La2Co7 ⇄ τ + La2Co3, LU5 + τ ⇄ (αCo,Fe) + La2Co3, LU6 + La2Co3 ⇄ (αCo,Fe) + La2Co1.7, and LU7 + (αCo,Fe) ⇄ La3Co + La2Co1.7, taking place at 527, 978, ~ 970, ~ 950, 818, 687, 673, 645, 569, and 515 °C, respectively. In the two-phase areas (γCo,Fe) + LaCo13 and (αCo,Fe) + La3Co, the solidus surface has maximum temperatures of ~ 1300 and 540 °C, respectively. All invariant equilibria are summarized in Table 6. Figure 8 shows the Scheil diagram for the solidification of Co-Fe-La alloys.

Table 6 Invariant equilibria in the Fe–Co–La system
Fig. 8
figure 8

Scheil diagram for solidification of La-Co-Fe alloys

Conclusions

Phase equilibria in the Fe–Co–La system over the whole concentration range during solidification have been studied using DTA, X-ray diffraction analysis, SEM, and electron probe microanalysis. Liquidus and solidus projections, as well as the melting diagram and a Scheil diagram were constructed for this system.

The ternary compound (Co,Fe)17La2 (τ) forms by peritectic reaction LP1 + LaCo13 + LaCo5 ⇄ τ at 978 °C and has a wide homogeneity range from 45.8 to 79.5 at.% Co at solidus temperature, which is linear and located along the isoconcentrate of ~ 11 at.% La.

The Co13La phase has the widest homogeneity range at solidus temperature and dissolves up to 43.3 at.% Fe.

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Acknowledgments

The study was supported by Russian Science Foundation project no. 18-73-10219.

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This invited article is part of a special tribute issue of the Journal of Phase Equilibria and Diffusion dedicated to the memory of Günter Effenberg. The special issue was organized by Andrew Watson, Coventry University, Coventry, United Kingdom; Svitlana Iljenko, MSI, Materials Science International Services GmbH, Stuttgart, Germany; and Rainer Schmid-Fetzer, Clausthal University of Technology, Clausthal-Zellerfield, Germany.

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Fartushna, I., Mardani, M., Khvan, A. et al. Experimental Investigation of Fe–Co–La System: Liquidus and Solidus Projections. J. Phase Equilib. Diffus. 41, 418–442 (2020). https://doi.org/10.1007/s11669-020-00800-w

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Keywords

  • Co–Fe–La
  • Co–La
  • Liquidus surface
  • Phase diagrams
  • Solidus surface