Experimental Investigation of Isothermal Section in the La–Co–Ni System at 723 K

Abstract

The isothermal section of the La–Co–Ni ternary system at 723 K has been constructed in this work by using X-ray diffraction (XRD), scanning electron microscopy, and energy dispersion spectroscopy techniques (SEM-EDS). The experimental results show no existence of ternary compounds at 723 K. The isothermal section consists of 16 two-phase regions and 8 three-phase regions. La₃Co and La₃Ni, La₂Co₃ and La₂Ni₃, La₂Co₇ and La₂Ni₇, and LaCo₅ and LaNi₅ form a continuous solid solution. The ternary solid solubility of Ni in LaCo₁₃ phase and La₂Co_1.7 phase was determined to be 15.61 at.% and 9.61 at.%, respectively. The solid solubility of Co in the LaNi₃, La₇Ni₃, and LaNi phases was 18.07 at.%, 5.62 at.%, and 8.49 at.%, respectively. The present experimental results are important for the design of La(Fe,Si)₁₃-based magnetic refrigeration materials.

1. Introduction

Magnetic cooling is a novel, energy-efficient, and environmentally friendly technology that aims to replace conventional vapor-compression technology for air conditioning, space heating, and domestic refrigerators/freezers. It is based on the magnetocaloric effect (MCE), which occurs in magnetic materials, in particular those undergoing a magnetic or magneto-structural phase transition [1,2,3]. The NaZn₁₃-type La(Fe,Si)₁₃-based alloy system has attracted great attention among reported magnetic refrigerants, as it exhibits tunable giant entropy change and small hysteresis at ambient conditions and contains a minimal amount of rare-earth elements with high Fe-content and high magnetic moment [4,5]. However, its phase transition temperature is around 200 K, so it is difficult to cool it at room temperature. The introduction of transition element Ni can effectively improve the stability of La(FeSi)₁₃ compounds, and Co can increase its magnetic phase transition temperature [6,7,8,9,10]. By optimizing the Fe/Co/Ni/Si ratio of the La(FeCoNiSi)₁₃ alloy, the magnetic phase transition temperature of the alloy can be adjusted to room temperature, which makes the application of magnetic refrigeration technology at room temperature very possible. Phase diagrams are very important for studying the effects of the elements and are beneficial for alloys in design and fabrication [11,12,13,14,15]. Up to now, however, the current report on the isothermal section of the La–Co–Ni ternary system is the La < 32.2% isothermal section of the La–Co–Ni ternary phase diagram determined by Zheng et al. [16] in 1981, and the phase diagram of La–Co–Ni is not complete. Therefore, here we experimentally determined the isothermal cross-section of the La–Co–Ni ternary system at 723 K.

2. Literature Review

2.1. The La–Co System

The phase diagram of the Co–La system was first studied in detail by Velge [17], who measured the phase diagram of the entire compositional region by metallography, X-ray diffraction, and thermal analysis. The phase formation of La₃Co, La₂Co_1.7, La₂Co₃, La₂Co₇, LaCo₅, and LaCo₁₃ was determined at 727 K, 843 K, 968 K, 1073 K, 1363 K, and 1458 K, respectively. After that, the phase diagram was re-investigated by Ray [18], and it was confirmed that La₂Co_1.7, La₂Co₃, La₂Co₇, LaCo₅, and LaCo₁₃ were formed by peritectic reaction and found one compound of the La5Co19, which [17] did not report, and the peritectic reaction of La₅Co₁₉ was confirmed at a temperature of about 1141 K. In 2006, Wang et al. [19] re-optimized the thermodynamics of the Co–La binary system, and the calculated data were consistent with the experimental results of Buschow and Velge [17]. The calculated phase diagram of the Co–La system is shown in Figure 1a. In 2019, Iwase et al. [20] reported the crystal structure of the compound La₅Co₁₉ in detail and found its good hydrogen storage properties.

2.2. The La–Ni System

In 1972, Buschow and Mal [23] conducted a detailed study of the Ni–La (50–100 at% Ni) system using thermal analysis, metallography, and X-ray diffraction techniques, and they confirmed the stability of five phases LaNi, LaNi_1.4, LaNi₂, LaNi₃, La₂Ni₇, and LaNi₅. In 1991, Zhang et al. [24] re-determined the Ni–La binary system phase diagram (50–83.3 at% Ni) and found that the LaNi₂ phase could not exist stably, and the LaNi_2.286 phase was confirmed to be the La₇Ni₁₆ phase. In 2003, Zhou et al. [25] studied the phase diagram of the Ni–La–Si ternary system and found that there are eight intermediate compounds at the Ni–La end, namely: La₃Ni, La₇Ni₃, LaNi, La₇Ni₁₆, La₂Ni₃, La₂Ni₇, LaNi₅, and LaNi₃. In 1998, Du et al. [21] studied the phase diagram of the Ni–La binary system and found nine intermetallic compounds, namely La₃Ni, La₇Ni₃, LaNi, La₇Ni₁₆, La₂Ni₃, La₂Ni₇, βLa₂Ni₇, LaNi₅, and LaNi₃. The calculated phase diagram of the Co–La system is shown in Figure 1b. In 2000, Dischinger and Schaller [26] used DSC technology to measure the heat capacity of all intermetallic compounds in the La–Ni system at 923~1023 K but did not consider the La₅Ni₁₉ phase. In addition, they reported a new phase (La₄Ni₁₇), but this conclusion was not confirmed by other researchers. In 2008, An et al. [27] re-studied the Ni–La binary system and discovered the La₅Ni₁₉ phase, which was formed by the peritectic reaction between the liquid phase and the LaNi₅ phase at a reaction temperature of 1276 K. In 2017, Subotenko et al. [28] found that LaNi₂ is stable at elevated temperatures and decomposes into the La₇Ni₁₆ phase below 1003.5 K.

2.3. The Co–Ni System

The Co–Ni system is a simple isomorphous system. It is completely mutually soluble in the whole concentration range of Co and Ni. In 2019, Zhou et al. [22] conducted a thermodynamic evaluation of the Co–Ni–Ta system, where the calculated phase diagram of the Co–Ni system was shown as Figure 1c.

2.4. The La–Co–Ni System

In 1981, a part of the ternary phase diagram of La–Co–Ni (La < 32.2% isothermal section at room temperature) was determined by Zheng [14]. Four single-phase regions were: αCo, βNi, La₃Co, and LaCo_5xNi_5-5x; five two-phase regions were: αCo + βNi, LaCo₁₃ + αCo, LaCo₁₃ + βNi, βNi + LaCo_5xNi_5-5x, LaCo₁₃ + LaCo_5xNi_5-5x; and two three-phase regions were: αCo + βNi + LaCo₁₃ and LaCo₁₃ + βNi + LaCo_5xNi_5-5x. Because the phase diagram of the La–Co–Ni system was not verified in detail due to the limitation of the early experimental conditions, this work is based on the study of the phase diagram of the La–Co–Ni system.

The known structures of the single and binary phases are listed in

Table 1. The data on the crystal structures of the compounds of the La–Ni, Ni–Co, and La–Co binary systems.

Compound	Pearson Notation	Space Group	Structure Type	Lattice Parameters		Reference
				a (nm)	c (nm)
LaCo13	F112	Fm-3c	NaZn13	a = 11.334(1)	—	[17]
LaCo5	hP6	P6/mmm	CaCu5	a = 5.100(5)	c = 3.968(5)	[17]
La5Co19	hR72	R-3m	Ce5Co19	a = 5.130(5)	c = 49.50(4)	[20]
La2Co7	hR54	R-3m	Gd2Co7	a = 5.101(5)	c = 24.511(4)	[17]
La2Co3	0S20	Cnca	La2Ni3	a = 10.34(1)	c = 7.811(5)	[17]
La2Co1.7	mS8	C2/m	La2Co1.7	a = 8.4536(1)	c = 4.2723(2)	[17]
La3Co	Op16	pnma	Fe3C	a = 7.277(9)	c = 6.575(8)	[17]
αCo	Cf4	Fm-3m	Cu	a = 3.5447	—	[16]
βLa	Cf4	Fm-3m	Cu	a = 3.7740	c = 12.171	[16]
La7Ni3	—	P63mc	Fe7Th3	a = 1.0130	c = 0.6462	[25]
LaNi	—	Cmcm	BCr	a = 0.3907	c = 0.4396	[25]
La2Ni3	0S20	Cmca	La2Ni3	a = 0.5118	c = 0.7907	[25]
La7Ni16	—	I42m	La7Ni16	a = 0.7355	c = 1.451	[25]
LaNi3	—	R3m	BeNb3	a = 0.5086	c = 2.501	[25]
La2Ni7	hR55	P63/mmc	Ce2Ni7	a = 0.5085	c = 2.471	[25]
LaNi5	—	P6/mmm	CaCu5	a = 0.5016	c = 0.3983	[25]
βNi	Cf4	Fm-3m	Cu	a = 3.5446	—	[25]

[16,17,20,25].

3. Experimental Procedure

The phase relationships of the La–Co–Ni ternary system were constructed by equilibrated alloys. High purity of La (99.99%), Co (99.99%), and Ni (99.99%) were used as raw materials. The alloys with the mass of 3 g were prepared by arc melting with a non-consumable tungsten electrode under the protection of a high-purity argon atmosphere. The samples were turned over and re-melted four times to improve alloy homogeneities. The melting loss was less than 1 mass.%. The alloys were annealed at 723 K for 1440 h. After that, the samples were taken from the muffle furnace and quenched quickly in ice water.

The phase consistence and crystal structure of the samples were identified by means of XRD (Rigaku D/max2550VB, Tokyo, Japan) using Cu Kα radiation. The diffractometer was operated at 40 kV and 40 mA, and the 2θ scan ranges from 20° to 90° with a step size of 0.02° and a counting time of 5 s per step. The microstructural features and the elemental distribution of the alloy were characterized using SEM-EDS and XRD methods.

4. Results and Discussion

4.1. Microstructure

The twenty-two La–Co–Ni alloys annealed at 723 K for 1440 h were examined by SEM-EDS and XRD. It should be noted that the accuracy of the EDS results is about 0.01%. The experimental results of phase compositions measured by EDS and phase identified by XRD are presented in

Table 2. Composition of the La–Co–Ni phases according to the EDS data.

Alloy	Nominal Composition (at.%)	EDS Measurement				XRD Results
		La (at.%)	Co (at.%)	Ni (at.%)	Phase
1	La9Co85Ni6	16.06	72.30	11.65	LaCo5	LaCo5
		0.01	95.31	4.68	αCo	αCo
		6.92	77.47	15.61	LaCo13	LaCo13
2	La7Co44Ni49	15.41	72.13	12.46	LaCo5	LaCo5
		0.40	95.03	4.56	αCo	αCo
		0.50	14.20	85.30	βNi	βNi
3	La12Co60Ni28	0.01	94.31	5.68	αCo	αCo
		16.06	71.3	12.65	LaCo5	LaCo5
4	La68Co26Ni6	53.79	43.07	3.13	La2Co1.7	La2Co1.7
		72.54	16.76	10.70	La3Co	La3Co
5	La85Co5Ni10	73.21	17.30	9.49	La3Co	La3Co
		94.53	4.21	1.26	βLa	βLa
6	La62Co6Ni32	68.79	5.62	25.59	La7Ni3	La7Ni3
		49.61	3.37	47.02	LaNi	LaNi
7	La51Co34Ni15	40.53	45.79	13.68	La2Co3	La2Co3
		49.73	8.49	41.78	LaNi	LaNi
		52.73	37.66	9.61	La2Co1.7	La2Co1.7
8	La44Co12Ni44	40.35	17.96	41.69	La2Ni3	La2Ni3
		50.67	8.49	40.84	LaNi	LaNi
9	La44Co49Ni7	53.15	41.63	5.22	La2Co1.7	La2Co1.7
		41.53	50.08	8.39	La2Co3	La2Co3
10	La70Co5Ni25	70.41	5.45	24.14	La7Ni3	La7Ni3
		72.54	10.76	16.70	La3Ni	La3Ni
		49.45	1.28	49.26	LaNi	LaNi
11	La72Co5Ni23	70.41	5.06	24.53	La7Ni3	La7Ni3
		72.86	5.62	21.53	La3Ni	La3Ni
12	La8Co10Ni82	17.51	25.20	57.29	LaNi5	LaNi5
		0.05	5.37	94.58	βNi	βNi
13	La8Co90Ni2	7.41	77.26	15.33	LaCo13	LaCo13
		0.40	95.03	4.56	αCo	αCo
14	La10Co88Ni2	16.06	70.30	13.66	LaCo5	LaCo5
		7.41	77.26	15.33	LaCo13	LaCo13
15	La33Co24Ni43	40.12	21.01	38.87	La2(Co,Ni)3	La2(Co,Ni)3
		22.96	29.03	48.01	La2(Co,Ni)7	La2(Co,Ni)7
16	La31Co42Ni27	39.80	41.12	19.08	La2(Co,Ni)3	La2(Co,Ni)3
		22.83	45.98	31.19	La2(Co,Ni)7	La2(Co,Ni)7
17	La30Co63Ni7	39.12	56.13	7.75	La2(Co,Ni)3	La2(Co,Ni)3
		22.67	69.83	7.50	La2(Co,Ni)7	La2(Co,Ni)7
18	La34Co11Ni55	40.20	10.10	49.70	La2Ni3	La2Ni3
		25.52	18.07	56.41	LaNi3	LaNi3
19	La36Co3Ni61	40.22	3.63	56.15	La2Ni3	La2Ni3
		30.50	2.82	66.68	La7Ni16	La7Ni16
20	La29Co4Ni67	30.49	4.98	64.53	La7Ni16	La7Ni16
		25.90	17.99	56.11	LaNi3	LaNi3
21	La60Co19Ni21	74.93	6.62	18.45	La3Ni	La3Ni
		50.12	6.71	43.17	LaNi	LaNi
		53.94	36.45	9.61	La2Co1.7	La2Co1.7
22	La57Co14Ni29	74.98	6.67	18.35	La3Ni	La3Ni
		50.67	8.49	40.84	LaNi	LaNi
		53.43	37.67	8.90	La2Co1.7	La2Co1.7

. Figure 2 shows SEM images and XRD patterns of the representative La–Co–Ni alloys annealed at 723 K for 1440 h. According to the SEM-EDS results and the XRD analysis, it can be seen in Figure 2a that alloy #5 (La₈₅Co₅Ni₁₀) was composed of La₃(Co,Ni) and βLa phases, which agrees with the results of XRD results. The light-gray phase and white phase in Figure 2a correspond to the βLa and La₃(Co,Ni) phases, respectively. Their compositions were La_73.21Co_17.30Ni_9.49 and La_94.53Co_4.21Ni_1.21, respectively. So, the alloy La₈₅Co₅Ni₁₀ is confirmed to be located in the two-phase region, La₃(Co,Ni) + βLa. The BSE image of the no. 6 (La₆₂Co₆Ni₃₂) sample is presented in Figure 2b, showing the co-existence of two-phase La₇(Co,Ni)₃ (white) + LaNi(gray). Their compositions are La_68.79Co_5.62Ni_25.59 and La_49.61Co_3.37Ni_47.02, respectively. The result is consistent with the XRD results. So, the alloy La₆₂Co₆Ni₃₂ is confirmed to be located in the two-phase region, La₇Ni₃ + LaNi. The BSE image of the no. 1 (La₉Co₈₅Ni₆) sample annealed at 723 K for 1440 h is presented in Figure 2c, showing the co-existence of the three-phase area LaCo₅(white) + LaCo₁₃(gray) + αCo(black). The result is consistent with the XRD results. Figure 2d presents the BSE image of the no. 4 (La₆₈Co₂₆Ni₆) sample alloy annealed at 723 K for 1440 h. The white and gray phases are the La₃(Co,Ni) phase and La₂Co_1.7 phase. The no. 18 (La₃₄Co₁₁Ni₅₅) sample displays the alloy that consists of the La₂Ni₃(white) and LaNi₃(gray) phase in Figure 2f. The XRD pattern identified the result, as can be seen in Figure 2e, and their compositions are La_40.22Co_3.63Ni_56.15, La_25.90Co_17.99Ni_56.11, and La_22.96Co_29.03Ni_48.01, respectively. Figure 2g,h shows BSE images and XRD patterns of the no. 12 (La₈Co₁₀Ni₈₂) sample annealed at 723 K for 1440 h. According to the SEM-EDS results and the XRD analysis, it can be seen in Figure 2h that alloy #12 (La₈Co₁₀Ni₈₂) was composed of LaNi₅ and βNi phases, which agrees with the results of XRD results, as can be seen in Figure 2g.

4.2. Isothermal Section for La–Co–Ni at 723 K

Based on the experimental results obtained in this work and the information of relevant binary systems taken from the literature, the isothermal section of the La–Co–Ni system at 723 K in the whole concentration region was constructed and is shown in Figure 3. La₃Co, La₂Co_1.7, La₂Co₃, La₂Co₇, LaCo₅, La₅Co₁₉, LaCo₁₃, La₃Ni, La₇Ni₃, LaNi, La₇Ni₁₆, La₂Ni₇, LaNi₅, and LaNi₃ phases were found to be stable at 723 K. Among them, La₃Co and La₃Ni, La₂Co₃ and La₂Ni₃, La₂Co₇ and La₂Ni₇, and LaCo₅ and LaNi₅ form continuous solid solutions. The ternary solid solubility of Ni in the LaCo₁₃ phase and La₂Co_1.7 phase is about 15.61 at.% and 9.61 at.%, respectively. In addition, the ternary solid solubility of Co in the LaNi₃ phase, La₇Ni₃ phase, and LaNi phase are about 18.07 at.%, 5.62at.%, and 8.49 at.%, respectively. No ternary compounds were found. Unfortunately, the phase region in the dotted line cannot be determined because the relevant sample is not balanced. The system consists of 16 two-phase regions and 8 three-phase regions. In contrast to the room-temperature isothermal cross-section [28], in the region of La < 32.2%, βNi in the single-phase region of αCo can be solubilized at room temperature, and the cross-section at this temperature is αCo + βNi + La(Co,Ni)₅ in the three-phase region. There is the LaCo₁₃ + βNi + LaCo₅ three-phase region at room temperature, which transforms the LaCo₁₃ + αCo + LaCo₅ three-phase region at 723 K. In addition, the two-phase region αCo + βNi and the three-phase region αCo + βNi + LaCo₁₃ disappeared at 723 K and were replaced by the two-phase region αCo + La(Co,Ni)₅. Furthermore, the solid solution limit of βNi in LaCo₁₃ is found to be 15.61 at.% by means of the no. 1 sample.

The eight three-phase regions are: La₃Ni + La₇Ni₃ + LaNi, La₇Ni₁₆ + La₂Ni₃ + LaNi₃, LaNi + La₂Co₃ + La₂Co_1.7, LaNi + La₃Ni + La₂Co_1.7, αCo + βNi + La(Co,Ni)₅, La₂Ni₃ + LaNi₃ + La₂Ni₇, LaCo₁₃ + αCo + LaCo₅, and La₂Co₇ + La₅Co₁₉ + LaCo₅. The 16 two-phase regions are: La₃(Co,Ni)₅ + βLa, La₃Ni + La₇Ni₃, La₇Ni₃ + LaNi, La₂Co₃ + La₂Co_1.7, La₃Co + La₂Co_1.7, LaNi + La₃Ni + La₂Co_1.7, La₂Ni₃ + LaNi, LaCo₁₃ + LaCo₅, La₂Co₇ + La₂Co₃, LaCo₁₃ + αCo, LaCo₅ + αCo, βNi + LaNi₅, La₂Ni₃ + La₇Ni₁₆, LaNi₃ + La₇Ni₁₆, La₂Ni₃ + La₂Ni₇, and LaNi₃ + La₂Ni₃.

5. Conclusions

In this work, phase equilibria in the La–Co–Ni system at 723 K were measured using equilibrated alloy samples. Eight three-phase regions and sixteen two-phase regions were determined in the isothermal sections of the La–Co–Ni ternary system. La₃Co and La₃Ni, La₂Co₃ and La₂Ni₃, La₂Co₇ and La₂Ni₇, and LaCo₅ and LaNi₅ form a continuous solid solution. The ternary solid solubility of Ni in the LaCo₁₃ phase and La₂Co_1.7 phase is about 15.61 at.% and 9.61 at.%, respectively. In addition, the ternary solid solubility of Co in the LaNi₃ phase, La₇Ni₃ phase, and LaNi phase is about 18.07 at.%, 5.62 at.%, and 8.49 at.%, respectively. No ternary compounds were found.