Experimental investigation of the Y-Fe-Ga ternary phase diagram: Phase equilibria and new isothermal section at 800 °C

Accepted Manuscript Experimental investigation of the Y-Fe-Ga ternary phase diagram: Phase equilibria and new isothermal section at 800 °C K. Nouri, M. Jemmali, S. Walha, A. Ben Salah, E. Dhahri, L. Bessais PII:

S0925-8388(17)31773-5

DOI:

10.1016/j.jallcom.2017.05.178

Reference:

JALCOM 41909

To appear in:

Journal of Alloys and Compounds

Received Date: 31 March 2017 Revised Date:

9 May 2017

Accepted Date: 17 May 2017

Please cite this article as: K. Nouri, M. Jemmali, S. Walha, A. Ben Salah, E. Dhahri, L. Bessais, Experimental investigation of the Y-Fe-Ga ternary phase diagram: Phase equilibria and new isothermal section at 800 °C, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.05.178. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

MANUSCRIPT Experimental investigationACCEPTED of the Y-Fe-Ga ternary phase diagram : Phase equilibria and new isothermal section at 800 ◦ C K. Nouri ∗ ,1, 2 M. Jemmali,3, 2 S. Walha,2 A. Ben Salah,2 E. Dhahri,4 and L. Bessais1 1

Universit´e Paris Est, ICMPE (UMR 7182), CNRS, UPEC, F-94320 THIAIS France 2

Laboratoire des Sciences des Mat´eriaux et de l’Environnement,

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Facult´e des Sciences de Sfax-Universit´e de Sfax, Sfax, BP 1171, 3018 Tunisie. Chemistry Departement, College of Science and Arts at Ar-Rass, Qassim University, P.O. Box 53, Saoudi Arabia 4

Laboratoire de Physique appliqu´e, D´epartement de physique,

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Facult´e des sciences de Sfax-Universit´e de Sfax, Sfax, BP 1171, 3018 Tunisie. The solid state phase equilibria in the Y-Fe-Ga ternary system at 800

◦

C were investigated by

means of X-ray diffraction, scanning electron microscope (SEM) and quantitative electron probe

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microanalysis (EDS). The isothermal section of the Y-Fe-Ga ternary phase diagram contains 15 single phase-regions, 32 two phase-regions, 12 three phase-regions and one liquid area. The phase diagram at this temperature is characterized by the formation of ten binary stable phases. Among these, there are six extensions of the binary compounds Y2 Fe17 , YFe3 , YFe2 , YGa2 , Y3 Ga5 and Y5 Ga3 into the ternary system Y2 Fe17−x Gax (x ≤ 2), YFe3−x Gax (x ≤ 0.8), YFe2−x Gax (x ≤ 0.14), YGa2−x Fex (x ≤ 0.5), Y3 Ga5−x Fex (x ≤ 0.15) and Y5 Ga3−x Fex (x ≤ 0.1), respectively. The

PACS numbers:

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homogeneity range of the YFe12−x Gax ternary line compound is between (1.5 ≤ x ≤ 9.75).

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Keywords: intermetallic; phase diagram; rare earth; X-ray diffraction

I.

INTRODUCTION

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The rare-earth iron intermetallic compounds are of interest from an application point of view and in understanding fundamental physics and metallurgical properties. The two main areas of application for magnetic materials are magnetic recording (semihard magnetic materials), dynamic and static permanent magnet applications (high density magnetic materials) and magnetic refrigeration [1–4]. The isothermal section is an important and valuable tool to improve the synthetic route of high purity samples,

∗

Author to whom correspondence should be addressed; electronic mail: [email protected]

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ACCEPTEDto MANUSCRIPT especially those not yet magnetically characterized, determine the phases relation and their stability [5–8]. The phases and phase relations in R-Fe-Ga ternary systems may be different for different rare earth elements. The isothermal section of Ho-Fe-Ga [9], Gd-Fe-Ga [10] and Tb-Fe-Ga [11] have been constructed at 500◦ C. The phase diagrams of Y-Fe-Cr [12] and Y-Fe-Ti [13] ternary systems have been reported in the literatures. In the last decade, the number of works on Y-Fe-based ternary systems [14–18], including main alloying elements such as Cr, Ti, B and Ni, sharply increased.

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Recent investigations of the crystal-chemistry and solid state phase equilibria in the R-Fe-M system were performed. In fact, the magnetic properties of the ternary compounds of R(Fe,M)12 and R3 (Fe,M)29 have been investigated intensively. In both the compounds, the presence of a third element M (M : Si, W [19, 20], Cr, V [21], Al [22], Mo [23], (Co, Ti) [24, 25], (Si, Ti)[26]) is necessary for the stabilization of the two phases [27–29]. These materials

saturation magnetization and magnetocrystalline anisotropy.

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are considered as potential permanent magnet materials due to their fairly high values of the Curie temperature,

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So far, in the literature, among the phases synthesized with YFeGa compositions, only pseudo-binary phases Y2 Fe17−x Gax (x = 1, 2) [30] and ternary phases YFe12−x Gax (x = 1, 2) [31] were investigated. Regarding the absence of investigation of the ternary Y-Fe-Ga diagram, and the importance of these intermetallics for magnetic applications, we undertook the assessment of the isothermal section at 800 ◦ C of the new Y-Fe-Ga system to provide phase diagram

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information for developing new magnetic alloys.

II.

EXPERIMENTAL

Each sample with a total weight of 0.5 g starting from the pure components (Y: 99.99%, Fe: 99.99%, and Ga:

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99.99%) was prepared by arc melting on a water-cooled copper tray with a non-consumable tungsten electrode under the protection of a pure argon atmosphere. The samples were turned over and re-melted four times to ensure adequately homogeneity.

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In total, 72 alloys buttons were prepared. Only those samples with less than 1 wt.% weight losses were used for phase analysis. During melting process, a piece of zirconium was used as an oxygen getter. The bulk was wrapped in a tantalum sheet. As-cast samples were sealed in vacuum quartz tubes and annealed at 800◦ C for one week in order to reach a good homogenization and to improve the atomic diffusion kinetics. This temperature was chosen as a good compromise between relatively fast diffusion kinetics and absence of reaction with the quartz tube. Phase analysis was carried out using the Powder-cell program package [32] and/or Rietveld refinement [33–35] on an X-ray powder diffraction pattern recorded on a Bruker D8 diffractometer (monochromatic Cu Kα radiation λ =

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vacuum-sealed samples

Agate mortar

Resistance furnace

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Arc melting

EDS spectrum

SEM image

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X-ray diffraction

FIG. 1: Photography of synthesis steps.

1.5406 ˚ A) with data collected by 0.015◦ step width for 13.5 s over a 2θ range from 20◦ to 80◦ .

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The microstructure of the samples was studied on polished surfaces using a Merlin Scanning Electron Microscopy (SEM) equipped with Silicon Drift Detector (SDD)-X-Max 50 from Oxford Instruments employed for the elemental

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analysis of the various phases. In order to view a metal specimen under a scanning electron microscopy, the sample must be prepared using several steps. It must be mounted onto sample-support (Metal conductor). After we fix the latter in the polisher equipped with abrasive papers, using finer and finer grits as well as polishing slurry, and finally it needs to use a diamond abrasive paper to make the surface shine. Elemental compositions were obtained by averaging the values of at least three EDS analyzed zones, from different regions of the sample. An estimated deviation from the mean value is about 0.5 at.%. The agreement between the targeted and the sample compositions was checked by measuring a large zone of the sample surface. The steps of the syntheses and characterizations are summarized in figure 1.

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ACCEPTED MANUSCRIPT Figure 2 presents all the prepared nominal compositions in the Gibbs triangle after the annealing at 800 ◦ C in the above mentioned conditions.

G a 1

F e G a

% Y

Y G a

%

2

Y 3 G a 5

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a (F e G a )

Y

1

Y

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2 3

2 F e 1 7

F e

F e

6

%

0 Y

0

Y F e 3

Y F e 2

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3

1

F e

G a

F e 3 G a 4

Y G a Y 5G a

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0

III.

LITERATURE DATA

A.

Binary systems

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FIG. 2: Prepared compositions of YFeGa alloys.

In order to construct the Y-Fe-Ga isothermal section, we will cite some binary and ternary compounds reported in the literature.

The Y-Fe binary phase diagram indicates the existence of four defined compounds. The YFe2 (MgCu2 -type structure) compound formed peritectically at 1125 ◦ C, the Y2 Fe17 (Th2 Zn17 -type structure), Y6 Fe23 (Th6 Mn23 -type structure) and YFe3 (PuNi3 -type structure) melts congruently at 1400, 1300 and 1335 ◦ C, respectively. The binary boundary Fe-Ga system was constructed by W. Koester et al. [36]. This system was subsequently

5 reconstructed by T. B. Massalski [37]ACCEPTED used the studiesMANUSCRIPT and the results of C. Dasarathy [38]. At 800 ◦ C, this binary boundary system announces the existence of three phases: Fe3 Ga4 (Fe3 Ga4 -type structure), Fe0.75 Ga0.25 (W-type structure) and FeGa3 (IrIn3 -type structure) (stable at high temperature). This phase diagram has been completely studied. It is characterized by very wide domains of homogeneity, covering the Fe-rich region extending from 57% to 100% of iron. TABLE I: Crystal structures and lattice parameters data for the binary compounds in Y-Fe-Ga ternary system at 800◦ C.

a

b

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Composition Type-structure Space group Unit-cell parameters (˚ A) Reference c

Y2 Fe17

Th2 Zn17

R¯ 3m

8.46 (4)

Y6 Fe23

Th6 Mn23

F m¯ 3m

12.082

YFe3

PuNi3

R¯ 3m

5.15

YFe2

MgCu2

F d¯ 3m

7.363

[42]

Fe0.75 Ga0.25

W

Im¯ 3m

2.907

[43]

FeGa3

IrIn3

Fe3 Ga4

Fe3 Ga4

Y5 Ga3

Mn5 Si3

YGa

TlI

Y3 Ga5

Tm3 Ga5

YGa2

AlB2

12.41(4)

[40]

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24.6

6.256

C12/m1

10.090

P 63 /mcm

8.576

Cmcm

4.304

P nma

11.279

P 6/mmm

4.211

[41]

6.56

[44]

7.866

[45]

6.479

[46]

10.868

4.065

[47]

9.556

5.987

[48]

4.112

[49]

7.666

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P42 /mnm

[39]

The Y-Ga system was constructed by S. P. Yatsenko et al. [50]. They found four defined binary phases : Y3 Ga5

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(Tm5 Ga5 -type structure), Y5 Ga3 (Mn5 Si3 -type structure and space group P 63 /mcm), YGa (TlI-type structure and space group Cmcm) and YGa2 ( AlB2 -type structure and space group P 6/mmm). This was re-examined by T.B.

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Massalski [51]. The crystallographic data of the unit-cell parameters are summarized in table I.

B.

Ternary systems

In the ternary system Y-Fe-Ga, the compounds YFe12−x Gax (x = 1, 2) [31] and Y2 Fe17−x Gax (x = 1, 2) [30] have been identified in the previous studies.

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ACCEPTED MANUSCRIPT IV. RESULTS AND DISCUSSION A.

Phase analysis and solid solubility

The phases in the concentration interval studied deeply extend into the ternary system, forming solid-solutions with significant changes in their lattice parameters. The extension of these solid solutions and the configurations of their homogeneity ranges are studied. The various concentrations of Fe/Ga were chosen to determine the limit of these

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extensions into the Y-Fe-Ga isothermal section and to find the structure type.

In order to obtain the crystal structure of the Y2 (Fe,Ga)17 solid solution at 800◦ C, we synthesized several samples on this solid solution and in the side regions. The existence of the extension of the Y2 Fe17−x Gax solid solution in the isothermal section is proven by the preparation of a sample with a nominal composition Y9.5-Fe76.4-Ga14.1. Figure

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3 presents the XRD analysis of the Y9.5-Fe76.4-Ga14.1 sample and the identification of the 2/17 phase. This result is confirmed by the EDS-SEM image (figure 4).

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To confirm the presence of this binary extension, the synthesis of a sample in region 8 of the Y-Fe-Ga ternary phase diagram with the Y2-Fe10.2-Ga6.8 nominal composition confirms the stability of the Y2 Fe17−x Gax solid solution. The SEM-EDS (Figure 5) and the X-ray analysis (Figure 6) prove the homogeneity domain at 800◦ C which extends from the Y2 Fe17−x Gax (x = 0) composition (a= 8.4621 (4) ˚ A and c = 12.412 (4) ˚ A) to the Y2 Fe17−x Gax (x = 2) composition (a= 8.5434 (1) ˚ A and c = 12.463 (3) ˚ A). Those unit cell parameters are refined by Rietveld method.

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The experimental diffractogram was indexed in three type structures, and shows the three-phase equilibrium between the following phases : YGa2−x Fex (AlB2 -type structure) + Y2 Fe17−x Gax (Th2 Zn17 -type structure)+ YFe12−x Gax (ThMn12 -type structure).

The Sm2 (Fe,M)17 (M: Ga) compounds were studied by L. Bessais et al. [52]. Rietveld analysis of these phases

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proves that the substitution of Ga, Si or Co by Fe was made in the 18h site. This study was confirmed by M¨ ossbauer spectroscopy [52]. Based on this study, the best agreement factor RB for the Rietveld analysis of the Y2 Fe17−x Gax

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samples has been obtained with Ga located in 18h site. Figure 7 shows, as an example, the Rietveld refinement for the Y2 Fe16.75 Ga0.25 composition. This analysis shows that this intermetallic adopt the rhombohedral Th2 Zn17 structure with R¯ 3m space group. For this Ga content, the unit cell parameters are : a = 8.4964(4) ˚ A and c= 12.443(3) ˚ A. In this structure, the yttrium occupied the 6c site and the Fe atoms are localized in the four non-equivalent sites: 18h, 18f , 6c and 9d. However, the Rietveld analysis shows that the preferred site substitution is the 18h. The increase of Ga content in the Y2 Fe17−x Gax compounds preserve the Th2 Zn17 -type structure up to the limit of this solid solution. Systematic studies of the gallium substituted series Y2 Fe17−x Gax have shown that the unit cell volume increases as a function of Ga content.

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2 ,0

Y 9 .5 -F e 7 6 .4 -G a 1 4 .1

1 ,8 1 ,6

1 ,2 1 ,0

T h M n

0 ,8

T h 2Z n

1 2 1 7

0 ,6 0 ,4 0 ,2

3 0

4 0

2θ(°) 5 0

6 0

7 0

8 0

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2 0

-T y p e s tru c tu re

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0 ,0

-T y p e s tru c tu re

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I n t e n s it y ( u .a )

1 ,4

FIG. 3: X-ray diffraction of the nominal composition Y9.5-Fe76.4-Ga14.1, showing the presence of Y2 Fe15 Ga2 and YFe10.5 Ga1.5

YFe10.5Ga1.5

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Y2Fe15Ga2

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phases.

FIG. 4: Backscattered electron image of the nominal composition Y9.5-Fe76.4-Ga14.1, showing the presence of Y2 Fe15 Ga2 and YFe10.5 Ga1.5 phases.

M. Valuanu et al [30] studied the Y2 Fe17−x Gax (x = 1, 2) compounds. They found that these materials crystallize

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ACCEPTED MANUSCRIPT YGa1.82Fe0.18

YFe6Ga6

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Y2Fe15Ga2

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FIG. 5: Backscattered electron image of Y2-Fe10.2-Ga6.8 showing the presence of Y2 Fe15 Ga2 , Y2 Fe6 Ga6 and YGa1.82 Fe0.18 phases. 2 0 0 0 0

Y 2 -F e 1 0 .2 -G a 6 .8 T h 2

Z n

1 7

1 2

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T h M n

1 0 0 0 0

5 0 0 0

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I n t e n s it y ( u .a )

1 5 0 0 0

Y

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0

2 0

3 0

2

F e 1 5 G a 2 (R -3 m

)

Y G a 1 .8 2 F e 0 .1 8 ( P 6 /m m m

Y F e 6G a

4 0

2 q (°) 5 0

6 0

6

)

(I 4 /m m m )

7 0

8 0

FIG. 6: X-ray diffraction of the Y2-Fe10.2-Ga6.8 nominal composition.

in the Th2 Ni17 -type structure (space group P 63 /mmc) without using any Rietveld analysis. The RM3 (R:rare earth and M: transition metal) compounds crystallize in two polymorphic type structure, the hexagonal CeNi3 (P 63 /mmc) and the rhombohedral PuNi3 (R¯3m). The first can be viewed as constructed of alternate

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1 0 0 0 0

Y o b s Y

8 0 0 0

c a l

Y

-Y

c a lc

B r a g g _ p o s itio n

6 0 0 0

4 0 0 0

2 0 0 0

0

-2 0 0 0 3 0

4 0

5 0

2 q (°)

6 0

7 0

8 0

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2 0

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I n t e n s it y ( u .a )

o b s

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FIG. 7: Rietveld analysis for X-ray diffraction pattern of Y2 Fe16.75 Ga0.25 compound.

stacking blocks along the c-direction of RM2 (MgCu2 -type)and RM5 (CaCu5 -type structure). The R atoms lie on two different crystallographic sites: R1 in the RM5 block has a uniaxial local symmetry (¯6m2), while the local symmetry of the R2 site (3m) in the RM2 block is quite cubic. The second phase adopt the PuNi3 type structure. The unit cell contains two non-equivalent crystallographic sites for the R ions, 3a and 6c , and three sites for M : 3b, 6c and 18h, corresponding to other equivalent sites in CaCu5 and MgCu2 type host structures.

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At this temperature, we confirmed the stability of YFe3−x Gax solid solution and delimited this extension using the MEB-EDS and the X-ray diffraction analyzes. During the study of this homogeneity domain, we noticed that this extension keeps the same type of structure (PuNi3 ). The synthesis of Y14.1-Fe72.1-Ga13.8 and Y20.8-Fe64.54-

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Ga14.57 compositions in the regions 4 and 5, reveals the limit of this solid solution (Figure 8 and Figure 9). The analyzes of samples in region 5 are in thermodynamic equilibrium with the binary Y6 Fe23 , the limit of solid solution

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Y2 Fe17−x Gax (x = 2) and the limit of YFe3−x Gax (x = 0.8). The 1/3 limit phase adopt the PuNi3 type structure with the following cell parameters: a = 5.1621 (1) ˚ A and c = 24.683 (1)˚ A. We found in the regions 1 and 7 the existence of two homogeneity domains having a Y/(Fe + Ga)= 1/2 ratio which are distributed between YFe2 and YGa2 . Various samples with a Y(FeGa)2 nominal composition and in the side region were prepared. X-ray diffraction analysis the samples as well as the SEM-EDS prove the existence of the two binary extensions. The YFe2−x Gax compound adopts the MgCu2 -type structure, and it extended from x = 0 to x = 0.14. However, an extension resulting from the binary YGa2 adopts the AlB2 -type structure. • From YFe2 to YFe1,86 Ga0,14 : Cubic type structure MgCu2 (F d¯3m).

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Y2Fe17-xGax Y6Fe23

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YFe3-xGax

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Y2Fe17-xGax

Y6Fe23

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YFe3-xGax

FIG. 8: Scanning electron micrograph of polished samples with Y14.1-Fe72.1-Ga13.8(below) and Y20.8-Fe64.54-Ga14.57(up)

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compositions.

• From YGa2 to YGa1,5 Fe0,5 : Hexagonal type structure AlB2 (P 6/mmm).

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In this diagram, we identified a new ternary line compound, which will be described in the following paragraph.

B.

Intermediate YFe12−x Gax solid solution

The ThMn12 structure derives from the CaCu5 structure and can be schematized by the following relations:

4 (RM5 ) - 2 R + 4 M (Dumbbell) 7−→ 2 (RM12 ) The lattice parameters of the tetragonal ThMn12 -type phase can be converted into those of hexagonal CaCu5 -type

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I n t e n s it y ( u .a )

1 ,0

0 ,8

Y20.8-Fe64.54-Ga14.57

0 ,6

0 ,4

Y14.1-Fe72.1-Ga13.8

0 ,2

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Y2Fe17-xGax (R-3m)

0 ,0

Y6Fe23 (Fm-3m)

YFe3-xFex (R-3m)

-0 ,2 3 0

4 0

2q (°)

5 0

6 0

7 0

8 0

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2 0

FIG. 9: X-ray diffraction patterns for Y14.1-Fe72.1-Ga13.8 and Y20.8-Fe64.54-Ga14.57 compositions.

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phase according to the following conversion equations:

c(1:12) = a(1:5)

c(1:5) = a(1:12)/2

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TABLE II: Wickoff positions of I4/mmm type structure. Atom Wykoff site Symmetry

(x, y, z)

2a

4/mmm

(0, 0,0)

M1

8i

m2m

(x, 0,0)

M2

8j

m2m

(x,1/2,0)

M3

8f

2/m

(1/4, 1/4, 1/4)

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R

The solubility and the domain of existence for the RFe12−x Mx phase (M: metalloid or transition metal) depend by the M metal. When the M atoms (M = Cr, V, Ti, Mo) substitute the Fe atoms on the 8i site, the cohesion energy decreases significantly for M atoms distributed into the 8j or 8f sites. In this structure, the rare earth atoms occupied the 2a(0,0,0) site and the Fe atoms occupy three crystallographic non-equivalent sites, denoted as Fe(8i), Fe(8j) and Fe(8f ) : 8i (x, 0, 0), 8j (x, 1/2, 0) and 8f (1/4, 1/4, 1/4) (See table II). The existence of this ternary line compound was revealed in annealed samples having several compositions. Many samples with nominal compositions in Y(Fe, Ga)12 system were prepared. The results of scanning electron microscopy

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FeGa3

YFe2.25Ga9.75

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YGa2

FIG. 10: Backscattered electron SEM image of the Y1-Fe2.3-Ga9.7 sample showing the equilibria between YFe2.25 Ga9.75 , YGa2 and FeGa3 compounds.

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2 5 0 0 0

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1 5 0 0 0

1 0 0 0 0

5 0 0 0

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0

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I n t e n s it y ( u .a )

2 0 0 0 0

Y 1 -F e 2 .3 -G a 9 .7

F e G a

(P 4 2 /m n m

Y G a 2(P 6 /m m m Y F e

-5 0 0 0

2 0

3

3 0

4 0

2 q (°) 5 0

6 0

2 ,2 5

G a

7 0

9 ,7 5

) )

(I4 /m m m )

8 0

FIG. 11: Experimental XRD pattern of the Y1-Fe2.3-Ga9.7 composition showing the equilibria between YFe2.25 Ga9.75 , YGa2 and FeGa3 phases.

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ACCEPTED MANUSCRIPT Y2Fe17

YFe10.5Ga1.5

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Fe

FIG. 12: SEM-EDS image of Y1-Fe11.5-Ga0.5 composition showing the three-phases equilibria between YFe10,5 Ga1,5 , αF e and the Y2 Fe17 compounds.

TABLE III: a and c cell parameters, RB and χ2 factors from Rietveld refinement of YFe12−x Gax (x = 5 and 6) as an example. Phases YFe6 Ga6 YFe7 Ga5 8.551(1) 8.564(2)

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a(˚ A)

5.062(1) 5.042(3)

x(8i)

0.342

0.339

x(8j)

0.275

0.273

χ2

3.8

4.1

RB

5.1

4.5

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c(˚ A)

coupled to the X-ray diffractograms of each sample allowed to delimit the YFe12−x Gax ternary solid solution. In this work, the XRD pattern confirms that this phase crystallizes in the tetragonal ThMn12 -type structure. The homogeneity of the YFe12−x Gax ternary solid solution is between (1.5 ≤ x ≤ 9.75). The SEM-EDS image (figure 10) for the Y1-Fe2.3-Ga9.7 sample (region 11) shows the thermodynamic equilibrium between the superior limit of the YFe12−x Gax solid solution : YFe2.25 Ga9.75 (with a cell parameters a = 8.621(2) ˚ A and c = 5.092(3)˚ A ) and the two binary phases YGa2 and FeGa3 . These results are confirmed by the XRD analysis (Figure 11).

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ACCEPTED MANUSCRIPT B r a g g _ p o s itio n x = 6

3 0 0 0

Y c a lc Y o b s Y o b s -Y c a lc

2 5 0 0

1 5 0 0 1 0 0 0 5 0 0 0 -5 0 0 -1 0 0 0 5 0

2 q (°)

6 0

7 0

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8 0

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I n t e n s it y ( u .a )

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FIG. 13: Observed (dots) and calculated (solid line) XRD patterns of YFe6 Ga6 compounds. Vertical bars represent the positions

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of the Bragg reflections. The observed-calculated difference is depicted at the bottom of the figure.

FIG. 14: crystal structure of Y(Fe,Ga)12 compound.

F. Weitzer et al. [53] found that the YFe6 Ga6 phase crystallizes in the orthorhombic ScFe6 Ga6 -type structure. Contrary to this result, and based on the Rietveld analysis, we have proved that this composition crystallizes in the ThMn12 type structure.

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ACCEPTED After the synthesis of a composition in the region MANUSCRIPT 9, the SEM-EDS analysis proved that the lower limit of the ThMn12 type structure is YFe10.5 Ga1.5 (with cell parameters a = 8.515 (1) and c = 4.913 (1) ˚ A), this later is in thermodynamic equilibrium with the αF e and the Y2 Fe17 binary compounds (figure 12). Figure 13 represents the XRD patterns corresponding to the intermetallic YFe12−x Gax (x = 6) compounds annealed at 800 ◦ C with its Rietveld refinement. It reveals that they consist of the tetragonal ThMn12 type structure with I4/mmm space group. This result is in good agreement with those found in [54]. Table III summarizes the values

structure of Y(Fe, Ga)12 compound.

Phase equilibria at 800◦ C

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of the crystallographic parameters as well as the reliability factors of the refinements. Figure 14 shows the crystal

The isothermal section at 800◦ C was studied in the whole concentration range with special attention. The phase

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equilibria are shown graphically in figure 16.

In the binary Y-Fe system the gallium solubility was evaluated at 4.6 at.%, 20 at.% and 9.4 at.%, respectively, for the binariy compounds YFe2 , YFe3 , Y2 Fe17 and adopt the following chemical formulae: YFe1.86 Ga0.14 , YFe2.2 Ga0.8 , Y2 Fe15 Ga2 . The ThMn12 type structure is stable in a large homogeneity domain. The superior and inferior limits of the intermediate solid solution Y(Fe,Ga)12 are, respectively: YFe2.25 Ga9.75 and YFe10.5 Ga1.5 . In the binary segment Y-Ga, the solubility of Fe in Ga was evaluated. The maximum of solid solubility of Fe in

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YGa2 is about 16.66 at.%, 1.8 at.% for the binary Y3 Ga5 , and 1.25 at.% for the Y5 Ga3 phase. The chemical formula for these solid solutions are, respectively, YGa1.5 Fe0.5 , Y3 Ga4.85 Fe0.15 and Y5 Ga2.9 Fe0.1 . In our work and after annealing the samples at 800

◦

C for one week, the X-ray diffraction analysis coupled with

SEM-EDS analysis show that from the binary Y2 Fe17 to the limit of this solid solution Y2 Fe15 Ga2 , theses intermetallics

unit cell parameters.

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preserve the same (Th2 Zn17 -R¯ 3m). So, the increasing of Ga content has no influence on the structure, just in the

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The binary compounds YGa2 and YFe2 show, with a constant composition of yttrium, a solubility of iron and gallium along two different regions: YFe2−x Gax (for x = 0.14) adopt the cubic type structure (MgCu2 ) and YGa2−x Fex (for x = 0.5) crystallize in the hexagonal type structure (AlB2 ). The MEB-EDS and X-ray diffraction analysis carried out on various samples demonstrate the homogeneity domain of type Y5 Ga3−x Fex which ranges from (x = 0 to x =0.1). The same characterizations show that the maximum solubility of Ga in the Y3 Ga5−x Fex system up to x = 0.15). This extension adopts the orthorhombic type structure (Tm3 Ga5 ). Finally, the yttrium has no solubility in the binary Fe-Ga segment.

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FIG. 15: Images of nominal compositions: Y0.02-Fe0.04-Ga0.94 and Y0.03-Fe0.03-Ga0.94 after annealing during one week at 800◦ (liquid region).

Four samples were synthesized in the Ga-rich region. These compositions show that this region is completely liquid.

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Figure 15 show the image of two liquid compositions in tantalum after annealing for one week at 800◦ C. Details of the phase components in three phase-regions are summarized in table IV and shown on the isothermal

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section in figure 16. In this work, particular attention has been assigned to evaluate the homogeneity domains of the different phases and their equilibria. The structural types reported for all phases were confirmed. By comparing and analyzing the XRD patterns of the samples and identifying the phases in each composition, we succeeded to construct

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the isothermal section of the Y-Fe-Ga ternary system at 800◦ C, as shown in Figure 16.

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G a 1

1 2 Y G a

Y G a Y 5G a

7 3 2

1 ,0

8

a (F e G a )

4

1

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5 9

Y 2 F e 1 7

F e

2

%

G a

%

F e 3 G a 4

Y F e 6 2 3 Y F e 3

0

6

1 0

Y F e

Y

3

2

Y 3 G a 5

3

F e G a

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Y

1 1

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%

L iq u id

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0 ,0

1

0

F e

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FIG. 16: The isothermal section of the Y-Fe-Ga ternary system at 800◦ C (three-phase-fields(white) and two-phase-fields(grey)).

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TABLE IV: Details of the three phase-regions of the Y-Fe-Ga isothermal section at 800◦ C . phase equilibria

1

Y + YFe3−x Gax + YFe2−x Gax

2

Y+ Y5 Ga3−x Fex + YFe3−x Gax

3

Y5 Ga3 + YGa + YFe3−x Gax

4

YFe3−x Gax + Y2 Fe17−x Gax +YGa

5

Y6 Fe23 + Y2 Fe17−x Gax + YFe3−x Gax

6

Y2 Fe17−x Gax + YGa + YFe2−x Gax

7

YGa2−x Fex + Y3 Ga5−x Fex + YGa

8

YGa2−x Fex + Y2 Fe17−x Gax + YFe12−x Gax

9

αF e + Y2 Fe17 + YFe12−x Gax

10

YFe12−x Gax +Fe3 Ga4 + (FeGa)-solid solution

11

FeGa3 + YFe12−x Gax + YGa2

12

FeGa3 + Liquid + YGa2

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Region number

V.

CONCLUSION

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The complete study of the Y-Fe-Ga phase diagram confirms the literature data and provide additional information relating to the phases already-known. This work has enabled us to characterize new solid solutions which have never been published and to determine the different equilibrium areas. The substitution mechanisms are due to mutual substitution between Fe and Ga.

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We have shown the formation of a homogeneity domain at 800◦ C of the YFe12−x Gax ternary line compound. This domain extends from x = 1.5 to x = 9.75 and crystallizes in the quadratic system I4/mmm. MEB-EDS analyses

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coupled with X-ray diffraction confirm the stability of this intermediate solid solution. Summarizing all the results obtained during this investigation at 800 ◦ C, a total number of 15 single-phase regions, 32 two-phase regions, 12 three-phase regions and 1 liquid area.

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Highlights

Binary, pseudo-binary and ternary of Y-Fe-Ga intermetallics alloys have synthesized.

•

A systematic XRD Rietveld refinement and SEM/EDS have been used to analyze those compounds.

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New isothermal section at 800°C of Y-Fe-Ga system was established.

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•