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Interdiffusion studies of rare earth metals with liquid gallium
Journal of the Less-Common
I~ERDIFFUSION LIQUID GALLIUM
Metals, 87 (1982)
87
87 - 98
STUDIES OF RARE EARTH METALS WITH
D. DAYAN and U. ATZMONY Nuclear Research Centre, Negev, P.O. Box 9001, Beer-Sheva (Israel) M. P. DARIEL Nuclear Research Centre, Negev, P.O. Box 9001, Beer-Sheva (Israel) and Department Materials Engineering, Ben Gurion University of the Negev, Beer-Sheva (Israel)
of
(Received April 6,1982)
Summary
The gallium-rich side of some rare earth-gallium binary systems was studied using a diffusion couple technique. Diffusion couples were formed by annealing rare earth metals immersed in liquid gallium at various temperatures. The nature and composition of the compound layers formed at the rare earth metal-gallium interface were determined by metahographic and electron microprobe analysis. The results indicate that in the light rare earth metals (R = La - Cd) the e phase extends from the stoichiometric composition RGa, to the gallium-rich compositions Ri -X Gaz +X. In these systems the RGa6 layer is dominant in the diffusion region. No RGa, layer is present in these systems, in contrast with the two heavy rare earth-gallium couples (Tb-Ga and Dy-Ga) in which the E phase appears at the stoichiometric composition only and the RGa, layer is dominant.
1. Introduction
Recent studies on the constitution of the light rare earth-gallium binary systems [l, 21 have shown that the RGaz phases (R = rare earth metal) are present over a wide range of homogeneity. These so-called e phases have a hexagonal AlB,-type structure. Their departure from stoichiometry can be accounted for in terms of the substitution of one rare earth atom in the basal plane of the unit cell by a pair of gallium atoms, It has indeed been observed that the range of existence of the E phase extends from the stoichiome~i~ composition to the gallium-rich compositions. Pelleg and Zevin [2] have also pointed out that the substitution of the rare earth atom by two gallium atoms is accompanied by a distortion of the original matrix and leads to the dependence of the width of the E phase in the phase diagram on the size ratio of the component elements. The next gallium-rich 0 Elsevier Sequoial~inted
in The Netherlands
88
compound RGa, has a stoichiometric composition and only appears in the heavy rare earth-gallium binary systems in a variety of crystal types [ 31. All previous studies of the extent of the f phase and the existence of the RGa, compounds [3, 41 have been carried out using conventional techniques, e.g. X-ray diffraction and optical and scanning electron microscopy, on cast samples which had undergone various annealing treatments. The object of the present work was to complete the data on the homogeneity range of the E phase and to study its systematics with respect to the presence of the RGa, compounds along the lanthanide series using a diffusion couple technique. Diffusion couples were formed by immersing rare earth metal samples or previously prepared RGa, samples in molten gallium for several hours at various temperatures.
2. Experimental
details
The rare earth metals lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium, terbium and dysprosium (nominal purity, 99.9%) and gallium (purity, 99.9%) were used to prepare the couples. Rare earth metal cylinders were prepared by arc melting under an argon atmosphere and casting into water-cooled split copper moulds of diameter 10 mm. The RGaz compounds were prepared in a similar manner. The cast samples were sectioned into discs 5 - 6 mm thick which were sealed in evacuated silica capsules and annealed at 600 “C. After careful cleaning of their external surfaces, the rare earth samples were inserted into pyrophillite crucibles. The free volume of the crucibles (approximately 0.15 cm3) was filled with molten gallium. The diffusion anneal was carried out in a horizontal vacuum furnace at temperatures which in most instances were below the peritectic temperature corresponding to the e + L + q (Q = RGa,) or RGa, + L + 7 reactions. After the diffusion anneal a cross section of the diffusion couple was examined by optical metallography and electron microprobe analysis. The results of the electron microprobe (Cameca II) scans and points analyses were corrected using a ZAF-based Magic computer program [ 51. Because of the low melting temperature of gallium, special precautions such as the use of precooled lubricant fluid had to be employed during the metallographic preparation procedures.
3. Results and discussion It is well established at present that the gallium-rich part of the rare earth-gallium binary systems conforms to one of the two schematic diagrams shown in Fig. 1. In the light rare earth-gallium systems (R E La - Gd) the congruently melting RGaz compound (e phase) exists over a range of compositions and it is followed by the peritectically melting RGa, compound (Fig. l(a)). In the heavy rare earth-gallium systems the e phase
89
WEIGHT
WEIGHT t*
PER CENT GALLIUM
ATOMIC
(4
PER CENT GALLIUM 50 60 7* 80 90 Km
PER
CENT
GALLIUM
(b)
Fig. 1. Schematic phase diagrams of the gallium-rich part of the rare earth-gallium systems: (a) light rare earth metals; (b) heavy rare earth metals.
binary
appears at the stoi~hiometric RGaz composition only and is followed at a higher gallium content by the two non-congruently melting RGa, and RGa, compounds (Fig. l(b)). During the present study we reacted a series of rare earth metals with liquid gallium. The majority of experiments consisted of immersing rare earth samples in liquid gallium at temperatures TP below the peritectic melting temperature of the RGa, compounds (Q phase). In some instances previously prepared RGa, compounds (R = Pr, Nd) or rare earth metals (samarium, gadolinium, terbium and dysprosium) were reacted with liquid gallium at temperatures higher than Tp. The experiments with the most significant observations are summarized in Table 1. When the light rare earth metals lithium, cerium, praseodymium, neodymium and samarium were annealed at a temperature below TP (see Fig. l(a)) a thick and mostly adherent RGa, compound layer was formed at the interface of the rare earth metal with liquid gallium. This layer was 5 - 8 mm thick for a typical diffusion anneal carried out at approximately 100 “C
360 360
560
370
623
2
3
2
1
1
1
2
2
La-Ga
Ce-Ga
Pr-Ga
PrGa*-Ga
Nd-Ga
NdGaTGa
Sm-Ga
Gd-Ga
100 pm
93 /_lm 65 W 1.9 mm
310 /Jm 170 pm 50 I_Lrn
NdGaza NdGaeb NdGaaa + NdGa6 plates precipitated in Ga matrix SmGaaa SmGab? porous layer SmGaaa + SmGa6 plates precipitated in Ga matrix GdGaza GdGaza GdGae porous layer
2 1
2 1
1 2
21 h 95 min
21 h 150 min
21 h 145 min
370
540
370 545
26 /..lrn 5mm
PrGae plates precipitated in Ga matrix
-
2h
8mm 50 /Jm 1.5 mm
PrGa$ PrGasa PrGaB
83 pm 200 I_tm
1 2
2h
390
CeGaaa CeGas + CesGas CeGaaa CeGaza CeGaB
10 /_lm
0.1 mm 0.42 mm 0.19 mm 8mm
Thickness of layers
20h 2h
5h 69 h
360 360
2
21 h
LaGaaa LaGahb LaGaaa LaGaeb
360
structure of layers
Number of layers 2
Dum tion 6h
(“C)
360
Tempemture
Diffusion anneal
Number of couples
System
Summary of observations of diffusion couples formed between rare earth metals and liquid gallium
TABLE 1
91
._
.I
; 0
m
E
E
d
m
92
below Tp for a duration of 20 h (Fig. 2). A microprobe analysis of this layer revealed a constant gallium content corresponding to the RGa, composition throughout the layer thickness. Progressing inward towards the rare earth
-Pr
-Pr
Fig. 2. Layer of PrGab formed at the interface of the rare earth metal with liquid gallium during an anneal at 360 “C for 20 h.
metal, the adjacent layer was much thinner and consisted of E phase corresponding to the composition Ri _,Ga 2 +% in which a strong concentration gradient was observed. By carefully analysing the composition of this layer in the immediate vicinity of the RGa, layer, we established the composition limit of the E phase on the gallium-rich side of the diagram. In all instances the composition of this E phase on its rare-earth-rich side corresponded to the stoichiometric composition. The results for the various binary systems examined are plotted in Fig. 3. In Sm-Ga couples annealed below Tp the external SmGa, layer was continuous with a thicker non-stoichiometric Sm, -xGaz +X layer (Fig. 4). In this system the layers showed an excellent mutual adherence and the planarity of the interfaces (Fig. 5) is noteworthy. We were unable to discern the presence of the GdGa, layer in the Gd-Ga couple. It is not clear whether the layer corresponding to this compound was not formed to begin with or whether it became detached during the preparation of the diffusion couple as a result of its poor adherence. As in all previous cases, we observed a concentration gradient in the thick E phase layer. In both the Sm-Ga and the Gd-Ga couples the highest gallium concentrations in this layer (about 77 at.%) were slightly lower than the
93
.w
5 Y
if
70 65L-‘--
‘E s <
------
I
”
60 55
I
1,
r
6
6
’
r
1
c
p
x
57
58
59
60
61
62
63
64
65
66
67
La
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
’
Atomic Number
Fig. 3. Dependence of the gallium solubility the rare earth component R.
limit in the f phase on the atomic
number
of
-SmGas -Sm
l-x%+x
-Sm
Fig. 4. Two compound layers formed at the interface of samarium with liquid gallium during an anneal at 370 “C for 20 h. The sample was etched in a 1:l :l mixture of HNOJ, glacial acetic acid and glycerine.
values (about 82 at.%) observed in the couples based on the lighter rare earths. Metallographic examination of the Gd-Ga couple revealed the retention in the newly formed e phase of the original substructure of the rare earth metal as shown by the network of oxide precipitates (Fig. 6).
-Sm
Fig. 5. Same sample as in Fig. 4 illustrating Sml _xGaz + X layer and samarium.
l-X%+X
the good mutual
adherence
between
the
Fig. 6. Oxide precipitates at the gadolinium grain boundary are retained and delineate the substructure of the E phase formed at the interface of gadolinium with liquid gallium during annealing at 370 “C for 21 h.
The two heavy rare earth-gallium couples (Tb-Ga and Dy-Ga) that were studied displayed a significantly different behaviour. The dominant layer that grew fastest was the stoichiometric well-adherent RGa, layer. In the Dy-Ga couple annealed below Tp there were indications of the formation of a porous ill-defined outermost DyGa, layer prior to the formation of the DyGa, layer. The E phase was observed only in the Tb-Ga couple annealed above Tp (at 560 “C) in the form of a thin (about 5 pm) layer (Fig. 7). Despite its reduced width, it was possible to establish that its composition corresponded to the stoichiometric RGa, composition.
95
-TbGa3
-TbGa2
Tb
Fig. 7. Microstructure 560 “C) for 105 min.
of the Tb-Ga
diffusion zone in a couple annealed above Tp (at
Our results indicate that the extent of the E phase range and the presence of the RGaa compound are closely linked and depend on the atomic number, i.e. the size of the rare earth component. The schematic diagrams of the Gibbs free energy G uersus the gallium content C shown in Fig. 8 illustrate this interrelationship. The free-energy curve for the E compound is asymmetrical with respect to its minimum at the stoichiometric composition.
1 64
96
72 ATOMIC
(a)
60 ltlCEll
66 6AttlUY
96
(b)
Fig. 8. Schematic representation of the dependence of the free energy on the gallium content for the gallium-rich portion of (a) the light rare earth-gallium binary systems and (b) the heavy rare earth-gallium binary systems.
96
Its branch on the gallium-rich side of the minimum has a relatively small slope for the light rare earth metals, suggesting the possibility of substitution of one rare earth atom by two gallium atoms. With increasing atomic number, i.e. smaller rare earth atoms, this substitution involves an increasingly large distortion of the lattice leading to a steeper slope of the G-C curve. For all rare earth metals up to gadolinium, the branch of the GC curve for the e phase or its common tangent with the GC curve for the RGa, compound lies lower than the G-C curve for the RGa, compound. Only for rare earth metals heavier than terbium does the relative position of the competing freeenergy curves lead to the presence of the RGa, compound at its stoichiometric composition (Fig. 8(b)). Both elementary rare earth metals and RGaz samples reacted rapidly with liquid gallium when annealed at temperatures higher than Tp. In these samples a significant dissolution of the solid rare earth metal in liquid gallium took place. Upon cooling to a low temperature, the RGa, phase precipitated from the oversaturated liquid solution. The precipitates appeared in the form of long thin rods distributed throughout the gallium matrix (Fig. 9). These rods are actually the traces of the intersection of the platelet-like precipitates with the polished plane of the sample. This particular and distinctive shape of the precipitates is indicative of a strong surface energy anisotropy of the interface of the RGa, precipitate with liquid gallium. The platelet-like shape is also consistent with the hexagonal symmetry of the RGa, structure and suggests that the crystallographic planes perpendicular to the unique axis have a lower interfacial energy with liquid gallium than the planes parallel to the unique axis.
-PrGa6
precipitate
plates
--PrGa2
Fig. 9. PrGad platelets precipitated from the oversaturated nealed at temperatures above Tp (560 “C) for 2 h.
liquid gallium solution an-
97
In addition to the gallium-rich compounds (RGa2, RGas and RGa,) mentioned so far, the rare earth-gallium binary systems contain several other compounds with lower gallium contents, i.e. RaGa, RsGaa and RGa 161. Nevertheless, no traces of these latter compounds were observed with the exception of the Ce-Ga couple annealed below T, for 69 h. In this couple two more layers were observed between the rare earth metal and the nonstoichiometric e compound layer (Fig. 10). Only the composition of the thicker layer could be analysed; it corresponded to a composition intermediate between those of the CeGa3 and the Ce,Ga, compounds.
----de
1
-xGa2cx
----Ce-rich
layers
--Ce
Fig. 10. A Ce-Ga couple annealed below TP (360 “C) for 68 h. The two thin layers are cerium-rich compound layers.
4. Conclusions (1) In the light rare earth-gallium systems the E phase extends from the stoichiometric composition RGa, to the g~lium-~ch compositions R, _xGaz +%. The range of homogeneity of this compound depends on the atomic number of the rare earth component. For terbium and the heavier rare earth metals, the E phase appears only at the stoichiometric compositions. (2) The RGa, compound is present only in the heavy rare earth gallium systems in which the E phase appears only at the stoichiometric RGa, composition. (3) For light rare earth metals immersed in liquid gallium and annealed below the peritectic melting temperature Tp of RGa,, this latter compound forms the dominant thickest layer in the diffusion zone.
We wish to thank Mr. M. Herman and Mr. R. Zerbib for their able technical assistance.
98
References 1 ‘2 3 4 5
G. Kimmel, D. Dayan, A. Grill and J. Pelleg, 3. Less-Common Met., 75 (1980) 133. J. Pelleg and L. Zevin, J. Less-Common Met,, 77 (1981) 197. S. Cirafici and E. Franceschi, J. Less-Common Met., 77 (1981) 269. J. Pelleg, G. Kimmel and D. Dayan, J. Less-Common Met., 81 (1981) 33. J. W. Colby, Magic-IV, A Computer Program for Quantitative Electron Microprobe Analysis, Bell Telephone Laboratories Inc., Allentown, PA, 1969. 6 S. P. Yatsenko, A. A. Semyannikov, B. G. Semenov and K. A. Chuntonov, J. LessCommon Met., 64 (19’79) 185.
I~ERDIFFUSION LIQUID GALLIUM
Metals, 87 (1982)
87
87 - 98
STUDIES OF RARE EARTH METALS WITH
D. DAYAN and U. ATZMONY Nuclear Research Centre, Negev, P.O. Box 9001, Beer-Sheva (Israel) M. P. DARIEL Nuclear Research Centre, Negev, P.O. Box 9001, Beer-Sheva (Israel) and Department Materials Engineering, Ben Gurion University of the Negev, Beer-Sheva (Israel)
of
(Received April 6,1982)
Summary
The gallium-rich side of some rare earth-gallium binary systems was studied using a diffusion couple technique. Diffusion couples were formed by annealing rare earth metals immersed in liquid gallium at various temperatures. The nature and composition of the compound layers formed at the rare earth metal-gallium interface were determined by metahographic and electron microprobe analysis. The results indicate that in the light rare earth metals (R = La - Cd) the e phase extends from the stoichiometric composition RGa, to the gallium-rich compositions Ri -X Gaz +X. In these systems the RGa6 layer is dominant in the diffusion region. No RGa, layer is present in these systems, in contrast with the two heavy rare earth-gallium couples (Tb-Ga and Dy-Ga) in which the E phase appears at the stoichiometric composition only and the RGa, layer is dominant.
1. Introduction
Recent studies on the constitution of the light rare earth-gallium binary systems [l, 21 have shown that the RGaz phases (R = rare earth metal) are present over a wide range of homogeneity. These so-called e phases have a hexagonal AlB,-type structure. Their departure from stoichiometry can be accounted for in terms of the substitution of one rare earth atom in the basal plane of the unit cell by a pair of gallium atoms, It has indeed been observed that the range of existence of the E phase extends from the stoichiome~i~ composition to the gallium-rich compositions. Pelleg and Zevin [2] have also pointed out that the substitution of the rare earth atom by two gallium atoms is accompanied by a distortion of the original matrix and leads to the dependence of the width of the E phase in the phase diagram on the size ratio of the component elements. The next gallium-rich 0 Elsevier Sequoial~inted
in The Netherlands
88
compound RGa, has a stoichiometric composition and only appears in the heavy rare earth-gallium binary systems in a variety of crystal types [ 31. All previous studies of the extent of the f phase and the existence of the RGa, compounds [3, 41 have been carried out using conventional techniques, e.g. X-ray diffraction and optical and scanning electron microscopy, on cast samples which had undergone various annealing treatments. The object of the present work was to complete the data on the homogeneity range of the E phase and to study its systematics with respect to the presence of the RGa, compounds along the lanthanide series using a diffusion couple technique. Diffusion couples were formed by immersing rare earth metal samples or previously prepared RGa, samples in molten gallium for several hours at various temperatures.
2. Experimental
details
The rare earth metals lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium, terbium and dysprosium (nominal purity, 99.9%) and gallium (purity, 99.9%) were used to prepare the couples. Rare earth metal cylinders were prepared by arc melting under an argon atmosphere and casting into water-cooled split copper moulds of diameter 10 mm. The RGaz compounds were prepared in a similar manner. The cast samples were sectioned into discs 5 - 6 mm thick which were sealed in evacuated silica capsules and annealed at 600 “C. After careful cleaning of their external surfaces, the rare earth samples were inserted into pyrophillite crucibles. The free volume of the crucibles (approximately 0.15 cm3) was filled with molten gallium. The diffusion anneal was carried out in a horizontal vacuum furnace at temperatures which in most instances were below the peritectic temperature corresponding to the e + L + q (Q = RGa,) or RGa, + L + 7 reactions. After the diffusion anneal a cross section of the diffusion couple was examined by optical metallography and electron microprobe analysis. The results of the electron microprobe (Cameca II) scans and points analyses were corrected using a ZAF-based Magic computer program [ 51. Because of the low melting temperature of gallium, special precautions such as the use of precooled lubricant fluid had to be employed during the metallographic preparation procedures.
3. Results and discussion It is well established at present that the gallium-rich part of the rare earth-gallium binary systems conforms to one of the two schematic diagrams shown in Fig. 1. In the light rare earth-gallium systems (R E La - Gd) the congruently melting RGaz compound (e phase) exists over a range of compositions and it is followed by the peritectically melting RGa, compound (Fig. l(a)). In the heavy rare earth-gallium systems the e phase
89
WEIGHT
WEIGHT t*
PER CENT GALLIUM
ATOMIC
(4
PER CENT GALLIUM 50 60 7* 80 90 Km
PER
CENT
GALLIUM
(b)
Fig. 1. Schematic phase diagrams of the gallium-rich part of the rare earth-gallium systems: (a) light rare earth metals; (b) heavy rare earth metals.
binary
appears at the stoi~hiometric RGaz composition only and is followed at a higher gallium content by the two non-congruently melting RGa, and RGa, compounds (Fig. l(b)). During the present study we reacted a series of rare earth metals with liquid gallium. The majority of experiments consisted of immersing rare earth samples in liquid gallium at temperatures TP below the peritectic melting temperature of the RGa, compounds (Q phase). In some instances previously prepared RGa, compounds (R = Pr, Nd) or rare earth metals (samarium, gadolinium, terbium and dysprosium) were reacted with liquid gallium at temperatures higher than Tp. The experiments with the most significant observations are summarized in Table 1. When the light rare earth metals lithium, cerium, praseodymium, neodymium and samarium were annealed at a temperature below TP (see Fig. l(a)) a thick and mostly adherent RGa, compound layer was formed at the interface of the rare earth metal with liquid gallium. This layer was 5 - 8 mm thick for a typical diffusion anneal carried out at approximately 100 “C
360 360
560
370
623
2
3
2
1
1
1
2
2
La-Ga
Ce-Ga
Pr-Ga
PrGa*-Ga
Nd-Ga
NdGaTGa
Sm-Ga
Gd-Ga
100 pm
93 /_lm 65 W 1.9 mm
310 /Jm 170 pm 50 I_Lrn
NdGaza NdGaeb NdGaaa + NdGa6 plates precipitated in Ga matrix SmGaaa SmGab? porous layer SmGaaa + SmGa6 plates precipitated in Ga matrix GdGaza GdGaza GdGae porous layer
2 1
2 1
1 2
21 h 95 min
21 h 150 min
21 h 145 min
370
540
370 545
26 /..lrn 5mm
PrGae plates precipitated in Ga matrix
-
2h
8mm 50 /Jm 1.5 mm
PrGa$ PrGasa PrGaB
83 pm 200 I_tm
1 2
2h
390
CeGaaa CeGas + CesGas CeGaaa CeGaza CeGaB
10 /_lm
0.1 mm 0.42 mm 0.19 mm 8mm
Thickness of layers
20h 2h
5h 69 h
360 360
2
21 h
LaGaaa LaGahb LaGaaa LaGaeb
360
structure of layers
Number of layers 2
Dum tion 6h
(“C)
360
Tempemture
Diffusion anneal
Number of couples
System
Summary of observations of diffusion couples formed between rare earth metals and liquid gallium
TABLE 1
91
._
.I
; 0
m
E
E
d
m
92
below Tp for a duration of 20 h (Fig. 2). A microprobe analysis of this layer revealed a constant gallium content corresponding to the RGa, composition throughout the layer thickness. Progressing inward towards the rare earth
-Pr
-Pr
Fig. 2. Layer of PrGab formed at the interface of the rare earth metal with liquid gallium during an anneal at 360 “C for 20 h.
metal, the adjacent layer was much thinner and consisted of E phase corresponding to the composition Ri _,Ga 2 +% in which a strong concentration gradient was observed. By carefully analysing the composition of this layer in the immediate vicinity of the RGa, layer, we established the composition limit of the E phase on the gallium-rich side of the diagram. In all instances the composition of this E phase on its rare-earth-rich side corresponded to the stoichiometric composition. The results for the various binary systems examined are plotted in Fig. 3. In Sm-Ga couples annealed below Tp the external SmGa, layer was continuous with a thicker non-stoichiometric Sm, -xGaz +X layer (Fig. 4). In this system the layers showed an excellent mutual adherence and the planarity of the interfaces (Fig. 5) is noteworthy. We were unable to discern the presence of the GdGa, layer in the Gd-Ga couple. It is not clear whether the layer corresponding to this compound was not formed to begin with or whether it became detached during the preparation of the diffusion couple as a result of its poor adherence. As in all previous cases, we observed a concentration gradient in the thick E phase layer. In both the Sm-Ga and the Gd-Ga couples the highest gallium concentrations in this layer (about 77 at.%) were slightly lower than the
93
.w
5 Y
if
70 65L-‘--
‘E s <
------
I
”
60 55
I
1,
r
6
6
’
r
1
c
p
x
57
58
59
60
61
62
63
64
65
66
67
La
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
’
Atomic Number
Fig. 3. Dependence of the gallium solubility the rare earth component R.
limit in the f phase on the atomic
number
of
-SmGas -Sm
l-x%+x
-Sm
Fig. 4. Two compound layers formed at the interface of samarium with liquid gallium during an anneal at 370 “C for 20 h. The sample was etched in a 1:l :l mixture of HNOJ, glacial acetic acid and glycerine.
values (about 82 at.%) observed in the couples based on the lighter rare earths. Metallographic examination of the Gd-Ga couple revealed the retention in the newly formed e phase of the original substructure of the rare earth metal as shown by the network of oxide precipitates (Fig. 6).
-Sm
Fig. 5. Same sample as in Fig. 4 illustrating Sml _xGaz + X layer and samarium.
l-X%+X
the good mutual
adherence
between
the
Fig. 6. Oxide precipitates at the gadolinium grain boundary are retained and delineate the substructure of the E phase formed at the interface of gadolinium with liquid gallium during annealing at 370 “C for 21 h.
The two heavy rare earth-gallium couples (Tb-Ga and Dy-Ga) that were studied displayed a significantly different behaviour. The dominant layer that grew fastest was the stoichiometric well-adherent RGa, layer. In the Dy-Ga couple annealed below Tp there were indications of the formation of a porous ill-defined outermost DyGa, layer prior to the formation of the DyGa, layer. The E phase was observed only in the Tb-Ga couple annealed above Tp (at 560 “C) in the form of a thin (about 5 pm) layer (Fig. 7). Despite its reduced width, it was possible to establish that its composition corresponded to the stoichiometric RGa, composition.
95
-TbGa3
-TbGa2
Tb
Fig. 7. Microstructure 560 “C) for 105 min.
of the Tb-Ga
diffusion zone in a couple annealed above Tp (at
Our results indicate that the extent of the E phase range and the presence of the RGaa compound are closely linked and depend on the atomic number, i.e. the size of the rare earth component. The schematic diagrams of the Gibbs free energy G uersus the gallium content C shown in Fig. 8 illustrate this interrelationship. The free-energy curve for the E compound is asymmetrical with respect to its minimum at the stoichiometric composition.
1 64
96
72 ATOMIC
(a)
60 ltlCEll
66 6AttlUY
96
(b)
Fig. 8. Schematic representation of the dependence of the free energy on the gallium content for the gallium-rich portion of (a) the light rare earth-gallium binary systems and (b) the heavy rare earth-gallium binary systems.
96
Its branch on the gallium-rich side of the minimum has a relatively small slope for the light rare earth metals, suggesting the possibility of substitution of one rare earth atom by two gallium atoms. With increasing atomic number, i.e. smaller rare earth atoms, this substitution involves an increasingly large distortion of the lattice leading to a steeper slope of the G-C curve. For all rare earth metals up to gadolinium, the branch of the GC curve for the e phase or its common tangent with the GC curve for the RGa, compound lies lower than the G-C curve for the RGa, compound. Only for rare earth metals heavier than terbium does the relative position of the competing freeenergy curves lead to the presence of the RGa, compound at its stoichiometric composition (Fig. 8(b)). Both elementary rare earth metals and RGaz samples reacted rapidly with liquid gallium when annealed at temperatures higher than Tp. In these samples a significant dissolution of the solid rare earth metal in liquid gallium took place. Upon cooling to a low temperature, the RGa, phase precipitated from the oversaturated liquid solution. The precipitates appeared in the form of long thin rods distributed throughout the gallium matrix (Fig. 9). These rods are actually the traces of the intersection of the platelet-like precipitates with the polished plane of the sample. This particular and distinctive shape of the precipitates is indicative of a strong surface energy anisotropy of the interface of the RGa, precipitate with liquid gallium. The platelet-like shape is also consistent with the hexagonal symmetry of the RGa, structure and suggests that the crystallographic planes perpendicular to the unique axis have a lower interfacial energy with liquid gallium than the planes parallel to the unique axis.
-PrGa6
precipitate
plates
--PrGa2
Fig. 9. PrGad platelets precipitated from the oversaturated nealed at temperatures above Tp (560 “C) for 2 h.
liquid gallium solution an-
97
In addition to the gallium-rich compounds (RGa2, RGas and RGa,) mentioned so far, the rare earth-gallium binary systems contain several other compounds with lower gallium contents, i.e. RaGa, RsGaa and RGa 161. Nevertheless, no traces of these latter compounds were observed with the exception of the Ce-Ga couple annealed below T, for 69 h. In this couple two more layers were observed between the rare earth metal and the nonstoichiometric e compound layer (Fig. 10). Only the composition of the thicker layer could be analysed; it corresponded to a composition intermediate between those of the CeGa3 and the Ce,Ga, compounds.
----de
1
-xGa2cx
----Ce-rich
layers
--Ce
Fig. 10. A Ce-Ga couple annealed below TP (360 “C) for 68 h. The two thin layers are cerium-rich compound layers.
4. Conclusions (1) In the light rare earth-gallium systems the E phase extends from the stoichiometric composition RGa, to the g~lium-~ch compositions R, _xGaz +%. The range of homogeneity of this compound depends on the atomic number of the rare earth component. For terbium and the heavier rare earth metals, the E phase appears only at the stoichiometric compositions. (2) The RGa, compound is present only in the heavy rare earth gallium systems in which the E phase appears only at the stoichiometric RGa, composition. (3) For light rare earth metals immersed in liquid gallium and annealed below the peritectic melting temperature Tp of RGa,, this latter compound forms the dominant thickest layer in the diffusion zone.
We wish to thank Mr. M. Herman and Mr. R. Zerbib for their able technical assistance.
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References 1 ‘2 3 4 5
G. Kimmel, D. Dayan, A. Grill and J. Pelleg, 3. Less-Common Met., 75 (1980) 133. J. Pelleg and L. Zevin, J. Less-Common Met,, 77 (1981) 197. S. Cirafici and E. Franceschi, J. Less-Common Met., 77 (1981) 269. J. Pelleg, G. Kimmel and D. Dayan, J. Less-Common Met., 81 (1981) 33. J. W. Colby, Magic-IV, A Computer Program for Quantitative Electron Microprobe Analysis, Bell Telephone Laboratories Inc., Allentown, PA, 1969. 6 S. P. Yatsenko, A. A. Semyannikov, B. G. Semenov and K. A. Chuntonov, J. LessCommon Met., 64 (19’79) 185.