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The Gd–Ni–Al system: Phases formation and isothermal sections at 500 °C and 800 °C
Intermetallics 45 (2014) 71e79
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Intermetallics journal homepage: www.elsevier.com/locate/intermet
The GdeNieAl system: Phases formation and isothermal sections at 500 C and 800 C S. Delsante*, G. Borzone Dipartimento di Chimica e Chimica Industriale, Università di Genova, Consorzio INSTM e UdR di Genova, Via Dodecaneso 31, 16146 Genova, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 August 2013 Received in revised form 24 September 2013 Accepted 10 October 2013 Available online
Phase relations in the GdeNieAl ternary system have been established for two isothermal sections (500 and 800 C) based on X-ray powder diffraction analysis (XRD), Light Optical Microscopy (LOM), Scanning Electron Microscopy (SEM) coupled with Energy dispersive Microprobe Analysis (EPMA) on about 31 annealed alloys in the 50e100 at.% Al region. Seven intermetallic phases have been identified in samples annealed at 500 C and 800 C: Gd3Ni5Al19 (oS108eGd3Ni5Al19), Gd4Ni6Al23 (mS66eY4Ni6Al23), GdNiAl4 (oS24eYNiAl4), GdNiAl3 (oP20eYNiAl3), Gd3Ni7Al14 (hP72eGd3Ni7Al14), GdNiAl2 (oS16eCuMgAl2) and GdNi2Al3 (hP18eGdNi2Al3). One additional ternary phase GdNi3Al9 (hR78eErNi3Al9) has been found only in samples annealed at 800 C. The isothermal sections have been determined and then compared, the crystal structures of the ternary phases have been confirmed and lattice parameters calculated. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Rare earth intermetallics A. Intermetallics B. Phase diagrams B. Crystal chemistry of intermetallics F. Electron microscopy, scanning
1. Introduction The synthesis, structure and properties of ReNi and ReNieAl alloys are of growing interest for fundamental aspects as well as for potential applications in various technological fields, because of their typical properties. These alloys are generally recognised as members of the hydrogen absorption alloy family [1e3]: the hydrogen sorption properties of GdNi5xAlx (0 < x 3) and NdNi5xAlx have been reported in Refs. [4,5], besides in Ref. [6] results about structural and hydrogen sorption of the same systems are presented. Moreover, Al-based amorphous alloys based on Al-transition metal-rare earth systems, are promising candidates as structural materials for industrial applications because of their high specific strength combined with good bending ductility [7,8]. In addition, these alloys also exhibit greater hardness [9] and wear resistance [10]. The mechanical properties can be further improved by ensuring uniform distribution of fcc-Al nanoparticles in the amorphous matrix by appropriate heat treatment [11]; e.g. this situation leads to high tensile strength materials with low density and extremely high corrosion resistance [12]. An extensive study has been made on glassy alloys and crystallization of the AleNieR amorphous alloys by different researchers [13e17]. The study and definition of the properties of intermetallic phases (stoichiometric
compounds, solid solutions) provides an important contribution to a significant part of inorganic chemistry, as well as to the consolidation and development of relevant topics in material science and engineering (structural alloys, functional intermetallics, etc). Among the properties to be studied in order to obtain a full characterization of the phases, the constitutional ones should take first place. Knowledge of the existence of the phases, of their composition extension, microstructure and crystal structure is a prerequisite to a better understanding of the bondemechanism and to the analysis of its implements as a material, to be prepared and used for certain purposes and in given conditions. A systematic study on constitutional properties of ternary alloys formed by rare earth (R) with aluminium and nickel has been in progress for a few years in our laboratory. Both thermochemical approach, such the determination of DfH0 of selected binary and ternary phase [18e20] and systematic phase equilibria analysis have been performed [21,22]. The results obtained during the investigation of the Al-rich part of the isothermal sections at 500 C and 800 C for the GdeNieAl are presented and discussed. 2. Literature data 2.1. Binary boundary systems
* Corresponding author. Tel.: þ39 010 3536160/6153; fax: þ39 010 3625051. E-mail address: [email protected] (S. Delsante). 0966-9795/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.intermet.2013.10.009
A brief summary of the literature data focusing on phase equilibria of the relevant binary GdeNieAl subsystems is presented in
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the following. A list of the solid phases formed in the two binary systems is given in Table 1. 2.1.1. NieAl The phase diagram has been well established since several experimental studies, assessments and thermodynamic optimization were published. A re-calculation of the diagram is reported by Huang and Chang [28]: in this paper the studies carried out by Ansara et al. [29] and Du et al. [30] are summarized. Two more recent assessments have been presented by Saltykov et al. [26] and by Schuster [31]. Five intermediate phases are reported; among them, only the Alerichest NiAl3 is a “line compound”, while the others present a certain solubility range. The solid solubility of Ni in Al is very limited, while Ni dissolves Al in solid solution up to a maximum of 21.2 at.% at 1371 C, which is the Ni-rich eutectic temperature. 2.1.2. GdeAl The GdeAl phase diagram was assessed by Gschneidner and Calderwood [32] mainly on the basis of the work of Buschow [33]; more recently Saccone et al. [27] re-investigated the 0e67 at.% Al region. Five intermetallic line compounds are present; only the GdAl2 phase melts congruently at 1520 C and shows a wide region of primary crystallization and the Al-richest formed phase is the GdAl3.A catatectic reaction bGd 4 L þ aGd is reported at 2.5 at.% Al and 1200 C. It could be worthwhile to remind that, in correspondence to Gd, we have the change in the alloying behaviour from the light rare earths to those typical of the heavy ones [34]. Indeed, as a consequence of a gradual property changes such as for instance the progressive decrease in the number of compounds and of the stoichiometries of the phases richest in Al, it has been recognised the subdivision of the trivalent rare earth-aluminium alloys into two groups, one given by the light earths (from La to Sm) and the other by the heavy ones (from Gd to Lu) [35,36]. 2.2. GdeNieAl ternary system The ternary phases reported in literature belonging to the Gde NieAl system and the solid solutions based on the binary phases are listed in Table 2 together with the crystal structures, the lattice parameters and details about the annealing temperatures. The XRD results obtained in this work are summarized in Table 2. The isothermal section at 800 C for the GdeNieAl system has been published by Refs. [41]; the Alerichest phase was found to be the GdNi3Al16 for which the complete structure determination was not given. Subsequently, the GdNi3Al16 phase was not observed by Ref. [38] and was replaced by the Gd3Ni5Al19. Besides, two new phases, Gd4Ni6Al23 and GdNi3Al9 were found [39,40] in samples Table 1 Binary boundary NieAl and GdeAl systems: literature data on crystal structures and transformation temperature data. Phase and T range/ C
at.% Al Pearson symbolprototype
Al < 660.45 100 GdAl3 < 950 75 NiAl3 < 856 75 GdAl2 < 1520 66.66 Ni2Al3 < 1138 60
cF4eCu hP8eNi3Sn oP16eFe3C cF24eMgCu2 hP5eNi2Al3
GdAl < 1070 NiAl < 1651
oP16eAlEr cP2eCsCl
50 50
Lattice parameters ( A) a
b
Ref.
c
e e [23,24] e 4.592 [25] 7.367 4.811 [26] [27] e 4.891 36.8O40.5 at.% Ni [24,26] 5.893 11.59 5.695 [27] 2.8872 e e 42O69.2 at.% Ni [24] at 50 at.% Ni [26] 4.0433 6.32 6.613 7.9060 4.028
annealed at 500 and 800 C, respectively. Furthermore, the existence of the Gd2Ni3Al7 phase proposed by Ref. [41] was not confirmed. More recently, two new phases have been found and characterized: GdNiAl3 [42] and Gd3Ni7Al14 [43] in samples annealed at 600 C and 800 C, respectively. In light of these studies, it clearly appears that a revision of the isothermal section at 800 C is needed. The thermodynamics of the GdeNieAl system in the Al-rich corner were assessed using the calculation of phase diagrams (CALPHAD) approach by Gao et al. [44] who reported a calculation of a small portion of the isothermal section at 500 C. However, they did not take into account the new information about the ternary phases discovered and characterized after the work by Rykhal’ et al. [41]. 3. Experimental Small pieces of pure metals (Gd rods with 99.9% purity e JM Alfa AesarÒ, Ni rods with 99.99% purity Newmet Koch Company and Al 99.999% e WAV AG “Kryal”) were polished, weighed in the proper amount and then melted to obtain the alloys synthesis. Two different methods have been employed: induction melting (in alumina crucibles) and arc-melting. Each alloy was re-melted two or three times in order to achieve a complete homogeneity. Generally no weight loss was observed after melting. No significant differences have been observed in samples synthesized using the two techniques. Each sample was afterwards divided into two pieces in order to perform the annealing at 500 C and 800 C; few samples where also annealed at 600 and 700 C. Alumina containers sealed in quartz tubes in argon atmosphere were employed. The annealing time was different considering the composition of the samples and the temperature: at 500 C in the range 40 O 120 days, at 800 C 20 O 40 days, 20 and 10 days at 600 and 700 C, respectively. At the end of the heat treatment, the samples were quenched in icy water. Microstructures of the alloys were systematically investigated by light optical microscopy (LOM) on smooth samples prepared by grinding, lapping and polishing the resin-mounted samples with diamond abrasive spray down to 1 mm grain size. A thorough investigation of each sample by using a Scanning Electron Microscope (SEM, Zeiss e EVO 40) was performed by using a backscattered electron detector (BSE) in order to reveal the compositional contrast between the different phases. Quantitative EPMA data were collected at 20 kV on a Link system Ltd. instrument equipped with an Energy Dispersive Spectroscopy (EDS) detector. A counting time of 100 s and a ZAF correction program were adopted. Certified pure elements were used as reference standards, while cobalt was adopted for calibration purposes. The software package Inca Energy (Oxford Instruments, Analytical Ltd., Bucks, U.K.) was employed to process X-ray spectra. Generally for each element an error within 0.5 at.% was ascribed. Powder X-ray diffraction (XRD) was performed using a Philips X’Pert MPD machine (Philips, Almeno, The Netherlands) equipped with a copper target, excited to 40 kV and 30 mA, and a solid state detector. Values of the lattice parameters were processed and refined through a least-square interpolation, by using the NelsoneRiley function [45]. The present study was carried out on the basis of 31 samples having a mass of about 1.5 g and a suitable composition to determine the isothermal sections in the 50 O 100 at.% Al region. 4. Results and discussion The compositions of the GdeNieAl alloys prepared are reported in Table 3, ordered with decreasing at.% of Al, together with the phases observed for each sample after the annealing at 500 C and at 800 C. Considering the observed differences in the determined
S. Delsante, G. Borzone / Intermetallics 45 (2014) 71e79
73
Table 2 Gd-Ni-Al system: experimental and literature data on crystal structures and lattice parameters of the ternary phases in the 50e100 at. % Al region. N.
Phase
Stoichiometric composition (at.%) Gd
s1
s2
s3
s4
s5
s6
s7
Ni
Al
Pearson symbol-prototype and method
Lattice parameters [ A] and [ ]
Ref/ Annealing temperature
a ¼ 4.0858(1) b ¼ 15.9821(2) c ¼ 27.0713(4) a ¼ 4.0893(7) b ¼ 15.993(2) c ¼ 27.092(4) a ¼ 15.876 (8) b ¼ 4.0827 (3) c ¼ 18.31 (1) b ¼ 113.01(2) a ¼ 15.856 (4) b ¼ 4.078(1) c ¼ 18.286(6) b ¼ 113.01(2) a ¼ 7.3006(9) b ¼ 27.478(5) a ¼ 7.2869(8) b ¼ 27.435(3) a ¼ 4.087(5) b ¼ 15.30(3) c ¼ 6.696 (3) a ¼ 4.072(1) b ¼ 15.473(7) c ¼ 6.610(1) a ¼ 8.164(2) b ¼ 4.0680 (9) c ¼ 10.666 (3) a ¼ 8.158(2) b ¼ 4.060(1) c ¼ 10.665(4) a ¼ 17.980(2) c ¼ 4.0460(5) a ¼ 17.962(8) c ¼ 4.047(2) a ¼ 4.912(5) c ¼ 3.964(5) (x ¼ 0) a ¼ 5.028(5) c ¼ 4.074(5) (x ¼ 1.74) a ¼ 8.769(5) c ¼ 4.113(5) (x ¼ 2.01) a ¼ 9.085(5) c ¼ 4.039(5) (x ¼ 3.07) a ¼ 4.080(5) b ¼ 10.14(4) c ¼ 6.93 (3) a ¼ 7.8900 (x ¼ 0.45)
T.W and [37] (#7R800) 800 C
Gd3Ni5Al19
11.1
18.5
70.4
oS108eGd3Ni5Al19 Powder diffraction
Gd3Ni5Al19
11.1
18.5
70.4
oS108eGd3Ni5Al19 Single crystal
Gd4Ni6Al23
12.12
18.18
69.7
mS66Y4Ni6Al23 Powder diffraction
Gd4Ni6Al23
12.12
18.18
69.7
mS66Y4Ni6Al23 Single crystal
GdNi3Al9
7.7
23.1
69.2
GdNi3Al9
7.7
23.1
69.2
GdNiAl4
16.67
16.67
66.66
hR78eErNi3Al9 Single crystal hR78eErNi3Al9 Powder diffraction oS24YNiAl4 Powder diffraction
GdNiAl4
16.67
16.67
66.66
oS24YNiAl4 Powder diffraction
GdNiAl3
20.0
20.0
60.0
oP20eYNiAl3 Powder diffraction
GdNiAl3
20.0
20.0
60.0
oP20eYNiAl3 Powder diffraction
Gd3Ni7Al14
12.5
29.2
58.3
Gd3Ni7Al14
12.5
29.2
58.3
GdNi5xAlx
16.67
hP72eGd3Ni7Al14 Single crystal hP72e Gd3Ni7Al14 Powder diffraction hP6CaCu5 Powder diffraction
83.33(x/6)%
(x/6)%
hP18HoNi2.6Ga2.4 Powder diffraction
s8
GdNiAl2
25.0
GdAl2-xNix
33.33
25.0
(x/3)%
50.0
oS16CuMgAl2 Powder diffraction
66.67(x/3)%
cF24Cu2Mg Powder diffraction
[38] 800 C T.W and [37] (#8 R800) 800 C [39] 500 C
[40] 800 C T.W (# 6 R800) 800 C [41] 800 C T.W (#15 R500) 500 C and 800 C [42] 600 C T.W (# 24 R500) 500 C and 800 C [43] T not given T.W (#30 R800) 500 and 800 C [6] 800 C
[41] 800 C [41] 800 C
T.W ¼ This work.
phase equilibria for the 65e75 at.% Al region, additional annealing at 600 C and 700 C have been performed for samples #6, 7 and 9 (see Table 4, where the same information are inserted). As far as the composition of the samples #1e4 and #6e9 the annealing at 800 C should be considered partly performed in the liquidus state even if only the shape of the sample #1 and 2 was macroscopically changed. In particular, samples #1 and 2 can be considered quenched from the liquid state and the other samples partially quenched from the liquid state. Seven intermetallic phases have been identified using SEM/EDS and XRD analysis in samples annealed at 500 C: Gd3Ni5Al19 (oS108eGd3Ni5Al19), Gd4Ni6Al23 (mS66eY4Ni6Al23), GdNiAl4 (oS24eYNiAl4), GdNiAl3 (oP20eYNiAl3), Gd3Ni7Al14 (hP72e Gd3Ni7Al14), GdNiAl2 (oS16eCuMgAl2) and GdNi2Al3 (hP18e GdNi2Al3). The same phases have been observed in samples
annealed at 800 C; nevertheless, the GdNi3Al9 (hR78eErNi3Al9) has been found to be stable only at this temperature and seems to decompose at lower temperature. The investigation of samples # 6, 7 and 9 (see Table 4) annealed at 600 C and 700 C showed that this phase in no more stable also at these temperatures. The peculiar feature of the determined phase equilibria is the high complexity observed due to the presence of numerous ternary intermetallic phases having very close compositions. The intermetallic nature of the samples results also in a high brittleness proved by the holes, cracks and porosities found during the observation of the alloys micrographs. In Fig. 1 is displayed the isothermal section at 500 C for the 50e 100 at.% Al region; the established phase equilibria result in 16 tietriangles. Concerning the Al-rich corner, we did not find any appreciable solubility of Ni and Gd in pure aluminium in agreement
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S. Delsante, G. Borzone / Intermetallics 45 (2014) 71e79
Table 3 XRD and SEM/EDS experimental results of the GdeNieAl alloys annealed at 500 and 800 C and then water quenched. Sample#
1
Gd at.%
5.0
Ni at.%
2.0
Al at.%
93.0
2
1.5
7.5
91.0
3
9.5
3.5
87.0
4
3.5
12.0
84.5
5
12.0
4.0
84.0
6
5.0
20.0
75.0
7
8.0
20.0
72.0
8
13.0
15.0
72.0
9
11.0
18.0
71.0
10
18.0
13.0
69.0
11
17.0
14.0
69.0
12
10.0
22.0
68.0
13
1.0
31.0
68.0
14
2.5
30.0
67.5
15
20.0
13.0
67.0
16
14.0
20.0
66.0
17
11.0
23.0
66.0
18
27.0
8.0
65.0
19
7.0
28.0
65.0
20
19.0
17.0
64.0
21
16.0
20.0
64.0
22
24.0
14.0
62.0
23
11.5
26.5
62.0
24
20.0
20.0
60.0
Table 3 (continued ) Sample#
Gd at.%
Ni at.%
Al at.%
Phase observed (SEM/EDS and XRD) After annealing at 500 C
After annealing at 800 C
GdAl3 Gd4Ni6Al23 Al Gd3Ni5Al19 NiAl3 Al Gd4Ni6Al23 GdAl3 Al Al NiAl3 Gd3Ni5Al19 GdAl3 Gd4Ni6Al23 Al Gd3Ni5Al19 NiAl3 Al Gd3Ni5Al19 NiAl3 Al Gd4Ni6Al23 Al GdAl3 Al NiAl3 Gd4Ni6Al23 GdAl3 GdNiAl4 Gd4Ni6Al23 e
GdAl3 Gd4Ni6Al23 L Gd3Ni5Al19 NiAl3 L Gd4Ni6Al23 GdAl3 L L NiAl3 GdNi3Al9 e
Ni2Al3 NiAl3 Gd4Ni6Al23 Ni2Al3 NiAl3 Gd4Ni6Al23 Ni2Al3 NiAl3 Gd4Ni6Al23 GdAl2 GdNiAl4 GdAl3 GdNiAl4 Gd4Ni6Al23 Ni2Al3 GdNiAl4 Gd4Ni6Al23 Ni2Al3 GdAl2 (ss) GdNiAl4 GdNiAl3 Gd4Ni6Al23 Ni2Al3 GdNiAl4 GdAl2 (ss) GdNiAl4 GdNiAl3 GdNiAl4 GdNiAl3 Gd3Ni7Al14 GdAl2 (ss) GdNiAl3 GdNiAl4 GdNiAl4 Gd3Ni7Al14 NiAl
GdNi3Al9 NiAl3 L Gd3Ni5Al19 GdNi3Al9 L Gd4Ni6Al23 GdAl3 L Gd4Ni6Al23 Gd3Ni5Al19 L GdAl3 GdNiAl4 Gd4Ni6Al23 GdAl3 GdNiAl4 Gd4Ni6Al23 Gd4Ni6Al23 Ni2Al3 GdNiAl4 Ni2Al3 NiAl3 GdNi3Al9 GdNi3Al9 Ni2Al3 NiAl3 GdAl2 GdNiAl4 GdAl3 GdNiAl4 Gd4Ni6Al23 Ni2Al3 GdNiAl4 Gd4Ni6Al23 Ni2Al3 GdAl2 GdNiAl4 GdNiAl3 Ni2Al3 Gd4Ni6Al23 GdNiAl4 GdAl2 GdNiAl4 GdNiAl3 GdNiAl4 GdNiAl3 Gd3Ni7Al14 GdAl2 GdNiAl3 GdNiAl4 GdNiAl4 Gd3Ni7Al14 NiAl
25
5.0
36.0
59.0
26
22.5
18.5
59.0
27
10.0
31.0
59.0
28
4.5
37.5
58.0
29
15.0
28.0
57.0
30
8.0
36.0
56.0
31
21.0
27.0
52.0
Phase observed (SEM/EDS and XRD) After annealing at 500 C
After annealing at 800 C
GdNiAl3 GdAl2 (ss) Gd3Ni7Al14 NiAl GdNiAl4 GdAl2 (ss) GdNiAl3 GdNi2Al3 GdNiAl4 Gd3Ni7Al14 NiAl NiAl GdNiAl4 Gd3Ni7Al14 GdNiAl3 GdNi2Al3 NiAl GdNi2Al3 Gd3Ni7Al14 GdNi2Al3 GdAl2 (ss) GdNiAl2
GdNiAl3 GdAl2 Gd3Ni7Al14 NiAl GdNiAl4 GdAl2 GdNiAl3 GdNi2Al3 NiAl GdNiAl4 Gd3Ni7Al14 NiAl GdNiAl4 Gd3Ni7Al14 GdNiAl3 GdNi2Al3 NiAl Gd3Ni7Al14 GdNi2Al3 GdNi2Al3 GdNiAl2 GdAl2 (ss)
(ss) ¼ solid solution of Ni in GdAl2 phase.
with what was observed in the binary subsystems [24,27,33]. The isothermal section at 800 C, in the same composition range, is inserted in Fig. 2 and shows 18 tie-triangles. Note that at 800 C, in the Al-rich corner the liquid phase is present and a dotted liquidus line has been sketched considering the liquidus composition reported for the binary systems NieAl (w 9 at.% Ni) [26] and GdeAl (w7 at.% Gd) [33]. The resulting isothermal sections at 500 and 800 C appear to be quite different especially in the 65e75 at.% Al region. The SEM/EDS and XRD analysis on samples annealed at 600 and 700 C (#6, 7 and 9) give an indication that in this small portion of the phase diagram the phase equilibria should be the same established at 500 C. In the very Al-rich part of the ternary system, a eutectic microstructure has been observed involving the phases Al þ GdAl3 þ Gd4Ni6Al23, in the left side and Al þ NiAl3 þ Gd3Ni5Al19 in the right side, see Fig. 3 (sample #1 annealed at 500 C) and Fig. 4 (sample #2 annealed at 500 C) in which the eutectic microstructure is visible. The presence of two ternary eutectics was claimed by Refs. [44], but they did not consider the existence of the ternary phases Gd4Ni6Al23 and Gd3Ni5Al19 but only a ternary phase having a composition Gd2Ni3Al15 (which means 10 at.% Gd, 15 at.% Ni and 75 at.% Al); for this reason they reported two ternary eutectics formed by (Al þ GdAl3 þ Gd2Ni3Al15) and by (Al þ NiAl3 þ Gd2Ni3Al15). In the following, the established phase equilibria at 500 and 800 C are commented together with the involved binary and ternary phases. For convenience, the phase diagram has been divided in two sub-regions. The GdAl corner has not been investigated. 4.1. The 65e100 at.% Al region Three ternary phases and three binary phases have been observed in samples annealed at 500 C: Gd3Ni5Al19, Gd4Ni6Al23, GdNiAl4, GdAl3, NiAl3 and GdAl2. At 800 C, in addition to the already mentioned phases, also the GdNi3Al9 has been found. See Tables 1 and 2 to have the atomic compositions and the structural details.
S. Delsante, G. Borzone / Intermetallics 45 (2014) 71e79
75
Table 4 XRD and SEM/EDS experimental results of the GdeNieAl alloys annealed at 600 and 700 C and then water quenched. Sample#
Gd at.%
Ni at.%
Al at.%
6
5.0
20.0
75.0
7
8.0
20.0
72.0
9
11.0
18.0
71.0
Phase observed (SEM/EDS and XRD) After annealing at 600 C
After annealing at 700 C
Gd3Ni5Al19 NiAl3 Al Gd3Ni5Al19 NiAl3 Al Gd3Ni5Al19 Gd4Ni6Al23 Al
Gd3Ni5Al19 NiAl3 L Gd3Ni5Al19 NiAl3 L Gd3Ni5Al19 Gd4Ni6Al23 L
4.1.1. Binary phases As for the NiAl3 and GdAl3 phases, on the basis of the results obtained by EPMA analysis, we can suppose that there is no appreciable solubility of Gd or Ni, respectively, at 500 C and at 800 C. The GdAl2 phase shows a wide field of primary crystallization in the binary system which extends into the ternary; a solubility range has been observed extending into the ternary field up to about 7 at.% Ni for samples annealed at 500 C. The cubic cell of the GdAl2 (cF24eMgCu2 type) phase becomes smaller when the Ni content increases. The values of the lattice parameter a of the cubic phase at different Ni content were determined in samples annealed at 500 C and are inserted in Table 5. Figs. 5 and 6 illustrate the micrographic appearance of samples # 15 and 22 annealed at 500 C: in both samples there is the GdAl2 bright phase as a primary crystals; sample #15 (Fig. 5) shows the GdAl3 (light grey) close to the GdAl2 and a matrix of GdNiAl4 (dark grey) whereas sample #22 (Fig. 6) has a matrix mainly formed by GdNiAl3 (light grey) and a few quantity of GdNiAl4 (dark grey). As for the same samples annealed at 800 C, we observed for the GdAl2 a smaller solubility of Ni which has a maximum value of about 4 at.% Ni (sample #31). See Figs. 1 and 2 to appreciate the resulting differences in the phase
Fig. 1. GdeNieAl system: isothermal section at 500 C in the 100< at.% Al <50 composition range. The existing ternary phases are reported: (s1) Gd3Ni5Al19; (s2) Gd4Ni6Al23; (s4) GdNiAl4; (s5) GdNiAl3; (s6) Gd3Ni7Al14; (s7) GdNi2Al3; (s8) GdNiAl2. The two-phase fields having a certain homogeneity range have been coloured in light-grey. The GdAl corner has not been investigated.
Fig. 2. GdeNieAl system: isothermal section at 800 C in the 100< at.% Al <50 composition range. The existing ternary phases are reported: (s1) Gd3Ni5Al19; (s2) Gd4Ni6Al23; (s3) GdNi3Al9; (s4) GdNiAl4; (s5) GdNiAl3; (s6) Gd3Ni7Al14; (s7) GdNi2Al3; (s8) GdNiAl2. A dotted liquidus line has been sketched (see text for details). The twophase fields having a certain homogeneity range have been coloured in light-grey. The GdAl corner has not been investigated.
equilibria involving the GdAl2 solid solution at 500 and 800 C. These outcomes are very different from those reported by Rykhal’ et al. [41] which reported at 800 C a solubility of Ni in GdAl2 up to around 15 at.%. 4.1.2. Ternary phases The Gd3Ni5Al19 phase was discovered using the single crystal analysis by Gladyshevskii [38] and then confirmed by powder diffraction by Ref. [37] to have an orthorhombic crystal structure (oS108eGd3Ni5Al19) in samples annealed at 800 C. The lattice parameters have been calculated in sample #7 annealed at 800 C. During this investigation, the phase was found to be stable also in samples annealed at 500, 600 and 700 C. At a very close composition, there is the Gd4Ni6Al23 phase (see Table 2) which has been observed is samples annealed at 500, 600,
Fig. 3. SEM image (BSE mode) of 5.0 at.% Gd, 2.0 at.% Ni, 93.0 at.% Al sample (#1 annealed at 500 , see Table 3). White phase ¼ GdAl3, s2 ¼ Gd4Ni6Al23, black phase ¼ Al and eutectic mixture Al þ GdAl3 þ Gd4Ni6Al23.
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Fig. 4. SEM image (BSE mode) of 1.5 at.% Gd, 7.5 at.% Ni, 91.0 at.% Al sample (#2 annealed at 500 , see Table 3). Dark-grey phase ¼ NiAl3, s1 ¼ Gd3Ni5Al19, black phase ¼ Al and the eutectic mixture Al þ NiAl3 þ Gd3Ni5Al19.
Fig. 6. SEM image (BSE mode) of 24.0 at.% Gd, 14.0 at.% Ni, 62.0 at.% Al sample (#22 annealed at 500 C, see Table 3). Bright phase ¼ GdAl2, s4 ¼ GdNiAl4, s5 ¼ GdNiAl3.
700 and 800 C. It has a monocline mS66eY4Ni6Al23 type structure [37,39]; the lattice parameters have been determined in the sample #8 annealed at 800 C. Both phases in our samples do not show any significant solubility by EDS analysis. As displayed in Figs. 1 and 2, a very narrow threeephase field involves these two ternary phases and the pure aluminium; in Fig. 7 it is possible to see the
micrographic appearance of sample # 9 annealed at 800 C: the dark grey phase is the Gd3Ni5Al19 and the light grey the Gd4Ni6Al23; the small quantity of Al is indicated by the arrow and is not easily recognizable from cracks and holes. Alloys representing threephase field involving Al, GdAl3 and Gd4Ni6Al23 have the typical microstructure displayed in Fig. 3; nevertheless, for composition not so close to the pure Al, the eutectic microstructure is less visible as can be seen in Fig. 8 (sample #5 annealed at 500 C): crystals of GdAl3 (bright phase) and Gd4Ni6Al23 (light grey phase) in a matrix of Al and eutectic mixture (Al þ GdAl3 þ Gd4Ni6Al23). The existence of the GdNi3Al9 phase was firstly reported for samples annealed at 800 C by Ref. [40]. Observing the SEM/EDS and XRD analysis of samples # 6, 7, 13 and 14 after the heat treatment at 500 and 800 C, it can be stated that this phase seems to be stable at 800 C but should undergo to a decomposition reaction below 800 C because is not present anymore at lower temperature. The analysis on alloys #6 and 7 after a further annealing at 600 and 700 C (see Table 4) proved that the GdNi3Al9 ternary phase is not stable also at these temperatures. Figs. 9 and 10 show the
Fig. 5. SEM image (BSE mode) of 20.0 at.% Gd, 13.0 at.% Ni, 67.0 at.% Al sample (#15 annealed at 500 , see Table 3). Bright phase ¼ GdAl2, Grey phase ¼ GdAl3, s4 ¼ GdNiAl4.
Fig. 7. SEM image (BSE mode) of 11.0 at.% Gd, 18.0 at.% Ni, 71.0 at.% Al sample (#9 annealed at 800 C, see Table 3). s1 ¼ Gd3Ni5Al19, s2 ¼ Gd4Ni6Al23, Al is indicated by the arrow. Crack and holes are also present.
Table 5 Lattice parameter a for the solid solution based on the binary cubic phase GdAl2 (cF24eMgCu2 type) at different Ni content for samples annealed at 500 C (see Table 3 and text comments). #
GdAl2xNix/x value
Lattice parameter a/ A
15 18 20 22 26 31
0 w0.03 w0.03 w0.03 w0.09 w0.21
7.900(1) 7.896(1) 7.891(1) 7.890(1) 7.881(1) 7.866(3)
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Fig. 8. SEM image (BSE mode) of 12.0 at.% Gd, 4.0 at.% Ni, 84.0 at.% Al sample (#5 annealed at 500 C, see Table 3). Bright phase ¼ GdAl3, s2 ¼ Gd4Ni6Al23, matrix of Al and eutectic mixture Al þ GdAl3 þ Gd4Ni6Al23.
Fig. 10. SEM image (BSE mode) of 2.5 at.% Gd, 30.0 at.% Ni, 67.5 at.% Al sample (#14 annealed at 800 C, see Table 3). Grey phase ¼ Ni2Al3, Dark-grey phase ¼ NiAl3, s3 ¼ GdNi3Al9.
micrographic appearance of alloys #14 annealed at 500 C and 800 C, respectively. In both samples the Ni2Al3 and NiAl3 binary phases are present while the bright phase is the ternary one which is Gd4Ni6Al23 at 500 C and GdNi3Al9 at 800 C. Comparing the XRD patterns of the sample #6 annealed at 700 and 800 C it is possible to observe that the reflections belonging to the GdNi3Al9, which are present in the pattern of the sample annealed at 800 C, disappear in the diffraction pattern of the sample annealed at 700 C. The comparison of the XRD patterns is inserted in Fig. 11; since they belong to three-phases samples, they appears quite complex and only a reduced range is displayed in order to underline the main peaks pertaining to the hexagonal GdNi3Al9 phase which are indicated with the asterisk. The GdNiAl4 phase, already reported by Rykhal’ [41], has been found at both annealing temperature having no significant solubility. The lattice parameters of this orthorhombic phase have been determined on sample #15 annealed at 500 C, see Table 2. It seems to have a primary formation, as can be seen in Fig. 12 which shows the micrograph of the alloy #11 annealed at 800 C: the light grey
GdNiAl4 ternary phase is surrounded by the Gd4Ni6Al23 which is mixed with the GdAl3 phase (bright one); wide cracks between the phases (in black) are also present.
Fig. 9. SEM image (BSE mode) of 2.5 at.% Gd, 30.0 at.% Ni, 67.5 at.% Al sample (#14 annealed at 500 C, see Table 3). Grey phase ¼ Ni2Al3, Dark-grey phase ¼ NiAl3, s2 ¼ Gd4Ni6Al23.
4.2. The 50e65 at.% Al region Four ternary phases and two binary phases have been observed in samples annealed both at 500 C and 800 C: GdNiAl3, GdNi2Al3, Gd3Ni7Al14, GdNiAl2, Ni2Al3 and NiAl. See Tables 1 and 2 for the atomic composition of the phases. 4.2.1. Binary phases The Ni2Al3 has been found to be the primary phase formed in samples # 13 and 14 surrounded by NiAl3 phase with a peritectic formation in agreement with the binary NieAl phase diagram [24,29]. See, as an example, Fig. 9. A dotted line has been traced due to a disagreement between the detected composition of the Ni2Al3 phase in samples #13 and #14. A wide field of primary crystallization was observed for the NiAl phase at both 500 C and 800 C. This behaviour is evident in the
Fig. 11. Comparison of the XRD patterns of the sample #6 annealed at 700 C (full line) and 800 C (dotted line). The main peaks pertaining to the hexagonal GdNi3Al9 phase (hR78eErNi3Al9) are indicated with the asterisk.
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parameters are in agreement with the literature data [46]. A negligible solubility of Gd in both Ni2Al3 and NiAl phases was detected.
Fig. 12. SEM image (BSE mode) of 17.0 at.% Gd, 14.0 at.% Ni, 69.0 at.% Al sample (#11 annealed at 800 C, see Table 3). s4 ¼ GdNiAl4, bright phase ¼ GdAl3, s2 ¼ Gd4Ni6Al23. Wide cracks are the black part of the alloy.
two-phase samples #25 and 28 annealed at 500 and 800 C; the samples show as primary phase the NiAl and a matrix of GdNiAl4 ternary phase (see Fig. 13, which refers to sample # 25 annealed at 500 C). The detected composition was w55 at.% Al (sample #28) and w56 at.% Al (sample #25); as a consequence of these results, the twoephase field NiAleGdNiAl4 has been traced, see Figs. 1 and 2. The calculated lattice parameter a for the cubic NiAl phase having the cP2eCsCl structure in the already mentioned samples annealed at 800 C was: - Sample #28 (w55 at.% Al): a ¼ 2.8613(8) A - Sample # 25 (w56 at.% Al): a ¼ 2.8538(8) A The decrease of the lattice parameter a with the increase of the Al content in the NiAl phase is due to the lattice vacancies produced in the Ni sites whereas the Al sites remain fully occupied by Al as extensively discussed in Ref. [46]. The obtained values of the lattice
Fig. 13. SEM image (BSE mode) of 5.0 at.% Gd, 36.0 at.% Ni, 59 at.% Al sample (#25 annealed at 500 C, see Table 3). Dark-grey phase ¼ NiAl, s4 ¼ GdNiAl4. Black holes are also present due to the brittleness of the sample.
4.2.2. Ternary phases The GdNiAl3 phase was reported among the ReNieAl systems for R ¼ Y, Sm, Gd, Tb and Dy [22,42,47]. The crystal structure has been confirmed to be oP20eYNiAl3 type. A slight homogeneity range, (comparable with microprobe error) might be attributed to this phase but is not considered in the drawn sections. The cell parameters have been calculated for sample #24 annealed at 500 C which was nearly oneephase sample. A study concerning the existence and the structural investigation of the Gd3Ni7Al14 ternary phase was presented during a conference [43]; it was found to have a hexagonal structure hP72e Gd3Ni7Al14. The presence of this phase at 500 and 800 C was confirmed; the lattice parameters have been calculated on sample #30 annealed at 800 C and inserted in Table 2. The threeephase field in which the Gd3Ni7Al14 is in equilibrium with GdNiAl4 and NiAl phases is represented in Fig. 14 (alloy #27 annealed at 500 C): the black phase is the NiAl (solid solution with w55 at.% Al) and the matrix is formed by Gd3Ni7Al14 (dark grey) and GdNiAl4 (light grey). The GdNi2Al3 (which was also reported in literature as GdNi5xAlx) has been observed by SEM/EDS analysis having a certain homogeneity range (a maximum content of Al w53 at.% at 500 C and w52 at.% at 800 C, along the isopleth at 16.7 at.% Gd). Different authors reported for RNi5xAlx (R ¼ Nd, Gd, Tb, Dy, Ho, Er and Y [5e6,48e50]) an hP6CaCu5 structure type for samples having composition with x < 2 and an hP18HoNi2.6Ga2.4 for samples having composition with x 2. The unit cell parameters of CaCu5p and ffiffiffi HoNi2.6Ga2.4 type structures are related by aHoNi2:6 Ga2:4 ¼ aCaCu5 3 and cHoNi2:6 Ga2:4 ¼ cCaCu5 and the HoNi2.6Ga2.4 type was considered as a P6/mmm hexagonal superstructure of the RNi5xAlx phase, which is located on the same R ¼ 16.67 at.% isopleth line, see Table 2. The XRD analysis performed on sample #31 annealed at 500 C shows that in our sample the diffraction pattern of the GdNi5xAlx phase (x ¼ 3.06 corresponding to an atomic composition w Gd16.7Ni32.3Al51) presents some additional intense reflections which could be indexed considering a larger unit cell having the hexagonal HoNi2.6Ga2.4 type structure [9].
Fig. 14. SEM image (BSE mode) of 10.0 at.% Gd, 31.0 at.% Ni, 59.0 at.% Al sample (#27 annealed at 500 C, see Table 3). Black phase ¼ NiAl, s4 ¼ GdNiAl4, s6 ¼ Gd3Ni7Al14.
S. Delsante, G. Borzone / Intermetallics 45 (2014) 71e79
The existence of RNiAl2 was previously reported for all the rare earths with the exception of Sc and Eu [51] and also reported by Ref. [41]. The formation of the GdNiAl2 phase has been confirmed in this work. 5. Conclusions The experimental determination of the isothermal sections of the GdeNieAl system at 500 C and 800 C confirmed the existence of eight ternary phases in the 50e100 at.% Al region: Gd3Ni5Al19 (oS108eGd3Ni5Al19), Gd4Ni6Al23 (mS66eY4Ni6Al23), GdNi3Al9 (hR78eErNi3Al9), GdNiAl4 (oS24eYNiAl4), GdNiAl3 (oP20eYNiAl3), Gd3Ni7Al14 (hP72eGd3Ni7Al14), GdNiAl2 (oS16eCuMgAl2) and GdNi2Al3 (hP18eGdNi2Al3). Experimental evidence proves that the GdNi3Al9 phase is not stable below 800 C. Only the GdAl2 binary phase seems to dissolve a significant amount of Ni: up to 7 at.% at 500 C and up to 4 at.% at 800 C even GdAl3, NiAl3, Ni2Al3 and NiAl do not dissolve the third element. The resulting isothermal sections appears to be quite different at the two investigated temperatures, however both of them show a high number of threeephase fields. Considering the high number of new ternary phases discovered after the publication by Rykhal’ et al. [41] and that we confirmed their existence during our investigation, it is selfeevident that the isothermal section at 800 C determined during this work is completely different from the one reported by these authors. The same consideration can be done regarding the partial isothermal section at 500 C calculated by Ref. [44] which has been constructed without considering the existence of the ternary phases reported by Refs. [38e40]. References [1] Buschow KHJ. In: Gschneidner Jr. KA, Eyring L, editors. Handbook on the physics and chemistry of rare earths, vol. 6. The Netherlands: North-Holland, Elsevier Science Publishers B.V; 1984pp.1e111. [2] Wang XL, Suda S. Effects of Alesubstitution on hydriding reaction rates of LaNi5xAlx. J Alloys Compd 1993;191:5e7. [3] Chandra D, Reilly JJ, Chellappa R. Metal hydrides for vehicular applications: the state of the art (metal hydrides). JOM 2006;58:26e32. [4] Takeshita T, Malik SK, Wallace WE. Hydrogen absorption in RNi4Al (R ¼ rare earth) ternary compounds. J Solid State Chem 1978;23:271e4. [5] Sorgic B, Drasner A, Blazina Z. On the structural and hydrogen sorption properties of the GdNi5xAlx system. J Alloys Compd 1995;221(1e2):169e73. [6] Bobet J, Pechev S, Chevalier B, Darriet B. Structural and hydrogen sorption studies of NdNi5xAlx and GdNi5xAlx. J Alloys Compd 1998;267:136e41. [7] Inoue A, Zhang T, Kita K, Masumoto T. Mechanical strength, thermal stability and electrical resistivity of aluminiumerare earth metal binary amorphous alloys. Mater Trans JIM 1989;30(11):870e7. [8] He Y, Poon SJ, Shiflet G. Synthesis and properties of metallic glasses that contain aluminum. Science 1988;241:1640e2. [9] Zhong ZC, Jiang XY, Greer AL. Nanocrystallization in Alebased amorphous alloys. Philos Mag 1997;B76:505e10. [10] Gloriant T, Greer AL. Alebased nanocrystalline composites by rapid solidification of AleNieSm alloys. Nanostruct Mater 1998;10(3):389e96. [11] Inoue A. Progress Mater Sci 1998;43:365e520 [and references therein]. [12] Gloriant T. Microhardness and abrasive wear resistance of metallic glasses and nanostructured composite materials. J NoneCryst Solids 2003;316:96e103. [13] Gangopadhyay AK, Croat TK, Kelton KF. The effect of phase separation on subsequent crystallization in Al88Gd6La2Ni4. Acta Mater 2000;48:4035e43. [14] Abrosimova GE, Aronin AS, Zver’-kova II, Kir’-yanov YV. Phase transformations upon crystallization of amorphous AleNieRE alloys. Phys Metals Metallogr 2002;94(1):102e7. [15] Vasiliev AL, Aindow M, Blackburn MJ, Watson TJ. Phase stability and microstructure in devitrified Alerich AleYeNi alloys. Intermetallics 2004;12:349e 62. [16] Zhu A, Poon SJ, Shiflet GJ. On glass formability of AleGdeNi (Fe). Scr Mater 2004;50:1451e5. [17] Zhang Z, Xiong XZ, Zhou W, Lin X, Inoue A, Li JF. Glass forming ability and crystallization behaviour of AleNieRE metallic glasses. Intermetallics 2013;42:21e31. [18] Borzone G, Raggio R, Delsante S, Ferro R. Chemical and thermodynamic properties of several AleNieR systems. Intermetallics 2003;11:1217e22.
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[19] Palumbo M, Cacciamani G, Borzone G, Delsante S, Parodi N, Ferro R, et al. Thermodynamic analysis and assessment of the CeeNi system. Intermetallics 2004;12:1367e72. [20] Delsante S, Stifanese R, Borzone G. Thermodynamic stability of RNi2 Laves phases. J Chem Thermodyn 2013;65:73e7. [21] Raggio R, Borzone G, Ferro R. The Alerich region in the YeNieAl system: microstructures and phase equilibria. Intermetallics 2000;8:247e57. [22] Delsante S, Raggio R, Borzone G. Phase relations of the SmeNieAl ternary system at 500 C in the 40e100 at.% Al region. Intermetallics 2008;16:1250e7. [23] Otte HM, Montague WG, Welch DO. Xeray diffractometer determination of the thermal expansion coefficient of aluminium near room temperature. J Appl Phys 1963;34:3149e50. [24] Massalski TB, et al., editors. Binary alloys phase diagrams. 2nd ed., vols. 1e3. Materials Park, OH, USA: TMS; 1990. [25] Van Vucht JHN, Buschow KHJ. The structures of the rareeearth trialuminides. J LessCommon Met 1966;10:98e107. [26] Saltykov P, Cornish L, Cacciamani G. AleNi(Aluminium-Nickel), MSIT binary. Evaluation program. to be published. In: Effenberg G, editor. Msit Workplace. Stuttgart: MSI, Material Science International Services GmbH; 2003. Equi. Diagram, Review, 164. [27] Saccone A, Cardinale AM, Delfino S, Ferro R. GdeAl and DyeAl systems: phase equilibria in the 0 to 66.7 at.% Al composition range. Z Metallkd 2000;91:17e23. [28] Huang W, Chang YA. A thermodynamic analysis of the NieAl system. Intermetallics 1998;6:487e98. [29] Ansara I, Dupin N, Lukas H-L, Sundman B. Thermodynamic assessment of the AleNi system. J Alloys Compd 1997;247:20e30. [30] Du Y, Clavaguera N. Thermodynamic assessment of the AleNi system. J Alloys Compd 1996;237:20e32. [31] Schuster JC. In: Paper presented at COST535 meeting, Paris March 2004. [32] Gschneidner KA, Calderwood FW. The AleGd (AluminumeGadolinium) system. Bull Alloy Phase Diagrams 1988;9(6):680e3. [33] Buschow KHJ. Phase relations and intermetallic compounds in the systems neodymium-aluminium and gadolinium-aluminium. J Lessecommon Metals 1965;9(6):452e6. [34] K.A. Jr , Gschneidner FW, Calderwood. In: Jr. Gschneidner KA, Eyring L, editors. Handbook of the physics and chemistry of rare earths, vol. 8. Amsterdam: North-Holland Publishing; 1986. [35] Saccone A, Cacciamani G, Macciò D, Borzone G, Ferro R. Contribution to the study of the alloys and intermetallic compounds of aluminium with the rareearth metals. Intermetallics 1998;6:201e15. [36] Borzone G, Raggio R, Ferro R. Thermochemistry and reactivity of rare earth metals. Phys Chem Chem Phys 1999;1:1487e500. [37] Delsante S, Richter KW, .Ipser H, Borzone G. Synthesis and structural characterization of ternary compounds belonging to the series RE2þm Ni4þmAl15þ4m (RE ¼ rare earth metal). Z Anorg Allg Chem 2009;635:365e8. [38] Gladyshevskii RE, Cenzual K, Parthé E. The crystal structure of orthorhombic Gd3Ni5Al19, a new representative of the structure series R2þmT4þmAl15þ4m. J Solid State Chem 1992;100:9e15. [39] Gladyshevskii RE, Parthé E. Crystal structure of tetragadolinium hexanickel icosatresaluminium, Gd4Ni6Al23 with Y4Ni6Al23 type. Z Kristallogr 1992;198: 171e2. [40] Gladyshevskii RE, Cenzual K, Flack HD, Parthé E. Acta Cryst 1993;B49:468e74. [41] Rykhal’ RM, Zarechnyuk OS, Marych OM. The isothermal section at 800 C of the GdeNieAl ternary system in the range 0e33.3 at.% of gadolinium. Dopov Akad Nauk Ukr RSR, Seriya A: Fizikomatematichni Ta Tekhnichni Nauki 1978;40(9):853e5. [42] Pukas S, Matselko O, Gladyshevskii RR. New compounds RNiAl3 (R ¼ Gd, Tb, Dy). Chem Metals Alloys 2010;3:35e41. [43] Pukas S, Matselko O, Gladyshevskii R. New ternary aluminides R3Ni7Al14. In: Xi international conference on crystal chemistry of intermetallic compounds, Lviv, Ukraine May 30eJune 2 2010. p. 125. [44] Gao MC, Hackenberg RE, Shiftlet GJ. 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The GdeNieAl system: Phases formation and isothermal sections at 500 C and 800 C S. Delsante*, G. Borzone Dipartimento di Chimica e Chimica Industriale, Università di Genova, Consorzio INSTM e UdR di Genova, Via Dodecaneso 31, 16146 Genova, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 August 2013 Received in revised form 24 September 2013 Accepted 10 October 2013 Available online
Phase relations in the GdeNieAl ternary system have been established for two isothermal sections (500 and 800 C) based on X-ray powder diffraction analysis (XRD), Light Optical Microscopy (LOM), Scanning Electron Microscopy (SEM) coupled with Energy dispersive Microprobe Analysis (EPMA) on about 31 annealed alloys in the 50e100 at.% Al region. Seven intermetallic phases have been identified in samples annealed at 500 C and 800 C: Gd3Ni5Al19 (oS108eGd3Ni5Al19), Gd4Ni6Al23 (mS66eY4Ni6Al23), GdNiAl4 (oS24eYNiAl4), GdNiAl3 (oP20eYNiAl3), Gd3Ni7Al14 (hP72eGd3Ni7Al14), GdNiAl2 (oS16eCuMgAl2) and GdNi2Al3 (hP18eGdNi2Al3). One additional ternary phase GdNi3Al9 (hR78eErNi3Al9) has been found only in samples annealed at 800 C. The isothermal sections have been determined and then compared, the crystal structures of the ternary phases have been confirmed and lattice parameters calculated. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Rare earth intermetallics A. Intermetallics B. Phase diagrams B. Crystal chemistry of intermetallics F. Electron microscopy, scanning
1. Introduction The synthesis, structure and properties of ReNi and ReNieAl alloys are of growing interest for fundamental aspects as well as for potential applications in various technological fields, because of their typical properties. These alloys are generally recognised as members of the hydrogen absorption alloy family [1e3]: the hydrogen sorption properties of GdNi5xAlx (0 < x 3) and NdNi5xAlx have been reported in Refs. [4,5], besides in Ref. [6] results about structural and hydrogen sorption of the same systems are presented. Moreover, Al-based amorphous alloys based on Al-transition metal-rare earth systems, are promising candidates as structural materials for industrial applications because of their high specific strength combined with good bending ductility [7,8]. In addition, these alloys also exhibit greater hardness [9] and wear resistance [10]. The mechanical properties can be further improved by ensuring uniform distribution of fcc-Al nanoparticles in the amorphous matrix by appropriate heat treatment [11]; e.g. this situation leads to high tensile strength materials with low density and extremely high corrosion resistance [12]. An extensive study has been made on glassy alloys and crystallization of the AleNieR amorphous alloys by different researchers [13e17]. The study and definition of the properties of intermetallic phases (stoichiometric
compounds, solid solutions) provides an important contribution to a significant part of inorganic chemistry, as well as to the consolidation and development of relevant topics in material science and engineering (structural alloys, functional intermetallics, etc). Among the properties to be studied in order to obtain a full characterization of the phases, the constitutional ones should take first place. Knowledge of the existence of the phases, of their composition extension, microstructure and crystal structure is a prerequisite to a better understanding of the bondemechanism and to the analysis of its implements as a material, to be prepared and used for certain purposes and in given conditions. A systematic study on constitutional properties of ternary alloys formed by rare earth (R) with aluminium and nickel has been in progress for a few years in our laboratory. Both thermochemical approach, such the determination of DfH0 of selected binary and ternary phase [18e20] and systematic phase equilibria analysis have been performed [21,22]. The results obtained during the investigation of the Al-rich part of the isothermal sections at 500 C and 800 C for the GdeNieAl are presented and discussed. 2. Literature data 2.1. Binary boundary systems
* Corresponding author. Tel.: þ39 010 3536160/6153; fax: þ39 010 3625051. E-mail address: [email protected] (S. Delsante). 0966-9795/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.intermet.2013.10.009
A brief summary of the literature data focusing on phase equilibria of the relevant binary GdeNieAl subsystems is presented in
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the following. A list of the solid phases formed in the two binary systems is given in Table 1. 2.1.1. NieAl The phase diagram has been well established since several experimental studies, assessments and thermodynamic optimization were published. A re-calculation of the diagram is reported by Huang and Chang [28]: in this paper the studies carried out by Ansara et al. [29] and Du et al. [30] are summarized. Two more recent assessments have been presented by Saltykov et al. [26] and by Schuster [31]. Five intermediate phases are reported; among them, only the Alerichest NiAl3 is a “line compound”, while the others present a certain solubility range. The solid solubility of Ni in Al is very limited, while Ni dissolves Al in solid solution up to a maximum of 21.2 at.% at 1371 C, which is the Ni-rich eutectic temperature. 2.1.2. GdeAl The GdeAl phase diagram was assessed by Gschneidner and Calderwood [32] mainly on the basis of the work of Buschow [33]; more recently Saccone et al. [27] re-investigated the 0e67 at.% Al region. Five intermetallic line compounds are present; only the GdAl2 phase melts congruently at 1520 C and shows a wide region of primary crystallization and the Al-richest formed phase is the GdAl3.A catatectic reaction bGd 4 L þ aGd is reported at 2.5 at.% Al and 1200 C. It could be worthwhile to remind that, in correspondence to Gd, we have the change in the alloying behaviour from the light rare earths to those typical of the heavy ones [34]. Indeed, as a consequence of a gradual property changes such as for instance the progressive decrease in the number of compounds and of the stoichiometries of the phases richest in Al, it has been recognised the subdivision of the trivalent rare earth-aluminium alloys into two groups, one given by the light earths (from La to Sm) and the other by the heavy ones (from Gd to Lu) [35,36]. 2.2. GdeNieAl ternary system The ternary phases reported in literature belonging to the Gde NieAl system and the solid solutions based on the binary phases are listed in Table 2 together with the crystal structures, the lattice parameters and details about the annealing temperatures. The XRD results obtained in this work are summarized in Table 2. The isothermal section at 800 C for the GdeNieAl system has been published by Refs. [41]; the Alerichest phase was found to be the GdNi3Al16 for which the complete structure determination was not given. Subsequently, the GdNi3Al16 phase was not observed by Ref. [38] and was replaced by the Gd3Ni5Al19. Besides, two new phases, Gd4Ni6Al23 and GdNi3Al9 were found [39,40] in samples Table 1 Binary boundary NieAl and GdeAl systems: literature data on crystal structures and transformation temperature data. Phase and T range/ C
at.% Al Pearson symbolprototype
Al < 660.45 100 GdAl3 < 950 75 NiAl3 < 856 75 GdAl2 < 1520 66.66 Ni2Al3 < 1138 60
cF4eCu hP8eNi3Sn oP16eFe3C cF24eMgCu2 hP5eNi2Al3
GdAl < 1070 NiAl < 1651
oP16eAlEr cP2eCsCl
50 50
Lattice parameters ( A) a
b
Ref.
c
e e [23,24] e 4.592 [25] 7.367 4.811 [26] [27] e 4.891 36.8O40.5 at.% Ni [24,26] 5.893 11.59 5.695 [27] 2.8872 e e 42O69.2 at.% Ni [24] at 50 at.% Ni [26] 4.0433 6.32 6.613 7.9060 4.028
annealed at 500 and 800 C, respectively. Furthermore, the existence of the Gd2Ni3Al7 phase proposed by Ref. [41] was not confirmed. More recently, two new phases have been found and characterized: GdNiAl3 [42] and Gd3Ni7Al14 [43] in samples annealed at 600 C and 800 C, respectively. In light of these studies, it clearly appears that a revision of the isothermal section at 800 C is needed. The thermodynamics of the GdeNieAl system in the Al-rich corner were assessed using the calculation of phase diagrams (CALPHAD) approach by Gao et al. [44] who reported a calculation of a small portion of the isothermal section at 500 C. However, they did not take into account the new information about the ternary phases discovered and characterized after the work by Rykhal’ et al. [41]. 3. Experimental Small pieces of pure metals (Gd rods with 99.9% purity e JM Alfa AesarÒ, Ni rods with 99.99% purity Newmet Koch Company and Al 99.999% e WAV AG “Kryal”) were polished, weighed in the proper amount and then melted to obtain the alloys synthesis. Two different methods have been employed: induction melting (in alumina crucibles) and arc-melting. Each alloy was re-melted two or three times in order to achieve a complete homogeneity. Generally no weight loss was observed after melting. No significant differences have been observed in samples synthesized using the two techniques. Each sample was afterwards divided into two pieces in order to perform the annealing at 500 C and 800 C; few samples where also annealed at 600 and 700 C. Alumina containers sealed in quartz tubes in argon atmosphere were employed. The annealing time was different considering the composition of the samples and the temperature: at 500 C in the range 40 O 120 days, at 800 C 20 O 40 days, 20 and 10 days at 600 and 700 C, respectively. At the end of the heat treatment, the samples were quenched in icy water. Microstructures of the alloys were systematically investigated by light optical microscopy (LOM) on smooth samples prepared by grinding, lapping and polishing the resin-mounted samples with diamond abrasive spray down to 1 mm grain size. A thorough investigation of each sample by using a Scanning Electron Microscope (SEM, Zeiss e EVO 40) was performed by using a backscattered electron detector (BSE) in order to reveal the compositional contrast between the different phases. Quantitative EPMA data were collected at 20 kV on a Link system Ltd. instrument equipped with an Energy Dispersive Spectroscopy (EDS) detector. A counting time of 100 s and a ZAF correction program were adopted. Certified pure elements were used as reference standards, while cobalt was adopted for calibration purposes. The software package Inca Energy (Oxford Instruments, Analytical Ltd., Bucks, U.K.) was employed to process X-ray spectra. Generally for each element an error within 0.5 at.% was ascribed. Powder X-ray diffraction (XRD) was performed using a Philips X’Pert MPD machine (Philips, Almeno, The Netherlands) equipped with a copper target, excited to 40 kV and 30 mA, and a solid state detector. Values of the lattice parameters were processed and refined through a least-square interpolation, by using the NelsoneRiley function [45]. The present study was carried out on the basis of 31 samples having a mass of about 1.5 g and a suitable composition to determine the isothermal sections in the 50 O 100 at.% Al region. 4. Results and discussion The compositions of the GdeNieAl alloys prepared are reported in Table 3, ordered with decreasing at.% of Al, together with the phases observed for each sample after the annealing at 500 C and at 800 C. Considering the observed differences in the determined
S. Delsante, G. Borzone / Intermetallics 45 (2014) 71e79
73
Table 2 Gd-Ni-Al system: experimental and literature data on crystal structures and lattice parameters of the ternary phases in the 50e100 at. % Al region. N.
Phase
Stoichiometric composition (at.%) Gd
s1
s2
s3
s4
s5
s6
s7
Ni
Al
Pearson symbol-prototype and method
Lattice parameters [ A] and [ ]
Ref/ Annealing temperature
a ¼ 4.0858(1) b ¼ 15.9821(2) c ¼ 27.0713(4) a ¼ 4.0893(7) b ¼ 15.993(2) c ¼ 27.092(4) a ¼ 15.876 (8) b ¼ 4.0827 (3) c ¼ 18.31 (1) b ¼ 113.01(2) a ¼ 15.856 (4) b ¼ 4.078(1) c ¼ 18.286(6) b ¼ 113.01(2) a ¼ 7.3006(9) b ¼ 27.478(5) a ¼ 7.2869(8) b ¼ 27.435(3) a ¼ 4.087(5) b ¼ 15.30(3) c ¼ 6.696 (3) a ¼ 4.072(1) b ¼ 15.473(7) c ¼ 6.610(1) a ¼ 8.164(2) b ¼ 4.0680 (9) c ¼ 10.666 (3) a ¼ 8.158(2) b ¼ 4.060(1) c ¼ 10.665(4) a ¼ 17.980(2) c ¼ 4.0460(5) a ¼ 17.962(8) c ¼ 4.047(2) a ¼ 4.912(5) c ¼ 3.964(5) (x ¼ 0) a ¼ 5.028(5) c ¼ 4.074(5) (x ¼ 1.74) a ¼ 8.769(5) c ¼ 4.113(5) (x ¼ 2.01) a ¼ 9.085(5) c ¼ 4.039(5) (x ¼ 3.07) a ¼ 4.080(5) b ¼ 10.14(4) c ¼ 6.93 (3) a ¼ 7.8900 (x ¼ 0.45)
T.W and [37] (#7R800) 800 C
Gd3Ni5Al19
11.1
18.5
70.4
oS108eGd3Ni5Al19 Powder diffraction
Gd3Ni5Al19
11.1
18.5
70.4
oS108eGd3Ni5Al19 Single crystal
Gd4Ni6Al23
12.12
18.18
69.7
mS66Y4Ni6Al23 Powder diffraction
Gd4Ni6Al23
12.12
18.18
69.7
mS66Y4Ni6Al23 Single crystal
GdNi3Al9
7.7
23.1
69.2
GdNi3Al9
7.7
23.1
69.2
GdNiAl4
16.67
16.67
66.66
hR78eErNi3Al9 Single crystal hR78eErNi3Al9 Powder diffraction oS24YNiAl4 Powder diffraction
GdNiAl4
16.67
16.67
66.66
oS24YNiAl4 Powder diffraction
GdNiAl3
20.0
20.0
60.0
oP20eYNiAl3 Powder diffraction
GdNiAl3
20.0
20.0
60.0
oP20eYNiAl3 Powder diffraction
Gd3Ni7Al14
12.5
29.2
58.3
Gd3Ni7Al14
12.5
29.2
58.3
GdNi5xAlx
16.67
hP72eGd3Ni7Al14 Single crystal hP72e Gd3Ni7Al14 Powder diffraction hP6CaCu5 Powder diffraction
83.33(x/6)%
(x/6)%
hP18HoNi2.6Ga2.4 Powder diffraction
s8
GdNiAl2
25.0
GdAl2-xNix
33.33
25.0
(x/3)%
50.0
oS16CuMgAl2 Powder diffraction
66.67(x/3)%
cF24Cu2Mg Powder diffraction
[38] 800 C T.W and [37] (#8 R800) 800 C [39] 500 C
[40] 800 C T.W (# 6 R800) 800 C [41] 800 C T.W (#15 R500) 500 C and 800 C [42] 600 C T.W (# 24 R500) 500 C and 800 C [43] T not given T.W (#30 R800) 500 and 800 C [6] 800 C
[41] 800 C [41] 800 C
T.W ¼ This work.
phase equilibria for the 65e75 at.% Al region, additional annealing at 600 C and 700 C have been performed for samples #6, 7 and 9 (see Table 4, where the same information are inserted). As far as the composition of the samples #1e4 and #6e9 the annealing at 800 C should be considered partly performed in the liquidus state even if only the shape of the sample #1 and 2 was macroscopically changed. In particular, samples #1 and 2 can be considered quenched from the liquid state and the other samples partially quenched from the liquid state. Seven intermetallic phases have been identified using SEM/EDS and XRD analysis in samples annealed at 500 C: Gd3Ni5Al19 (oS108eGd3Ni5Al19), Gd4Ni6Al23 (mS66eY4Ni6Al23), GdNiAl4 (oS24eYNiAl4), GdNiAl3 (oP20eYNiAl3), Gd3Ni7Al14 (hP72e Gd3Ni7Al14), GdNiAl2 (oS16eCuMgAl2) and GdNi2Al3 (hP18e GdNi2Al3). The same phases have been observed in samples
annealed at 800 C; nevertheless, the GdNi3Al9 (hR78eErNi3Al9) has been found to be stable only at this temperature and seems to decompose at lower temperature. The investigation of samples # 6, 7 and 9 (see Table 4) annealed at 600 C and 700 C showed that this phase in no more stable also at these temperatures. The peculiar feature of the determined phase equilibria is the high complexity observed due to the presence of numerous ternary intermetallic phases having very close compositions. The intermetallic nature of the samples results also in a high brittleness proved by the holes, cracks and porosities found during the observation of the alloys micrographs. In Fig. 1 is displayed the isothermal section at 500 C for the 50e 100 at.% Al region; the established phase equilibria result in 16 tietriangles. Concerning the Al-rich corner, we did not find any appreciable solubility of Ni and Gd in pure aluminium in agreement
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S. Delsante, G. Borzone / Intermetallics 45 (2014) 71e79
Table 3 XRD and SEM/EDS experimental results of the GdeNieAl alloys annealed at 500 and 800 C and then water quenched. Sample#
1
Gd at.%
5.0
Ni at.%
2.0
Al at.%
93.0
2
1.5
7.5
91.0
3
9.5
3.5
87.0
4
3.5
12.0
84.5
5
12.0
4.0
84.0
6
5.0
20.0
75.0
7
8.0
20.0
72.0
8
13.0
15.0
72.0
9
11.0
18.0
71.0
10
18.0
13.0
69.0
11
17.0
14.0
69.0
12
10.0
22.0
68.0
13
1.0
31.0
68.0
14
2.5
30.0
67.5
15
20.0
13.0
67.0
16
14.0
20.0
66.0
17
11.0
23.0
66.0
18
27.0
8.0
65.0
19
7.0
28.0
65.0
20
19.0
17.0
64.0
21
16.0
20.0
64.0
22
24.0
14.0
62.0
23
11.5
26.5
62.0
24
20.0
20.0
60.0
Table 3 (continued ) Sample#
Gd at.%
Ni at.%
Al at.%
Phase observed (SEM/EDS and XRD) After annealing at 500 C
After annealing at 800 C
GdAl3 Gd4Ni6Al23 Al Gd3Ni5Al19 NiAl3 Al Gd4Ni6Al23 GdAl3 Al Al NiAl3 Gd3Ni5Al19 GdAl3 Gd4Ni6Al23 Al Gd3Ni5Al19 NiAl3 Al Gd3Ni5Al19 NiAl3 Al Gd4Ni6Al23 Al GdAl3 Al NiAl3 Gd4Ni6Al23 GdAl3 GdNiAl4 Gd4Ni6Al23 e
GdAl3 Gd4Ni6Al23 L Gd3Ni5Al19 NiAl3 L Gd4Ni6Al23 GdAl3 L L NiAl3 GdNi3Al9 e
Ni2Al3 NiAl3 Gd4Ni6Al23 Ni2Al3 NiAl3 Gd4Ni6Al23 Ni2Al3 NiAl3 Gd4Ni6Al23 GdAl2 GdNiAl4 GdAl3 GdNiAl4 Gd4Ni6Al23 Ni2Al3 GdNiAl4 Gd4Ni6Al23 Ni2Al3 GdAl2 (ss) GdNiAl4 GdNiAl3 Gd4Ni6Al23 Ni2Al3 GdNiAl4 GdAl2 (ss) GdNiAl4 GdNiAl3 GdNiAl4 GdNiAl3 Gd3Ni7Al14 GdAl2 (ss) GdNiAl3 GdNiAl4 GdNiAl4 Gd3Ni7Al14 NiAl
GdNi3Al9 NiAl3 L Gd3Ni5Al19 GdNi3Al9 L Gd4Ni6Al23 GdAl3 L Gd4Ni6Al23 Gd3Ni5Al19 L GdAl3 GdNiAl4 Gd4Ni6Al23 GdAl3 GdNiAl4 Gd4Ni6Al23 Gd4Ni6Al23 Ni2Al3 GdNiAl4 Ni2Al3 NiAl3 GdNi3Al9 GdNi3Al9 Ni2Al3 NiAl3 GdAl2 GdNiAl4 GdAl3 GdNiAl4 Gd4Ni6Al23 Ni2Al3 GdNiAl4 Gd4Ni6Al23 Ni2Al3 GdAl2 GdNiAl4 GdNiAl3 Ni2Al3 Gd4Ni6Al23 GdNiAl4 GdAl2 GdNiAl4 GdNiAl3 GdNiAl4 GdNiAl3 Gd3Ni7Al14 GdAl2 GdNiAl3 GdNiAl4 GdNiAl4 Gd3Ni7Al14 NiAl
25
5.0
36.0
59.0
26
22.5
18.5
59.0
27
10.0
31.0
59.0
28
4.5
37.5
58.0
29
15.0
28.0
57.0
30
8.0
36.0
56.0
31
21.0
27.0
52.0
Phase observed (SEM/EDS and XRD) After annealing at 500 C
After annealing at 800 C
GdNiAl3 GdAl2 (ss) Gd3Ni7Al14 NiAl GdNiAl4 GdAl2 (ss) GdNiAl3 GdNi2Al3 GdNiAl4 Gd3Ni7Al14 NiAl NiAl GdNiAl4 Gd3Ni7Al14 GdNiAl3 GdNi2Al3 NiAl GdNi2Al3 Gd3Ni7Al14 GdNi2Al3 GdAl2 (ss) GdNiAl2
GdNiAl3 GdAl2 Gd3Ni7Al14 NiAl GdNiAl4 GdAl2 GdNiAl3 GdNi2Al3 NiAl GdNiAl4 Gd3Ni7Al14 NiAl GdNiAl4 Gd3Ni7Al14 GdNiAl3 GdNi2Al3 NiAl Gd3Ni7Al14 GdNi2Al3 GdNi2Al3 GdNiAl2 GdAl2 (ss)
(ss) ¼ solid solution of Ni in GdAl2 phase.
with what was observed in the binary subsystems [24,27,33]. The isothermal section at 800 C, in the same composition range, is inserted in Fig. 2 and shows 18 tie-triangles. Note that at 800 C, in the Al-rich corner the liquid phase is present and a dotted liquidus line has been sketched considering the liquidus composition reported for the binary systems NieAl (w 9 at.% Ni) [26] and GdeAl (w7 at.% Gd) [33]. The resulting isothermal sections at 500 and 800 C appear to be quite different especially in the 65e75 at.% Al region. The SEM/EDS and XRD analysis on samples annealed at 600 and 700 C (#6, 7 and 9) give an indication that in this small portion of the phase diagram the phase equilibria should be the same established at 500 C. In the very Al-rich part of the ternary system, a eutectic microstructure has been observed involving the phases Al þ GdAl3 þ Gd4Ni6Al23, in the left side and Al þ NiAl3 þ Gd3Ni5Al19 in the right side, see Fig. 3 (sample #1 annealed at 500 C) and Fig. 4 (sample #2 annealed at 500 C) in which the eutectic microstructure is visible. The presence of two ternary eutectics was claimed by Refs. [44], but they did not consider the existence of the ternary phases Gd4Ni6Al23 and Gd3Ni5Al19 but only a ternary phase having a composition Gd2Ni3Al15 (which means 10 at.% Gd, 15 at.% Ni and 75 at.% Al); for this reason they reported two ternary eutectics formed by (Al þ GdAl3 þ Gd2Ni3Al15) and by (Al þ NiAl3 þ Gd2Ni3Al15). In the following, the established phase equilibria at 500 and 800 C are commented together with the involved binary and ternary phases. For convenience, the phase diagram has been divided in two sub-regions. The GdAl corner has not been investigated. 4.1. The 65e100 at.% Al region Three ternary phases and three binary phases have been observed in samples annealed at 500 C: Gd3Ni5Al19, Gd4Ni6Al23, GdNiAl4, GdAl3, NiAl3 and GdAl2. At 800 C, in addition to the already mentioned phases, also the GdNi3Al9 has been found. See Tables 1 and 2 to have the atomic compositions and the structural details.
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75
Table 4 XRD and SEM/EDS experimental results of the GdeNieAl alloys annealed at 600 and 700 C and then water quenched. Sample#
Gd at.%
Ni at.%
Al at.%
6
5.0
20.0
75.0
7
8.0
20.0
72.0
9
11.0
18.0
71.0
Phase observed (SEM/EDS and XRD) After annealing at 600 C
After annealing at 700 C
Gd3Ni5Al19 NiAl3 Al Gd3Ni5Al19 NiAl3 Al Gd3Ni5Al19 Gd4Ni6Al23 Al
Gd3Ni5Al19 NiAl3 L Gd3Ni5Al19 NiAl3 L Gd3Ni5Al19 Gd4Ni6Al23 L
4.1.1. Binary phases As for the NiAl3 and GdAl3 phases, on the basis of the results obtained by EPMA analysis, we can suppose that there is no appreciable solubility of Gd or Ni, respectively, at 500 C and at 800 C. The GdAl2 phase shows a wide field of primary crystallization in the binary system which extends into the ternary; a solubility range has been observed extending into the ternary field up to about 7 at.% Ni for samples annealed at 500 C. The cubic cell of the GdAl2 (cF24eMgCu2 type) phase becomes smaller when the Ni content increases. The values of the lattice parameter a of the cubic phase at different Ni content were determined in samples annealed at 500 C and are inserted in Table 5. Figs. 5 and 6 illustrate the micrographic appearance of samples # 15 and 22 annealed at 500 C: in both samples there is the GdAl2 bright phase as a primary crystals; sample #15 (Fig. 5) shows the GdAl3 (light grey) close to the GdAl2 and a matrix of GdNiAl4 (dark grey) whereas sample #22 (Fig. 6) has a matrix mainly formed by GdNiAl3 (light grey) and a few quantity of GdNiAl4 (dark grey). As for the same samples annealed at 800 C, we observed for the GdAl2 a smaller solubility of Ni which has a maximum value of about 4 at.% Ni (sample #31). See Figs. 1 and 2 to appreciate the resulting differences in the phase
Fig. 1. GdeNieAl system: isothermal section at 500 C in the 100< at.% Al <50 composition range. The existing ternary phases are reported: (s1) Gd3Ni5Al19; (s2) Gd4Ni6Al23; (s4) GdNiAl4; (s5) GdNiAl3; (s6) Gd3Ni7Al14; (s7) GdNi2Al3; (s8) GdNiAl2. The two-phase fields having a certain homogeneity range have been coloured in light-grey. The GdAl corner has not been investigated.
Fig. 2. GdeNieAl system: isothermal section at 800 C in the 100< at.% Al <50 composition range. The existing ternary phases are reported: (s1) Gd3Ni5Al19; (s2) Gd4Ni6Al23; (s3) GdNi3Al9; (s4) GdNiAl4; (s5) GdNiAl3; (s6) Gd3Ni7Al14; (s7) GdNi2Al3; (s8) GdNiAl2. A dotted liquidus line has been sketched (see text for details). The twophase fields having a certain homogeneity range have been coloured in light-grey. The GdAl corner has not been investigated.
equilibria involving the GdAl2 solid solution at 500 and 800 C. These outcomes are very different from those reported by Rykhal’ et al. [41] which reported at 800 C a solubility of Ni in GdAl2 up to around 15 at.%. 4.1.2. Ternary phases The Gd3Ni5Al19 phase was discovered using the single crystal analysis by Gladyshevskii [38] and then confirmed by powder diffraction by Ref. [37] to have an orthorhombic crystal structure (oS108eGd3Ni5Al19) in samples annealed at 800 C. The lattice parameters have been calculated in sample #7 annealed at 800 C. During this investigation, the phase was found to be stable also in samples annealed at 500, 600 and 700 C. At a very close composition, there is the Gd4Ni6Al23 phase (see Table 2) which has been observed is samples annealed at 500, 600,
Fig. 3. SEM image (BSE mode) of 5.0 at.% Gd, 2.0 at.% Ni, 93.0 at.% Al sample (#1 annealed at 500 , see Table 3). White phase ¼ GdAl3, s2 ¼ Gd4Ni6Al23, black phase ¼ Al and eutectic mixture Al þ GdAl3 þ Gd4Ni6Al23.
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S. Delsante, G. Borzone / Intermetallics 45 (2014) 71e79
Fig. 4. SEM image (BSE mode) of 1.5 at.% Gd, 7.5 at.% Ni, 91.0 at.% Al sample (#2 annealed at 500 , see Table 3). Dark-grey phase ¼ NiAl3, s1 ¼ Gd3Ni5Al19, black phase ¼ Al and the eutectic mixture Al þ NiAl3 þ Gd3Ni5Al19.
Fig. 6. SEM image (BSE mode) of 24.0 at.% Gd, 14.0 at.% Ni, 62.0 at.% Al sample (#22 annealed at 500 C, see Table 3). Bright phase ¼ GdAl2, s4 ¼ GdNiAl4, s5 ¼ GdNiAl3.
700 and 800 C. It has a monocline mS66eY4Ni6Al23 type structure [37,39]; the lattice parameters have been determined in the sample #8 annealed at 800 C. Both phases in our samples do not show any significant solubility by EDS analysis. As displayed in Figs. 1 and 2, a very narrow threeephase field involves these two ternary phases and the pure aluminium; in Fig. 7 it is possible to see the
micrographic appearance of sample # 9 annealed at 800 C: the dark grey phase is the Gd3Ni5Al19 and the light grey the Gd4Ni6Al23; the small quantity of Al is indicated by the arrow and is not easily recognizable from cracks and holes. Alloys representing threephase field involving Al, GdAl3 and Gd4Ni6Al23 have the typical microstructure displayed in Fig. 3; nevertheless, for composition not so close to the pure Al, the eutectic microstructure is less visible as can be seen in Fig. 8 (sample #5 annealed at 500 C): crystals of GdAl3 (bright phase) and Gd4Ni6Al23 (light grey phase) in a matrix of Al and eutectic mixture (Al þ GdAl3 þ Gd4Ni6Al23). The existence of the GdNi3Al9 phase was firstly reported for samples annealed at 800 C by Ref. [40]. Observing the SEM/EDS and XRD analysis of samples # 6, 7, 13 and 14 after the heat treatment at 500 and 800 C, it can be stated that this phase seems to be stable at 800 C but should undergo to a decomposition reaction below 800 C because is not present anymore at lower temperature. The analysis on alloys #6 and 7 after a further annealing at 600 and 700 C (see Table 4) proved that the GdNi3Al9 ternary phase is not stable also at these temperatures. Figs. 9 and 10 show the
Fig. 5. SEM image (BSE mode) of 20.0 at.% Gd, 13.0 at.% Ni, 67.0 at.% Al sample (#15 annealed at 500 , see Table 3). Bright phase ¼ GdAl2, Grey phase ¼ GdAl3, s4 ¼ GdNiAl4.
Fig. 7. SEM image (BSE mode) of 11.0 at.% Gd, 18.0 at.% Ni, 71.0 at.% Al sample (#9 annealed at 800 C, see Table 3). s1 ¼ Gd3Ni5Al19, s2 ¼ Gd4Ni6Al23, Al is indicated by the arrow. Crack and holes are also present.
Table 5 Lattice parameter a for the solid solution based on the binary cubic phase GdAl2 (cF24eMgCu2 type) at different Ni content for samples annealed at 500 C (see Table 3 and text comments). #
GdAl2xNix/x value
Lattice parameter a/ A
15 18 20 22 26 31
0 w0.03 w0.03 w0.03 w0.09 w0.21
7.900(1) 7.896(1) 7.891(1) 7.890(1) 7.881(1) 7.866(3)
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77
Fig. 8. SEM image (BSE mode) of 12.0 at.% Gd, 4.0 at.% Ni, 84.0 at.% Al sample (#5 annealed at 500 C, see Table 3). Bright phase ¼ GdAl3, s2 ¼ Gd4Ni6Al23, matrix of Al and eutectic mixture Al þ GdAl3 þ Gd4Ni6Al23.
Fig. 10. SEM image (BSE mode) of 2.5 at.% Gd, 30.0 at.% Ni, 67.5 at.% Al sample (#14 annealed at 800 C, see Table 3). Grey phase ¼ Ni2Al3, Dark-grey phase ¼ NiAl3, s3 ¼ GdNi3Al9.
micrographic appearance of alloys #14 annealed at 500 C and 800 C, respectively. In both samples the Ni2Al3 and NiAl3 binary phases are present while the bright phase is the ternary one which is Gd4Ni6Al23 at 500 C and GdNi3Al9 at 800 C. Comparing the XRD patterns of the sample #6 annealed at 700 and 800 C it is possible to observe that the reflections belonging to the GdNi3Al9, which are present in the pattern of the sample annealed at 800 C, disappear in the diffraction pattern of the sample annealed at 700 C. The comparison of the XRD patterns is inserted in Fig. 11; since they belong to three-phases samples, they appears quite complex and only a reduced range is displayed in order to underline the main peaks pertaining to the hexagonal GdNi3Al9 phase which are indicated with the asterisk. The GdNiAl4 phase, already reported by Rykhal’ [41], has been found at both annealing temperature having no significant solubility. The lattice parameters of this orthorhombic phase have been determined on sample #15 annealed at 500 C, see Table 2. It seems to have a primary formation, as can be seen in Fig. 12 which shows the micrograph of the alloy #11 annealed at 800 C: the light grey
GdNiAl4 ternary phase is surrounded by the Gd4Ni6Al23 which is mixed with the GdAl3 phase (bright one); wide cracks between the phases (in black) are also present.
Fig. 9. SEM image (BSE mode) of 2.5 at.% Gd, 30.0 at.% Ni, 67.5 at.% Al sample (#14 annealed at 500 C, see Table 3). Grey phase ¼ Ni2Al3, Dark-grey phase ¼ NiAl3, s2 ¼ Gd4Ni6Al23.
4.2. The 50e65 at.% Al region Four ternary phases and two binary phases have been observed in samples annealed both at 500 C and 800 C: GdNiAl3, GdNi2Al3, Gd3Ni7Al14, GdNiAl2, Ni2Al3 and NiAl. See Tables 1 and 2 for the atomic composition of the phases. 4.2.1. Binary phases The Ni2Al3 has been found to be the primary phase formed in samples # 13 and 14 surrounded by NiAl3 phase with a peritectic formation in agreement with the binary NieAl phase diagram [24,29]. See, as an example, Fig. 9. A dotted line has been traced due to a disagreement between the detected composition of the Ni2Al3 phase in samples #13 and #14. A wide field of primary crystallization was observed for the NiAl phase at both 500 C and 800 C. This behaviour is evident in the
Fig. 11. Comparison of the XRD patterns of the sample #6 annealed at 700 C (full line) and 800 C (dotted line). The main peaks pertaining to the hexagonal GdNi3Al9 phase (hR78eErNi3Al9) are indicated with the asterisk.
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S. Delsante, G. Borzone / Intermetallics 45 (2014) 71e79
parameters are in agreement with the literature data [46]. A negligible solubility of Gd in both Ni2Al3 and NiAl phases was detected.
Fig. 12. SEM image (BSE mode) of 17.0 at.% Gd, 14.0 at.% Ni, 69.0 at.% Al sample (#11 annealed at 800 C, see Table 3). s4 ¼ GdNiAl4, bright phase ¼ GdAl3, s2 ¼ Gd4Ni6Al23. Wide cracks are the black part of the alloy.
two-phase samples #25 and 28 annealed at 500 and 800 C; the samples show as primary phase the NiAl and a matrix of GdNiAl4 ternary phase (see Fig. 13, which refers to sample # 25 annealed at 500 C). The detected composition was w55 at.% Al (sample #28) and w56 at.% Al (sample #25); as a consequence of these results, the twoephase field NiAleGdNiAl4 has been traced, see Figs. 1 and 2. The calculated lattice parameter a for the cubic NiAl phase having the cP2eCsCl structure in the already mentioned samples annealed at 800 C was: - Sample #28 (w55 at.% Al): a ¼ 2.8613(8) A - Sample # 25 (w56 at.% Al): a ¼ 2.8538(8) A The decrease of the lattice parameter a with the increase of the Al content in the NiAl phase is due to the lattice vacancies produced in the Ni sites whereas the Al sites remain fully occupied by Al as extensively discussed in Ref. [46]. The obtained values of the lattice
Fig. 13. SEM image (BSE mode) of 5.0 at.% Gd, 36.0 at.% Ni, 59 at.% Al sample (#25 annealed at 500 C, see Table 3). Dark-grey phase ¼ NiAl, s4 ¼ GdNiAl4. Black holes are also present due to the brittleness of the sample.
4.2.2. Ternary phases The GdNiAl3 phase was reported among the ReNieAl systems for R ¼ Y, Sm, Gd, Tb and Dy [22,42,47]. The crystal structure has been confirmed to be oP20eYNiAl3 type. A slight homogeneity range, (comparable with microprobe error) might be attributed to this phase but is not considered in the drawn sections. The cell parameters have been calculated for sample #24 annealed at 500 C which was nearly oneephase sample. A study concerning the existence and the structural investigation of the Gd3Ni7Al14 ternary phase was presented during a conference [43]; it was found to have a hexagonal structure hP72e Gd3Ni7Al14. The presence of this phase at 500 and 800 C was confirmed; the lattice parameters have been calculated on sample #30 annealed at 800 C and inserted in Table 2. The threeephase field in which the Gd3Ni7Al14 is in equilibrium with GdNiAl4 and NiAl phases is represented in Fig. 14 (alloy #27 annealed at 500 C): the black phase is the NiAl (solid solution with w55 at.% Al) and the matrix is formed by Gd3Ni7Al14 (dark grey) and GdNiAl4 (light grey). The GdNi2Al3 (which was also reported in literature as GdNi5xAlx) has been observed by SEM/EDS analysis having a certain homogeneity range (a maximum content of Al w53 at.% at 500 C and w52 at.% at 800 C, along the isopleth at 16.7 at.% Gd). Different authors reported for RNi5xAlx (R ¼ Nd, Gd, Tb, Dy, Ho, Er and Y [5e6,48e50]) an hP6CaCu5 structure type for samples having composition with x < 2 and an hP18HoNi2.6Ga2.4 for samples having composition with x 2. The unit cell parameters of CaCu5p and ffiffiffi HoNi2.6Ga2.4 type structures are related by aHoNi2:6 Ga2:4 ¼ aCaCu5 3 and cHoNi2:6 Ga2:4 ¼ cCaCu5 and the HoNi2.6Ga2.4 type was considered as a P6/mmm hexagonal superstructure of the RNi5xAlx phase, which is located on the same R ¼ 16.67 at.% isopleth line, see Table 2. The XRD analysis performed on sample #31 annealed at 500 C shows that in our sample the diffraction pattern of the GdNi5xAlx phase (x ¼ 3.06 corresponding to an atomic composition w Gd16.7Ni32.3Al51) presents some additional intense reflections which could be indexed considering a larger unit cell having the hexagonal HoNi2.6Ga2.4 type structure [9].
Fig. 14. SEM image (BSE mode) of 10.0 at.% Gd, 31.0 at.% Ni, 59.0 at.% Al sample (#27 annealed at 500 C, see Table 3). Black phase ¼ NiAl, s4 ¼ GdNiAl4, s6 ¼ Gd3Ni7Al14.
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The existence of RNiAl2 was previously reported for all the rare earths with the exception of Sc and Eu [51] and also reported by Ref. [41]. The formation of the GdNiAl2 phase has been confirmed in this work. 5. Conclusions The experimental determination of the isothermal sections of the GdeNieAl system at 500 C and 800 C confirmed the existence of eight ternary phases in the 50e100 at.% Al region: Gd3Ni5Al19 (oS108eGd3Ni5Al19), Gd4Ni6Al23 (mS66eY4Ni6Al23), GdNi3Al9 (hR78eErNi3Al9), GdNiAl4 (oS24eYNiAl4), GdNiAl3 (oP20eYNiAl3), Gd3Ni7Al14 (hP72eGd3Ni7Al14), GdNiAl2 (oS16eCuMgAl2) and GdNi2Al3 (hP18eGdNi2Al3). Experimental evidence proves that the GdNi3Al9 phase is not stable below 800 C. Only the GdAl2 binary phase seems to dissolve a significant amount of Ni: up to 7 at.% at 500 C and up to 4 at.% at 800 C even GdAl3, NiAl3, Ni2Al3 and NiAl do not dissolve the third element. The resulting isothermal sections appears to be quite different at the two investigated temperatures, however both of them show a high number of threeephase fields. Considering the high number of new ternary phases discovered after the publication by Rykhal’ et al. [41] and that we confirmed their existence during our investigation, it is selfeevident that the isothermal section at 800 C determined during this work is completely different from the one reported by these authors. The same consideration can be done regarding the partial isothermal section at 500 C calculated by Ref. [44] which has been constructed without considering the existence of the ternary phases reported by Refs. [38e40]. References [1] Buschow KHJ. In: Gschneidner Jr. KA, Eyring L, editors. Handbook on the physics and chemistry of rare earths, vol. 6. The Netherlands: North-Holland, Elsevier Science Publishers B.V; 1984pp.1e111. [2] Wang XL, Suda S. Effects of Alesubstitution on hydriding reaction rates of LaNi5xAlx. J Alloys Compd 1993;191:5e7. [3] Chandra D, Reilly JJ, Chellappa R. Metal hydrides for vehicular applications: the state of the art (metal hydrides). JOM 2006;58:26e32. [4] Takeshita T, Malik SK, Wallace WE. Hydrogen absorption in RNi4Al (R ¼ rare earth) ternary compounds. J Solid State Chem 1978;23:271e4. [5] Sorgic B, Drasner A, Blazina Z. On the structural and hydrogen sorption properties of the GdNi5xAlx system. J Alloys Compd 1995;221(1e2):169e73. [6] Bobet J, Pechev S, Chevalier B, Darriet B. Structural and hydrogen sorption studies of NdNi5xAlx and GdNi5xAlx. J Alloys Compd 1998;267:136e41. [7] Inoue A, Zhang T, Kita K, Masumoto T. Mechanical strength, thermal stability and electrical resistivity of aluminiumerare earth metal binary amorphous alloys. Mater Trans JIM 1989;30(11):870e7. [8] He Y, Poon SJ, Shiflet G. Synthesis and properties of metallic glasses that contain aluminum. Science 1988;241:1640e2. [9] Zhong ZC, Jiang XY, Greer AL. Nanocrystallization in Alebased amorphous alloys. Philos Mag 1997;B76:505e10. [10] Gloriant T, Greer AL. Alebased nanocrystalline composites by rapid solidification of AleNieSm alloys. Nanostruct Mater 1998;10(3):389e96. [11] Inoue A. Progress Mater Sci 1998;43:365e520 [and references therein]. [12] Gloriant T. Microhardness and abrasive wear resistance of metallic glasses and nanostructured composite materials. J NoneCryst Solids 2003;316:96e103. [13] Gangopadhyay AK, Croat TK, Kelton KF. The effect of phase separation on subsequent crystallization in Al88Gd6La2Ni4. Acta Mater 2000;48:4035e43. [14] Abrosimova GE, Aronin AS, Zver’-kova II, Kir’-yanov YV. Phase transformations upon crystallization of amorphous AleNieRE alloys. Phys Metals Metallogr 2002;94(1):102e7. [15] Vasiliev AL, Aindow M, Blackburn MJ, Watson TJ. Phase stability and microstructure in devitrified Alerich AleYeNi alloys. Intermetallics 2004;12:349e 62. [16] Zhu A, Poon SJ, Shiflet GJ. On glass formability of AleGdeNi (Fe). Scr Mater 2004;50:1451e5. [17] Zhang Z, Xiong XZ, Zhou W, Lin X, Inoue A, Li JF. Glass forming ability and crystallization behaviour of AleNieRE metallic glasses. Intermetallics 2013;42:21e31. [18] Borzone G, Raggio R, Delsante S, Ferro R. Chemical and thermodynamic properties of several AleNieR systems. Intermetallics 2003;11:1217e22.
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