Powder Metallurgy and Metal Ceramics, Vol. 50, Nos. 1-2, May, 2011 (Russian Original Vol. 50, Nos. 1-2, Jan.-Feb., 2011)
PHYSICOCHEMICAL MATERIALS RESEARCH THE Al–Cr–Fe PHASE DIAGRAM. I. PHASE EQUILIBRIA AT SUBSOLIDUS TEMPERATURES OVER COMPOSITION RANGE 58–100 AT.% Al V. G. Khoruzha,1 K. E. Kornienko,1,3 D. V. Pavlyuchkov,1 B. Grushko,2 and T. Ya. Velikanova1 UDC 669.715′26′11 Based on transmission and scanning electron microscopy, x-ray diffraction, electron microprobe and differential thermal analyses, the solidus surface of the ternary Al–Cr–Fe system is constructed for the first time on the concentration triangle over composition range 58–100 at.% Al. Four ternary compounds, D3, O1, H, and ε, with decagonal, orthorhombic base-centered, hexagonal, and orthorhombic primitive lattices participate in phase equilibria on the solidus surface. Solid solutions based on aluminum and binary compounds as well as ternary phases form 12 single-phase surfaces, 25 ruled surfaces of two-phase equilibria bounding two-phase regions, and 14 three-phase isothermal planes corresponding to invariant four-phase equilibria on the solidus surface. Keywords: solidus surface, compound, ruled surface, isothermal plane.
INTRODUCTION Alloys of aluminum with high-valence d-metals form quasicrystalline phases or their approximants that exhibit high corrosion resistance, low friction coefficient, and high resistivity. Quasicrystals with these properties are used as dispersoids in commercial alloys, solar energy absorbers, hydrogen storages, thermoelectric elements, etc. Quasicrystals are metastable in binary systems and stable in ternary systems, in which they commonly represent high-temperature phases that often exist over a limited temperature range. Most of these phases form by peritectic reactions, so they are difficult to obtain. A careful analysis of alloys annealed at high temperatures and processes that occur during their crystallization depending on conditions (first of all, alloy cooling rate) may promote the identification of new quasicrystalline phases even when they exist over narrow temperature and concentration ranges. Hence, the melting/crystallization regions of the phase diagrams where quasicrystals or their approximants are expected to form (or are known to exist) need to be examined. Such examinations primarily focus on the liquidus surfaces and processes that occur during crystallization of alloys according to the reaction scheme. In this paper we have plotted the solidus surface of the Al–Cr–Fe system for composition range 58– 100 at.% Al onto the concentration triangle.
1Frantsevich
Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, Kiev, Ukraine. 2Institute of Microstructural Research, Research Center Jülich, Germany. 3To
whom correspondence should be addressed; e-mail:
[email protected];
[email protected].
Translated from Poroshkovaya Metallurgiya, Vol. 50, No. 1–2 (477), pp. 106–123, 2011. Original article submitted June 19, 2009. 1068-1302/11/0102-0083 ©2011 Springer Science+Business Media, Inc.
83
The literature shows that a ternary phase diagram of this system has not been constructed to date. However, three stable ternary phases were known to exist in the system: quasicrystalline phase, D3, and periodic phases, O1 and H [1–3]. According to [1, 2], the decagonal phase, D3, forms over a narrow concentration range close to Al72Cr16Fe12 (at.%) and has a melting point of 1090°C. Based on electron diffraction, its parameter along the periodic axis is about 1.2 nm. In addition, it is established in [2] that samples of Al76Cr19Fe5, Al76Cr16Fe8, and TABLE 1. Crystallographic Characteristics of Intermediate Phases in the Al–Cr–Fe System at 58–100 at.% Al Phase: symbol/formula
Structural type
Space group
Lattice parameters, nm
Comments
Ref.
θ, Al13Cr2
Al45V7
C2/m
−
[7]
η, Al11Cr2
Al11Cr2
μ, Al4Cr
μ-Al4Mn
P63/mmc
γ1, Al1–xCrx
Zn8Cu5
I 4 3m
a = 2.0595 b = 0.7574 c = 1.0949 β = 107.34° a = 1.77348(10) b = 3.04555(17) c = 1.77344(10) β = 91.0520(12)° a = 2.01 c = 2.48 a = 0.9104(1)–0.9047(1)
γ1, Al1–xFex
Zn8Cu5
I 4 3m
γ2, Al1–xCrx
Al8Cr5
R 3m
Al13Fe4
Al13Fe4
C2/m
Al5Fe2
Al5Fe2
Cmcm
Al2Fe
Al2Fe
P1
AlFe D3, ~Al72Cr16Fe12 O1, Al75–76Cr16–9Fe5–9
CsCl
Pm 3 m Decagonal quasicrystal
H, ~Al82.5Cr11.5Fe6 ε, ~Al77.5Cr20Fe2.5
84
C2/c
a = 1.27696(11) c = 0.79580(5) a = 1.5492(2) b = 0.8078(2) c = 1.2471(1) β = 107.69(1)° a = 0.76559(8) b = 0.64154(6) c = 0.42184(4) a = 0.4878 b = 0.6461 c = 0.8800 α = 91.75° β = 73.27° γ = 96.89° a = 0.2909–0.28953 n ≈ 1.2 nm along the periodic axis
Orthorhombic a ≈ 3.27 base-centered b ≈ 1.24 c ≈ 2.34 Hexagonal a = 1.74 c = 4.14 Orthorhombic a ≈ 3.46 primitive b ≈ 2.00 c ≈ 1.24
Single crystal, Al83.8Cr16.2 phase [10]
[8]
Single crystal [11]
[9]
[10] Alloys quenched from melt, 0.3 ≤ ≤ x ≤ 0.42, x-ray powder diffraction Study in situ with high-temperature [11, 12] neutron diffraction [13]; 0.35 ≤ x ≤ ≤ 0.42 [14] x = 0.36, annealing at 800°C, x-ray [13] powder diffraction; 0.3 ≤ x ≤ 0.42 76 at.% Al, single crystal [14]
71.5 at.% Al, single crystal
[15]
66.9 at.% Al, single crystal
[16]
50.0–63.8 at.% Fe SEM, EMPA, TEM
[17] [1, 2]
SEM, EMPA, TEM
[2]
SEM, EMPA, TEM
[3]
SEM, EMPA, TEM
This paper
o
T, C o
T, C
L
1250
L +
1231 1146 1155 1169
L+ γ 1 +γ1 + 1095 1000
L
1320 1250 γ1
γ1 + γ2
1149
1060 1040
1000
γ2
750
(μ)
~865 795 (η)
750 660.452
655
658
660.452
(θ)
500
60
70
80 Al, at.%
a
90
Al
500
60
70
80 Al, at.%
90
Al
b
Fig. 1. Phase diagrams of the Al–Fe (a) and Al–Cr (b) boundary binary systems over the range between 58 and 100 at.% Al Al75Cr16Fe9 alloys annealed at 1000°C contain one phase. The phase denoted by O1 has a complex base-centered orthorhombic structure with parameters a ≈ 3.27 nm, b ≈ 1.24 nm, and c ≈ 2.34 nm. An orthorhombic compound with the similar structure was previously found in [4] in as-cast Al82.5Cr11.5Fe6 alloy, which contained other phases as well. The constitution of Al–Cr–Fe alloys containing 80–90 at.% Al and 5–15 at.% Cr annealed at 800°C was examined in [3]. It is shown that there is a ternary phase, approximately Al82.5Cr11.5Fe6, denoted by H, which has a hexagonal structure with parameters a = 1.74 nm and c = 4.14 nm and, as assumed in the paper, melts incongruently at 953°C. A ternary phase close to the above composition (approximately Al81Cr11Fe8), denoted by ν, with the similar structure was revealed previously in [5] in as-cast Al12CrFe2 alloy. Its space group is reported as P63/m and lattice parameters as a = 4.000 nm and c = 1.240 nm. The data on phase equilibria in the Al–Cr–Fe ternary system at 50–100 at.% Al are limited to the partial isothermal section at 1000°C [6], no ternary compounds being revealed. The literature data on the ternary intermediate phases are summarized in Table 1. The Cr–Fe and Al–Fe binary systems have been studied in detail. Figure 1a shows our version of the Al–Fe phase diagram over the range of aluminum-rich compositions. It is based on the paper [12]. The temperatures of invariant processes L + ⇔ γ1, L ⇔ γ1 + , L + ⇔ , γ1 ⇔ + , and γ1 + ⇔ are based on the data from [18] that were specified later. Figure 1b shows our version of the Al–Cr phase diagram over the range between 58 and 100 at.% Al based on the data from [13, 19, 20]. Table 1 summarizes data on the crystal structure of the intermediate phases. There are discrepancies between different data on temperature ranges for a number of phases. For example, the paper [21] does not confirm the existence of the Al11Cr2 phase, which was reported in [22]. The evaluation [23] gives preference to temperatures at which intermetallic compounds form, according to [22]. Based on [24], it is concluded that Al11Cr2 exists over a narrow temperature range (from 940 to about 785°C). According to [25], Al4Cr has three structural modifications: stable μ-Al4Cr (reported previously in [9]) with a hexagonal structure of μ-Al4Mn type and two metastable modifications, ε-Al4Cr and ε′-Al4Cr, with orthorhombic base-centered and orthorhombic
85
primitive structures, respectively. The more recent papers [13, 19, 20] additionally examined the Al–Cr system with alloys produced by arc melting or levitation induction melting (for most binary alloys) from the starting materials with the same purity as we used in our experiment to make ternary alloys. It is ascertained that Al13Cr2, Al11Cr2, and Al4Cr exist in the system. Their incongruent formation temperatures are 795, about 865, and 1040°C; temperature of eutectic reaction L ⇔ + is 658°C. Solid solutions are accepted to exist over the range between 25 and 42 at.% Cr. They are denoted by γ and represent two modifications: high-temperature cubic γ1 and low-temperature γ2 with a rhombohedral distorted structure of γbrass type. Reactions L + ⇔ γ1 and L + γ1 ⇔ γ2 proceed at 1320 and 1060°C. This paper examines the structure of the alloys at solidus temperatures and considers the solidus surface constructed in the ternary system over the aluminum-rich range based on experimental data and phase diagrams of the Al–Fe and Al–Cr bounding binary systems (Fig. 1).
EXPERIMENTAL PROCEDURE To examine the Al–Cr–Fe ternary system and a number of reference points in the Al–Cr bounding binary system, alloys with 48 compositions were arc melted in titanium-gettered argon from the following materials: A-995 aluminum (99.995 wt.%), refined chromium metal powder (99.93 wt.%), and high-purity powder-like carbonyl iron (99.82 wt.%), which was preliminary compacted and melted. Levitation melting was used to make alloys with 88 compositions from compact metals of high purity (99.999 wt.% Al, 99.99 wt.% Fe, 99.99 wt.% Cr). Although preventive measures were taken, there was a risk that the ingots would be inhomogeneous as the components had largely different specific weights. Hence, to check the composition of the samples, a metallographic section was examined with scanning electron microscopy (SEM). To homogenize the arc-melted alloys, we subjected them to stage annealing from 600°C to near-solidus temperatures of the alloys as shown in Table 2. The levitation-melted alloys were annealed in as-cast state at near-solidus temperatures determined with differential thermal analysis (DTA). The chemical composition of all analyzed alloys is shown with circles in Fig. 2; the results for selected compositions are shown in Table 2. The as-cast and annealed alloys were examined with DTA, x-ray diffraction (XRD), microstructural analysis (MSA), SEM, and electron microprobe analysis (EMPA). The DTA was carried out in inert gas (argon or Al
10 79 ~8 65
at.%
20
H
η O1
980
1075
9 114
1026 1085
80
998
ε
104
0
μ
1085 D3
1105
1125
, 1169 70
5 115
1045
γ2
at.%
985
1035 γ2 (Al−Cr), 30 1060
40 γ1 (Al−Cr), 1320
90
θ 660
Al,
Cr,
657
655
5
750
65
8
-1 -2
γ1
60 γ (Al−Fe), 1231 1
Fig. 2. Projection of the Al–Cr–Fe solidus surface at 58–100 at.% Al: 1) composition of analyzed alloys, 2) composition of binary phases
86
87
γ2 + γ2 +
600 (10), 720 (10), 920 (10), 1000 (185) 600 (10), 720 (10), 920 (10), 1000 (185) 600 (10), 720 (10), 920 (10), 1000 (185)
27.0
28.0
11.0
7.0
11.0
17.5
30.0
11.0
21.5
15.0
18.3
18.0
62.0
65.0
65.0
65.0
65.0
68.5
68.5
70.0
70.0
70.5
10.5
11.7
15.0
10.0
20.5
5.0
17.5
γ2 +
γ1 + γ2*
γ1 + γ2*
1065 (66)
1065 (66)
γ2 + D3
γ2 + D 3
γ2 + D3 600 (10), 720 (10), 920 (10), 1000 (185) 600 (10), 720 (10), γ2 + D3+ 920 (10), 1000 (185)
600 (10), 720 (10), 920 (10), 1000 (185)
600 (10), 720 (10), 920 (10), 1000 (185)
γ1 + γ2* +
600 (10), 720 (10), 920 (10), 1000 (185)
29.0
9.9
61.1
24.0
γ1 + γ2*
1100 (29)
γ1 + γ2*
5.0
35.0
60.0
γ1 + γ2*
600 (10), 720 (10), 920 (10), 1000 (185) 600 (10), 720 (10), 920 (10), 1000 (185)
20.0
20.0
60.0
γ1 + γ2*
1065 (72)
36.0
4.0
60.0
γ1 + γ2*
600 (10), 720 (10), 920 (10), 1000 (185)
32.0
10.0
58.0
Fe
Cr
Al
Phase composition
Thermal treatment, °С (h)
Alloy composition, at.%
γ2 D3 γ2 D3 γ2 D3 γ2 D3
γ2 γ2 γ1 γ2* γ2
γ1 γ2* γ1 γ2* γ1 γ2* γ1 γ2 * γ1 γ2* γ1 γ2* γ1 γ 2*
Phase
60.90 − 70.00
66.20 70.40 66.70 71.80
67.30 72.00 66.70 71.80 74.80
64.30 73.80
64.10 72.60 66.40 −
22.90 17.60 23.60 16.40
24.00 17.20 23.90 16.50 5.90
20.40 4.70
20.60 4.10 28.60 −
17.20 3.80
12.10 − 3.00
60.90 − − −
63.10 71.30
29.20 − − −
9.90 − − −
60.75 −
8.70 10.80 9.40 11.60 19.30 10.90 12.00 9.70 11.80
15.30 21.50
15.30 23.30 5.00 −
19.70 24.90
27.00 − 27.00
19.13 − 4.74 −
33.57
− −
Fe
19.65 − 34.51 −
61.22 −
4.06
− −
− − 62.37
Cr
Al
Phase composition, at.%
1.276 − 1.542 − − − −
1.272 −
1.277 0.768 0.910 1.271 1.264 0.770
1.264 0.768
0.902 1.254 0.905 1.261 0.767
0.899 1.252 − − 0.905 1.258 0.908 1.266 − −
a, nm
− − − − 0.811 − − − −
0.642 − 0.643 − − − 0.645
0.643 −
− − − − − − − − − − − − − −
b, nm
0.796 − 1.237 − − − −
0.793 −
0.802 0.420 − 0.794 0.792 0.419
0.422
0.792
0.785 0.421
0.784 −
0.793 − − −
− −
− 0.781 − − − 0.788 −
− − − − 10710 − − − −
− − − −
− −
− −
− − − − − − − − − − − − −
β, o c, nm
Lattice parameters
TABLE 2. Phase Composition According to SEM–EMPA and XRD and Lattice Parameters of the Phases in Annealed Alloys in the Al–Cr–Fe System at 58–100 at.% Al
88 1065 (24)
9.5
16.6
9.7
14.0
5.0
24.0
15.1
19.5
12.1
18.8
14.0
22.0
2.8
10.0
3.0
12.5
14.5
19.3 20.6
14.6
19.9 20.0 16.9
71.0
71.3
71.5
72.0
73.0
73.2
74.9
75.0
75.0
75.0
76.5 77.5
77.8
77.9 77.9 78.2
2.2 2.1 4.9
1032 (111) 1032 (19) 600 (106), 700 (58)
965 (137)
7.6
4.2 1.9
600 (30), 720 (30), 900 (2900) 1000 (111) 1000 (73)
600 (30), 720 (30), 900 (2900) 600 (30), 720 (30), 900 (2900)
1065 (24)
11.5
12.5
22.0
1065 (26)
24.0
5.0
71.0
D3 + γ2 + O1
ε ε η + O1
H + O1
O1 ε + γ2 + μ
O1 +
O1 +
O1 +
O1 O1 ε γ2 μ H O1 ε ε η O1
O1
γ2 O1 D3 γ2 D3 D3 γ2 O1 D3 O1 γ2 O1
γ2 + O1 + D3 γ2 + D3+
Phase
Phase composition
600 (10), 720 (10), D3 + 920 (10), 1000 (185) 600 (10), 720 (10), O1 + γ2 920 (10), 1000 (185) + 1065 (191)
600 (10), 720 (10), 920 (10), 1000 (185) 1032 (95)
Fe
Cr
Thermal treatment, °С (h)
Al
Alloy composition, at.%
77.00 77.40 76.60 77.60 71.10 78.00 79.60 78.00 77.80 78.00 78.70 77.20
76.80 77.90
65.50 69.80 74.00 71.50 67.00 74.50 69.80 74.00 75.00 69.90 75.10 72.60 74.50 76.10 77.40
67.90 72.90 −
72.30
Al
15.50 5.30 19.30 20.60 27.60 20.30 13.60 14.70 20.20 20.00 17.40 17.90
15.70 5.10
22.00 16.50 6.00 18.00 25.00 16.00 16.50 6.00 19.80 27.30 2.30 3.00 14.70 5.70 2.60
24.60 17.10 −
4.40
Cr
− 1.248 − − − − − − − − − −
− 0.807 − − − − − − − − − − − 1.555 − − − − − − − − − −
7.50 17.00 7.50 17.30 4,10 1.80 1.30 1.70 6.80 7.30 2.00 2.00 3.90 4.90
− 1.250
− 0.811 − 1.554
12.50 13.70 20.00 10.50 8.00 9.50 13.70 20.00 5.20 2.80 22.60 24.40 10.80 18.20 20.00
− 0.423
− − − − − − − − − −
108.03
−
107.51
−
107.66
− − − − − − − − − − 107.79 − − − − − 107.56
β, o c, nm
− − − − − − − − − − 1.247 − 0.795 − − − 1.250 1.242
0.644
b, nm
− − − − − − − − − − 0.808 − − − − − 0.807 0.809
0.770
a, nm
Lattice parameters
− − − − − − − − − − 1.550 − 1.280 − − − 1.551 1.554 7.50 10.00 −
23.30
Fe
Phase composition, at.%
TABLE 2. Continued
89
H H
η H +
910 (83) 800 (91)
H + η+H
600 (106) 600 (106), 700 (58) 865 (64), 910 (82,5) 600 (106), 700 (58) 865 (94) 800 (122) 600 (106), 700 (58) 860 (67) 600 (106)
6.1
3.8
9.2
15.0
5.0
3.0 9.5
3.9
7.3
2.0
1.0
8.0
5.0
14.6
16.9
11.2
5.0
15.0
16.1 9.5
15.0
10.9
16.0
16.7
8.0
11.0
11.0
5.0
79.3
79.3
79.6
80.0
80.0
80.9 81.0
81.1
81.8
82.0
82.3
84.0
84.0
87.0
90.0
86.40 82.50 − 85.70 − 81.70 77.40 98.10
θ H θ H
θ + H +
θ +
600 (106)
600 (106) 600 (106)
2.0
5.0
* The phase formed in solid state during cooling of the alloy after annealing.
H++
80.60 75.20 99.40
η H
η
η
H +
η
η+H
H +
η+μ
H++
980 (87)
1000 (73)
79.50 − 81.20 82.00 77.00 81.20 81.70 81.90 78.70 81.70 −
80.40 77.20 82.70 77.50
Η η η
Η+η
2.0
η + O1
19.1
965 (82)
78.9
4.9
16.8
η H η H η H H η
− − − − 79.00 79.20 79.90 − −
ε+η ε η η O1 η μ
980 (40)
78.3
2.8
19.0
78.2
11.60 0.80 0.60
12.20 −
11.10 10.60 −
11.80 1.40 0.50
16.50 − 15.90 11.00 1.60 15.80 14.00 11.20 4.50 16.60 −
10.70 0.80
12.80 4.20
14.30 − −
− − − − 19.00 19.50
6.70 21.80 1.30
2.10 −
2.50 6.90 −
0.10
7.60 23.40
7.00 21.40 3.00 4.30 6.90 16.80 1.70 −
4.00 − 2.90
6.80 18.60 6.60 21.70
− − − − 2.00 1.30 5.80 − −
− − − −
− − − − − − − 0.808 −
0.405 − − − 0.405
− 0.405 − 1.559 0.405
− 1.245 −
− −
1.250 − 0.808 −
1.551
1.771 −
1.253 − − − − 1.250 − − − − − 3.054 −
0.808 − − − − 0.808 − − − − −
1.552 − − − − 1.549 − − − − −
− 1.769 1.770 − − − − − 1.772 − − −
1.772 −
− 3.061 3.060 − − − − − 3.064 − − −
− 1.768 1.767 − − − − − 1.771 − − −
−
−
108.18
−
− −
− − − −
−
107.91
−
90.85
− − − − −
107.72
− − − −
107.59
− − −
90.10
− − − − −
90.12 90.07
helium) in Al2O3 crucibles. Differential tungsten/tungsten–rhenium or platinum/platinum–rhodium thermocouples were a temperature sensor. The heating and cooling rate of the samples was 20 or 30°C/min. The x-ray diffraction patterns of the alloys were obtained with the Debye–Scherrer method using a Debye camera (diameter 57.3 mm) with chromium or copper Kα radiation. The phases were identified with the Lasy Pulverix and X-ray Powder Analysis software. The lattice parameters were calculated with the Gitter software based on the least-squares method. The electron microprobe analysis of the alloys was carried out with a Superprobe-733 analyzer (JEOL, Japan) with the maximum instrumental error of ±2 at.% and with an EDAX energy-dispersive x-ray analyzer combined with a JEOL-840A scanning electron microscope (instrumental error was about 0.5 at.%). Given the high reproducibility of phase composition measurements (within 0.2 at.%) and the small difference between the amounts of components in alloys and phases that exist in equilibria, the tables show the composition of alloys and phases with a precision to one tenth of a percent.
EXPERIMENTAL RESULTS AND DISCUSSION Using the data on the phase composition of alloys annealed from as-cast state at solidus and subsolidus temperatures and on their solidus temperatures, we constructed the solidus surface of the Al–Cr–Fe system at 58– 100 at.% Al on the concentration triangle (Fig. 2). In addition to solid solutions based on binary systems that exist in the Al–Cr and Al–Fe bounding systems, three ternary compounds (D3, O1, and H) are found on the solidus surface in the ternary system. These compounds have quite extensive homogeneity ranges at subsolidus temperatures. Their lattices are decagonal, orthorhombic base-centered, and hexagonal [1–3]. At about 20 at.% Cr and 2.5 at.% Fe, we revealed a phase denoted by ε with the structure similar to that of the metastable ε-Al4Cr binary phase [25]. This is orthorhombic primitive structure with parameters close to a ≈ 3.46 nm, b ≈ 2.00 nm, and c ≈ ≈ 1.24 nm (Table 1). Aluminum-depleted phases γ1 in the Al–Cr and Al–Fe systems have cubic lattices and close lattice parameters, according to [11] (the relevant phase in the Al–Fe system is denoted by ε). A continuous series of solid solutions has been confirmed to exist between them in the ternary system (γ1). For example, the solidus temperatures of the alloys containing about 60 at.% Al show that the solidus surface of the γ1 phase in the ternary system is smooth over the range from 10 to 20 at.% Cr. The homogeneity range of the γ1 phase above aluminum has a slight bend at compositions close to minimum on its solidus surface. There is a very narrow two-phase γ1 + γ2 range in the Al–Cr system at subsolidus temperatures; according to [20], this range leads to a three-phase γ1 + γ2 + + equilibrium on the solidus surface of the ternary system, which is a component of the relevant fourphase invariant process involving the liquid phase. X-ray diffraction shows that the crystallization of the alloys containing the γ1 and phases (Table 2) is nonequilibrium. This results in an effect on the heating curves for these alloys at 1125°C, corresponding to the above-mentioned invariant process (Table 3). According to EMPA, the alloys containing about 70 at.% Al and 10 at.% Cr have γ2 + , γ2 + + , and + equilibria (the microstructure of the alloys is shown in Fig. 3a, b), leading to the three-phase γ2 + + equilibrium (Fig. 3c). The invariant effect on the heating curve of the three-phase 70.0 Al–10.0 Cr–20.0 Fe alloy observed at 1105°C (Fig. 4a) determines the temperature of this equilibrium. The three-phase structure (γ2 + D3 + ) of the 70.0 Al–15.0 Cr–15.0 Fe and 71.3 Al–12.2 Cr–16.5 Fe alloys (Fig. 4d) and invariant effects at temperature close to 1085°C on their heating curves (Table 3, Fig. 4b) determine the position and temperature of the relevant three-phase triangle. It has been established that the decagonal D3 phase has a noticeable homogeneity range (at about 2 at.% for all three components) and is present in two three-phase equilibria at subsolidus temperatures, specifically: D3 + γ2 + + O1 and D3 + + O1 (Fig. 3e–g). Positions of the relevant triangles are determined from EMPA of the two-phase and three-phase alloys. Although the compositions of the D3 and O1 phases involved in these equilibria
90
TABLE 3. Solidus Temperatures of Al–Cr–Fe Alloys at 58–100 at.% Al Alloy composition, at.% Al
Cr
Fe
58.00 58.70 59.00 59.00 59.10 60.00 60.00 60.10 60.60 61.10 62.00 62.00 63.00 65.00 65.00 65.00 65.90 67.00 69.94 70.00 70.50 71.00 71.00 71.10 71.30 71.60 72.00 73.00 73.00 75.00 75.00 75.00 75.00 75.00 75.00 75.00
10.0 6.0 8.2 10.0 2.4 20.0 35.0 3.9 15.6 19.7 11.0 28.4 2.0 11.0 17.5 30.0 6.2 8.2 9.8 15.0 10.0 5.0 23.0 18.7 12.2 16.3 14.0 17.0 22.0 1.0 3.0 5.0 5.1 8.0 10.0 12.5
32.00 35.30 32.80 31.00 38.50 20.00 5.00 36.00 23.80 19.20 27.00 9.60 35.00 24.00 17.50 5.00 27.90 24.80 20.26 15.00 19.50 24.00 6.00 10.20 16.50 12.10 14.00 10.00 5.00 24.00 22.00 20.00 19.90 17.00 15.00 12.50
Solidus temperature, °C 1220 1190 1186 1187 1196 1173 1250 1194 1166 1171 1148 1202 1150 1120 1100 1200 1133 1118 1105 1085 1103 1135 1070 1085 1090 1084 1090 1051 1045 1100 1085 1070 1051 1020 1022 1020
Alloy composition, at.% Al
Cr
Fe
75.0 75.0 76.0 76.0 76.2 76.8 77.0 77.3 77.6 77.9 78.0 78.2 78.3 79.0 79.2 79.3 80.0 80.0 80.2 80.4 80.6 81.0 81.0 81.2 81.3 81.5 82.2 82.4 83.0 84.0 84.0 85.0 87.0 88.0 90.0
14.0 16.0 15.8 19.0 16.0 16.6 19.0 14.7 16.3 14.9 15.8 18.9 16.8 18.7 13.7 16.9 5.0 15.0 16.9 13.1 15.6 9.5 11.0 13.0 16.9 15.5 16.1 16.6 15.0 8.0 11.0 5.0 11.0 6.0 5.0
11.0 9.0 8.2 5.0 7.8 6.6 4.0 8.0 6.1 7.2 6.2 2.9 4.9 2.3 7.1 3.8 15.0 5.0 2.9 6.5 3.8 9.5 8.0 5.8 1.8 3.0 1.7 1.0 2.0 8.0 5.0 10.0 2.0 6.0 5.0
Solidus temperature, °C 1023 1087 1028 1050 1025 1004 1026 1000 990 996 980 996 1000 983 999 978 657 920 931 870 961 905 910 999 890 870 878 990 769 655 660 657 660 657 657
are close and, hence, their images in backscattered electrons are hardly differentiated and cannot be recognized visually, they are reliably identified by EMPA. These equilibria involving the liquid phase exist at 1085 and about 1075°C (Table 4, Fig. 4c, d). The most extensive homogeneity range among the ternary compounds that exist in the system at subsolidus temperatures has been found for the O1 phase (5 at.% Al, 4 at.% Cr, and 6 at.% Fe), while the ε phase has the narrowest range (about 1 at.% for each component). The homogeneity range of the latter is close to the Al–Cr side of the concentration triangle and thereby limits the solubility of iron in μ (the phase based on hexagonal Al4Cr stable in the ternary system) to 1.5 at.%. We emphasize again that the ternary ε phase results from iron stabilization of the metastable orthorhombic modification of Al4Cr. The O1 + γ2 and ε + γ2 equilibria (Fig. 3h, i) identified in the two-phase alloys with EMPA and XRD with close compositions of the γ2 phase determine the position of the three-
91
a
b
c
d
e
f
Fig. 3. Microstructure of annealed alloys in the Al–Cr–Fe ternary system containing from 58 to 100 at.% Al: a) 65.0 Al–11.0 Cr–24.0 Fe (600°C, 10 h; 720°C, 10 h; 920°C, 10 h; 1000°C, 185 h), γ2 + ; b) 71.74 Al–9.57 Cr–18.68 Fe (1065°C, 72 h), γ2 + ; c) 68.5 Al–11.0 Cr–20.5 Fe (600°C, 10 h; 720°C, 10 h; 920°C, 10 h; 1000°C, 185 h), γ2 + + ; d) 71.3 Al–12.2 Cr–16.5 Fe (1065°C, 26 h), γ2 + D3 + ; e) 73.0 Al–14.0 Cr–13.0 Fe (1032°C, 95 h), D3 + + + O1; f) 68.5 Al–21.5 Cr–10.0 Fe (600°C, 10 h; 720°C, 10 h; 920°C, 10 h; 1000°C, 185 h), γ2 + D3
92
g
h
i
j
k
Fig. 3. Finished: g) 71.34 Al–19.51 Cr–9.14 Fe (after DTA), γ2 + O1; h) 73.0 Al–22.0 Cr–5.0 Fe (after DTA), γ2 + O1; i) 76.8 Al–21.1 Cr–2.1 Fe (1000°C, 848 h), ε + γ2; j) 84.0 Al–11.0 Cr–5.0 Fe (600°C, 106 h), H + θ + ; k) 84.0 Al–8.0 Cr–8.0 Fe (600°C, 106 h), H + + phase O1 + ε + γ2 and ε + μ + γ2 triangles. The temperatures of the respective equilibria are determined using heating curves for cast two-phase alloys, which show invariant processes involving these phases due to nonequilibrium crystallization. They correspond to 1045 and 1035°C.
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TABLE 4. Coordinates of Three-Phase Ranges in the Al–Cr–Fe System at 58–100 at.% Al at Subsolidus Temperatures Phase composition, at.% Phase range γ1 + γ2 +
T, °C
1125
Phase Al
Cr
Fe
γ1
63.1
13.0
23.9
γ2
63.2 70.2
13.3 4.3
23.5 25.5
γ2 + +
1105
γ2
64.8 71.2 74.0
18.4 5.3 4.5
16.8 23.5 21.5
γ2 + D3 +
1085
γ2 D3
γ2 + D3 + O1
1085
γ2 D3 O1
66.0 69.8 74.0 67.0 71.5 ~72.8
21.9 16.4 5.2 23.8 18.0 ~18.3
12.1 13.8 20.8 9.2 10.5 ~8.9
D3 + O1 +
~ 1075
γ2 + O1 + ε
1045
D3 O1 γ2 O1
72.4 74.0 75.5 ~69.5 ~76.0 ~77.0
15.3 15.0 6.0 ~27.2 ~19.7 ~20.0
12.3 11.0 18.5 ~3.3 ~4.3 ~3.0
γ2 + ε + μ
1035
70.3 77.2 78.0
27.7 20.7 20.3
2.0 2.1 1.7
~77.8 ~77.1 ~78.7
~19.2 ~18.1 ~17.9
~3.0 ~4.8 ~3.4
77.0 80.0 77.5 78.0 79.0 79.0
14.8 12.0 5.0 19.5 19.6 18.1
8.2 8.0 17.5 2.5 1.4 2.9
~77.9 ~79.0 ~79.4 82.3 83.0 86.1
~15.1 ~16.1 ~13.6 14.2 16.0 12.9
~7.0 ~4.9 ~7.0 3.5 1.0 1.0
82.6 86.1 ~99.85 82.6 78.0 ~99.90
11.0 11.9 ~0.13 10.4 1.6 ~0.08
6.4 2.0 ~0.02 7.0 20.4 ~0.02
ε γ2 ε μ
94
ε + O1 + η
1026
O1 + H +
998
ε+μ+η
985
O1 + η + H
980
H+η+θ
750
H + θ +
660
H + +
657
ε O1 η O1 H ε μ η O1 η H H η θ H θ H
1085 1105 1188
c
1115 1128
1051
а
1085 1076
1160
b
d
1101
Fig. 4. Heating curves for Al–Cr–Fe alloys at 50–100 at.% Al annealed at subsolidus temperatures: a) 70.0 Al–10.0 Cr–20.0 Fe (1065°C, 72 h); b) 70.0 Al–15.0 Cr–15.0 Fe (600°C, 10 h; 720°C, 10 h; 920°C, 10 h; 1000°C, 185 h); c) 71.34 Al–19.51 Cr–9.14 Fe (1065°C, 26 h); d) 75.0 Al–10.0 Cr–15.0 Fe (1000°C, 144 h) As distinct from the μ phase, the solubility of iron in the η phase of the Al–Cr binary system reaches 5 at.% when aluminum content decreases to 79 at.%. The solidus temperatures of the alloys increase from about 865°C in the binary system to 1026°C in the ternary one (Table 3). The homogeneity ranges of the μ, η, ε, and O1 phases are located closely, thus leading to very narrow three-phase O1 + ε + η and ε + μ + η fields, whose temperatures of existence (1026 and 985°C) have also been determined from the heating curves for the two-phase as-cast alloys where invariant processes involving the liquid phase result from nonequilibrium crystallization. The tie lines O1H, O1η, and ηH are located closely and lead to another three-phase O1 + η + H field, whose temperature on the solidus surface is 980°C. The phase based on the ternary compound (H phase) found in the system has a homogeneity range at about 4 at. % Al and Fe. In addition to equilibria with the O1 and η phases, this phase also participates in equilibria with the , θ phase (based on Al13Cr2), and aluminum-based solid solution. The three-phase O1 + H + + and H + η + θ fields exist at 998 and 750°C. The positions of the respective tie-line triangles are determined with EMPA for two-phase alloys from the ranges bounding these triangles. Three-phase H + θ + and H + + equilibria have been established to exist in the aluminum-rich range (more than 75 at.% Al) at subsolidus temperatures: 660 and 657°C. According to EMPA of the three-phase alloys from these ranges (Fig. 3j, k), we have determined coordinates of the respective tie-line triangles (Fig. 2).
CONCLUSIONS The solidus surface of the Al–Cr–Fe ternary system at 58–100 at.% Al is rather complex. Four ternary compounds lead to 14 three-phase isothermal planes, which are included into the planes of four-phase invariant equilibria involving the liquid phase (Table 4). There are 25 ruled surfaces that bound the two-phase regions. Some of them have a fold of maximum temperatures.
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It is established that the solidus surfaces of solid solutions based on Al–Cr and Al–Fe binary compounds go down into the ternary system, excepting the η phase, which is stabilized by iron in the ternary system to higher temperatures (to 1026°C versus 865°C in the binary system). The solubility of chromium in both binary phases of the Al–Fe system reaches 6 at.% at subsolidus temperatures, while the solubility of iron in phases of the Al–Cr system changes from 1.5 at.% for the μ phase to 5 at.% for the η phase.
ACKNOWLEDGEMENT The authors are grateful to D. O. Kapush for the electron microprobe analysis and to L. A. Duma for the xray diffraction.
REFERENCES 1. 2. 3. 4. 5.
D. Pavlyuchkov, S. Balanetskyy, W. Kowalski, et al., “Stable decagonal quasicrystals in the Al–Fe–Cr and Al–Fe–Mn alloy systems,” J. All Comp., 477, L41–L44 (2009). D. V. Pavlyuchkov, B. Przepiorzynski, B. Grushko, and T. Ya. Velikanova, “Complex orthorhombic phase in the Al–Cr–Fe system,” Powder Metall. Met. Ceram., 47, No. 11–12, 698–701 (2008). D. V. Pavlyuchkov, B. Przepiorzynski, B. Grushko, and T. Ya. Velikanova, “Hexagonal phase in the Al– Cr–Fe system,” Metallofiz. Noveish. Tekhnol., 30, No. 10, 1423–1428 (2008). V. Demange, J. S. Wu, V. Brien et al., “New approximant phases in Al–Cr–Fe,” Mat. Sci. Eng., 294–296, 79–81 (2000). H. X. Sui, X. Z. Li, and K. H. Kuo, “Hexagonal Al81Fe8Cr11 with a = 4.00 nm and c = 1.24 nm,” Philos.
7.
Mag. Lett., 79, No. 4, 181–185 (1999). M. Palm, “The Al–Cr–Fe system—phases and phase equilibria in the Al-rich corner,” J. All. Comp., 252, 192–200 (1997). Z. B. He, B. S. Zou, and K. H. Kuo, “The monoclinic Al45Cr7 revisited,” J. All. Comp., 417, No. 1–2, L4–
8.
L8 (2006). B. B. Cao and K. H. Kuo, “Crystal structure of the η-Al11Cr2,” J. All. Comp., 458, 238–247 (2008).
6.
9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
96
L. A. Bendersky, R. S. Roth, J. T. Ramon, and D. Shechtman, “Crystallographic characterization of some intermetallic compounds in the Al–Cr system,” Met. Trans. A, 22A, 5–10 (1991). M. Ellner, J. Braun, and B. Predel, “Roentgenographische Untersuchung an Cr–Al–Phasen der W-Familie,” Z. Metallkd., 80, No. 5, 374–383 (1989). M. Eumann, G. Sauthoff, and M. Palm, “Phase equilibria in the Fe–Al–Mo system. Part II: Isothermal sections at 1000 and 1150°C,” Intermetallics, 16, 834–846 (2008). U. R. Kattner and B. P. Burton, “Aluminum–iron,” in: Phase Diagrams of Binary Iron Alloys, ASM, Metals Park, Ohio (1993), pp. 12–28. B. Grushko, E. Kowalska-Strzeciwilk, B. Przepiorzynski, and M. Surowiec, “Investigation of the Al-Cr γrange,” J. All. Comp., 402, 98–104 (2005). J. Grin, U. Burkhardt, M. Ellner, and K. Peters, “Refinement of the Fe4Al13 structure and its relationship to quasihomological homeotypical structures,” Z. Kristallogr., 209, 479–487 (1994). U. Burkhardt, J. Grin, M. Ellner, and K. Peters, “Structure refinement of the iron-aluminum phase with the approximate composition Fe2Al5,” Acta Cryst. B, B50, 313–316 (1994). R. N. Corby and P. J. Black, “The structure of FeAl2 by anomalous dispersion methods,” Acta Cryst. B, 29, 2669–2677 (1973). A. Taylor and R. M. Jones, “Constitution and magnetic properties of iron-rich iron–aluminum alloys,” J. Phys. Chem. Sol., No. 6, 16–37 (1958). F. Stein and M. Palm, “Redetermination of transition temperatures in the Fe–Al system by differential thermal analysis,” Int. J. Mat. Res., 98, No. 7, 580–588 (2007).
19. 20. 21. 22. 23.
24. 25.
B. Grushko, B. Przepiorzynski, and D. Pavlyuchkov, “On the constitution of the high-Al region of the Al– Cr alloy system,” J. All. Comp., 454, 214–220 (2008). S. Balanetskyy, W. Kowalski, and B. Grushko, “Liquidus, solidus and reaction scheme of the Al-rich part of Al–Cr–Mn,” J. All. Comp., 474, 147–151 (2009). J. G. Costa Neto, S. Gama, and C. A. Ribeiro, “Experimental study of the Al–Cr equilibrium diagram,” J. All. Comp., 182, 271–280 (1992). W. Koester, E. Wachtel, and K. Grube, “Aufbau und magnetische Eigenschaften der Aluminium-Chrom Legierungen,” Z. Metallkd., 54, 393–401 (1963). L. Cornish, P. Saltykov, G. Cacciamani, and T. Velikanova, “Al–Cr (aluminum–chromium),” in: G. Effenberg (ed.), MSIT Binary Evaluation Program, MSIT Workplace Materials Science International Services GmbH, Stuttgart (2003). K. Mahdouk and J.-C. Gachon, “Thermodynamic investigation of the aluminum–chromium system,” J. Phase Equil., 21, No. 2, 157–166 (2000). K. Y. Wen, Y. L. Chen, and K. H. Kuo, “Crystallographic relationships of the Al4Cr crystalline and quasicrystalline phases,” Met. Trans. A, 23A, 2437–2445 (1992).
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