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Solidification phases and liquidus surface of the Al–Ni–Ru system above 50 at.% aluminium
Journal of Alloys and Compounds 308 (2000) 205–215
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Solidification phases and liquidus surface of the Al–Ni–Ru system above 50 at.% aluminium a, b a c J. Hohls *, L.A. Cornish , P. Ellis , M.J. Witcomb a b
Advanced Materials Group, Physical Metallurgy Division, MINTEK, Private Bag X3015, Randburg 2125, South Africa School of Process and Materials Engineering, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa c Electron Microscope Unit, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa Received 30 March 2000; accepted 11 April 2000
Abstract The Al–Ni–Ru system above 50 at.% aluminium was studied using scanning electron microscopy with energy dispersive X-ray spectroscopy, and X-ray diffraction analyses of as-cast material. The liquidus surface was deduced and the invariant reactions derived. The composition range of the |RuNi 2 Al 14 ternary phase was |7–11 at.% Ru and |70–81 at.% Al, which is wider than hitherto reported. The liquidus surface of the |RuNi 2 Al 14 phase was also larger than previously reported, lying between |75–95 at.% Al and |2–5 at.% Ru. 2000 Elsevier Science S.A. All rights reserved. Keywords: Transition metal alloys; Phase diagram; Scanning electron microscopy; X-ray diffraction
1. Introduction Much work has been done on the study of materials for turbine engines [1,2]. The efficiency of turbine engines is related to the combustion temperature. However, the higher the temperature, the more aggressive become the conditions within the engine. Therefore, the turbine performance is limited by the ability of the materials used in the manufacture of the engines to withstand oxidation, hot corrosion and creep, among other factors. In order to increase the combustion temperature and thereby improve the efficiency of turbines, materials that can withstand these conditions for reasonable periods of time need to be found. At present, thermal barrier coatings are used to protect the components in hot sections of turbines, thus allowing higher working temperatures and increasing the efficiency. Unfortunately, the thermal expansion of the coatings and the substrates are not perfectly matched, leading to adhesion problems. A possible solution to this problem is to find new materials capable of bonding the thermal barrier coatings to the underlying alloy. The Al– Cr–Ni–Ru system has been identified as having potential for bond coating applications, and could thus increase both the efficiency and lifetime of relevant components of *Corresponding author. E-mail address: [email protected] (J. Hohls).
turbines. No information is available on the quaternary system, and only limited information for the Al–Ni–Ru system. The scope of the present work is the characterisation of the aluminium-rich corner (above 50 at.% Al) of the Al–Ni–Ru ternary system.
2. Previous work The binary phase diagrams for Al–Ni and Ni–Ru are well established [3]. The Al–Ru binary phase diagram has not been assessed by computer calculation, but the most recent phase diagram is by Boniface and Cornish [4]. Only the binary phase diagram reactions pertinent to the present investigation are described below. In the binary Al–Ni system [3], NiAl forms congruently at 16388C and 50 at.% Ni. In addition, there is a peritectoid reaction at |7008C, forming Ni 5 Al 3 . Below 50 at.% Ni, there are two peritectic reactions, forming Ni 2 Al 3 at 11338C and |27 at.% Ni, and NiAl 3 at |15 at.% Ni and 8548C. At approximately 3 at.% Ni and 639.98C, there is a eutectic reaction forming NiAl 3 and the Al solid solution. The Ni–Ru binary system is simple, with a single peritectic reaction occurring at |28 at.% Ru and 15508C [3]. In this reaction, (Ni) containing 34.5 at.% Ru forms from (Ru) containing |50 at.% Ni. However, the phase solubilities of (Ru) and (Ni) decrease rapidly with decreas-
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ing temperature, becoming less than 6 at.% at room temperature. In the Al–Ru binary system, above 50 at.% Ru, a eutectic reaction occurs between RuAl and (Ru) at |70 at.% Ru and 19208C [4,5]. Between RuAl and (Al) there is a cascade of four peritectic reactions, followed by a eutectic reaction [4,6,7]. The products of the peritectic reactions are as follows: Ru 2 Al 3 at |34 at.% Ru [4]; RuAl 2 at |14608C and 28 at.% Ru [4]; Ru 4 Al 13 at 14038C and |17 at.% Ru [6]; and RuAl 6 at 7238C and |1 at.% Ru [6]. The eutectic reaction between RuAl 6 and (Al) occurs below 1 at.% Ru at 6528C [6]. The Al–Ni–Ru ternary system has been previously studied by various workers [8–16] at different temperatures, with varying interpretations. In the present work, the approach used by Horner et al. [15] was followed, together with the use of binary formulae for extensions of the binary phases into the ternary system. The symbol B2 is used to indicate the compound that extends across the ternary system from NiAl to RuAl [16]. The order of the elements in compounds is given by following the Pettifor notation [17]. Investigations of the B2 region in the Al–Ni–Ru system have yielded varying results. Tsurikov et al. [8] found that at 5508C up to 5 at.% of Ru was soluble in NiAl and a maximum of 8 at.% of Ni was soluble in RuAl, with a two-phase region between NiAl and RuAl. Petrovoj [9] reported an isostructural B2 phase extending from NiAl to RuAl at 8008C. While initially studying the Al–Cr–Ni–Ru system, Chakravorty et al. [10] assumed a high mutual solubility between RuAl and NiAl, but in later studies [11,12] reported a miscibility gap, even at high temperatures. Wolff and Sauthoff [18] undertook mechanical tests of alloys within the Al–Ni–Ru system, and their nominal compositions appear to agree with Chakravorty and West [11,12]. Horner et al. [13] reported that most alloys in the |50 at.% Al region appeared to show coring rather than two-phase structures, and pointed out that the sloping liquidus would encourage coring. Subsequently, these workers [16] deduced the presence of the isostructural Ru x Ni 12x Al compound, B2, which was confirmed by the continuous variation in lattice parameter from RuAl to NiAl [19], and a smooth trend in microhardness with a maximum. They concluded that it was unlikely for both of these trends to be observed if the system contained a two-phase region. In addition, they interpreted that the samples were cored due to indistinct boundaries between the phases, and the gradual variation of the component compositions with alloy composition. At 5508C, Tsurikov et al. [8] reported that three-phase fields in the Al–Ni–Ru ternary system above 50 at.% Al were between: NiAl1Ni 2 Al 3 1RuAl; Ni 2 Al 3 1RuAl1 RuAl 2 ; Ni 2 Al 3 1RuAl 2 1Ru 4 Al 13 ; Ni 2 Al 3 1Ru 4 Al 13 1 NiAl 3 ; NiAl 3 1Ru 4 Al 13 1RuAl 6 and NiAl 3 1RuAl 6 1(Al). They showed little extension of the binary phases into the ternary system, except for Ni 2 Al 3 which showed up to 5
at.% solubility of Ru. No exclusively ternary compounds were reported. At 8008C in the Al-rich region, Petrovoj [9] reported almost the same three-phase fields as those given above, with the exception of RuAl 2 , which showed a slightly greater extension of approximately 3 at.% Ni. Horner et al. [14] found the ternary extensions of Ru 2 Al 3 , RuAl 2 , RuAl 6 , Ni 2 Al 3 and NiAl 3 to be less than 1 at.%, and the Al-rich solid solution, (Al), showed negligible solubility for Ru or Ni. They reported that only the Ru 4 Al 13 phase extended significantly into the ternary phase diagram, with a maximum of |10 at.% Ni. In addition, they found a previously unreported ternary phase with a composition of |RuNi 2 Al 14 , which formed peritectically. Horner et al. [14] also derived a schematic liquidus surface, with eight ternary invariant reactions: L 1 Ru 2 Al 3 → B2 1 RuAl 2
(1)
L 1 B2 1 RuAl 2 → Ni 2 Al 3
(2)
L 1 RuAl 2 → Ni 2 Al 3 1 Ru 4 Al 13
(3)
L 1 Ni 2 Al 3 1 Ru 4 Al 13 → | RuNi 2 Al 14
(4)
L 1 Ni 2 Al 3 1 | RuNi 2 Al 14 → NiAl 3
(5)
L 1 Ru 4 Al 13 → | RuNi 2 Al 14 1 (Al)
(6)
L 1 RuAl 6 → Ru 4 Al 13 1 (Al)
(7)
L 1 | RuNi 2 Al 14 → NiAl 3 1 (Al)
(8)
Sabariz and Taylor [20] reported that a two-phase (RuNi)Al alloy was a result of a spinodal decomposition into NiAl with |12 at.% Ru, and RuAl with |18 at.% Ni, stating this as further evidence of the peritectic reaction (L1ternary RuAl→ternary NiAl) suggested by Chakravorty and West [12]. However, spinodal decomposition is usually only observable at much higher magnification than that of their micrograph.
3. Experimental procedure Various alloy compositions were prepared by arc-melting small solid pieces or foils of the pure (99.9%) metals in an argon atmosphere on a water-cooled copper hearth. Buttons of 5 g were produced, and these were inverted and re-melted twice in order to attempt homogeneity. The buttons were cut in half, mounted and polished to 0.25 mm. The polished samples were examined were under the optical microscope and by scanning electron microscopy (Jeol JSM-840, Jeol JSM-840A). The compositions of the phases were determined by energy dispersive X-ray spectroscopy (EDS) using elemental standards. X-ray diffrac-
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207
tion was undertaken using Mo K a radiation in order to identify the phase structures.
B2 and RuAl 2 , and here the B2 phase had a different composition from the dendrites. The XRD spectrum clearly showed the presence of B2 and RuAl 2 .
4. Results
4.1.3. Al51 : Ni19 : Ru30 The |Al 51 :Ni 19 :Ru 30 alloy was composed mainly of cored B2 and a little B21RuAl 2 eutectic. The XRD analysis confirmed the presence of the B2 phase.
The compositions and morphologies of the alloys studied are given in the following section, with the EDS analyses summarised in Table 1.
4.1. Alloys with primary phase B2 4.1.1. Al51.5 : Ni3.5 : Ru45 and Al55 : Ni43 : Ru2 The alloys |Al 51 :Ni 3.5 :Ru 45.5 and |Al 55 :Ni 43 :Ru 2 were composed entirely of B2 with very little variation in composition. XRD analysis confirmed the presence of the B2 phase. 4.1.2. Al56 : Ni7 : Ru37 The |Al 56 :Ni 7 :Ru 37 alloy comprised dendrites of B2 with RuAl 2 . There were also areas of a eutectic mixture of
4.1.4. Al58 : Ni14.5 : Ru27.5 The |Al 58 :Ni 14.5 :Ru 27.5 alloy is shown in Fig. 1a,b. The B2 dendrites formed first, and show a subsequent solidstate precipitation, yielding needles. A fine eutectic of RuAl 2 and the darker B2 phase (which had much less Ru than the dendritic B2 phase) can be seen more clearly in Fig. 1b. The composition of the needles could not be verified from the analyses due to their size. The XRD analysis showed clearly the presence of the B2 phase. As very few of the remaining peaks matched with those of other alloys in this investigation containing Ru 4 Al 13 , most of the peaks were assumed to be from
Table 1 Summary of EDS analyses of the phase compositions of the alloys Nominal composition
Phase compositions Al:Ni:Ru (at.%)
Al 51.5 :Ni 3.5 :Ru 45
51.3:3.6:45.1
Al 56 :Ni 7 :Ru 37 Al 51 :Ni 19 :Ru 30
56.3:6.6:37.1 51.2:18.7:30.1
Al 58 :Ni 14.5 :Ru 27.5
58.0:14.5:27.5
Al 60 :Ni 35 :Ru 5 Al 64 :Ni 31 :Ru 5 Al 55 :Ni 43 :Ru 2
60.4:34.6:5.0 64:31:5 55.2:43.2:1.6
Al 64.5 :Ni 13.5 :Ru 22
64.6:13.4:22
Al 59 :Ni 23 :Ru 18 Al 65 :Ni 24.5 :Ru 10.5 Al 71 :Ni 15 :Ru 14
59:23:18 64.7:24.6:10.7 71.3:14.7:14.0
Al 76 :Ni 17.5 :Ru 6.5 Al 81 :Ni 6 :Ru 13 Al 83 :Ni 11 :Ru 6 Al 65 :Ni 28 :Ru 7 Al 74 :Ni 21.5 :Ru 4.5
76.2:17.3:6.5 80.7:6.2:13.1 83:11:6 65.2:27.9:6.9 74.0:21.5:4.5
75.6:23.6:0.8
60.3:38.9:0.8 61.0:38.5:0.5
Al 69 :Ni 28 :Ru 3
69:28:3
76.3:22.9:0.8
62.1:37.3:0.6
Al 72 :Ni 26 :Ru 2
72:26:2
62.0:37.4:0.6
Al 80 :Ni 16 :Ru 4 Al 85 :Ni 12 :Ru 3 Al 91.5 :Ni 7 :Ru 1.5
79.9:16.0:4.1 85.5:11.6:2.9 91.5:7.0:1.5
74.3:24.5:1.2 75.8:23.4:0.8 75.6:23.3:1.1 72.4:21.8:5.8 75.7:23.8:0.5
Al 91.5 :Ni 6.5 :Ru 2
91.7:6.5:1.8
79:19:2
Al 92 :Ni 7.5 :Ru 0.5
92.3:7.2:0.5
76.3:23.1:0.6
Overall
NiAl 3
Ni 2 Al 3
B2
Ru 4 Al 13
50.5:3.0:46.5 51.7:4.7:43.6 54:5:41 50.5:17.7:31.8 53.6:24.7:21.7 53.5:13.3:33.2 53.6:40:6.4 60.1:39.0:0.9 61.0:38.3:0.7
75.7:23.3:1.0 76.3:22.8:0.9 75.3:23.6:1.1
|RuNi 2 Al 14
65.8:0.5:33.7
66.1:1.0:32.9 72.4:10.9:16.7 74:9:17
56:41:3 55.7:42.6:1.7 58.6:36.0:5.4 61:30:9 56:37:7 57.3:38.5:4.2 60.2:38.9:0.9
RuAl 2
72.4:9.1:18.5
72.7:9.7:17.6 72.3:10.4:17.3 73.9:9.4:16.7 75.1:7.4:17.5 76.0:1.4:22.6 75.7:6.0:18.3 72.5:10.5:17.0
72:17:11
65.4:1:33.6 65.2:1.6:33.2 65.6:2.2:32.2 68.8:20.7:10.5 72.4:21.8:5.8 82.2:12.7:5.1 71.7:17.8:10.5 74.3:16.5:9.2 72.5:21.7:5.8 73.8:14.0:12.2 74.1:15.6:10.3 73.9:16.6:9.5 73.4:19.7:6.9 78:13:9 79.1:13.4:7.5 77.0:12:11 82.7:13.6:3.7 77.9:11.0:11.1 85:11:4
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of slightly different d-spacings were superimposed, and hence the B2 phase was deduced to be present.
4.1.6. Al64 : Ni31 : Ru5 The |Al 64 :Ni 31 :Ru 5 alloy is shown in Fig. 2. A supersaturated B2 phase formed first, followed by the peritectic formation of the dark Ni 2 Al 3 phase. Then, the coarse Ni 2 Al 3 and Ru 4 Al 13 eutectic formed, followed by the peritectic formation of the ternary phase |RuNi 2 Al 14 from Ru 4 Al 13 . Subsequently, the supersaturated B2 phase precipitated Ru 4 Al 13 . XRD analysis of this alloy very clearly showed the presence of Ni 2 Al 3 , while the major Ru 4 Al 13 peak was evident, but very small. There was a possible indication of the ternary |RuNi 2 Al 14 phase. It should be noted that the Ni 2 Al 3 peaks showed some shift. The shapes of the peaks indicated that both B2 and Ni 2 Al 3 were present. 4.2. Alloys with RuAl2 primary phase
Fig. 1. (a) SEM image in backscattered electron mode of the |Al 58 :Ni 14.5 :Ru 27.5 alloy, showing the B2 dendrites (grey) with a coarse eutectic comprising RuAl 2 (light) and B2 (dark). Bar represents 10 mm. (b) SEM image in backscattered electron mode of the |Al 58 :Ni 14.5 :Ru 27.5 alloy, shows more clearly the fine eutectic RuAl 2 (light)1B2 (dark) and needles formed by later solid state precipitation. Bar represents 10 mm.
4.2.1. Al64.5 : Ni13.5 : Ru22 and Al59 : Ni23 : Ru18 Fig. 3 shows the alloy |Al 64.5 :Ni 13.5 :Ru 22 , with the primary dendrites of RuAl 2 , and peritectically formed Ru 4 Al 13 . Finally, a coarse eutectic of RuAl 2 and supersaturated B2 formed, followed by some local solid-state precipitation of Ru 4 Al 13 in B2. XRD analysis confirmed the presence of RuAl 2 and there were many peaks corresponding to those deduced to be Ru 4 Al 13 in other alloys. In addition, there was a fairly good fit for the B2 peaks. The |Al 59 :Ni 23 :Ru 18 alloy was similar to the previous alloy, but without the solid state precipitation of Ru 4 Al 13 . XRD confirmed the B2 and RuAl 2 phases. 4.2.2. Al65 : Ni24.5 : Ru10.5 The |Al 65 :Ni 24.5 :Ru 10.5 alloy is shown in Fig. 4. The RuAl 2 phase formed first, followed by the peritectic formation of Ru 4 Al 13 and, lastly, a eutectic of B2 and
RuAl 2 or B2. Data for RuAl 2 [21] are available only for low angles, where they show good agreement.
4.1.5. Al60 : Ni35 : Ru5 The |Al 60 :Ni 35 :Ru 5 alloy solidified with the supersaturated B2 phase initially, followed by the peritectic formation of Ni 2 Al 3 , and finally a eutectic between Ni 2 Al 3 and Ru 4 Al 13 . Subsequently, there was solid state precipitation of Ru 4 Al 13 within the B2 phase. XRD analysis of this alloy clearly indicated the presence of Ni 2 Al 3 . A small peak at the position of the major Ru 4 Al 13 peak (observed in the Al 81 :Ni 6 :Ru 13 sample, which was mainly Ru 4 Al 13 ) showed that some Ru 4 Al 13 was also present. Unfortunately, all the B2 peaks have similar positions to Ni 2 Al 3 peaks, making it difficult to show definitively the presence of B2. However, the shape and relative heights of these peaks revealed that two peaks
Fig. 2. SEM image in backscattered electron mode of the |Al 64 :Ni 31 :Ru 5 alloy, showing B2 (dark with lighter precipitation within), Ru 4 Al 13 (light), Ni 2 Al 3 (dark) and |RuNi 2 Al 14 (light grey). Bar represents 10 mm.
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209
4.3.2. Al76 : Ni17.5 : Ru6.5 The |Al 76 :Ni 17.5 :Ru 6.5 alloy formed Ru 4 Al 13 needles first, then |RuNi 2 Al 14 by a peritectic reaction. Another peritectic reaction produced NiAl 3 , and the last reaction was a degenerate eutectic comprising (Al)1NiAl 3 . The fact that there were four phases present indicates nonequilibrium cooling of the sample, which can be expected from an arc-melted sample. The XRD spectrum gave a good fit for NiAl 3 , and the assumed ternary peaks corresponded well with those of other alloys. There was some evidence for the presence of Ru 4 Al 13 . The proportion of (Al) in the sample was too low to register any significant peaks. Fig. 3. SEM image in backscattered electron mode of the |Al 64.5 :Ni 13.5 :Ru 22 alloy, showing RuAl 2 (light), Ru 4 Al 13 (grey), and B2 (dark) with eutectic RuAl 2 (light)1B2 (dark). Bar represents 10 mm.
RuAl 2 . XRD analysis showed that this alloy probably contained B2 (more rich in NiAl than RuAl), and possibly RuAl 2 , but there were very few peaks available as data for Ru 4 Al 13 for comparison. The uncertainty regarding the RuAl 2 phase is due to the low concentration of this phase in the alloy.
4.3. Alloys with primary phase Ru4 Al13 4.3.1. Al71 : Ni15 : Ru14 The |Al 71 :Ni 15 :Ru 14 alloy solidified to form cored Ru 4 Al 13 , with a peritectic reaction forming the ternary phase |RuNi 2 Al 14 around it. There was also a eutectic comprising Ni 2 Al 3 1 |RuNi 2 Al 14 . XRD analysis for this alloy indicated the presence of Ni 2 Al 3 and Ru 4 Al 13 , although the peaks for the latter are not very clear. A few peaks indicated that |RuNi 2 Al 14 was present, by comparison with spectra from the other alloys containing that phase.
Fig. 4. SEM image in backscattered electron mode of the |Al 65 :Ni 24.5 :Ru 10.5 alloy, showing RuAl 2 (light), Ru 4 Al 13 (grey), and B2 (dark), with a eutectic of RuAl 2 1B2. Bar represents 10 mm.
4.3.3. Al81 : Ni6 : Ru13 In the |Al 81 :Ni 6 :Ru 13 alloy, it is apparent that the Ru 4 Al 13 needles formed first, followed by a peritectic reaction to form NiAl 3 . Finally, there was a degenerate eutectic (Al)1NiAl 3 . The XRD analysis confirmed NiAl 3 and (Al), although the peaks were small due to the low proportions of these phases. Ru 4 Al 13 is the major phase, and the peaks not corresponding to NiAl 3 or (Al) were assumed to result from this phase. These peaks were used to compare with other alloys containing this phase in smaller amounts. 4.3.4. Al83 : Ni11 : Ru6 The |Al 83 :Ni 11 :Ru 6 alloy is shown in Fig. 5. The Ru 4 Al 13 dendrites formed first, followed by a peritectic reaction giving NiAl 3 . There was a eutectic of (Al)1 NiAl 3 , which was not always discernable in some areas. The XRD scan indicated the presence of (Al), and some peaks corresponded well with the assumed ternary peaks of other alloys, indicating |RuNi 2 Al 14 . The peaks for NiAl 3 were not as intense as would be expected, considering the amount of this phase present. The peaks assumed to
Fig. 5. SEM image in backscattered electron mode of the |Al 83 :Ni 11 :Ru 6 alloy, showing Ru 4 Al 13 dendrites (light), NiAl 3 (grey), with a sparse eutectic of NiAl 3 1(Al) (dark). Bar represents 10 mm.
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correspond to Ru 4 Al 13 are also not a very good match, but this is likely to be a result of the fairly high percentage of Ni (|6%) present in this phase, which would shift them from the binary values.
4.4. Alloys with primary phase Ni2 Al3 4.4.1. Al65 : Ni28 : Ru7 The |Al 65 :Ni 28 :Ru 7 alloy is shown in Fig. 6a,b. Supersaturated Ni 2 Al 3 formed first and subsequently experienced solid state precipitation of Ru 4 Al 13. There was also a coarse eutectic between these phases. The |RuNi 2 Al 14 phase, which formed peritectically from Ru 4 Al 13 was present only in small and sparse proportions. The lamellar cellular precipitation products in the primary phase can be seen more clearly in Fig. 6b. XRD analysis showed good peaks for Ni 2 Al 3 , and indicated a small amount of the ternary phase. The remaining peaks appear to correspond to Ru 4 Al 13 .
4.4.2. Al74 : Ni21.5 : Ru4.5 In nominal |Al 74 :Ni 21.5 :Ru 4.5 , the Ni 2 Al 3 phase formed first, then |RuNi 2 Al 14 formed peritectically and was cored. The NiAl 3 phase appeared to have formed either peritectically or eutectically. There was also a sparse eutectic of (Al)1NiAl 3 . The ternary phase and NiAl 3 were the major phases, and both showed strong peaks in the XRD spectrum. The ternary phase peaks were taken to be those which agreed with those of other alloys containing the |RuNi 2 Al 14 phase. Some peaks indicated the presence of Ni 2 Al 3 . There was very little evidence of (Al), as a result of the low proportion of (Al) in the sample. 4.4.3. Al69 : Ni28 : Ru3 The |Al 69 :Ni 28 :Ru 3 alloy formed Ni 2 Al 3 first, followed by the peritectic formation of |RuNi 2 Al 14 , which was cored. The NiAl 3 phase formed a degenerate eutectic with the ternary phase. XRD analysis of this alloy confirmed the presence of all three phases. 4.4.4. Al72 : Ni26 : Ru2 The Ni 2 Al 3 phase formed first in the |Al 72 :Ni 26 :Ru 2 alloy, followed by the peritectic formation of |RuNi 2 Al 14 , and the latter was cored. The NiAl 3 phase formed a degenerate eutectic with the ternary phase. There was some variation in the shading of this dark phase, suggesting coring. The XRD analysis of this alloy also confirmed the presence of all three phases: Ni 2 Al 3 , |RuNi 2 Al 14 , and NiAl 3 . 4.5. Alloys with | RuNi2 Al14 primary phase 4.5.1. Al80 : Ni16 : Ru4 In the |Al 80 :Ni 16 :Ru 4 alloy, the |RuNi 2 Al 14 phase formed initially, followed by a peritectic reaction to form NiAl 3 . The (Al) phase formed last as a degenerate eutectic with NiAl 3 . The XRD spectrum obtained from this alloy was not very clear, and NiAl 3 , the major component of the system, yielded a much smaller signal than expected. Both the ternary phase and the (Al) phase were present in small amounts. 4.5.2. Al85 : Ni12 : Ru3 In |Al 85 :Ni 12 :Ru 3 , the |RuNi 2 Al 14 phase formed first, and then NiAl 3 formed peritectically. There was a degenerate eutectic of (Al) and NiAl 3 . The presence of (Al) and NiAl 3 was clearly confirmed by the XRD spectrum. Peaks corresponding to those present in other alloys containing the ternary phase also indicated the presence of |RuNi 2 Al 14 .
Fig. 6. (a) SEM image in backscattered electron mode of the |Al 65 :Ni 28 :Ru 7 alloy, showing Ru 4 Al 13 (light), Ni 2 Al 3 (dark), and |RuNi 2 Al 14 (light grey), with a coarse eutectic Ru 4 Al 13 1Ni 2 Al 3 . Bar represents 10 mm. (b) SEM image in backscattered electron mode of the |Al 65 :Ni 28 :Ru 7 alloy, showing the very fine cellular precipitation of Ru 4 Al 13 (light) within Ni 2 Al 3 (dark). Bar represents 1 mm.
4.5.3. Al91.5 : Ni7 : Ru1.5 The microstructure of the |Al 91.5 :Ni 7 :Ru 1.5 alloy is shown in Fig. 7. The |RuNi 2 Al 14 phase solidified first with coring, and next was a peritectic reaction forming
J. Hohls et al. / Journal of Alloys and Compounds 308 (2000) 205 – 215
211
gave a good fit for (Al) and showed peaks which may correspond to the ternary phase |RuNi 2 Al 14 .
4.6. Alloys with NiAl3 primary phase 4.6.1. Al92 : Ni7.5 : Ru0.5 The |Al 92 :Ni 7.5 :Ru 0.5 alloy formed NiAl 3 dendrites in an NiAl 3 1(Al) eutectic. XRD analysis of the alloy gave a good fit for (Al), and a less favourable fit for the NiAl 3 phase.
5. Discussion
Fig. 7. SEM image in backscattered electron mode of the |Al 91.5 :Ni 7 :Ru 1.5 alloy, showing |RuNi 2 Al 14 (light grey), NiAl 3 (medium grey) and NiAl 3 1(Al) (dark) eutectic. Bar represents 10 mm.
NiAl 3 (which exhibits similar grey contrast to |RuNi 2 Al 14 ). Lastly, there was a eutectic (Al)1NiAl 3. XRD analysis clearly showed the presence of (Al) and, from comparison with other alloys containing the ternary phase, the |RuNi 2 Al 14 phase also appeared to be present. There was some indication of peaks corresponding to NiAl 3 , although these were slightly shifted.
4.5.4. Al91.5 : Ni6.5 : Ru2 The |Al 91.5 :Ni 6.5 :Ru 2 alloy was similar to the previous alloy, but was locally inhomogeneous, showing a small region with the peritectic formation of NiAl 3 from |RuNi 2 Al 14 . The fine eutectic observed was probably (Al)1 |RuNi 2 Al 14 , although the morphology is very similar to the (Al)1NiAl 3 eutectic, as well as to the (Al)1 | RuNi 2 Al 14 eutectic of Horner et al. [14]. A very small amount of a light grey phase of different composition to the light grey dendrites was found and is inferred, from EDS, to be NiAl 3 , but this is probably formed as a result of localised inhomogeneity. XRD analysis of the alloy
NiAl 3 consistently gave a weak signal, even when a large proportion of the phase was present. The X-ray spectra peaks of samples containing the ternary |RuNi 2 Al 14 phase were compared, and the remaining unidentified peaks were assumed to belong to this phase (Table 2). The differences are due to the different compositions of the individual phases in the specimens giving some shift in the peaks. The data for Ru 4 Al 13 from Edshammer in the JC-PDS database [21] are only for low angles, and were thus of limited use in confirming the presence of this phase. The Al 81 :Ni 6 :Ru 13 specimen had a very high proportion of Ru 4 Al 13 , so this was used as a basis for comparison with the X-ray spectra of other alloys after removing the peaks of the other known phases. The Crystallographica package [22] was also used to model the peaks of Ru 4 Al 13 , and the modelled spectrum showed good agreement with that of Al 81 :Ni 6 :Ru 13 . Table 3 lists the peaks identified to be due to Ru 4 Al 13 and their deduced intensities (after compensating for peaks of other phases). There was some difficulty with the third large peak ˚ because it was consistently superposed on (|2.070 A) high-intensity peaks of other phases. Data for the Al 64 :Ni 31 :Ru 5 and Al 60 :Ni 35 :Ru 5 alloys were not included because, having lower proportions of Ru 4 Al 13 , the signals were not good. There is reasonable agreement for the
Table 2 d-Spacings and relative intensities (as percentage of highest peak) of peaks originating from the ternary phase |RuNi 2 Al 14 ˚ Relative d-Spacing (A) intensity Al 76 :Ni 17.5 :Ru 6.5 Al 85 :Ni 12 :Ru 3 Al 74 :Ni 21.5 :Ru 4.5 Al 83 :Ni 11 :Ru 6 Al 91.5 :Ni 7 :Ru 1.5 Al 91.5 :Ni 6.5 :Ru 2 Al 65 :Ni 28 :Ru 7 Al 71 :Ni 15 :Ru 14 Al 64 :Ni 31 :Ru 5 Al 72 :Ni 26 :Ru 2 Al 69 :Ni 28 :Ru 3 100 20 |5–15 |5 |5–15 |6 |2–14 |8–15 |9–18 |8–15 |1–9
2.062
2.062
2.055
1.565
1.574
1.524 1.467 1.365 1.344
1.530 g 1.368 1.346 i
1.572 1.562 1.532 1.466 1.362 1.348
1.058
1.060 1.029
1.064 1.029
2.089 1.594 a 1.574 b 1.546 c
2.060
2.058
2.060
2.066
1.568 1.542 1.530
1.571
1.565 1.545
1.367 d 1.346 e
1.358 h 1.346 j
1.068 f
1.066 k
1.357 1.346 1.140 1.067 1.029 l
1.530
2.063
2.053
1.563
1.561
1.566
1.533 1.501
1.531 1.470 1.361 1.346 1.137 1.063 1.029
1.530 1.479 1.359 1.344 1.137 1.060 1.027
1.363 1.345
1.344
1.067 1.029
1.067 1.032
2.073
1.077
The superscripts denote peaks for which the relative intensity differed from those found in the other alloys. The relative intensities of the marked peaks are: (a) 21, (b) 39, (c) 21, (d) 21, (e) 11, (f) 25, (g) 38, (h) 15, (i) 37, (j) 83, (k) 19 and (l) 16.
J. Hohls et al. / Journal of Alloys and Compounds 308 (2000) 205 – 215
212
Table 3 ˚ and relative intensities (as percentage of highest peak) of peaks originating from the phase based on Ru 4 Al 13 d-Spacing (A) Alloy composition and nickel content of Ru 4 Al 13 (at.%) Al 81 :Ni 6 :Ru 13 (1.4 at.% Ni)
3.787 (50) 3.321 3.192 3.029 2.652 2.416
(45) (20) (20) (15) (15)
2.300 2.275 2.184 2.149 2.122 2.098 2.025 1.987 1.896 1.813 1.783 1.732 1.659 1.625 1.530 1.516 1.478 1.438 1.410
(,10) (,10) (25) (50) (80) (100) (25) (15) (10) (15) (10) (15) (10) (,10) (25) (60) (25) (15) (15)
1.322 (15) 1.274 (60)
1.224 1.205 1.118 1.101
(20) (25) (15) (20)
1.045 (,10) 0.9787 (15) 0.9697 (15) 0.9158 (15) 0.8283 (,10) 0.7950 (10) a
Al 83 :Ni 11 :Ru 6 (6 at.% Ni)
Al 76 :Ni 17.5 :Ru 6.5 (7.4 at.% Ni)
3.923 3.757 3.467 3.311 3.166 3.029 2.664 2.420 2.375 2.299 2.265
3.882 (50) 3.769 (20)
(80) (80) (10) (15) (10) (,10) (15) (20) (15) (,10) (,10)
2.114 (100) 2.023 (80)
Al 64.5 :Ni 13.5 :Ru 22 (9.1 at.% Ni)
Al 65 :Ni 24.5 :Ru 10.5 (9.7 at.% Ni)
Al 71 :Ni 15 :Ru 14 (9.4 / 10.4 a at.% Ni)
Al 65 :Ni 28 :Ru 7 (10.5 at.% Ni)
3.760 (40) 3.468 (20)
3.721 (15)
3.868 (30) 3.734 (15) 3.604 (15)
3.843 (10) 3.651 (10)
3.179 (,10) 3.051 (10) 3.006 (,10)
3.172 (30)
3.164 (15)
3.146 (20)
3.273 (15) 3.011 (20) 2.401 (20)
2.394 (10) 2.309 (10)
2.156 2.125 2.062 2.042 2.003
(30) (20) (100) (100) (20)
2.219 2.156 2.093 2.047
(20) (20) (100) (100)
2.141 (80) 2.073 (100) 2.032 (100)
2.177 2.127 2.085 2.066
(15) (60) (100) (100)
2.132 (80) 2.075 (100) 2.020 (100)
1.888 (20) 1.804 (,10)
1.653 (10) 1.632 (10) 1.530 (15)
1.453 (20)
1.325 1.278 1.264 1.247
(,10) (20) (10) (20)
1.115 (10) 1.093 (20)
1.653 (20) 1.625 (,10) 1.531 (20) 1.524 1.467 1.427 1.412
(20) (20) (40) (25)
1.323 1.278 1.262 1.244 1.234 1.204 1.118 1.106
(,10) (,10) (10) (10) (10) (,10) (,10) (10)
1.044 (,10)
1.325 1.271 1.250 1.236
(,10) (,10) (30) (20)
1.205 (10) 1.117 (30) 1.096 (10)
1.078 (30)
0.9107 (15)
1.544 (15) 1.512 (15) 1.484 (25) 1.406 1.364 1.321 1.284 1.250
(25) (10) (,10) (,10) (40)
1.545 (10) 1.504 (10) 1.466 (20) 1.446 (25) 1.402 (15)
1.287 (,10)
1.325 (20) 1.280 (30) 1.247 (40)
1.246 (15) 1.202 (,10) 1.174 (60) 1.101 (10) 1.089 (10) 1.080 (10) 1.069 (10) 1.057 (10) 1.050 (10) 0.9744 (,10) 0.9085 (10)
1.193 (15)
1.194 (15) 1.166 (70)
1.077 (10) 1.078 (10) 1.055 (10) 1.048 (20) 1.011 (20) 0.9030 (20)
0.8274 (10)
Cored.
different samples, taking into account the varying Ni contents and the fact that since the phase is not cubic, the peak order might change as the shape of the unit cell changes. Fig. 8 shows a solidification projection of the Al–Ni–Ru ternary above 50 at.% Al. Some analyses need to be extrapolated back to the actual phase and extrapolated
away from the composition of the precipitated phase. This is because the analyses were undertaken on phases in which solid-state precipitation had occurred, and thus the results were affected by the presence of the second phase. The asterisks represent the overall eutectic compositions where analysis was possible. The boundary of the |RuNi 2 Al 14 phase has been drawn with a dotted line
J. Hohls et al. / Journal of Alloys and Compounds 308 (2000) 205 – 215
213
Fig. 8. Solidification projection of the Al–Ni–Ru ternary above 50 at.% Al. Dotted lines represent tie-lines; solid squares represent overall analyses of specimens; solid circles represent phases analysed; asterisks represent multi-phase compositions; and question marks represent analyses of phases within which were solid-state precipitations.
because some of the analyses were on more irregular morphologies and could have been affected by surrounding phases, rendering the boundary uncertain. It can be seen that all of the phase boundaries agree with those of Horner et al. [14], except for the ternary |RuNi 2 Al 14 phase, which has a far greater extent than previously reported. The extent of the new phase boundary for |RuNi 2 Al 14 is between |7–11 at.% Ru and |70–81 at.% Al. The |RuNi 2 Al 14 phase shows a wider range of compositions because of coring. From the microstructures and morphologies of the alloys, the solidification sequences were deduced as described in the results section. The reactions are listed in Table 4, together with the nominal alloy compositions. In addition, Table 5 details the alloys in which solid-state precipitation occurred due to supersaturation of the primary phase. These reactions are not likely to be equilibrium reactions. From these solidification reactions, the following ternary invariant reactions can be inferred, where the numbers indicate the invariant reactions marked on Fig. 9: L 1 Ru 2 Al 3 → B2 1 RuAl 2
(1)
L 1 B2 1 Ni 2 Al 3 → Ru 4 Al 13
(2)
L 1 Ru 4 Al 13 → B2 1 RuAl 2
(3)
L 1 Ni 2 Al 3 1 Ru 4 Al 13 → | RuNi 2 Al 14
(4)
L 1 Ni 2 Al 3 → | RuNi 2 Al 14 1 NiAl 3
(5)
L 1 RuAl 6 → Ru 4 Al 13 1 (Al)
(6)
L 1 Ru 4 Al 13 → | RuNi 2 Al 14 1 (Al)
(7)
L 1 | RuNi 2 Al 14 → NiAl 3 1 (Al)
(8)
The reactions labelled (2) and (3) disagree with those of Horner et al. [14], whereas the others agree. Using the equations above, the overall alloy compositions and the primary phases associated with them, together with the solidification sequence shown in Table 4, a liquidus surface was drawn. This is given in Fig. 9. In order for the reactions observed in the |Al 64.5 :Ni 13.5 :Ru 22 alloy to occur, there must be a minimum between the junction of the B2 and RuAl 2 liquidus surfaces at approximately Al 59 :Ni 27 :Ru 14 . The |Al 91.5 :Ni 6.5 :Ru 2 and |Al 92 :Ni 7.5 :Ru 0.5 alloys indicate that near the higher Al content boundary, the direction of maximum slope of the |RuNi 2 Al 14 liquidus surface is parallel to its length. This liquidus surface is similar to that of Horner et al. [14], although there are some differences. Firstly, the liquidus surface of the ternary phase has a greater extent than previously detailed. Horner et al. [14] reported a range of |85–95 at.% Al and |0.5–4.5 at.% Ru. However, in this study, the range was found to exist from |75–95 at.% Al and |2–5 at.% Ru. This investigation was based on more alloys, and thus gives a more complete study. Secondly,
J. Hohls et al. / Journal of Alloys and Compounds 308 (2000) 205 – 215
214 Table 4 Reaction sequences in the alloys Alloy
Reaction sequence
Al 56 :Ni 7 :Ru 37 Al 51 :Ni 19 :Ru 30 Al 58 :Ni 14.5 :Ru 27.5 Al 59 :Ni 23 :Ru 18
L→B21RuAl 2
Al 60 :Ni 35 :Ru 5
L1B2→Ni 2 Al 3 L→Ni 2 Al 3 1Ru 4 Al 13
Al 64 :Ni 31 :Ru 5
L1B2→Ni 2 Al 3 L→Ni 2 Al 3 1Ru 4 Al 13 L1Ru 4 Al 13 → |RuNi 2 Al 14
Al 64.5 :Ni 13.5 :Ru 22
L1RuAl 2 →Ru 4 Al 13 L→RuAl 2 1B2 (supersaturated)
Al 65 :Ni 24.5 :Ru 10.5
L1RuAl 2 →Ru 4 Al 13 L→RuAl 2 1B2
Al 71 :Ni 15 :Ru 14
L1Ru 4 Al 13 → |RuNi 2 Al 14 L→ |RuNi 2 Al 14 1Ni 2 Al 3
Al 76 :Ni 17.5 :Ru 6.5
L1Ru 4 Al 13 → |RuNi 2 Al 14 L1 |RuNi 2 Al 14 →NiAl 3 L→NiAl 3 1(Al)
Al 81 :Ni 6 :Ru 13
L1Ru 4 Al 13 → |RuNi 2 Al 14 L1 |RuNi 2 Al 14 →NiAl 3 (|RuNi 2 Al 14 completely consumed) L→NiAl 3 1(Al)
Al 83 :Ni 11 :Ru 6
L1Ru 4 Al 13 → |RuNi 2 Al 14 L1 |RuNi 2 Al 14 →NiAl 3 L→NiAl 3 1(Al) (limited amount)
Al 65 :Ni 28 :Ru 7
L→Ni 2 Al 3 1Ru 4 Al 13 L1Ru 4 Al 13 → |RuNi 2 Al 14 (small amount)
Al 74 :Ni 21.5 :Ru 4.5
L1Ni 2 Al 3 → |RuNi 2 Al 14 (cored) L1 |RuNi 2 Al 14 →NiAl 3 OR L→ |RuNi 2 Al 14 1NiAl 3 L→NiAl 3 1(Al) (limited amount)
Al 69 :Ni 28 :Ru 3 Al 72 :Ni 26 :Ru 2
L1Ni 2 Al 3 → |RuNi 2 Al 14 L→ |RuNi 2 Al 14 1NiAl 3
Al 80 :Ni 16 :Ru 4 Al 85 :Ni 12 :Ru 3 Al 91.5 :Ni 7 :Ru 1.5
L1 |RuNi 2 Al 14 →NiAl 3 L→NiAl 3 1(Al)
Al 91.5 :Ni 6.5 :Ru 2
L→ |RuNi 2 Al 14 1(Al)
Al 92 :Ni 7.5 :Ru 0.5
L→NiAl 3 1(Al)
the RuAl 2 liquidus surface was found to extend to lower Ru compositions, particularly for compositions with higher Ni content. Horner et al. [14] reported the lower boundary Table 5 Solid-state precipitation in the alloys Alloy
Solid-state reaction
Al 58 :Ni 14.5 :Ru 27.5
Ru 4 Al 13 or RuAl 2 precipitated in B2
Al 60 :Ni 35 :Ru 5 Al 64 :Ni 31 :Ru 5 Al 64.5 :Ni 13.5 :Ru 22
Ru 4 Al 13 precipitated in B2
Al 65 :Ni 28 :Ru 7
Ru 4 Al 13 precipitated in Ni 2 Al 3
of the RuAl 2 liquidus surface to be approximately 15 at.% Ru, whereas the results of the present study indicate a minimum of |10 at.% Ru. Finally, the boundary of the Ni 2 Al 3 liquidus surface no longer adjoins the RuAl 2 liquidus surface. Rather, the Ru 4 Al 13 liquidus surface extends to join the B2 boundary and lies between the surfaces of Ni 2 Al 3 and RuAl 2 , providing the eutectic between Ni 2 Al 3 and Ru 4 Al 13 . It should be noted that the valley between the |RuNi 2 Al 14 and Ni 2 Al 3 surfaces is peritectic on the more Ni-rich side and becomes eutectic near the ternary invariant point (5). Similarly, the valley between |RuNi 2 Al 14 and NiAl 3 is eutectic at the invariant point (5) and becomes peritectic as the Al content increases. The analysed phase compositions of Wolff and Sauthoff [18] agreed with the primary B2 and RuAl 2 , and were consistent with a eutectic between B2 (of different composition) and RuAl 2 , of this investigation. The two-phase microstructure of Sabariz and Taylor [20] is not in disagreement with the current findings, although their interpretation is. The interdendritic B2 phase is likely to be part of the irregular B21RuAl 2 eutectic found in the alloys |Al 58 :Ni 15 :Ru 27, |Al 65 :Ni 24 :Ru 11 , |Al 56 :Ni 7 :Ru 37 and |Al 59 :Ni 23 :Ru 18 . Since the B2 eutectic component forms at lower temperatures, and hence at different compositions to the primary B2, it is not surprising that there are two apparent B2 phase compositions. It is surprising that Sabariz and Taylor [20] dub their microstructure ‘spinodal’, since these structures are usually so fine that they can generally be resolved only with a transmission electron microscope.
6. Conclusions The study has shown good agreement with Horner et al. [14], except that the liquidus surfaces of RuAl 2 and |RuNi 2 Al 14 are larger and two of the invariant reactions differed. The ternary invariant reactions have been deduced to be: L 1 Ru 2 Al 3 → B2 1 RuAl 2 L 1 B2 1 Ni 2 Al 3 → Ru 4 Al 13 (disagrees with Horner et al. [14]) L 1 Ru 4 Al 13 → B2 1 RuAl 2 (disagrees with Horner et al. [14]) L 1 Ni 2 Al 3 1 Ru 4 Al 13 → | RuNi 2 Al 14 L 1 Ni 2 Al 3 → | RuNi 2 Al 14 1 NiAl 3 L 1 RuAl 6 → Ru 4 Al 13 1 (Al) L 1 Ru 4 Al 13 → | RuNi 2 Al 14 1 (Al)
J. Hohls et al. / Journal of Alloys and Compounds 308 (2000) 205 – 215
215
Fig. 9. Schematic liquidus surface of the Al–Ni–Ru system above 50 at.% Al. The solid squares are the overall compositions of the alloys in this study. The numbers represent the ternary invariant reactions given in the text. The liquidus slopes down from (7) to (8).
L 1 | RuNi 2 Al 14 → NiAl 3 1 (Al)
Acknowledgements The authors would like to acknowledge Impala Platinum for the provision of ruthenium for this study, and the Department of Arts, Culture, Science and Technology (DACST) for their support of this project. The authors are grateful to Dr. J.C. Zhao (GE-CRD) for guidance in this work. The encouragement of I.M. Wolff and assistance of P.J. Hill, S.S. Taylor, T. Biggs, L. Glaner and H.E. Muhuma of Mintek, and the MRSP at the University of the Witwatersrand is much appreciated. This paper is published by permission of Mintek.
References [1] D.M. Dimiduk, D.B. Miracle, C.H. Ward, Mater. Sci. Tech. 8 (1992) 367. [2] F.H. Froes, C. Suryanarayana, W. Quist, E. Lavernia, B.I. Bondarev, N.F. Anoshkin, I.S. Polkin, O.K. Fatkullin, V. Samarov, A.B. Notkin, Key Eng. Mater. 77–78 (1993) 1. [3] William W. Scott Jr. in: T.B. Massalski (Ed.-in-Chief), H. Okamoto, B.R. Subramanian, L. Kacprzak (Eds.), Binary Alloy Phase Diagrams, 2nd Edition, ASM International, OH, 1990.
[4] T.D. Boniface, L.A. Cornish, J. Alloys Comp. 234 (1996) 275. [5] W. Obrowski, Metallwiss. Teck. (Berlin) 17 (1963) 108. [6] S.M. Anlage, P. Nash, R. Ramachandran, J. Schwarz, J. LessCommon Met. 136 (1988) 237. [7] T.D. Boniface, L.A. Cornish, J. Alloys Comp. 233 (1996) 241. [8] V.F. Tsurikov, E.M. Sokolovskaya, E.F. Kazakova, Vesnik Moskovkogo Univ. Khim. 35 (5) (1980) 113. [9] L.A. Petrovoj, in: N.V. Ageeva (Ed.), Diagrammy Sostoyaniya Metallicheskikh Sistem, Vol. 30, Viniti, Moscow, 1985, p. 323. [10] S. Chakravorty, H. Hashim, D.R.F. West, J. Mater. Sci. 20 (1985) 2313. [11] S. Chakravorty, D.R.F. West, Scripta Metall. 19 (1985) 1355. [12] S. Chakravorty, D.R.F. West, J. Mater. Sci. 21 (1986) 2721. [13] I.J. Horner, M.J. Witcomb, L.A. Cornish, Proc. Electron Microsc. Soc. South Africa 25 (1995) 13. [14] I.J. Horner, L.A. Cornish, M.J. Witcomb, J. Alloys Comp. 256 (1997) 213. [15] I.J. Horner, L.A. Cornish, M.J. Witcomb, J. Alloys Comp. 256 (1997) 221. [16] I.J. Horner, L.A. Cornish, M.J. Witcomb, J. Alloys Comp. 264 (1998) 173. [17] D.G. Pettifor, J. Phase Equilibria 17 (5) (1996) 384. [18] I.M. Wolff, G. Sauthoff, Metall. Mater. Trans. A 27A (1996) 1395. [19] A.S. Harte, P.M. Hung, I.J. Horner, N. Hall, L.A. Cornish, M.J. Witcomb, in: J.V. Gilfrich (Ed.), Advances in X-ray Analysis, Vol. 39, Plenum Press, New York, 1997, p. 747. [20] A.L.R. Sabariz, G. Taylor, Mater. Res. Soc. Symp. Proc. 460 (1997) 611. [21] JC-PDS International Centre for Diffraction Data, Version 2.16, Newtown Square, PA 19073, USA, 1995. [22] Crystallographica, Oxford Cryosystems, 1995–1999.
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Solidification phases and liquidus surface of the Al–Ni–Ru system above 50 at.% aluminium a, b a c J. Hohls *, L.A. Cornish , P. Ellis , M.J. Witcomb a b
Advanced Materials Group, Physical Metallurgy Division, MINTEK, Private Bag X3015, Randburg 2125, South Africa School of Process and Materials Engineering, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa c Electron Microscope Unit, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa Received 30 March 2000; accepted 11 April 2000
Abstract The Al–Ni–Ru system above 50 at.% aluminium was studied using scanning electron microscopy with energy dispersive X-ray spectroscopy, and X-ray diffraction analyses of as-cast material. The liquidus surface was deduced and the invariant reactions derived. The composition range of the |RuNi 2 Al 14 ternary phase was |7–11 at.% Ru and |70–81 at.% Al, which is wider than hitherto reported. The liquidus surface of the |RuNi 2 Al 14 phase was also larger than previously reported, lying between |75–95 at.% Al and |2–5 at.% Ru. 2000 Elsevier Science S.A. All rights reserved. Keywords: Transition metal alloys; Phase diagram; Scanning electron microscopy; X-ray diffraction
1. Introduction Much work has been done on the study of materials for turbine engines [1,2]. The efficiency of turbine engines is related to the combustion temperature. However, the higher the temperature, the more aggressive become the conditions within the engine. Therefore, the turbine performance is limited by the ability of the materials used in the manufacture of the engines to withstand oxidation, hot corrosion and creep, among other factors. In order to increase the combustion temperature and thereby improve the efficiency of turbines, materials that can withstand these conditions for reasonable periods of time need to be found. At present, thermal barrier coatings are used to protect the components in hot sections of turbines, thus allowing higher working temperatures and increasing the efficiency. Unfortunately, the thermal expansion of the coatings and the substrates are not perfectly matched, leading to adhesion problems. A possible solution to this problem is to find new materials capable of bonding the thermal barrier coatings to the underlying alloy. The Al– Cr–Ni–Ru system has been identified as having potential for bond coating applications, and could thus increase both the efficiency and lifetime of relevant components of *Corresponding author. E-mail address: [email protected] (J. Hohls).
turbines. No information is available on the quaternary system, and only limited information for the Al–Ni–Ru system. The scope of the present work is the characterisation of the aluminium-rich corner (above 50 at.% Al) of the Al–Ni–Ru ternary system.
2. Previous work The binary phase diagrams for Al–Ni and Ni–Ru are well established [3]. The Al–Ru binary phase diagram has not been assessed by computer calculation, but the most recent phase diagram is by Boniface and Cornish [4]. Only the binary phase diagram reactions pertinent to the present investigation are described below. In the binary Al–Ni system [3], NiAl forms congruently at 16388C and 50 at.% Ni. In addition, there is a peritectoid reaction at |7008C, forming Ni 5 Al 3 . Below 50 at.% Ni, there are two peritectic reactions, forming Ni 2 Al 3 at 11338C and |27 at.% Ni, and NiAl 3 at |15 at.% Ni and 8548C. At approximately 3 at.% Ni and 639.98C, there is a eutectic reaction forming NiAl 3 and the Al solid solution. The Ni–Ru binary system is simple, with a single peritectic reaction occurring at |28 at.% Ru and 15508C [3]. In this reaction, (Ni) containing 34.5 at.% Ru forms from (Ru) containing |50 at.% Ni. However, the phase solubilities of (Ru) and (Ni) decrease rapidly with decreas-
0925-8388 / 00 / $ – see front matter 2000 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 00 )00898-7
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J. Hohls et al. / Journal of Alloys and Compounds 308 (2000) 205 – 215
ing temperature, becoming less than 6 at.% at room temperature. In the Al–Ru binary system, above 50 at.% Ru, a eutectic reaction occurs between RuAl and (Ru) at |70 at.% Ru and 19208C [4,5]. Between RuAl and (Al) there is a cascade of four peritectic reactions, followed by a eutectic reaction [4,6,7]. The products of the peritectic reactions are as follows: Ru 2 Al 3 at |34 at.% Ru [4]; RuAl 2 at |14608C and 28 at.% Ru [4]; Ru 4 Al 13 at 14038C and |17 at.% Ru [6]; and RuAl 6 at 7238C and |1 at.% Ru [6]. The eutectic reaction between RuAl 6 and (Al) occurs below 1 at.% Ru at 6528C [6]. The Al–Ni–Ru ternary system has been previously studied by various workers [8–16] at different temperatures, with varying interpretations. In the present work, the approach used by Horner et al. [15] was followed, together with the use of binary formulae for extensions of the binary phases into the ternary system. The symbol B2 is used to indicate the compound that extends across the ternary system from NiAl to RuAl [16]. The order of the elements in compounds is given by following the Pettifor notation [17]. Investigations of the B2 region in the Al–Ni–Ru system have yielded varying results. Tsurikov et al. [8] found that at 5508C up to 5 at.% of Ru was soluble in NiAl and a maximum of 8 at.% of Ni was soluble in RuAl, with a two-phase region between NiAl and RuAl. Petrovoj [9] reported an isostructural B2 phase extending from NiAl to RuAl at 8008C. While initially studying the Al–Cr–Ni–Ru system, Chakravorty et al. [10] assumed a high mutual solubility between RuAl and NiAl, but in later studies [11,12] reported a miscibility gap, even at high temperatures. Wolff and Sauthoff [18] undertook mechanical tests of alloys within the Al–Ni–Ru system, and their nominal compositions appear to agree with Chakravorty and West [11,12]. Horner et al. [13] reported that most alloys in the |50 at.% Al region appeared to show coring rather than two-phase structures, and pointed out that the sloping liquidus would encourage coring. Subsequently, these workers [16] deduced the presence of the isostructural Ru x Ni 12x Al compound, B2, which was confirmed by the continuous variation in lattice parameter from RuAl to NiAl [19], and a smooth trend in microhardness with a maximum. They concluded that it was unlikely for both of these trends to be observed if the system contained a two-phase region. In addition, they interpreted that the samples were cored due to indistinct boundaries between the phases, and the gradual variation of the component compositions with alloy composition. At 5508C, Tsurikov et al. [8] reported that three-phase fields in the Al–Ni–Ru ternary system above 50 at.% Al were between: NiAl1Ni 2 Al 3 1RuAl; Ni 2 Al 3 1RuAl1 RuAl 2 ; Ni 2 Al 3 1RuAl 2 1Ru 4 Al 13 ; Ni 2 Al 3 1Ru 4 Al 13 1 NiAl 3 ; NiAl 3 1Ru 4 Al 13 1RuAl 6 and NiAl 3 1RuAl 6 1(Al). They showed little extension of the binary phases into the ternary system, except for Ni 2 Al 3 which showed up to 5
at.% solubility of Ru. No exclusively ternary compounds were reported. At 8008C in the Al-rich region, Petrovoj [9] reported almost the same three-phase fields as those given above, with the exception of RuAl 2 , which showed a slightly greater extension of approximately 3 at.% Ni. Horner et al. [14] found the ternary extensions of Ru 2 Al 3 , RuAl 2 , RuAl 6 , Ni 2 Al 3 and NiAl 3 to be less than 1 at.%, and the Al-rich solid solution, (Al), showed negligible solubility for Ru or Ni. They reported that only the Ru 4 Al 13 phase extended significantly into the ternary phase diagram, with a maximum of |10 at.% Ni. In addition, they found a previously unreported ternary phase with a composition of |RuNi 2 Al 14 , which formed peritectically. Horner et al. [14] also derived a schematic liquidus surface, with eight ternary invariant reactions: L 1 Ru 2 Al 3 → B2 1 RuAl 2
(1)
L 1 B2 1 RuAl 2 → Ni 2 Al 3
(2)
L 1 RuAl 2 → Ni 2 Al 3 1 Ru 4 Al 13
(3)
L 1 Ni 2 Al 3 1 Ru 4 Al 13 → | RuNi 2 Al 14
(4)
L 1 Ni 2 Al 3 1 | RuNi 2 Al 14 → NiAl 3
(5)
L 1 Ru 4 Al 13 → | RuNi 2 Al 14 1 (Al)
(6)
L 1 RuAl 6 → Ru 4 Al 13 1 (Al)
(7)
L 1 | RuNi 2 Al 14 → NiAl 3 1 (Al)
(8)
Sabariz and Taylor [20] reported that a two-phase (RuNi)Al alloy was a result of a spinodal decomposition into NiAl with |12 at.% Ru, and RuAl with |18 at.% Ni, stating this as further evidence of the peritectic reaction (L1ternary RuAl→ternary NiAl) suggested by Chakravorty and West [12]. However, spinodal decomposition is usually only observable at much higher magnification than that of their micrograph.
3. Experimental procedure Various alloy compositions were prepared by arc-melting small solid pieces or foils of the pure (99.9%) metals in an argon atmosphere on a water-cooled copper hearth. Buttons of 5 g were produced, and these were inverted and re-melted twice in order to attempt homogeneity. The buttons were cut in half, mounted and polished to 0.25 mm. The polished samples were examined were under the optical microscope and by scanning electron microscopy (Jeol JSM-840, Jeol JSM-840A). The compositions of the phases were determined by energy dispersive X-ray spectroscopy (EDS) using elemental standards. X-ray diffrac-
J. Hohls et al. / Journal of Alloys and Compounds 308 (2000) 205 – 215
207
tion was undertaken using Mo K a radiation in order to identify the phase structures.
B2 and RuAl 2 , and here the B2 phase had a different composition from the dendrites. The XRD spectrum clearly showed the presence of B2 and RuAl 2 .
4. Results
4.1.3. Al51 : Ni19 : Ru30 The |Al 51 :Ni 19 :Ru 30 alloy was composed mainly of cored B2 and a little B21RuAl 2 eutectic. The XRD analysis confirmed the presence of the B2 phase.
The compositions and morphologies of the alloys studied are given in the following section, with the EDS analyses summarised in Table 1.
4.1. Alloys with primary phase B2 4.1.1. Al51.5 : Ni3.5 : Ru45 and Al55 : Ni43 : Ru2 The alloys |Al 51 :Ni 3.5 :Ru 45.5 and |Al 55 :Ni 43 :Ru 2 were composed entirely of B2 with very little variation in composition. XRD analysis confirmed the presence of the B2 phase. 4.1.2. Al56 : Ni7 : Ru37 The |Al 56 :Ni 7 :Ru 37 alloy comprised dendrites of B2 with RuAl 2 . There were also areas of a eutectic mixture of
4.1.4. Al58 : Ni14.5 : Ru27.5 The |Al 58 :Ni 14.5 :Ru 27.5 alloy is shown in Fig. 1a,b. The B2 dendrites formed first, and show a subsequent solidstate precipitation, yielding needles. A fine eutectic of RuAl 2 and the darker B2 phase (which had much less Ru than the dendritic B2 phase) can be seen more clearly in Fig. 1b. The composition of the needles could not be verified from the analyses due to their size. The XRD analysis showed clearly the presence of the B2 phase. As very few of the remaining peaks matched with those of other alloys in this investigation containing Ru 4 Al 13 , most of the peaks were assumed to be from
Table 1 Summary of EDS analyses of the phase compositions of the alloys Nominal composition
Phase compositions Al:Ni:Ru (at.%)
Al 51.5 :Ni 3.5 :Ru 45
51.3:3.6:45.1
Al 56 :Ni 7 :Ru 37 Al 51 :Ni 19 :Ru 30
56.3:6.6:37.1 51.2:18.7:30.1
Al 58 :Ni 14.5 :Ru 27.5
58.0:14.5:27.5
Al 60 :Ni 35 :Ru 5 Al 64 :Ni 31 :Ru 5 Al 55 :Ni 43 :Ru 2
60.4:34.6:5.0 64:31:5 55.2:43.2:1.6
Al 64.5 :Ni 13.5 :Ru 22
64.6:13.4:22
Al 59 :Ni 23 :Ru 18 Al 65 :Ni 24.5 :Ru 10.5 Al 71 :Ni 15 :Ru 14
59:23:18 64.7:24.6:10.7 71.3:14.7:14.0
Al 76 :Ni 17.5 :Ru 6.5 Al 81 :Ni 6 :Ru 13 Al 83 :Ni 11 :Ru 6 Al 65 :Ni 28 :Ru 7 Al 74 :Ni 21.5 :Ru 4.5
76.2:17.3:6.5 80.7:6.2:13.1 83:11:6 65.2:27.9:6.9 74.0:21.5:4.5
75.6:23.6:0.8
60.3:38.9:0.8 61.0:38.5:0.5
Al 69 :Ni 28 :Ru 3
69:28:3
76.3:22.9:0.8
62.1:37.3:0.6
Al 72 :Ni 26 :Ru 2
72:26:2
62.0:37.4:0.6
Al 80 :Ni 16 :Ru 4 Al 85 :Ni 12 :Ru 3 Al 91.5 :Ni 7 :Ru 1.5
79.9:16.0:4.1 85.5:11.6:2.9 91.5:7.0:1.5
74.3:24.5:1.2 75.8:23.4:0.8 75.6:23.3:1.1 72.4:21.8:5.8 75.7:23.8:0.5
Al 91.5 :Ni 6.5 :Ru 2
91.7:6.5:1.8
79:19:2
Al 92 :Ni 7.5 :Ru 0.5
92.3:7.2:0.5
76.3:23.1:0.6
Overall
NiAl 3
Ni 2 Al 3
B2
Ru 4 Al 13
50.5:3.0:46.5 51.7:4.7:43.6 54:5:41 50.5:17.7:31.8 53.6:24.7:21.7 53.5:13.3:33.2 53.6:40:6.4 60.1:39.0:0.9 61.0:38.3:0.7
75.7:23.3:1.0 76.3:22.8:0.9 75.3:23.6:1.1
|RuNi 2 Al 14
65.8:0.5:33.7
66.1:1.0:32.9 72.4:10.9:16.7 74:9:17
56:41:3 55.7:42.6:1.7 58.6:36.0:5.4 61:30:9 56:37:7 57.3:38.5:4.2 60.2:38.9:0.9
RuAl 2
72.4:9.1:18.5
72.7:9.7:17.6 72.3:10.4:17.3 73.9:9.4:16.7 75.1:7.4:17.5 76.0:1.4:22.6 75.7:6.0:18.3 72.5:10.5:17.0
72:17:11
65.4:1:33.6 65.2:1.6:33.2 65.6:2.2:32.2 68.8:20.7:10.5 72.4:21.8:5.8 82.2:12.7:5.1 71.7:17.8:10.5 74.3:16.5:9.2 72.5:21.7:5.8 73.8:14.0:12.2 74.1:15.6:10.3 73.9:16.6:9.5 73.4:19.7:6.9 78:13:9 79.1:13.4:7.5 77.0:12:11 82.7:13.6:3.7 77.9:11.0:11.1 85:11:4
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of slightly different d-spacings were superimposed, and hence the B2 phase was deduced to be present.
4.1.6. Al64 : Ni31 : Ru5 The |Al 64 :Ni 31 :Ru 5 alloy is shown in Fig. 2. A supersaturated B2 phase formed first, followed by the peritectic formation of the dark Ni 2 Al 3 phase. Then, the coarse Ni 2 Al 3 and Ru 4 Al 13 eutectic formed, followed by the peritectic formation of the ternary phase |RuNi 2 Al 14 from Ru 4 Al 13 . Subsequently, the supersaturated B2 phase precipitated Ru 4 Al 13 . XRD analysis of this alloy very clearly showed the presence of Ni 2 Al 3 , while the major Ru 4 Al 13 peak was evident, but very small. There was a possible indication of the ternary |RuNi 2 Al 14 phase. It should be noted that the Ni 2 Al 3 peaks showed some shift. The shapes of the peaks indicated that both B2 and Ni 2 Al 3 were present. 4.2. Alloys with RuAl2 primary phase
Fig. 1. (a) SEM image in backscattered electron mode of the |Al 58 :Ni 14.5 :Ru 27.5 alloy, showing the B2 dendrites (grey) with a coarse eutectic comprising RuAl 2 (light) and B2 (dark). Bar represents 10 mm. (b) SEM image in backscattered electron mode of the |Al 58 :Ni 14.5 :Ru 27.5 alloy, shows more clearly the fine eutectic RuAl 2 (light)1B2 (dark) and needles formed by later solid state precipitation. Bar represents 10 mm.
4.2.1. Al64.5 : Ni13.5 : Ru22 and Al59 : Ni23 : Ru18 Fig. 3 shows the alloy |Al 64.5 :Ni 13.5 :Ru 22 , with the primary dendrites of RuAl 2 , and peritectically formed Ru 4 Al 13 . Finally, a coarse eutectic of RuAl 2 and supersaturated B2 formed, followed by some local solid-state precipitation of Ru 4 Al 13 in B2. XRD analysis confirmed the presence of RuAl 2 and there were many peaks corresponding to those deduced to be Ru 4 Al 13 in other alloys. In addition, there was a fairly good fit for the B2 peaks. The |Al 59 :Ni 23 :Ru 18 alloy was similar to the previous alloy, but without the solid state precipitation of Ru 4 Al 13 . XRD confirmed the B2 and RuAl 2 phases. 4.2.2. Al65 : Ni24.5 : Ru10.5 The |Al 65 :Ni 24.5 :Ru 10.5 alloy is shown in Fig. 4. The RuAl 2 phase formed first, followed by the peritectic formation of Ru 4 Al 13 and, lastly, a eutectic of B2 and
RuAl 2 or B2. Data for RuAl 2 [21] are available only for low angles, where they show good agreement.
4.1.5. Al60 : Ni35 : Ru5 The |Al 60 :Ni 35 :Ru 5 alloy solidified with the supersaturated B2 phase initially, followed by the peritectic formation of Ni 2 Al 3 , and finally a eutectic between Ni 2 Al 3 and Ru 4 Al 13 . Subsequently, there was solid state precipitation of Ru 4 Al 13 within the B2 phase. XRD analysis of this alloy clearly indicated the presence of Ni 2 Al 3 . A small peak at the position of the major Ru 4 Al 13 peak (observed in the Al 81 :Ni 6 :Ru 13 sample, which was mainly Ru 4 Al 13 ) showed that some Ru 4 Al 13 was also present. Unfortunately, all the B2 peaks have similar positions to Ni 2 Al 3 peaks, making it difficult to show definitively the presence of B2. However, the shape and relative heights of these peaks revealed that two peaks
Fig. 2. SEM image in backscattered electron mode of the |Al 64 :Ni 31 :Ru 5 alloy, showing B2 (dark with lighter precipitation within), Ru 4 Al 13 (light), Ni 2 Al 3 (dark) and |RuNi 2 Al 14 (light grey). Bar represents 10 mm.
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4.3.2. Al76 : Ni17.5 : Ru6.5 The |Al 76 :Ni 17.5 :Ru 6.5 alloy formed Ru 4 Al 13 needles first, then |RuNi 2 Al 14 by a peritectic reaction. Another peritectic reaction produced NiAl 3 , and the last reaction was a degenerate eutectic comprising (Al)1NiAl 3 . The fact that there were four phases present indicates nonequilibrium cooling of the sample, which can be expected from an arc-melted sample. The XRD spectrum gave a good fit for NiAl 3 , and the assumed ternary peaks corresponded well with those of other alloys. There was some evidence for the presence of Ru 4 Al 13 . The proportion of (Al) in the sample was too low to register any significant peaks. Fig. 3. SEM image in backscattered electron mode of the |Al 64.5 :Ni 13.5 :Ru 22 alloy, showing RuAl 2 (light), Ru 4 Al 13 (grey), and B2 (dark) with eutectic RuAl 2 (light)1B2 (dark). Bar represents 10 mm.
RuAl 2 . XRD analysis showed that this alloy probably contained B2 (more rich in NiAl than RuAl), and possibly RuAl 2 , but there were very few peaks available as data for Ru 4 Al 13 for comparison. The uncertainty regarding the RuAl 2 phase is due to the low concentration of this phase in the alloy.
4.3. Alloys with primary phase Ru4 Al13 4.3.1. Al71 : Ni15 : Ru14 The |Al 71 :Ni 15 :Ru 14 alloy solidified to form cored Ru 4 Al 13 , with a peritectic reaction forming the ternary phase |RuNi 2 Al 14 around it. There was also a eutectic comprising Ni 2 Al 3 1 |RuNi 2 Al 14 . XRD analysis for this alloy indicated the presence of Ni 2 Al 3 and Ru 4 Al 13 , although the peaks for the latter are not very clear. A few peaks indicated that |RuNi 2 Al 14 was present, by comparison with spectra from the other alloys containing that phase.
Fig. 4. SEM image in backscattered electron mode of the |Al 65 :Ni 24.5 :Ru 10.5 alloy, showing RuAl 2 (light), Ru 4 Al 13 (grey), and B2 (dark), with a eutectic of RuAl 2 1B2. Bar represents 10 mm.
4.3.3. Al81 : Ni6 : Ru13 In the |Al 81 :Ni 6 :Ru 13 alloy, it is apparent that the Ru 4 Al 13 needles formed first, followed by a peritectic reaction to form NiAl 3 . Finally, there was a degenerate eutectic (Al)1NiAl 3 . The XRD analysis confirmed NiAl 3 and (Al), although the peaks were small due to the low proportions of these phases. Ru 4 Al 13 is the major phase, and the peaks not corresponding to NiAl 3 or (Al) were assumed to result from this phase. These peaks were used to compare with other alloys containing this phase in smaller amounts. 4.3.4. Al83 : Ni11 : Ru6 The |Al 83 :Ni 11 :Ru 6 alloy is shown in Fig. 5. The Ru 4 Al 13 dendrites formed first, followed by a peritectic reaction giving NiAl 3 . There was a eutectic of (Al)1 NiAl 3 , which was not always discernable in some areas. The XRD scan indicated the presence of (Al), and some peaks corresponded well with the assumed ternary peaks of other alloys, indicating |RuNi 2 Al 14 . The peaks for NiAl 3 were not as intense as would be expected, considering the amount of this phase present. The peaks assumed to
Fig. 5. SEM image in backscattered electron mode of the |Al 83 :Ni 11 :Ru 6 alloy, showing Ru 4 Al 13 dendrites (light), NiAl 3 (grey), with a sparse eutectic of NiAl 3 1(Al) (dark). Bar represents 10 mm.
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correspond to Ru 4 Al 13 are also not a very good match, but this is likely to be a result of the fairly high percentage of Ni (|6%) present in this phase, which would shift them from the binary values.
4.4. Alloys with primary phase Ni2 Al3 4.4.1. Al65 : Ni28 : Ru7 The |Al 65 :Ni 28 :Ru 7 alloy is shown in Fig. 6a,b. Supersaturated Ni 2 Al 3 formed first and subsequently experienced solid state precipitation of Ru 4 Al 13. There was also a coarse eutectic between these phases. The |RuNi 2 Al 14 phase, which formed peritectically from Ru 4 Al 13 was present only in small and sparse proportions. The lamellar cellular precipitation products in the primary phase can be seen more clearly in Fig. 6b. XRD analysis showed good peaks for Ni 2 Al 3 , and indicated a small amount of the ternary phase. The remaining peaks appear to correspond to Ru 4 Al 13 .
4.4.2. Al74 : Ni21.5 : Ru4.5 In nominal |Al 74 :Ni 21.5 :Ru 4.5 , the Ni 2 Al 3 phase formed first, then |RuNi 2 Al 14 formed peritectically and was cored. The NiAl 3 phase appeared to have formed either peritectically or eutectically. There was also a sparse eutectic of (Al)1NiAl 3 . The ternary phase and NiAl 3 were the major phases, and both showed strong peaks in the XRD spectrum. The ternary phase peaks were taken to be those which agreed with those of other alloys containing the |RuNi 2 Al 14 phase. Some peaks indicated the presence of Ni 2 Al 3 . There was very little evidence of (Al), as a result of the low proportion of (Al) in the sample. 4.4.3. Al69 : Ni28 : Ru3 The |Al 69 :Ni 28 :Ru 3 alloy formed Ni 2 Al 3 first, followed by the peritectic formation of |RuNi 2 Al 14 , which was cored. The NiAl 3 phase formed a degenerate eutectic with the ternary phase. XRD analysis of this alloy confirmed the presence of all three phases. 4.4.4. Al72 : Ni26 : Ru2 The Ni 2 Al 3 phase formed first in the |Al 72 :Ni 26 :Ru 2 alloy, followed by the peritectic formation of |RuNi 2 Al 14 , and the latter was cored. The NiAl 3 phase formed a degenerate eutectic with the ternary phase. There was some variation in the shading of this dark phase, suggesting coring. The XRD analysis of this alloy also confirmed the presence of all three phases: Ni 2 Al 3 , |RuNi 2 Al 14 , and NiAl 3 . 4.5. Alloys with | RuNi2 Al14 primary phase 4.5.1. Al80 : Ni16 : Ru4 In the |Al 80 :Ni 16 :Ru 4 alloy, the |RuNi 2 Al 14 phase formed initially, followed by a peritectic reaction to form NiAl 3 . The (Al) phase formed last as a degenerate eutectic with NiAl 3 . The XRD spectrum obtained from this alloy was not very clear, and NiAl 3 , the major component of the system, yielded a much smaller signal than expected. Both the ternary phase and the (Al) phase were present in small amounts. 4.5.2. Al85 : Ni12 : Ru3 In |Al 85 :Ni 12 :Ru 3 , the |RuNi 2 Al 14 phase formed first, and then NiAl 3 formed peritectically. There was a degenerate eutectic of (Al) and NiAl 3 . The presence of (Al) and NiAl 3 was clearly confirmed by the XRD spectrum. Peaks corresponding to those present in other alloys containing the ternary phase also indicated the presence of |RuNi 2 Al 14 .
Fig. 6. (a) SEM image in backscattered electron mode of the |Al 65 :Ni 28 :Ru 7 alloy, showing Ru 4 Al 13 (light), Ni 2 Al 3 (dark), and |RuNi 2 Al 14 (light grey), with a coarse eutectic Ru 4 Al 13 1Ni 2 Al 3 . Bar represents 10 mm. (b) SEM image in backscattered electron mode of the |Al 65 :Ni 28 :Ru 7 alloy, showing the very fine cellular precipitation of Ru 4 Al 13 (light) within Ni 2 Al 3 (dark). Bar represents 1 mm.
4.5.3. Al91.5 : Ni7 : Ru1.5 The microstructure of the |Al 91.5 :Ni 7 :Ru 1.5 alloy is shown in Fig. 7. The |RuNi 2 Al 14 phase solidified first with coring, and next was a peritectic reaction forming
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gave a good fit for (Al) and showed peaks which may correspond to the ternary phase |RuNi 2 Al 14 .
4.6. Alloys with NiAl3 primary phase 4.6.1. Al92 : Ni7.5 : Ru0.5 The |Al 92 :Ni 7.5 :Ru 0.5 alloy formed NiAl 3 dendrites in an NiAl 3 1(Al) eutectic. XRD analysis of the alloy gave a good fit for (Al), and a less favourable fit for the NiAl 3 phase.
5. Discussion
Fig. 7. SEM image in backscattered electron mode of the |Al 91.5 :Ni 7 :Ru 1.5 alloy, showing |RuNi 2 Al 14 (light grey), NiAl 3 (medium grey) and NiAl 3 1(Al) (dark) eutectic. Bar represents 10 mm.
NiAl 3 (which exhibits similar grey contrast to |RuNi 2 Al 14 ). Lastly, there was a eutectic (Al)1NiAl 3. XRD analysis clearly showed the presence of (Al) and, from comparison with other alloys containing the ternary phase, the |RuNi 2 Al 14 phase also appeared to be present. There was some indication of peaks corresponding to NiAl 3 , although these were slightly shifted.
4.5.4. Al91.5 : Ni6.5 : Ru2 The |Al 91.5 :Ni 6.5 :Ru 2 alloy was similar to the previous alloy, but was locally inhomogeneous, showing a small region with the peritectic formation of NiAl 3 from |RuNi 2 Al 14 . The fine eutectic observed was probably (Al)1 |RuNi 2 Al 14 , although the morphology is very similar to the (Al)1NiAl 3 eutectic, as well as to the (Al)1 | RuNi 2 Al 14 eutectic of Horner et al. [14]. A very small amount of a light grey phase of different composition to the light grey dendrites was found and is inferred, from EDS, to be NiAl 3 , but this is probably formed as a result of localised inhomogeneity. XRD analysis of the alloy
NiAl 3 consistently gave a weak signal, even when a large proportion of the phase was present. The X-ray spectra peaks of samples containing the ternary |RuNi 2 Al 14 phase were compared, and the remaining unidentified peaks were assumed to belong to this phase (Table 2). The differences are due to the different compositions of the individual phases in the specimens giving some shift in the peaks. The data for Ru 4 Al 13 from Edshammer in the JC-PDS database [21] are only for low angles, and were thus of limited use in confirming the presence of this phase. The Al 81 :Ni 6 :Ru 13 specimen had a very high proportion of Ru 4 Al 13 , so this was used as a basis for comparison with the X-ray spectra of other alloys after removing the peaks of the other known phases. The Crystallographica package [22] was also used to model the peaks of Ru 4 Al 13 , and the modelled spectrum showed good agreement with that of Al 81 :Ni 6 :Ru 13 . Table 3 lists the peaks identified to be due to Ru 4 Al 13 and their deduced intensities (after compensating for peaks of other phases). There was some difficulty with the third large peak ˚ because it was consistently superposed on (|2.070 A) high-intensity peaks of other phases. Data for the Al 64 :Ni 31 :Ru 5 and Al 60 :Ni 35 :Ru 5 alloys were not included because, having lower proportions of Ru 4 Al 13 , the signals were not good. There is reasonable agreement for the
Table 2 d-Spacings and relative intensities (as percentage of highest peak) of peaks originating from the ternary phase |RuNi 2 Al 14 ˚ Relative d-Spacing (A) intensity Al 76 :Ni 17.5 :Ru 6.5 Al 85 :Ni 12 :Ru 3 Al 74 :Ni 21.5 :Ru 4.5 Al 83 :Ni 11 :Ru 6 Al 91.5 :Ni 7 :Ru 1.5 Al 91.5 :Ni 6.5 :Ru 2 Al 65 :Ni 28 :Ru 7 Al 71 :Ni 15 :Ru 14 Al 64 :Ni 31 :Ru 5 Al 72 :Ni 26 :Ru 2 Al 69 :Ni 28 :Ru 3 100 20 |5–15 |5 |5–15 |6 |2–14 |8–15 |9–18 |8–15 |1–9
2.062
2.062
2.055
1.565
1.574
1.524 1.467 1.365 1.344
1.530 g 1.368 1.346 i
1.572 1.562 1.532 1.466 1.362 1.348
1.058
1.060 1.029
1.064 1.029
2.089 1.594 a 1.574 b 1.546 c
2.060
2.058
2.060
2.066
1.568 1.542 1.530
1.571
1.565 1.545
1.367 d 1.346 e
1.358 h 1.346 j
1.068 f
1.066 k
1.357 1.346 1.140 1.067 1.029 l
1.530
2.063
2.053
1.563
1.561
1.566
1.533 1.501
1.531 1.470 1.361 1.346 1.137 1.063 1.029
1.530 1.479 1.359 1.344 1.137 1.060 1.027
1.363 1.345
1.344
1.067 1.029
1.067 1.032
2.073
1.077
The superscripts denote peaks for which the relative intensity differed from those found in the other alloys. The relative intensities of the marked peaks are: (a) 21, (b) 39, (c) 21, (d) 21, (e) 11, (f) 25, (g) 38, (h) 15, (i) 37, (j) 83, (k) 19 and (l) 16.
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Table 3 ˚ and relative intensities (as percentage of highest peak) of peaks originating from the phase based on Ru 4 Al 13 d-Spacing (A) Alloy composition and nickel content of Ru 4 Al 13 (at.%) Al 81 :Ni 6 :Ru 13 (1.4 at.% Ni)
3.787 (50) 3.321 3.192 3.029 2.652 2.416
(45) (20) (20) (15) (15)
2.300 2.275 2.184 2.149 2.122 2.098 2.025 1.987 1.896 1.813 1.783 1.732 1.659 1.625 1.530 1.516 1.478 1.438 1.410
(,10) (,10) (25) (50) (80) (100) (25) (15) (10) (15) (10) (15) (10) (,10) (25) (60) (25) (15) (15)
1.322 (15) 1.274 (60)
1.224 1.205 1.118 1.101
(20) (25) (15) (20)
1.045 (,10) 0.9787 (15) 0.9697 (15) 0.9158 (15) 0.8283 (,10) 0.7950 (10) a
Al 83 :Ni 11 :Ru 6 (6 at.% Ni)
Al 76 :Ni 17.5 :Ru 6.5 (7.4 at.% Ni)
3.923 3.757 3.467 3.311 3.166 3.029 2.664 2.420 2.375 2.299 2.265
3.882 (50) 3.769 (20)
(80) (80) (10) (15) (10) (,10) (15) (20) (15) (,10) (,10)
2.114 (100) 2.023 (80)
Al 64.5 :Ni 13.5 :Ru 22 (9.1 at.% Ni)
Al 65 :Ni 24.5 :Ru 10.5 (9.7 at.% Ni)
Al 71 :Ni 15 :Ru 14 (9.4 / 10.4 a at.% Ni)
Al 65 :Ni 28 :Ru 7 (10.5 at.% Ni)
3.760 (40) 3.468 (20)
3.721 (15)
3.868 (30) 3.734 (15) 3.604 (15)
3.843 (10) 3.651 (10)
3.179 (,10) 3.051 (10) 3.006 (,10)
3.172 (30)
3.164 (15)
3.146 (20)
3.273 (15) 3.011 (20) 2.401 (20)
2.394 (10) 2.309 (10)
2.156 2.125 2.062 2.042 2.003
(30) (20) (100) (100) (20)
2.219 2.156 2.093 2.047
(20) (20) (100) (100)
2.141 (80) 2.073 (100) 2.032 (100)
2.177 2.127 2.085 2.066
(15) (60) (100) (100)
2.132 (80) 2.075 (100) 2.020 (100)
1.888 (20) 1.804 (,10)
1.653 (10) 1.632 (10) 1.530 (15)
1.453 (20)
1.325 1.278 1.264 1.247
(,10) (20) (10) (20)
1.115 (10) 1.093 (20)
1.653 (20) 1.625 (,10) 1.531 (20) 1.524 1.467 1.427 1.412
(20) (20) (40) (25)
1.323 1.278 1.262 1.244 1.234 1.204 1.118 1.106
(,10) (,10) (10) (10) (10) (,10) (,10) (10)
1.044 (,10)
1.325 1.271 1.250 1.236
(,10) (,10) (30) (20)
1.205 (10) 1.117 (30) 1.096 (10)
1.078 (30)
0.9107 (15)
1.544 (15) 1.512 (15) 1.484 (25) 1.406 1.364 1.321 1.284 1.250
(25) (10) (,10) (,10) (40)
1.545 (10) 1.504 (10) 1.466 (20) 1.446 (25) 1.402 (15)
1.287 (,10)
1.325 (20) 1.280 (30) 1.247 (40)
1.246 (15) 1.202 (,10) 1.174 (60) 1.101 (10) 1.089 (10) 1.080 (10) 1.069 (10) 1.057 (10) 1.050 (10) 0.9744 (,10) 0.9085 (10)
1.193 (15)
1.194 (15) 1.166 (70)
1.077 (10) 1.078 (10) 1.055 (10) 1.048 (20) 1.011 (20) 0.9030 (20)
0.8274 (10)
Cored.
different samples, taking into account the varying Ni contents and the fact that since the phase is not cubic, the peak order might change as the shape of the unit cell changes. Fig. 8 shows a solidification projection of the Al–Ni–Ru ternary above 50 at.% Al. Some analyses need to be extrapolated back to the actual phase and extrapolated
away from the composition of the precipitated phase. This is because the analyses were undertaken on phases in which solid-state precipitation had occurred, and thus the results were affected by the presence of the second phase. The asterisks represent the overall eutectic compositions where analysis was possible. The boundary of the |RuNi 2 Al 14 phase has been drawn with a dotted line
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Fig. 8. Solidification projection of the Al–Ni–Ru ternary above 50 at.% Al. Dotted lines represent tie-lines; solid squares represent overall analyses of specimens; solid circles represent phases analysed; asterisks represent multi-phase compositions; and question marks represent analyses of phases within which were solid-state precipitations.
because some of the analyses were on more irregular morphologies and could have been affected by surrounding phases, rendering the boundary uncertain. It can be seen that all of the phase boundaries agree with those of Horner et al. [14], except for the ternary |RuNi 2 Al 14 phase, which has a far greater extent than previously reported. The extent of the new phase boundary for |RuNi 2 Al 14 is between |7–11 at.% Ru and |70–81 at.% Al. The |RuNi 2 Al 14 phase shows a wider range of compositions because of coring. From the microstructures and morphologies of the alloys, the solidification sequences were deduced as described in the results section. The reactions are listed in Table 4, together with the nominal alloy compositions. In addition, Table 5 details the alloys in which solid-state precipitation occurred due to supersaturation of the primary phase. These reactions are not likely to be equilibrium reactions. From these solidification reactions, the following ternary invariant reactions can be inferred, where the numbers indicate the invariant reactions marked on Fig. 9: L 1 Ru 2 Al 3 → B2 1 RuAl 2
(1)
L 1 B2 1 Ni 2 Al 3 → Ru 4 Al 13
(2)
L 1 Ru 4 Al 13 → B2 1 RuAl 2
(3)
L 1 Ni 2 Al 3 1 Ru 4 Al 13 → | RuNi 2 Al 14
(4)
L 1 Ni 2 Al 3 → | RuNi 2 Al 14 1 NiAl 3
(5)
L 1 RuAl 6 → Ru 4 Al 13 1 (Al)
(6)
L 1 Ru 4 Al 13 → | RuNi 2 Al 14 1 (Al)
(7)
L 1 | RuNi 2 Al 14 → NiAl 3 1 (Al)
(8)
The reactions labelled (2) and (3) disagree with those of Horner et al. [14], whereas the others agree. Using the equations above, the overall alloy compositions and the primary phases associated with them, together with the solidification sequence shown in Table 4, a liquidus surface was drawn. This is given in Fig. 9. In order for the reactions observed in the |Al 64.5 :Ni 13.5 :Ru 22 alloy to occur, there must be a minimum between the junction of the B2 and RuAl 2 liquidus surfaces at approximately Al 59 :Ni 27 :Ru 14 . The |Al 91.5 :Ni 6.5 :Ru 2 and |Al 92 :Ni 7.5 :Ru 0.5 alloys indicate that near the higher Al content boundary, the direction of maximum slope of the |RuNi 2 Al 14 liquidus surface is parallel to its length. This liquidus surface is similar to that of Horner et al. [14], although there are some differences. Firstly, the liquidus surface of the ternary phase has a greater extent than previously detailed. Horner et al. [14] reported a range of |85–95 at.% Al and |0.5–4.5 at.% Ru. However, in this study, the range was found to exist from |75–95 at.% Al and |2–5 at.% Ru. This investigation was based on more alloys, and thus gives a more complete study. Secondly,
J. Hohls et al. / Journal of Alloys and Compounds 308 (2000) 205 – 215
214 Table 4 Reaction sequences in the alloys Alloy
Reaction sequence
Al 56 :Ni 7 :Ru 37 Al 51 :Ni 19 :Ru 30 Al 58 :Ni 14.5 :Ru 27.5 Al 59 :Ni 23 :Ru 18
L→B21RuAl 2
Al 60 :Ni 35 :Ru 5
L1B2→Ni 2 Al 3 L→Ni 2 Al 3 1Ru 4 Al 13
Al 64 :Ni 31 :Ru 5
L1B2→Ni 2 Al 3 L→Ni 2 Al 3 1Ru 4 Al 13 L1Ru 4 Al 13 → |RuNi 2 Al 14
Al 64.5 :Ni 13.5 :Ru 22
L1RuAl 2 →Ru 4 Al 13 L→RuAl 2 1B2 (supersaturated)
Al 65 :Ni 24.5 :Ru 10.5
L1RuAl 2 →Ru 4 Al 13 L→RuAl 2 1B2
Al 71 :Ni 15 :Ru 14
L1Ru 4 Al 13 → |RuNi 2 Al 14 L→ |RuNi 2 Al 14 1Ni 2 Al 3
Al 76 :Ni 17.5 :Ru 6.5
L1Ru 4 Al 13 → |RuNi 2 Al 14 L1 |RuNi 2 Al 14 →NiAl 3 L→NiAl 3 1(Al)
Al 81 :Ni 6 :Ru 13
L1Ru 4 Al 13 → |RuNi 2 Al 14 L1 |RuNi 2 Al 14 →NiAl 3 (|RuNi 2 Al 14 completely consumed) L→NiAl 3 1(Al)
Al 83 :Ni 11 :Ru 6
L1Ru 4 Al 13 → |RuNi 2 Al 14 L1 |RuNi 2 Al 14 →NiAl 3 L→NiAl 3 1(Al) (limited amount)
Al 65 :Ni 28 :Ru 7
L→Ni 2 Al 3 1Ru 4 Al 13 L1Ru 4 Al 13 → |RuNi 2 Al 14 (small amount)
Al 74 :Ni 21.5 :Ru 4.5
L1Ni 2 Al 3 → |RuNi 2 Al 14 (cored) L1 |RuNi 2 Al 14 →NiAl 3 OR L→ |RuNi 2 Al 14 1NiAl 3 L→NiAl 3 1(Al) (limited amount)
Al 69 :Ni 28 :Ru 3 Al 72 :Ni 26 :Ru 2
L1Ni 2 Al 3 → |RuNi 2 Al 14 L→ |RuNi 2 Al 14 1NiAl 3
Al 80 :Ni 16 :Ru 4 Al 85 :Ni 12 :Ru 3 Al 91.5 :Ni 7 :Ru 1.5
L1 |RuNi 2 Al 14 →NiAl 3 L→NiAl 3 1(Al)
Al 91.5 :Ni 6.5 :Ru 2
L→ |RuNi 2 Al 14 1(Al)
Al 92 :Ni 7.5 :Ru 0.5
L→NiAl 3 1(Al)
the RuAl 2 liquidus surface was found to extend to lower Ru compositions, particularly for compositions with higher Ni content. Horner et al. [14] reported the lower boundary Table 5 Solid-state precipitation in the alloys Alloy
Solid-state reaction
Al 58 :Ni 14.5 :Ru 27.5
Ru 4 Al 13 or RuAl 2 precipitated in B2
Al 60 :Ni 35 :Ru 5 Al 64 :Ni 31 :Ru 5 Al 64.5 :Ni 13.5 :Ru 22
Ru 4 Al 13 precipitated in B2
Al 65 :Ni 28 :Ru 7
Ru 4 Al 13 precipitated in Ni 2 Al 3
of the RuAl 2 liquidus surface to be approximately 15 at.% Ru, whereas the results of the present study indicate a minimum of |10 at.% Ru. Finally, the boundary of the Ni 2 Al 3 liquidus surface no longer adjoins the RuAl 2 liquidus surface. Rather, the Ru 4 Al 13 liquidus surface extends to join the B2 boundary and lies between the surfaces of Ni 2 Al 3 and RuAl 2 , providing the eutectic between Ni 2 Al 3 and Ru 4 Al 13 . It should be noted that the valley between the |RuNi 2 Al 14 and Ni 2 Al 3 surfaces is peritectic on the more Ni-rich side and becomes eutectic near the ternary invariant point (5). Similarly, the valley between |RuNi 2 Al 14 and NiAl 3 is eutectic at the invariant point (5) and becomes peritectic as the Al content increases. The analysed phase compositions of Wolff and Sauthoff [18] agreed with the primary B2 and RuAl 2 , and were consistent with a eutectic between B2 (of different composition) and RuAl 2 , of this investigation. The two-phase microstructure of Sabariz and Taylor [20] is not in disagreement with the current findings, although their interpretation is. The interdendritic B2 phase is likely to be part of the irregular B21RuAl 2 eutectic found in the alloys |Al 58 :Ni 15 :Ru 27, |Al 65 :Ni 24 :Ru 11 , |Al 56 :Ni 7 :Ru 37 and |Al 59 :Ni 23 :Ru 18 . Since the B2 eutectic component forms at lower temperatures, and hence at different compositions to the primary B2, it is not surprising that there are two apparent B2 phase compositions. It is surprising that Sabariz and Taylor [20] dub their microstructure ‘spinodal’, since these structures are usually so fine that they can generally be resolved only with a transmission electron microscope.
6. Conclusions The study has shown good agreement with Horner et al. [14], except that the liquidus surfaces of RuAl 2 and |RuNi 2 Al 14 are larger and two of the invariant reactions differed. The ternary invariant reactions have been deduced to be: L 1 Ru 2 Al 3 → B2 1 RuAl 2 L 1 B2 1 Ni 2 Al 3 → Ru 4 Al 13 (disagrees with Horner et al. [14]) L 1 Ru 4 Al 13 → B2 1 RuAl 2 (disagrees with Horner et al. [14]) L 1 Ni 2 Al 3 1 Ru 4 Al 13 → | RuNi 2 Al 14 L 1 Ni 2 Al 3 → | RuNi 2 Al 14 1 NiAl 3 L 1 RuAl 6 → Ru 4 Al 13 1 (Al) L 1 Ru 4 Al 13 → | RuNi 2 Al 14 1 (Al)
J. Hohls et al. / Journal of Alloys and Compounds 308 (2000) 205 – 215
215
Fig. 9. Schematic liquidus surface of the Al–Ni–Ru system above 50 at.% Al. The solid squares are the overall compositions of the alloys in this study. The numbers represent the ternary invariant reactions given in the text. The liquidus slopes down from (7) to (8).
L 1 | RuNi 2 Al 14 → NiAl 3 1 (Al)
Acknowledgements The authors would like to acknowledge Impala Platinum for the provision of ruthenium for this study, and the Department of Arts, Culture, Science and Technology (DACST) for their support of this project. The authors are grateful to Dr. J.C. Zhao (GE-CRD) for guidance in this work. The encouragement of I.M. Wolff and assistance of P.J. Hill, S.S. Taylor, T. Biggs, L. Glaner and H.E. Muhuma of Mintek, and the MRSP at the University of the Witwatersrand is much appreciated. This paper is published by permission of Mintek.
References [1] D.M. Dimiduk, D.B. Miracle, C.H. Ward, Mater. Sci. Tech. 8 (1992) 367. [2] F.H. Froes, C. Suryanarayana, W. Quist, E. Lavernia, B.I. Bondarev, N.F. Anoshkin, I.S. Polkin, O.K. Fatkullin, V. Samarov, A.B. Notkin, Key Eng. Mater. 77–78 (1993) 1. [3] William W. Scott Jr. in: T.B. Massalski (Ed.-in-Chief), H. Okamoto, B.R. Subramanian, L. Kacprzak (Eds.), Binary Alloy Phase Diagrams, 2nd Edition, ASM International, OH, 1990.
[4] T.D. Boniface, L.A. Cornish, J. Alloys Comp. 234 (1996) 275. [5] W. Obrowski, Metallwiss. Teck. (Berlin) 17 (1963) 108. [6] S.M. Anlage, P. Nash, R. Ramachandran, J. Schwarz, J. LessCommon Met. 136 (1988) 237. [7] T.D. Boniface, L.A. Cornish, J. Alloys Comp. 233 (1996) 241. [8] V.F. Tsurikov, E.M. Sokolovskaya, E.F. Kazakova, Vesnik Moskovkogo Univ. Khim. 35 (5) (1980) 113. [9] L.A. Petrovoj, in: N.V. Ageeva (Ed.), Diagrammy Sostoyaniya Metallicheskikh Sistem, Vol. 30, Viniti, Moscow, 1985, p. 323. [10] S. Chakravorty, H. Hashim, D.R.F. West, J. Mater. Sci. 20 (1985) 2313. [11] S. Chakravorty, D.R.F. West, Scripta Metall. 19 (1985) 1355. [12] S. Chakravorty, D.R.F. West, J. Mater. Sci. 21 (1986) 2721. [13] I.J. Horner, M.J. Witcomb, L.A. Cornish, Proc. Electron Microsc. Soc. South Africa 25 (1995) 13. [14] I.J. Horner, L.A. Cornish, M.J. Witcomb, J. Alloys Comp. 256 (1997) 213. [15] I.J. Horner, L.A. Cornish, M.J. Witcomb, J. Alloys Comp. 256 (1997) 221. [16] I.J. Horner, L.A. Cornish, M.J. Witcomb, J. Alloys Comp. 264 (1998) 173. [17] D.G. Pettifor, J. Phase Equilibria 17 (5) (1996) 384. [18] I.M. Wolff, G. Sauthoff, Metall. Mater. Trans. A 27A (1996) 1395. [19] A.S. Harte, P.M. Hung, I.J. Horner, N. Hall, L.A. Cornish, M.J. Witcomb, in: J.V. Gilfrich (Ed.), Advances in X-ray Analysis, Vol. 39, Plenum Press, New York, 1997, p. 747. [20] A.L.R. Sabariz, G. Taylor, Mater. Res. Soc. Symp. Proc. 460 (1997) 611. [21] JC-PDS International Centre for Diffraction Data, Version 2.16, Newtown Square, PA 19073, USA, 1995. [22] Crystallographica, Oxford Cryosystems, 1995–1999.