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Equilibrium phases at the Ir-rich corner of the Ir–Nb–Ni–Al quaternary system
Intermetallics 10 (2002) 893–902 www.elsevier.com/locate/intermet
Equilibrium phases at the Ir-rich corner of the Ir–Nb–Ni–Al quaternary system C. Huang*, K. Nishida, Y. Yamabe-Mitarai, H. Harada National Institute for Materials Science (NIMS), Sengen 1-2-1, Tsukuba, Ibaraki 305-0047, Japan Received 21 May 2002; received in revised form 8 July 2002; accepted 9 July 2002
Abstract The equilibrium phases of three Ir–Nb–Ni–Al quaternary alloys at the Ir-rich corner were investigated in some detail. The phases were determined by X-ray profile as well as microstructure observation. Three phases of fcc/L12-Ir3Nb/B2-IrAl were found in two samples, and four phases of fcc/L12-Ir3Nb/L12-Ni3Al/B2-IrAl were characterized in one sample. The electron probe microanalysis (EPMA) technique was applied for analyzing the phase composition of this quaternary system. A partial quaternary phase diagram at the Ir-rich side was plotted according to the EPMA results. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Intermetallics, miscellaneous; B. Alloy design; B. Phase diagram; B. Phase identification
1. Introduction Nickel-base superalloys are the most widely used high-temperature materials in gas turbine engines, and they are now operating at a temperature of 1100 C, which is up to 90% of their melting point. It is clear that the melting point of nickel imposes a natural ceiling to their potential, and, hence, further improvement is limited [1]. However, the demand for materials capable of attaining even higher operating temperatures is increasing because higher temperature can afford the designer much greater flexibility in trading off performance gains against major improvements in engine durability, fuel savings, and overall engine life-cycle costs [2]. Among platinum group metals (PGMs), iridium has an fcc crystal structure and some noticeable properties, such as a high melting point and excellent corrosionresistance, which show superior characteristics for hightemperature application. Yamabe-Mitarai et al. [3–6] developed an ‘‘Ir-base refractory superalloy’’, which is a promising material for ultra high-temperature applications. Binary alloys Ir–x (x=Nb, Zr, Hf, Ti, V. . .) as well as ternary alloys Ir–Nb–x (x=Ni) were systematically * Corresponding author. Tel.: +81-298-59-2527; fax: +81-298-592501. E-mail address: [email protected] (C. Huang).
investigated. The results showed that these alloys had a coherent fcc/L12-Ir3x (x=Nb, Zr, Hf. . .) two-phase structure, which was analogous to that of a Ni-base superalloy, yet with greater high-temperature strength. However, the ductility of these alloys was not satisfactory. The binary and ternary alloys usually presented an inter-granular fracture [7]. In order to improve the ductility, quaternary Ir–Nb– Ni–Al alloys were developed by adding some Al. More than 15 quaternary alloys have been investigated [8–10]. Four alloys were at around the Ir-rich side (the content of Ir was equal to or more than that of Ni), and the remaining ones were around the Ni-rich corner. The results indicated that the quaternary alloys presented an excellent balance of high-temperature strength and ductility both at the Ir-rich and Ni-rich regions. On the Nirich side, the strength at 1200 C was above 100 MPa, three times that of the Ni–Al binary alloy, and more than twice that of commercial Ni-base superalloys MarM247. More promising characteristics came from the two alloys F5 and F10 at the Ir-rich side with a coherent fcc-Ir/L12-Ir3Nb two-phase structure [10]. They exhibited above 1000 MPa high-temperature strength at 1200 C with compressive ductility of more than 14%; both the strength and ductility were higher or larger than those of the Ir-base binary and ternary alloys. For the microstructure of quaternary alloys, it
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was initially thought that combining two kinds of binary alloys, both of which had an fcc/L12 (Ir/Ir3Nb and Ni/Ni3Al) two-phase structure, would yield the fcc/L12 two-phase structure in quaternary alloys. However, the two-phase regions did not connect with each other from the Ir-rich to the Ni-rich side. The fcc/L12 two-phase structure formed at the Ir-rich or Ni-rich corners only, and a three-phase fcc/L12-Ir3Nb/L12-Ni3Al region was observed between the two two-phase regions. One sample even showed four phases of fcc/L12-Ir3Nb/L12Ni3Al/B2-IrAl. The phase constitution affects the mechanical properties of the material; the fcc-Ir/L12Ir3Nb two-phase coherent structure seems to be the most promising in this quaternary system. In order to enhance the understanding of the phase relations, a phase diagram is needed. In the previous investigations, only a schematic phase diagram [11,12] was obtained because of the difficulty in obtaining the phase composition. Previously, energy dispersive analysis X-ray spectroscopy (EDX) was used for analyzing the phase composition of Ir–Nb alloys. However, it was very difficult to obtain reasonable results. The main problem came from the Nb in the Ir-base alloys. An example of peaks detected in an Ir–Nb–Ni–Al quaternary alloy is shown in Fig. 1, and it was obtained by EDX under a low magnification of 50. In this figure, the peak of Ir-M (the X-ray energy was 1.98 keV) was almost completely overlapped by Nb-La (the X-ray energy was 2.17 keV). From the composition result, which could reflect the
Fig. 1. The peaks detected in one Ir–Nb–Ni–Al quaternary alloy by EDX.
average composition of this alloy because of the low magnification used, the Nb content was 28.12 at.%, more than twice the nominal content of 13.5 at.%. In this work, three other Ir–Nb–Ni–Al quaternary alloys at the Ir-rich corner were investigated in order to add more information of the phase construction in the Irrich region because emphasis had been previously given to the alloys at the Ni-rich side. Instead of EDX, the EPMA method was tried to test the phase composition of Ir-Nb quaternary alloys. The phase constituent was discussed, and a phase diagram of the Ir-rich corner at 1400 C was drawn according to the results of EPMA.
2. Experimental procedure Three ingot samples, 20 g for each, were produced by the arc melting method in an Ar atmosphere. The compositions of the sample were 42.75Ir–4.5Nb–42.75Ni– 10Al, 49.5Ir–13.5Nb–27Ni–10Al, and 67.5Ir-13.5Nb– 9Ni–10Al. (The composition expressions were all in atomic percentages.) All the samples have the same atomic Al content of 10%. After being arc-melted, one sample was analyzed by the fluorescence X-ray method to determine its actual composition. It was found to be very close to the nominal value. Cylindrical samples that were 3 mm in diameter and 5 6 mm in length were cut with an electron discharge machine (EDM) from each ingot for heat treatment. Three temperatures of 1250, 1400, and 1600 C were selected for heat treatment of 42.75Ir–4.5Nb–42.75Ni–10Al, four temperatures of 1250, 1400, 1600, and 1800 C were chosen for 49.5Ir–13.5Nb–27Ni–10Al, and five temperatures with an addition of 2000 C were used for 67.5Ir–13.5Nb– 9Ni–10Al. All the heat treatments were conducted in a vacuum furnace for 168 h. The heat-treated samples were polished for X-ray diffraction (XRD) analysis. Scanning electron microscopy (SEM) was applied for microstructure observation. For composition analysis by EPMA, the samples were diamond-polished to a final 0.5 mm without etching. The composition of each phase was analyzed using an accelerating voltage of 20 kV and a current of 5108A. Four standard samples made of pure metals were characterized prior to the analysis. According to the X-ray energy values of peaks of each element, crystals PET and TAP were chosen for detecting Nb and Al, and crystal LiF was applied for probing Ir and Ni. From the results of a standard sample, the Ir-M peak was separated into two peaks of Ir-Ma and Ir-Mb, and these two peaks were clearly differentiated from the Nb-La peak, as shown in Fig. 2. Four obviously distinguished peaks of Ir-La, Nb-La, Ni-Ka, and Al-Ka were selected as the characterized peaks for identifying the elements of Ir, Nb, Ni, and Al in the investigated alloys. The adjacent background of the four peaks was also determined from
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67.5Ir–13.5Nb–9Ni–10Al [Fig. 3(c)]: Three phase peaks of fcc, L12-Ir3Nb, and B2-IrAl were observed in 1250, 1400, and 1600 C heat-treated samples. Two phase peaks of L12-Ir3Nb and B2-IrAl were detected in the 1800 C heat-treated samples, and only the L12Ir3Nb phase peak was observed in the 2000 C heattreated sample. The phases formed at different heattreatment temperatures are summarized in Table 1. In order to obtain a whole understanding of the phase formation at the Ir-rich corner of this quaternary system, the previous results of four quaternary alloys with the composition at the Ir-rich side are also shown there [8–10]. The numbers of the four samples presented in the literature are indicated below the compositions of the samples. 3.2. Microstructure observation
Fig. 2. The peaks detected in pure standard samples by EPMA.
this figure for the background subtraction process. A classical correction program ZAF (atomic number, absorption, and fluorescence) was applied for correcting the results. Nine points were detected for each phase, and the final results were the average value. When the phase size was not large enough for point analysis, a map analysis was used instead.
3. Results 3.1. X-ray profiles The XRD patterns of the three samples at different heat-treatment conditions are shown in Fig. 3. 42.75Ir–4.5Nb–42.75Ni–10Al [Fig. 3(a)]: Four phase peaks of fcc, L12-Ir3Nb, L12-Ni3Al, and B2-IrAl were characterized in 1250 and 1400 C heat-treated samples; however, the fcc and L12-Ni3Al phase peaks were overlapped. In the 1600 C heat-treated sample, the B2 phase disappeared, and the overlapped peaks of the fcc and L12-Ni3Al phases were separated into two peaks. 49.5Ir–13.5Nb–27Ni–10Al [Fig. 3(b)]: Three phase peaks of fcc, L12-Ir3Nb, and B2-IrAl were found in 1250 and 1400 C heat-treated samples. In the 1600 C heat-treated samples, the L12-Ir3Nb and B2-IrAl phases were found; on the other hand, in the 1800 C heattreated sample, only the L12-Ir3Nb phase peak remained.
The microstructures of the samples after being heattreated at 1250, 1400, and 1600 C are shown in Fig. 4. In 42.75Ir–4.5Nb-42.75Ni–10Al, four phases with different contrasts were observed in the 1250 and 1400 C heat-treated samples. The square blocks with the brightest contrast corresponded to the L12-Ir3Nb phase. The matrix was an fcc phase with different partition ratios of Ir and Ni, and the matrix with brighter contrast contained more Ir and less Ni than the matrix with darker contrast. Gray blocks marked the B2 phase. Lots of small precipitates were found within the matrix. In 1250 and 1400 C heat-treated samples, the L12Ir3Nb became coarsened, and small precipitates still existed. Because the size of the precipitates was too small to examine the composition by point analysis in EPMA, the map analysis was used. It was determined to be Ni3Al. A dendritic structure formed in 49.5Ir–13.5Nb–27Ni– 10Al. Three phases were clearly defined in the 1250 and 1400 C heat-treated samples, which were the L12Ir3Nb, B2-IrAl, and fcc-Ni (Ir) phases. The phase with a dendritic structure was L12-Ir3Nb. Between the dendritic arms, the phase showing the gray contrast was B2–IrAl, and the phase with the dark contrast was fccNi (Ir). In the 1600 C heat-treated sample, the coarse L12-Ir3Nb dendritic phase, as well as a few parts of the B2-IrAl phase, could be observed; however, the fccNi(Ir) disappeared, and many holes formed at the position of the original fcc-Ni(Ir) phase. The holes were believed to have formed by the heat treatment. In the 1800 C heat-treated sample, only the single phase of the L12-Ir3Nb was observed, as well as many holes. Three phases were observed in the 1250, 1400, and 1600 C heat-treated samples in 67.5Ir–13.5Nb–9Ni– 10Al. The matrix was the L12-Ir3Nb phase. Around the grain boundary, a smaller area with a brightness contrast was the fcc-Ir(Ni) phase, and the phase with a dark contrast was B2-IrAl. In the 1800 C heat-treated sample, L12-Ir3Nb and B2-IrAl phases were found; however, in
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Fig. 3. The XRD patterns of the samples: (a) 42.75Ir–4.5Nb–42.75Ni–10Al, (b) 49.5Ir–13.5Nb-27Ni–10Al, and (c) 67.5Ir–13.5Nb–9Ni–10Al.
the 2000 C heat-treated sample, only the single phase L12-Ir3Nb remained. Many big holes were observed in samples heat-treated at 1800 and 2000 C. 3.3. Dissolving temperature of the phases The melting temperature (MT) is a very important value, especially for high-temperature materials. The MT range of the alloys could be primarily determined from the heat-treatment process, which was < 1800 C for sample 42.75Ir–4.5Nb–42.75Ni–10Al (which melted
at 1800 C), < 2000 C for sample 49.5Ir–13.5Nb–27Ni– 10Al (which melted at 2000 C), and > 2000 C for sample 67.5Ir-13.5Nb–9Ni–10Al (which stood up to 2000 C). The dissolving temperatures of the phases in alloys could also be defined from X-ray analysis and observation of the microstructure of the samples that were heat-treated at different temperatures. Since some phases that existed in a sample that was heat-treated at low temperature disappeared in a sample that was heattreated at a higher temperature, it was indicated that the dissolving temperature of the phase was between two
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C. Huang et al. / Intermetallics 10 (2002) 893–902 Table 1 Phases detected in samples at different heat-treatment temperatures [8–10] Temperature ( C)
Phase
42.75Ir–4.5Nb– 42.75Ni–10Al
49.5Ir–13.5Nb– 27Ni–10Al
67.5Ir–13.5Nb– 9Ni–10Al
1250
fcc L12-Ir3Nb B2 L12-Ni3Al
* * * *
* * *
* * *
1300
fcc L12-Ir3Nb B2 L12-Ni3Al
1400
1600
1800
2000
fcc L12-Ir3Nb B2 L12-Ni3Al
* * * *
fcc L12-Ir3Nb B2
* *
L12-Ni3Al fcc L12-Ir3Nb B2 fcc
*
L12-Ir3Nb B2
* * *
* *
* * *
*
* *
60Ir–15Nb– 20.8Ni–4.2Al (S750)
40Ir–10Nb– 41.6Ni–8.4Al (S550)
80.7Ir–14.3Nb– 4Ni–1Al (F5)
76.5Ir–13.5Nb– 8.1Ni–1.9Al (F10)
* *
* *
* *
*
* * * *
* *
* *
* *
* *
*
*
*
heat-treatment temperatures. For example, in sample 42.75Ir–4.5Nb–42.75Ni–10Al, the B2-IrAl phase disappeared in the 1600 C heat-treated sample, which meant that the dissolving temperature of the B2–IrAl phase was below 1600 C. In 49.5Ir–13.5Nb–27Ni– 10Al, the fcc-Ir(Ni) phase vanished in the sample heattreated at 1600 C. For 67.5Ir–13.5Nb–9Ni–10Al, it was also the fcc-Ir(Ni) phase that first disappeared in the sample heat-treated at 1800 C. The dissolving temperatures of the fcc phase were below 1600 and 1800 C for samples 49.5Ir–13.5Nb–27Ni–10Al and 67.5Ir– 13.5Nb–9Ni–10Al, respectively. The L12-Ir3Nb phase had the highest dissolving temperature of the group. 3.4. Phase diagram 3.4.1. Previous study of a phase diagram for an Ir-base alloy In this investigation, we attempted to determine an Ir–Nb–Ni–Al quaternary phase diagram at 1400 C around the Ir-rich side. Prior to the determination of the phase diagram, it was necessary to collect reliable data on the phase equilibra existing in the binary subsystems Ir–Nb, Ir–Ni, and Ir–Al and the ternary subsystems Ir– Nb–Ni, Ir–Nb–Al, and Ir–Ni–Al. 3.4.1.1. Binary subsystems. Okamoto [13] had determined the Ir–Nb binary phase diagram. At 1400 C,
around the Ir-rich side, the fcc-Ir phase existed with a maximum Nb solubility of 11%; the L12-Ir3Nb phase occurred from 24 to 32% of Nb, and the fcc-Ir/L12-Ir3Nb two-phase area was between these two single phases. The Ir–Al binary phase diagram had been investigated by Axler et al. [13]. From the phase diagram, the fcc-Ir/B2-IrAl two-phase area occupied a large part, from 52 to 90%, of the Ir at 1400 C, and the B2-IrAl phase area was comparably narrow between 48 and 52% of the Al. The melting temperature of the B2-IrAl phase was above 2000 C. The Ir–Ni binary phase diagram was determined by Yang et al. [13]. A continuous series of fcc-(Ir,Ni) solid solutions was found. The melting temperature showed a nearly linear dependence on the composition. 3.4.1.2. Ternary subsystems. Little information is available on the ternary phase diagram including Ir. Only the isothermal section of the Ir–Nb–Ni system at 1300 C was investigated experimentally by Gu [5]; it is shown here as Fig. 5(a). Around the Ir-rich side, two singlephase (fcc-Ir and L12-Ir3Nb) regions and one two-phase (fcc-Ir/L12-Ir3Nb) region between the two single-phase regions were schematically determined. Because the fcc and L12-Ir3Nb phases are stable [13], no obvious differences in this ternary phase diagram were thought to occur on the edge of these phases between 1300 and 1400 C.
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Fig. 4. The microstructures of the samples (a) 42.75Ir–4.5Nb–42.75Ni–10Al, (b) 49.5Ir–13.5Nb-27Ni–10Al, and (c) 67.5Ir–13.5Nb-9Ni–10Al. (a), (b), and (c) are from the samples heat-treated at 1250 C, (a0 ), (b0 ), (c0 ) are from the samples heat-treated at 1400 C, and (a00 ), (b00 ), and (c00 ) are from the samples heat-treated at 1600 C. (a), (a0 ), (a00 ), (b), (b0 ), and (b00 ) are back scattered imagines, (c), (c0 ), and (c00 ) are secondary electron images.
Fig. 5. Phase diagrams in literatures, (a) Ir–Nb–Ni ternary phase diagram at 1300 C [5], (b) Ir–Nb–Ni–Al quaternary phase diagram at 1400 C [12].
Horyn [14] had studied the Al–Ir–Nb system, but only at the Nb-rich part. The composition range was up to 45 at.% Ir and Al and up to 70 at.% Al and 55 at.% Ir. However, it was not in the composition range of this work.
3.4.1.3. Quaternary system. The schematic phase diagram of Ir–Nb–Ni–Al from the Ir-rich side to the Nirich side at 1400 C was determined by Yu [11,12] and is shown here as Fig. 5(b). Three single-phase [fcc-Ir(Ni), L12-Ir3Nb, and L12-Ni3Al] regions were characterized.
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Two single-phase (L12-Ir3Nb and L12-Ni3Al) parts were linked by the L12-Ir3Nb/L12-Ni3Al two-phase area. Between the single phases of fcc and L12, a two-phase area of fcc-Ir(Ni)/L12-Ir3Nb, a three-phase area of fccIr(Ni)/L12-Ir3Nb/L12-Ni3Al, and a two-phase region of the fcc-Ir(Ni)/L12-Ni3Al area were determined from the Ir-rich side to the Ni-rich side. Although the B2-IrAl phase had been found in the sample S750 (as shown in Table 1), the B2-IrAl phase region was not predicted in the phase diagram.
with 10 at.% of Al. Two samples, 49.5Ir–13.5Nb–27Ni– 10Al and 67.5Ir–13.5Nb–9Ni–10Al, showed three phases of fcc, L12-Ir3Nb, and B2-IrAl, and one sample, 42.75Ir–4.5Nb–42.75Ni-10Al, exhibited four phases of fcc, L12-Ir3Nb, B2-IrAl, and L12-Ni3Al. As the composition of L12-Ni3Al was not known, the phase was not included in this figure.
3.4.2. Quaternary phase diagram investigation The compositions of phases in the heat-treated samples are indicated in Table 2. The composition of the L12-Ni3Al phase in 42.75Ir–4.5Nb–42.75Ni–10Al was unavailable because the phase was too small. According to the EPMA results, the equilibrium quaternary phase diagram at 1400 C was drawn (Fig. 6) showing only the three phases of fcc, L12-Ir3Nb, and B2-IrAl. The method for plotting the point indicating the alloy composition is shown schematically in the upper right part of Fig. 6. For example, the point position indicating the composition of 49.5Ir–13.5Nb– 27Ni–10Al can be determined by the following three steps: (1) drawing the first plane abc parallel to the triangle base plane Ir–Ni–Nb, the distance of the abc plane to the Ir–Ni–Nb plane was 10. Every point on the abc plane indicates an alloy composition with an Al content of 10 at.%; (2) drawing the second plane def parallel to the triangle base plane Ni–Nb–Al, the distance of the def plane to the Ni–Nb–Al plane was 49.5; the abc and def planes intersected at line AB; (3) drawing the third plane ghl parallel to the triangle base plane Ir-Ni-Al, the distance of the ghl plane to the Ir–Ni–Al plane was 13.5; this plane and the abc plane intersected at line CD. Lines AB and CD intersected at point E, which was the indication point of this sample. The points of the three samples were on the shadow plane
4.1. Analysis of the EPMA results
4. Discussion
From the composition results of the phase in Ir–Nb– Ni–Al quaternary alloys obtained by EPMA, the distribution of elements in every phase could be understood. Nb showed a strong preference to partition into the L12-Ir3Nb phase. The partitioning ratio between the fccIr(Ni) and L12-Ir3Nb phases was about 0.1 0.3; moreover, this ratio value presented the same decreasing trend in all three samples when the temperature of the heat treatment was increased, as shown in Fig. 7. This was different from that in the Ir–Nb binary alloy system, where the solubility of Nb in the fcc-Ir phase increased slightly along with the temperature. This meant that the volume faction of the L12-Ir3Nb phase would increase along with the temperature in Ir–Nb– Ni–Al alloys. This outcome was consistent with the coarsening phenomenon of the L12-Ir3Nb phase observed in the microstructure of the samples of 42.75Ir-4.5Nb-42.75Ni-10Al and 49.5Ir–13.5Nb–27Ni– 10Al. The coarsening of the L12-Ir3Nb phase was not obvious in sample 67.5Ir–13.5Nb–9Ni–10Al because the L12-Ir3Nb phase had already occupied a large portion in the sample heat-treated at low temperature. In the B2-IrAl phase, Nb exhibited extremely low solubility. Through all the results, the maximum solubility of Nb in the B2-IrAl phase was 0.3 at.%. This implied that the
Table 2 The composition of every phase in the samples heat-treated at 1250, 1400, and 1600 C 1250 C
1400 C
1600 C
fcc
L12-Ir3Nb
B2
fcc
L12-Ir3Nb
B2
fcc
L12-Ir3Nb
48.5 2.1 42.2 7.2
69.1 21.5 6.4 3.0
42.75Ir–4.5Nb–42.75Ni–10Al
Ir Nb Ni Al
45.7 2.6 42.8 8.8
60.3 12.2 21.2 6.3
45.7 0.0 13.2 41.0
43.5 2.8 44.7 9.1
62.7 15.5 16.4 5.5
45.6 0.2 13.2 41.2
49.5Ir–13.5Nb–27Ni–10Al
Ir Nb Ni Al
24.8 2.7 58.7 13.7
68.7 23.4 5.6 2.4
42.3 0.3 16.3 41.1
24.4 1.4 62.4 11.8
65.4 22.6 9.5 2.5
43.2 0.0 16.4 40.5
67.5Ir–13.5Nb–9Ni–10Al
Ir Nb Ni Al
78.1 4.7 2.5 14.6
75.3 19.0 0.4 5.3
50.8 0.2 3.7 45.4
82.0 4.6 3.6 9.8
74.9 20.4 0.4 4.3
50.1 0.2 3.9 45.8
82.0 3.3 5.0 9.7
1800 C B2
L12-Ir3Nb
71.4 22.1 3.6 2.4
49.6 0.0 3.2 47.1
70.5 22.8 4.3 2.4
76.2 17.0 0.6 6.2
50.6 0.0 3.9 45.5
75.7 15.5 0.7 8.1
2000 C B2
L12-Ir3Nb
50.2 0.0 1.9 47.9
74.6 15.4 0.7 9.2
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Fig. 6. The quaternary phase diagram of Ir–Nb–Ni–Al at Ir-rich side at 1400 C.
B2-IrAl phase only occupied a flat region in the Ir–Nb– Ni–Al quaternary phase diagram, which was almost entirely on the Ir–Ni–Al plane. The Al exhibited a strong preferential partitioning in the B2-IrAl phase. About 70 at.% of all Al partitioned in the B2-IrAl phase. Between the remaining phases of fcc-Ir(Ni) and L12-Ir3Nb, Al showed preferential partitioning into fccIr(Ni). The Ni partitioned mainly in the fcc-Ir(Ni) phase and showed some solution in the B2-IrAl phase. The solubility of other elements in the B2-IrAl and fcc-Ir(Ni) phases greatly affected their dissolving temperatures. Although the B2-IrAl and fcc-Ir phases without alloying element had very high MTs of about 2120 and 2447 C [13], respectively, the dissolving temperatures of these two phases decreased drastically in the quaternary systems. From the results of the composition, the decrement occurred because of the presence of the solution of other elements in the phases. Since the B2-IrAl phase had a very limited solubility of Nb, the
change in the dissolving temperature of the B2-IrAl phase came mainly from the effect of the Ni. A comparison of the B2-IrAl phase in all the three samples indicated that the B2-IrAl phase in 42.75Ir–4.5Nb– 42.75Ni–10Al with a Ni content of about 13 at.% disappeared at 1600 C, the B2-IrAl phase in 49.5Ir– 13.5Nb–27Ni–10Al with about 3 at.% of Ni stood up to 1600 C and disappeared at 1800 C, and the B2-IrAl phase in 67.5Ir–13.5Nb–9Ni–10Al with about 2 at.% of Ni held up to 1800 C. This indicated that the solidus temperature of the B2-IrAl phase decreased with increasing the solution of Ni in the B2-IrAl phase. For the fcc-Ir phase, the solubility of Nb in the fcc phase decreased to less than 5 at.% compared with 10 at.% in the Ir–Nb binary alloys, and the solution of Nb in Ir had little effect on the dissolving temperature of the fccIr phase, according to the Ir–Nb binary phase diagram. Therefore, the decrease of the dissolving temperature was assumed to be due, mainly, to the Ni and Al. From
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901
Al), was found to have four phases of fcc, L12-Ir3Nb, Ni3Al, and B2-IrAl in the 1300 C heat-treated sample, and the B2-IrAl phase disappeared in the 1400 C heattreated sample [8]. In Fig. 6, the position of S550 was near that of the sample 42.75Ir–4.5Nb–42.75Ni–10Al, which also had four phases. This implied that the edge plane of the four-phase region was between sample S550 and 42.75Ir–4.5Nb–42.75Ni–10Al. The sample, S750, had an Al content of 4.2 at.%. The position of the sample in Fig. 6 was significantly below the shadowed plane. Three phases of fcc-Ir(Ni), L12-Ir3Nb, and Ni3Al were found in the sample heat-treated at 1400 C. The lower content of Al in S750 prevented the formation of the B2-IrAl phase. This indicated that the three-phase (fcc-Ir(Ni), L12-Ir3Nb, and Ni3Al) region near the Ir-rich side was at the low part in the phase diagram. 4.3. Some views on alloy development
Fig. 7. Solubility ratio of Nb in fcc-Ir(Ni) phase to that in L12-Ir3Nb phase.
the binary phase diagram of the Ir–Ni, the melting temperature of the fcc-Ir(Ni) phase decreased almost linearly with increasing the Ni. According to this phase diagram and the content of Ir and Ni in the fcc phase in the three alloys, the dissolving temperature was determined to be approximately 1900, 1700, and 2400 C for 42.75Ir–4.5Nb–42.75Ni-10Al,49.5Ir–13.5Nb–27Ni–10Al, and 67.5Ir–13.5Nb–9Ni–10Al, respectively. All these values were higher than those found in the fcc-Ir(Ni) phases of the three samples. The additional decrement was caused by the solution of Al in the fcc phases. In conclusion, the decrease of the dissolving temperature of the B2-IrAl phase was due mainly to the solution of Ni in it, and the combined Ni+Al affected the dissolving temperature of the fcc-Ir(Ni) phase. 4.2. Comparison with the previous results of Ir–Nb–Ni–Al In a previous study, four alloys with a composition around the Ir-rich side were investigated (Table 1). Because the phase composition could not be obtained, the samples were plotted in Fig. 6 only according to their composition. F5 and F10, with lower Al contents of 1 and 1.9 at.%, respectively, were found to have only an fcc/L12-Ir3Nb two-phase structure. In Fig. 6, the positions of F5 and F10 were only slightly above the Ir– Ni–Nb base plane and might have settled in the fcc/L12Ir3Nb two-phase region. These results showed good agreement with those obtained in this work. Another sample, S550, with a higher content of Al (8.4 at.% of
The coherent fcc/L12 two-phase structure was believed to be a key structure for the Ni-base superalloy, which was assumed to be an outstanding high-temperature material. The initial idea for developing an Ir-base alloy was to develop a new alloy with a coherent fcc/ L12-Ir3Nb two-phase structure, analogous to the Nibase superalloy. From this viewpoint, the B2 phase was not the expected phase and would have to be controlled. However, all three samples showed a B2 phase in this work. Compared with previous researches, the main reason for this result was that the content of Al (10 at.%) was high. This could also be seen from the phase diagram in Fig. 6. Although the edge of the phase region in this quaternary phase diagram had not been expressed, a comparably large area with fcc/L12-Ir3Nb/ B2 three-phase structure was predicted with a high content of Al. A phase diagram could provide a guide for choosing a suitable alloy composition to obtain an expected microstructure. Along with the improvement of the phase diagram, the Al content could be carefully controlled to avoid the formation of a B2-IrAl phase in this alloy system. In addition, in Ni-base superalloys, the L12-Ni3Al phase precipitated within the fcc-Ni matrix and formed a coherent fcc/L12-Ni3Al two-phase structure. This meant that the fcc-Ni phase has a higher solidus temperature than the L12-Ni3Al phase, and, during the solidification, fcc-Ni formed as the primary phase from the melt and the L12-Ni3Al phase precipitated by suitable heat-treatment. In other words, the fcc matrix should be the primary phase grown so that the expected microstructure is achieved. From the Ir-Nb binary phase diagram, the fcc-Ir and L12-Ir3Nb phases had solidification characteristics that were similar to those of Ni–Al. That is the reason that a similar structure was found in the Ir–Nb binary alloys. However, the addition of extra Ni and Al into the Ir–Nb system changed the solidification process, as found in this
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work. The solidus temperature of the fcc-Ir(Ni) phase decreased drastically, and the L12-Ir3Nb phase had the highest dissolving temperature. It meant that, during the solidification it was L12-Ir3Nb phase that solidified at first, and the primary L12-Ir3Nb phase grow freely into the melt, tended to be large. This implied that the extra content of Ni and Al destroyed the formation process of the coherent fcc/L12-Ir3Nb two-phase structure. Therefore, not only the Al content but also the Ni+Al content should be limited. Work on the phase diagram at the Ni-rich corner is underway, and connection of the two parts of the Ir-rich and Ni-rich sides in this quaternary system will be studied in the future.
5. Summary The equilibrium phases of the Ir–Nb–Ni–Al quaternary system at the Ir-rich corner were investigated in some detail in this work. Four kinds of phases of fcc, L12-Ir3Nb, L12-Ni3Al, and B2 were determined in these samples. The EPMA technique was applied to analyze the composition of phases in the Ir-base quaternary alloys. The results were more reliable than those of EDX. Nb showed extremely limited solubility in the B2-IrAl phase. A partial phase diagram around the Irrich corner was drawn according to the EPMA results. This phase diagram will be of great help for controlling the phase in this quaternary system.
Acknowledgements The authors would like to thank Dr. T. Yokokawa for helping with the EPMA tests and Dr. Y. Gu for his participation in helpful discussions.
References [1] Goulette MJ. The future costs less-high temperature materials from an aeroengine perspective. Superalloys 1996, vol. 3. The Minerals, Metals & Materials Society 1996;3:6. [2] Bement AL Jr. Trends and needs for high-temperature materials. In: Advanced high-temperature alloys: processing and properties, vol. 141/146, June. ASM; 1985. [3] Yamabe Y, Koizumi Y, Murakami H, Ro Y, Maruko T, Harada H. Development of Ir-base refractory superalloys. Scripta Materialiar 1996;35(2):211–5. [4] Yamabe-Mitarai Y, Ro Y, Maruko T, Harada H. Ir-based refractory superalloys for ultra-high temperatures. Metallurgical and Materials Ttransactions A 1998;29:537–49. [5] Gu Y, Yamabe-Mitarai Y, Ro Y, Yokokawa T, Harada H. Microstructures and compressive properties of Ir-15Nb refractory superalloys containing nickel. Scripta Materialiar 1998; 39(6):723–8. [6] Gu Y, Yamabe-Mitarai Y, Ro Y, Yokokawa T, Harada H. Properties of the Ir3Nb15 two-phase refractory superalloys with nickel additions. Metallurgical and Materials Transactions A 1999;30A:2629–39. [7] Gu Y, Yamabe-Mitarai Y, Ro Y, Harada H. Microstructure and deformation behavior of Ir-Nb two-phase refractory superalloys for various Nb content. Scripta Materialiar 1999;40(11):1313–9. [8] Yu XH, Yamabe-Mitarai Y, Ro Y, Harada H. Investigation on microstructure and fracture of quaternary Ir-based alloys. Key Engineering Materials 2000;171–4:677–84. [9] Yu XH, Yamabe-Mitarai Y, Yokokawa T, Osawa M, Ro Y, Harada H. Microstructure and mechanical properties of quaternary Ir-Nb-Ni-Al alloys, iridium. In: Ohriner EK, Lanam RD, Panfilov P, Harada H. Nashville, TN: TMS; 2000. p. 61–71. [10] Yu XH, Yamabe-Mitarai Y, Nakazawa S, Ro Y, Harada H. Mechanical properties of Ir-Nb alloys containing Ni and Al, Metallurgical and materials transactions. 32A. ; 2001 1347–53. [11] Yu XH, Yamabe-Mitarai Y, Ro Y, Harada H. Design of quaternary Ir-Nb-Ni-Al refractory superalloys. Metallurgical and materials transactions A 2000;31A:173–8. [12] Yu XH, Yamabe-Mitarai Y, Ro Y, Harada H. Phase separation in two kinds of L12-Ni3Al and Ir3Nb of refractory superalloys. Scripta Materialiar 1999;41(11):1153–9. [13] Massalski TB, editor. Binary alloy phase diagrams. ASM; 1992. [14] Petzow G, Effenberg G, editors. Ternary phase diagram, vol. 6. VCH; 1993. p. 290–2.
Equilibrium phases at the Ir-rich corner of the Ir–Nb–Ni–Al quaternary system C. Huang*, K. Nishida, Y. Yamabe-Mitarai, H. Harada National Institute for Materials Science (NIMS), Sengen 1-2-1, Tsukuba, Ibaraki 305-0047, Japan Received 21 May 2002; received in revised form 8 July 2002; accepted 9 July 2002
Abstract The equilibrium phases of three Ir–Nb–Ni–Al quaternary alloys at the Ir-rich corner were investigated in some detail. The phases were determined by X-ray profile as well as microstructure observation. Three phases of fcc/L12-Ir3Nb/B2-IrAl were found in two samples, and four phases of fcc/L12-Ir3Nb/L12-Ni3Al/B2-IrAl were characterized in one sample. The electron probe microanalysis (EPMA) technique was applied for analyzing the phase composition of this quaternary system. A partial quaternary phase diagram at the Ir-rich side was plotted according to the EPMA results. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Intermetallics, miscellaneous; B. Alloy design; B. Phase diagram; B. Phase identification
1. Introduction Nickel-base superalloys are the most widely used high-temperature materials in gas turbine engines, and they are now operating at a temperature of 1100 C, which is up to 90% of their melting point. It is clear that the melting point of nickel imposes a natural ceiling to their potential, and, hence, further improvement is limited [1]. However, the demand for materials capable of attaining even higher operating temperatures is increasing because higher temperature can afford the designer much greater flexibility in trading off performance gains against major improvements in engine durability, fuel savings, and overall engine life-cycle costs [2]. Among platinum group metals (PGMs), iridium has an fcc crystal structure and some noticeable properties, such as a high melting point and excellent corrosionresistance, which show superior characteristics for hightemperature application. Yamabe-Mitarai et al. [3–6] developed an ‘‘Ir-base refractory superalloy’’, which is a promising material for ultra high-temperature applications. Binary alloys Ir–x (x=Nb, Zr, Hf, Ti, V. . .) as well as ternary alloys Ir–Nb–x (x=Ni) were systematically * Corresponding author. Tel.: +81-298-59-2527; fax: +81-298-592501. E-mail address: [email protected] (C. Huang).
investigated. The results showed that these alloys had a coherent fcc/L12-Ir3x (x=Nb, Zr, Hf. . .) two-phase structure, which was analogous to that of a Ni-base superalloy, yet with greater high-temperature strength. However, the ductility of these alloys was not satisfactory. The binary and ternary alloys usually presented an inter-granular fracture [7]. In order to improve the ductility, quaternary Ir–Nb– Ni–Al alloys were developed by adding some Al. More than 15 quaternary alloys have been investigated [8–10]. Four alloys were at around the Ir-rich side (the content of Ir was equal to or more than that of Ni), and the remaining ones were around the Ni-rich corner. The results indicated that the quaternary alloys presented an excellent balance of high-temperature strength and ductility both at the Ir-rich and Ni-rich regions. On the Nirich side, the strength at 1200 C was above 100 MPa, three times that of the Ni–Al binary alloy, and more than twice that of commercial Ni-base superalloys MarM247. More promising characteristics came from the two alloys F5 and F10 at the Ir-rich side with a coherent fcc-Ir/L12-Ir3Nb two-phase structure [10]. They exhibited above 1000 MPa high-temperature strength at 1200 C with compressive ductility of more than 14%; both the strength and ductility were higher or larger than those of the Ir-base binary and ternary alloys. For the microstructure of quaternary alloys, it
0966-9795/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(02)00092-4
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was initially thought that combining two kinds of binary alloys, both of which had an fcc/L12 (Ir/Ir3Nb and Ni/Ni3Al) two-phase structure, would yield the fcc/L12 two-phase structure in quaternary alloys. However, the two-phase regions did not connect with each other from the Ir-rich to the Ni-rich side. The fcc/L12 two-phase structure formed at the Ir-rich or Ni-rich corners only, and a three-phase fcc/L12-Ir3Nb/L12-Ni3Al region was observed between the two two-phase regions. One sample even showed four phases of fcc/L12-Ir3Nb/L12Ni3Al/B2-IrAl. The phase constitution affects the mechanical properties of the material; the fcc-Ir/L12Ir3Nb two-phase coherent structure seems to be the most promising in this quaternary system. In order to enhance the understanding of the phase relations, a phase diagram is needed. In the previous investigations, only a schematic phase diagram [11,12] was obtained because of the difficulty in obtaining the phase composition. Previously, energy dispersive analysis X-ray spectroscopy (EDX) was used for analyzing the phase composition of Ir–Nb alloys. However, it was very difficult to obtain reasonable results. The main problem came from the Nb in the Ir-base alloys. An example of peaks detected in an Ir–Nb–Ni–Al quaternary alloy is shown in Fig. 1, and it was obtained by EDX under a low magnification of 50. In this figure, the peak of Ir-M (the X-ray energy was 1.98 keV) was almost completely overlapped by Nb-La (the X-ray energy was 2.17 keV). From the composition result, which could reflect the
Fig. 1. The peaks detected in one Ir–Nb–Ni–Al quaternary alloy by EDX.
average composition of this alloy because of the low magnification used, the Nb content was 28.12 at.%, more than twice the nominal content of 13.5 at.%. In this work, three other Ir–Nb–Ni–Al quaternary alloys at the Ir-rich corner were investigated in order to add more information of the phase construction in the Irrich region because emphasis had been previously given to the alloys at the Ni-rich side. Instead of EDX, the EPMA method was tried to test the phase composition of Ir-Nb quaternary alloys. The phase constituent was discussed, and a phase diagram of the Ir-rich corner at 1400 C was drawn according to the results of EPMA.
2. Experimental procedure Three ingot samples, 20 g for each, were produced by the arc melting method in an Ar atmosphere. The compositions of the sample were 42.75Ir–4.5Nb–42.75Ni– 10Al, 49.5Ir–13.5Nb–27Ni–10Al, and 67.5Ir-13.5Nb– 9Ni–10Al. (The composition expressions were all in atomic percentages.) All the samples have the same atomic Al content of 10%. After being arc-melted, one sample was analyzed by the fluorescence X-ray method to determine its actual composition. It was found to be very close to the nominal value. Cylindrical samples that were 3 mm in diameter and 5 6 mm in length were cut with an electron discharge machine (EDM) from each ingot for heat treatment. Three temperatures of 1250, 1400, and 1600 C were selected for heat treatment of 42.75Ir–4.5Nb–42.75Ni–10Al, four temperatures of 1250, 1400, 1600, and 1800 C were chosen for 49.5Ir–13.5Nb–27Ni–10Al, and five temperatures with an addition of 2000 C were used for 67.5Ir–13.5Nb– 9Ni–10Al. All the heat treatments were conducted in a vacuum furnace for 168 h. The heat-treated samples were polished for X-ray diffraction (XRD) analysis. Scanning electron microscopy (SEM) was applied for microstructure observation. For composition analysis by EPMA, the samples were diamond-polished to a final 0.5 mm without etching. The composition of each phase was analyzed using an accelerating voltage of 20 kV and a current of 5108A. Four standard samples made of pure metals were characterized prior to the analysis. According to the X-ray energy values of peaks of each element, crystals PET and TAP were chosen for detecting Nb and Al, and crystal LiF was applied for probing Ir and Ni. From the results of a standard sample, the Ir-M peak was separated into two peaks of Ir-Ma and Ir-Mb, and these two peaks were clearly differentiated from the Nb-La peak, as shown in Fig. 2. Four obviously distinguished peaks of Ir-La, Nb-La, Ni-Ka, and Al-Ka were selected as the characterized peaks for identifying the elements of Ir, Nb, Ni, and Al in the investigated alloys. The adjacent background of the four peaks was also determined from
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67.5Ir–13.5Nb–9Ni–10Al [Fig. 3(c)]: Three phase peaks of fcc, L12-Ir3Nb, and B2-IrAl were observed in 1250, 1400, and 1600 C heat-treated samples. Two phase peaks of L12-Ir3Nb and B2-IrAl were detected in the 1800 C heat-treated samples, and only the L12Ir3Nb phase peak was observed in the 2000 C heattreated sample. The phases formed at different heattreatment temperatures are summarized in Table 1. In order to obtain a whole understanding of the phase formation at the Ir-rich corner of this quaternary system, the previous results of four quaternary alloys with the composition at the Ir-rich side are also shown there [8–10]. The numbers of the four samples presented in the literature are indicated below the compositions of the samples. 3.2. Microstructure observation
Fig. 2. The peaks detected in pure standard samples by EPMA.
this figure for the background subtraction process. A classical correction program ZAF (atomic number, absorption, and fluorescence) was applied for correcting the results. Nine points were detected for each phase, and the final results were the average value. When the phase size was not large enough for point analysis, a map analysis was used instead.
3. Results 3.1. X-ray profiles The XRD patterns of the three samples at different heat-treatment conditions are shown in Fig. 3. 42.75Ir–4.5Nb–42.75Ni–10Al [Fig. 3(a)]: Four phase peaks of fcc, L12-Ir3Nb, L12-Ni3Al, and B2-IrAl were characterized in 1250 and 1400 C heat-treated samples; however, the fcc and L12-Ni3Al phase peaks were overlapped. In the 1600 C heat-treated sample, the B2 phase disappeared, and the overlapped peaks of the fcc and L12-Ni3Al phases were separated into two peaks. 49.5Ir–13.5Nb–27Ni–10Al [Fig. 3(b)]: Three phase peaks of fcc, L12-Ir3Nb, and B2-IrAl were found in 1250 and 1400 C heat-treated samples. In the 1600 C heat-treated samples, the L12-Ir3Nb and B2-IrAl phases were found; on the other hand, in the 1800 C heattreated sample, only the L12-Ir3Nb phase peak remained.
The microstructures of the samples after being heattreated at 1250, 1400, and 1600 C are shown in Fig. 4. In 42.75Ir–4.5Nb-42.75Ni–10Al, four phases with different contrasts were observed in the 1250 and 1400 C heat-treated samples. The square blocks with the brightest contrast corresponded to the L12-Ir3Nb phase. The matrix was an fcc phase with different partition ratios of Ir and Ni, and the matrix with brighter contrast contained more Ir and less Ni than the matrix with darker contrast. Gray blocks marked the B2 phase. Lots of small precipitates were found within the matrix. In 1250 and 1400 C heat-treated samples, the L12Ir3Nb became coarsened, and small precipitates still existed. Because the size of the precipitates was too small to examine the composition by point analysis in EPMA, the map analysis was used. It was determined to be Ni3Al. A dendritic structure formed in 49.5Ir–13.5Nb–27Ni– 10Al. Three phases were clearly defined in the 1250 and 1400 C heat-treated samples, which were the L12Ir3Nb, B2-IrAl, and fcc-Ni (Ir) phases. The phase with a dendritic structure was L12-Ir3Nb. Between the dendritic arms, the phase showing the gray contrast was B2–IrAl, and the phase with the dark contrast was fccNi (Ir). In the 1600 C heat-treated sample, the coarse L12-Ir3Nb dendritic phase, as well as a few parts of the B2-IrAl phase, could be observed; however, the fccNi(Ir) disappeared, and many holes formed at the position of the original fcc-Ni(Ir) phase. The holes were believed to have formed by the heat treatment. In the 1800 C heat-treated sample, only the single phase of the L12-Ir3Nb was observed, as well as many holes. Three phases were observed in the 1250, 1400, and 1600 C heat-treated samples in 67.5Ir–13.5Nb–9Ni– 10Al. The matrix was the L12-Ir3Nb phase. Around the grain boundary, a smaller area with a brightness contrast was the fcc-Ir(Ni) phase, and the phase with a dark contrast was B2-IrAl. In the 1800 C heat-treated sample, L12-Ir3Nb and B2-IrAl phases were found; however, in
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Fig. 3. The XRD patterns of the samples: (a) 42.75Ir–4.5Nb–42.75Ni–10Al, (b) 49.5Ir–13.5Nb-27Ni–10Al, and (c) 67.5Ir–13.5Nb–9Ni–10Al.
the 2000 C heat-treated sample, only the single phase L12-Ir3Nb remained. Many big holes were observed in samples heat-treated at 1800 and 2000 C. 3.3. Dissolving temperature of the phases The melting temperature (MT) is a very important value, especially for high-temperature materials. The MT range of the alloys could be primarily determined from the heat-treatment process, which was < 1800 C for sample 42.75Ir–4.5Nb–42.75Ni–10Al (which melted
at 1800 C), < 2000 C for sample 49.5Ir–13.5Nb–27Ni– 10Al (which melted at 2000 C), and > 2000 C for sample 67.5Ir-13.5Nb–9Ni–10Al (which stood up to 2000 C). The dissolving temperatures of the phases in alloys could also be defined from X-ray analysis and observation of the microstructure of the samples that were heat-treated at different temperatures. Since some phases that existed in a sample that was heat-treated at low temperature disappeared in a sample that was heattreated at a higher temperature, it was indicated that the dissolving temperature of the phase was between two
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C. Huang et al. / Intermetallics 10 (2002) 893–902 Table 1 Phases detected in samples at different heat-treatment temperatures [8–10] Temperature ( C)
Phase
42.75Ir–4.5Nb– 42.75Ni–10Al
49.5Ir–13.5Nb– 27Ni–10Al
67.5Ir–13.5Nb– 9Ni–10Al
1250
fcc L12-Ir3Nb B2 L12-Ni3Al
* * * *
* * *
* * *
1300
fcc L12-Ir3Nb B2 L12-Ni3Al
1400
1600
1800
2000
fcc L12-Ir3Nb B2 L12-Ni3Al
* * * *
fcc L12-Ir3Nb B2
* *
L12-Ni3Al fcc L12-Ir3Nb B2 fcc
*
L12-Ir3Nb B2
* * *
* *
* * *
*
* *
60Ir–15Nb– 20.8Ni–4.2Al (S750)
40Ir–10Nb– 41.6Ni–8.4Al (S550)
80.7Ir–14.3Nb– 4Ni–1Al (F5)
76.5Ir–13.5Nb– 8.1Ni–1.9Al (F10)
* *
* *
* *
*
* * * *
* *
* *
* *
* *
*
*
*
heat-treatment temperatures. For example, in sample 42.75Ir–4.5Nb–42.75Ni–10Al, the B2-IrAl phase disappeared in the 1600 C heat-treated sample, which meant that the dissolving temperature of the B2–IrAl phase was below 1600 C. In 49.5Ir–13.5Nb–27Ni– 10Al, the fcc-Ir(Ni) phase vanished in the sample heattreated at 1600 C. For 67.5Ir–13.5Nb–9Ni–10Al, it was also the fcc-Ir(Ni) phase that first disappeared in the sample heat-treated at 1800 C. The dissolving temperatures of the fcc phase were below 1600 and 1800 C for samples 49.5Ir–13.5Nb–27Ni–10Al and 67.5Ir– 13.5Nb–9Ni–10Al, respectively. The L12-Ir3Nb phase had the highest dissolving temperature of the group. 3.4. Phase diagram 3.4.1. Previous study of a phase diagram for an Ir-base alloy In this investigation, we attempted to determine an Ir–Nb–Ni–Al quaternary phase diagram at 1400 C around the Ir-rich side. Prior to the determination of the phase diagram, it was necessary to collect reliable data on the phase equilibra existing in the binary subsystems Ir–Nb, Ir–Ni, and Ir–Al and the ternary subsystems Ir– Nb–Ni, Ir–Nb–Al, and Ir–Ni–Al. 3.4.1.1. Binary subsystems. Okamoto [13] had determined the Ir–Nb binary phase diagram. At 1400 C,
around the Ir-rich side, the fcc-Ir phase existed with a maximum Nb solubility of 11%; the L12-Ir3Nb phase occurred from 24 to 32% of Nb, and the fcc-Ir/L12-Ir3Nb two-phase area was between these two single phases. The Ir–Al binary phase diagram had been investigated by Axler et al. [13]. From the phase diagram, the fcc-Ir/B2-IrAl two-phase area occupied a large part, from 52 to 90%, of the Ir at 1400 C, and the B2-IrAl phase area was comparably narrow between 48 and 52% of the Al. The melting temperature of the B2-IrAl phase was above 2000 C. The Ir–Ni binary phase diagram was determined by Yang et al. [13]. A continuous series of fcc-(Ir,Ni) solid solutions was found. The melting temperature showed a nearly linear dependence on the composition. 3.4.1.2. Ternary subsystems. Little information is available on the ternary phase diagram including Ir. Only the isothermal section of the Ir–Nb–Ni system at 1300 C was investigated experimentally by Gu [5]; it is shown here as Fig. 5(a). Around the Ir-rich side, two singlephase (fcc-Ir and L12-Ir3Nb) regions and one two-phase (fcc-Ir/L12-Ir3Nb) region between the two single-phase regions were schematically determined. Because the fcc and L12-Ir3Nb phases are stable [13], no obvious differences in this ternary phase diagram were thought to occur on the edge of these phases between 1300 and 1400 C.
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Fig. 4. The microstructures of the samples (a) 42.75Ir–4.5Nb–42.75Ni–10Al, (b) 49.5Ir–13.5Nb-27Ni–10Al, and (c) 67.5Ir–13.5Nb-9Ni–10Al. (a), (b), and (c) are from the samples heat-treated at 1250 C, (a0 ), (b0 ), (c0 ) are from the samples heat-treated at 1400 C, and (a00 ), (b00 ), and (c00 ) are from the samples heat-treated at 1600 C. (a), (a0 ), (a00 ), (b), (b0 ), and (b00 ) are back scattered imagines, (c), (c0 ), and (c00 ) are secondary electron images.
Fig. 5. Phase diagrams in literatures, (a) Ir–Nb–Ni ternary phase diagram at 1300 C [5], (b) Ir–Nb–Ni–Al quaternary phase diagram at 1400 C [12].
Horyn [14] had studied the Al–Ir–Nb system, but only at the Nb-rich part. The composition range was up to 45 at.% Ir and Al and up to 70 at.% Al and 55 at.% Ir. However, it was not in the composition range of this work.
3.4.1.3. Quaternary system. The schematic phase diagram of Ir–Nb–Ni–Al from the Ir-rich side to the Nirich side at 1400 C was determined by Yu [11,12] and is shown here as Fig. 5(b). Three single-phase [fcc-Ir(Ni), L12-Ir3Nb, and L12-Ni3Al] regions were characterized.
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Two single-phase (L12-Ir3Nb and L12-Ni3Al) parts were linked by the L12-Ir3Nb/L12-Ni3Al two-phase area. Between the single phases of fcc and L12, a two-phase area of fcc-Ir(Ni)/L12-Ir3Nb, a three-phase area of fccIr(Ni)/L12-Ir3Nb/L12-Ni3Al, and a two-phase region of the fcc-Ir(Ni)/L12-Ni3Al area were determined from the Ir-rich side to the Ni-rich side. Although the B2-IrAl phase had been found in the sample S750 (as shown in Table 1), the B2-IrAl phase region was not predicted in the phase diagram.
with 10 at.% of Al. Two samples, 49.5Ir–13.5Nb–27Ni– 10Al and 67.5Ir–13.5Nb–9Ni–10Al, showed three phases of fcc, L12-Ir3Nb, and B2-IrAl, and one sample, 42.75Ir–4.5Nb–42.75Ni-10Al, exhibited four phases of fcc, L12-Ir3Nb, B2-IrAl, and L12-Ni3Al. As the composition of L12-Ni3Al was not known, the phase was not included in this figure.
3.4.2. Quaternary phase diagram investigation The compositions of phases in the heat-treated samples are indicated in Table 2. The composition of the L12-Ni3Al phase in 42.75Ir–4.5Nb–42.75Ni–10Al was unavailable because the phase was too small. According to the EPMA results, the equilibrium quaternary phase diagram at 1400 C was drawn (Fig. 6) showing only the three phases of fcc, L12-Ir3Nb, and B2-IrAl. The method for plotting the point indicating the alloy composition is shown schematically in the upper right part of Fig. 6. For example, the point position indicating the composition of 49.5Ir–13.5Nb– 27Ni–10Al can be determined by the following three steps: (1) drawing the first plane abc parallel to the triangle base plane Ir–Ni–Nb, the distance of the abc plane to the Ir–Ni–Nb plane was 10. Every point on the abc plane indicates an alloy composition with an Al content of 10 at.%; (2) drawing the second plane def parallel to the triangle base plane Ni–Nb–Al, the distance of the def plane to the Ni–Nb–Al plane was 49.5; the abc and def planes intersected at line AB; (3) drawing the third plane ghl parallel to the triangle base plane Ir-Ni-Al, the distance of the ghl plane to the Ir–Ni–Al plane was 13.5; this plane and the abc plane intersected at line CD. Lines AB and CD intersected at point E, which was the indication point of this sample. The points of the three samples were on the shadow plane
4.1. Analysis of the EPMA results
4. Discussion
From the composition results of the phase in Ir–Nb– Ni–Al quaternary alloys obtained by EPMA, the distribution of elements in every phase could be understood. Nb showed a strong preference to partition into the L12-Ir3Nb phase. The partitioning ratio between the fccIr(Ni) and L12-Ir3Nb phases was about 0.1 0.3; moreover, this ratio value presented the same decreasing trend in all three samples when the temperature of the heat treatment was increased, as shown in Fig. 7. This was different from that in the Ir–Nb binary alloy system, where the solubility of Nb in the fcc-Ir phase increased slightly along with the temperature. This meant that the volume faction of the L12-Ir3Nb phase would increase along with the temperature in Ir–Nb– Ni–Al alloys. This outcome was consistent with the coarsening phenomenon of the L12-Ir3Nb phase observed in the microstructure of the samples of 42.75Ir-4.5Nb-42.75Ni-10Al and 49.5Ir–13.5Nb–27Ni– 10Al. The coarsening of the L12-Ir3Nb phase was not obvious in sample 67.5Ir–13.5Nb–9Ni–10Al because the L12-Ir3Nb phase had already occupied a large portion in the sample heat-treated at low temperature. In the B2-IrAl phase, Nb exhibited extremely low solubility. Through all the results, the maximum solubility of Nb in the B2-IrAl phase was 0.3 at.%. This implied that the
Table 2 The composition of every phase in the samples heat-treated at 1250, 1400, and 1600 C 1250 C
1400 C
1600 C
fcc
L12-Ir3Nb
B2
fcc
L12-Ir3Nb
B2
fcc
L12-Ir3Nb
48.5 2.1 42.2 7.2
69.1 21.5 6.4 3.0
42.75Ir–4.5Nb–42.75Ni–10Al
Ir Nb Ni Al
45.7 2.6 42.8 8.8
60.3 12.2 21.2 6.3
45.7 0.0 13.2 41.0
43.5 2.8 44.7 9.1
62.7 15.5 16.4 5.5
45.6 0.2 13.2 41.2
49.5Ir–13.5Nb–27Ni–10Al
Ir Nb Ni Al
24.8 2.7 58.7 13.7
68.7 23.4 5.6 2.4
42.3 0.3 16.3 41.1
24.4 1.4 62.4 11.8
65.4 22.6 9.5 2.5
43.2 0.0 16.4 40.5
67.5Ir–13.5Nb–9Ni–10Al
Ir Nb Ni Al
78.1 4.7 2.5 14.6
75.3 19.0 0.4 5.3
50.8 0.2 3.7 45.4
82.0 4.6 3.6 9.8
74.9 20.4 0.4 4.3
50.1 0.2 3.9 45.8
82.0 3.3 5.0 9.7
1800 C B2
L12-Ir3Nb
71.4 22.1 3.6 2.4
49.6 0.0 3.2 47.1
70.5 22.8 4.3 2.4
76.2 17.0 0.6 6.2
50.6 0.0 3.9 45.5
75.7 15.5 0.7 8.1
2000 C B2
L12-Ir3Nb
50.2 0.0 1.9 47.9
74.6 15.4 0.7 9.2
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Fig. 6. The quaternary phase diagram of Ir–Nb–Ni–Al at Ir-rich side at 1400 C.
B2-IrAl phase only occupied a flat region in the Ir–Nb– Ni–Al quaternary phase diagram, which was almost entirely on the Ir–Ni–Al plane. The Al exhibited a strong preferential partitioning in the B2-IrAl phase. About 70 at.% of all Al partitioned in the B2-IrAl phase. Between the remaining phases of fcc-Ir(Ni) and L12-Ir3Nb, Al showed preferential partitioning into fccIr(Ni). The Ni partitioned mainly in the fcc-Ir(Ni) phase and showed some solution in the B2-IrAl phase. The solubility of other elements in the B2-IrAl and fcc-Ir(Ni) phases greatly affected their dissolving temperatures. Although the B2-IrAl and fcc-Ir phases without alloying element had very high MTs of about 2120 and 2447 C [13], respectively, the dissolving temperatures of these two phases decreased drastically in the quaternary systems. From the results of the composition, the decrement occurred because of the presence of the solution of other elements in the phases. Since the B2-IrAl phase had a very limited solubility of Nb, the
change in the dissolving temperature of the B2-IrAl phase came mainly from the effect of the Ni. A comparison of the B2-IrAl phase in all the three samples indicated that the B2-IrAl phase in 42.75Ir–4.5Nb– 42.75Ni–10Al with a Ni content of about 13 at.% disappeared at 1600 C, the B2-IrAl phase in 49.5Ir– 13.5Nb–27Ni–10Al with about 3 at.% of Ni stood up to 1600 C and disappeared at 1800 C, and the B2-IrAl phase in 67.5Ir–13.5Nb–9Ni–10Al with about 2 at.% of Ni held up to 1800 C. This indicated that the solidus temperature of the B2-IrAl phase decreased with increasing the solution of Ni in the B2-IrAl phase. For the fcc-Ir phase, the solubility of Nb in the fcc phase decreased to less than 5 at.% compared with 10 at.% in the Ir–Nb binary alloys, and the solution of Nb in Ir had little effect on the dissolving temperature of the fccIr phase, according to the Ir–Nb binary phase diagram. Therefore, the decrease of the dissolving temperature was assumed to be due, mainly, to the Ni and Al. From
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901
Al), was found to have four phases of fcc, L12-Ir3Nb, Ni3Al, and B2-IrAl in the 1300 C heat-treated sample, and the B2-IrAl phase disappeared in the 1400 C heattreated sample [8]. In Fig. 6, the position of S550 was near that of the sample 42.75Ir–4.5Nb–42.75Ni–10Al, which also had four phases. This implied that the edge plane of the four-phase region was between sample S550 and 42.75Ir–4.5Nb–42.75Ni–10Al. The sample, S750, had an Al content of 4.2 at.%. The position of the sample in Fig. 6 was significantly below the shadowed plane. Three phases of fcc-Ir(Ni), L12-Ir3Nb, and Ni3Al were found in the sample heat-treated at 1400 C. The lower content of Al in S750 prevented the formation of the B2-IrAl phase. This indicated that the three-phase (fcc-Ir(Ni), L12-Ir3Nb, and Ni3Al) region near the Ir-rich side was at the low part in the phase diagram. 4.3. Some views on alloy development
Fig. 7. Solubility ratio of Nb in fcc-Ir(Ni) phase to that in L12-Ir3Nb phase.
the binary phase diagram of the Ir–Ni, the melting temperature of the fcc-Ir(Ni) phase decreased almost linearly with increasing the Ni. According to this phase diagram and the content of Ir and Ni in the fcc phase in the three alloys, the dissolving temperature was determined to be approximately 1900, 1700, and 2400 C for 42.75Ir–4.5Nb–42.75Ni-10Al,49.5Ir–13.5Nb–27Ni–10Al, and 67.5Ir–13.5Nb–9Ni–10Al, respectively. All these values were higher than those found in the fcc-Ir(Ni) phases of the three samples. The additional decrement was caused by the solution of Al in the fcc phases. In conclusion, the decrease of the dissolving temperature of the B2-IrAl phase was due mainly to the solution of Ni in it, and the combined Ni+Al affected the dissolving temperature of the fcc-Ir(Ni) phase. 4.2. Comparison with the previous results of Ir–Nb–Ni–Al In a previous study, four alloys with a composition around the Ir-rich side were investigated (Table 1). Because the phase composition could not be obtained, the samples were plotted in Fig. 6 only according to their composition. F5 and F10, with lower Al contents of 1 and 1.9 at.%, respectively, were found to have only an fcc/L12-Ir3Nb two-phase structure. In Fig. 6, the positions of F5 and F10 were only slightly above the Ir– Ni–Nb base plane and might have settled in the fcc/L12Ir3Nb two-phase region. These results showed good agreement with those obtained in this work. Another sample, S550, with a higher content of Al (8.4 at.% of
The coherent fcc/L12 two-phase structure was believed to be a key structure for the Ni-base superalloy, which was assumed to be an outstanding high-temperature material. The initial idea for developing an Ir-base alloy was to develop a new alloy with a coherent fcc/ L12-Ir3Nb two-phase structure, analogous to the Nibase superalloy. From this viewpoint, the B2 phase was not the expected phase and would have to be controlled. However, all three samples showed a B2 phase in this work. Compared with previous researches, the main reason for this result was that the content of Al (10 at.%) was high. This could also be seen from the phase diagram in Fig. 6. Although the edge of the phase region in this quaternary phase diagram had not been expressed, a comparably large area with fcc/L12-Ir3Nb/ B2 three-phase structure was predicted with a high content of Al. A phase diagram could provide a guide for choosing a suitable alloy composition to obtain an expected microstructure. Along with the improvement of the phase diagram, the Al content could be carefully controlled to avoid the formation of a B2-IrAl phase in this alloy system. In addition, in Ni-base superalloys, the L12-Ni3Al phase precipitated within the fcc-Ni matrix and formed a coherent fcc/L12-Ni3Al two-phase structure. This meant that the fcc-Ni phase has a higher solidus temperature than the L12-Ni3Al phase, and, during the solidification, fcc-Ni formed as the primary phase from the melt and the L12-Ni3Al phase precipitated by suitable heat-treatment. In other words, the fcc matrix should be the primary phase grown so that the expected microstructure is achieved. From the Ir-Nb binary phase diagram, the fcc-Ir and L12-Ir3Nb phases had solidification characteristics that were similar to those of Ni–Al. That is the reason that a similar structure was found in the Ir–Nb binary alloys. However, the addition of extra Ni and Al into the Ir–Nb system changed the solidification process, as found in this
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work. The solidus temperature of the fcc-Ir(Ni) phase decreased drastically, and the L12-Ir3Nb phase had the highest dissolving temperature. It meant that, during the solidification it was L12-Ir3Nb phase that solidified at first, and the primary L12-Ir3Nb phase grow freely into the melt, tended to be large. This implied that the extra content of Ni and Al destroyed the formation process of the coherent fcc/L12-Ir3Nb two-phase structure. Therefore, not only the Al content but also the Ni+Al content should be limited. Work on the phase diagram at the Ni-rich corner is underway, and connection of the two parts of the Ir-rich and Ni-rich sides in this quaternary system will be studied in the future.
5. Summary The equilibrium phases of the Ir–Nb–Ni–Al quaternary system at the Ir-rich corner were investigated in some detail in this work. Four kinds of phases of fcc, L12-Ir3Nb, L12-Ni3Al, and B2 were determined in these samples. The EPMA technique was applied to analyze the composition of phases in the Ir-base quaternary alloys. The results were more reliable than those of EDX. Nb showed extremely limited solubility in the B2-IrAl phase. A partial phase diagram around the Irrich corner was drawn according to the EPMA results. This phase diagram will be of great help for controlling the phase in this quaternary system.
Acknowledgements The authors would like to thank Dr. T. Yokokawa for helping with the EPMA tests and Dr. Y. Gu for his participation in helpful discussions.
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