Design of Quaternary Ir-Nb-Ni-Al Refractory Superalloys

X.H. YU, Y. YAMABE-MITARAI, Y. RO, and H. HARADA

We propose a method for developing new quaternary Ir-Nb-Ni-Al refractory superalloys for ultra-high-temperature uses, by mixing two types of binary alloys, Ir-20 at. pct Nb and Ni-16.8 at. pct Al,which contain fcc/L12 two-phase coherent structures. For alloys of various Ir-Nb/Ni-Al compositions,we analyzed the microstructure and measured the compressive strengths. Phase analysis indicatedthat three-phase equilibria—fcc, Ir3Nb-L12, and Ni3Al-L12—existed in Ir-5Nb-62.4Ni-12.6Al (at. pct)(alloy A), Ir-10Nb-41.6Ni-8.4Al (at. pct) (alloy B), and Ir-15Nb-20.8Ni-4.2Al (at. pct) (alloy C) at1400 8C; at 1300 8C, three phase equilibria—fcc, Ir3Nb, and Ni3Al—existed in alloys A and C andfour-phase equilibria—fcc, Ir3Nb, Ni3Al, and IrAl-B2—existed in alloy B. The fcc/L12 coherentstructure was examined by using transmission electron microscopy (TEM). At a temperature of 12008C, the compressive strength of these quaternary alloys was between 130 and 350 MPa, which washigher than that of commercial Ni-based superalloys, such as MarM247 (50 MPa), and lower thanthat of Ir-based binary alloys (500 MPa). Compared to Ir-based alloys, the compressive strain ofthese quaternary alloys was greatly improved. The potential of the quaternary alloys for ultra-high-temperature use is also discussed.

I. INTRODUCTION alloys and superior properties compared to Ir-based alloys.The objective was to combine the high-temperature strengthTHE temperature capability of Ni-based superalloys hasof Ir-based alloys with the high ductility, low density (aboutbeen improved by more than 300 8C in the last 50 years[1]

8.5 g/cm3, compared to 22.65 g/cm3 for Ir), and relativelyand is approaching 1100 8C for single-crystal superalloyslow cost of Ni-based alloys. Figure 1 shows a sketch of aconsisting of g-fcc and g 8-L12 phases.[2] In Ni-based super-portion of the Ir-Nb-Ni-Al quaternary phase equilibriumalloys, the g 8 phase is formed with a coherent interface. Thediagram. Initially, two binary alloys (Ir- and Ni-based binarycoherent structures become obstacles to dislocation move-alloys, indicated by I and N, respectively) were mixed toment and are therefore one of the important features of Ni-prepare quaternary Ir-Nb-Ni-Al alloys (for example, Ir-based superalloys that allow them to maintain sufficientlybased:Ni-based 5 25:75, 50:50, and 75:25 are indicated aslow creep rates at high temperatures. Many attempts havealloys A, B, and C, respectively). It is expected that fcc/L12also been made to develop alloys superior to Ni-based super-regions exist over the entire range of mixture ratios, fromalloys, by using intermetallic and refractory alloys.[3,4] Plati-pure Ni-based to pure Ir-based alloys. In particular, the coex-num-group metals have been considered because of theiristence of the fcc/L12-Ni3Al and fcc/L12-Ir3Nb coherenthigher melting temperatures (Ir: 2447 8C) and superior oxi-structures is both desirable and expected. The fcc/L12 coher-dation resistance.[5,6] We propose a new class of superalloysent structures are required for high strength at high tempera-using platinum group metals, called “Refractory Super-ture. The coherent structures and high melting temperaturesalloys.”[7,8] Our experimental results show that Ir-basedof Ir-based alloys give the quaternary alloys useful high-refractory superalloys have fcc/L12 coherent structure andtemperature strength. A decrease in density and cost arehave a superior high-temperature strength compared to Ni-achieved by using Ni-based alloys. From the available Ir-based superalloys. However, the density and expense of Ir-based binary alloys, we selected the Ir-Nb binary alloybased alloys are much higher than those of Ni-based super-because of its high strength at high temperature (over 500alloys. The ductility of Ir-based refractory superalloys mustMPa at 1200 8C).[8] For the Ni-based alloy, the Ni-Al alloyalso be improved. Increased expense is never desirable, andwas selected because it contained fcc/L12 coherentincreased density is particularly undesirable for applicationsstructures.[9]

in rotating equipment where inertial stresses are increasedThe aim of the present research was to characterize theand also in airborne equipment due to reduced carrying

microstructures and mechanical properties of quaternary Ir-capacity.Nb-Ni-Al alloys with various proportions of Ir-20 at. pctNb and Ni-16.8 at. pct Al alloys. The results are discussedfor the potential use of quaternary refractory superalloys atII. ALLOY DESIGNtemperatures above 1500 8C.

Based on the known properties of Ir- and Ni-based super-alloys, we developed a series of quaternary Ir-Nb-Ni-Alalloys, which have superior strength compared to Ni-based

III. EXPERIMENTAL PROCEDURES

Three quaternary alloys were prepared by arc melting Ir-X.H. YU, Visiting Researcher, Y. YAMABE-MITARAI and Y. RO, 20 at. pct Nb and Ni-16.8 at. pct Al master alloys. Both Ir-

Senior Researchers, and H. HARADA, Project Leader and Senior 20 at. pct Nb and Ni-16.8 at. pct Al alloys have fcc and L12Researcher, are with the High Temperature Materials 21 Project, Nationaltwo-phase structures. The nominal compositions (at. pct)Research Institute for Metals, Ibaraki 305-0047, Japan.

Manuscript submitted March 11, 1999. is Ir-5Nb-62.4Ni-12.6Al (alloy A), Ir-10Nb-41.6Ni-8.4Al

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in the Ni- and Ir-based binary alloys were clearly differenti-ated as fcc and Ni3Al and fcc and Ir3Nb two-phase structures,respectively (Figures 2(f) and (j)).[9,5] Coherent cuboidal L12

phases (Ni3Al or Ir3Nb type) could be observed in the matrix.Only one type of fcc/L12 coherent structure existed in theNi-based and Ir-based binary alloys. The phases in the qua-ternary Ir-Nb-Ni-Al alloys were determined by XRD andEDX. The XRD phase analysis indicated that the fcc andtwo types of L12 phases existed in all of the quaternaryalloys. Lamellar structures formed by Ir3Nb containing Niand Al (bright contrast)/fcc(dark) were observed in the cellsof fcc dendrites (Figure 2(g)). Figure 2(g) also shows thatNi3Al containing Ir and Nb had less contrast in the fcc matrixand in the lamellar structure. In alloy B, the dendrite armswith bright contrast were Ir3Nb (Figure 2(h)). The phasebetween the dendrite arms was the fcc phase. Fine Ni3Aland IrAl 10-mm grains containing Ni and Nb formed in thefcc matrix. In alloy C, dendrite arms were Ir3Nb, the same

Fig. 1—A Sketch of a portion of the Ir-Nb-Ni-Al quaternary phase diagram as in alloy B (Figure 2(i)). Fine Ni3Al phases were formedat 1400 8C.

in the fcc phase between the dendrite arms.The TEM observation of alloy A heat treated at 1300 8C

for 168 hours showed 200-nm cubic precipitates formed(alloy B), and Ir-15Nb-20.8Ni-4.2Al (alloy C). The micro- in the fcc matrix (Figure 3); additional 30-mm L12-phasestructure of the Ir-20 at. pct Nb and Ni-16.8 at. pct Al alloys precipitates, which could be observed with SEM (Figurewas investigated. These alloys were heated at 1300 8C and 2(g)), were seen. The cuboidal phases had a L12 structure.1400 8C for 168 hours in a vacuum furnace. The microstruc- Similar cuboidal L12 precipitates were also observed inture of the as-cast and heated samples was investigated by alloys B and C.scanning electron microscopy (SEM) and transmission elec- The detailed compositional analysis is given in Table I.tron microscopy (TEM). Micrographs were taken either by For alloys B and C, the secondary Ni3Al phase concentrationusing secondary electron imaging or by using back scattered is not listed because it was too low to determine accurately.imaging (BSI). These samples were etched with 5 pct HCl The compositional analysis indicates that the primary Ni3Alethanol solution at AC 10 V for 10 minutes. The phase in alloy A contained about 23 at. pct Ir. The fcc solid mixturecompositions were measured by using energy dispersive X- of three kinds of alloys was mainly formed by Ni and Irray spectroscopy (EDX). The phase structures of the Ir-Nb- (from alloys A, B, to C, the concentration of Ni varied fromNi-Al alloys were examined by using X-ray diffractometry 60 to 44 at. pct and Ir varied from 17 to 38 at. pct). The Ir(XRD). To understand the relationship of the phases, differ- content in the fcc solid increased with increasing content ofential thermal analysis (DTA) was used for alloy A using the base Ir alloy. The Nb content in the fcc matrix was aboutthe following procedure. Alloy A was heated from 1000 8C 5 at. pct. Note that the composition of Al in the fcc phaseto 1750 8C at a rate of 5 8C/min and cooled to 1000 8C at was a little higher. This is because some Ni3Al particlesa rate of 25 8C/min. The samples for compression test were were included in the fcc phase. Figure 3 indicates that it isheated at 1400 8C for 168 hours. The compressive strength difficult to separate the fcc from the L12 phase and to accu-at 1200 8C was measured, using an initial strain rate of 4 3 rately detect fcc by EDX.1023/s. Figure 4 shows that after heat treatment at 1400 8C for

168 hours, dendrite structures remained in all alloys. Fig-ure 4(a) shows that in alloy A, lamellar structures con-IV. RESULTSsisting of Ir3Nb and fcc phases also remained. Fine Ir3Nb

A. Microstructure Evolution and Ni3Al precipitates, indicated by bright contrast andvarious shapes and sizes, were more numerous in the fccFigure 2 shows the microstructures for the as-cast alloysmatrix compared with microstructures heat treated at 1300and alloys heat treated at 1300 8C for 168 hours. Figure 2(a)8C. Figure 4(b) shows that IrAl disappeared from alloy B.shows that in the matrix of the as-cast Ni-based binary alloy,The microstructure characteristics of alloy C remainedthere were no dendrite structures, but only dispersed fineconstant. Figure 4(c) indicates that the volume fraction ofparticles. For alloy A, the microstructure was dendrite, simi-fine precipitates in the fcc matrix was more than in thelar to that of Ni-based superalloys. The dendrites were wellmatrix shown in Figure 2(i).developed and some secondary dendrite arms formed cellu-

Figure 5 shows the portion of DTA cooling traces of thelar structures. Figure 2(b) shows that there are two typesas-cast alloy A. The curve is representative. The similar L12of particles in the interdendritic areas. Figure 2(c) and (d)precipitation should also be observed in alloys B and C.indicated that with increasing Ir-based alloy proportion (up

After the initial precipitation temperature of 1467 8C wasto 75 pct), dendrite structures were developed and were thereached, the slope of the DTA trace changed at 1450 8Cprimary phase. For pure Ir-based binary alloys, the micro-and another sharp peak, corresponding to higher heat flow,structure was also dendritic, and the grain boundary couldappeared at 1427 8C. This means two phase transformationsbe clearly observed.

After heat treatment at 1300 8C for 168 hours, the phases occurred. Based on the known characteristics of this alloy,

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Fig. 2—Photographs of Ni-and Ir-based binary (SEM) and quaternary Ir-Nb-Ni-Al alloys (BSI) ordered according to the proportion of Ir-based alloy from0 to 100 pct. (a) through (e) as-cast; and ( f ) through ( j ) heated at 1300 8C for 168 h. (a) and (f) Ni-16.8 at. pct Al, (b) and (g) Ir-5Nb-62.4Ni-12.6Al (at.pct) (alloy A), (c) and (h) Ir-10Nb-41.6Ni-8.4Al (at. pct) (alloy B), (d) and (i) Ir-15Nb-20.8Ni-4.2Al (at. pct) (alloy C), and (e) and (j) Ir-20 at. pct Nb.

we conclude that the transformations at 1450 8C and 1427 B. Compressive Strength8C correspond to the formation of Ir3Nb and Ni3Al phases,

Figure 6 shows the compressive strengths of Ir-Nb-Ni-Alrespectively. The peak at 1467 8C corresponds to the solidifi-cation of the fcc matrix. alloys at 1200 8C. For reference, we also show the strengths

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 31A, JANUARY 2000—175

Fig. 5—Portion of DTA cooling trace of alloy A.

Fig. 3—Cuboidal L12 precipitates formed in the fcc matrix in alloy A.

Table I. Composition of Phases in Quaternary Ir-Nb-Ni-Al Alloys Heat treated at 1300 8C for 168 Hours

(Atomic Percent)

Phase andAlloy Structure Ir Ni Nb Al

Alloy A L12-Ir3Nb 50.39 21.39 19.06 9.16L12-Ni3Al 23.10 42.59 10.22 24.10fcc 1 L12 17.63 59.94 5.67 16.75

Alloy B L12-Ir3Nb 65.55 6.50 19.25 8.70B2-IrAl 39.42 12.38 3.62 44.57

fcc 1 L12 23.60 52.33 5.96 18.11Alloy C L12-Ir3Nb 63.55 3.35 22.30 10.80

fcc(1L12) 38.92 44.31 5.61 11.16

at 1200 8C of the commercial Ni-based superalloys,MarM247,[10] and Ir-based alloys, Ir-20 at. pct Nb[8] and Fig. 6—Compressive strength at 1200 8C of Ir-Nb-Ni-Al and Ir-Ta-Ni-Al

alloys[11] heat treated at 1400 8C for 168 h.Ir-Ta-Ni-Al.[11] Compared to the compressive strength ofMarM247 of about 50 MPa, the compressive strength at

Fig. 4—Backscattered images of Ir-Nb-Ni-Al alloys heated at 1400 8C for 168 h: (a) Ir-5Nb-62.4Ni-12.6Al (at. pct) (alloy A), (b) Ir-10Nb-41.6Ni-8.4Al(at. pct) (alloy B), and (c) Ir-15Nb-20.8Ni-4.2Al (at. pct) (alloy C).

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1200 8C of quaternary Ir-Nb-Ni-Al was from 130 to 350 strengths of Ni-and Ir-based binary alloys. The compressivestrength of quaternary Ir-Nb-Ni-Al refractory superalloysMPa. The compressive strength increased with increasing

concentration of the Ir-based alloy, but did not obtain the increased by 600 pct compared to Ni-based superalloys, butdecreased by 30 pct compared with Ir-based superalloyscompressive strength of Ir-20 at. pct Nb binary alloys of 500

MPa. During the compression test, these samples exhibited Converts of the compressive strain of quaternary Ir-Nb-Ni-Al alloys increased greatly.excellent compressive strain characteristics. There were no

macrocracks on the surface of the sample after compression Similarly to the characteristics of Ni-based superalloysthat contribute to their compressive strength, the compres-tests, and they were not fractured, even after reaching 90

pct strain. Compared with Ir-based alloys (below 10 pct), sive strength of quaternary Ir-Nb-Ni-Al alloys can be attrib-uted to the precipitation and solid solution hardening as wellthe quaternary alloys showed superior compressive strain.as to the coherent interface strengthening. The larger L12

phase with 30-mm precipitates exists up to 1400 8C and isV. DISCUSSION easily deformed during compression tests. This explains why

the compressive strength of quaternary alloys is lower thanA. Microstructure Evolutionthat of Ir-based alloys.

The most important aspect of the microstructure in super- The strength of quaternary alloys increased with increas-alloys is the coherency of the interface between the fcc and ing concentration of Ir-based alloys. If we obtained a micro-L12 phases. Even though two kinds of alloys with fcc/L12 structure containing coherent structures, the compressivetwo-phase coherent structures were combined, our studies strength would also increase. The shaded region in Figureindicated that, contrary to expectations, the quaternary alloys 6 shows the hypothetical strength. We therefore expectdid not show L12 and fcc two-phase structures. Rather, three- improved strength if the microstructure can be preciselyphase equilibrium—two types of L12 and fcc phases— controlled.coexisted in alloys A, B, and C at 1400 8C. In addition to Compared with other quaternary Ir-Ta-Ni-Al alloys,[11]

the L12 and fcc phases, the B2-IrAl phase existed in alloy the compressive strength of alloy A, which included 25 pctB at 1300 8C. The B2 phase occurred because the composi- Ir-based alloy, was about 60 pct higher, but that of alloy C,tion of alloy B was slightly outside of the three-phase equilib- which included 75 pct Ir-based alloy, was about 43 pct lower.rium region. For Ir-Ta-Ni-Al alloys, there were many larger precipitate

The ideal microstructure in Ni-based superalloys should particles at the grain boundaries in alloy A and these particlesbe obtained by dissolving the larger L12 phase and reprecipi- weakened the strength of the grain boundary. This is becausetating fine cuboidal L12 phases by using heat treatment. the fcc matrix belongs to the Ir-based solid mixture in alloyHowever, as shown in Figure 4, the as-cast primary L12 C in quaternary Ir-Ta-Ni-Al alloys. Therefore, the strengthphases in quaternary Ir-Nb-Ni-Al refractory superalloys per- of alloy C in the Ir-Ta-Ni-Al quaternary alloy was highersisted, even in samples heat treated at 1400 8C for 168 hours. than that of alloy C in the Ir-Nb-Ni-Al quaternary alloy.These results can be explained as follows. From the resultsof DTA (Figure 5), the precipitating temperature of the large

C. Potential for Ultra-High-Temperature Materialsand fine cuboidal L12 phases is 1427 8C and 1450 8C, respec-tively. Ideally, the dissolving temperature should be higher To increase the high-temperature compressive strengththan the precipitation temperature. The dissolving tempera- of Ni-based superalloys, increasing amounts of refractoryture of the Ni3Al type L12 phase with about 23 at. pct Ir is elements, such as Mo, W, and Re, were added to Ni-basedhigher than 1427 8C, which is higher than that of Ni-based super alloys. The third generation Ni-based superalloys aresuperalloys, which dissolve at about 1300 8C. In this study, represented by the level of Re content, up to 6 wt pct.[12,13]

the samples were heated to 1400 8C. In this case, the primary But excess addition of alloying elements may reduce theL12 phase could not be dissolved by heat treatment at 1400 microstructural stability and therefore enhance the precipita-8C for 168 hours. We expect that similar behavior occurs in tion of the topologically closed-packed phase.[14] Murakamialloys B and C. Considering the three-phase equilibrium of et al.[15] tested other refractory elements for enhancing Ni-alloy A, if heat treatment of the fcc region is done and based superalloys. They investigated the Ni-based super-proceeds by a proper aging treatment, only fine precipitates alloys containing 2 at. pct Ir and indicated that Ir atoms areshould be obtained. Therefore, two possibilities for improv- expected to act as solid solution hardeners of both the g anding the microstructure of quaternary Ir-Nb-Ni-Al alloys g 8 phases, without reducing the microstructural stability.exist. One is to decrease the amount of the larger primary L12 The Ir-bearing alloys are therefore promising, especially inphase by decreasing the amount of the L12 phase-forming terms of phase stability, and will become candidates for theelement, such as Nb and Al. Another possibility is to change next generation of alloys.the ingot-preparation method. These two methods are sub- The Ni-based superalloys contain only one kind of fcc/jects of ongoing research. We will report these result in L12 coherent structure and only one kind of L12 phase.future articles. Unlike Ni-based superalloys, quaternary Ir-Nb-Ni-Al refrac-

tory superalloys contain both Ir3Nb and Ni3Al phases. Thefcc/Ir3Nb or fcc/Ni3Al coherent structures are also found in

B. Compressive Strength the matrix of Ir-Nb-Ni-Al refractory superalloys. Althoughthe quaternary alloys did not show fcc/L12 two-phase struc-In general, the law of mixture strength of composite mate-

rials can estimate the strength of quaternary alloys. This is ture, depending on mixture ratios and heat treatment, fcc/Ir3Nb, fcc/Ni3Al, and fcc/(Ir3Nb 1 Ni3Al) coherent structurea linear combination of the strength of the two types of

component binary alloys. From Figure 6, we see that the may exist in these quaternary refractory superalloys. Whenthe microstructures are controlled, this new type of coherentstrengths of quaternary alloys are located between the

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 31A, JANUARY 2000—177

structure will be more effective for strengthening compared SETARAM Corp., for DTA measurements and RIGAKUInternational Corp. for introducing the SETARAM DTAwith that of fcc/Ni3Al coherent structures found in Ni-

based superalloys. apparatus.Quaternary alloys are promising for applications at tem-

peratures above 1500 8C. Components can potentially be REFERENCESadjusted to produce alloys meeting specified target require-

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Nb and Ni-Al alloys, both having fcc and L12 two-phase in Structural Intermetallics 1997, M.V. Nathal, R. Darolia, C.T. Liu,structures. The Ir-Nb-Ni-Al quaternary alloys were heat P.L. Martin, D.B. Miracle, R. Wagner, and M. Ramaguchi, eds., TMS,

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Subramanian, and L. Kacprzak, eds., William W. Scott, Jr., 1992,phases, the B2 phase existed in alloy B. The compressive vol. 1, p. 183.strengths of the quaternary Ir-Nb-Ni-Al alloys at 1200 8C 10. W.F. Brown, Jr., H. Mindin, and C.Y. Ho: Aerospace Structural Metals

Handbook, CINDAS/Purdue University, West Lafayette, IN, 1992,were 600 pct higher than that of commercial Ni-based super-vol. 5, p. 4218.alloys, MarM247, and were 30 pct lower than that or Ir-20

11. Xihong Yu, Yoko Yamabe-Mitarai, Yoshikazu Ro, and Hiroshi Harada:at. pct Nb alloys. The quaternary alloys exhibited excellent unpublished research, 1998.compressive strain. Therefore, improved properties can be 12. W.S. Walston, K.S. O’Hara, E.W. Ross, T.M. Pollock, and W.H. Mur-expected in the future. phy: in Superalloys 1996, R.D. Kissinger, D.J. Deye, D.L. Anton,

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13. Y. Koizumi, T. Kobayashi, T. Yokokawa, T. Kimura, M. Osawa, and H.ACKNOWLEDGMENTSHarada: Materials for Advanced Power Engineering 1998, J. Lecomte-Beckers, F. Schubert, and P.J. Ennis, eds., 1998, vol. 5, part II, p. 1098.We are grateful to Dr. Y. Gu, Mr. T. Yokokawa, Dr. H.

14. W. S. Walston, J.C. Schaeffer, and W.H. Murphy: in Superalloys 1996,Murakami, and Dr. M. Osawa for their assistance with theR.D. Kissinger, D.J. Deye, D.L. Anton, A.D. Cetel, M.V. Nathal, T.M.experiments and for useful discussions. We also thank Pollock, and D.A. Woodford, TMS, Warrendale, PA, 1996, p. 9.

Messrs. S. Nishikawa and T. Maruko, Furuya Metal Co. 15. H. Murakami, Y. Koizumi, T. Yokokawa, Y. Yamabe-Mitarai, T. Yama-gata, and H. Harada: Mater. Sci. Eng., 1998, vol. A250, p. 109.Ltd., for providing Ir-based alloys. We thank Mr. L. Benois,

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