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Production scale processing of a new intermetallic NiAl–Ta–Cr alloy for high-temperature application
Journal of Materials Processing Technology 136 (2003) 114–119
Production scale processing of a new intermetallic NiAl–Ta–Cr alloy for high-temperature application Part II. Powder metallurgical production of bolts by hot isostatic pressing M. Palma,*, J. Preuhsb,1, G. Sauthoffa a
b
Max-Planck-Institut fu¨r Eisenforschung GmbH, D-40074 Du¨sseldorf, Germany DONCASTERS Precision Castings - Bochum GmbH, D-44725 Bochum, Germany Received 9 May 2001; accepted 21 November 2002
Abstract A series of cylindrical bolts of 80 mm diameter and 115 mm height each weighing about 3.5 kg were produced by hot isostatic pressing (HIP) from pre-alloyed powder of the new intermetallic NiAl–Ta–Cr alloy for high-temperature application with nominal composition 45 Ni, 45 Al, 7.5 Cr and 2.5 Al (at.%). The HIP parameters (temperature, pressure, and time) were chosen according to production scale processing conditions for obtaining specific microstructures. To avoid reactions between the NiAl–Ta–Cr alloy and the Ni capsule, a special coating technique was employed. The microstructure of the compacts was characterised by light optical and scanning electron microscopy and the compositions of the constituent alloy phases were determined by energy dispersive analysis. The temperature dependence of the 0.2% yield stress and the stress and temperature dependence of creep were checked and compared with those of the corresponding laboratory scale material. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Intermetallics; Powder metallurgy; Hot isostatic pressing
1. Introduction The new intermetallic NiAl–Ta–Cr alloy with strengthening Laves phase with nominal composition 45 Ni, 45 Al, 7.5 Cr and 2.5 Ta (at.%)—IP75 for short—[1–9] is attractive for high-temperature applications because of its higher melting temperature, lower density, higher oxidation resistance and thermal conductivity compared to Ni-base superalloys. A problem is the comparatively high brittle-to-ductile transition temperature (BDTT) of this intermetallic alloy which precludes the application of forming processes at intermediate and low temperatures. A possibility for producing components is investment casting, which was subject of Part I of this study. Alternatively components may be produced by powder metallurgical methods [10–12] which is subject *
Corresponding author. Tel.: þ49-221-6792-226; fax: þ49-221-6792-537. E-mail address: [email protected] (M. Palm). 1 Present address: MAN Turbomaschinen AG - GHH BORSIG, D-46145 Oberhausen, Germany.
of the present Part II. Powder metallurgical methods additionally offer the possibility of refining the microstructure, which is advantageous for improving the toughness. It was shown previously that powder metallurgical processing reduces the BDTT of IP75 alloys substantially with simultaneous reduction of creep strength [5]. The present work is part of a larger cooperative development project which aims at using intermetallic NiAl-base alloys as combustor liner panel material for industrial gas turbines [13].
2. Experimental 2.1. Powder production The powder of composition 45 at.% Ni, 45 at.% Al, 7.5 at.% Cr and 2.5 at.% Ta was produced by gas atomisation by H.C. Starck, Laufenburg. Melts were produced from the pure elements and atomised at about 1750 8C using argon. Several batches each weighing about 150 kg were molten and the gain of powder was between 96.3 and 99.5%.
0924-0136/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0924-0136(02)01104-4
M. Palm et al. / Journal of Materials Processing Technology 136 (2003) 114–119 Table 1 Used HIP parameters Bolt no.
Temperature Pressure (8C) (MPa)
Time (h)
Bolt diameter Ø and height
Comments
7567
1400
160
4
Ø 50 mm, 50 mm
7569
1400
150
4
Ø 50 mm, 50 mm
Partially molten Partially molten
7559 7538 7568 7570 7571 7537 7847 7885 7886 7848 7849 7850
1385 1350 1350 1360 1250 1250 1250 1250 1250 1250 1250 1250
160 150 200 200 200 150 150 150 150 150 139 150
4 4 4 4 4 4 4 4 4 4 4 4
Ø Ø Ø Ø Ø Ø Ø Ø Ø Ø Ø Ø
50 mm, 50 mm, 50 mm, 50 mm, 50 mm, 50 mm, 80 mm, 20 mm, 20 mm, 80 mm, 80 mm, 80 mm,
50 mm 50 mm 50 mm 50 mm 50 mm 150 mm 115 mm 50 mm 50 mm 115 mm 115 mm 115 mm
115
A special ceramic coating for the inner surface of the capsule was used to avoid unacceptable reactions of the prealloyed NiAl–Ta–Cr powder with the Ni container during HIP. The coating was produced by plasma deposition of an Al2O3 film. After HIP the Ni capsule was removed by cutting off and grinding. 2.3. Microstructure characterisation and mechanical testing The microstructures of the obtained bolts were analysed and the mechanical properties were determined as described in Part I.
3. Results and discussion 3.1. Impurities
Impurities were less than typically 0.06 wt.% Fe, 0.015 wt.% C, 0.001 wt.% S, 0.04 wt.% O2, 0.01 wt.% N2 and 0.02 wt.% B. The tap density of the powder was 4.2 g/cm3 while the measured density of the powder was 6.32 g/cm3, which is about the theoretical density of IP 75.
The impurity content was determined for bolt no. 7537 (Table 1) with HIP at 1250 8C and 150 MPa for 4 h. Less than 0.07 wt.% Fe, 0.015 wt.% C, 0.001 wt.% S, 0.05 wt.% O2, 0.01 wt.% N2, 0.02 wt.% B and 0.1 wt.% Si were found. This is in agreement with the impurity content of the used powder except a minimal increase in the oxygen content, i.e. no impurities were introduced by the HIP process.
2.2. Hot isostatic pressing (HIP) 3.2. Microstructure A number of cylindrical bolts of 80 mm diameter, 115 mm height and 3.5 kg weight were produced by using a near-net-shape technique with HIP. The used HIP parameters are shown in Table 1 and the grain size distribution of the used powders is given in Table 2. For HIP both steel and Ni capsules were used. The bolts for mechanical testing were fabricated by HIP at 1250 8C with 150 MPa for 240 min. The obtained material is designated as PM75-DPC.
The produced PM bolts were found to show no porosity. The scanning electron microscopy (SEM) image in Fig. 1 shows the microstructure of the PM75-DPC material which was produced by HIP at 1250 8C and 150 MPa for 4 h. The microstructure is homogeneous and the average grain sizes
Table 2 Grain size distribution of the powder used for HIP as determined according to ASTM B 214 and using a Microtac II SRA device ASTM B 214
Microtac II SRA
Particle diameter (mm)
Frequency (%)
Particle diameter (mm)
Frequency (%)
>150 150–125 125–90 90–75 75–63 63–45 <45
5.8 10.5 20.9 12.6 10.2 18.1 21.9
<248.90 <176.00 <124.45 <88.00 <62.23 <44.00 <31.11 <22.00 <15.56
100.0 91.8 72.0 49.6 33.1 19.3 7.8 1.3 0.0
<33.54 <88.54 <171.23
10 50 90
Fig. 1. SEM micrograph (back-scattered electron (BSE) contrast) of PM75-DPC material produced with optimised HIP parameters at 1250 8C with 150 MPa for 4 h (grey matrix: NiAl; light precipitates: Laves phase with C14 structure).
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Fig. 2. Light optical micrograph of a section of an HIP capsule after HIP at 1350 8C with 150 MPa for 4 h. Between the Ni capsule and the HIPped NiAl–Ta– Cr alloy (PM75-DPC) a reaction zone is observed from which an extended crack system originates.
are 10 mm and about 2 mm of the NiAl phase and the Laves phase, respectively. 3.3. Reactions of PM75-DPC with capsules Fig. 2 shows a light optical micrograph of a section through an HIP capsule after HIP at 1350 8C and 150 MPa for 4 h. A reaction zone formed between the Ni capsule and the HIPped NiAl–Ta–Cr alloy PM75-DPC. Closer inspection by SEM revealed that Ni3Al formed in the reaction zone and in addition precipitates of Ni3Al were found in an adjacent zone of 300 mm thickness (Fig. 3). This gave rise to an
extended crack system. The crack originated from the Ni3Al zone and propagated along the Ni3Al grains into the HIPped alloy (Fig. 3). The reaction zone formation was avoided by coating the Ni capsules with Al2O3. Fig. 4 shows an SEM micrograph of the contact region between the Al2O3-coated Ni capsule and PM75-DPC after HIP at 1250 8C and 150 MPa for 4 h. No reaction zone formed during HIP, i.e. the employed coating technique effectively prevents the formation of reaction zones. For the first HIP tests a steel sleeve was used as filler pipe in the upper part of the Ni capsule. Fig. 5 is a light optical micrograph of the steel sleeve after HIP at 1350 8C and 200 MPa for 4 h. During HIP a reaction zone between the steel sleeve and the HIPped NiAl–Ta–Cr alloy PM75-DPC formed. SEM investigation of the respective region revealed that a crack formed between the sleeve and PM75-DPC and there is only partial coherence between the powder particles (Fig. 6). Energy dispersive spectroscopy (EDS) analyses of the respective area showed that the PM75-DPC material was contaminated by Fe in a surface layer of several hundred micrometre depth. This reaction was avoided by using Ni sleeves in subsequent HIP tests. 3.4. Mechanical properties
Fig. 3. SEM micrograph of the reaction zone with crack system of Fig. 2 (grey matrix: NiAl; light precipitates and reaction zone: Ni3Al; light grey phase on the right: Ni capsule (BSE contrast).
The 0.2% flow stress as a function of temperature and secondary creep rates were determined—primarily in compression with some additional tensile tests at 500 and 1000 8C—for comparing the present production scale
M. Palm et al. / Journal of Materials Processing Technology 136 (2003) 114–119
117
Fig. 4. SEM micrograph of the contact region between Al2O3-coated Ni capsule and PM75-DPC. No reaction zone has formed during HIP at 1250 8C with 150 MPa for 4 h (BSE contrast).
material PM75-DPC with material, which was previously produced on a laboratory scale, i.e. PM75-MPI [5]. The present material exhibits somewhat lower 0.2% flow stresses compared to PM75-MPI except at 1200 8C as is
Fig. 5. Light optical micrograph of a section of the upper part of an HIP capsule after HIP at 1350 8C with 200 MPa for 4 h. Reaction zones have formed between the Ni capsule and PM75-DPC as well as between the steel sleeve (‘‘steel’’) and PM75-DPC.
visible in Fig. 7. The differences in the 0.2% flow stresses are believed to be attributed to different levels of impurities since PM75-DPC and PM75-MPI show similar microstructures. Indeed significant effects of impurities on BDTT on PM75-MPI material were observed previously [5]. It is noted that the data for the tensile 0.2% flow stress at 1000 8C are only slightly lower than those for the compression tests of PM75-DPC. At 500 8C no tensile data could be obtained because of premature brittle cracking.
Fig. 6. SEM micrograph (BSE contrast) of the reaction zone between PM75-DPC and steel sleeve of Fig. 5. In the upper part various precipitates were formed during HIP. Below the crack the PM75 powder is hardly consolidated and there are pores. EDS analyses revealed about 4 at.% Fe in the NiAl matrix (grey phase) in this area.
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Fig. 7. The 0.2% flow stress of PM75-MPI and PM 75-DPC as a function of temperature (tests with 104 s1 deformation rate in compression (comp.) and tension (tens.)).
Fig. 8. Secondary creep rate (in compression) at various temperatures as a function of applied stress for PM75-MPI and PM75-DPC.
Secondary creep rates were determined in compression as a function of applied stress by stepwise loading between 900 and 1200 8C. The data for PM75-DPC and PM75-MPI (Fig. 8) are in satisfying agreement at higher temperatures above 1000 8C. Only at the lower temperature of 900 8C the production scale material shows lower creep rates, i.e. higher creep resistances, than the laboratory scale material. This effect is believed to be a result of differing initial microstructures.
4. Conclusions The intermetallic NiAl-base Ni–45Al–2.5Ta–7.5Cr alloy IP75 (at.%) was processed by HIP of pre-alloyed powders using various HIP conditions. HIP at 1250 8C with 150 MPa for 240 min in Al2O3-coated Ni capsules produced homogeneous material without porosity and cracks. The obtained compacts may be used for fabricating combustor liner panels for industrial gas turbines. Respective test panels
M. Palm et al. / Journal of Materials Processing Technology 136 (2003) 114–119
119
Fig. 9. Test panels for combustion tests produced from the HIP material PM75-DPC by electro-discharge machining (EDM).
for combustion tests have been already produced from the HIP material PM75-DPC by electro-discharge machining (Fig. 9). Acknowledgements The financial support by the Bundesministerium fu¨ r Forschung und Technologie within the MaTech program (project 03N20009) is gratefully acknowledged. References [1] B.A. Zeumer, Zur Entwicklung von Laves-Phasen-versta¨ rkten NiAlBasis-Legierungen fu¨ r Anwendungen bei hohen Temperaturen, VDI Verlag, Du¨ sseldorf, 1994, pp. 1–140. [2] B. Zeumer, G. Sauthoff, Intermetallic NiAl–Ta alloys with strengthening laves phase for high-temperature applications. I. Basic properties, Intermetallics 5 (1997) 563–577. [3] B. Zeumer, G. Sauthoff, Deformation behaviour of intermetallic NiAl–Ta alloys with strengthening laves phase for high-temperature applications. II. Effects of alloying with Nb and other elements, Intermetallics 5 (1997) 641–649. [4] B. Zeumer, G. Sauthoff, Deformation behaviour of intermetallic NiAl–Ta alloys with strengthening laves phase for high-temperature applications. III. Effects of alloying with Cr, Intermetallics 6 (1998) 451–460. [5] B. Zeumer, W. Sanders, G. Sauthoff, Deformation behaviour of intermetallic NiAl–Ta alloys with strengthening laves phase for hightemperature applications. IV. Effects of processing, Intermetallics 7 (1999) 889–899.
[6] M. Palm, G. Sauthoff, Werkstoffcharakterisierung und -optimierung von NiAl–Ta–Cr-Legierungen fu¨ r Anwendungen im Gasturbinenbau, in: R. Kopp, P. Beiss, K. Herfurth, D. Bo¨ hme, R. Bormann, E. Arzt, H. Riedel (Eds.), Werkstoffwoche’98 - Band VI: Symposium 8: Metalle, Symposium 14: Simulation Metalle, Wiley–VCH, Weinheim, 1999, pp. 503–508. [7] G. Sauthoff, B. Zeumer, Nickel–aluminium–legierung, German Patent DE 4 417 936 C1 (7–12–1995). [8] G. Sauthoff, B. Zeumer, Intermetallic nickel–aluminum base alloy and material formed of the alloy, US Patent 5 935 349 (10–8– 1999). [9] G. Sauthoff, B. Zeumer, Intermetallitsheskiy splav na osnove nikelalyuminiya, Russian Federation Patent 2 148 671 (10–5–2000). [10] J.C. Borofka, J.K. Tien, R.D. Kissinger, Powder metallurgy and oxide dispersion processing of superalloys, in: J.K. Tien, T. Caulfield (Eds.), Superalloys, Supercomposites and Superceramics, Academic Press, Boston, 1989, pp. 237–284. [11] R.M. German, R.G. Iacocca, Powder metallurgy processing, in: N.S. Stoloff, V.K. Sikka (Eds.), Physical Metallurgy and Processing of Intermetallic Compounds, Chapman & Hall, London, 1996, pp. 605–654. [12] K.G. Shaw, J.H. Reinshagen, Powder production of nickel and iron aluminides, in: S.C. Deevi, P.J. Maziasz, V.K. Sikka, R.W. Cahn (Eds.), Proceedings of the International Symposium on Nickel and Iron Aluminides: Processing, Properties, and Applications, ASM International, Materials Park, 1997, pp. 301–311. [13] W. Hermann, U. Paul, M. Palm, G. Sauthoff, R. Rablbauer, G. Frommeyer, H. Rothe, F. Scheppe, P.R. Sahm, J. Preuhs, Intermetallische NiAl-Komponenten fu¨ r Systeme zur umweltfreundlichen Energiewandlung, in: A. Kranzmann, U. Gramberg (Eds.), Werkstoffwoche’98 - Band III: Symposium 3: Werkstoffe fu¨ r die Energietechnik, Symposium 7: Werkstoffe und Korrosion, Wiley– VCH, Weinheim, 1999, pp. 43–48.
Production scale processing of a new intermetallic NiAl–Ta–Cr alloy for high-temperature application Part II. Powder metallurgical production of bolts by hot isostatic pressing M. Palma,*, J. Preuhsb,1, G. Sauthoffa a
b
Max-Planck-Institut fu¨r Eisenforschung GmbH, D-40074 Du¨sseldorf, Germany DONCASTERS Precision Castings - Bochum GmbH, D-44725 Bochum, Germany Received 9 May 2001; accepted 21 November 2002
Abstract A series of cylindrical bolts of 80 mm diameter and 115 mm height each weighing about 3.5 kg were produced by hot isostatic pressing (HIP) from pre-alloyed powder of the new intermetallic NiAl–Ta–Cr alloy for high-temperature application with nominal composition 45 Ni, 45 Al, 7.5 Cr and 2.5 Al (at.%). The HIP parameters (temperature, pressure, and time) were chosen according to production scale processing conditions for obtaining specific microstructures. To avoid reactions between the NiAl–Ta–Cr alloy and the Ni capsule, a special coating technique was employed. The microstructure of the compacts was characterised by light optical and scanning electron microscopy and the compositions of the constituent alloy phases were determined by energy dispersive analysis. The temperature dependence of the 0.2% yield stress and the stress and temperature dependence of creep were checked and compared with those of the corresponding laboratory scale material. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Intermetallics; Powder metallurgy; Hot isostatic pressing
1. Introduction The new intermetallic NiAl–Ta–Cr alloy with strengthening Laves phase with nominal composition 45 Ni, 45 Al, 7.5 Cr and 2.5 Ta (at.%)—IP75 for short—[1–9] is attractive for high-temperature applications because of its higher melting temperature, lower density, higher oxidation resistance and thermal conductivity compared to Ni-base superalloys. A problem is the comparatively high brittle-to-ductile transition temperature (BDTT) of this intermetallic alloy which precludes the application of forming processes at intermediate and low temperatures. A possibility for producing components is investment casting, which was subject of Part I of this study. Alternatively components may be produced by powder metallurgical methods [10–12] which is subject *
Corresponding author. Tel.: þ49-221-6792-226; fax: þ49-221-6792-537. E-mail address: [email protected] (M. Palm). 1 Present address: MAN Turbomaschinen AG - GHH BORSIG, D-46145 Oberhausen, Germany.
of the present Part II. Powder metallurgical methods additionally offer the possibility of refining the microstructure, which is advantageous for improving the toughness. It was shown previously that powder metallurgical processing reduces the BDTT of IP75 alloys substantially with simultaneous reduction of creep strength [5]. The present work is part of a larger cooperative development project which aims at using intermetallic NiAl-base alloys as combustor liner panel material for industrial gas turbines [13].
2. Experimental 2.1. Powder production The powder of composition 45 at.% Ni, 45 at.% Al, 7.5 at.% Cr and 2.5 at.% Ta was produced by gas atomisation by H.C. Starck, Laufenburg. Melts were produced from the pure elements and atomised at about 1750 8C using argon. Several batches each weighing about 150 kg were molten and the gain of powder was between 96.3 and 99.5%.
0924-0136/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0924-0136(02)01104-4
M. Palm et al. / Journal of Materials Processing Technology 136 (2003) 114–119 Table 1 Used HIP parameters Bolt no.
Temperature Pressure (8C) (MPa)
Time (h)
Bolt diameter Ø and height
Comments
7567
1400
160
4
Ø 50 mm, 50 mm
7569
1400
150
4
Ø 50 mm, 50 mm
Partially molten Partially molten
7559 7538 7568 7570 7571 7537 7847 7885 7886 7848 7849 7850
1385 1350 1350 1360 1250 1250 1250 1250 1250 1250 1250 1250
160 150 200 200 200 150 150 150 150 150 139 150
4 4 4 4 4 4 4 4 4 4 4 4
Ø Ø Ø Ø Ø Ø Ø Ø Ø Ø Ø Ø
50 mm, 50 mm, 50 mm, 50 mm, 50 mm, 50 mm, 80 mm, 20 mm, 20 mm, 80 mm, 80 mm, 80 mm,
50 mm 50 mm 50 mm 50 mm 50 mm 150 mm 115 mm 50 mm 50 mm 115 mm 115 mm 115 mm
115
A special ceramic coating for the inner surface of the capsule was used to avoid unacceptable reactions of the prealloyed NiAl–Ta–Cr powder with the Ni container during HIP. The coating was produced by plasma deposition of an Al2O3 film. After HIP the Ni capsule was removed by cutting off and grinding. 2.3. Microstructure characterisation and mechanical testing The microstructures of the obtained bolts were analysed and the mechanical properties were determined as described in Part I.
3. Results and discussion 3.1. Impurities
Impurities were less than typically 0.06 wt.% Fe, 0.015 wt.% C, 0.001 wt.% S, 0.04 wt.% O2, 0.01 wt.% N2 and 0.02 wt.% B. The tap density of the powder was 4.2 g/cm3 while the measured density of the powder was 6.32 g/cm3, which is about the theoretical density of IP 75.
The impurity content was determined for bolt no. 7537 (Table 1) with HIP at 1250 8C and 150 MPa for 4 h. Less than 0.07 wt.% Fe, 0.015 wt.% C, 0.001 wt.% S, 0.05 wt.% O2, 0.01 wt.% N2, 0.02 wt.% B and 0.1 wt.% Si were found. This is in agreement with the impurity content of the used powder except a minimal increase in the oxygen content, i.e. no impurities were introduced by the HIP process.
2.2. Hot isostatic pressing (HIP) 3.2. Microstructure A number of cylindrical bolts of 80 mm diameter, 115 mm height and 3.5 kg weight were produced by using a near-net-shape technique with HIP. The used HIP parameters are shown in Table 1 and the grain size distribution of the used powders is given in Table 2. For HIP both steel and Ni capsules were used. The bolts for mechanical testing were fabricated by HIP at 1250 8C with 150 MPa for 240 min. The obtained material is designated as PM75-DPC.
The produced PM bolts were found to show no porosity. The scanning electron microscopy (SEM) image in Fig. 1 shows the microstructure of the PM75-DPC material which was produced by HIP at 1250 8C and 150 MPa for 4 h. The microstructure is homogeneous and the average grain sizes
Table 2 Grain size distribution of the powder used for HIP as determined according to ASTM B 214 and using a Microtac II SRA device ASTM B 214
Microtac II SRA
Particle diameter (mm)
Frequency (%)
Particle diameter (mm)
Frequency (%)
>150 150–125 125–90 90–75 75–63 63–45 <45
5.8 10.5 20.9 12.6 10.2 18.1 21.9
<248.90 <176.00 <124.45 <88.00 <62.23 <44.00 <31.11 <22.00 <15.56
100.0 91.8 72.0 49.6 33.1 19.3 7.8 1.3 0.0
<33.54 <88.54 <171.23
10 50 90
Fig. 1. SEM micrograph (back-scattered electron (BSE) contrast) of PM75-DPC material produced with optimised HIP parameters at 1250 8C with 150 MPa for 4 h (grey matrix: NiAl; light precipitates: Laves phase with C14 structure).
116
M. Palm et al. / Journal of Materials Processing Technology 136 (2003) 114–119
Fig. 2. Light optical micrograph of a section of an HIP capsule after HIP at 1350 8C with 150 MPa for 4 h. Between the Ni capsule and the HIPped NiAl–Ta– Cr alloy (PM75-DPC) a reaction zone is observed from which an extended crack system originates.
are 10 mm and about 2 mm of the NiAl phase and the Laves phase, respectively. 3.3. Reactions of PM75-DPC with capsules Fig. 2 shows a light optical micrograph of a section through an HIP capsule after HIP at 1350 8C and 150 MPa for 4 h. A reaction zone formed between the Ni capsule and the HIPped NiAl–Ta–Cr alloy PM75-DPC. Closer inspection by SEM revealed that Ni3Al formed in the reaction zone and in addition precipitates of Ni3Al were found in an adjacent zone of 300 mm thickness (Fig. 3). This gave rise to an
extended crack system. The crack originated from the Ni3Al zone and propagated along the Ni3Al grains into the HIPped alloy (Fig. 3). The reaction zone formation was avoided by coating the Ni capsules with Al2O3. Fig. 4 shows an SEM micrograph of the contact region between the Al2O3-coated Ni capsule and PM75-DPC after HIP at 1250 8C and 150 MPa for 4 h. No reaction zone formed during HIP, i.e. the employed coating technique effectively prevents the formation of reaction zones. For the first HIP tests a steel sleeve was used as filler pipe in the upper part of the Ni capsule. Fig. 5 is a light optical micrograph of the steel sleeve after HIP at 1350 8C and 200 MPa for 4 h. During HIP a reaction zone between the steel sleeve and the HIPped NiAl–Ta–Cr alloy PM75-DPC formed. SEM investigation of the respective region revealed that a crack formed between the sleeve and PM75-DPC and there is only partial coherence between the powder particles (Fig. 6). Energy dispersive spectroscopy (EDS) analyses of the respective area showed that the PM75-DPC material was contaminated by Fe in a surface layer of several hundred micrometre depth. This reaction was avoided by using Ni sleeves in subsequent HIP tests. 3.4. Mechanical properties
Fig. 3. SEM micrograph of the reaction zone with crack system of Fig. 2 (grey matrix: NiAl; light precipitates and reaction zone: Ni3Al; light grey phase on the right: Ni capsule (BSE contrast).
The 0.2% flow stress as a function of temperature and secondary creep rates were determined—primarily in compression with some additional tensile tests at 500 and 1000 8C—for comparing the present production scale
M. Palm et al. / Journal of Materials Processing Technology 136 (2003) 114–119
117
Fig. 4. SEM micrograph of the contact region between Al2O3-coated Ni capsule and PM75-DPC. No reaction zone has formed during HIP at 1250 8C with 150 MPa for 4 h (BSE contrast).
material PM75-DPC with material, which was previously produced on a laboratory scale, i.e. PM75-MPI [5]. The present material exhibits somewhat lower 0.2% flow stresses compared to PM75-MPI except at 1200 8C as is
Fig. 5. Light optical micrograph of a section of the upper part of an HIP capsule after HIP at 1350 8C with 200 MPa for 4 h. Reaction zones have formed between the Ni capsule and PM75-DPC as well as between the steel sleeve (‘‘steel’’) and PM75-DPC.
visible in Fig. 7. The differences in the 0.2% flow stresses are believed to be attributed to different levels of impurities since PM75-DPC and PM75-MPI show similar microstructures. Indeed significant effects of impurities on BDTT on PM75-MPI material were observed previously [5]. It is noted that the data for the tensile 0.2% flow stress at 1000 8C are only slightly lower than those for the compression tests of PM75-DPC. At 500 8C no tensile data could be obtained because of premature brittle cracking.
Fig. 6. SEM micrograph (BSE contrast) of the reaction zone between PM75-DPC and steel sleeve of Fig. 5. In the upper part various precipitates were formed during HIP. Below the crack the PM75 powder is hardly consolidated and there are pores. EDS analyses revealed about 4 at.% Fe in the NiAl matrix (grey phase) in this area.
118
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Fig. 7. The 0.2% flow stress of PM75-MPI and PM 75-DPC as a function of temperature (tests with 104 s1 deformation rate in compression (comp.) and tension (tens.)).
Fig. 8. Secondary creep rate (in compression) at various temperatures as a function of applied stress for PM75-MPI and PM75-DPC.
Secondary creep rates were determined in compression as a function of applied stress by stepwise loading between 900 and 1200 8C. The data for PM75-DPC and PM75-MPI (Fig. 8) are in satisfying agreement at higher temperatures above 1000 8C. Only at the lower temperature of 900 8C the production scale material shows lower creep rates, i.e. higher creep resistances, than the laboratory scale material. This effect is believed to be a result of differing initial microstructures.
4. Conclusions The intermetallic NiAl-base Ni–45Al–2.5Ta–7.5Cr alloy IP75 (at.%) was processed by HIP of pre-alloyed powders using various HIP conditions. HIP at 1250 8C with 150 MPa for 240 min in Al2O3-coated Ni capsules produced homogeneous material without porosity and cracks. The obtained compacts may be used for fabricating combustor liner panels for industrial gas turbines. Respective test panels
M. Palm et al. / Journal of Materials Processing Technology 136 (2003) 114–119
119
Fig. 9. Test panels for combustion tests produced from the HIP material PM75-DPC by electro-discharge machining (EDM).
for combustion tests have been already produced from the HIP material PM75-DPC by electro-discharge machining (Fig. 9). Acknowledgements The financial support by the Bundesministerium fu¨ r Forschung und Technologie within the MaTech program (project 03N20009) is gratefully acknowledged. References [1] B.A. Zeumer, Zur Entwicklung von Laves-Phasen-versta¨ rkten NiAlBasis-Legierungen fu¨ r Anwendungen bei hohen Temperaturen, VDI Verlag, Du¨ sseldorf, 1994, pp. 1–140. [2] B. Zeumer, G. Sauthoff, Intermetallic NiAl–Ta alloys with strengthening laves phase for high-temperature applications. I. Basic properties, Intermetallics 5 (1997) 563–577. [3] B. Zeumer, G. Sauthoff, Deformation behaviour of intermetallic NiAl–Ta alloys with strengthening laves phase for high-temperature applications. II. Effects of alloying with Nb and other elements, Intermetallics 5 (1997) 641–649. [4] B. Zeumer, G. Sauthoff, Deformation behaviour of intermetallic NiAl–Ta alloys with strengthening laves phase for high-temperature applications. III. Effects of alloying with Cr, Intermetallics 6 (1998) 451–460. [5] B. Zeumer, W. Sanders, G. Sauthoff, Deformation behaviour of intermetallic NiAl–Ta alloys with strengthening laves phase for hightemperature applications. IV. Effects of processing, Intermetallics 7 (1999) 889–899.
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