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Creep and tensile tests on refractory metals at extremely high temperatures
International Journal of Refractory Metals & Hard Materials 24 (2006) 292–297 www.elsevier.com/locate/ijrmhm
Creep and tensile tests on refractory metals at extremely high temperatures Bernd Fischer a, Stefan Vorberg a, Rainer Völkl b, Manuel Beschliesser c,¤, Andreas HoVmann c a
FH Jena—University of Applied Sciences, Jena, Germany b University Bayreuth, Germany c PLANSEE Aktiengesellschaft, Technology Centre, 6600 Reutte, Austria Received 10 August 2005; accepted 29 October 2005
Abstract The need for mechanical properties at elevated temperatures is high for Wnite element modelling, process optimization, research and development or quality assurance purposes. Obtaining of this data is diYcult, for refractory materials such as molybdenum or tungsten reliable data including precise strain measurement is required up to 2500 °C. Over the last years a cooperation between PLANSEE and the University of Applied Sciences, Jena, Germany was built up. Within the internal research program “Basisdaten” (basic materials data) creep data and mechanical properties of molybdenum, tungsten and their alloys at very high temperatures could be achieved. After a description of the unique test equipment at the University of Applied Sciences, Jena, the results of creep and tensile tests on molybdenum and tungsten sheet material are presented. © 2005 Elsevier Ltd. All rights reserved. Keywords: Mechanical testing; Non-contact strain measurement; Tensile strength; Creep strength
1. Introduction In the Weld of high-temperature applications, an increasing demand can be observed for the use of refractory metals as engineering materials. Therefore, a reliable determination of the high-temperature mechanical properties is essential. The high-temperature mechanical properties are required for the development of optimized alloys and processes, speciWcations in design engineering, modelling of component performance in industrial application and for quality assurance in production. In many cases no data for very high temperatures is available and calculations and constructions are performed on the basis of experience and knowledge. With that no reli-
*
Corresponding author. Tel.: +43 5672 600 2766; fax: +43 5672 600 536. E-mail address: [email protected] (M. Beschliesser).
0263-4368/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2005.10.011
ability and safety calculations can be performed. For research and development activities improvements in properties cannot be captured on a quantitative basis. Because no commercial measuring system exists for creep and tensile tests conducted on metals at temperatures up to 3000 °C, special devices were developed and constructed at the University of Applied Sciences in Jena, Germany. The samples are heated directly by an electric current. Temperature measurement and control are achieved contact-free by a pyrometer and a PID-controller with a temperature tolerance of §5 K. The strain measurement is also accomplished contact-free by a high resolution CCD-camera. Obtained data then are analyzed by the image processing program “SuperCreep” which has also been developed at the University of Applied Sciences. Results gained from these devices have been transferred to industrial partners over the last years which gave proof of the reliability of this testing equipment.
B. Fischer et al. / International Journal of Refractory Metals & Hard Materials 24 (2006) 292–297
293
In cooperation with PLANSEE uniaxial creep and tensile tests on sheet material of molybdenum, tungsten and their alloys which were taken from the commercial production line of PLANSEE have been performed. The Wrst part of this paper covers tensile tests of tungsten sheet material (thickness 1 mm). These tests should give answer to the question whether the results obtained in Jena Wt to the results obtained at PLANSEE. Therefore, tensile tests on a sheet of the same batch were performed in Jena (testing temperature 1800–2500 °C) and in the Testing Laboratories in the Technology Centre of PLANSEE (testing temperature 1400–2100 °C). The second part investigates the creep performance of Mo and ML (molybdenum based material, doped with La2O3 particles) sheet material (thickness 2 mm) at 1400 °C and 1600 °C up to 200 h. 2. Experimental 2.1. General design of test facilities at the University of Applied Sciences, Jena The interest of the research group in Jena is focused on metallic materials for ultra-high-temperature applications. Envisaged test temperatures range up to 3000 °C for Re/W alloys [1]. Commercial test facilities either do not have the required speciWcations or are too expensive. This is the reason why these test facilities [2–6] were designed and built at the University of Applied Sciences, Jena. Ohmic heating was chosen for easy access to the sample, fast heating and cooling cycles, and simplicity in design and operation. Low temperatures at the grips allow the use of inexpensive copper without the need for an active cooling system. Usually specimens in the form of thin wires or strips are tested. The test facilities permit tests either in air or under a protective gas atmosphere. All functions are computer controlled with the software LabView and SuperCreep [4–6], the later developed at the University of Applied Sciences, Jena for strain measurements by means of digital image analysis. Both constant tensile load creep tests and high-temperature tensile tests can be executed. For constant engineering stress creep tests, the load is applied to the sample through a steel pull rod by means of calibrated weights. Alternatively, the specimen chamber can be mounted in a commercial test machine. The steel pull rod is then connected with the load cell at the crosshead of the test machine. A schematic diagram of the test facilities is given in Fig. 1. The temperature is measured with a pyrometer. An adjustable response time down to 1 ms guarantees secure temperature control at high heating rates. A problem often encountered in pyrometry is that the spectral emissivity of the investigated material has to be known as a function of temperature, time and wavelength. Pt/Rh alloys show very slow oxidation at low temperatures. At temperatures above about 1000 °C their oxide scales evaporate. Neuer et al. [7,8] therefore recommend Pt/Rh alloys as reference materials.
Fig. 1. Test facility to measure tensile and creep properties of metallic materials at temperatures up to 3000 °C.
Hence simple calibration of the test system for materials with unknown emissivity can be performed with a thin foil of a Pt/Rh alloy pasted on the specimen. Strain is measured with a video extensometer controlled by the software SuperCreep. A variable exposure time of the CCD camera from 1 to 1000 ms allows images to be grabbed up to 2000 °C without introducing Wlters in the optical path. Telecentric lenses are used to avoid perspective distortions. SuperCreep continuously determines the distances between corresponding markers in the central zone of the specimen where the temperature is uniform. Suitable markers for high temperature tests can be made by laser cutting samples with small shoulders from the sheet material (Fig. 2a), or simply by winding thin wires around the specimen (Fig. 2b). 2.2. Performance of test facilities The temperature distribution was measured by a second pyrometer on a 100 mm long strip of a Pt–10% Rh alloy [5]. The maximum temperature was 1500 °C at the specimen centre. In a zone 30 mm around the specimen centre the temperature was 1500 § 5 °C. In a zone 10 mm between the markers the temperature was within 1500 § 2 °C. During a following creep test the temperature between the markers could be held to 1500 § 3 °C until necking occurred. Due to ohmic heating the temperature outside a necked region is always lower, whereas in the necked region the temperature is kept at the desired value. The test facility allows fast heating and cooling cycles. A maximum temperature of 1506 °C at the centre of the Pt– 10% Rh DPH strip was reached 12 s after turning on power. After 15 s the maximum temperature was held at 1500 § 1 °C. After switching oV the power, the specimen took about 20 s to cool down to 750 °C. Heating and cooling rates of +100 °C/s and ¡30 °C/s, respectively, can be reached [5].
294
B. Fischer et al. / International Journal of Refractory Metals & Hard Materials 24 (2006) 292–297
Fig. 3. Measured thermal strain of pure Pt compared to literature data according to Beck [9] and Arblaster [10].
min which corresponds to an initial strain rate of 2 £ 10¡2 s¡1. The strain was measured in a direct mode with a CCD camera and the Software SuperCreep. Fig. 2. Images of self-radiating specimens with markers for the video extensometer. The initial gauge length deWned by two corresponding markers is approximately 10 mm. (a) Pt–10% Rh specimen at 1500 °C with four small shoulders laser-cut out of sheet. (b) Pt-wires wound round a specimen as markers.
With SuperCreep and the CCD camera the gauge length from the digital image can be determined to a subpixel accuracy of 0.2 pixels. At a maximum measurable strain of t 60%, a maximum error t §0.07% of the strain was determined [6]. The repeatability therefore is less than 0.25% of full-range output. The accuracy can be improved by increasing the initial distance between the markers. However the maximum measurable strain then decreases. To verify the performance of the video extensometer the thermal expansion of pure Pt was studied [6]. The temperature was increased in Wve steps from 1000 °C up to 1250 °C. Two hundred to three hundred measurements were performed at each temperature. The standard deviations D §(0.04–0.05)%, given in Fig. 3, are in good agreement with the estimated error of t §0.07%. Underlying zero strain at 1000 °C, Fig. 3 shows that the measured mean thermal strains lie always in between the values reported by Beck [9] and Arblaster [10]. 2.3. Experimental details Jena The measurements on refractory metals were executed in water-cooled chambers under reducing atmosphere (Ar–5% H2). Samples for creep and tensile testing were produced by laser cutting and grinding. Specimen geometry was 120 £ 4 £ 1 mm (length £ width £ thickness). The gauge length was 10 mm. For tensile tests the testing speed was 10 mm/
2.4. Experimental details PLANSEE Tensile tests have been performed in a high-temperature testing apparatus in vacuum in the Testing Laboratories in the Technology Centre of PLANSEE. Samples for tensile testing were produced by spark erosion and grinding. Specimen geometry was 80 £ 16 £ 1 mm (length £ width £ thickness). The gauge length was 20 mm, testing speed was set to 20 mm/min which corresponds to an initial strain rate of 2 £ 10¡2 s¡1. The strain was recorded in a non-direct mode via cross-head movement. 2.5. Material Tungsten (W) has the highest melting point (Tm D 3420 °C) and lowest vapour pressure (p D 1 £ 10¡5 Pa at 2500 K) of all metals [11]. Therefore, tungsten is commonly used in high-vacuum industry and applications at high temperatures. Tungsten sheet material for this investigation has been produced using powder metallurgy route (O < 5 g/g). Final thickness of 1 mm has been achieved by rolling. Tensile tests have been conducted on stress-relieved W sheet material. Pure molybdenum (Mo) exhibits high melting point (Tm D 2620 °C), low vapour pressure (p D 7 £ 10¡2 Pa at 2500 K) and high corrosion resistance to molten glass and metals [11]. For example, it is used in lightning technology, electronics, automotive industry and high-temperature furnace industry. Additions of lanthanum oxide, La2O3 particles (0.7 wt.%) to pure molybdenum (PLANSEE trade name: ML) result in formation of a layered, Wbrous structure upon recrystalli-
B. Fischer et al. / International Journal of Refractory Metals & Hard Materials 24 (2006) 292–297
295
Table 1 Number of tests performed on stress-relieved tungsten sheet material (thickness 1 mm) in Jena and PLANSEE in the temperature range of 1400–2500 °C Tester
Testing speed [mm/min]
Strain rate [s¡1]
1400 °C
1600 °C
1800 °C
2000 °C
2100 °C
2200 °C
2500 °C
PLANSEE Jena
20 10
2 £ 10¡2 2 £ 10¡2
1
2
2 3
2 3
2 3
3
4
Table 2 Mechanical properties (tensile strength, Rm and yield strength, Rp0.2) of stress-relieved tungsten sheet material generated in tensile tests in facilities of University of Applied Sciences, Jena
Table 3 Mechanical properties (tensile strength, Rm and yield strength, Rp0.2) of stress-relieved tungsten sheet material generated in tensile tests in facilities of Testing Laboratories in the Technology Centre of PLANSEE
Temperature [°C]
Rm [MPa]
Rp0.2 [MPa]
Temperature [°C]
Proben
Rm [MPa]
Rp0.2 [MPa]
A [%]
1800 1800 1800 2000 2000 2000 2100 2100 2100 2200 2200 2200 2500 2500 2500 2500
103.5 105 105.8 76.5 78 78.2 66 66 63.5 53 54.5 54.3 25.3 24.2 20.5 19
61.5 62.5 62.5 51 51 52 46.5 46.3 47.5 38.5 41 41.8 17.5 20 14.3 14
1400 1600 1600 1800 1800 2000 2000 2100 2100
3 3 4 3 4 3 4 3 4
183 138 141 113 114 80 33 63 42a
82 57 59 53 51 40 45 32 30
41.10 45.10 52.80 54.70 54.70 52.30 62.40 38.10 29.00
a Indicates an experiment, where the test start was delayed although the sample had reached the testing temperature.
zation. ML sheet material has excellent creep properties. Examples of application are furnace parts and wires for heating elements. Creep tests have been conducted on stress-relieved Mo and ML sheets which have been rolled to 2 mm thickness and further ground to 1 mm (Mo: O D 24 g/g, C D 8 g/g, ML: O D 1035 g/g, C D 6 g/g). Testing temperature was 1400 °C and 1600 °C which corresponds to homologous temperatures T/Tm of 0.58 and 0.65 respectively. It can be assumed that steady-state creep predominates at these temperatures. 3. Results and discussion 3.1. Tensile tests
Fig. 4. Mean values of mechanical properties of stress-relieved tungsten sheet material (thickness 1 mm) between 1400 °C and 2500 °C determined in uniaxial tensile tests with an initial strain rate of 2 £ 10¡2 s¡1.
Table 1 lists the number of tested tensile-specimens at the given testing temperature. Note the overlap of tests between 1800 °C and 2100 °C to achieve quantitative judgement if a transfer of data recorded on diVerent testing equipments can be done. Results of the tensile tests conducted on the stressrelieved tungsten sheet (thickness 1 mm, initial strain rate 2 £ 10¡2 s¡1) are presented in Tables 2 and 3. Mean values are shown in Fig. 4. The elongation to fracture is not listed due to the fact that the methods of strain-measurement (Jena: direct via SuperCreep, PLANSEE: non-direct via cross-head movement) lead to diVerent i.e. non-comparable results. Tensile strength of tungsten sheet material linearly decreases with increasing temperature in the investigated
temperature range. The data recorded in Jena Wt into the data which have been recorded at PLANSEE. However, the evolution of the yield strength is diVerent although the absolute deviation is only about 10 MPa. In the overlapregion the data recorded in Jena appears to be higher than the one determined at PLANSEE. Since determination of 0.2% plastic yield strength involves strain measurement the origin of error could be found in diVerent types of strain measurements at Jena and PLANSEE. It is unlikely that the eVect is caused by a combination of deviations in strain measurements and the known strain rate sensitivity of the yield strength of body-centred cubic metals since this eVect appears at lower temperatures (approx. <1000 °C for tungsten) [12,13].
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3.2. Creep tests Output data of creep tests are simple strain–time curves which can be further transferred into stress–rupture curves, deformation–time curves and creep rate–stress curves (i.e. Norton plots). The results of the experiments on pure molybdenum and ML sheet material are shown in Figs. 5 and 6. The results show that the addition of obstructions to dislocation movement as realized by adding La2O3 particles to molybdenum (ML) has a beneWcial eVect on both the creep rupture behavior and the minimum creep rate. For example, creep stress for life to rupture of 10 h (see Fig. 5) increases from about 32 to 89 MPa at 1400 °C and from 15 to 43 MPa at 1600 °C (corresponds to an increase by a factor of 2.8 in both cases). However, it has to be stated that a quantitative judgement and description of the creep curves
of Mo and ML is diYcult since parameters such as (sub-) grain size and dislocation density in the starting condition of Mo and ML are diVerent. The Mo sheet material undergoes recrystallization during the creep experiment, moreover, grain growth is likely to occur since no second phase is present. For ML sheet material, recrystallization is delayed due the presence of La2O3 particles. The creep stress sensitivity of the minimum creep rate (Norton exponent n, see Fig. 6) is of the same order for Mo and MLR at 1400 °C and 1600 °C respectively. A Norton exponent between 3 and 8 describes that the creep mechanisms is mainly controlled by dislocation movement (power-law creep). This means that the deformation mechanism is of the same nature in the investigated materials. However, the stress to activate the creep deformation is much higher for ML sheet material. For example, stress to obtain a minimum creep rate of 5 £ 10¡8 s¡1 is 10 MPa for Mo and 40 MPa for ML sheet material. 4. Conclusions
Fig. 5. Creep rupture plot of Mo and ML (stress-relieved) sheet material (thickness 2 mm) tested under constant load conditions in ArH2 atmosphere at 1400 °C and 1600 °C.
The investigation revealed that the test equipment at the University of Applied Sciences, Jena is highly capable of generating creep and tensile testing data of molybdenum and tungsten sheet material up to high temperatures. The results of uniaxial tensile tests on tungsten sheet material (thickness 1 mm) are in good agreement with data recorded in the Testing Laboratories in the Technology Centre of PLANSEE. With the aid of the high-temperature testing equipment mechanical data of tungsten sheet material could be generated up to 2500 °C. The results of creep experiments on molybdenum and La2O3 particles doped molybdenum sheet material (thickness 2 mm) at 1400 °C and 1600 °C showed the beneWcial eVect of particles on both creep rupture strength and minimum creep rate. Acknowledgment Financial support came from PLANSEE’s research project “Basisdaten” which is installed to gain materials data of PLANSEE products in diVerent product forms. References
Fig. 6. Norton plot of Mo and ML (stress-relieved) sheet material (thickness 2 mm) tested under constant load conditions in ArH2 atmosphere at 1400 °C and 1600 °C.
[1] Fischer B, Freund D, Lupton DF. Stress–rupture strength of rhenium at very high temperatures. In: Proceedings of the international symposium on rhenium and rhenium alloys, TMS annual meeting, Orlando, 1997. p. 311–20. [2] Fischer B, Helmich R, Töpfer H. Anordnung zur Warmfestigkeitsprüfung hochschmelzender, elektrisch leitender WerkstoVe. DD Patent 245576 A3, May 1982. [3] Fischer B, Töpfer H, Helmich R. Gerät für Warmfestigkeitsmessungen an hochschmelzenden Metallen nach Einwirkung silikatischer Schmelzen. Silikattechnik 1984;35(11):329–31. [4] Völkl R, Freund D, Fischer B, Gohlke D. Berührungslose Dehnungsaufnahme an widerstandsbeheizten Metallzugproben mit Hilfe digitaler Bildverarbeitung bei Prüftemperaturen bis 3000 °C. In: Proceedings of the conference WerkstoVprüfung 1998, Bad Nauheim,
B. Fischer et al. / International Journal of Refractory Metals & Hard Materials 24 (2006) 292–297
[5] [6] [7]
[8]
Germany, Deutscher Verband für Materialforschung und -prüfung e.V., Berlin, Germany, WerkstoVprüfung Berichtsbände, ISSN 09415300, No. 638, 1998. p. 211–8. Völkl R, Freund D, Fischer B. Economic creep testing of ultra-high temperature alloys. J Testing Eval 2003;31(1):35–43. Völkl R, Fischer B. Mechanical testing of ultra-high temperature alloys. Exp Mech 2004;44(2):121–8. Neuer G, Pohlmann P, Schreiber E. Gesamtemissionsgrad und spektraler Emissionsgrad von Hochtemperaturmaterialien. Technical Report IKE 5-249, Stuttgart, 1998. ISSN 0173-6892. Neuer G, Jaroma-Weiland G. Spectral and total emissivity of high temperature materials. Int J Thermophys 1998;19(3):917–29.
297
[9] Beck G. In: Degussa AG, editor. Edelmetall-Taschenbuch. Hüthig, Heidelberg, Frankfurt Main, 1995. ISBN 3-7785-2448-8. [10] Arblaster JW. Crystallographic properties of platinum. Platinum Metals Rev 1997;41(1):12–21. [11] Martienssen W, Warlimont H. Springer handbook of condensed matter and materials data. Berlin, Heidelberg: Springer-Verlag; 2005. [12] Stickler Ch. Mikroplastizität und zyklisches Spannungs-Dehnungsverhalten von Ta und Mo bei Temperaturen unter 0,2 Tm, Wien, 1998. [13] L. Hollang, Fließspannung und Verfestigung hochreiner MolybdänEinkristalle. Stuttgart, 1996.
Creep and tensile tests on refractory metals at extremely high temperatures Bernd Fischer a, Stefan Vorberg a, Rainer Völkl b, Manuel Beschliesser c,¤, Andreas HoVmann c a
FH Jena—University of Applied Sciences, Jena, Germany b University Bayreuth, Germany c PLANSEE Aktiengesellschaft, Technology Centre, 6600 Reutte, Austria Received 10 August 2005; accepted 29 October 2005
Abstract The need for mechanical properties at elevated temperatures is high for Wnite element modelling, process optimization, research and development or quality assurance purposes. Obtaining of this data is diYcult, for refractory materials such as molybdenum or tungsten reliable data including precise strain measurement is required up to 2500 °C. Over the last years a cooperation between PLANSEE and the University of Applied Sciences, Jena, Germany was built up. Within the internal research program “Basisdaten” (basic materials data) creep data and mechanical properties of molybdenum, tungsten and their alloys at very high temperatures could be achieved. After a description of the unique test equipment at the University of Applied Sciences, Jena, the results of creep and tensile tests on molybdenum and tungsten sheet material are presented. © 2005 Elsevier Ltd. All rights reserved. Keywords: Mechanical testing; Non-contact strain measurement; Tensile strength; Creep strength
1. Introduction In the Weld of high-temperature applications, an increasing demand can be observed for the use of refractory metals as engineering materials. Therefore, a reliable determination of the high-temperature mechanical properties is essential. The high-temperature mechanical properties are required for the development of optimized alloys and processes, speciWcations in design engineering, modelling of component performance in industrial application and for quality assurance in production. In many cases no data for very high temperatures is available and calculations and constructions are performed on the basis of experience and knowledge. With that no reli-
*
Corresponding author. Tel.: +43 5672 600 2766; fax: +43 5672 600 536. E-mail address: [email protected] (M. Beschliesser).
0263-4368/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2005.10.011
ability and safety calculations can be performed. For research and development activities improvements in properties cannot be captured on a quantitative basis. Because no commercial measuring system exists for creep and tensile tests conducted on metals at temperatures up to 3000 °C, special devices were developed and constructed at the University of Applied Sciences in Jena, Germany. The samples are heated directly by an electric current. Temperature measurement and control are achieved contact-free by a pyrometer and a PID-controller with a temperature tolerance of §5 K. The strain measurement is also accomplished contact-free by a high resolution CCD-camera. Obtained data then are analyzed by the image processing program “SuperCreep” which has also been developed at the University of Applied Sciences. Results gained from these devices have been transferred to industrial partners over the last years which gave proof of the reliability of this testing equipment.
B. Fischer et al. / International Journal of Refractory Metals & Hard Materials 24 (2006) 292–297
293
In cooperation with PLANSEE uniaxial creep and tensile tests on sheet material of molybdenum, tungsten and their alloys which were taken from the commercial production line of PLANSEE have been performed. The Wrst part of this paper covers tensile tests of tungsten sheet material (thickness 1 mm). These tests should give answer to the question whether the results obtained in Jena Wt to the results obtained at PLANSEE. Therefore, tensile tests on a sheet of the same batch were performed in Jena (testing temperature 1800–2500 °C) and in the Testing Laboratories in the Technology Centre of PLANSEE (testing temperature 1400–2100 °C). The second part investigates the creep performance of Mo and ML (molybdenum based material, doped with La2O3 particles) sheet material (thickness 2 mm) at 1400 °C and 1600 °C up to 200 h. 2. Experimental 2.1. General design of test facilities at the University of Applied Sciences, Jena The interest of the research group in Jena is focused on metallic materials for ultra-high-temperature applications. Envisaged test temperatures range up to 3000 °C for Re/W alloys [1]. Commercial test facilities either do not have the required speciWcations or are too expensive. This is the reason why these test facilities [2–6] were designed and built at the University of Applied Sciences, Jena. Ohmic heating was chosen for easy access to the sample, fast heating and cooling cycles, and simplicity in design and operation. Low temperatures at the grips allow the use of inexpensive copper without the need for an active cooling system. Usually specimens in the form of thin wires or strips are tested. The test facilities permit tests either in air or under a protective gas atmosphere. All functions are computer controlled with the software LabView and SuperCreep [4–6], the later developed at the University of Applied Sciences, Jena for strain measurements by means of digital image analysis. Both constant tensile load creep tests and high-temperature tensile tests can be executed. For constant engineering stress creep tests, the load is applied to the sample through a steel pull rod by means of calibrated weights. Alternatively, the specimen chamber can be mounted in a commercial test machine. The steel pull rod is then connected with the load cell at the crosshead of the test machine. A schematic diagram of the test facilities is given in Fig. 1. The temperature is measured with a pyrometer. An adjustable response time down to 1 ms guarantees secure temperature control at high heating rates. A problem often encountered in pyrometry is that the spectral emissivity of the investigated material has to be known as a function of temperature, time and wavelength. Pt/Rh alloys show very slow oxidation at low temperatures. At temperatures above about 1000 °C their oxide scales evaporate. Neuer et al. [7,8] therefore recommend Pt/Rh alloys as reference materials.
Fig. 1. Test facility to measure tensile and creep properties of metallic materials at temperatures up to 3000 °C.
Hence simple calibration of the test system for materials with unknown emissivity can be performed with a thin foil of a Pt/Rh alloy pasted on the specimen. Strain is measured with a video extensometer controlled by the software SuperCreep. A variable exposure time of the CCD camera from 1 to 1000 ms allows images to be grabbed up to 2000 °C without introducing Wlters in the optical path. Telecentric lenses are used to avoid perspective distortions. SuperCreep continuously determines the distances between corresponding markers in the central zone of the specimen where the temperature is uniform. Suitable markers for high temperature tests can be made by laser cutting samples with small shoulders from the sheet material (Fig. 2a), or simply by winding thin wires around the specimen (Fig. 2b). 2.2. Performance of test facilities The temperature distribution was measured by a second pyrometer on a 100 mm long strip of a Pt–10% Rh alloy [5]. The maximum temperature was 1500 °C at the specimen centre. In a zone 30 mm around the specimen centre the temperature was 1500 § 5 °C. In a zone 10 mm between the markers the temperature was within 1500 § 2 °C. During a following creep test the temperature between the markers could be held to 1500 § 3 °C until necking occurred. Due to ohmic heating the temperature outside a necked region is always lower, whereas in the necked region the temperature is kept at the desired value. The test facility allows fast heating and cooling cycles. A maximum temperature of 1506 °C at the centre of the Pt– 10% Rh DPH strip was reached 12 s after turning on power. After 15 s the maximum temperature was held at 1500 § 1 °C. After switching oV the power, the specimen took about 20 s to cool down to 750 °C. Heating and cooling rates of +100 °C/s and ¡30 °C/s, respectively, can be reached [5].
294
B. Fischer et al. / International Journal of Refractory Metals & Hard Materials 24 (2006) 292–297
Fig. 3. Measured thermal strain of pure Pt compared to literature data according to Beck [9] and Arblaster [10].
min which corresponds to an initial strain rate of 2 £ 10¡2 s¡1. The strain was measured in a direct mode with a CCD camera and the Software SuperCreep. Fig. 2. Images of self-radiating specimens with markers for the video extensometer. The initial gauge length deWned by two corresponding markers is approximately 10 mm. (a) Pt–10% Rh specimen at 1500 °C with four small shoulders laser-cut out of sheet. (b) Pt-wires wound round a specimen as markers.
With SuperCreep and the CCD camera the gauge length from the digital image can be determined to a subpixel accuracy of 0.2 pixels. At a maximum measurable strain of t 60%, a maximum error t §0.07% of the strain was determined [6]. The repeatability therefore is less than 0.25% of full-range output. The accuracy can be improved by increasing the initial distance between the markers. However the maximum measurable strain then decreases. To verify the performance of the video extensometer the thermal expansion of pure Pt was studied [6]. The temperature was increased in Wve steps from 1000 °C up to 1250 °C. Two hundred to three hundred measurements were performed at each temperature. The standard deviations D §(0.04–0.05)%, given in Fig. 3, are in good agreement with the estimated error of t §0.07%. Underlying zero strain at 1000 °C, Fig. 3 shows that the measured mean thermal strains lie always in between the values reported by Beck [9] and Arblaster [10]. 2.3. Experimental details Jena The measurements on refractory metals were executed in water-cooled chambers under reducing atmosphere (Ar–5% H2). Samples for creep and tensile testing were produced by laser cutting and grinding. Specimen geometry was 120 £ 4 £ 1 mm (length £ width £ thickness). The gauge length was 10 mm. For tensile tests the testing speed was 10 mm/
2.4. Experimental details PLANSEE Tensile tests have been performed in a high-temperature testing apparatus in vacuum in the Testing Laboratories in the Technology Centre of PLANSEE. Samples for tensile testing were produced by spark erosion and grinding. Specimen geometry was 80 £ 16 £ 1 mm (length £ width £ thickness). The gauge length was 20 mm, testing speed was set to 20 mm/min which corresponds to an initial strain rate of 2 £ 10¡2 s¡1. The strain was recorded in a non-direct mode via cross-head movement. 2.5. Material Tungsten (W) has the highest melting point (Tm D 3420 °C) and lowest vapour pressure (p D 1 £ 10¡5 Pa at 2500 K) of all metals [11]. Therefore, tungsten is commonly used in high-vacuum industry and applications at high temperatures. Tungsten sheet material for this investigation has been produced using powder metallurgy route (O < 5 g/g). Final thickness of 1 mm has been achieved by rolling. Tensile tests have been conducted on stress-relieved W sheet material. Pure molybdenum (Mo) exhibits high melting point (Tm D 2620 °C), low vapour pressure (p D 7 £ 10¡2 Pa at 2500 K) and high corrosion resistance to molten glass and metals [11]. For example, it is used in lightning technology, electronics, automotive industry and high-temperature furnace industry. Additions of lanthanum oxide, La2O3 particles (0.7 wt.%) to pure molybdenum (PLANSEE trade name: ML) result in formation of a layered, Wbrous structure upon recrystalli-
B. Fischer et al. / International Journal of Refractory Metals & Hard Materials 24 (2006) 292–297
295
Table 1 Number of tests performed on stress-relieved tungsten sheet material (thickness 1 mm) in Jena and PLANSEE in the temperature range of 1400–2500 °C Tester
Testing speed [mm/min]
Strain rate [s¡1]
1400 °C
1600 °C
1800 °C
2000 °C
2100 °C
2200 °C
2500 °C
PLANSEE Jena
20 10
2 £ 10¡2 2 £ 10¡2
1
2
2 3
2 3
2 3
3
4
Table 2 Mechanical properties (tensile strength, Rm and yield strength, Rp0.2) of stress-relieved tungsten sheet material generated in tensile tests in facilities of University of Applied Sciences, Jena
Table 3 Mechanical properties (tensile strength, Rm and yield strength, Rp0.2) of stress-relieved tungsten sheet material generated in tensile tests in facilities of Testing Laboratories in the Technology Centre of PLANSEE
Temperature [°C]
Rm [MPa]
Rp0.2 [MPa]
Temperature [°C]
Proben
Rm [MPa]
Rp0.2 [MPa]
A [%]
1800 1800 1800 2000 2000 2000 2100 2100 2100 2200 2200 2200 2500 2500 2500 2500
103.5 105 105.8 76.5 78 78.2 66 66 63.5 53 54.5 54.3 25.3 24.2 20.5 19
61.5 62.5 62.5 51 51 52 46.5 46.3 47.5 38.5 41 41.8 17.5 20 14.3 14
1400 1600 1600 1800 1800 2000 2000 2100 2100
3 3 4 3 4 3 4 3 4
183 138 141 113 114 80 33 63 42a
82 57 59 53 51 40 45 32 30
41.10 45.10 52.80 54.70 54.70 52.30 62.40 38.10 29.00
a Indicates an experiment, where the test start was delayed although the sample had reached the testing temperature.
zation. ML sheet material has excellent creep properties. Examples of application are furnace parts and wires for heating elements. Creep tests have been conducted on stress-relieved Mo and ML sheets which have been rolled to 2 mm thickness and further ground to 1 mm (Mo: O D 24 g/g, C D 8 g/g, ML: O D 1035 g/g, C D 6 g/g). Testing temperature was 1400 °C and 1600 °C which corresponds to homologous temperatures T/Tm of 0.58 and 0.65 respectively. It can be assumed that steady-state creep predominates at these temperatures. 3. Results and discussion 3.1. Tensile tests
Fig. 4. Mean values of mechanical properties of stress-relieved tungsten sheet material (thickness 1 mm) between 1400 °C and 2500 °C determined in uniaxial tensile tests with an initial strain rate of 2 £ 10¡2 s¡1.
Table 1 lists the number of tested tensile-specimens at the given testing temperature. Note the overlap of tests between 1800 °C and 2100 °C to achieve quantitative judgement if a transfer of data recorded on diVerent testing equipments can be done. Results of the tensile tests conducted on the stressrelieved tungsten sheet (thickness 1 mm, initial strain rate 2 £ 10¡2 s¡1) are presented in Tables 2 and 3. Mean values are shown in Fig. 4. The elongation to fracture is not listed due to the fact that the methods of strain-measurement (Jena: direct via SuperCreep, PLANSEE: non-direct via cross-head movement) lead to diVerent i.e. non-comparable results. Tensile strength of tungsten sheet material linearly decreases with increasing temperature in the investigated
temperature range. The data recorded in Jena Wt into the data which have been recorded at PLANSEE. However, the evolution of the yield strength is diVerent although the absolute deviation is only about 10 MPa. In the overlapregion the data recorded in Jena appears to be higher than the one determined at PLANSEE. Since determination of 0.2% plastic yield strength involves strain measurement the origin of error could be found in diVerent types of strain measurements at Jena and PLANSEE. It is unlikely that the eVect is caused by a combination of deviations in strain measurements and the known strain rate sensitivity of the yield strength of body-centred cubic metals since this eVect appears at lower temperatures (approx. <1000 °C for tungsten) [12,13].
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3.2. Creep tests Output data of creep tests are simple strain–time curves which can be further transferred into stress–rupture curves, deformation–time curves and creep rate–stress curves (i.e. Norton plots). The results of the experiments on pure molybdenum and ML sheet material are shown in Figs. 5 and 6. The results show that the addition of obstructions to dislocation movement as realized by adding La2O3 particles to molybdenum (ML) has a beneWcial eVect on both the creep rupture behavior and the minimum creep rate. For example, creep stress for life to rupture of 10 h (see Fig. 5) increases from about 32 to 89 MPa at 1400 °C and from 15 to 43 MPa at 1600 °C (corresponds to an increase by a factor of 2.8 in both cases). However, it has to be stated that a quantitative judgement and description of the creep curves
of Mo and ML is diYcult since parameters such as (sub-) grain size and dislocation density in the starting condition of Mo and ML are diVerent. The Mo sheet material undergoes recrystallization during the creep experiment, moreover, grain growth is likely to occur since no second phase is present. For ML sheet material, recrystallization is delayed due the presence of La2O3 particles. The creep stress sensitivity of the minimum creep rate (Norton exponent n, see Fig. 6) is of the same order for Mo and MLR at 1400 °C and 1600 °C respectively. A Norton exponent between 3 and 8 describes that the creep mechanisms is mainly controlled by dislocation movement (power-law creep). This means that the deformation mechanism is of the same nature in the investigated materials. However, the stress to activate the creep deformation is much higher for ML sheet material. For example, stress to obtain a minimum creep rate of 5 £ 10¡8 s¡1 is 10 MPa for Mo and 40 MPa for ML sheet material. 4. Conclusions
Fig. 5. Creep rupture plot of Mo and ML (stress-relieved) sheet material (thickness 2 mm) tested under constant load conditions in ArH2 atmosphere at 1400 °C and 1600 °C.
The investigation revealed that the test equipment at the University of Applied Sciences, Jena is highly capable of generating creep and tensile testing data of molybdenum and tungsten sheet material up to high temperatures. The results of uniaxial tensile tests on tungsten sheet material (thickness 1 mm) are in good agreement with data recorded in the Testing Laboratories in the Technology Centre of PLANSEE. With the aid of the high-temperature testing equipment mechanical data of tungsten sheet material could be generated up to 2500 °C. The results of creep experiments on molybdenum and La2O3 particles doped molybdenum sheet material (thickness 2 mm) at 1400 °C and 1600 °C showed the beneWcial eVect of particles on both creep rupture strength and minimum creep rate. Acknowledgment Financial support came from PLANSEE’s research project “Basisdaten” which is installed to gain materials data of PLANSEE products in diVerent product forms. References
Fig. 6. Norton plot of Mo and ML (stress-relieved) sheet material (thickness 2 mm) tested under constant load conditions in ArH2 atmosphere at 1400 °C and 1600 °C.
[1] Fischer B, Freund D, Lupton DF. Stress–rupture strength of rhenium at very high temperatures. In: Proceedings of the international symposium on rhenium and rhenium alloys, TMS annual meeting, Orlando, 1997. p. 311–20. [2] Fischer B, Helmich R, Töpfer H. Anordnung zur Warmfestigkeitsprüfung hochschmelzender, elektrisch leitender WerkstoVe. DD Patent 245576 A3, May 1982. [3] Fischer B, Töpfer H, Helmich R. Gerät für Warmfestigkeitsmessungen an hochschmelzenden Metallen nach Einwirkung silikatischer Schmelzen. Silikattechnik 1984;35(11):329–31. [4] Völkl R, Freund D, Fischer B, Gohlke D. Berührungslose Dehnungsaufnahme an widerstandsbeheizten Metallzugproben mit Hilfe digitaler Bildverarbeitung bei Prüftemperaturen bis 3000 °C. In: Proceedings of the conference WerkstoVprüfung 1998, Bad Nauheim,
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[9] Beck G. In: Degussa AG, editor. Edelmetall-Taschenbuch. Hüthig, Heidelberg, Frankfurt Main, 1995. ISBN 3-7785-2448-8. [10] Arblaster JW. Crystallographic properties of platinum. Platinum Metals Rev 1997;41(1):12–21. [11] Martienssen W, Warlimont H. Springer handbook of condensed matter and materials data. Berlin, Heidelberg: Springer-Verlag; 2005. [12] Stickler Ch. Mikroplastizität und zyklisches Spannungs-Dehnungsverhalten von Ta und Mo bei Temperaturen unter 0,2 Tm, Wien, 1998. [13] L. Hollang, Fließspannung und Verfestigung hochreiner MolybdänEinkristalle. Stuttgart, 1996.