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The fatigue behavior of a zirconium-based bulk metallic glass in vacuum and air
Journal of Non-Crystalline Solids 317 (2003) 187–192 www.elsevier.com/locate/jnoncrysol
The fatigue behavior of a zirconium-based bulk metallic glass in vacuum and air W.H. Peter a, R.A. Buchanan a, C.T. Liu b, P.K. Liaw
a,*
a
b
Department of Materials Science and Engineering, University of Tennessee, 434 Dougherty Engineering Building, Knoxville, TN 37996-2200, USA Oak Ridge National Laboratory, Metals and Ceramics Division, P.O. Box 2008, MS 6115, Oak Ridge, TN 37831-6115, USA
Abstract Early fatigue studies on bulk metallic glasses (BMGs) have indicated fatigue strengths lower than anticipated. It was suspected that environmental effects degraded the fatigue life. In the present study, a zirconium-based BMG, BMG-11 (Zr–10Al–5Ti–17.9Cu–14.6Ni, at.%), was tested in vacuum and air. This comparison was made to define any detrimental environmental effects due to water vapor on the fatigue lifetime of BMG-11. The limited results obtained so far suggest that water vapor in air does not affect the lifetime. Moreover, the fatigue endurance limit for BMG-11 in air was found to be quite high, 907 MPa, and comparable to, or better than, those of many conventional high-strength alloys. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 81.05.Kf; 62.20.Mk
1. Introduction Recent advances in the field of metallic glasses have generated major scientific interest. Metallic glasses are alloys with no long-range atomic periodic lattice structure [1,2]. Traditionally, fabrication limitations have kept the thicknesses of manufactured specimens below 1 mm [3,4]. In the early 1990s, a large breakthrough in innovative alloy compositions allowed the production of metallic glasses with thicknesses over 1 mm. Alloys consisting of these new compositions, which can be produced by conventional casting techniques, have been labeled Ôbulk metallic glassesÕ (BMGs). Since *
Corresponding author. Tel.: +1-615 974 6356; fax: +1-615 974 4115. E-mail address: [email protected] (P.K. Liaw).
then, samples with diameters up to several centimeters have been produced [4]. BMGs are of structural interest because of their high strengths and low YoungÕs moduli. Fatigue and fracture specimens when tested in tension undergo elastic elongation with very little bulk plastic deformation. Tensile samples of zirconium (Zr)-based BMG tensile samples have shown only around 0.5% plastic elongation [3]. The lack of plastic deformation may indicate the possibility for a perfect amorphous alloy, without constraint, to exhibit a fatigue limit that approaches the ultimate tensile strength [5]. This idealistic theory has not been realized. In fact, fatigue studies have shown the fatigue endurance limit of a Zr-based BMG containing beryllium, Zr–13.8Ti–12.5Cu–10Ni– 22.5Be (atomic percent, at.%), to be as low as 6– 8% of the tensile strength [6]. The values for this
0022-3093/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-3093(02)02009-4
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W.H. Peter et al. / Journal of Non-Crystalline Solids 317 (2003) 187–192
BMG are very low compared to conventional crystalline alloys, such as high-strength steels and aluminum alloys. In the last few years, experiments conducted at the Oak Ridge National Laboratory (ORNL) [3] and the University of California, Berkeley [6,7], have addressed the environmental effects on the fracture and fatigue behavior of Zr-based BMGs. Ritchie et al. studied the fatigue behavior of a metallic glass in air, water, and a 0.5 M NaCl solution to observe their effects on fatigue behavior [7]. The results showed a slight loss of lifetime in water relative to air, but a large loss of lifetime, by several magnitudes, in the NaCl solution relative to air. These results indicate that the environment will influence the fatigue lifetime of a Zr-based BMG component. However, Liu et al. at ORNL conducted tensile tests on a Zr-based BMG in air, water, vacuum, and dry oxygen at room temperature and found no influence of these environments on tensile strength [3]. It is noted that the testing times of tensile experiments are generally shorter than fatigue experiments. Therefore, the environmental effects on the mechanical behavior of Zr-based BMGs appear to be dependent on time as well as the specific environment. The fatigue behavior of a Zr-based BMG in vacuum has not been reported. The overall purpose of the present investigation was to study the fatigue characteristics of BMG-11, a Zr-based BMG whose fatigue properties have not been determined, in air and vacuum. In this way, any detrimental or beneficial effects associated with water vapor and oxygen in air could be evaluated. It was suspected that hydrogen production through the electrochemical reaction of the BMG11 with water vapor in air could lead to hydrogen embrittlement and reduced fatigue lifetimes.
2. Experimental procedures BMG-11 was developed at the California Institute of Technology, Pasadena, CA; the chemical composition is Zr–10Al–5Ti–17.9Cu–14.6Ni (at.%) [3]. Drop-cast round-bar ingots of BMG-11, 6.4 mm in diameter by 76 mm in length, were fabricated at ORNL. The ingots were cut into fatigue, corrosion,
and metallography samples with an electrical discharge machine. Metallography samples, 6.4 mm in diameter by 1.6 to 3.2 mm in thickness, of the amorphous alloy were polished to a 5 lm finish. Some of these samples were etched with a nitric acid solution and observed with optical microscopy for crystalline regions. X-ray diffraction analyses were conducted on other samples using a Philips XÕpert X-Ray Diffractometer to characterize the structure. Radiographs were taken of the round-bar ingots to locate any porosity, and to ensure that no porosity would be present in the testing section of the fatigue specimens. Sections that were to be used for fatigue specimens were cut to length and machined. Once the samples were returned, they were ground to a 9 lm finish within 1–2 h of initiating the fatigue tests. Fig. 1 shows the button-head geometry that was used for the majority of the fatigue specimens. The gripping system and the specimen geometry are modified versions of that used by Dr Ken Liu (ORNL) for testing ceramics and other brittle materials. A Material Test System (MTS) Model 810 load frame was used for fatigue testing. The machine was aligned prior to use, and as required. Samples were tested at various stress levels with a load ratio (R), the minimum stress (rmin ) divided by the maximum stress (rmax ), equal to 0.1, i.e., R ¼ 0:1, and a frequency of 10 Hz. Peak and valley load values for individual cycles were intermittently recorded for verification. Upon failure, samples were removed and stored for later examination by scanning electron microscopy. Prior to testing the specimens in vacuum, the chamber was evacuated and backfilled with ultrapure argon or helium three times or more. A mechanical pump and a diffusion pump in series were used to evacuate the chamber. A gettering oven was employed to remove any impurities from the inert gas before entering the chamber. After the third backfill, the chamber was evacuated to 105 – 106 Torr. The samples were then tested in the same manner as the air-data specimens and at the same stress ranges. In order to provide electrochemical data on the possible corrosion effects of condensed water vapor, cyclic and dynamic polarization tests, and scratch tests, were conducted in distilled water
W.H. Peter et al. / Journal of Non-Crystalline Solids 317 (2003) 187–192
189
Grip Pieces (Copper)
Fatigue Specimen BMG-11 5.33 mm
Grip Holder (Rene 41)
R 0.40 mm 4.00 mm
0.51 mm
R 1.27 mm
31.75 mm 2.03 mm Testing ø: 2.98 mm Shaft ø: 4.00 mm Button ø: 5.33 mm
3.81 mm
Bottom View
Specimen
Bottom View
Gripping System
Fig. 1. BMG-11 specimen geometry for fatigue testing in air and vacuum.
containing 0.05 M Na2 SO4 . The small amount of sodium sulfate was added to increase the conductivity of the distilled water while not increasing its aggressiveness. Samples used for the corrosion studies were drilled and hand tapped approximately 2 mm into the specimen. A brass screw was placed into the sample. The corrosion samples were then placed in a mold with resin, cured, and ground to a 9 lm finish – the same finish as the fatigue samples. The samples were ground within an hour of starting the corrosion testing to ensure a controlled thickness of the passive film. A nonconductive epoxy was placed along the metal–resin interface. The exposed surface of the metallic glass sample was measured, and the surface area was calculated. The potentiostat employed was an EG&G Princeton Applied Research Model 263A Potentiostat/Galvanostat. It was controlled using a computer with EG&G 352 SoftCorr III software. The potential was measured relative to a saturated calomel reference electrode (SCE), and the counter electrode consisted of a rectangular sheet of platinum. The natural corrosion potential was allowed to stabilize for approximately 1 h. The potential scan was then initiated 50 mV below the corrosion potential, and ramped at a rate of 0.17 mV/s. Prior
to conducting a scratch test, two cyclic polarization tests were performed. The potential was taken to an approximate value of 1700 mV (SCE), then cycled back down to the initial potential. The third test was a dynamic polarization curve. The potential was scanned to 750 mV (SCE). At 100 mV above the corrosion potential, the sample was scratched with a diamond stylus. The polarization curve was then observed for repassivation. The sample was scratched thereafter every 100 mV for approximately 10 s.
3. Results Results of the metallography and X-ray diffraction indicated that the fabricated BMG-11 was an amorphous material, i.e., either no or very little crystallinity was present. The limited crystallinity was in the form of micro-starlets forming thin spirals just inside the circumference of the sample. This was consistent with BMG-11 samples previously fabricated and observed at ORNL, and was deemed acceptable for the present study. The crystalline locations were largely removed during machining due to their location. X-ray diffraction patterns were obtained from two amorphous
190
W.H. Peter et al. / Journal of Non-Crystalline Solids 317 (2003) 187–192
metallography samples. The patterns contained no crystalline peaks. However, a wide peak identifying limited short-range order was present. The fatigue results for the BMG-11 samples in air and vacuum are presented in Fig. 2, where the stress-range values reflect the stress concentration factor (Kt ) of 1.55 [8,9] at the notched section for the majority of the specimens. The two samples tested at 1066 MPa had a Kt of 1.48. The fatigue endurance limit (rL ), based on the applied stress range (rmin rmax ), for the BMG-11 samples tested in air was approximately 907 MPa. The fatigue ratio (rL divided by the ultimate tensile strength) for the BMG-11 was 0.53, using the ultimate tensile strength of 1700 MPa. The stress-to-lifetime relationship for the air data was Dr ¼ 233:1 logðNf Þ þ 2052;
Fig. 3. At the natural-corrosion and higher potentials, the BMG-11 sample formed a stable passive film. The passive current density was observed to be approximately 106 A/cm2 . These results are in agreement with the cyclic polarization curves where the passive film was found to be stable at potentials up to 1700 mV (SCE). The results of scratch tests conducted on the sample during the dynamic polarization can be observed in Fig. 3. These tests are indicated by the drastic spikes in current density approximately every 100 mV. As the passive film was scratched from the surface of the sample, the current density and the corrosion rate quickly increased. Upon stopping the diamond-stylus scratching process, the BMG11 sample immediately repassivated.
ð1Þ
where Dr is the stress range in MPa, and Nf is the number of cycles to failure. This equation was found to fit the data with a R-squared value of 0.88. The average relative humidity for the tests performed in air was 41%, and ranged from 18% to 61%. The scatter in the vacuum data made it difficult to determine a stress-to-life relationship. The majority of the vacuum data displayed lifetimes shorter than the air data for any given stress amplitude. The dynamic polarization curve for BMG-11 in water containing 0.05 M Na2 SO4 is presented in
4. Discussion At a frequency of 10 Hz, no detrimental effects of water vapor in air were observed on the fatigue properties of BMG-11. In fact, the fatigue behavior in air appears to be longer lifetimes and less scatter than in vacuum. The source for this phenomenon is not currently known, but tests are presently being conducted to determine if adiabatic heating, frequency effects, lack of beneficial environmental effects, and/or alignment issues may be causing the lower lifetimes in vacuum.
Fig. 2. Stress–amplitude/fatigue-life data of Notched BMG-11 specimens tested in air and vacuum with R ¼ 0:1 and a frequency of 10 Hz.
W.H. Peter et al. / Journal of Non-Crystalline Solids 317 (2003) 187–192
191
Fig. 3. Dynamic polarization curve for BMG-11 in distilled water containing 0.05 M Na2 SO4 with scratch tests performed at 100 mV intervals for 10 s.
The results of the dynamic polarization and scratch tests indicated that this Zr-based BMG was highly capable of quickly re-establishing a passive film after mechanical damage. In the fatigue testing of BMGs, mechanical damage of the passive film may be present due to the shear-band development at the surface, corrosion fatigue, and/ or crack propagation. The highly passive nature of this material in water at the corrosion potential and the ability of BMG-11 to quickly reform a passive film limit the influence of corrosion mechanisms on the fatigue lifetime of this metallic
glass. However, at high frequencies, films may still not have enough time to reform. The results of the corrosion tests, in conjunction with the fatigue data, illustrate the masking and/or ineffectiveness of water-vapor corrosion to decrease the fatigue lifetime at this frequency of 10 Hz. The original purpose of this research was to better understand the low fatigue endurance limit found for a Zr-based BMG [6]. For BMG-11, the humidity in air was not found to be detrimental, and the fatigue ratio, 0.53, was much higher than other previously reported values of 0.06–0.08 [6].
Fig. 4. Stress–amplitude/fatigue-life data of notched BMG-11 specimens tested in air and compared with the fatigue endurance limits (rL ) of high-strength alloys [2,7,9,10].
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W.H. Peter et al. / Journal of Non-Crystalline Solids 317 (2003) 187–192
When comparing BMG-11 with high-strength alloys and crystalline zirconium, Fig. 4, the fatigue endurance limit of BMG-11 is comparable or higher than the other materials. Also, the fatigue ratio for BMG-11, 0.53, is a relatively high value. Most alloy steels have a fatigue ratio between 0.35 and 0.60 [8]. BMG-11 is at the higher end of this range. Many high-strength steels with comparable ultimate tensile strengths have lower fatigue ratios, as shown in Fig. 4 [11].
managed by Dr Wyn Jennings and Dr Larry Goldberg, and to the Division of Materials Science and Engineering, Department of Energy under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory (ORNL) operated by UT-Battelle, LLC for support of this research. Also, special thanks are given to Dr Charlie Brooks of the University of Tennessee and Dr Ken Liu, Mr Cecil Carmichael, Jr., and Mr Joseph Wright of ORNL for their advice and consultation.
5. Conclusions Based on the experiments conducted and discussed on BMG-11, the following conclusions can be made: 1. Water vapor in air is generally not detrimental to the fatigue lifetime performance of BMG11 at the tested relative humidity and at a cyclic frequency of 10 Hz. 2. Corrosion testing of BMG-11 seems to further support little to no impact of humidity on the fatigue lifetime. The passive film of the material in water rapidly reforms after being mechanically removed [12]. 3. The high fatigue endurance limit of BMG-11, 907 MPa, is approximately 53% of the ultimate tensile strength. This is comparable to conventional high-strength alloys. Acknowledgements The authors are grateful to the National Science Foundation Integrative Graduate Education and Research Training (IGERT) Program on Materials Lifetime Science and Engineering,
References [1] F. Luborsky, Amorphous Metallic Alloys, Butterworths Monographs in Materials, Butterworths, 1983, p. 1. [2] ASM Handbook, Properties and Selections: Nonferrous Alloys and Special Purpose Materials, ASM, Metals Park, OH, vol. 2, 1990, p. 186, 667, 804. [3] C.T. Liu, L. Heatherly, D.S. Easton, C.A. Carmichael, J.H. Schneibel, C.H. Chen, J.L. Wright, M.H. Yoo, J.A. Horton, A. Inoue, Metall. Mater. Trans. 29A (1998) 1811. [4] A. Inoue, Intermetallics 8 (2000) 455. [5] L. Davis, Metallic Glasses, American Society for Metals, Metals Park, OH, 1978, p. 190. [6] C.J. Gilbert, J.M. Lippmann, R.O. Ritchie, Scripta Mater. 38 (1998) 537. [7] R.O. Ritchie, V. Schroeder, C.J. Gilbert, Intermetallics 8 (2000) 469. [8] R. Hertzberg, Deformation and Fracture Mechanics of Engineering Materials, 3rd Ed., John Wiley, New York, 1989, p. 244, 477. [9] ASME Handbook, Metals Engineering – Design, McGraw-Hill, New York, 1953. [10] Structural Alloys Handbook, Mechanical Properties Data Center, Traverse City, MI, 1977. [11] W.H. Peter, P.K. Liaw, R.A. Buchanan, C.T. Liu, C.R. Brooks, J.A. Horton Jr., C.A. Carmichael Jr., J.L. Wright, Intermetallics 10 (11–12) (2002) 1125. [12] W.H. Peter, R.A. Buchanan, C.T. Liu, P.K. Liaw, M.L. Morrison, J.A. Horton Jr., C.A. Carmichael Jr., J.L. Wright, Intermetallics 10 (11–12) (2002) 1157.
The fatigue behavior of a zirconium-based bulk metallic glass in vacuum and air W.H. Peter a, R.A. Buchanan a, C.T. Liu b, P.K. Liaw
a,*
a
b
Department of Materials Science and Engineering, University of Tennessee, 434 Dougherty Engineering Building, Knoxville, TN 37996-2200, USA Oak Ridge National Laboratory, Metals and Ceramics Division, P.O. Box 2008, MS 6115, Oak Ridge, TN 37831-6115, USA
Abstract Early fatigue studies on bulk metallic glasses (BMGs) have indicated fatigue strengths lower than anticipated. It was suspected that environmental effects degraded the fatigue life. In the present study, a zirconium-based BMG, BMG-11 (Zr–10Al–5Ti–17.9Cu–14.6Ni, at.%), was tested in vacuum and air. This comparison was made to define any detrimental environmental effects due to water vapor on the fatigue lifetime of BMG-11. The limited results obtained so far suggest that water vapor in air does not affect the lifetime. Moreover, the fatigue endurance limit for BMG-11 in air was found to be quite high, 907 MPa, and comparable to, or better than, those of many conventional high-strength alloys. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 81.05.Kf; 62.20.Mk
1. Introduction Recent advances in the field of metallic glasses have generated major scientific interest. Metallic glasses are alloys with no long-range atomic periodic lattice structure [1,2]. Traditionally, fabrication limitations have kept the thicknesses of manufactured specimens below 1 mm [3,4]. In the early 1990s, a large breakthrough in innovative alloy compositions allowed the production of metallic glasses with thicknesses over 1 mm. Alloys consisting of these new compositions, which can be produced by conventional casting techniques, have been labeled Ôbulk metallic glassesÕ (BMGs). Since *
Corresponding author. Tel.: +1-615 974 6356; fax: +1-615 974 4115. E-mail address: [email protected] (P.K. Liaw).
then, samples with diameters up to several centimeters have been produced [4]. BMGs are of structural interest because of their high strengths and low YoungÕs moduli. Fatigue and fracture specimens when tested in tension undergo elastic elongation with very little bulk plastic deformation. Tensile samples of zirconium (Zr)-based BMG tensile samples have shown only around 0.5% plastic elongation [3]. The lack of plastic deformation may indicate the possibility for a perfect amorphous alloy, without constraint, to exhibit a fatigue limit that approaches the ultimate tensile strength [5]. This idealistic theory has not been realized. In fact, fatigue studies have shown the fatigue endurance limit of a Zr-based BMG containing beryllium, Zr–13.8Ti–12.5Cu–10Ni– 22.5Be (atomic percent, at.%), to be as low as 6– 8% of the tensile strength [6]. The values for this
0022-3093/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-3093(02)02009-4
188
W.H. Peter et al. / Journal of Non-Crystalline Solids 317 (2003) 187–192
BMG are very low compared to conventional crystalline alloys, such as high-strength steels and aluminum alloys. In the last few years, experiments conducted at the Oak Ridge National Laboratory (ORNL) [3] and the University of California, Berkeley [6,7], have addressed the environmental effects on the fracture and fatigue behavior of Zr-based BMGs. Ritchie et al. studied the fatigue behavior of a metallic glass in air, water, and a 0.5 M NaCl solution to observe their effects on fatigue behavior [7]. The results showed a slight loss of lifetime in water relative to air, but a large loss of lifetime, by several magnitudes, in the NaCl solution relative to air. These results indicate that the environment will influence the fatigue lifetime of a Zr-based BMG component. However, Liu et al. at ORNL conducted tensile tests on a Zr-based BMG in air, water, vacuum, and dry oxygen at room temperature and found no influence of these environments on tensile strength [3]. It is noted that the testing times of tensile experiments are generally shorter than fatigue experiments. Therefore, the environmental effects on the mechanical behavior of Zr-based BMGs appear to be dependent on time as well as the specific environment. The fatigue behavior of a Zr-based BMG in vacuum has not been reported. The overall purpose of the present investigation was to study the fatigue characteristics of BMG-11, a Zr-based BMG whose fatigue properties have not been determined, in air and vacuum. In this way, any detrimental or beneficial effects associated with water vapor and oxygen in air could be evaluated. It was suspected that hydrogen production through the electrochemical reaction of the BMG11 with water vapor in air could lead to hydrogen embrittlement and reduced fatigue lifetimes.
2. Experimental procedures BMG-11 was developed at the California Institute of Technology, Pasadena, CA; the chemical composition is Zr–10Al–5Ti–17.9Cu–14.6Ni (at.%) [3]. Drop-cast round-bar ingots of BMG-11, 6.4 mm in diameter by 76 mm in length, were fabricated at ORNL. The ingots were cut into fatigue, corrosion,
and metallography samples with an electrical discharge machine. Metallography samples, 6.4 mm in diameter by 1.6 to 3.2 mm in thickness, of the amorphous alloy were polished to a 5 lm finish. Some of these samples were etched with a nitric acid solution and observed with optical microscopy for crystalline regions. X-ray diffraction analyses were conducted on other samples using a Philips XÕpert X-Ray Diffractometer to characterize the structure. Radiographs were taken of the round-bar ingots to locate any porosity, and to ensure that no porosity would be present in the testing section of the fatigue specimens. Sections that were to be used for fatigue specimens were cut to length and machined. Once the samples were returned, they were ground to a 9 lm finish within 1–2 h of initiating the fatigue tests. Fig. 1 shows the button-head geometry that was used for the majority of the fatigue specimens. The gripping system and the specimen geometry are modified versions of that used by Dr Ken Liu (ORNL) for testing ceramics and other brittle materials. A Material Test System (MTS) Model 810 load frame was used for fatigue testing. The machine was aligned prior to use, and as required. Samples were tested at various stress levels with a load ratio (R), the minimum stress (rmin ) divided by the maximum stress (rmax ), equal to 0.1, i.e., R ¼ 0:1, and a frequency of 10 Hz. Peak and valley load values for individual cycles were intermittently recorded for verification. Upon failure, samples were removed and stored for later examination by scanning electron microscopy. Prior to testing the specimens in vacuum, the chamber was evacuated and backfilled with ultrapure argon or helium three times or more. A mechanical pump and a diffusion pump in series were used to evacuate the chamber. A gettering oven was employed to remove any impurities from the inert gas before entering the chamber. After the third backfill, the chamber was evacuated to 105 – 106 Torr. The samples were then tested in the same manner as the air-data specimens and at the same stress ranges. In order to provide electrochemical data on the possible corrosion effects of condensed water vapor, cyclic and dynamic polarization tests, and scratch tests, were conducted in distilled water
W.H. Peter et al. / Journal of Non-Crystalline Solids 317 (2003) 187–192
189
Grip Pieces (Copper)
Fatigue Specimen BMG-11 5.33 mm
Grip Holder (Rene 41)
R 0.40 mm 4.00 mm
0.51 mm
R 1.27 mm
31.75 mm 2.03 mm Testing ø: 2.98 mm Shaft ø: 4.00 mm Button ø: 5.33 mm
3.81 mm
Bottom View
Specimen
Bottom View
Gripping System
Fig. 1. BMG-11 specimen geometry for fatigue testing in air and vacuum.
containing 0.05 M Na2 SO4 . The small amount of sodium sulfate was added to increase the conductivity of the distilled water while not increasing its aggressiveness. Samples used for the corrosion studies were drilled and hand tapped approximately 2 mm into the specimen. A brass screw was placed into the sample. The corrosion samples were then placed in a mold with resin, cured, and ground to a 9 lm finish – the same finish as the fatigue samples. The samples were ground within an hour of starting the corrosion testing to ensure a controlled thickness of the passive film. A nonconductive epoxy was placed along the metal–resin interface. The exposed surface of the metallic glass sample was measured, and the surface area was calculated. The potentiostat employed was an EG&G Princeton Applied Research Model 263A Potentiostat/Galvanostat. It was controlled using a computer with EG&G 352 SoftCorr III software. The potential was measured relative to a saturated calomel reference electrode (SCE), and the counter electrode consisted of a rectangular sheet of platinum. The natural corrosion potential was allowed to stabilize for approximately 1 h. The potential scan was then initiated 50 mV below the corrosion potential, and ramped at a rate of 0.17 mV/s. Prior
to conducting a scratch test, two cyclic polarization tests were performed. The potential was taken to an approximate value of 1700 mV (SCE), then cycled back down to the initial potential. The third test was a dynamic polarization curve. The potential was scanned to 750 mV (SCE). At 100 mV above the corrosion potential, the sample was scratched with a diamond stylus. The polarization curve was then observed for repassivation. The sample was scratched thereafter every 100 mV for approximately 10 s.
3. Results Results of the metallography and X-ray diffraction indicated that the fabricated BMG-11 was an amorphous material, i.e., either no or very little crystallinity was present. The limited crystallinity was in the form of micro-starlets forming thin spirals just inside the circumference of the sample. This was consistent with BMG-11 samples previously fabricated and observed at ORNL, and was deemed acceptable for the present study. The crystalline locations were largely removed during machining due to their location. X-ray diffraction patterns were obtained from two amorphous
190
W.H. Peter et al. / Journal of Non-Crystalline Solids 317 (2003) 187–192
metallography samples. The patterns contained no crystalline peaks. However, a wide peak identifying limited short-range order was present. The fatigue results for the BMG-11 samples in air and vacuum are presented in Fig. 2, where the stress-range values reflect the stress concentration factor (Kt ) of 1.55 [8,9] at the notched section for the majority of the specimens. The two samples tested at 1066 MPa had a Kt of 1.48. The fatigue endurance limit (rL ), based on the applied stress range (rmin rmax ), for the BMG-11 samples tested in air was approximately 907 MPa. The fatigue ratio (rL divided by the ultimate tensile strength) for the BMG-11 was 0.53, using the ultimate tensile strength of 1700 MPa. The stress-to-lifetime relationship for the air data was Dr ¼ 233:1 logðNf Þ þ 2052;
Fig. 3. At the natural-corrosion and higher potentials, the BMG-11 sample formed a stable passive film. The passive current density was observed to be approximately 106 A/cm2 . These results are in agreement with the cyclic polarization curves where the passive film was found to be stable at potentials up to 1700 mV (SCE). The results of scratch tests conducted on the sample during the dynamic polarization can be observed in Fig. 3. These tests are indicated by the drastic spikes in current density approximately every 100 mV. As the passive film was scratched from the surface of the sample, the current density and the corrosion rate quickly increased. Upon stopping the diamond-stylus scratching process, the BMG11 sample immediately repassivated.
ð1Þ
where Dr is the stress range in MPa, and Nf is the number of cycles to failure. This equation was found to fit the data with a R-squared value of 0.88. The average relative humidity for the tests performed in air was 41%, and ranged from 18% to 61%. The scatter in the vacuum data made it difficult to determine a stress-to-life relationship. The majority of the vacuum data displayed lifetimes shorter than the air data for any given stress amplitude. The dynamic polarization curve for BMG-11 in water containing 0.05 M Na2 SO4 is presented in
4. Discussion At a frequency of 10 Hz, no detrimental effects of water vapor in air were observed on the fatigue properties of BMG-11. In fact, the fatigue behavior in air appears to be longer lifetimes and less scatter than in vacuum. The source for this phenomenon is not currently known, but tests are presently being conducted to determine if adiabatic heating, frequency effects, lack of beneficial environmental effects, and/or alignment issues may be causing the lower lifetimes in vacuum.
Fig. 2. Stress–amplitude/fatigue-life data of Notched BMG-11 specimens tested in air and vacuum with R ¼ 0:1 and a frequency of 10 Hz.
W.H. Peter et al. / Journal of Non-Crystalline Solids 317 (2003) 187–192
191
Fig. 3. Dynamic polarization curve for BMG-11 in distilled water containing 0.05 M Na2 SO4 with scratch tests performed at 100 mV intervals for 10 s.
The results of the dynamic polarization and scratch tests indicated that this Zr-based BMG was highly capable of quickly re-establishing a passive film after mechanical damage. In the fatigue testing of BMGs, mechanical damage of the passive film may be present due to the shear-band development at the surface, corrosion fatigue, and/ or crack propagation. The highly passive nature of this material in water at the corrosion potential and the ability of BMG-11 to quickly reform a passive film limit the influence of corrosion mechanisms on the fatigue lifetime of this metallic
glass. However, at high frequencies, films may still not have enough time to reform. The results of the corrosion tests, in conjunction with the fatigue data, illustrate the masking and/or ineffectiveness of water-vapor corrosion to decrease the fatigue lifetime at this frequency of 10 Hz. The original purpose of this research was to better understand the low fatigue endurance limit found for a Zr-based BMG [6]. For BMG-11, the humidity in air was not found to be detrimental, and the fatigue ratio, 0.53, was much higher than other previously reported values of 0.06–0.08 [6].
Fig. 4. Stress–amplitude/fatigue-life data of notched BMG-11 specimens tested in air and compared with the fatigue endurance limits (rL ) of high-strength alloys [2,7,9,10].
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W.H. Peter et al. / Journal of Non-Crystalline Solids 317 (2003) 187–192
When comparing BMG-11 with high-strength alloys and crystalline zirconium, Fig. 4, the fatigue endurance limit of BMG-11 is comparable or higher than the other materials. Also, the fatigue ratio for BMG-11, 0.53, is a relatively high value. Most alloy steels have a fatigue ratio between 0.35 and 0.60 [8]. BMG-11 is at the higher end of this range. Many high-strength steels with comparable ultimate tensile strengths have lower fatigue ratios, as shown in Fig. 4 [11].
managed by Dr Wyn Jennings and Dr Larry Goldberg, and to the Division of Materials Science and Engineering, Department of Energy under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory (ORNL) operated by UT-Battelle, LLC for support of this research. Also, special thanks are given to Dr Charlie Brooks of the University of Tennessee and Dr Ken Liu, Mr Cecil Carmichael, Jr., and Mr Joseph Wright of ORNL for their advice and consultation.
5. Conclusions Based on the experiments conducted and discussed on BMG-11, the following conclusions can be made: 1. Water vapor in air is generally not detrimental to the fatigue lifetime performance of BMG11 at the tested relative humidity and at a cyclic frequency of 10 Hz. 2. Corrosion testing of BMG-11 seems to further support little to no impact of humidity on the fatigue lifetime. The passive film of the material in water rapidly reforms after being mechanically removed [12]. 3. The high fatigue endurance limit of BMG-11, 907 MPa, is approximately 53% of the ultimate tensile strength. This is comparable to conventional high-strength alloys. Acknowledgements The authors are grateful to the National Science Foundation Integrative Graduate Education and Research Training (IGERT) Program on Materials Lifetime Science and Engineering,
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