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Fatigue behavior of Zr52.5Al10Ti5Cu17.9Ni14.6 bulk metallic glass
Intermetallics 10 (2002) 1125–1129 www.elsevier.com/locate/intermet
Fatigue behavior of Zr52.5Al10Ti5Cu17.9Ni14.6 bulk metallic glass W.H. Petera, P.K. Liawa, R.A. Buchanana,*, C.T. Liub, C.R. Brooksa, J.A. Horton Jr.b, C.A. Carmichael Jr.b, J.L. Wrightb a
University of Tennessee, Department of Materials Science and Engineering, 434 Dougherty Engineering Building, Knoxville, TN 37996-2200, USA b Oak Ridge National Laboratory, Metals & Ceramics Division, P.O. Box 2008, MS 6115, Oak Ridge, TN 37831-6115, USA Received in revised form 7 August 2002; accepted 7 August 2002
Abstract In the present study, fatigue tests were conducted on a zirconium-based bulk metallic glass (BMG), BMG-11 (Zr–10Al–5Ti– 17.9Cu–14.6Ni, atomic percent), in air and vacuum to elucidate the possible environmental effects. In air, the fatigue endurance limit and the fatigue ratio were found to be 907 MPa and 0.53, respectively. These values are better than many conventional highstrength crystalline alloys. Unexpectedly, the fatigue lifetimes in vacuum were found to be lower than in air. Additional testing indicated that dissociation of residual water vapor to atomic hydrogen in the vacuum via a hot-tungsten-filament ionization gauge, and subsequent hydrogen embrittlement of the BMG-11, could have been a factor causing the lower fatigue lifetimes observed in vacuum. Published by Elsevier Science Ltd. Keywords: B. Glasses, metallic; Fatigue
1. Introduction Enhanced scientific interest in metallic glasses has been generated by new compositions resulting in great increases in sample sizes. 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– 6]. In the early 1990s, a 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 ‘‘bulk metallic glasses’’ (BMGs). Since then, samples with diameters up to several centimeters have been produced [6]. BMGs are of structural interest because of their high strengths and low Young’s moduli. Fatigue and fracture specimens when tested in tension undergo elastic elongation with very little bulk plastic deformation. Tensile tests of a zirconium (Zr)-based BMG have shown only * Corresponding author. Tel.: +1-865-974-4858; fax: +1-865-9744115. E-mail address: [email protected] (R.A. Buchanan). 0966-9795/02/$ - see front matter Published by Elsevier Science Ltd. PII: S0966-9795(02)00152-8
around 0.5% plastic elongation [5]. The lack of plastic deformation may indicate the possibility for a perfect amorphous alloy, without constraint, to exhibit a fatigue endurance limit that approaches the ultimate tensile strength [7]. This idealistic hypothesis has not been realized. In fact, certain fatigue studies have shown the fatigue endurance limit of a Zr-based BMG to be as low as 6–8% of the tensile strength [8]. These values are very low compared to conventional crystalline alloys, such as high-strength steels and aluminum alloys, whose fatigue limits are typically 30–40% of the ultimate tensile strengths. Other factors must play a paramount role in affecting the fatigue behavior of amorphous alloys. In the last few years, experiments conducted at the Oak Ridge National Laboratory (ORNL) [5] and the University of California, Berkeley [8,9], have emphasized the environmental effects on the fracture and fatigue behaviors of Zr-based BMGs. Ritchie et al. studied the fatigue crack-growth behavior of a metallic glass in a 0.5 M NaCl solution, water, and air to observe their effects on fatigue characteristics [9]. The results showed a slight loss of lifetime in water relative to air, but a large loss of lifetime, by several magnitudes, in a NaCl solution relative to air. These results indicate that the environment will influence the fatigue lifetime of a
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Zr-based BMG component. However, Liu et al. at ORNL conducted simple 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 [5]. 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 next step toward understanding the corrosion-fatigue phenomenon is to carefully compare fatigue results 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 BMG-11 with water vapor in air could lead to hydrogen embrittlement and reduced fatigue lifetimes.
2. Experimental procedures Bulk metallic glass 11, BMG-11, was the Zr-based metallic glass used in these experiments. Developed at California Institute of Technology, Pasadena, California, the composition in atomic percent (at.%) is Zr– 10Al–5Ti–17.9Cu–14.6Ni [3–5]. 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 for various tests (note that fatigue specimens were fabricated by grinding). Metallography samples, 6.4 mm in diameter by 1.6–3.2 mm in thickness, of the amorphous alloy were polished to a 5 mm finish. Some of these samples were etched with a nitric-hydrofluoric acid solution and observed with optical microscopy for crystalline regions. X-ray diffraction analyses were conducted on other samples using a Philips X’pert 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. The samples were then polished to a 9 mm finish, which corresponded to the surface finish of the specimens in the ongoing corrosion study [10,11], within 1–2 h of initiating the fatigue tests. Fig. 1 shows the modified button-head geometry that was used for the majority of the fatigue specimens. The gripping system and the specimen geometry were similar to that used at ORNL for testing ceramics and other brittle materials [12]. Initial tests with a conventional button-head gripping system resulted in premature
fracture of the specimen near the grips (not at the notched section) at low stress ranges ( 670 MPa). Cracks would initiate at the localized points of contact of the gripping system before cracks initiated in the notched section of the test specimen. One air test and one vacuum test performed with the conventional gripping system at a stress range of 1066 MPa were recorded because the stress level was high enough that this problem was not encountered. The modified gripping system largely eliminated the premature fractures near the grips by increasing the area of contact using soft copper inserts. 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 ranges with a load ratio (R), the minimum load divided by the maximum load, equal to 0.1 (i.e. R=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 pure 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 minimize any impurities in the inert gas before entering the chamber. After the final backfill, the chamber was evacuated to 105 to 106 torr. An ionization gauge with a hot tungsten filament open to the chamber environment was used to monitor the vacuum at low pressure. The vacuum samples were tested in the same manner as the air specimens and at the same stress ranges. Subsequent experiments were performed on fatigue specimens in vacuum without the prolonged use of the ionization gauge. The ionization gauge was turned off after realizing the vacuum, or was never used. In the first test of this series, a thermocouple was attached to the testing gauge section of the fatigue specimen. The temperature was monitored by a digital read-out, and then recorded every 2000 cycles and upon fracture.
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
W.H. Peter et al. / Intermetallics 10 (2002) 1125–1129
patterns were obtained from two amorphous samples. The patterns contained no crystalline peaks; instead, a wide peak corresponding to an amorphous structure was present. The fatigue results for the BMG-11 samples in air and vacuum are presented in Fig. 2. The stress-range values reflect the stress concentration factor (Kt) of 1.55 [13,14] 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 ( L), based on the applied stress range, for the BMG-11 samples tested in air was approximately 907 MPa [10]. The fatigue ratio ( L divided by the ultimate tensile strength) for the BMG-11 was 0.53, based on the ultimate tensile strength of 1700 MPa. The stress-to-lifetime relationship for the air data was: ¼ 233:1logðNf Þ þ 2052
ð1Þ
1127
where 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 correlation coefficient (R-squared value) of 0.88. The average relative humidity for the tests performed in air was 41%, and ranged from 18 to 61%. Although the fatigue lifetimes in vacuum were expected to be longer than those in air, the opposite was found, as shown in Fig. 2. The majority of the vacuum data displayed lifetimes shorter than the air data for any given stress range. Furthermore, the tests conducted in vacuum without the prolonged use of the ionization gauge resulted in improved fatigue lifetimes at 986 MPa. The average fatigue lifetime for these tests was 27,757 cycles. This value was still lower than the average fatigue lifetime of the air data at 986 MPa, which was 40,570 cycles, but significantly higher than the average fatigue lifetime of the vacuum data with the ionization gauge on, which was 12,341 cycles. The higher fatigue life in air suggests a possible beneficial effect of oxygen (in air) that promotes the formation of oxide layers on the specimen surfaces during the air tests. Further studies are needed to verify the oxygen effect. The possibility of detrimental effects in vacuum due to adiabatic heating was evaluated. The fatigue sample tested at 986 MPa that failed at 31,994 cycles had a thermocouple attached to the notched section. During the entire test, the temperature was recorded as 25 C.
4. Discussion
Fig. 1. BMG-11 specimen geometry for fatigue testing in air and vacuum.
At a frequency of 10 Hz, the fatigue lifetimes of BMG-11 in vacuum appear to be shorter than in air. Possible unfavorable conditions associated with the
Fig. 2. Stress-range/fatigue-life data of notched BMG-11 specimens tested in air and vacuum with R=0.1 and a frequency of 10 Hz.
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vacuum could be adiabatic heating from the lack of convective heat transfer, complexities in the vacuum process that could generate detrimental species, and/or lack of beneficial environmental effects. The low fatigue lifetimes in vacuum do not appear to be caused by adiabatic heating. There was concern that localized plastic deformation (at/near the fracture surface) created heat that could not be removed by convection, only by conduction. The heat would then accelerate the crack growth. However, the temperature of the notched section remained at 25 C, indicating that no significant adiabatic heating occurred. With regard to the vacuum process, the ionization gauge monitors the pressure under high-vacuum conditions, and is open to the testing chamber. The hot tungsten filament can dissociate residual water vapor into atomic hydrogen and oxygen [15]. The test material
can absorb the limited atomic hydrogen. Even in small amounts, the absorbed hydrogen could embrittle certain materials. This embrittlement could lower the time for crack initiation, and, hence, reduce the observed lifetime of the sample. George and Liu observed a similar phenomenon with Ni3Al [15,16]. By turning off the ionization gauge, this potential source of atomic hydrogen, and the resulting decrease in fatigue life, would be eliminated. As shown in Fig. 2, the vacuum tests conducted at 986 MPa with the ionization gauge off did show an increase in number of cycles compared to the results with the ionization gauge on, and was within the range of the air data. These limited results indicate an interaction between the ionization gauge and the premature failure of the samples in vacuum, and suggest a hydrogen embrittlement phenomenon. Indeed, Suh and Dauskardt have recently reported embrittlement of a
Table 1 Fatigue endurance limits and fatigue ratios based on the stress ranges of high-strength alloys, Zr-based BMGs, and pure grade zirconium Material
Yield strength (MPa)
Ultimate tensile strength (MPa)
Fatigue endurance limit (MPa)
Fatigue ratio
BMG-11 300 M Steel [13,18] D6AC Steel Q&T (260 C) [13,18] Ti–6Al–4V [13,19] 2090-T81 Al–Li Alloy [2] BMG (Zr–Ti–Cu–Ni–Be) [8] Zirconium, Grade 702 [2]
1700 1670 1720 885 483 1900 310
1,700 2,000 2,000 1035 517 1,900 430
907 800 690 515 250 152 145
0.534 0.479 0.345 0.498 0.484 0.080 0.429
Fig. 3. Stress-range/fatigue-life data of notched BMG-11 specimens tested in air and compared with the fatigue endurance limits ( L) of highstrength alloys [2,8,12,16,17].
W.H. Peter et al. / Intermetallics 10 (2002) 1125–1129
Zr-based BMG that had been pre-charged with hydrogen [17]. When comparing the fatigue properties of BMG-11 in air with those of high-strength alloys and crystalline zirconium, Fig. 3, the fatigue endurance limit of BMG11, 907 MPa, 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 [13,18]. 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 Table 1. The fatigue properties of BMG-11 in air were determined to be considerably better than those reported by Gilbert et al. [8] for a Zr-based BMG containing beryllium (Be), Zr–13.8Ti–12.5Cu–10Ni–22.5Be (at.%). Based on stress-range values, the fatigue endurance limits for BMG-11 and the Be-containing BMG were 907 and 110–150 MPa, respectively, and the fatigue ratios were 0.53 and 0.06–0.08, respectively. Possible explanations for the significant differences in fatigue results are: (1) the differences in chemical compositions, which certainly could influence the inherent fatigue properties, and (2) the differences in specimen geometries and testing procedures. With BMG-11, the tests were conducted on round, notched specimens in uniaxial tension; whereas, with the Be-containing BMG, tests were performed on rectangular-cross-sectional plate specimens in bending. To determine if the differences in specimen geometries and testing procedures were the sources of the apparent discrepancy in results, arrangements have been made to study the fatigue properties of the Be-containing BMG under exactly the same conditions as those used for BMG-11.
5. Conclusions The following conclusions can be made based on the experiments conducted and discussed regarding the bulk metallic glass, BMG-11, Zr–10Al–5Ti–17.9Cu–14.6Ni (at.%): 1. A high fatigue endurance limit, 907 MPa, was observed for BMG-11 in air. 2. A high fatigue ratio, 0.53, was observed for BMG-11 in air, which is in the range demonstrated by most conventional alloy steels. 3. The better fatigue properties determined for BMG-11 in air, as compared to those reported for a Be-containing BMG, could be related to the differences in chemical composition and/or the differences in specimen geometries and testing procedures. 4. The fatigue lifetimes for BMG-11 were found to be lower in vacuum than in air.
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5. Limited results suggest that hydrogen embrittlement, resulting from the dissociation of residual water vapor at the hot tungsten filament of the ionization gauge, could be an environmental effect causing the lower fatigue lifetimes in vacuum than in air.
Acknowledgements The authors are grateful to the National Science Foundation Integrative Graduate Education and Research Training (IGERT) Program on Materials Lifetime Science and Engineering, 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 the Oak Ridge National Laboratory (ORNL), operated by UT-Battelle, LLC., for support of this research. Also, the authors wish to thank Dr. Ken Liu of ORNL for his advice and consultation in the design of the fatigue gripping system.
References [1] Luborsky F. Amorphous metallic alloys. London: Butterworth Publishers Ltd; 1983. [2] ASM Handbook. Properties and selections: nonferrous alloys and special purpose materials, vol. 2. Metals Park, OH: ASM; 1990. [3] Pecker A, Johnson WL. Appl Phys Letter 1993;63:2242. [4] Lin XH, Johnson WL. J Appl Phys 1995;78:6514. [5] Liu CT, Heatherly L, Easton DS, Carmichael CA, Schneibel JH, Chen CH, et al. Metall Mater Trans A 1998;29A:1811. [6] Inoue A. Intermetallics 2000;8:455. [7] Davis L. Metallic glasses. Metals Park, Ohio: American Society for Metals; 1978. [8] Gilbert CJ, Lippmann JM, Ritchie RO. Scripta Mater 1998;38:537. [9] Ritchie RO, Schroeder V, Gilbert CJ. Intermetallics 2000;8:469. [10] Peter WH, Buchanan RA, Liu CT, Liaw PK, The fatigue behavior of a zirconium-based bulk metallic glass in vacuum and air. Journal of Non-Crystalline Solids in press. [11] Peter WH, Buchanan RA, Liu CT, Liaw PK, Morrison ML, Horton JA, et al. Localized corrosion behavior of a zirconiumbased bulk metallic glass relative to its crystalline state. Intermetallics in press. [12] Liu KC, Stevens CO, Brinkman CR, Holshauser NE, J. Eng Gas Turbines Power, Transactions of the ASME, 1997;119(3):500, and private communication. [13] Hertzberg R. Deformation and fracture mechanics of engineering materials. 3rd ed. John Wiley & Sons; 1989. [14] ASME Handbook. Metals engineering—design. New York: McGraw-Hill; 1953. [15] George EP, Liu CT. Structural intermetallics. Warrendale, Pennsylvania: The Minerals, Metals & Materials Society; 1997. [16] Cohron JW, George EP, Heatherly L, Liu CT, Zee RH. Intermetallics 1996;4:997. [17] Suh D, Dauskardt RH. Materials Transactions 2001;42(4):638. [18] Structural alloys handbook. Traverse City, MI: Mechanical Properties Data Center, 1977. [19] Materials engineering. Cleveland, OH: Penton/IPC Publication, 1981.
Fatigue behavior of Zr52.5Al10Ti5Cu17.9Ni14.6 bulk metallic glass W.H. Petera, P.K. Liawa, R.A. Buchanana,*, C.T. Liub, C.R. Brooksa, J.A. Horton Jr.b, C.A. Carmichael Jr.b, J.L. Wrightb a
University of Tennessee, Department of Materials Science and Engineering, 434 Dougherty Engineering Building, Knoxville, TN 37996-2200, USA b Oak Ridge National Laboratory, Metals & Ceramics Division, P.O. Box 2008, MS 6115, Oak Ridge, TN 37831-6115, USA Received in revised form 7 August 2002; accepted 7 August 2002
Abstract In the present study, fatigue tests were conducted on a zirconium-based bulk metallic glass (BMG), BMG-11 (Zr–10Al–5Ti– 17.9Cu–14.6Ni, atomic percent), in air and vacuum to elucidate the possible environmental effects. In air, the fatigue endurance limit and the fatigue ratio were found to be 907 MPa and 0.53, respectively. These values are better than many conventional highstrength crystalline alloys. Unexpectedly, the fatigue lifetimes in vacuum were found to be lower than in air. Additional testing indicated that dissociation of residual water vapor to atomic hydrogen in the vacuum via a hot-tungsten-filament ionization gauge, and subsequent hydrogen embrittlement of the BMG-11, could have been a factor causing the lower fatigue lifetimes observed in vacuum. Published by Elsevier Science Ltd. Keywords: B. Glasses, metallic; Fatigue
1. Introduction Enhanced scientific interest in metallic glasses has been generated by new compositions resulting in great increases in sample sizes. 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– 6]. In the early 1990s, a 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 ‘‘bulk metallic glasses’’ (BMGs). Since then, samples with diameters up to several centimeters have been produced [6]. BMGs are of structural interest because of their high strengths and low Young’s moduli. Fatigue and fracture specimens when tested in tension undergo elastic elongation with very little bulk plastic deformation. Tensile tests of a zirconium (Zr)-based BMG have shown only * Corresponding author. Tel.: +1-865-974-4858; fax: +1-865-9744115. E-mail address: [email protected] (R.A. Buchanan). 0966-9795/02/$ - see front matter Published by Elsevier Science Ltd. PII: S0966-9795(02)00152-8
around 0.5% plastic elongation [5]. The lack of plastic deformation may indicate the possibility for a perfect amorphous alloy, without constraint, to exhibit a fatigue endurance limit that approaches the ultimate tensile strength [7]. This idealistic hypothesis has not been realized. In fact, certain fatigue studies have shown the fatigue endurance limit of a Zr-based BMG to be as low as 6–8% of the tensile strength [8]. These values are very low compared to conventional crystalline alloys, such as high-strength steels and aluminum alloys, whose fatigue limits are typically 30–40% of the ultimate tensile strengths. Other factors must play a paramount role in affecting the fatigue behavior of amorphous alloys. In the last few years, experiments conducted at the Oak Ridge National Laboratory (ORNL) [5] and the University of California, Berkeley [8,9], have emphasized the environmental effects on the fracture and fatigue behaviors of Zr-based BMGs. Ritchie et al. studied the fatigue crack-growth behavior of a metallic glass in a 0.5 M NaCl solution, water, and air to observe their effects on fatigue characteristics [9]. The results showed a slight loss of lifetime in water relative to air, but a large loss of lifetime, by several magnitudes, in a NaCl solution relative to air. These results indicate that the environment will influence the fatigue lifetime of a
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Zr-based BMG component. However, Liu et al. at ORNL conducted simple 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 [5]. 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 next step toward understanding the corrosion-fatigue phenomenon is to carefully compare fatigue results 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 BMG-11 with water vapor in air could lead to hydrogen embrittlement and reduced fatigue lifetimes.
2. Experimental procedures Bulk metallic glass 11, BMG-11, was the Zr-based metallic glass used in these experiments. Developed at California Institute of Technology, Pasadena, California, the composition in atomic percent (at.%) is Zr– 10Al–5Ti–17.9Cu–14.6Ni [3–5]. 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 for various tests (note that fatigue specimens were fabricated by grinding). Metallography samples, 6.4 mm in diameter by 1.6–3.2 mm in thickness, of the amorphous alloy were polished to a 5 mm finish. Some of these samples were etched with a nitric-hydrofluoric acid solution and observed with optical microscopy for crystalline regions. X-ray diffraction analyses were conducted on other samples using a Philips X’pert 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. The samples were then polished to a 9 mm finish, which corresponded to the surface finish of the specimens in the ongoing corrosion study [10,11], within 1–2 h of initiating the fatigue tests. Fig. 1 shows the modified button-head geometry that was used for the majority of the fatigue specimens. The gripping system and the specimen geometry were similar to that used at ORNL for testing ceramics and other brittle materials [12]. Initial tests with a conventional button-head gripping system resulted in premature
fracture of the specimen near the grips (not at the notched section) at low stress ranges ( 670 MPa). Cracks would initiate at the localized points of contact of the gripping system before cracks initiated in the notched section of the test specimen. One air test and one vacuum test performed with the conventional gripping system at a stress range of 1066 MPa were recorded because the stress level was high enough that this problem was not encountered. The modified gripping system largely eliminated the premature fractures near the grips by increasing the area of contact using soft copper inserts. 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 ranges with a load ratio (R), the minimum load divided by the maximum load, equal to 0.1 (i.e. R=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 pure 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 minimize any impurities in the inert gas before entering the chamber. After the final backfill, the chamber was evacuated to 105 to 106 torr. An ionization gauge with a hot tungsten filament open to the chamber environment was used to monitor the vacuum at low pressure. The vacuum samples were tested in the same manner as the air specimens and at the same stress ranges. Subsequent experiments were performed on fatigue specimens in vacuum without the prolonged use of the ionization gauge. The ionization gauge was turned off after realizing the vacuum, or was never used. In the first test of this series, a thermocouple was attached to the testing gauge section of the fatigue specimen. The temperature was monitored by a digital read-out, and then recorded every 2000 cycles and upon fracture.
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
W.H. Peter et al. / Intermetallics 10 (2002) 1125–1129
patterns were obtained from two amorphous samples. The patterns contained no crystalline peaks; instead, a wide peak corresponding to an amorphous structure was present. The fatigue results for the BMG-11 samples in air and vacuum are presented in Fig. 2. The stress-range values reflect the stress concentration factor (Kt) of 1.55 [13,14] 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 ( L), based on the applied stress range, for the BMG-11 samples tested in air was approximately 907 MPa [10]. The fatigue ratio ( L divided by the ultimate tensile strength) for the BMG-11 was 0.53, based on the ultimate tensile strength of 1700 MPa. The stress-to-lifetime relationship for the air data was: ¼ 233:1logðNf Þ þ 2052
ð1Þ
1127
where 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 correlation coefficient (R-squared value) of 0.88. The average relative humidity for the tests performed in air was 41%, and ranged from 18 to 61%. Although the fatigue lifetimes in vacuum were expected to be longer than those in air, the opposite was found, as shown in Fig. 2. The majority of the vacuum data displayed lifetimes shorter than the air data for any given stress range. Furthermore, the tests conducted in vacuum without the prolonged use of the ionization gauge resulted in improved fatigue lifetimes at 986 MPa. The average fatigue lifetime for these tests was 27,757 cycles. This value was still lower than the average fatigue lifetime of the air data at 986 MPa, which was 40,570 cycles, but significantly higher than the average fatigue lifetime of the vacuum data with the ionization gauge on, which was 12,341 cycles. The higher fatigue life in air suggests a possible beneficial effect of oxygen (in air) that promotes the formation of oxide layers on the specimen surfaces during the air tests. Further studies are needed to verify the oxygen effect. The possibility of detrimental effects in vacuum due to adiabatic heating was evaluated. The fatigue sample tested at 986 MPa that failed at 31,994 cycles had a thermocouple attached to the notched section. During the entire test, the temperature was recorded as 25 C.
4. Discussion
Fig. 1. BMG-11 specimen geometry for fatigue testing in air and vacuum.
At a frequency of 10 Hz, the fatigue lifetimes of BMG-11 in vacuum appear to be shorter than in air. Possible unfavorable conditions associated with the
Fig. 2. Stress-range/fatigue-life data of notched BMG-11 specimens tested in air and vacuum with R=0.1 and a frequency of 10 Hz.
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vacuum could be adiabatic heating from the lack of convective heat transfer, complexities in the vacuum process that could generate detrimental species, and/or lack of beneficial environmental effects. The low fatigue lifetimes in vacuum do not appear to be caused by adiabatic heating. There was concern that localized plastic deformation (at/near the fracture surface) created heat that could not be removed by convection, only by conduction. The heat would then accelerate the crack growth. However, the temperature of the notched section remained at 25 C, indicating that no significant adiabatic heating occurred. With regard to the vacuum process, the ionization gauge monitors the pressure under high-vacuum conditions, and is open to the testing chamber. The hot tungsten filament can dissociate residual water vapor into atomic hydrogen and oxygen [15]. The test material
can absorb the limited atomic hydrogen. Even in small amounts, the absorbed hydrogen could embrittle certain materials. This embrittlement could lower the time for crack initiation, and, hence, reduce the observed lifetime of the sample. George and Liu observed a similar phenomenon with Ni3Al [15,16]. By turning off the ionization gauge, this potential source of atomic hydrogen, and the resulting decrease in fatigue life, would be eliminated. As shown in Fig. 2, the vacuum tests conducted at 986 MPa with the ionization gauge off did show an increase in number of cycles compared to the results with the ionization gauge on, and was within the range of the air data. These limited results indicate an interaction between the ionization gauge and the premature failure of the samples in vacuum, and suggest a hydrogen embrittlement phenomenon. Indeed, Suh and Dauskardt have recently reported embrittlement of a
Table 1 Fatigue endurance limits and fatigue ratios based on the stress ranges of high-strength alloys, Zr-based BMGs, and pure grade zirconium Material
Yield strength (MPa)
Ultimate tensile strength (MPa)
Fatigue endurance limit (MPa)
Fatigue ratio
BMG-11 300 M Steel [13,18] D6AC Steel Q&T (260 C) [13,18] Ti–6Al–4V [13,19] 2090-T81 Al–Li Alloy [2] BMG (Zr–Ti–Cu–Ni–Be) [8] Zirconium, Grade 702 [2]
1700 1670 1720 885 483 1900 310
1,700 2,000 2,000 1035 517 1,900 430
907 800 690 515 250 152 145
0.534 0.479 0.345 0.498 0.484 0.080 0.429
Fig. 3. Stress-range/fatigue-life data of notched BMG-11 specimens tested in air and compared with the fatigue endurance limits ( L) of highstrength alloys [2,8,12,16,17].
W.H. Peter et al. / Intermetallics 10 (2002) 1125–1129
Zr-based BMG that had been pre-charged with hydrogen [17]. When comparing the fatigue properties of BMG-11 in air with those of high-strength alloys and crystalline zirconium, Fig. 3, the fatigue endurance limit of BMG11, 907 MPa, 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 [13,18]. 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 Table 1. The fatigue properties of BMG-11 in air were determined to be considerably better than those reported by Gilbert et al. [8] for a Zr-based BMG containing beryllium (Be), Zr–13.8Ti–12.5Cu–10Ni–22.5Be (at.%). Based on stress-range values, the fatigue endurance limits for BMG-11 and the Be-containing BMG were 907 and 110–150 MPa, respectively, and the fatigue ratios were 0.53 and 0.06–0.08, respectively. Possible explanations for the significant differences in fatigue results are: (1) the differences in chemical compositions, which certainly could influence the inherent fatigue properties, and (2) the differences in specimen geometries and testing procedures. With BMG-11, the tests were conducted on round, notched specimens in uniaxial tension; whereas, with the Be-containing BMG, tests were performed on rectangular-cross-sectional plate specimens in bending. To determine if the differences in specimen geometries and testing procedures were the sources of the apparent discrepancy in results, arrangements have been made to study the fatigue properties of the Be-containing BMG under exactly the same conditions as those used for BMG-11.
5. Conclusions The following conclusions can be made based on the experiments conducted and discussed regarding the bulk metallic glass, BMG-11, Zr–10Al–5Ti–17.9Cu–14.6Ni (at.%): 1. A high fatigue endurance limit, 907 MPa, was observed for BMG-11 in air. 2. A high fatigue ratio, 0.53, was observed for BMG-11 in air, which is in the range demonstrated by most conventional alloy steels. 3. The better fatigue properties determined for BMG-11 in air, as compared to those reported for a Be-containing BMG, could be related to the differences in chemical composition and/or the differences in specimen geometries and testing procedures. 4. The fatigue lifetimes for BMG-11 were found to be lower in vacuum than in air.
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5. Limited results suggest that hydrogen embrittlement, resulting from the dissociation of residual water vapor at the hot tungsten filament of the ionization gauge, could be an environmental effect causing the lower fatigue lifetimes in vacuum than in air.
Acknowledgements The authors are grateful to the National Science Foundation Integrative Graduate Education and Research Training (IGERT) Program on Materials Lifetime Science and Engineering, 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 the Oak Ridge National Laboratory (ORNL), operated by UT-Battelle, LLC., for support of this research. Also, the authors wish to thank Dr. Ken Liu of ORNL for his advice and consultation in the design of the fatigue gripping system.
References [1] Luborsky F. Amorphous metallic alloys. London: Butterworth Publishers Ltd; 1983. [2] ASM Handbook. Properties and selections: nonferrous alloys and special purpose materials, vol. 2. Metals Park, OH: ASM; 1990. [3] Pecker A, Johnson WL. Appl Phys Letter 1993;63:2242. [4] Lin XH, Johnson WL. J Appl Phys 1995;78:6514. [5] Liu CT, Heatherly L, Easton DS, Carmichael CA, Schneibel JH, Chen CH, et al. Metall Mater Trans A 1998;29A:1811. [6] Inoue A. Intermetallics 2000;8:455. [7] Davis L. Metallic glasses. Metals Park, Ohio: American Society for Metals; 1978. [8] Gilbert CJ, Lippmann JM, Ritchie RO. Scripta Mater 1998;38:537. [9] Ritchie RO, Schroeder V, Gilbert CJ. Intermetallics 2000;8:469. [10] Peter WH, Buchanan RA, Liu CT, Liaw PK, The fatigue behavior of a zirconium-based bulk metallic glass in vacuum and air. Journal of Non-Crystalline Solids in press. [11] Peter WH, Buchanan RA, Liu CT, Liaw PK, Morrison ML, Horton JA, et al. Localized corrosion behavior of a zirconiumbased bulk metallic glass relative to its crystalline state. Intermetallics in press. [12] Liu KC, Stevens CO, Brinkman CR, Holshauser NE, J. Eng Gas Turbines Power, Transactions of the ASME, 1997;119(3):500, and private communication. [13] Hertzberg R. Deformation and fracture mechanics of engineering materials. 3rd ed. John Wiley & Sons; 1989. [14] ASME Handbook. Metals engineering—design. New York: McGraw-Hill; 1953. [15] George EP, Liu CT. Structural intermetallics. Warrendale, Pennsylvania: The Minerals, Metals & Materials Society; 1997. [16] Cohron JW, George EP, Heatherly L, Liu CT, Zee RH. Intermetallics 1996;4:997. [17] Suh D, Dauskardt RH. Materials Transactions 2001;42(4):638. [18] Structural alloys handbook. Traverse City, MI: Mechanical Properties Data Center, 1977. [19] Materials engineering. Cleveland, OH: Penton/IPC Publication, 1981.