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Environmental embrittlement of two-phase Fe30Ni20Mn35Al15
Intermetallics 19 (2011) 1533e1537
Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Environmental embrittlement of two-phase Fe30Ni20Mn35Al15 Yifeng Liao 1, Fanling Meng, Ian Baker* Thayer School of Engineering, Dartmouth College, Hanover, NH 03755, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 January 2011 Received in revised form 7 May 2011 Accepted 28 May 2011 Available online 22 June 2011
It is shown that the ductility of lamellae-structured Fe30Ni20Mn35Al15 (in at. %), which consists of B2 and f.c.c. phases, is influenced by testing environment. Tensile tests performed in air at strain rates ranging from 3 106 to 3 101 s1 showed that the elongation to fracture and ultimate tensile strength (UTS) increased with increasing strain rates below 3 103 s1, and were independent of strain rate at w10.5% and 840 MPa for strain rates 3 103 s1. In order to understand this strain-rate sensitive behavior, tensile tests were also performed in either dry oxygen or 4% hydrogen þ nitrogen at different strain rates. The elongation and UTS in oxygen were insensitive to strain rate and close to those tested at 3 103 s1 in air, whereas the elongation in hydrogen was 4% for strain rates 3 103 s1 and increased to w10.8% at 3 101 s1. The reduction of ductility in air and hydrogen-charged environment at low strain rate is attributed to hydrogen embrittlement. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: A. composites B. environmental embrittlement C. mechanical testing
1. Introduction
2Al þ 3H2 O/Al2 O3 þ 6Hþ þ 6e
A recently discovered quaternary compound with the nominal composition Fe30Ni20Mn35Al15 was found to have a yield strength of 600 MPa in the as-cast state and an elongation to failure of w8% when tested at 5 104 s1 in air [1]. The alloy was found to consist of very fine B2 (ordered b.c.c.) and f.c.c. lamellae with widths of 200e500 nm [1]. Plastic deformation was accommodated by glide of <110> dislocations within the f.c.c. phase; the B2 phase showed little sign of plastic deformation. A previous investigation [2] showed that the ductility of this two-phase alloy was dependent on lamellar size, with the elongation increasing to 12% as the lamellae widths were increased to w10 mm. In addition to the intrinsic microstructural properties, extrinsic factors, for instance testing environment, have to be considered in an effort to understand the mechanical behavior of this alloy. It is well known that low carbon steel [3] and many intermetallics [4] are subject to environmental embrittlement. Liu et al. [4] first reported that B2 iron aluminide was embrittled by water vapor. Takasugi and Izumi [5] and George et al. [6] found the mechanical properties of Co3Ti and Ni3Al, both having a L12 (ordered f.c.c.) structure, were also affected by the testing environment. In aluminum-containing alloys reactive hydrogen atoms can be formed from moisture in the air via the following reaction [7]:
The resulting atomic hydrogen can diffuse into metal rapidly, due to the very small size of the hydrogen atoms [7], and assist crack nucleation, for instance, on {100} in polycrystalline B2 FeAl [8]. The decrease in ductility due to hydrogen is significant: Fe39.8%Al single crystal has an intrinsic ductility (elongation to fracture) of w10%, a value obtained from tensile tests in dry oxygen, but shows poor ductility of w0.7% elongation when tested in air [9,10]. Nagpal and Baker [11] noted that such hydrogen embrittlement should be strain-rate dependent and that the ductility was sensitive to strain rate. It is worth noting that in a recent study, it was reported that moisture-induced embrittlement in ferritic-steel can be suppressed by coating [12]. The intermetallics subjected to hydrogen embrittlement that have been studied so far are mostly single-phase materials, particularly B2 FeAl [9e11,13e15]. In this paper, the influences of moisture on the mechanical properties of two-phase drop-cast Fe30Ni20Mn35Al15 are examined by performing tensile tests at different strain rates in air, oxygen and hydrogen environments.
* Corresponding author. Tel.: þ1 603 646 2184. E-mail address: [email protected] (I. Baker). 1 Present address: Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, USA. 0966-9795/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2011.05.023
2. Experimental A drop-cast Fe30Ni20Mn35Al15 rod of 25.4 mm (D) 152.4 mm (L) was provided by Dr. Easo George of the Oak Ridge National Laboratory. A 5 mm thick slab was cut using a high-speed saw and mounted in a phenolic resin. The surface was polished to a mirror finish using 0.3 mm alumina powder followed by etching with aqueous 4% nitric acid. The microstructure was examined using
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Fig. 1. SEM micrograph showing the lamellar structure of drop-cast Fe30Ni20Mn35Al15. The bright and dark regions are the B2 and f.c.c. phases, respectively.
a FEI XL30 field emission gun scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy operated at 15 kV. Flat tensile specimens with gauge lengths of 12.7 mm were cut using an electro-discharge machine. The specimens were polished using 420 grit SiC paper and finished with 0.3 mm alumina powder to reduce defects on the specimens’ surfaces. Tensile tests were performed using an MTS at initial strain rates ranging from 3 106 s1 to 3 101 s1. Prior to the tests, the specimens were preloaded to w150 N. A set of tensile tests was performed for comparison under either a dry oxygen atmosphere (99.99% purity) or 4% hydrogen in nitrogen in a polyacetylene chamber. The strain rate of the tests ranged from 3 106 s1 to 3 103 s1 for dry oxygen and 3 106 s1 to 3 101 s1 for hydrogen þ nitrogen environment. The chamber was flushed with either oxygen or hydrogen þ nitrogen for 30 min prior to testing to remove the air. Gauge lengths were measured before and after the tests using an optical microscope to determine elongation. Fracture surfaces were examined in the SEM.
Fig. 3. Stressestrain curves for specimens tested at different strain rates in dry oxygen.
The chemical compositions of the B2 and f.c.c. phases were Fe9Ni42Mn17Al32 and Fe43Ni12Mn37Al8, respectively. It has been reported previously that, based on TEM electron diffraction studies, the two phases have a Kurjumov-Sachs orientation relationship in the drop-cast state [1], i.e. f.c.c. [111 ]//B2[011 ]; f.c.c.[011]//B2[111].
3. Results The drop-cast Fe30Ni20Mn35Al15 had a fine lamellar structure, as shown in the SEM micrograph in Fig. 1 where the B2 (bright region) and f.c.c. (dark region) lamellae of 200e500 nm wide are evident.
Fig. 2. Stressestrain curves for specimens tested at different strain rates in air.
Fig. 4. (a) Elongation to fracture and (b) ultimate tensile stress and yield stress as a function of strain rate.
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The stress-strain curves for specimens tested in air at different strain rates are shown in Fig. 2. The elongation to fracture exhibits a strong strain rate dependence below 3 103 s1. The elongation was only 0.7% in air at the lowest strain rate (3 106 s1), and gradually increased with increasing the strain rate to 10.5% at a strain rate of 3 103 s1. The elongation was insensitive to strain rate between 3 103 s1 and 3 101 s1. Correspondingly, the UTS in air increased from w640 MPa at 3 106 s1 to w850 MPa at 3 103 s1 and was constant at w850 MPa between 3 103 s1 and 3 101 s1. The stress-strain curves for the tensile tests performed in dry oxygen are shown in Fig. 3. The elongation was w10.5% and independent of strain rate over the range 3 105 to 3 103 s1. The value is essentially identical to specimens tested at high strain rates in air. The elongation in oxygen was slightly reduced to 8.4% at 3 106 s1, suggesting that some air was still present in the testing chamber. The ductility and UTS in air and oxygen are plotted as a function of strain rate in Fig. 4a and b, respectively. The elongation and UTS in oxygen were greater than in air at low strain rates (<3 103 s1) and were comparable to the ones tested in air at a strain rate of 3 103 s1. In contrast to the ductility and UTS, the yield stress was not affected by either the strain rate or the testing environment, see Fig. 4b. The tensile tests in hydrogen environment showed embrittlement at low strain rates. Fig. 5a shows stress-strain curves of the
tests in hydrogen environment for different strain rates. The elongations to fracture are plotted in Fig. 5b. The enlongation was w4% when the strain rate was 3 103 s1 or lower, and increased gradually with increasing strain rate. The elongation at 3 101 s1 was w10.8%, which is essentially the same value as tests performed in oxygen and air at high strain rates. Fracture surfaces from the specimens tested at 3 101 s1 and 3 106 s1 in air are shown in Fig. 6a and Fig. 6b, respectively. The fracture surfaces under the two strain rates were quite similar, showing ductile tearing. A few microvoids were observed, as arrowed in both micrographs. Fig. 7 shows the fracture surfaces from specimens tested at 3 103 s1 and 3 106 s1 in oxygen. Again, both of these show ductile tearing. Microvoids are arrowed in both micrographs. The fracture morphology in oxygen was essentially identical to that strained in air. Neither the strain rate nor environment affected the fracture mode significantly.
Fig. 5. (a) Stressestrain curves for specimens tested at difference strain rates in 4% hydrogen þ nitrogen environment. (b) Elongation as a function of strain rate.
Fig. 6. Fracture surfaces for specimens tested at (a) 3 101 s1, and (b) 3 106 s1 in air. Some microvoids are arrowed.
4. Discussion The deformation of Fe30Ni20Mn35Al15 is inhomogeneous and the alloy can be considered as a composite material composed of a ductile f.c.c. matrix and a strong B2 strength enhancer [1]. The yielding is controlled by the f.c.c. phase, whereas cracks initiate from the B2 phase by elastic fracture. Since the elongation and UTS rather than the yield stress were affected by the strain rate and environment, it appears that the hydrogen embrittlement of the B2
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B2 Fe-40Al, however, no discernible plateau of ductility was reported but the yield stress showed a plateau above 1 103 s1 [17,18]. It is clear that the hydrogen embrittlement can be significantly suppressed in an oxygen atmosphere. The strain rate dependence of the elongation in oxygen was insignificant. The elongation and UTS in oxygen were greater than in air below 3 103 s1, and eventually converged to the values in air at high strain rates, which are presumably the intrinsic plasticity. This is similar to the findings that the elongation of Fe-40Al converged to w15% in both air and vacuum at high strain rates [18] and that the UTS of Co3Ti tested in vacuum, air and hydrogen-charged environment above 1 103 s1 were all w1200 MPa [5]. A few intermetallics change their fracture mode when subjected to hydrogen environment, resulting in a reduction in ductility. George et al. [6] showed that hydrogen had a detrimental effect on grain boundary strength of Ni3Al, leading to an intergranular fracture. The fracture mode for Fe-40Al may also change from transgranular to intergranular fracture at low strain rates [7]. This did not occur in Fe30Ni20Mn35Al15, however, as the fracture mode was similar in both air and oxygen at different strain rates. This is similar to the behavior of Fe3Al in which intergranular fracture prevails unaffected by environment [7]. 5. Conclusion
Fig. 7. Fracture surfaces tested at (a) 3 103 s1, and (b) 3 106 s1 in oxygen. Some microvoids are arrowed.
The ductility of the two-phase Fe30Ni20Mn35Al15 was influenced by the strain rate in both air and hydrogen. Upon testing in air, the elongation and UTS decreased from 10.5% to 0.7% as strain rate decreased from 3 103 to 3 106 s1 but was independent of strain rate for tests at 3 103 s1. Similarly, the elongation tested in hydrogen environment showed limited ductility of w4% when the strain rate was 3 103 s1, and increased to w10.8% at 3 101 s1. In contrast, the ductility and UTS were independent of strain rate for tests performed in oxygen. The embrittlement did not affect the yield stress or the fracture mode. Acknowledgments
phase is more significant. This is the phase that contains most of the aluminum. The elongation in air is sensitive to the strain rate at strain rates below 3 103 s1 as hydrogen embrittlement involves timedependent processes. The ductility in hydrogen-charged environment was low (w4%) at low strain rates and increased to w10.8% with increasing strain rate to 3 103 s1. The results are similar to the observation of hydrogen embrittlement in Fe-40Al, where elongation was reduced in air (2.2%) and hydrogen (5.5%e6.1%) compared to that in oxygen (11.3%) [4]. Stoloff and Liu [7] suggested that aluminum oxide was formed in the presence of water vapor, releasing atomic hydrogen which penetrated the material. Kasul and Heldt [16] calculated the room temperature diffusion coefficient of hydrogen in Fe-35Al to be w4 1012 cm2/s by extrapolating the data of 200 C and 400 C using the Arrhenius equation. Although the exact reaction rate of hydrogen with Al is unknown, it is evident the ductility is dependent on the strain rate [11]. Interestingly, the elongation showed a somewhat linear dependence on the log scale of strain rate, a feature also seen in polycrystalline Ni3Al [6], and Fe-40Al [17,18]. In the range 3 103 s1 to 3 101 s1, the elongation and UTS in air are insensitive to the strain rate, possibly because the atomic hydrogen cannot keep up with the crack propagation, as suggested by George et al. [6]. Similar ductility plateaus were reported in many intermetallics at relatively high strain rates. For instance, Co3Ti had a stable elongation of 40% above 1 103 s1 in air and vacuum [5], a strain rate close to the one in this study. For Ni3Al, a ductility plateau occurred above 5 102 s1 in vacuum and 5 101 s1 in air [6]. For
This research was supported by National Science Foundation Grant DMR 0905229. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation or the U.S. Government. References [1] Liao Y, Baker I. On the room-temperature deformation mechanisms of lamellar-structured Fe30Ni20Mn35Al15. Materials Science and Engineering A 2010;528:3998e4008. [2] Liao Y, Baker I. Evolution of the microstructure and mechanical properties of eutectic Fe30Ni20Mn35Al15. Journal of Materials Science 2010;46:2009e17. [3] Hirth JP. Institute of metals lecture the Metallurgical-Society-of-Aime - Effects of hydrogen on the properties of iron and steel. Metallurgical Transactions APhysical Metallurgy and Materials Science 1980;11:861e90. [4] Liu CT, Lee EH, Mckamey CG. An environmental-effect as the major cause for room-temperature embrittlement in FeAl. Scripta Metallurgica 1989;23:875e80. [5] Takasugi T, Izumi O. Factors affecting the intergranular hydrogen embrittlement of Co3Ti. Acta Metallurgica 1986;34:607e18. [6] George EP, Liu CT, Pope DP. Mechanical behavior of Ni3Al: effects of environment, strain rate, temperature and Boron doping. Acta Materialia 1996;44: 1757e63. [7] Stoloff NS, Liu CT. Environmental embrittlement of iron aluminides. Intermetallics 1994;2:75e87. [8] Li JCM, Liu CT. Crack nucleation in hydrogen embrittlement. Scripta Metallurgica Et Materialia 1992;27:1701e5. [9] Nathal MV, Liu CT. Intrinsic ductility of FeAl single-crystals. Intermetallics 1995;3:77e81. [10] Gaydosh DJ, Draper SL, Noebe RD, Nathal MV. Room-temperature flow and fracture of Fe-40 at-percent-Al alloys. Materials Science and Engineering A 1992;150:7e20.
Y. Liao et al. / Intermetallics 19 (2011) 1533e1537 [11] Nagpal P, Baker I. The effect of strain rate on the room-temperature ductility of FeAl. Scripta Metallurgica Et Materialia 1991;25:2577e80. [12] Zhang ZW, Liu CT, Guo S, Cheng JL, Chen G, Fujita T, et al. Boron effects on the ductility of a nano-cluster-strengthened ferritic steel. Materials Science and Engineering A 2011;528:855e9. [13] Baker I, Wu D, Kruijver SO, George EP. The effects of environment on the room-temperature mechanical behavior of single-slip oriented FeAl single crystals. Materials Science and Engineering A 2002;329:729e33. [14] Lynch RJ, Gee KA, Heldt LA. Environmental embrittlement of single-crystal and thermomechanically processed B2-ordered iron aluminides. Scripta Metallurgica Et Materialia 1994;30:945e50.
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[15] Cohron JW, Lin Y, Zee RH, George EP. Room-temperature mechanical behavior of FeAl: effects of stoichiometry, environment, and Boron addition. Acta Materialia 1998;46:6245e56. [16] Kasul DB, Heldt LA. Embrittlement of B2 iron aluminide by water-vapor and by hydrogen. Metallurgical and Materials Transactions A-Physical Metallurgy and Materials Science 1994;25:1285e90. [17] Pike LM, Liu CT. Environmental and strain rate effects on the ductility and yield strength of Fe-40Al. Scripta Materialia 1998;38:1475e80. [18] Wu D, Baker I. The effect of environment and strain rate on the room temperature tensile properties of FeAl single crystals. Intermetallics 2001;9: 57e65.
Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Environmental embrittlement of two-phase Fe30Ni20Mn35Al15 Yifeng Liao 1, Fanling Meng, Ian Baker* Thayer School of Engineering, Dartmouth College, Hanover, NH 03755, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 January 2011 Received in revised form 7 May 2011 Accepted 28 May 2011 Available online 22 June 2011
It is shown that the ductility of lamellae-structured Fe30Ni20Mn35Al15 (in at. %), which consists of B2 and f.c.c. phases, is influenced by testing environment. Tensile tests performed in air at strain rates ranging from 3 106 to 3 101 s1 showed that the elongation to fracture and ultimate tensile strength (UTS) increased with increasing strain rates below 3 103 s1, and were independent of strain rate at w10.5% and 840 MPa for strain rates 3 103 s1. In order to understand this strain-rate sensitive behavior, tensile tests were also performed in either dry oxygen or 4% hydrogen þ nitrogen at different strain rates. The elongation and UTS in oxygen were insensitive to strain rate and close to those tested at 3 103 s1 in air, whereas the elongation in hydrogen was 4% for strain rates 3 103 s1 and increased to w10.8% at 3 101 s1. The reduction of ductility in air and hydrogen-charged environment at low strain rate is attributed to hydrogen embrittlement. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: A. composites B. environmental embrittlement C. mechanical testing
1. Introduction
2Al þ 3H2 O/Al2 O3 þ 6Hþ þ 6e
A recently discovered quaternary compound with the nominal composition Fe30Ni20Mn35Al15 was found to have a yield strength of 600 MPa in the as-cast state and an elongation to failure of w8% when tested at 5 104 s1 in air [1]. The alloy was found to consist of very fine B2 (ordered b.c.c.) and f.c.c. lamellae with widths of 200e500 nm [1]. Plastic deformation was accommodated by glide of <110> dislocations within the f.c.c. phase; the B2 phase showed little sign of plastic deformation. A previous investigation [2] showed that the ductility of this two-phase alloy was dependent on lamellar size, with the elongation increasing to 12% as the lamellae widths were increased to w10 mm. In addition to the intrinsic microstructural properties, extrinsic factors, for instance testing environment, have to be considered in an effort to understand the mechanical behavior of this alloy. It is well known that low carbon steel [3] and many intermetallics [4] are subject to environmental embrittlement. Liu et al. [4] first reported that B2 iron aluminide was embrittled by water vapor. Takasugi and Izumi [5] and George et al. [6] found the mechanical properties of Co3Ti and Ni3Al, both having a L12 (ordered f.c.c.) structure, were also affected by the testing environment. In aluminum-containing alloys reactive hydrogen atoms can be formed from moisture in the air via the following reaction [7]:
The resulting atomic hydrogen can diffuse into metal rapidly, due to the very small size of the hydrogen atoms [7], and assist crack nucleation, for instance, on {100} in polycrystalline B2 FeAl [8]. The decrease in ductility due to hydrogen is significant: Fe39.8%Al single crystal has an intrinsic ductility (elongation to fracture) of w10%, a value obtained from tensile tests in dry oxygen, but shows poor ductility of w0.7% elongation when tested in air [9,10]. Nagpal and Baker [11] noted that such hydrogen embrittlement should be strain-rate dependent and that the ductility was sensitive to strain rate. It is worth noting that in a recent study, it was reported that moisture-induced embrittlement in ferritic-steel can be suppressed by coating [12]. The intermetallics subjected to hydrogen embrittlement that have been studied so far are mostly single-phase materials, particularly B2 FeAl [9e11,13e15]. In this paper, the influences of moisture on the mechanical properties of two-phase drop-cast Fe30Ni20Mn35Al15 are examined by performing tensile tests at different strain rates in air, oxygen and hydrogen environments.
* Corresponding author. Tel.: þ1 603 646 2184. E-mail address: [email protected] (I. Baker). 1 Present address: Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, USA. 0966-9795/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2011.05.023
2. Experimental A drop-cast Fe30Ni20Mn35Al15 rod of 25.4 mm (D) 152.4 mm (L) was provided by Dr. Easo George of the Oak Ridge National Laboratory. A 5 mm thick slab was cut using a high-speed saw and mounted in a phenolic resin. The surface was polished to a mirror finish using 0.3 mm alumina powder followed by etching with aqueous 4% nitric acid. The microstructure was examined using
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Fig. 1. SEM micrograph showing the lamellar structure of drop-cast Fe30Ni20Mn35Al15. The bright and dark regions are the B2 and f.c.c. phases, respectively.
a FEI XL30 field emission gun scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy operated at 15 kV. Flat tensile specimens with gauge lengths of 12.7 mm were cut using an electro-discharge machine. The specimens were polished using 420 grit SiC paper and finished with 0.3 mm alumina powder to reduce defects on the specimens’ surfaces. Tensile tests were performed using an MTS at initial strain rates ranging from 3 106 s1 to 3 101 s1. Prior to the tests, the specimens were preloaded to w150 N. A set of tensile tests was performed for comparison under either a dry oxygen atmosphere (99.99% purity) or 4% hydrogen in nitrogen in a polyacetylene chamber. The strain rate of the tests ranged from 3 106 s1 to 3 103 s1 for dry oxygen and 3 106 s1 to 3 101 s1 for hydrogen þ nitrogen environment. The chamber was flushed with either oxygen or hydrogen þ nitrogen for 30 min prior to testing to remove the air. Gauge lengths were measured before and after the tests using an optical microscope to determine elongation. Fracture surfaces were examined in the SEM.
Fig. 3. Stressestrain curves for specimens tested at different strain rates in dry oxygen.
The chemical compositions of the B2 and f.c.c. phases were Fe9Ni42Mn17Al32 and Fe43Ni12Mn37Al8, respectively. It has been reported previously that, based on TEM electron diffraction studies, the two phases have a Kurjumov-Sachs orientation relationship in the drop-cast state [1], i.e. f.c.c. [111 ]//B2[011 ]; f.c.c.[011]//B2[111].
3. Results The drop-cast Fe30Ni20Mn35Al15 had a fine lamellar structure, as shown in the SEM micrograph in Fig. 1 where the B2 (bright region) and f.c.c. (dark region) lamellae of 200e500 nm wide are evident.
Fig. 2. Stressestrain curves for specimens tested at different strain rates in air.
Fig. 4. (a) Elongation to fracture and (b) ultimate tensile stress and yield stress as a function of strain rate.
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The stress-strain curves for specimens tested in air at different strain rates are shown in Fig. 2. The elongation to fracture exhibits a strong strain rate dependence below 3 103 s1. The elongation was only 0.7% in air at the lowest strain rate (3 106 s1), and gradually increased with increasing the strain rate to 10.5% at a strain rate of 3 103 s1. The elongation was insensitive to strain rate between 3 103 s1 and 3 101 s1. Correspondingly, the UTS in air increased from w640 MPa at 3 106 s1 to w850 MPa at 3 103 s1 and was constant at w850 MPa between 3 103 s1 and 3 101 s1. The stress-strain curves for the tensile tests performed in dry oxygen are shown in Fig. 3. The elongation was w10.5% and independent of strain rate over the range 3 105 to 3 103 s1. The value is essentially identical to specimens tested at high strain rates in air. The elongation in oxygen was slightly reduced to 8.4% at 3 106 s1, suggesting that some air was still present in the testing chamber. The ductility and UTS in air and oxygen are plotted as a function of strain rate in Fig. 4a and b, respectively. The elongation and UTS in oxygen were greater than in air at low strain rates (<3 103 s1) and were comparable to the ones tested in air at a strain rate of 3 103 s1. In contrast to the ductility and UTS, the yield stress was not affected by either the strain rate or the testing environment, see Fig. 4b. The tensile tests in hydrogen environment showed embrittlement at low strain rates. Fig. 5a shows stress-strain curves of the
tests in hydrogen environment for different strain rates. The elongations to fracture are plotted in Fig. 5b. The enlongation was w4% when the strain rate was 3 103 s1 or lower, and increased gradually with increasing strain rate. The elongation at 3 101 s1 was w10.8%, which is essentially the same value as tests performed in oxygen and air at high strain rates. Fracture surfaces from the specimens tested at 3 101 s1 and 3 106 s1 in air are shown in Fig. 6a and Fig. 6b, respectively. The fracture surfaces under the two strain rates were quite similar, showing ductile tearing. A few microvoids were observed, as arrowed in both micrographs. Fig. 7 shows the fracture surfaces from specimens tested at 3 103 s1 and 3 106 s1 in oxygen. Again, both of these show ductile tearing. Microvoids are arrowed in both micrographs. The fracture morphology in oxygen was essentially identical to that strained in air. Neither the strain rate nor environment affected the fracture mode significantly.
Fig. 5. (a) Stressestrain curves for specimens tested at difference strain rates in 4% hydrogen þ nitrogen environment. (b) Elongation as a function of strain rate.
Fig. 6. Fracture surfaces for specimens tested at (a) 3 101 s1, and (b) 3 106 s1 in air. Some microvoids are arrowed.
4. Discussion The deformation of Fe30Ni20Mn35Al15 is inhomogeneous and the alloy can be considered as a composite material composed of a ductile f.c.c. matrix and a strong B2 strength enhancer [1]. The yielding is controlled by the f.c.c. phase, whereas cracks initiate from the B2 phase by elastic fracture. Since the elongation and UTS rather than the yield stress were affected by the strain rate and environment, it appears that the hydrogen embrittlement of the B2
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B2 Fe-40Al, however, no discernible plateau of ductility was reported but the yield stress showed a plateau above 1 103 s1 [17,18]. It is clear that the hydrogen embrittlement can be significantly suppressed in an oxygen atmosphere. The strain rate dependence of the elongation in oxygen was insignificant. The elongation and UTS in oxygen were greater than in air below 3 103 s1, and eventually converged to the values in air at high strain rates, which are presumably the intrinsic plasticity. This is similar to the findings that the elongation of Fe-40Al converged to w15% in both air and vacuum at high strain rates [18] and that the UTS of Co3Ti tested in vacuum, air and hydrogen-charged environment above 1 103 s1 were all w1200 MPa [5]. A few intermetallics change their fracture mode when subjected to hydrogen environment, resulting in a reduction in ductility. George et al. [6] showed that hydrogen had a detrimental effect on grain boundary strength of Ni3Al, leading to an intergranular fracture. The fracture mode for Fe-40Al may also change from transgranular to intergranular fracture at low strain rates [7]. This did not occur in Fe30Ni20Mn35Al15, however, as the fracture mode was similar in both air and oxygen at different strain rates. This is similar to the behavior of Fe3Al in which intergranular fracture prevails unaffected by environment [7]. 5. Conclusion
Fig. 7. Fracture surfaces tested at (a) 3 103 s1, and (b) 3 106 s1 in oxygen. Some microvoids are arrowed.
The ductility of the two-phase Fe30Ni20Mn35Al15 was influenced by the strain rate in both air and hydrogen. Upon testing in air, the elongation and UTS decreased from 10.5% to 0.7% as strain rate decreased from 3 103 to 3 106 s1 but was independent of strain rate for tests at 3 103 s1. Similarly, the elongation tested in hydrogen environment showed limited ductility of w4% when the strain rate was 3 103 s1, and increased to w10.8% at 3 101 s1. In contrast, the ductility and UTS were independent of strain rate for tests performed in oxygen. The embrittlement did not affect the yield stress or the fracture mode. Acknowledgments
phase is more significant. This is the phase that contains most of the aluminum. The elongation in air is sensitive to the strain rate at strain rates below 3 103 s1 as hydrogen embrittlement involves timedependent processes. The ductility in hydrogen-charged environment was low (w4%) at low strain rates and increased to w10.8% with increasing strain rate to 3 103 s1. The results are similar to the observation of hydrogen embrittlement in Fe-40Al, where elongation was reduced in air (2.2%) and hydrogen (5.5%e6.1%) compared to that in oxygen (11.3%) [4]. Stoloff and Liu [7] suggested that aluminum oxide was formed in the presence of water vapor, releasing atomic hydrogen which penetrated the material. Kasul and Heldt [16] calculated the room temperature diffusion coefficient of hydrogen in Fe-35Al to be w4 1012 cm2/s by extrapolating the data of 200 C and 400 C using the Arrhenius equation. Although the exact reaction rate of hydrogen with Al is unknown, it is evident the ductility is dependent on the strain rate [11]. Interestingly, the elongation showed a somewhat linear dependence on the log scale of strain rate, a feature also seen in polycrystalline Ni3Al [6], and Fe-40Al [17,18]. In the range 3 103 s1 to 3 101 s1, the elongation and UTS in air are insensitive to the strain rate, possibly because the atomic hydrogen cannot keep up with the crack propagation, as suggested by George et al. [6]. Similar ductility plateaus were reported in many intermetallics at relatively high strain rates. For instance, Co3Ti had a stable elongation of 40% above 1 103 s1 in air and vacuum [5], a strain rate close to the one in this study. For Ni3Al, a ductility plateau occurred above 5 102 s1 in vacuum and 5 101 s1 in air [6]. For
This research was supported by National Science Foundation Grant DMR 0905229. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation or the U.S. Government. References [1] Liao Y, Baker I. On the room-temperature deformation mechanisms of lamellar-structured Fe30Ni20Mn35Al15. Materials Science and Engineering A 2010;528:3998e4008. [2] Liao Y, Baker I. Evolution of the microstructure and mechanical properties of eutectic Fe30Ni20Mn35Al15. Journal of Materials Science 2010;46:2009e17. [3] Hirth JP. Institute of metals lecture the Metallurgical-Society-of-Aime - Effects of hydrogen on the properties of iron and steel. Metallurgical Transactions APhysical Metallurgy and Materials Science 1980;11:861e90. [4] Liu CT, Lee EH, Mckamey CG. An environmental-effect as the major cause for room-temperature embrittlement in FeAl. Scripta Metallurgica 1989;23:875e80. [5] Takasugi T, Izumi O. Factors affecting the intergranular hydrogen embrittlement of Co3Ti. Acta Metallurgica 1986;34:607e18. [6] George EP, Liu CT, Pope DP. Mechanical behavior of Ni3Al: effects of environment, strain rate, temperature and Boron doping. Acta Materialia 1996;44: 1757e63. [7] Stoloff NS, Liu CT. Environmental embrittlement of iron aluminides. Intermetallics 1994;2:75e87. [8] Li JCM, Liu CT. Crack nucleation in hydrogen embrittlement. Scripta Metallurgica Et Materialia 1992;27:1701e5. [9] Nathal MV, Liu CT. Intrinsic ductility of FeAl single-crystals. Intermetallics 1995;3:77e81. [10] Gaydosh DJ, Draper SL, Noebe RD, Nathal MV. Room-temperature flow and fracture of Fe-40 at-percent-Al alloys. Materials Science and Engineering A 1992;150:7e20.
Y. Liao et al. / Intermetallics 19 (2011) 1533e1537 [11] Nagpal P, Baker I. The effect of strain rate on the room-temperature ductility of FeAl. Scripta Metallurgica Et Materialia 1991;25:2577e80. [12] Zhang ZW, Liu CT, Guo S, Cheng JL, Chen G, Fujita T, et al. Boron effects on the ductility of a nano-cluster-strengthened ferritic steel. Materials Science and Engineering A 2011;528:855e9. [13] Baker I, Wu D, Kruijver SO, George EP. The effects of environment on the room-temperature mechanical behavior of single-slip oriented FeAl single crystals. Materials Science and Engineering A 2002;329:729e33. [14] Lynch RJ, Gee KA, Heldt LA. Environmental embrittlement of single-crystal and thermomechanically processed B2-ordered iron aluminides. Scripta Metallurgica Et Materialia 1994;30:945e50.
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[15] Cohron JW, Lin Y, Zee RH, George EP. Room-temperature mechanical behavior of FeAl: effects of stoichiometry, environment, and Boron addition. Acta Materialia 1998;46:6245e56. [16] Kasul DB, Heldt LA. Embrittlement of B2 iron aluminide by water-vapor and by hydrogen. Metallurgical and Materials Transactions A-Physical Metallurgy and Materials Science 1994;25:1285e90. [17] Pike LM, Liu CT. Environmental and strain rate effects on the ductility and yield strength of Fe-40Al. Scripta Materialia 1998;38:1475e80. [18] Wu D, Baker I. The effect of environment and strain rate on the room temperature tensile properties of FeAl single crystals. Intermetallics 2001;9: 57e65.