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Glass forming ability and mechanical properties characterization on Mg58Cu31Y11−xGdx bulk metallic glasses
Journal of Alloys and Compounds 483 (2009) 40–43
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Glass forming ability and mechanical properties characterization on Mg58 Cu31 Y11−x Gdx bulk metallic glasses P.J. Hsieh ∗ , S.C. Lin, H.C. Su, J.S.C. Jang Department of Materials Science and Engineering, I-Shou University Kaohsiung 840, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 30 August 2007 Received in revised form 29 July 2008 Accepted 10 August 2008 Available online 14 November 2008 Keywords: Mg-based BMG Glass forming ability Vein pattern
a b s t r a c t Mg–Cu–Y–(Gd) alloy rods are made by arc-melting and injection casting methods in this research. The improvement of glass forming ability and mechanical properties by using Gd to substitute Y in Mg58 Cu31 Y11 bulk metallic glasses (BMGs) is of interest. The results of thermal analysis present that the Mg–Cu–Y base alloys with the addition of 6 and 8 at% Gd are the best BMG former. The Vickers indentation tests and the compression tests are carried out in order to explore the mechanical properties of alloys. It reveals that there is no obvious change in Young’s modulus (∼45 GPa) of the Gd-containing Mg-based BMG, in contrast with the base alloys. Vickers (micro-) indentation fracture toughness measurements are performed for comparison. Shear bands and the corner cracks around the inverted pyramind mark are showed. An average fracture toughness of Mg–Cu–Y–Gd alloy is calculated as ∼4 MPa m1/2 , which is a little higher than that of base alloys studied in the paper. Meanwhile, the fracture surface of Mg-based BMGs is dominant by featureless mirror-like and river-like pattern. Only nano-scaled shear bands and vein patterns are displayed, indicating that the plasticity of the Mg–Cu–Y–Gd BMGs are shown in nano-scale indeed. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The metallic glasses with the lack of long-range atomic ordering, are expected to possess unique properties [1,2]. Recently, Mg-based bulk metallic glasses with large specimen size and high glass forming ability (GFA) have attracted attention for the studies of their mechanical behavior. The glass forming ability of bulk metallic glasses is very important to the glass metal preparation. Hence, there are some parameters can be used to estimate the glass forming ability of BMGs, such as the reduced glass transition temperature [3–5], Trg (=Tg /Tl , Tg : glass transition temperature; Tl : liquidus temperature), (=Tx /(Tg + Tl )) [6], m (=(2Tx − Tg )/Tl ) [7] and the supercooled liquid regime, Tx (=Tx − Tg , Tx : crystallization temperature). Among the reported Mg-based amorphous alloy systems, a lot of works have been attended to the Mg–Cu–Y system. The injection casting Mg–Cu–Y alloy systems are reported to reveal high tensile stress (∼800 MPa) [8] and the outstanding GFA with large critical size ∼9 mm in diameter [9]. However, the brittleness of Mg-based BMG is a reproached problem even now. No obvious macroscopic plasticity can be observed in Mg–Cu–Y BMGs, due to very local-
∗ Corresponding author. Tel.: +886 7 6577711x3120; fax: +886 7 6578444. E-mail address: [email protected] (P.J. Hsieh). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.08.124
ized shear bands predominated the brittle failure behavior of alloys. For further improvement of the glass forming ability and the ductility in the Mg–Cu–Y system, the rare-earth metal Gd was selected to partially substitute for Y, because Gd and Y have similar atomic radius and heat of mixing against Mg and Cu [10]. In this study, BMGs with composition Mg58 Cu31 Y11 were prepared by injection casting as the base alloys. The effects of rare-earth substitution on GFA and fracture mechanism of Mg58 Cu31 Y11−x Gdx quaternary alloys are discussed in this research. 2. Experimental procedure Bulk metallic glasses with compositions of Mg58 Cu31 Y11−x Gdx (x = 0, 2, 4, 6, 8, 10, 11) were studied and the Mg58 Cu31 Y11 (x = 0) alloys are regarded as the base materials. The purity of all elemental materials is above 99.99at%. The ingots of Cu–Y(–Gd) alloys were first prepared by arc melting process under the Ti-gettered argon atmosphere. Then theses ingots were melted with pure Mg in an induction furnace under a purified argon atmosphere. The master alloy was prepared by using arc melting, induction melting and injection casting processes to form Mg58 Cu31 Y11−x Gdx BMG rods with the size of 2–5 mm in diameters and 5 cm in length. The X-ray diffractometer (Scintag X-400 XRD) and the differential scanning calorimeter (DSC 2920) were used for the phase identification and the thermal analysis of alloys. Besides, indentation tests and compression tests (at strain rate 5 × 10−4 s−1 , RT) were executed to estimate the mechanical properties of alloys. The specimens for compression testing were machined from cylindrical master alloys, with size of 2 mm in diameter and 4 mm in height. The microstructure of the desired alloy systems was observed by field-emission scanning electric microscopy (FE-SEM, Hitachi S-4700).
P.J. Hsieh et al. / Journal of Alloys and Compounds 483 (2009) 40–43
41
Table 2 Fracture toughness of various Mg58 Cu31 Y11−x Gdx (x = 0, 6, 8, 11) BMGs. Gd content (at%)
KIc (MPa m1/2 )
x=0 x=6 x=8 x = 11
2.91 2.92 4.02 2.89
Fig. 1. The XRD patterns of injection-casting Mg58 Cu31 Y11−x Gdx (x = 0, 6, 8, 11) alloy specimens with 3 mm in diameter. Table 1 Tg , Tx, Tl , Trg , and m for the Mg58 Cu31 Y11−x Gdx (x = 0, 6, 8, 11) BMGs. (at%)
Tg (K)
Tx (K)
Tl (K)
Tx
Trg
m
x=0 x=6 x=8 x = 11
424 422 420 422
495 493 491 479
790 747 740 780
71 71 71 57
0.536 0.564 0.567 0.533
0.407 0.421 0.423 0.398
0.716 0.755 0.759 0.687
3. Results and discussions Fig. 1 is the XRD patterns taken from the Mg58 Cu31 Y11−x Gdx (x = 0, 6, 8, 11) cylindrical alloy specimens with 3 mm in diameter. No crystalline phases superimposed on broadening hump were detected in the XRD patterns of the Mg58 Cu31 Y5 Gd6 (x = 6) and the Mg58 Cu31 Y3 Gd8 (x = 8) specimens. It reflects that the partial substituting effect of rare-earth metal Gd on the glass forming ability is contributive Table 1 summaries Tg , Tx and Tl for the Mg58 Cu31 Y11−x Gdx alloys based on the non-isothermal DSC curves (with a heating rate of 20 K/min) shown in Fig. 2. Tl slightly decreased (by 10–50 K) with increasing Gd content, indicating that the liquid phase is stabi-
Fig. 2. Non-isothermal DSC curves for Mg58 Cu31 Y11−x Gdx (x = 0, 6, 8, 11) BMG rods (3 mm in diameter) with a heating rate of 20 K/min.
Fig. 3. Half-circular patterns of shear bands in the vicinity of the deformed crack region for the Mg58 Cu31 Y5 Gd6 BMG.
lized by rare-earth metal Gd, especially for the specimen of the Mg58 Cu31 Y3 Gd8 (x = 8) BMG. The estimated parameters of GFA, Trg , and m , are increased with the decreasing of Tl . Reviewing three GFA parameters included in Table 1, it is found that m is the most sensitive parameter to display the deviation in GFA of these Mg58 Cu31 Y11−x Gdx BMGs. The alloys with 6 and 8 at% Gd content (Mg58 Cu31 Y5 Gd6 , Mg58 Cu31 Y3 Gd8 ) provided the great GFA and the high thermal stability ∼71 K. Broad supercooled liquid region implies that the Mg58 Cu31 Y11−x Gdx (x = 6 and 8) BMG possessed a high resistance to crystallization, leading to the high thermal stability and the high GFA. For further investigation of mechanical properties of the best two BMG former (x = 6, 8), the indentation tests are followed. The fracture toughness (KIc ) determination [11] is applied by a Vickers pyramidal hardness indenter (Akashi, MVK-H11). The Vickers hardness of the Mg58 Cu31 Y11−x Gdx BMGs are in the range of 284–306 Hv . Averaged hardness of the Mg-based BMGs is four times higher than
Fig. 4. The compressive stress–strain curve for the Mg58 Cu31 Y11−x Gdx (x = 0, 6, 8, 11) BMGs.
42
P.J. Hsieh et al. / Journal of Alloys and Compounds 483 (2009) 40–43
Table 3 The mechanical properties of the Mg58 Cu31 Y11−x Gdx (x = 0, 6, 8, 11) BMGs via compression tests at room temperature. Gd content (at%)
Fracture stress f (MPa)
Young’s modulus E (GPa)
Elastic strain εe (%)
x=0 x=6 x=8 x = 11
714 855 899 772
44 44.9 45.2 44.1
1.71 1.98 2.04 1.76
Fig. 7. Striation patterns with different orientations and nano-scaled vein-like pattern upon the fracture surfaces of the compressive Mg58 Cu31 Y3 Gd8 BMG.
Fig. 5. Vein pattern upon the fracture surfaces of the compressive Mg58 Cu31 Y3 Gd8 specimen.
Fig. 6. (a) Mirror-like and (b) river-like patterns upon the fracture surfaces of the compressive Mg58 Cu31 Y11−x Gdx (x = 6 and 8) BMGs.
that of the AZ91 Mg alloys. Meanwhile, an empirical calibration [12,13] is used to estimate the KIc of the BMGs and the results are listed in Table 2. The highest KIc value is ∼4.02 MPa m1/2 for the Mg58 Cu31 Y3 Gd8 alloy, which is higher than that of the base alloys. A few half-circular patterns of shear bands were observed (in Fig. 3) in the vicinity of the deformed crack region, illustrating the plastic deformation of the Mg–Cu–Y–Gd amorphous alloys. Uniaxial compression tests were performed by using material test system (MTS 810) to explore the fracture stress, the elastic strain, and the elastic constant. Fig. 4 reveals the compressive stress–strain curve for the Mg58 Cu31 Y11−x Gdx BMGs (x = 0, 6, 8 and 11), and the mechanical properties were extracted as listed in Table 3. It is concluded that the addition of Gd improves the fracture strength of the base alloys, but no macroscopic plastic deformation occurred in the Mg–Cu–Y(–Gd) BMGs. Meanwhile, the fracture surfaces of the compressive specimens were examined by using SEM. Only a few vein-patterns (Fig. 5) can be observed in the fracture surface of the Mg58 Cu31 Y3 Gd8 BMG. The major morphologies of the fracture surface are mirror-like (Fig. 6(a)) and river-like pattern (Fig. 6(b)), indicating that the dominant fracture behavior is brittle [14,15]. Further examination of the mirror region found that the featureless area is composed of the striation patterns with different orientations indeed (in Fig. 7). Each striation pattern corresponds to a set of individual shear bands. Besides, a
Fig. 8. The typical load-displacement curve for the Mg58 Cu31 Y3 Gd8 BMG.
P.J. Hsieh et al. / Journal of Alloys and Compounds 483 (2009) 40–43
nano-scaled vein-like pattern could be found inside the striation structure. It implies that the plastic deformation can take place in nano-scale. The spacing inside the striation structure is about 50 nm, which is comparable with the serrated flow in the load-displacement curves of nanoindentation measurement in Fig. 8. The serration appeared in the load-displacement curve was caused by the discrete plastic deformation during nanoindentation test. The interrupting of plasticity is associated with the awakening of shear bands. Fig. 8 is the typical load-displacement curve at the loading rate of 1 mN/s for Mg58 Cu31 Y3 Gd8 specimen. Averaged step of the discrete interrupting is near 50 nm as well. 4. Conclusions (i) For the Mg58 Cu31 Y11 alloy, a suitable Gd content in substitution for Y improves the GFA and thermal stability. The best BMG former is alloy with composition Mg58 Cu31 Y5 Gd6 and Mg58 Cu31 Y3 Gd8 in this study. (ii) The fracture toughness, KIc , from the Vickers indentation fracture test is ∼4.02 MPa m1/2 for the Mg58 Cu31 Y3 Gd8 alloy. It exhibits that the fracture resistance is enhanced by the addition of Gd in Mg58 Cu31 Y11 alloy systems. (iii) The dominant morphologies of the fracture surface are mirrorlike and river-like pattern, indicating that the fracture behavior is brittle. However, the featureless mirror region is composed of the striation pattern (shear bans) and the vein-like pattern
43
in nano-scale actually. It implies that the plastic deformation can take place in nano-scale as well. Acknowledgements The authors are gratefully acknowledge the sponsorship by National Science Council of Taiwan (ROC) under the project no. NSC 95-2221-E-214-016-MY3 and the Micro- and Nano-Structure Analysis Lab in I-Shou university. References [1] A. Inoue (Ed.), Bulk Amorphous Alloys, Trans Tech Publication Ltd., Switzerland, 1998. [2] W.L. Johnson, Mater. Sci. Forum 225–227 (1996) 35. [3] A. Brenner, D.E. Couch, E.K. Williams, J. Res. Natl. Bur. Stand. 44 (1950) 109. [4] D. Turnbull, Solid State Phys. 3 (1956) 225. [5] D. Turnbull, J. Phys. 35 (1974) 1. [6] Q. Zheng, H. Ma, J. Xu, Scripta Mater. 55 (2006) 541. [7] X.H. Du, J.C. Huang, C.T. Liu, Z.P. Liu, J. Appl. Phys. 101 (2007) 086108. [8] A. Inoue, A. Kato, T. Zhang, S.G. Kim, T. Masumoto, Mater. Trans., JIM 32 (1991) 609. [9] H. Ma, Q. Zeheng, J. Xu, Y. Li, E. Ma, J. Mater. Res. 20 (2005) 2252. [10] H. Men, W.T. Kim, D.H. Kim, J. Non-Cryst. Solids 337 (2004) 29. [11] G.D. Quinn, R.C. Bradt, J. Am. Ceram. Soc. 90 (2007) 673. [12] B.R. Lawn, T.R. Wilshaw, J. Mater. Sci. 10 (1975) 1049. [13] A.G. Evans, T.R. Wilshaw, Acta Mater. 24 (1979) 939. [14] D.G. Pan, W.Y. Liu, H.F. Zhang, A.M. Wang, Z.Q. Hu, J. Alloys Compd. 438 (2007) 142. [15] D.G. Pan, H.F. Zhang, A.M. Wang, Z.G. Wang, Z.Q. Hu, J. Alloys Compd. 438 (2007) 145.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Glass forming ability and mechanical properties characterization on Mg58 Cu31 Y11−x Gdx bulk metallic glasses P.J. Hsieh ∗ , S.C. Lin, H.C. Su, J.S.C. Jang Department of Materials Science and Engineering, I-Shou University Kaohsiung 840, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 30 August 2007 Received in revised form 29 July 2008 Accepted 10 August 2008 Available online 14 November 2008 Keywords: Mg-based BMG Glass forming ability Vein pattern
a b s t r a c t Mg–Cu–Y–(Gd) alloy rods are made by arc-melting and injection casting methods in this research. The improvement of glass forming ability and mechanical properties by using Gd to substitute Y in Mg58 Cu31 Y11 bulk metallic glasses (BMGs) is of interest. The results of thermal analysis present that the Mg–Cu–Y base alloys with the addition of 6 and 8 at% Gd are the best BMG former. The Vickers indentation tests and the compression tests are carried out in order to explore the mechanical properties of alloys. It reveals that there is no obvious change in Young’s modulus (∼45 GPa) of the Gd-containing Mg-based BMG, in contrast with the base alloys. Vickers (micro-) indentation fracture toughness measurements are performed for comparison. Shear bands and the corner cracks around the inverted pyramind mark are showed. An average fracture toughness of Mg–Cu–Y–Gd alloy is calculated as ∼4 MPa m1/2 , which is a little higher than that of base alloys studied in the paper. Meanwhile, the fracture surface of Mg-based BMGs is dominant by featureless mirror-like and river-like pattern. Only nano-scaled shear bands and vein patterns are displayed, indicating that the plasticity of the Mg–Cu–Y–Gd BMGs are shown in nano-scale indeed. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The metallic glasses with the lack of long-range atomic ordering, are expected to possess unique properties [1,2]. Recently, Mg-based bulk metallic glasses with large specimen size and high glass forming ability (GFA) have attracted attention for the studies of their mechanical behavior. The glass forming ability of bulk metallic glasses is very important to the glass metal preparation. Hence, there are some parameters can be used to estimate the glass forming ability of BMGs, such as the reduced glass transition temperature [3–5], Trg (=Tg /Tl , Tg : glass transition temperature; Tl : liquidus temperature), (=Tx /(Tg + Tl )) [6], m (=(2Tx − Tg )/Tl ) [7] and the supercooled liquid regime, Tx (=Tx − Tg , Tx : crystallization temperature). Among the reported Mg-based amorphous alloy systems, a lot of works have been attended to the Mg–Cu–Y system. The injection casting Mg–Cu–Y alloy systems are reported to reveal high tensile stress (∼800 MPa) [8] and the outstanding GFA with large critical size ∼9 mm in diameter [9]. However, the brittleness of Mg-based BMG is a reproached problem even now. No obvious macroscopic plasticity can be observed in Mg–Cu–Y BMGs, due to very local-
∗ Corresponding author. Tel.: +886 7 6577711x3120; fax: +886 7 6578444. E-mail address: [email protected] (P.J. Hsieh). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.08.124
ized shear bands predominated the brittle failure behavior of alloys. For further improvement of the glass forming ability and the ductility in the Mg–Cu–Y system, the rare-earth metal Gd was selected to partially substitute for Y, because Gd and Y have similar atomic radius and heat of mixing against Mg and Cu [10]. In this study, BMGs with composition Mg58 Cu31 Y11 were prepared by injection casting as the base alloys. The effects of rare-earth substitution on GFA and fracture mechanism of Mg58 Cu31 Y11−x Gdx quaternary alloys are discussed in this research. 2. Experimental procedure Bulk metallic glasses with compositions of Mg58 Cu31 Y11−x Gdx (x = 0, 2, 4, 6, 8, 10, 11) were studied and the Mg58 Cu31 Y11 (x = 0) alloys are regarded as the base materials. The purity of all elemental materials is above 99.99at%. The ingots of Cu–Y(–Gd) alloys were first prepared by arc melting process under the Ti-gettered argon atmosphere. Then theses ingots were melted with pure Mg in an induction furnace under a purified argon atmosphere. The master alloy was prepared by using arc melting, induction melting and injection casting processes to form Mg58 Cu31 Y11−x Gdx BMG rods with the size of 2–5 mm in diameters and 5 cm in length. The X-ray diffractometer (Scintag X-400 XRD) and the differential scanning calorimeter (DSC 2920) were used for the phase identification and the thermal analysis of alloys. Besides, indentation tests and compression tests (at strain rate 5 × 10−4 s−1 , RT) were executed to estimate the mechanical properties of alloys. The specimens for compression testing were machined from cylindrical master alloys, with size of 2 mm in diameter and 4 mm in height. The microstructure of the desired alloy systems was observed by field-emission scanning electric microscopy (FE-SEM, Hitachi S-4700).
P.J. Hsieh et al. / Journal of Alloys and Compounds 483 (2009) 40–43
41
Table 2 Fracture toughness of various Mg58 Cu31 Y11−x Gdx (x = 0, 6, 8, 11) BMGs. Gd content (at%)
KIc (MPa m1/2 )
x=0 x=6 x=8 x = 11
2.91 2.92 4.02 2.89
Fig. 1. The XRD patterns of injection-casting Mg58 Cu31 Y11−x Gdx (x = 0, 6, 8, 11) alloy specimens with 3 mm in diameter. Table 1 Tg , Tx, Tl , Trg , and m for the Mg58 Cu31 Y11−x Gdx (x = 0, 6, 8, 11) BMGs. (at%)
Tg (K)
Tx (K)
Tl (K)
Tx
Trg
m
x=0 x=6 x=8 x = 11
424 422 420 422
495 493 491 479
790 747 740 780
71 71 71 57
0.536 0.564 0.567 0.533
0.407 0.421 0.423 0.398
0.716 0.755 0.759 0.687
3. Results and discussions Fig. 1 is the XRD patterns taken from the Mg58 Cu31 Y11−x Gdx (x = 0, 6, 8, 11) cylindrical alloy specimens with 3 mm in diameter. No crystalline phases superimposed on broadening hump were detected in the XRD patterns of the Mg58 Cu31 Y5 Gd6 (x = 6) and the Mg58 Cu31 Y3 Gd8 (x = 8) specimens. It reflects that the partial substituting effect of rare-earth metal Gd on the glass forming ability is contributive Table 1 summaries Tg , Tx and Tl for the Mg58 Cu31 Y11−x Gdx alloys based on the non-isothermal DSC curves (with a heating rate of 20 K/min) shown in Fig. 2. Tl slightly decreased (by 10–50 K) with increasing Gd content, indicating that the liquid phase is stabi-
Fig. 2. Non-isothermal DSC curves for Mg58 Cu31 Y11−x Gdx (x = 0, 6, 8, 11) BMG rods (3 mm in diameter) with a heating rate of 20 K/min.
Fig. 3. Half-circular patterns of shear bands in the vicinity of the deformed crack region for the Mg58 Cu31 Y5 Gd6 BMG.
lized by rare-earth metal Gd, especially for the specimen of the Mg58 Cu31 Y3 Gd8 (x = 8) BMG. The estimated parameters of GFA, Trg , and m , are increased with the decreasing of Tl . Reviewing three GFA parameters included in Table 1, it is found that m is the most sensitive parameter to display the deviation in GFA of these Mg58 Cu31 Y11−x Gdx BMGs. The alloys with 6 and 8 at% Gd content (Mg58 Cu31 Y5 Gd6 , Mg58 Cu31 Y3 Gd8 ) provided the great GFA and the high thermal stability ∼71 K. Broad supercooled liquid region implies that the Mg58 Cu31 Y11−x Gdx (x = 6 and 8) BMG possessed a high resistance to crystallization, leading to the high thermal stability and the high GFA. For further investigation of mechanical properties of the best two BMG former (x = 6, 8), the indentation tests are followed. The fracture toughness (KIc ) determination [11] is applied by a Vickers pyramidal hardness indenter (Akashi, MVK-H11). The Vickers hardness of the Mg58 Cu31 Y11−x Gdx BMGs are in the range of 284–306 Hv . Averaged hardness of the Mg-based BMGs is four times higher than
Fig. 4. The compressive stress–strain curve for the Mg58 Cu31 Y11−x Gdx (x = 0, 6, 8, 11) BMGs.
42
P.J. Hsieh et al. / Journal of Alloys and Compounds 483 (2009) 40–43
Table 3 The mechanical properties of the Mg58 Cu31 Y11−x Gdx (x = 0, 6, 8, 11) BMGs via compression tests at room temperature. Gd content (at%)
Fracture stress f (MPa)
Young’s modulus E (GPa)
Elastic strain εe (%)
x=0 x=6 x=8 x = 11
714 855 899 772
44 44.9 45.2 44.1
1.71 1.98 2.04 1.76
Fig. 7. Striation patterns with different orientations and nano-scaled vein-like pattern upon the fracture surfaces of the compressive Mg58 Cu31 Y3 Gd8 BMG.
Fig. 5. Vein pattern upon the fracture surfaces of the compressive Mg58 Cu31 Y3 Gd8 specimen.
Fig. 6. (a) Mirror-like and (b) river-like patterns upon the fracture surfaces of the compressive Mg58 Cu31 Y11−x Gdx (x = 6 and 8) BMGs.
that of the AZ91 Mg alloys. Meanwhile, an empirical calibration [12,13] is used to estimate the KIc of the BMGs and the results are listed in Table 2. The highest KIc value is ∼4.02 MPa m1/2 for the Mg58 Cu31 Y3 Gd8 alloy, which is higher than that of the base alloys. A few half-circular patterns of shear bands were observed (in Fig. 3) in the vicinity of the deformed crack region, illustrating the plastic deformation of the Mg–Cu–Y–Gd amorphous alloys. Uniaxial compression tests were performed by using material test system (MTS 810) to explore the fracture stress, the elastic strain, and the elastic constant. Fig. 4 reveals the compressive stress–strain curve for the Mg58 Cu31 Y11−x Gdx BMGs (x = 0, 6, 8 and 11), and the mechanical properties were extracted as listed in Table 3. It is concluded that the addition of Gd improves the fracture strength of the base alloys, but no macroscopic plastic deformation occurred in the Mg–Cu–Y(–Gd) BMGs. Meanwhile, the fracture surfaces of the compressive specimens were examined by using SEM. Only a few vein-patterns (Fig. 5) can be observed in the fracture surface of the Mg58 Cu31 Y3 Gd8 BMG. The major morphologies of the fracture surface are mirror-like (Fig. 6(a)) and river-like pattern (Fig. 6(b)), indicating that the dominant fracture behavior is brittle [14,15]. Further examination of the mirror region found that the featureless area is composed of the striation patterns with different orientations indeed (in Fig. 7). Each striation pattern corresponds to a set of individual shear bands. Besides, a
Fig. 8. The typical load-displacement curve for the Mg58 Cu31 Y3 Gd8 BMG.
P.J. Hsieh et al. / Journal of Alloys and Compounds 483 (2009) 40–43
nano-scaled vein-like pattern could be found inside the striation structure. It implies that the plastic deformation can take place in nano-scale. The spacing inside the striation structure is about 50 nm, which is comparable with the serrated flow in the load-displacement curves of nanoindentation measurement in Fig. 8. The serration appeared in the load-displacement curve was caused by the discrete plastic deformation during nanoindentation test. The interrupting of plasticity is associated with the awakening of shear bands. Fig. 8 is the typical load-displacement curve at the loading rate of 1 mN/s for Mg58 Cu31 Y3 Gd8 specimen. Averaged step of the discrete interrupting is near 50 nm as well. 4. Conclusions (i) For the Mg58 Cu31 Y11 alloy, a suitable Gd content in substitution for Y improves the GFA and thermal stability. The best BMG former is alloy with composition Mg58 Cu31 Y5 Gd6 and Mg58 Cu31 Y3 Gd8 in this study. (ii) The fracture toughness, KIc , from the Vickers indentation fracture test is ∼4.02 MPa m1/2 for the Mg58 Cu31 Y3 Gd8 alloy. It exhibits that the fracture resistance is enhanced by the addition of Gd in Mg58 Cu31 Y11 alloy systems. (iii) The dominant morphologies of the fracture surface are mirrorlike and river-like pattern, indicating that the fracture behavior is brittle. However, the featureless mirror region is composed of the striation pattern (shear bans) and the vein-like pattern
43
in nano-scale actually. It implies that the plastic deformation can take place in nano-scale as well. Acknowledgements The authors are gratefully acknowledge the sponsorship by National Science Council of Taiwan (ROC) under the project no. NSC 95-2221-E-214-016-MY3 and the Micro- and Nano-Structure Analysis Lab in I-Shou university. References [1] A. Inoue (Ed.), Bulk Amorphous Alloys, Trans Tech Publication Ltd., Switzerland, 1998. [2] W.L. Johnson, Mater. Sci. Forum 225–227 (1996) 35. [3] A. Brenner, D.E. Couch, E.K. Williams, J. Res. Natl. Bur. Stand. 44 (1950) 109. [4] D. Turnbull, Solid State Phys. 3 (1956) 225. [5] D. Turnbull, J. Phys. 35 (1974) 1. [6] Q. Zheng, H. Ma, J. Xu, Scripta Mater. 55 (2006) 541. [7] X.H. Du, J.C. Huang, C.T. Liu, Z.P. Liu, J. Appl. Phys. 101 (2007) 086108. [8] A. Inoue, A. Kato, T. Zhang, S.G. Kim, T. Masumoto, Mater. Trans., JIM 32 (1991) 609. [9] H. Ma, Q. Zeheng, J. Xu, Y. Li, E. Ma, J. Mater. Res. 20 (2005) 2252. [10] H. Men, W.T. Kim, D.H. Kim, J. Non-Cryst. Solids 337 (2004) 29. [11] G.D. Quinn, R.C. Bradt, J. Am. Ceram. Soc. 90 (2007) 673. [12] B.R. Lawn, T.R. Wilshaw, J. Mater. Sci. 10 (1975) 1049. [13] A.G. Evans, T.R. Wilshaw, Acta Mater. 24 (1979) 939. [14] D.G. Pan, W.Y. Liu, H.F. Zhang, A.M. Wang, Z.Q. Hu, J. Alloys Compd. 438 (2007) 142. [15] D.G. Pan, H.F. Zhang, A.M. Wang, Z.G. Wang, Z.Q. Hu, J. Alloys Compd. 438 (2007) 145.