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Effect of strain rate on compressive behavior of Ti-based bulk metallic glass at room temperature
Journal of Alloys and Compounds 472 (2009) 214–218
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effect of strain rate on compressive behavior of Ti-based bulk metallic glass at room temperature Weifeng Ma, Hongchao Kou, Jinshan Li ∗ , Hui Chang, Lian Zhou State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China
a r t i c l e
i n f o
Article history: Received 27 September 2007 Received in revised form 8 April 2008 Accepted 9 April 2008 Available online 23 May 2008 Keywords: Bulk metallic glasses Shear band Strain rate effect Compressive behavior
a b s t r a c t Compressive deformation behavior of the Ti40 Zr25 Ni8 Cu9 Be18 bulk metallic glass was investigated over a wide strain rate ranging from 10−4 to 103 s−1 at room temperature. Fracture stress was found to increase and fracture strain decrease with increasing applied strain rate, which were due to the decrease of the density of shear bands. Serrated flow was observed at lower strain rate. The appearance of a large number of shear bands was probably associated with flow serration observed during compression. At high strain rates, the rate of shear band nucleation was not sufficient to accommodate the applied strain rate and thus caused an early fracture of the test sample. The positive strain rate dependence of compressive strength might be associated with the different microstructure on the atomic scale in the Ti-based amorphous matrix. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Bulk metallic glasses (BMGs) have attracted much more attention for a potential application as structural materials. Compared to conventional engineering alloys, they show unique properties such as high strength and hardness, large elastic strains (∼2%), high fracture toughness, and excellent wear resistance [1–5]. But their applicability is limited by their near-zero tensile ductility resulting from work-softening and shear localization. Although the mechanical behavior of BMGs has been extensively studied, the precise nature of the deformation mechanisms of metallic glasses remains unclear [6–8]. However, the effects of strain rate in BMGs are dependent on the loading conditions and chemical components of the material itself. BMGs with different chemical components manifest different strain rate effect. Bruck et al. [9] and Lu et al. [10] reported that the fracture strength of a Zr-based BMG was independent of the strain rate in compression. Recently, the positive and negative strain rate sensitivity was found in Zr-based [11,12] and Nd-based [13] BMGs, respectively. These results suggest that the mechanical behavior under different strain rates can be influenced significantly by many factors, such as level of amorphization, loading mode, chemical compositions of BMGs. To date, the limited data is available for the strain rate effect on the mechanical behavior of BMGs. Therefore, more experimental
∗ Corresponding author. Tel.: +86 29 8849 3484; fax: +86 29 8848 0294. E-mail address: [email protected] (J. Li). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.04.043
data are required to fully understand the deformation mechanism of BMGs. In this paper, we investigate the effect of strain rate on the compression strength, plastic strain and fracture behavior of Ti40 Zr25 Ni8 Cu9 Be18 BMG. No data is currently available on dynamic constitutive behavior for this class of BMGs. The experimental results can aid to better understand the deformation nature in metallic glasses over such a wide range of strain rate. 2. Experimental procedures Ingots of Ti40 Zr25 Ni8 Cu9 Be18 were prepared by arc-melting the constituents with a purity ranging from 99.5 to 99.99% under a Ti-gettered argon atmosphere. BMG specimens were prepared by drop casting into a Cu mold and the obtained cylindrical specimens have diameters of 3 and 1 mm. The glassy nature of as-prepared alloy rods was confirmed by X-ray diffraction (XRD). Cylindrical specimens with 1:1 and 2:1 aspect ratio were prepared for dynamic and quasi-static compression. Uniaxial quasi-static compression tests were conducted using an Instron 5567 mechanical instrument at room temperature. Dynamic compression tests were performed using the Split Hopkinson Pressure Bar (SHPB) at strain rates of 102 –103 s−1 . In order to protect the end of the bars and limit the final strain of the specimen, some modifications such as steel inserts and lantern ring were used in dynamic compression tests. A MoS2 lubricant was used to reduce friction between testing samples and the platen of the machine. After compression, the fracture surfaces were investigated using LEO1450 scanning electron microscope (SEM).
3. Results and discussion Fig. 1 presents the compression stress–strain curves at different strain rates for the BMGs with different diameters at room temperature. For each sample, the stress–strain relation is linear up to about 2% elastic strain, followed by a clear yield point and macroscopic plastic strain without strain hardening prior to fracture in the tested
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Fig. 2 shows the SEM images of shear bands on the deformed samples with diameter of 1 mm. Considering that the plastic deformation achieved by BMGs is confined at narrow region near shear bands, these specimens with different plasticities should differ in both densities and shape of the shear bands. Fig. 2(a) shows the side surface of the sample deformed at a strain rate of 10−4 s−1 , dense and multiple shear bands are visible near the fracture surface. Careful observation indicates that the shear bands have a pronounced tendency to branch when they propagate through the specimen, and that some shear band paths are highly curved. In contrast, only sparse and scattered shear bands can be seen on the side surface of the sample deformed at a strain rate of 10−2 s−1 . The results illustrate that the density of shear bands decreases significantly with the increasing strain rates, and the average spacing of primary shear bands increase with the increasing strain rates, thus representing the decreasing plastic strain. However, the decrease in the density of shear bands at high strain rate reported here are completely contrary to observations in previous reports. Sergueeva et al. [16] and Mukai et al. [17] found that multiple parallel shear bands and a rougher fracture surface were observed at high strain rates in tensile tests. The same experimental trends at high strain rates in compressive tests were reported by Nieh et al. [18]. Fig. 3 shows the fracture surface of the specimen with diameter of 3 mm deformed at the strain rates of 10−4 and 4 × 103 s−1 . The fracture surface of samples tests at a strain rate of 10−4 s−1 is uneven
Fig. 1. Compressive stress–strain curves of Ti40 Zr25 Ni8 Cu9 Be18 BMGs at different strain rates with diameters of 1 mm (a) and 3 mm (b).
strain rate range. However, the plastic strain is significantly different for the alloy with different strain rates. With decreasing strain rate, the plastic strain prior to failure increasing monotonically. This indicates that the stain rate plays a crucial role in achieving the plasticity of metallic glass materials. At the same time, serrated flow phenomenon begins after yield point in the stress–strain curve. It is noted that the serrated flow at 10−2 s−1 is markedly different from those at lower strain rates. The yield strength and fracture strength distinctly increases, and the flow serration phenomenon trends vanish at 10−2 s−1 . The disappearance of serrated flow at high strain rates has also been observed in nanoindentation studies by Schuh and Nieh [14] and Xing et al. [15]. The character of serrations is strongly dependent on the loading rate. The slower strain rates promote more conspicuous serrations, and rapid loading suppresses serrated flow. The phenomenon has been assumed to be associated with the initiation and propagation of individual shear bends during quasi-static loading. These results represent a transition from deformation controlled by the discrete operation of individual shear bands at low strain rate, to the simultaneous operation of multiple shear bands at higher rates. A single shear band cannot accommodate the imposed strain rapidly enough at high rates, and consequently multiple shear bands must operate simultaneously. In the present work, the decrease in serrated flow with increasing strain rate is also attributed as outlined above.
Fig. 2. SEM images showing the side view of the fractured samples with diameter of 1 mm deformed at the strain rates of 10−4 s−1 (a) and 10−2 s−1 (b).
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Fig. 3. The fracture surface of the sample with diameter of 3 mm deformed at the strain rates of 10−4 s−1 (a), (b) and 4 × 103 s−1 (c), (d).
(Fig. 3(a)). The uneven surface is caused by a simultaneous operation of multiple shear bands. In comparison, the fracture surface of samples deformed at a strain rate of 4 × 103 s−1 appears to be relatively rough (Fig. 3(c)), there is no clear sign showing the operation of other shear bands. The formation and propagation of one major shear band dominates the fracture of the sample. Fracture surfaces in the vicinity of the sample edge are shown in Fig. 3(b) and (d). In the case of deforming at the quasi-static strain rate, the progressive sliding region is readily seen before the onset of the final fracture (i.e. the vein pattern). The progressive sliding is probably associated with flow serration observed during compression (Fig. 1). For the case of dynamic loading, on the other hand, a short sliding region is observed. Fracture processes are evidently quite different at the two strain rates. The results suggest that flow serration is a strain accommodation or stress relaxation process. Details of fracture surfaces of deformed samples at various strain rates are shown in Fig. 4. The typical veinlike patterns indicate that a local viscous flow occurs during the fracture process. Molten droplets are also visible over the fractured surfaces, which illustrate the occurrence of local melting even at a low strain rate. It is worthy to notice that the vein patterns have a pronounced tendency to narrow with increasing the strain rate along the shear direction (as marked by arrows in the three figures), suggesting a higher nucle-
ation rate of vein patterns at the higher strain rate. The finding is probably associated with a distinctly increase in the fracture stress observed during compression (Fig. 1). The variation of maximum stress as a function of strain rate for Ti40 Zr25 Ni8 Cu9 Be18 is shown in Fig. 5. Also included in the figure are data from the previous reported results. The present experimental result indicates that the maximum stress increases with increasing strain rate, which is in contrast to other experimental observation for Zr-based and Pd-based. This difference demonstrates that BMGs deform by a shear banding mechanism is material-related. As is illustrated in Fig. 5, BMGs with different chemical components manifest different strain rate effect. Compressive strength of bulk Pd40 Ni20 P20 decreases slightly at strain rates below 2.3 × 100 s−1 but decreases rapidly at a higher strain rate [18]. By contrast, Lu et al. [10] and Bruck et al. [9] reported the compressive strength of bulk Vitreloy 1 was essentially independent of the strain rate. In tensile tests, the fracture strength of ribbon specimen (i.e. Zr65 Al10 Ni10 Cu15 [19] in Fig. 5), however, rapidly decreased with the strain rate. The sudden reduction in the strength of the material at strain rates higher than 10−1 s−1 is probably attributed to the fact that the strain of a thin ribbon sample is sensitive to surface defects. Mukai et al. [17] observed that the tensile fracture stress of bulk Pd40 Ni20 P20 was virtually independent of the strain rate.
W. Ma et al. / Journal of Alloys and Compounds 472 (2009) 214–218
217
Fig. 5. Variation of compressive strength as a function of strain rate for four bulk metallic glasses.
the amorphous matrix, such as 1–2 nm scale medium-range ordering or phase separation into nm-scale regions. The shear bands might be forced to be deflected or branched when they reached the region of the energy barriers. It has been observed that nucleation and branching of the shear bands during deformation promotes plasticity [21]. The increase of the total area of shear band leads to macroscopic plasticity at low strain rate. At high strain rate, the nucleation rate of shear bands is not sufficient to accommodate the applied strain rate and thus causes an early fracture of the test sample [22]. The energy barriers embedded in the amorphous matrix play an effective strengthening role and explain why the maximum stress increases at high strain rate. Variations in chemical compositions, microstructure, level of amorphization and sample geometry of the metallic glasses were found in the literature. Because an overall experimental data on inhomogeneous deformation of metallic glass is lacked, it is difficult to make the formulation of a final conclusion on the effects of shear band or fracture behavior on mechanical behaviors of metallic glasses at present. More work is needed in this area to fully understand the nature of deformation of BMGs. 4. Conclusions
Fig. 4. The vein pattern morphology on fracture surface of the sample with diameter of 1 mm deformed at the strain rate of 10−4 s−1 (a), 10−3 s−1 (b) and 10−2 s−1 (c).
The material-related mechanism of shear banding in BMGs may be contributed to the microstructure difference in different materials with different chemical components, e.g. cluster structure. The degree of randomness is fully related to the experimental techniques and the alloy composition. Park et al. [20] have confirmed that there are some evidences for local ordering in the present Ti-based BMG. At the same time, a possible reason for the enhancement for plasticity is the existence of energy barriers embedded in
The compressive deformation behavior of Ti40 Zr25 Ni8 Cu9 Be18 BMG was characterized within the strain rates ranging from 1 × 10−4 to 4 × 103 s−1 . The fracture stress increased and the plastic strain decreased due to the decrease of the density of shear bands. Serrated flow was observed in the stress–strain curves at strain rates of 10−4 and 10−3 s−1 , but diminished at a strain rate of 10−2 s−1 . The large numbers of shear bands were probably associated with flow serration observed during compression. The shear band and flow serrations were a strain accommodation mechanism for deformation. At high strain rate, the nucleation rate of shear bands was not sufficient to accommodate the applied strain rate and thus caused an early fracture of the test sample. The positive strain rate dependence of compressive strength might be associated with the different microstructure on the atomic scale in the Ti-based amorphous matrix. Acknowledgements The author is indebted to Prof. Liqian Xing for valuable suggestions. This research was funded by New Century Excellent Person Supporting Project of Ministry of Education of China and Innovation
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and Technology foundation for Young Scholars of Northwestern Polytechnical University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
A. Inoue, W. Zhang, T. Zhang, K. Kurosaka, Acta Mater. 49 (2001) 2645. W.H. Wang, R.J. Wang, D.Q. Zhao, M.X. Pan, Appl. Phys. Lett. 74 (1999) 1803. W.L. Johnson, MRS Bull. 24 (1999) 42. W.B. Kim, B.J. Ye, S. Yi, Met. Mater. Int. 10 (2004) 1. E.S. Park, D.H. Kim, Met. Mater. Int. 11 (2004) 19. Z.F. Zhang, J. Eckert, L. Schultz, Acta Mater. 51 (2003) 1167. J. Schroers, W.L. Johnson, Phys. Rev. Lett. 93 (2004) 255506. J.J. Lewandowski, W.H. Wang, A.L. Greer, Philos. Mag. Lett. 85 (2005) 77. H.A. Bruck, A.J. Rosakis, W.L. Johnson, J. Mater. Res. 11 (1996) 503. J. Lu, G. Ravichandran, W.L. Johnson, Acta Mater. 51 (2003) 3249.
[11] H. Li, G. Subhash, X.L. Gao, L.J. Kecskes, R.J. Dowding, Scr. Mater. 49 (2003) 1087. [12] T.C. Hufnagel, T. Jiao, Y. Li, L.Q. Xing, K.T. Ramesh, J. Mater. Res. 17 (2002) 1441. [13] L.F. liu, L.H. Dai, Y.L. Bai, B.C. Wei, G.S. Yu, Intermetallics 13 (2005) 827. [14] C.A. Schuh, T.G. Nieh, Acta Mater. 51 (2003) 87. [15] D.M. Xing, T.H. Zhang, W.H. Li, B.C. Wei, J. Alloys Compd. 433 (2007) 318. [16] A.V. Sergueeva, N.A. Mara, D.J. Branagan, A.K. Mukherjee, Scr. Mater. 50 (2004) 1303. [17] T. Mukai, T.G. Nieh, Y. Kawamura, A. Inoue, K. Higashi, Scr. Mater. 46 (2002) 43. [18] T. Mukai, T.G. Nieh, Y. Kawamura, A. Inoue, K. Higashi, Intermetallics 10 (2002) 1071. [19] Y. Kawamura, T. Shibata, A. Inoue, T. Masumoto, Scr. Mater. 37 (1997) 431. [20] J.M. Park, H.J. Chang, K.H. Han, W.T. Kim, D.H. Kim, Scr. Mater. 53 (2005) 1. [21] L.Q. Xing, Y. Li, K.T. Ramesh, J. Li, T.C. Hufnagel, Phys. Rev. Lett. 64 (2001) 180201–180211. [22] L.F. Liu, L.H. Dai, Y.L. Bai, B.C. Wei, G.S. Yu, Intermetallics 13 (2005) 827.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effect of strain rate on compressive behavior of Ti-based bulk metallic glass at room temperature Weifeng Ma, Hongchao Kou, Jinshan Li ∗ , Hui Chang, Lian Zhou State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China
a r t i c l e
i n f o
Article history: Received 27 September 2007 Received in revised form 8 April 2008 Accepted 9 April 2008 Available online 23 May 2008 Keywords: Bulk metallic glasses Shear band Strain rate effect Compressive behavior
a b s t r a c t Compressive deformation behavior of the Ti40 Zr25 Ni8 Cu9 Be18 bulk metallic glass was investigated over a wide strain rate ranging from 10−4 to 103 s−1 at room temperature. Fracture stress was found to increase and fracture strain decrease with increasing applied strain rate, which were due to the decrease of the density of shear bands. Serrated flow was observed at lower strain rate. The appearance of a large number of shear bands was probably associated with flow serration observed during compression. At high strain rates, the rate of shear band nucleation was not sufficient to accommodate the applied strain rate and thus caused an early fracture of the test sample. The positive strain rate dependence of compressive strength might be associated with the different microstructure on the atomic scale in the Ti-based amorphous matrix. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Bulk metallic glasses (BMGs) have attracted much more attention for a potential application as structural materials. Compared to conventional engineering alloys, they show unique properties such as high strength and hardness, large elastic strains (∼2%), high fracture toughness, and excellent wear resistance [1–5]. But their applicability is limited by their near-zero tensile ductility resulting from work-softening and shear localization. Although the mechanical behavior of BMGs has been extensively studied, the precise nature of the deformation mechanisms of metallic glasses remains unclear [6–8]. However, the effects of strain rate in BMGs are dependent on the loading conditions and chemical components of the material itself. BMGs with different chemical components manifest different strain rate effect. Bruck et al. [9] and Lu et al. [10] reported that the fracture strength of a Zr-based BMG was independent of the strain rate in compression. Recently, the positive and negative strain rate sensitivity was found in Zr-based [11,12] and Nd-based [13] BMGs, respectively. These results suggest that the mechanical behavior under different strain rates can be influenced significantly by many factors, such as level of amorphization, loading mode, chemical compositions of BMGs. To date, the limited data is available for the strain rate effect on the mechanical behavior of BMGs. Therefore, more experimental
∗ Corresponding author. Tel.: +86 29 8849 3484; fax: +86 29 8848 0294. E-mail address: [email protected] (J. Li). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.04.043
data are required to fully understand the deformation mechanism of BMGs. In this paper, we investigate the effect of strain rate on the compression strength, plastic strain and fracture behavior of Ti40 Zr25 Ni8 Cu9 Be18 BMG. No data is currently available on dynamic constitutive behavior for this class of BMGs. The experimental results can aid to better understand the deformation nature in metallic glasses over such a wide range of strain rate. 2. Experimental procedures Ingots of Ti40 Zr25 Ni8 Cu9 Be18 were prepared by arc-melting the constituents with a purity ranging from 99.5 to 99.99% under a Ti-gettered argon atmosphere. BMG specimens were prepared by drop casting into a Cu mold and the obtained cylindrical specimens have diameters of 3 and 1 mm. The glassy nature of as-prepared alloy rods was confirmed by X-ray diffraction (XRD). Cylindrical specimens with 1:1 and 2:1 aspect ratio were prepared for dynamic and quasi-static compression. Uniaxial quasi-static compression tests were conducted using an Instron 5567 mechanical instrument at room temperature. Dynamic compression tests were performed using the Split Hopkinson Pressure Bar (SHPB) at strain rates of 102 –103 s−1 . In order to protect the end of the bars and limit the final strain of the specimen, some modifications such as steel inserts and lantern ring were used in dynamic compression tests. A MoS2 lubricant was used to reduce friction between testing samples and the platen of the machine. After compression, the fracture surfaces were investigated using LEO1450 scanning electron microscope (SEM).
3. Results and discussion Fig. 1 presents the compression stress–strain curves at different strain rates for the BMGs with different diameters at room temperature. For each sample, the stress–strain relation is linear up to about 2% elastic strain, followed by a clear yield point and macroscopic plastic strain without strain hardening prior to fracture in the tested
W. Ma et al. / Journal of Alloys and Compounds 472 (2009) 214–218
215
Fig. 2 shows the SEM images of shear bands on the deformed samples with diameter of 1 mm. Considering that the plastic deformation achieved by BMGs is confined at narrow region near shear bands, these specimens with different plasticities should differ in both densities and shape of the shear bands. Fig. 2(a) shows the side surface of the sample deformed at a strain rate of 10−4 s−1 , dense and multiple shear bands are visible near the fracture surface. Careful observation indicates that the shear bands have a pronounced tendency to branch when they propagate through the specimen, and that some shear band paths are highly curved. In contrast, only sparse and scattered shear bands can be seen on the side surface of the sample deformed at a strain rate of 10−2 s−1 . The results illustrate that the density of shear bands decreases significantly with the increasing strain rates, and the average spacing of primary shear bands increase with the increasing strain rates, thus representing the decreasing plastic strain. However, the decrease in the density of shear bands at high strain rate reported here are completely contrary to observations in previous reports. Sergueeva et al. [16] and Mukai et al. [17] found that multiple parallel shear bands and a rougher fracture surface were observed at high strain rates in tensile tests. The same experimental trends at high strain rates in compressive tests were reported by Nieh et al. [18]. Fig. 3 shows the fracture surface of the specimen with diameter of 3 mm deformed at the strain rates of 10−4 and 4 × 103 s−1 . The fracture surface of samples tests at a strain rate of 10−4 s−1 is uneven
Fig. 1. Compressive stress–strain curves of Ti40 Zr25 Ni8 Cu9 Be18 BMGs at different strain rates with diameters of 1 mm (a) and 3 mm (b).
strain rate range. However, the plastic strain is significantly different for the alloy with different strain rates. With decreasing strain rate, the plastic strain prior to failure increasing monotonically. This indicates that the stain rate plays a crucial role in achieving the plasticity of metallic glass materials. At the same time, serrated flow phenomenon begins after yield point in the stress–strain curve. It is noted that the serrated flow at 10−2 s−1 is markedly different from those at lower strain rates. The yield strength and fracture strength distinctly increases, and the flow serration phenomenon trends vanish at 10−2 s−1 . The disappearance of serrated flow at high strain rates has also been observed in nanoindentation studies by Schuh and Nieh [14] and Xing et al. [15]. The character of serrations is strongly dependent on the loading rate. The slower strain rates promote more conspicuous serrations, and rapid loading suppresses serrated flow. The phenomenon has been assumed to be associated with the initiation and propagation of individual shear bends during quasi-static loading. These results represent a transition from deformation controlled by the discrete operation of individual shear bands at low strain rate, to the simultaneous operation of multiple shear bands at higher rates. A single shear band cannot accommodate the imposed strain rapidly enough at high rates, and consequently multiple shear bands must operate simultaneously. In the present work, the decrease in serrated flow with increasing strain rate is also attributed as outlined above.
Fig. 2. SEM images showing the side view of the fractured samples with diameter of 1 mm deformed at the strain rates of 10−4 s−1 (a) and 10−2 s−1 (b).
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Fig. 3. The fracture surface of the sample with diameter of 3 mm deformed at the strain rates of 10−4 s−1 (a), (b) and 4 × 103 s−1 (c), (d).
(Fig. 3(a)). The uneven surface is caused by a simultaneous operation of multiple shear bands. In comparison, the fracture surface of samples deformed at a strain rate of 4 × 103 s−1 appears to be relatively rough (Fig. 3(c)), there is no clear sign showing the operation of other shear bands. The formation and propagation of one major shear band dominates the fracture of the sample. Fracture surfaces in the vicinity of the sample edge are shown in Fig. 3(b) and (d). In the case of deforming at the quasi-static strain rate, the progressive sliding region is readily seen before the onset of the final fracture (i.e. the vein pattern). The progressive sliding is probably associated with flow serration observed during compression (Fig. 1). For the case of dynamic loading, on the other hand, a short sliding region is observed. Fracture processes are evidently quite different at the two strain rates. The results suggest that flow serration is a strain accommodation or stress relaxation process. Details of fracture surfaces of deformed samples at various strain rates are shown in Fig. 4. The typical veinlike patterns indicate that a local viscous flow occurs during the fracture process. Molten droplets are also visible over the fractured surfaces, which illustrate the occurrence of local melting even at a low strain rate. It is worthy to notice that the vein patterns have a pronounced tendency to narrow with increasing the strain rate along the shear direction (as marked by arrows in the three figures), suggesting a higher nucle-
ation rate of vein patterns at the higher strain rate. The finding is probably associated with a distinctly increase in the fracture stress observed during compression (Fig. 1). The variation of maximum stress as a function of strain rate for Ti40 Zr25 Ni8 Cu9 Be18 is shown in Fig. 5. Also included in the figure are data from the previous reported results. The present experimental result indicates that the maximum stress increases with increasing strain rate, which is in contrast to other experimental observation for Zr-based and Pd-based. This difference demonstrates that BMGs deform by a shear banding mechanism is material-related. As is illustrated in Fig. 5, BMGs with different chemical components manifest different strain rate effect. Compressive strength of bulk Pd40 Ni20 P20 decreases slightly at strain rates below 2.3 × 100 s−1 but decreases rapidly at a higher strain rate [18]. By contrast, Lu et al. [10] and Bruck et al. [9] reported the compressive strength of bulk Vitreloy 1 was essentially independent of the strain rate. In tensile tests, the fracture strength of ribbon specimen (i.e. Zr65 Al10 Ni10 Cu15 [19] in Fig. 5), however, rapidly decreased with the strain rate. The sudden reduction in the strength of the material at strain rates higher than 10−1 s−1 is probably attributed to the fact that the strain of a thin ribbon sample is sensitive to surface defects. Mukai et al. [17] observed that the tensile fracture stress of bulk Pd40 Ni20 P20 was virtually independent of the strain rate.
W. Ma et al. / Journal of Alloys and Compounds 472 (2009) 214–218
217
Fig. 5. Variation of compressive strength as a function of strain rate for four bulk metallic glasses.
the amorphous matrix, such as 1–2 nm scale medium-range ordering or phase separation into nm-scale regions. The shear bands might be forced to be deflected or branched when they reached the region of the energy barriers. It has been observed that nucleation and branching of the shear bands during deformation promotes plasticity [21]. The increase of the total area of shear band leads to macroscopic plasticity at low strain rate. At high strain rate, the nucleation rate of shear bands is not sufficient to accommodate the applied strain rate and thus causes an early fracture of the test sample [22]. The energy barriers embedded in the amorphous matrix play an effective strengthening role and explain why the maximum stress increases at high strain rate. Variations in chemical compositions, microstructure, level of amorphization and sample geometry of the metallic glasses were found in the literature. Because an overall experimental data on inhomogeneous deformation of metallic glass is lacked, it is difficult to make the formulation of a final conclusion on the effects of shear band or fracture behavior on mechanical behaviors of metallic glasses at present. More work is needed in this area to fully understand the nature of deformation of BMGs. 4. Conclusions
Fig. 4. The vein pattern morphology on fracture surface of the sample with diameter of 1 mm deformed at the strain rate of 10−4 s−1 (a), 10−3 s−1 (b) and 10−2 s−1 (c).
The material-related mechanism of shear banding in BMGs may be contributed to the microstructure difference in different materials with different chemical components, e.g. cluster structure. The degree of randomness is fully related to the experimental techniques and the alloy composition. Park et al. [20] have confirmed that there are some evidences for local ordering in the present Ti-based BMG. At the same time, a possible reason for the enhancement for plasticity is the existence of energy barriers embedded in
The compressive deformation behavior of Ti40 Zr25 Ni8 Cu9 Be18 BMG was characterized within the strain rates ranging from 1 × 10−4 to 4 × 103 s−1 . The fracture stress increased and the plastic strain decreased due to the decrease of the density of shear bands. Serrated flow was observed in the stress–strain curves at strain rates of 10−4 and 10−3 s−1 , but diminished at a strain rate of 10−2 s−1 . The large numbers of shear bands were probably associated with flow serration observed during compression. The shear band and flow serrations were a strain accommodation mechanism for deformation. At high strain rate, the nucleation rate of shear bands was not sufficient to accommodate the applied strain rate and thus caused an early fracture of the test sample. The positive strain rate dependence of compressive strength might be associated with the different microstructure on the atomic scale in the Ti-based amorphous matrix. Acknowledgements The author is indebted to Prof. Liqian Xing for valuable suggestions. This research was funded by New Century Excellent Person Supporting Project of Ministry of Education of China and Innovation
218
W. Ma et al. / Journal of Alloys and Compounds 472 (2009) 214–218
and Technology foundation for Young Scholars of Northwestern Polytechnical University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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