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Super-high compressive plastic deformation behaviors of Zr-based metallic glass at room temperature
Materials Science and Engineering A 541 (2012) 199–203
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Super-high compressive plastic deformation behaviors of Zr-based metallic glass at room temperature F.F. Wu a,b , W. Zheng a , J.W. Deng c , D.D. Qu a , J. Shen a,∗ a b c
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China School of Materials Science and Engineering, Liaoning University of Technology, Jinzhou 121001, China Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
a r t i c l e
i n f o
Article history: Received 22 July 2011 Received in revised form 19 December 2011 Accepted 7 February 2012 Available online 14 February 2012 Keywords: Metallic glass Free volume Plastic deformation Shear transformation zone
a b s t r a c t The plastic deformation behavior of a Zr-based metallic glass with super-high compressive ductility was investigated. It was found that large amount of excess free volume would induce large shear transformation zone (STZ), which could stimulate multiple primary shear bands and stabilize their propagation. However, the sample with small STZ showed unstable single shear banding, and an obvious stress valley was found in the true stress–strain curve. This finding is important for understanding the plastic deformation behaviors of various metallic glasses. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Bulk metallic glasses have been paid extensive attention in the past decades due to their novel physical properties, such as high strength, high hardness, and excellent corrosion resistance [1,2]. However, as a potential structural material, their poor ductility originating from the shear unstability in highly localized region is a fatal drawback for metallic glasses: usually, under uniaxial tension, the global plastic deformation of monolithic metallic glass is almost zero; under uniaxial compression, most monolithic metallic glasses only display very limited plasticity (smaller than 2%) [3,4]. Therefore, the reinforcement and ductilization of metallic glass have been a hot topic in the recent years [5–15]. Recently, a super-high compressive plastic deformation behavior was found in some bulk metallic glasses, such as Zr64.13 Cu15.75 Ni10.12 Al10 [10], Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 [4,16], Pd81 Si19 [14], and Ti40 Zr25 Cu12 Ni3 Be20 [17]. Therefore, it is necessary to reconsider the mechanisms and factors governing the brittleness and ductility of metallic glasses. Here in the present letter, we investigated the plastic deformation of a Zr-based metallic glass with a super-high compressive ductility. It was found that, the cooling rate during solidification can subtly influence the structure (i.e., the amount of free volume and thus, the size of
∗ Corresponding author. Tel.: +86 451 86403196; fax: +86 451 86403196. E-mail address: [email protected] (J. Shen). 0921-5093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2012.02.025
shear transformation zone (STZ)) of metallic glasses, leading to significantly different super-high plastic deformation behaviors in metallic glasses with identical chemical composition. 2. Experimental methods Ingots with a composition of Zr50.7 Cu28 Ni9 Al12.3 were prepared by arc melting a mixture of pure elements on a water-cooled copper plate [18]. The final ingots had the shape of rectangular plates with dimensions of 50 mm × 20 mm × 2.0 mm (sample I) for small cooling rate and 20 mm × 5 mm × 1.0 mm (sample II) for large cooling rate. The thermal analysis was performed using differential scanning calorimetry (DSC) with a heat rate of 20 K/s in a flow of purified argon gas. The microstructure and the phase of the prepared ingots were characterized by X-ray diffraction using a Rigaku diffractometer with Cu K␣ radiation. The final ingots showed only broad diffraction maxima and no peaks of crystalline phases were detected, revealing the fully amorphous structure of the samples. High-energy X-ray diffraction at Hasylab BW5 beamline (DESY Hamburg, Germany) and a Philips F30 High-resolution transmission electron microscope (HRTEM) with 300 kV were applied to analyze the fine structure of both samples. The high-energy X-ray diffraction details were same as that documented in Ref. [22]. The HTEM samples were prepared by chemical jet using a solution of 10% perchloric acid in methanol at a temperature 233 K. The compressive samples with an aspect ratio (height divided by diameter or width) of 2 and the final dimension of 1.0 mm × 1.0 mm × 2.0 mm
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Fig. 1. Engineering (a) and true (b) stress–strain curves for the samples I with a lower cooling rate. Engineering (c) and true (d) stress–strain curves for the samples II with a higher cooling rate. The insets in (b) and (d) are the enlarged true stress–strain curves at the initial plastic deformation region (εp ≤ 2%), showing zero strain-hardening (d/dε = 0) for the sample Ia and an obvious strain-hardening (d/dε = 2595 MPa) for the sample IIa, respectively.
Fig. 2. SEM images showing the lateral surface feature of sample Ia (a–c) and sample IIa (d–f) after plastic deformation. (a) Single primary shear band formed in the sample Ia with plastic strain of about 25%. (b and c) SEM images showing that the sample Ia was finally compressed into a disc. (d) Multiple primary shear bands formed in the sample IIa with plastic strain of about 25%. (e and f) SEM images showing that the sample IIa was finally compressed into a disc.
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Fig. 3. HRTEM images showing the microstructure of sample Ia (a) and sample IIa (b) after compressed into discs at strain rate of 2 × 10−4 s−1 . Diffraction patterns of the undeformed samples I (c) and II(d) recorded by using high energy X-ray.
were obtained from samples I and II, respectively. All laterial surfaces of compression samples were finally polished by 1.5 m diamond paste. Uniaxial compression tests were performed with an MTS810 testing machine at room temperature using constant strain rates ranging from 1 × 10−4 s−1 to 1 × 10−1 s−1 .
3. Results Fig. 1 shows the engineering and true stress–strain curves for the samples I and II with different strain rates. For the samples I, extensive compressive plastic deformation occurred in all tested strain rates from 2 × 10−4 s−1 to 3.75 × 10−3 s−1 , as shown in Fig. 1a. Close examination also showed there are some stress valleys at the strain of about 25%, as indicated by the arrow in Fig. 1a. The corresponding true stress–strain curves showed an obvious strain softening in the subsequent plastic deformation, as shown in Fig. 1b. And also no indication of strain hardening (d/dε = 0) in the initial plastic deformation region of εp < 2% can be seen in the stress–strain curves (the inset of Fig. 1b). Extensive compressive plastic deformation also occurred in all samples II, as shown in Fig. 1c. However, no stress valley was observed in the stress–strain curves of these samples. And the stress started to rise quickly when the plastic strain is close to 25%. The true stress–strain curves of the samples II confirm this strain-hardening-like phenomenon (see Fig. 1d). It is obvious that there is a weak strain hardening in the initial plastic deformation region εp < 2% (as seen in the inset of Fig. 1d), though showing a very slight strain softening in the subsequent plastic deformation. It is clear that the plastic deformation of the samples II is more stable than that of the samples I.
Fig. 2a shows the SEM image of one of the samples I (hereafter referred to as sample Ia) deformed with plastic strain of about 25% at strain rate of 2 × 10−4 s−1 . It was found that there was only one primary shear band, as indicated by the arrow in Fig. 2a. And a crack has formed in the primary shear band, corresponding to the stress valley in Fig. 1a. Besides the primary shear band, few second shear bands were formed in the sample Ia. The sample Ia was finally compressed into a disc (Fig. 2b), and squama-like morphology was formed on the lateral surface with profuse shear bands cutting through the sample, as shown in Fig. 2c. Similar morphology was also found in other samples I. However, as to one of the samples II deformed with plastic strain of about 25% at strain rate of 2 × 10−4 s−1 (hereafter referred to as sample IIa), multiple primary shear bands were formed, as indicated by arrows in Fig. 2d, and the crack in these primary shear bands was significantly slighter than that in Fig. 2a, which was consistent with the smooth stress–strain curves without stress valley in Fig. 1c. Finally, the sample IIa was also compressed into a disc (Fig. 2e), and profuse shear bands can be observed on its lateral surface of the sample IIa. Other samples II also showed identical deformation morphology. Fig. 3a and b shows the HRTEM images of the samples Ia and IIa after the super-high plastic deformation, respectively. It is shown that the structures of both samples after plastic deformation are still amorphous, and no nanocrystallization induced by plastic deformation can be detected.
4. Discussion From the experimental results above, it is noted that both samples Ia and IIa can display super-high compressive plasticity.
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Fig. 4. Pair-distribution function [G(r)] determined by high energy X-ray diffraction. The insets show the enlarged difference between the undeformed sample I with a lower cooling rate and the undeformed sample II with a higher cooling rate.
However, both of samples are chemically identical and fully amorphous in structure (Fig. 3c and d), therefore, it is interesting to know, why did the samples I exhibit unstable deformation behavior with only one single primary shear band, while the samples IIa behave in stable deformation with multiple primary shear bands? During cooling of a glass-forming material from the liquid state, some excess free volume as a structural defect will be frozen into the glassy state, and the quantity of excess free volume is determined by the cooling rate [8]. A faster cooling rate means that the mobility of the atoms are more limited, thereby a more disordered packing atomic structure forms during cooling from the melting, and the obtained glassy sample possesses more free volume [8]. The cooling rate for the sample II with thickness of 1 mm is higher than that of the sample I with thickness of 2 mm. Therefore, the amount of free volume in the samples II should be larger than that of the samples I. Moreover, by analyzing the pair-distribution function obtained by the High-energy X-ray diffraction, it is also found that the strong correlation between atoms in the first coordination shell in the sample IIa is a little weaker than that of the sample Ia, as shown in Fig. 4. Similar relation was also found in the second and third coordination shells. It means that the average coordination number of the sample IIa is relatively smaller than that of the sample Ia. And the small coordination number can to some extent reflect that the amount of excess free volume in the samples II is larger than that in the samples I [19]. Therefore, the packing density of the samples II with a fast cooling rate is smaller than that of samples I. In other words, the atom cluster in the samples II is looser than the samples I, showing a more disorder structure in the samples II. Furthermore, the pair-distribution function [G(r)] at larger rvalues representing the medium-range order (MRO) of metallic glasses can be well approximated by a damped oscillation [20,21]. G(r) =
A rˇ
r
exp −
sin
2r D
+ +1
disordered in the medium range. Therefore, this shows that the amount of excess free volume of the samples II should be larger than the samples I [19,22]. The plastic deformation of metallic glasses is commonly interpreted in terms of flow defects, i.e., free volume. However, the plastic deformation should be described as the local redistribution of atom cluster, but not just the cooperation of single atoms. Therefore, the shear transformation zone (STZ) was more suitable than free volume as the basic unit for plastic deformation in metallic glass [23,24]. It was found that the STZ size could significantly influence the plastic deformation of metallic glass [5,25] because the STZ is closely related to the amount of free volume in metallic glass, i.e., larger amount free volume should lead to larger STZ [25]. Therefore, the STZ in the samples II should be larger than that in the sample I. Even a tiny change in STZ could induce a dramatic effect on the flow behavior [25]. The areas with large STZ in the samples II are expected to have lower strength and are susceptive to the shear stress and, accordingly, can be considered as preexisting weakened regions. The differences in the elastic properties between these regions and the surroundings may introduce stress concentrations during plastic deformation, favoring the initial nucleation of shear bands. Furthermore, a large STZ volume in the samples II, when compared with a small one in the samples I, enables a less number of STZs to be activated for the nucleation of a shear band. Moreover, the large-size flow units in plastic deformation can produce large internal concentration of the applied stress where thermally activated flow becomes easy. Thus, large STZ in the samples II can reinforce the shear capability of metallic glass and promote the formation of multiple shear bands [25]. Moreover, for larger STZ, there is more space for its densification. As the stress increases, the STZ will be compressed into denser, and the densified regions will be stronger, where larger stress is needed to activate further plastic deformation. Therefore, new shear bands will form in other weaker regions. This is consistent with the initial work-hardening in Fig. 1d and the observed multiple shear bands in Fig. 2d. Moreover, it is also proposed that there exists mechanical heterogeneity (soft and hard zones) in metallic glass [10,26,27]. The soft zones can be considered as the regions with more excess free volume, which is a potential home for STZ; and the hard zone is the region with less excess free volume, which is not prone to the formation of STZ. It was found that the metallic glass with a high mechanical heterogeneity was more ductile [26]. Usually, for most metallic glasses, the volume fraction of the soft regions is very small and the soft regions are isolated, it is difficult for the single shear band to interact with each other to form multiple primary shear bands, leading to severe plastic deformation in single shear band and catastrophic failure in these brittle metallic glasses. Here, the soft regions in the sample II are more likely to link themselves to form a continuous network, the soft regions enable multiple shear bands to concurrently initiate at different regions to prevent few primary shear bands from carrying severe plastic deformation and inducing catastrophic fracture [26].
5. Summary (1)
where A is a normalization constant of 6.2, ˇ is an exponent of 0.69, is a decay exponent, D is the oscillation frequency, and is the phase shift constant. The parameters D and were deter˚ The parameter mined by a fit of Eq. (1) to the G(r) for r ≥ 6.3 A. in Eq. (1) is a measurement of the correlation length describing to what extent a correlation between a central atom and its surrounding atoms exists. The fit results show the decay exponents are 3.053 A˚ and 3.025 A˚ for the undeformed samples I and II, respectively. It implies that the atomic structure of the samples II is more
Different compressive plastic deformation behaviors were found in chemically identical Zr-based metallic glass samples with different cast cooling rates. The structure of atom cluster is looser in metallic glass with a faster cooling rate during solidification, displaying smaller coordination number and larger amount of excess free volume. It is suggested the large amount of free volume will increase the size of shear transformation zone (STZ), which can significantly promote the formation of multiple primary shear bands and stabilize the shear plastic deformation in metallic glass. These findings are helpful for us to understand the plastic deformation
F.F. Wu et al. / Materials Science and Engineering A 541 (2012) 199–203
behaviors of metallic glasses and further to design new metallic glasses with better mechanical performance properties. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (NSFC) under Grant nos. 50771040, 10732010, 50901038, 51025415, and 50931005. References [1] [2] [3] [4] [5]
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Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Super-high compressive plastic deformation behaviors of Zr-based metallic glass at room temperature F.F. Wu a,b , W. Zheng a , J.W. Deng c , D.D. Qu a , J. Shen a,∗ a b c
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China School of Materials Science and Engineering, Liaoning University of Technology, Jinzhou 121001, China Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
a r t i c l e
i n f o
Article history: Received 22 July 2011 Received in revised form 19 December 2011 Accepted 7 February 2012 Available online 14 February 2012 Keywords: Metallic glass Free volume Plastic deformation Shear transformation zone
a b s t r a c t The plastic deformation behavior of a Zr-based metallic glass with super-high compressive ductility was investigated. It was found that large amount of excess free volume would induce large shear transformation zone (STZ), which could stimulate multiple primary shear bands and stabilize their propagation. However, the sample with small STZ showed unstable single shear banding, and an obvious stress valley was found in the true stress–strain curve. This finding is important for understanding the plastic deformation behaviors of various metallic glasses. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Bulk metallic glasses have been paid extensive attention in the past decades due to their novel physical properties, such as high strength, high hardness, and excellent corrosion resistance [1,2]. However, as a potential structural material, their poor ductility originating from the shear unstability in highly localized region is a fatal drawback for metallic glasses: usually, under uniaxial tension, the global plastic deformation of monolithic metallic glass is almost zero; under uniaxial compression, most monolithic metallic glasses only display very limited plasticity (smaller than 2%) [3,4]. Therefore, the reinforcement and ductilization of metallic glass have been a hot topic in the recent years [5–15]. Recently, a super-high compressive plastic deformation behavior was found in some bulk metallic glasses, such as Zr64.13 Cu15.75 Ni10.12 Al10 [10], Zr52.5 Cu17.9 Ni14.6 Al10 Ti5 [4,16], Pd81 Si19 [14], and Ti40 Zr25 Cu12 Ni3 Be20 [17]. Therefore, it is necessary to reconsider the mechanisms and factors governing the brittleness and ductility of metallic glasses. Here in the present letter, we investigated the plastic deformation of a Zr-based metallic glass with a super-high compressive ductility. It was found that, the cooling rate during solidification can subtly influence the structure (i.e., the amount of free volume and thus, the size of
∗ Corresponding author. Tel.: +86 451 86403196; fax: +86 451 86403196. E-mail address: [email protected] (J. Shen). 0921-5093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2012.02.025
shear transformation zone (STZ)) of metallic glasses, leading to significantly different super-high plastic deformation behaviors in metallic glasses with identical chemical composition. 2. Experimental methods Ingots with a composition of Zr50.7 Cu28 Ni9 Al12.3 were prepared by arc melting a mixture of pure elements on a water-cooled copper plate [18]. The final ingots had the shape of rectangular plates with dimensions of 50 mm × 20 mm × 2.0 mm (sample I) for small cooling rate and 20 mm × 5 mm × 1.0 mm (sample II) for large cooling rate. The thermal analysis was performed using differential scanning calorimetry (DSC) with a heat rate of 20 K/s in a flow of purified argon gas. The microstructure and the phase of the prepared ingots were characterized by X-ray diffraction using a Rigaku diffractometer with Cu K␣ radiation. The final ingots showed only broad diffraction maxima and no peaks of crystalline phases were detected, revealing the fully amorphous structure of the samples. High-energy X-ray diffraction at Hasylab BW5 beamline (DESY Hamburg, Germany) and a Philips F30 High-resolution transmission electron microscope (HRTEM) with 300 kV were applied to analyze the fine structure of both samples. The high-energy X-ray diffraction details were same as that documented in Ref. [22]. The HTEM samples were prepared by chemical jet using a solution of 10% perchloric acid in methanol at a temperature 233 K. The compressive samples with an aspect ratio (height divided by diameter or width) of 2 and the final dimension of 1.0 mm × 1.0 mm × 2.0 mm
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F.F. Wu et al. / Materials Science and Engineering A 541 (2012) 199–203
Fig. 1. Engineering (a) and true (b) stress–strain curves for the samples I with a lower cooling rate. Engineering (c) and true (d) stress–strain curves for the samples II with a higher cooling rate. The insets in (b) and (d) are the enlarged true stress–strain curves at the initial plastic deformation region (εp ≤ 2%), showing zero strain-hardening (d/dε = 0) for the sample Ia and an obvious strain-hardening (d/dε = 2595 MPa) for the sample IIa, respectively.
Fig. 2. SEM images showing the lateral surface feature of sample Ia (a–c) and sample IIa (d–f) after plastic deformation. (a) Single primary shear band formed in the sample Ia with plastic strain of about 25%. (b and c) SEM images showing that the sample Ia was finally compressed into a disc. (d) Multiple primary shear bands formed in the sample IIa with plastic strain of about 25%. (e and f) SEM images showing that the sample IIa was finally compressed into a disc.
F.F. Wu et al. / Materials Science and Engineering A 541 (2012) 199–203
201
Fig. 3. HRTEM images showing the microstructure of sample Ia (a) and sample IIa (b) after compressed into discs at strain rate of 2 × 10−4 s−1 . Diffraction patterns of the undeformed samples I (c) and II(d) recorded by using high energy X-ray.
were obtained from samples I and II, respectively. All laterial surfaces of compression samples were finally polished by 1.5 m diamond paste. Uniaxial compression tests were performed with an MTS810 testing machine at room temperature using constant strain rates ranging from 1 × 10−4 s−1 to 1 × 10−1 s−1 .
3. Results Fig. 1 shows the engineering and true stress–strain curves for the samples I and II with different strain rates. For the samples I, extensive compressive plastic deformation occurred in all tested strain rates from 2 × 10−4 s−1 to 3.75 × 10−3 s−1 , as shown in Fig. 1a. Close examination also showed there are some stress valleys at the strain of about 25%, as indicated by the arrow in Fig. 1a. The corresponding true stress–strain curves showed an obvious strain softening in the subsequent plastic deformation, as shown in Fig. 1b. And also no indication of strain hardening (d/dε = 0) in the initial plastic deformation region of εp < 2% can be seen in the stress–strain curves (the inset of Fig. 1b). Extensive compressive plastic deformation also occurred in all samples II, as shown in Fig. 1c. However, no stress valley was observed in the stress–strain curves of these samples. And the stress started to rise quickly when the plastic strain is close to 25%. The true stress–strain curves of the samples II confirm this strain-hardening-like phenomenon (see Fig. 1d). It is obvious that there is a weak strain hardening in the initial plastic deformation region εp < 2% (as seen in the inset of Fig. 1d), though showing a very slight strain softening in the subsequent plastic deformation. It is clear that the plastic deformation of the samples II is more stable than that of the samples I.
Fig. 2a shows the SEM image of one of the samples I (hereafter referred to as sample Ia) deformed with plastic strain of about 25% at strain rate of 2 × 10−4 s−1 . It was found that there was only one primary shear band, as indicated by the arrow in Fig. 2a. And a crack has formed in the primary shear band, corresponding to the stress valley in Fig. 1a. Besides the primary shear band, few second shear bands were formed in the sample Ia. The sample Ia was finally compressed into a disc (Fig. 2b), and squama-like morphology was formed on the lateral surface with profuse shear bands cutting through the sample, as shown in Fig. 2c. Similar morphology was also found in other samples I. However, as to one of the samples II deformed with plastic strain of about 25% at strain rate of 2 × 10−4 s−1 (hereafter referred to as sample IIa), multiple primary shear bands were formed, as indicated by arrows in Fig. 2d, and the crack in these primary shear bands was significantly slighter than that in Fig. 2a, which was consistent with the smooth stress–strain curves without stress valley in Fig. 1c. Finally, the sample IIa was also compressed into a disc (Fig. 2e), and profuse shear bands can be observed on its lateral surface of the sample IIa. Other samples II also showed identical deformation morphology. Fig. 3a and b shows the HRTEM images of the samples Ia and IIa after the super-high plastic deformation, respectively. It is shown that the structures of both samples after plastic deformation are still amorphous, and no nanocrystallization induced by plastic deformation can be detected.
4. Discussion From the experimental results above, it is noted that both samples Ia and IIa can display super-high compressive plasticity.
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Fig. 4. Pair-distribution function [G(r)] determined by high energy X-ray diffraction. The insets show the enlarged difference between the undeformed sample I with a lower cooling rate and the undeformed sample II with a higher cooling rate.
However, both of samples are chemically identical and fully amorphous in structure (Fig. 3c and d), therefore, it is interesting to know, why did the samples I exhibit unstable deformation behavior with only one single primary shear band, while the samples IIa behave in stable deformation with multiple primary shear bands? During cooling of a glass-forming material from the liquid state, some excess free volume as a structural defect will be frozen into the glassy state, and the quantity of excess free volume is determined by the cooling rate [8]. A faster cooling rate means that the mobility of the atoms are more limited, thereby a more disordered packing atomic structure forms during cooling from the melting, and the obtained glassy sample possesses more free volume [8]. The cooling rate for the sample II with thickness of 1 mm is higher than that of the sample I with thickness of 2 mm. Therefore, the amount of free volume in the samples II should be larger than that of the samples I. Moreover, by analyzing the pair-distribution function obtained by the High-energy X-ray diffraction, it is also found that the strong correlation between atoms in the first coordination shell in the sample IIa is a little weaker than that of the sample Ia, as shown in Fig. 4. Similar relation was also found in the second and third coordination shells. It means that the average coordination number of the sample IIa is relatively smaller than that of the sample Ia. And the small coordination number can to some extent reflect that the amount of excess free volume in the samples II is larger than that in the samples I [19]. Therefore, the packing density of the samples II with a fast cooling rate is smaller than that of samples I. In other words, the atom cluster in the samples II is looser than the samples I, showing a more disorder structure in the samples II. Furthermore, the pair-distribution function [G(r)] at larger rvalues representing the medium-range order (MRO) of metallic glasses can be well approximated by a damped oscillation [20,21]. G(r) =
A rˇ
r
exp −
sin
2r D
+ +1
disordered in the medium range. Therefore, this shows that the amount of excess free volume of the samples II should be larger than the samples I [19,22]. The plastic deformation of metallic glasses is commonly interpreted in terms of flow defects, i.e., free volume. However, the plastic deformation should be described as the local redistribution of atom cluster, but not just the cooperation of single atoms. Therefore, the shear transformation zone (STZ) was more suitable than free volume as the basic unit for plastic deformation in metallic glass [23,24]. It was found that the STZ size could significantly influence the plastic deformation of metallic glass [5,25] because the STZ is closely related to the amount of free volume in metallic glass, i.e., larger amount free volume should lead to larger STZ [25]. Therefore, the STZ in the samples II should be larger than that in the sample I. Even a tiny change in STZ could induce a dramatic effect on the flow behavior [25]. The areas with large STZ in the samples II are expected to have lower strength and are susceptive to the shear stress and, accordingly, can be considered as preexisting weakened regions. The differences in the elastic properties between these regions and the surroundings may introduce stress concentrations during plastic deformation, favoring the initial nucleation of shear bands. Furthermore, a large STZ volume in the samples II, when compared with a small one in the samples I, enables a less number of STZs to be activated for the nucleation of a shear band. Moreover, the large-size flow units in plastic deformation can produce large internal concentration of the applied stress where thermally activated flow becomes easy. Thus, large STZ in the samples II can reinforce the shear capability of metallic glass and promote the formation of multiple shear bands [25]. Moreover, for larger STZ, there is more space for its densification. As the stress increases, the STZ will be compressed into denser, and the densified regions will be stronger, where larger stress is needed to activate further plastic deformation. Therefore, new shear bands will form in other weaker regions. This is consistent with the initial work-hardening in Fig. 1d and the observed multiple shear bands in Fig. 2d. Moreover, it is also proposed that there exists mechanical heterogeneity (soft and hard zones) in metallic glass [10,26,27]. The soft zones can be considered as the regions with more excess free volume, which is a potential home for STZ; and the hard zone is the region with less excess free volume, which is not prone to the formation of STZ. It was found that the metallic glass with a high mechanical heterogeneity was more ductile [26]. Usually, for most metallic glasses, the volume fraction of the soft regions is very small and the soft regions are isolated, it is difficult for the single shear band to interact with each other to form multiple primary shear bands, leading to severe plastic deformation in single shear band and catastrophic failure in these brittle metallic glasses. Here, the soft regions in the sample II are more likely to link themselves to form a continuous network, the soft regions enable multiple shear bands to concurrently initiate at different regions to prevent few primary shear bands from carrying severe plastic deformation and inducing catastrophic fracture [26].
5. Summary (1)
where A is a normalization constant of 6.2, ˇ is an exponent of 0.69, is a decay exponent, D is the oscillation frequency, and is the phase shift constant. The parameters D and were deter˚ The parameter mined by a fit of Eq. (1) to the G(r) for r ≥ 6.3 A. in Eq. (1) is a measurement of the correlation length describing to what extent a correlation between a central atom and its surrounding atoms exists. The fit results show the decay exponents are 3.053 A˚ and 3.025 A˚ for the undeformed samples I and II, respectively. It implies that the atomic structure of the samples II is more
Different compressive plastic deformation behaviors were found in chemically identical Zr-based metallic glass samples with different cast cooling rates. The structure of atom cluster is looser in metallic glass with a faster cooling rate during solidification, displaying smaller coordination number and larger amount of excess free volume. It is suggested the large amount of free volume will increase the size of shear transformation zone (STZ), which can significantly promote the formation of multiple primary shear bands and stabilize the shear plastic deformation in metallic glass. These findings are helpful for us to understand the plastic deformation
F.F. Wu et al. / Materials Science and Engineering A 541 (2012) 199–203
behaviors of metallic glasses and further to design new metallic glasses with better mechanical performance properties. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (NSFC) under Grant nos. 50771040, 10732010, 50901038, 51025415, and 50931005. References [1] [2] [3] [4] [5]
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