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Bending plasticity of Zr55Al10Ni5Cu30 bulk metallic glass with monolithic amorphous structure
Journal of Alloys and Compounds 688 (2016) 620e625
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Bending plasticity of Zr55Al10Ni5Cu30 bulk metallic glass with monolithic amorphous structure Y. Hu a, b, H.H. Yan a, J.F. Li b, *, Y.H. Zhou b a
Shanxi Key Laboratory of Metallic Materials Forming Theory and Technology, School of Materials Science and Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, People's Republic of China b State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 May 2016 Received in revised form 19 July 2016 Accepted 21 July 2016 Available online 25 July 2016
In order to unravel the effect of free volume on the bending plasticity of Zr55Al10Ni5Cu30 bulk metallic glass (BMG) with monolithic amorphous structure, a series of samples with different free volume content and structural disorder degree were prepared by varying the remelting times of the master alloy ingots and annealing the as-cast samples at 673 K. It is shown that both free volume and structural disorder degree influence the plasticity of the monolithic BMGs. Annihilating the excessive free volume from the BMG has a larger influence on the plasticity of the BMG that is more disordered in structure. © 2016 Elsevier B.V. All rights reserved.
Keywords: Metallic glasses Bending plasticity Free volume Structural disorder degree
1. Introduction Multi-component bulk metallic glasses (BMGs) with excellent mechanical properties [1e5] have been attracting much attention in recent years due to the feasibility that the materials can be cast using conventional solidification techniques at cooling rates as low as 1e100 K/s. However, during compressive or tensile testing at a temperature far below the glass transition temperature, BMGs quite often break abruptly along a very narrow shear band, and exhibit limited global plasticity [3,6]. Such a catastrophic fracture property greatly restricts their application as structural materials. Therefore, how to improve the ductility of BMGs has become one of the hottest issues in BMG research community [7e12]. Controlling the internal structure of BMGs is thought to be an effective way to improve the ductility. For example, Huang et al. [8] and Chen et al. [13] proposed that more free volume in BMGs might result in better plasticity. Louzguine-Luzgin [14] also demonstrated that smaller samples exhibit better plasticity because they contain more free volume. But Li et al. [15] believes that for a group of samples with different diameters, the microstructure and sample size rather than the free volume dominate the ductility. Jiang et al. [16] further
* Corresponding author. E-mail address: jfl[email protected] (J.F. Li). http://dx.doi.org/10.1016/j.jallcom.2016.07.228 0925-8388/© 2016 Elsevier B.V. All rights reserved.
found that free volume has significant effect on bending plasticity but not on compressive plasticity. To clarify what plays a greater role in the plasticity of BMGs, the bending tests of monolithic amorphous Zr55Al10Ni5Cu30 BMG with different free volume content and structural disorder degree (i.e. the deviation degree of an amorphous structure from crystal structure) were carried out in the present work, and the relationship between plasticity and microstructure were systematically discussed.
2. Experimental procedures The master alloy ingots of Zr55Al10Ni5Cu30 (atomic percent) were prepared by arc melting the mixture of pure Zr (99.9 wt%), Al (99.99 wt%), Ni (99.98 wt%) and Cu (99.98 wt%) in a water-cooled copper crucible in a Ti-gettered argon atmosphere. A set of ingots were remelted 4 times, and another one 10 times. The duration of each melting operation was 60 s. The alloy ingots were subsequently suction cast into a water-cooled copper mould to form plates of 1 10 60 mm3. Hereafter the plates produced from the ingots remelted 4 times were simply called as as-cast R4, and the ones produced from the ingots remelted 10 times as as-cast R10. As shown in Ref. [17], the as-cast R10 possesses more free volume content and higher structural disorder degree than the as-cast R4. In order to reduce the free volume content as much as possible
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3. Experimental results
Fig. 1. XRD patterns of the as-cast and as-annealed Zr55Al10Ni5Cu30 samples.
without obviously changing the structural disorder degree, part of the samples, named as-annealed R4 and as-annealed R10, were sealed in an evacuated quartz capsule and annealed for 120 s at 673 K, about ten degrees below the glass transition temperature Tg that equals 680 K for as-cast R4 and 683 K for as-cast R10 at the heating rate of 20 K/min. Rectangular samples with a dimension of 1 3 40 mm3 for bending were cut from the as-cast and asannealed plates using a diamond saw and then their surfaces were carefully polished. The structural natures of the as-cast and as-annealed samples were examined using a Thermo ARL X-ray diffractometer (XRD) with Cu-Ka radiation and a JEOL JEM-2100F HRTEM. The thermal analyses were performed using a Perkin-Elmer Pyris Diamond differential scanning calorimeter (DSC) at 20 K/min under a flow of high-purity Ar atmosphere. A Zwick/Roell Z10 tensile testing machine was used for the three-point bending test. The supports span and cross head speed in the bending test were 20 mm and 0.64 mm/min, respectively. After the bending, the samples were observed by a JEOL JSM-6460 scanning electron microscope (SEM).
Fig. 1 shows the XRD patterns of the as-cast and as-annealed Zr55Al10Ni5Cu30 samples. It can be seen that the XRD patterns of the samples consist of two broad diffraction peaks without any sharp Bragg peaks. In order to further ascertain that the as-cast samples are amorphous in nature, HRTEM examinations were conducted, and the results are shown in Fig. 2. No lattice fringes were found in the HRTEM images, indicating amorphous structures. Therefore, we can confirm that the as-cast R4 and R10 samples are monolithic amorphous. The DSC curves of the as-cast and as-annealed Zr55Al10Ni5Cu30 samples are shown in Fig. 3. All samples exhibit very similar thermal behavior with a distinct glass transition and a wide supercooled liquid region before crystallization. However, the exothermic signals (structural relaxation enthalpy) before glass transition in DSC curves are different among these samples, as indicated in the inset of Fig. 3. It is well known that structural relaxation enthalpy is related to the existence of free volume in the BMGs [18]. The structural relaxation enthalpies are 5.6 and 6.9 J/g for the as-cast R4 and R10 samples, respectively, suggesting that the as-cast R10 sample contains more free volume than the as-cast R4 sample. The DSC curves of the as-annealed R4 and R10 samples, in contrast, do not display exothermic signal any more in the same temperature region, indicating that the excessive free volume in the samples has been annealed out. In addition, the structural disorder degree is also different from each other between the as-cast R4 and R10 samples. Although there isn't a direct way to accurately evaluate the structural disorder degree of metallic glasses, it is believed that the crystallization enthalpy of metallic glasses relates to the structural disorder degree [19,20]. A larger crystallization enthalpy means a higher structural disorder degree or a severer deviation of the metallic glass from the crystalline counterpart in structure. The crystallization enthalpies are 54.6 and 58.0 J/g for the as-cast R4 and R10 samples, respectively, indicating that the R10 sample is more disordered in structure. The structural disorder degree can also be evaluated by the values of FWHM (full width half maximum) of the first broad diffraction peak in the XRD patterns of the samples [21]. The FWHM (fitted with Gaussian function) are 6.45 and 6.75 for the as-cast R4 and R10 samples, respectively. These results also indicate that the
Fig. 2. HRTEM images of the as-cast R4 (a) and R10 (b) samples.
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Fig. 3. DSC curves of the as-cast and as-annealed Zr55Al10Ni5Cu30 samples at a heating rate of 20 K/min. The inset is the exothermic signals (structural relaxation enthalpy) before glass transition in DSC curves.
R10 sample possesses a higher structural disorder degree. As shown in Fig. 3, annealing treatment almost has no effect on the crystallization enthalpy, meaning that the annealing treatment doesn't change the structural disorder degree of the as-cast samples. In Ref. [22], the microstructures of Zr48Cu45Al7 amorphous rods and ribbons were examined by synchrotron-radiation, and it was revealed that the lower cooling rate in the rod sample leads to the higher packing efficiency and the higher regularity of clusters. Obviously, the metallic glass with a higher structural disorder degree possesses the lower regularity of clusters, and perhaps lower
atomic packing efficiency. Fig. 4 shows the bending stress-displacement curves of the samples. All of them exhibit a similar elastic displacement De (about 1.8 mm) but different average plastic displacement Dp prior to failure. For the as-cast samples, increasing the remelting times induced a significant change in the fracture flexibility. The as-cast R4 samples break when the plastic displacement Dp is 2.8 mm, while the as-cast R10 samples do not fracture even when the bending displacement reaches 10 mm (the corresponding plastic displacement Dp is 8.2 mm), the maximum displacement of the current testing machine. As discussed in Ref. [17], such a bending plasticity improvement can be attributed to the more free volume, more disordered and more homogeneous microstructure of the BMG with more remelting times. The Dp significantly decreases after the annealing treatment. It is 2.2 mm for as-annealed R4 and 4.8 mm for as-annealed R10. The reduction reaches as high as 21% and more than 42%, respectively. These results indicate that the ductility of BMGs decreases as either free volume content or structural disorder degree decreases, and furthermore the decrease in plasticity with free volume is dependent on the structural disorder degree. The higher the structural disorder degree of BMGs, the larger the effect of free volume content on the bending plasticity, and vice versa. Scanning electron microscopy (SEM) analysis revealed the fracture features of the bending samples. Shear band distributions on the side surface of the samples subjected to bending test are shown in Fig. 5aed. Besides secondary shear bands between the primary shear bands, several distinct shear offsets along the primary shear bands on the tensile side are also observed. The density of shear bands on the side surface of as-cast R4 is very similar to that of as-annealed R4 (Fig. 5a and b), while the shear bands on the side surface of as-cast R10 (Fig. 5c) is distinctly denser than that of as-annealed R10 (Fig. 5d). A higher bending plasticity corresponds
Fig. 4. Bending stress-displacement curves of the samples.
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Fig. 5. SEM images of the side surface of as-cast R4 (a), as-annealed R4 (b), as-cast R10 (c) and as-annealed R10 (d) samples after testing.
to a larger shear band density. During bending tests, the tensile side takes the main responsibility for the failure of the sample. Therefore, the initial shear region on the fracture surface was examined, and the results are shown in Fig. 6aed. Apart from as-cast R10, a smooth region and a vein-like pattern region are found on the fracture surfaces of all other samples. The smooth region was caused by shear sliding [12] while the vein-like pattern was caused by the subsequent catastrophic failure [23]. The width of the smooth region is equal to the critical shear offset that is a parameter directly reflecting the stable shear capability [12]. The shear offsets of as-cast R4 and asannealed R4 are about 90 mm and 80 mm (Fig. 6a and b), respectively, while the shear offset of as-annealed R10 is about 150 mm (Fig. 6d). The as-cast R10 did not fracture in the present experiment, but a mass of shear offsets are found, and several shear offsets are even up to about 200 mm (Fig. 6c). The results indicate that the sliding along shear bands is also an important deformation mechanism of BMGs. In consistency with the bending stressdisplacement curves, the SEM images also suggest that the annealing treatment resulted in a larger change in the plastic deformation behavior of the BMGs with higher structural disorder degree.
4. Discussion Free volume is employed to explain the plasticity of metallic glasses based on the following three points of view [16,24e26]. Firstly, the sites with higher free volume content favor the initial nucleation and branching of shear bands. The concurrent nucleation of multiple shear bands and high branching of shear bands provide the material a larger plastic strain. Secondly, more free volume enhances the mobility of atoms, which alleviates the stress concentration and prevents from cracking and breaking. Thirdly, viscous flow is the main energy dissipation mechanism at the tip of the crack in metallic glasses, and the enhanced atomic rearrangement processes may hinder the extension of cracks. The cracks in the sample with high free volume concentration can extend a longer distance before catastrophic failure. Therefore, compared with the sample with low content of free volume, the sample containing large amounts of free volume can provide more sites for shear bands to nucleate and longer time for them to sustain shear strain. In the present study, the as-cast R10 contains the most free volume and correspondingly has the highest density of shear bands and the largest shear offset, leading to the best bending plasticity. However, the as-annealed R4 and as-annealed R10 that have
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Fig. 6. SEM images of the initial shear region in tensile side of as-cast R4 (a), as-annealed R4 (b), as-cast R10 (c) and as-annealed R10 (d) samples after testing.
similar free volume content, exhibit different bending plasticity. This is because free volume is not the only factor to affect the plastic deformation of the metallic glasses with monolithic amorphous structure. Structural disorder degree, which is inversely proportional to the fractions of the various SRO atomic clusters or the degree of short-to-medium range orderings [27e29], should also be considered. Recently, the structure of metallic liquid at different temperature [30] and atomic arrangements of metallic glass formed at different cooling rates [22], were investigated by applying the stateof-art advanced synchrotron radiation coupled with a series of calculations. It was reported that the metallic melts contain many low-coordinated polyhedral at high temperatures, but highcoordinated polyhedra of icosahedrons-like clusters at low temperatures, while more free volumes are created between the polyhedra with increasing temperature [30]. It was also demonstrated that a higher cooling rate can lead to a lower packing efficiency and lower regularity of clusters [22]. Similarly, the remelting operation may change the microstructures of the ingots. The increase in remelting time results in finer crystal grains in the alloy ingots, which inherits a smaller average size of clusters in the liquid alloy and the glassy phase [31]. All of these indicate that the as-cast R10 sample has a higher fraction of low-coordinated polyhedral or higher structural disorder degree than the as-cast R4 sample. In addition, it has been demonstrated that full icosahedral
clusters as the key local structural motif constitute the main resistance for plastic flow in Zr- and Cu-based metallic glasses [32,33], due to their higher local stiffness and yield resistance (stability). Shear transformation preferentially nucleates in the other more disordered regions where the local stiffness and stability are low [34]. A more disordered structure indicates more flow regions and numerous shear transformation events in the metallic glass, and consequently a larger global plastic strain can be achieved before catastrophic failure [33]. As mentioned above, the as-annealed R4 and as-annealed R10 have similar free volume content but the latter is more disordered in structure than the former, due to which the as-annealed R10 exhibits a larger plasticity. More importantly, the annealing treatment at 673 K induced a more obvious decrease in plasticity in the as-cast R10 sample than in the as-cast R4 sample, meaning that the influence of free volume on the plasticity of monolithic amorphous BMGs greatly depends on the structural disorder degree. For the BMGs with higher structural disorder degree, a larger proportion of sites for shear band nucleation vanished during the annealing treatment, resulting in a larger decrease in plasticity. 5. Conclusions The bending plasticity of monolithic amorphous Zr55Al10Ni5Cu30 BMG with different structural disorder degree and free volume
Y. Hu et al. / Journal of Alloys and Compounds 688 (2016) 620e625
content was investigated. It is found that both free volume and structural disorder degree influence the plasticity of monolithic amorphous BMGs. In addition, the effect of free volume on the plasticity of monolithic BMGs highly depends on the structural disorder degree. For the BMGs with higher structural disorder degree, the variation of free volume content result in a larger change in the plasticity. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 51471108 and 51204118), and the Program for the Top Young Academic Leaders of Higher Learning Institutions of Shanxi Province, China (2013), and the Research Project supported by Shanxi Scholarship Council of China (Grant No. 2013-094). References [1] A. Inoue, Stabilization of metallic supercooled liquid and bulk amorphous alloys, Acta Mater. 48 (2000) 279e306. [2] W.H. Wang, C. Dong, C.H. Shek, Bulk metallic glasses, Mater. Sci. Eng. A 44 (2004) 45e89. [3] C.A. Schuh, T.C. Hufnagel, U. Ramamurty, Mechanical behavior of amorphous alloys, Acta Mater. 55 (2007) 4067e4109. [4] A.R. Yavari, J.J. Lewandowski, J. Eckert, Mechanical properties of bulk metallic glasses, MRS Bull. 32 (2007) 635e638. [5] R.T. Qu, Z.Q. Liu, R.F. Wang, Z.F. Zhang, Yield strength and yield strain of metallic glasses and their correlations with glass transition temperature, J. Alloys Compd. 637 (2015) 44e54. vez, C. Chang, X. Wang, R.-W. Li, Evolution of shear [6] Y.-Y. Zhao, G. Zhang, D. Este bands into cracks in metallic glasses, J. Alloys Compd. 621 (2015) 238e243. [7] X.D. Wang, L. Yang, J.Z. Jiang, K. Saksl, H. Franz, H.J. Fecht, Y.G. Liu, H.S. Xian, Enhancement of plasticity in Zr-based bulk metallic glasses, J. Mater. Res. 22 (2007) 2454e2459. [8] Y.J. Huang, J. Shen, J.F. Sun, Bulk metallic glasses: smaller is softer, Appl. Phys. Lett. 90 (2007) 081919. [9] E.S. Park, H.J. Chang, J.Y. Lee, D.H. Kim, Improvement of plasticity by tailoring combination of constituent elements in Ti-rich Ti-Zr-Be-Cu-Ni bulk metallic glasses, J. Mater. Res. 22 (2007) 3440e3447. [10] S. Xie, E.P. George, Size-dependent plasticity and fracture of a metallic glass in compression, Intermetallics 16 (2008) 485e489. [11] W.F. Wu, Y. Li, C.A. Schuh, Strength, plasticity and brittleness of bulk metallic glasses under compression: statistical and geometric effects, Philos. Mag. 88 (2008) 71e89. [12] F.F. Wu, Z.F. Zhang, S.X. Mao, Size-dependent shear fracture and global tensile plasticity of metallic glasses, Acta Mater. 57 (2009) 257e266. [13] L.Y. Chen, Z.D. Fu, G.Q. Zhang, X.P. Hao, Q.K. Jiang, X.D. Wang, Q.P. Cao, H. Franz, Y.G. Liu, H.S. Xie, S.L. Zhang, B.Y. Wang, Y.W. Zeng, J.Z. Jiang, New class of plastic bulk metallic glass, Phys. Rev. Lett. 100 (2008) 075501. [14] D.V. Louzguine-Luzgin, N. Chen, V.Y. Zadorozhnyy, I. Seki, A. Inoue, Pd40Ni40Si5P15 bulk metallic glass properties variation as a function of sample thickness, Intermetallics 33 (2013) 67e72.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Bending plasticity of Zr55Al10Ni5Cu30 bulk metallic glass with monolithic amorphous structure Y. Hu a, b, H.H. Yan a, J.F. Li b, *, Y.H. Zhou b a
Shanxi Key Laboratory of Metallic Materials Forming Theory and Technology, School of Materials Science and Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, People's Republic of China b State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 May 2016 Received in revised form 19 July 2016 Accepted 21 July 2016 Available online 25 July 2016
In order to unravel the effect of free volume on the bending plasticity of Zr55Al10Ni5Cu30 bulk metallic glass (BMG) with monolithic amorphous structure, a series of samples with different free volume content and structural disorder degree were prepared by varying the remelting times of the master alloy ingots and annealing the as-cast samples at 673 K. It is shown that both free volume and structural disorder degree influence the plasticity of the monolithic BMGs. Annihilating the excessive free volume from the BMG has a larger influence on the plasticity of the BMG that is more disordered in structure. © 2016 Elsevier B.V. All rights reserved.
Keywords: Metallic glasses Bending plasticity Free volume Structural disorder degree
1. Introduction Multi-component bulk metallic glasses (BMGs) with excellent mechanical properties [1e5] have been attracting much attention in recent years due to the feasibility that the materials can be cast using conventional solidification techniques at cooling rates as low as 1e100 K/s. However, during compressive or tensile testing at a temperature far below the glass transition temperature, BMGs quite often break abruptly along a very narrow shear band, and exhibit limited global plasticity [3,6]. Such a catastrophic fracture property greatly restricts their application as structural materials. Therefore, how to improve the ductility of BMGs has become one of the hottest issues in BMG research community [7e12]. Controlling the internal structure of BMGs is thought to be an effective way to improve the ductility. For example, Huang et al. [8] and Chen et al. [13] proposed that more free volume in BMGs might result in better plasticity. Louzguine-Luzgin [14] also demonstrated that smaller samples exhibit better plasticity because they contain more free volume. But Li et al. [15] believes that for a group of samples with different diameters, the microstructure and sample size rather than the free volume dominate the ductility. Jiang et al. [16] further
* Corresponding author. E-mail address: jfl[email protected] (J.F. Li). http://dx.doi.org/10.1016/j.jallcom.2016.07.228 0925-8388/© 2016 Elsevier B.V. All rights reserved.
found that free volume has significant effect on bending plasticity but not on compressive plasticity. To clarify what plays a greater role in the plasticity of BMGs, the bending tests of monolithic amorphous Zr55Al10Ni5Cu30 BMG with different free volume content and structural disorder degree (i.e. the deviation degree of an amorphous structure from crystal structure) were carried out in the present work, and the relationship between plasticity and microstructure were systematically discussed.
2. Experimental procedures The master alloy ingots of Zr55Al10Ni5Cu30 (atomic percent) were prepared by arc melting the mixture of pure Zr (99.9 wt%), Al (99.99 wt%), Ni (99.98 wt%) and Cu (99.98 wt%) in a water-cooled copper crucible in a Ti-gettered argon atmosphere. A set of ingots were remelted 4 times, and another one 10 times. The duration of each melting operation was 60 s. The alloy ingots were subsequently suction cast into a water-cooled copper mould to form plates of 1 10 60 mm3. Hereafter the plates produced from the ingots remelted 4 times were simply called as as-cast R4, and the ones produced from the ingots remelted 10 times as as-cast R10. As shown in Ref. [17], the as-cast R10 possesses more free volume content and higher structural disorder degree than the as-cast R4. In order to reduce the free volume content as much as possible
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3. Experimental results
Fig. 1. XRD patterns of the as-cast and as-annealed Zr55Al10Ni5Cu30 samples.
without obviously changing the structural disorder degree, part of the samples, named as-annealed R4 and as-annealed R10, were sealed in an evacuated quartz capsule and annealed for 120 s at 673 K, about ten degrees below the glass transition temperature Tg that equals 680 K for as-cast R4 and 683 K for as-cast R10 at the heating rate of 20 K/min. Rectangular samples with a dimension of 1 3 40 mm3 for bending were cut from the as-cast and asannealed plates using a diamond saw and then their surfaces were carefully polished. The structural natures of the as-cast and as-annealed samples were examined using a Thermo ARL X-ray diffractometer (XRD) with Cu-Ka radiation and a JEOL JEM-2100F HRTEM. The thermal analyses were performed using a Perkin-Elmer Pyris Diamond differential scanning calorimeter (DSC) at 20 K/min under a flow of high-purity Ar atmosphere. A Zwick/Roell Z10 tensile testing machine was used for the three-point bending test. The supports span and cross head speed in the bending test were 20 mm and 0.64 mm/min, respectively. After the bending, the samples were observed by a JEOL JSM-6460 scanning electron microscope (SEM).
Fig. 1 shows the XRD patterns of the as-cast and as-annealed Zr55Al10Ni5Cu30 samples. It can be seen that the XRD patterns of the samples consist of two broad diffraction peaks without any sharp Bragg peaks. In order to further ascertain that the as-cast samples are amorphous in nature, HRTEM examinations were conducted, and the results are shown in Fig. 2. No lattice fringes were found in the HRTEM images, indicating amorphous structures. Therefore, we can confirm that the as-cast R4 and R10 samples are monolithic amorphous. The DSC curves of the as-cast and as-annealed Zr55Al10Ni5Cu30 samples are shown in Fig. 3. All samples exhibit very similar thermal behavior with a distinct glass transition and a wide supercooled liquid region before crystallization. However, the exothermic signals (structural relaxation enthalpy) before glass transition in DSC curves are different among these samples, as indicated in the inset of Fig. 3. It is well known that structural relaxation enthalpy is related to the existence of free volume in the BMGs [18]. The structural relaxation enthalpies are 5.6 and 6.9 J/g for the as-cast R4 and R10 samples, respectively, suggesting that the as-cast R10 sample contains more free volume than the as-cast R4 sample. The DSC curves of the as-annealed R4 and R10 samples, in contrast, do not display exothermic signal any more in the same temperature region, indicating that the excessive free volume in the samples has been annealed out. In addition, the structural disorder degree is also different from each other between the as-cast R4 and R10 samples. Although there isn't a direct way to accurately evaluate the structural disorder degree of metallic glasses, it is believed that the crystallization enthalpy of metallic glasses relates to the structural disorder degree [19,20]. A larger crystallization enthalpy means a higher structural disorder degree or a severer deviation of the metallic glass from the crystalline counterpart in structure. The crystallization enthalpies are 54.6 and 58.0 J/g for the as-cast R4 and R10 samples, respectively, indicating that the R10 sample is more disordered in structure. The structural disorder degree can also be evaluated by the values of FWHM (full width half maximum) of the first broad diffraction peak in the XRD patterns of the samples [21]. The FWHM (fitted with Gaussian function) are 6.45 and 6.75 for the as-cast R4 and R10 samples, respectively. These results also indicate that the
Fig. 2. HRTEM images of the as-cast R4 (a) and R10 (b) samples.
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Fig. 3. DSC curves of the as-cast and as-annealed Zr55Al10Ni5Cu30 samples at a heating rate of 20 K/min. The inset is the exothermic signals (structural relaxation enthalpy) before glass transition in DSC curves.
R10 sample possesses a higher structural disorder degree. As shown in Fig. 3, annealing treatment almost has no effect on the crystallization enthalpy, meaning that the annealing treatment doesn't change the structural disorder degree of the as-cast samples. In Ref. [22], the microstructures of Zr48Cu45Al7 amorphous rods and ribbons were examined by synchrotron-radiation, and it was revealed that the lower cooling rate in the rod sample leads to the higher packing efficiency and the higher regularity of clusters. Obviously, the metallic glass with a higher structural disorder degree possesses the lower regularity of clusters, and perhaps lower
atomic packing efficiency. Fig. 4 shows the bending stress-displacement curves of the samples. All of them exhibit a similar elastic displacement De (about 1.8 mm) but different average plastic displacement Dp prior to failure. For the as-cast samples, increasing the remelting times induced a significant change in the fracture flexibility. The as-cast R4 samples break when the plastic displacement Dp is 2.8 mm, while the as-cast R10 samples do not fracture even when the bending displacement reaches 10 mm (the corresponding plastic displacement Dp is 8.2 mm), the maximum displacement of the current testing machine. As discussed in Ref. [17], such a bending plasticity improvement can be attributed to the more free volume, more disordered and more homogeneous microstructure of the BMG with more remelting times. The Dp significantly decreases after the annealing treatment. It is 2.2 mm for as-annealed R4 and 4.8 mm for as-annealed R10. The reduction reaches as high as 21% and more than 42%, respectively. These results indicate that the ductility of BMGs decreases as either free volume content or structural disorder degree decreases, and furthermore the decrease in plasticity with free volume is dependent on the structural disorder degree. The higher the structural disorder degree of BMGs, the larger the effect of free volume content on the bending plasticity, and vice versa. Scanning electron microscopy (SEM) analysis revealed the fracture features of the bending samples. Shear band distributions on the side surface of the samples subjected to bending test are shown in Fig. 5aed. Besides secondary shear bands between the primary shear bands, several distinct shear offsets along the primary shear bands on the tensile side are also observed. The density of shear bands on the side surface of as-cast R4 is very similar to that of as-annealed R4 (Fig. 5a and b), while the shear bands on the side surface of as-cast R10 (Fig. 5c) is distinctly denser than that of as-annealed R10 (Fig. 5d). A higher bending plasticity corresponds
Fig. 4. Bending stress-displacement curves of the samples.
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Fig. 5. SEM images of the side surface of as-cast R4 (a), as-annealed R4 (b), as-cast R10 (c) and as-annealed R10 (d) samples after testing.
to a larger shear band density. During bending tests, the tensile side takes the main responsibility for the failure of the sample. Therefore, the initial shear region on the fracture surface was examined, and the results are shown in Fig. 6aed. Apart from as-cast R10, a smooth region and a vein-like pattern region are found on the fracture surfaces of all other samples. The smooth region was caused by shear sliding [12] while the vein-like pattern was caused by the subsequent catastrophic failure [23]. The width of the smooth region is equal to the critical shear offset that is a parameter directly reflecting the stable shear capability [12]. The shear offsets of as-cast R4 and asannealed R4 are about 90 mm and 80 mm (Fig. 6a and b), respectively, while the shear offset of as-annealed R10 is about 150 mm (Fig. 6d). The as-cast R10 did not fracture in the present experiment, but a mass of shear offsets are found, and several shear offsets are even up to about 200 mm (Fig. 6c). The results indicate that the sliding along shear bands is also an important deformation mechanism of BMGs. In consistency with the bending stressdisplacement curves, the SEM images also suggest that the annealing treatment resulted in a larger change in the plastic deformation behavior of the BMGs with higher structural disorder degree.
4. Discussion Free volume is employed to explain the plasticity of metallic glasses based on the following three points of view [16,24e26]. Firstly, the sites with higher free volume content favor the initial nucleation and branching of shear bands. The concurrent nucleation of multiple shear bands and high branching of shear bands provide the material a larger plastic strain. Secondly, more free volume enhances the mobility of atoms, which alleviates the stress concentration and prevents from cracking and breaking. Thirdly, viscous flow is the main energy dissipation mechanism at the tip of the crack in metallic glasses, and the enhanced atomic rearrangement processes may hinder the extension of cracks. The cracks in the sample with high free volume concentration can extend a longer distance before catastrophic failure. Therefore, compared with the sample with low content of free volume, the sample containing large amounts of free volume can provide more sites for shear bands to nucleate and longer time for them to sustain shear strain. In the present study, the as-cast R10 contains the most free volume and correspondingly has the highest density of shear bands and the largest shear offset, leading to the best bending plasticity. However, the as-annealed R4 and as-annealed R10 that have
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Fig. 6. SEM images of the initial shear region in tensile side of as-cast R4 (a), as-annealed R4 (b), as-cast R10 (c) and as-annealed R10 (d) samples after testing.
similar free volume content, exhibit different bending plasticity. This is because free volume is not the only factor to affect the plastic deformation of the metallic glasses with monolithic amorphous structure. Structural disorder degree, which is inversely proportional to the fractions of the various SRO atomic clusters or the degree of short-to-medium range orderings [27e29], should also be considered. Recently, the structure of metallic liquid at different temperature [30] and atomic arrangements of metallic glass formed at different cooling rates [22], were investigated by applying the stateof-art advanced synchrotron radiation coupled with a series of calculations. It was reported that the metallic melts contain many low-coordinated polyhedral at high temperatures, but highcoordinated polyhedra of icosahedrons-like clusters at low temperatures, while more free volumes are created between the polyhedra with increasing temperature [30]. It was also demonstrated that a higher cooling rate can lead to a lower packing efficiency and lower regularity of clusters [22]. Similarly, the remelting operation may change the microstructures of the ingots. The increase in remelting time results in finer crystal grains in the alloy ingots, which inherits a smaller average size of clusters in the liquid alloy and the glassy phase [31]. All of these indicate that the as-cast R10 sample has a higher fraction of low-coordinated polyhedral or higher structural disorder degree than the as-cast R4 sample. In addition, it has been demonstrated that full icosahedral
clusters as the key local structural motif constitute the main resistance for plastic flow in Zr- and Cu-based metallic glasses [32,33], due to their higher local stiffness and yield resistance (stability). Shear transformation preferentially nucleates in the other more disordered regions where the local stiffness and stability are low [34]. A more disordered structure indicates more flow regions and numerous shear transformation events in the metallic glass, and consequently a larger global plastic strain can be achieved before catastrophic failure [33]. As mentioned above, the as-annealed R4 and as-annealed R10 have similar free volume content but the latter is more disordered in structure than the former, due to which the as-annealed R10 exhibits a larger plasticity. More importantly, the annealing treatment at 673 K induced a more obvious decrease in plasticity in the as-cast R10 sample than in the as-cast R4 sample, meaning that the influence of free volume on the plasticity of monolithic amorphous BMGs greatly depends on the structural disorder degree. For the BMGs with higher structural disorder degree, a larger proportion of sites for shear band nucleation vanished during the annealing treatment, resulting in a larger decrease in plasticity. 5. Conclusions The bending plasticity of monolithic amorphous Zr55Al10Ni5Cu30 BMG with different structural disorder degree and free volume
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content was investigated. It is found that both free volume and structural disorder degree influence the plasticity of monolithic amorphous BMGs. In addition, the effect of free volume on the plasticity of monolithic BMGs highly depends on the structural disorder degree. For the BMGs with higher structural disorder degree, the variation of free volume content result in a larger change in the plasticity. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 51471108 and 51204118), and the Program for the Top Young Academic Leaders of Higher Learning Institutions of Shanxi Province, China (2013), and the Research Project supported by Shanxi Scholarship Council of China (Grant No. 2013-094). References [1] A. Inoue, Stabilization of metallic supercooled liquid and bulk amorphous alloys, Acta Mater. 48 (2000) 279e306. [2] W.H. Wang, C. Dong, C.H. Shek, Bulk metallic glasses, Mater. Sci. Eng. A 44 (2004) 45e89. [3] C.A. Schuh, T.C. Hufnagel, U. Ramamurty, Mechanical behavior of amorphous alloys, Acta Mater. 55 (2007) 4067e4109. [4] A.R. Yavari, J.J. Lewandowski, J. Eckert, Mechanical properties of bulk metallic glasses, MRS Bull. 32 (2007) 635e638. [5] R.T. Qu, Z.Q. Liu, R.F. Wang, Z.F. Zhang, Yield strength and yield strain of metallic glasses and their correlations with glass transition temperature, J. Alloys Compd. 637 (2015) 44e54. vez, C. Chang, X. Wang, R.-W. Li, Evolution of shear [6] Y.-Y. Zhao, G. Zhang, D. Este bands into cracks in metallic glasses, J. Alloys Compd. 621 (2015) 238e243. [7] X.D. Wang, L. Yang, J.Z. Jiang, K. Saksl, H. Franz, H.J. Fecht, Y.G. Liu, H.S. Xian, Enhancement of plasticity in Zr-based bulk metallic glasses, J. Mater. Res. 22 (2007) 2454e2459. [8] Y.J. Huang, J. Shen, J.F. Sun, Bulk metallic glasses: smaller is softer, Appl. Phys. Lett. 90 (2007) 081919. [9] E.S. Park, H.J. Chang, J.Y. Lee, D.H. Kim, Improvement of plasticity by tailoring combination of constituent elements in Ti-rich Ti-Zr-Be-Cu-Ni bulk metallic glasses, J. Mater. Res. 22 (2007) 3440e3447. [10] S. Xie, E.P. George, Size-dependent plasticity and fracture of a metallic glass in compression, Intermetallics 16 (2008) 485e489. [11] W.F. Wu, Y. Li, C.A. Schuh, Strength, plasticity and brittleness of bulk metallic glasses under compression: statistical and geometric effects, Philos. Mag. 88 (2008) 71e89. [12] F.F. Wu, Z.F. Zhang, S.X. Mao, Size-dependent shear fracture and global tensile plasticity of metallic glasses, Acta Mater. 57 (2009) 257e266. [13] L.Y. Chen, Z.D. Fu, G.Q. Zhang, X.P. Hao, Q.K. Jiang, X.D. Wang, Q.P. Cao, H. Franz, Y.G. Liu, H.S. Xie, S.L. Zhang, B.Y. Wang, Y.W. Zeng, J.Z. Jiang, New class of plastic bulk metallic glass, Phys. Rev. Lett. 100 (2008) 075501. [14] D.V. Louzguine-Luzgin, N. Chen, V.Y. Zadorozhnyy, I. Seki, A. Inoue, Pd40Ni40Si5P15 bulk metallic glass properties variation as a function of sample thickness, Intermetallics 33 (2013) 67e72.
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