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Microalloying-induced large plasticity in La-Al-C bulk metallic glass
Journal of Non-Crystalline Solids 447 (2016) 55–58
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Microalloying-induced large plasticity in La-Al-C bulk metallic glass Yangyang Cheng, Chen Chen, Xuan Li, Tao Zhang ⁎ Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China
a r t i c l e
i n f o
Article history: Received 26 December 2015 Received in revised form 13 February 2016 Accepted 11 May 2016 Available online xxxx Keywords: Amorphous materials Microalloying Mechanical properties
a b s t r a c t Bulk metallic glasses (BMGs) generally exhibit poor plastic deformability, especially for rare earth-based BMGs. In this paper, using the microalloying technique, we found that in La-Al-C BMG through the addition of 0.5 at.% element Zr/Mo the plastic strain is significantly improved to 4%/1% from nearly zero under uniaxial compression, and the fracture process changes from rapid fracture to stable fracture. The beneficial effect of Zr/Mo microalloying on the deformation is explained by the atomic bonding character. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Bulk metallic glasses (BMGs) generally fail in a brittle manner without obvious macroscopic deformation [1]. This raises a serious reliability issue for the potential application of BMGs as structural materials. Therefore, the mechanical behavior of BMGs has been the subject of intense research. It is widely accepted that the lack of ductility in BMGs is resulted from the localization of shear strain into shear bands and the instable propagation of shear bands which can initiate cracking [1,2]. Simulative and experimental studies have demonstrated that the nucleation and propagation of shear bands are closely related to the cooperative motion of atomic clusters, thus significantly affecting the mechanical behavior of BMGs [3–9]. Therefore, one strategy to optimize the mechanical properties of BMGs is microalloying which can tune their structure, i.e., the way that atoms are mixed, packed and bonded [5,10–12]. For instance, it has been reported that the plasticity of Tibased and Zr-based BMGs can be enhanced through the proper addition of elements In and Sn which have low Young's modulus and large Poisson's ratio [12,13]. Besides, the addition of element possessing a positive heat of mixing with the base element can lead to the formation of BMG composite and improve the plasticity, such as addition of element Nb for Cu-Zr-based BMG and element Zr for Mg-based BMG [11, 14]. For rare-earth-based BMGs, to the best of our knowledge, only Wang et al. studied the effect of microalloying on the glass-forming ability of Ce-Al-Cu BMG and the effect on the elastic constants of Pr-Al-NiCu BMG [15,16], but no reports have been published thus far about the influence of microalloying on the microstructure and mechanical properties.
⁎ Corresponding author. E-mail address: [email protected] (T. Zhang).
http://dx.doi.org/10.1016/j.jnoncrysol.2016.05.026 0022-3093/© 2016 Elsevier B.V. All rights reserved.
Therefore, the recently developed La57.5Al32.5C10 BMG is selected as a base composition [17]. Considering that the early transition element Zr/ Mo has a positive heat of mixing with the base element La (+ 13/ + 31 kJ mol−1) and a negative heat of mixing with Al and C (− 44/ − 5 and − 131/− 67 kJ mol− 1) [18], 0.5 at.% element Zr/Mo is added through replacing element La. 2. Material and methods Alloy ingots with a nominal composition of La57Al32.5C10Zr0.5 (at.%) and La57Al32.5C10Mo0.5 (at.%) were prepared by arc-melting the elements of high purity in a Ti-gettered argon atmosphere. From the ingots, cylindrical rods with a diameter of 1 mm and length of ∼40 mm were obtained by copper mold casting method. The structure was evaluated by X-ray diffraction (XRD) using Dmax2500PC X-ray diffractometer with CuKα radiation, and the detailed microstructure was investigated on JEM 2100 transmission electron microscope (TEM). For TEM and high-resolution TEM observation, the specimen was mechanically polished and then ion milled until perforation. The thermal stability of the specimen was characterized by NETZSCH 404C differential scanning calorimeter (DSC) under an Ar gas atmosphere at a heating rate of 0.33 K/s. Uniaxial compressive testing was carried out on 1 mm diameter specimen with an aspect ratio of 2:1 at a strain rate of 8 × 10−4 s−1 using SANS 5504 testing machine at room temperature. Five specimens were tested to ensure the reproducibility of the results. Morphologies of the deformed and fractured specimens were observed by CamScan 3400 scanning electron microscope (SEM). The density was measured by Archimedes's method using a Sartorius balance. The simulation of the structure in the present amorphous alloys was conducted via ab initio molecular dynamics (AIMD) calculations based on density functional theory using the Vienna ab initio simulation package. The generalized gradient approximation derived from Perdew-Wang was used
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to the calculate exchange-correlation potentials [19]. The simulation was carried out on a 200-atom cubic supercell with period boundary condition. The size of the box is determined from the experimental density at room temperature. The alloys were first melted at 1500 K and kept for 4000 steps to equilibrate (each timestep = 2 fs), and then quenched to 300 K at a rate of 5 × 1014 K/s. In order to obtain reliable statistical information of pair distribution function (PDF), a 3000 steps simulation was carried out at 300 K. 3. Results Fig. 1(a) shows the XRD patterns of as-cast Zr/Mo microalloyed specimens with diameter of 1 mm. No crystalline Bragg peaks were detected on the broad diffraction hump, indicating an amorphous structure within the resolution of XRD. Fig. 1(b) displays the DSC curves of the microalloyed specimens exhibiting a distinct endothermic peak followed by an exothermic peak corresponding to the glass transition and crystallization, respectively. It can be seen that as compared to the La57.5Al32.5C10 BMG the minor addition of element Zr/Mo does not significantly change the thermal behavior [17]. In order to further ensure the fully amorphous nature of the specimens, the detailed microstructure of the Zr-containing specimen was investigated by TEM. The homogeneous contrast in the bright-field image, the broad halo ring in the corresponding selected-area electron diffraction and the maze-like pattern in the HRTEM image (Fig. 1(c, d)) demonstrate that no phase separation and nanocrystalline exist. For comparison, engineering stress-strain curves of 1 mm diameter specimens with and without addition of element Zr/Mo are both presented in Fig. 2. Obviously, the base specimen (La57.5Al32.5C10) fails catastrophically without any plastic strain and the stress rapidly drops to zero. On the other hand, the 0.5 at.% Zr/Mo-containing specimens exhibit an apparent yielding plateau accompanied by numerous serrations which correspond to the intermittent shear-band operation, and the plateau of the Zr-containing specimen is greater than that of the Mocontaining specimen. Besides, after yielding there exists obvious
Fig. 2. Engineering stress-strain curves of the La57.5Al32.5C10 and La57Al32.5C10(Zr/Mo)0.5 BMGs.
increase in flow stress although the yielding strength is slightly decreased. When deformed about 4% and 1% for the Zr-containing and the Mo-containing specimens, respectively, the catastrophic failure does not occur, but instead the stress gradually decreases. Generally, the onset of the stress drop indicates the start of the specimen fracture, i.e., the formation of a crack [2]. Therefore, the Zr/Mo-containing specimen demonstrates a stable fracture process. 4. Discussion In order to explain the phenomenon, the fractured base specimen and incompletely fractured Zr-containing specimen were observed by SEM. It was found that both the specimens deform along the shear plane which deflects about 45° angle to the direction of the applied loading. However, the base specimen ruptures along the shear plane and breaks into pieces (Fig. 3 (a)). In contrast, the Zr-containing specimen is divided into two segments by the shear plane, and the segments move relatively along the shear plane (Fig. 3 (d)). Therefore, the Zr-
Fig. 1. XRD pattern (a), DSC curve (b), TEM bright-field image with selected area electron diffraction patterns (c) and high-resolution image (d) of the La57Al32.5C10(Zr/Mo)0.5 BMG.
Y. Cheng et al. / Journal of Non-Crystalline Solids 447 (2016) 55–58
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Fig. 3. Morphologies of lateral and fracture surface of the La57.5Al32.5C10 (a, b, c) and La57Al32.5C10Zr0.5 (d, e, f) BMGs.
containing specimen shows a stable fracture process. In addition, for the base specimen only limited shear bands are visible on the surface, while for the Zr-containing specimen with large plasticity numerous multiple and intersecting shear bands are formed. This is in agreement with the previous studies that the formation of profuse shear bands has a significant contribution to the plasticity of BMGs [20,21]. After the complete failure, the fracture morphology of the base specimen consists of veinlike patterns (Fig. 3 (b, c)), yet for the Zr-containing one the fracture morphology is constituted by dimples and a zigzag crack (Fig. 3 (e, f)). It has been reported that the fracture surface morphology is closely related to the velocity of the crack propagation, and the low crack propagation speed can generate smooth fracture surface [2,22]. In this work, it is reasonable to deduce that after the minor addition of Zr, the stable fracture process corresponds with the lower crack propagation speed, thus producing the smooth dimple-like pattern. A considerable amount of research suggests a close relationship between the elastic constants and the mechanical properties, and demonstrates that the BMG possessing low ratio of shear modulus to bulk modulus and high Poisson's ratio can exhibit outstanding plasticity [1, 10,12–14,23]. Accordingly, the positive effect of the microalloying on the plasticity is discussed in detail in terms of the change in elastic constants [10,12–14,23]. However, in this work, there is no pronounced difference in elastic constants between the base alloy and the Zr/Mocontaining alloy. To clarify the mechanism of enhanced plasticity, the La-Al, La-C and Al-C partial PDFs of the amorphous alloys with and without the addition of element Zr were analyzed via AIMD (Fig. 4). Although only 200 atoms are used, the bulk moduli of the La57.5Al32.5C10
and La57Al32.5C10Zr0.5 BMGs calculated by the simulation (41.2 and 40.1 GPa) are very close to the experimental values (43.2 and 42.1 GPa), which indicates the validity of the simulation. It can be seen that between the base alloy and the Zr-containing alloy the overall peak position, shape and intensity of the La-Al and La-C partial PDFs are almost identical, while the second maximum peak of Al-C partial PDF is split from two subpeaks into three subpeaks after the addition of Zr. It has been suggested that the splitting of the second maximum peak is derived from the unevenness of polyhedral connecting style [24,25]. Therefore, it is reasonable to infer that the added Zr/Mo may lead to the change in atomic bonding character. Recently, in order to underpin our understanding of the structural origin of mechanical properties in BMGs, the internal structure of BMGs is viewed as composed of clusters/superclusters where the atoms are tightly bonded and solvent-rich junctions with relatively loose bonding [26,27]. It is suggested that the strength and plasticity are mainly determined by the solvent elements [26–28]. From the experimental result, it should be expected that the base La57.5Al32.5C10 BMG consists of Al/C-centered clusters and La-rich junctions, and because the solvent element La exhibits a large negative heat of mixing with the solute elements Al and C (− 38 and − 116 kJ mol− 1) [18], some Al and C are likely to participate in the construction of the junction, thus weakening its compliant character. As a result, the strength of the base alloy is higher than that of other La-based BMGs, but it fails catastrophically without any plastic strain [29]. Considering that the junction may be relatively unstable compared to the clusters from a potential energy perspective [30,31] and the element Zr/Mo has a
Fig. 4. Simulative La-Al, La-C and Al-C partial PDFs of the La57.5Al32.5C10 and La57Al32·5C10Zr0.5 BMGs.
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large negative heat of mixing with the solute elements Al and C [18], the added Zr/Mo would be more prone to interact with the solute elements Al and C in the junctions, thus leaving behind weak and compliant junctions which tend to become preferential sites for shear transformation. In this scenario, the slightly decreased strength and significant enhancement of plasticity via the addition of Zr/Mo are ascribed to the change in configuration of the junction. On the other hand, considering that the Zr has more negative heat of mixing with Al and C (− 44 and −131 kJ mol−1) than that of Mo (−5 and −67 kJ mol−1) [18], the effect of the added Zr on weakening the junction is more significant leading to the larger plasticity. 5. Conclusions Extensive studies indicate that the mechanical properties of BMGs are mainly determined by the solvent elements [26–28]. Based on the model that the structure of BMGs is viewed as composed of solute-centered clusters and solvent-rich junctions and the affinity among constituent elements, we infer that the brittleness of La-Al-C BMG stems from the less compliant solvent-rich junctions, and after the addition of Zr/ Mo the junction becomes weak and compliant to serve as preferential sites for shear transformation, which facilitates the improvement of plasticity. Acknowledgements This work was supported by the Shanxi Provincial Science and Technology Major Project (Grant Nos. 322469 and 322402). References [1] C. Schuh, T. Hufnagel, U. Ramamurty, Mechanical behavior of amorphous alloys, Acta Mater. 55 (2007) 4067–4109. [2] B.A. Sun, J. Tan, S. Pauly, U. Kuhn, J. Eckert, Stable fracture of a malleable Zr-based bulk metallic glass, J. Appl. Phys. 112 (2012) 103533. [3] A.J. Cao, Y.Q. Cheng, E. Ma, Structural processes that initiate shear localization in metallic glass, Acta Mater. 57 (2009) 5146–5155. [4] A.L. Greer, Y.Q. Cheng, E. Ma, Shear bands in metallic glasses, Mater. Sci. Eng. R 74 (2013) 71–132. [5] O. Gendelman, A. Joy, P. Mishra, I. Procaccia, K. Samwer, On the effect of microalloying on the mechanical properties of metallic glasses, Acta Mater. 63 (2014) 209–215. [6] C.C. Yuan, J.F. Xiang, X.K. Xi, W.H. Wang, NMR signature of evolution of ductile-tobrittle transition in bulk metallic glasses, Phys. Rev. Lett. 107 (2011) 236403. [7] Y.J. Huang, J.C. Khong, T. Connolley, J. Mi, In situ study of the evolution of atomic strain of bulk metallic glass and its effects on shear band formation, Scr. Mater. 69 (2013) 207–210.
[8] E.R. Homer, Examining the initial stages of shear localization in amorphous metals, Acta Mater. 63 (2014) 44–53. [9] S.R. Jian, J.B. Li, K.W. Chen, J.S.C. Jang, J.Y. Juang, P.J. Wei, J.F. Lin, Mechanical responses of Mg-based bulk metallic glasses, Intermetallics 18 (2010) 1930–1935. [10] W.H. Wang, Roles of minor additions in formation and properties of bulk metallic glasses, Prog. Mater. Sci. 52 (2007) 540–596. [11] G. Chen, X.L. Zhang, C.T. Liu, High strength and plastic strain of Mg-based bulk metallic glass composite containing in situ formed intermetallic phases, Scr. Mater. 68 (2013) 150–153. [12] N. Zheng, R.T. Qu, S. Pauly, M. Calin, T. Gemming, Z.F. Zhang, et al., Design of ductile bulk metallic glasses by adding ‘soft’ atoms, Appl. Phys. Lett. 100 (2012) 141901. [13] C.C. Yuan, X.X. Xia, K.H. Jiang, D.Q. Zhao, X.K. Xi, Effect of Sn additions on the damage tolerance of a ZrCuNiAl bulk metallic glass, Metall. Mater. Trans. A 44 (2013) 819–826. [14] S.S. Chen, H.R. Zhang, I. Todd, Phase-separation-enhanced plasticity in a Cu47.2Zr46.5Al5.5Nb0.8 bulk metallic glass, Scr. Mater. 72–73 (2014) 47–50. [15] B. Zhang, D.Q. Zhao, M.X. Pan, R.J. Wang, W.H. Wang, Formation of cerium-based bulk metallic glasses, Acta Mater. 54 (2006) 3025–3032. [16] Y.T. Wang, Z.Y. Pang, R.J. Wang, D.Q. Zhao, M.X. Pan, B.S. Han, W.L. Wang, W.H. Wang, Doping-induced formation of bulk nanocrystalline alloy from metallic glass with controllable microstructure and properties, J. Non-Cryst. Solids 352 (2006) 444–449. [17] L.L. Zhang, R. Li, T. Xu, H.Y. Zhang, T. Zhang, Ternary La-Al-C bulk metallic glasses, Intermetallics 52 (2014) 92–96. [18] A. Takeuchi, A. Inoue, Classification of bulk metallic glasses by atomic size difference, heat of mixing and period of constituent elements and its application to characterization of the main alloying element, Mater. Trans. 46 (2005) 2817–2829. [19] J.P. Perdew, Atoms, molecules, solids, and surfaces: applications of the generalized gradient approximation for exchange and correlation, Phys. Rev. B 46 (1992) 6671–6687. [20] Z.F. Zhang, H. Zhang, X.F. Pan, J. Das, J. Eckert, Effect of aspect ratio on the compressive deformation and fracture behaviour of Zr-based bulk metallic glass, Phil. Mag. Lett. 85 (2005) 513–521. [21] Y.H. Lai, C.J. Lee, Y.T. Cheng, H.S. Chou, H.M. Chen, X.H. Du, C.I. Chang, J.C. Huang, S.R. Jian, J.S.C. Jang, T.G. Nieh, Bulk and microscale compressive behavior of a Zr-based metallic glass, Scr. Mater. 58 (2008) 890–893. [22] M. Gao, D.W. Ding, D.Q. Zhao, H.Y. Bai, W.H. Wang, Fracture morphology pattern transition dominated by the crack tip curvature radius in brittle metallic glasses, Mater. Sci. Eng. A 617 (2014) 89–96. [23] X.J. Gu, A.G. McDermott, S.J. Poon, G.J. Shiflet, Critical Poisson's ratio for plasticity in Fe-Mo-C-B-Ln bulk amorphous steel, Appl. Phys. Lett. 88 (2006) 211905. [24] S.P. Pan, J.Y. Qin, W.M. Wang, T.K. Gu, Origin of splitting of the second peak in the pair-distribution function for metallic glasses, Phys. Rev. B 84 (2011) 092201. [25] H. Wang, T. Hu, T. Zhang, Local structure of Co55Ta10B35 amorphous alloy investigated by ab-initio molecular dynamics, Sci. China 56 (2013) 904–909. [26] D. Ma, A.D. Stoica, X.L. Wang, Z.P. Lu, B. Clausen, et al., Elastic moduli inheritance and the weakest link in bulk metallic glasses, Phys. Rev. Lett. 108 (2012) 085501. [27] Z.Q. Liu, Z.F. Zhang, Universal softening and intrinsic local fluctuations in metallic glasses, Appl. Phys. Lett. 103 (2013) 181909. [28] W.H. Wang, Metallic glasses: family traits, Nat. Mater. 11 (2012) 275–276. [29] R. Li, Q. Yang, S. Pang, C. Ma, T. Zhang, Misch metal based metallic glasses, J. Alloys Compd. 450 (2008) 181–184. [30] Y.Q. Cheng, E. Ma, Atomic-level structure and structure-property relationship in metallic glasses, Prog. Mater. Sci. 56 (2011) 379–473. [31] W.H. Wang, The elastic properties, elastic models and elastic perspectives of metallic glasses, Prog. Mater. Sci. 57 (2012) 487–656.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Microalloying-induced large plasticity in La-Al-C bulk metallic glass Yangyang Cheng, Chen Chen, Xuan Li, Tao Zhang ⁎ Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China
a r t i c l e
i n f o
Article history: Received 26 December 2015 Received in revised form 13 February 2016 Accepted 11 May 2016 Available online xxxx Keywords: Amorphous materials Microalloying Mechanical properties
a b s t r a c t Bulk metallic glasses (BMGs) generally exhibit poor plastic deformability, especially for rare earth-based BMGs. In this paper, using the microalloying technique, we found that in La-Al-C BMG through the addition of 0.5 at.% element Zr/Mo the plastic strain is significantly improved to 4%/1% from nearly zero under uniaxial compression, and the fracture process changes from rapid fracture to stable fracture. The beneficial effect of Zr/Mo microalloying on the deformation is explained by the atomic bonding character. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Bulk metallic glasses (BMGs) generally fail in a brittle manner without obvious macroscopic deformation [1]. This raises a serious reliability issue for the potential application of BMGs as structural materials. Therefore, the mechanical behavior of BMGs has been the subject of intense research. It is widely accepted that the lack of ductility in BMGs is resulted from the localization of shear strain into shear bands and the instable propagation of shear bands which can initiate cracking [1,2]. Simulative and experimental studies have demonstrated that the nucleation and propagation of shear bands are closely related to the cooperative motion of atomic clusters, thus significantly affecting the mechanical behavior of BMGs [3–9]. Therefore, one strategy to optimize the mechanical properties of BMGs is microalloying which can tune their structure, i.e., the way that atoms are mixed, packed and bonded [5,10–12]. For instance, it has been reported that the plasticity of Tibased and Zr-based BMGs can be enhanced through the proper addition of elements In and Sn which have low Young's modulus and large Poisson's ratio [12,13]. Besides, the addition of element possessing a positive heat of mixing with the base element can lead to the formation of BMG composite and improve the plasticity, such as addition of element Nb for Cu-Zr-based BMG and element Zr for Mg-based BMG [11, 14]. For rare-earth-based BMGs, to the best of our knowledge, only Wang et al. studied the effect of microalloying on the glass-forming ability of Ce-Al-Cu BMG and the effect on the elastic constants of Pr-Al-NiCu BMG [15,16], but no reports have been published thus far about the influence of microalloying on the microstructure and mechanical properties.
⁎ Corresponding author. E-mail address: [email protected] (T. Zhang).
http://dx.doi.org/10.1016/j.jnoncrysol.2016.05.026 0022-3093/© 2016 Elsevier B.V. All rights reserved.
Therefore, the recently developed La57.5Al32.5C10 BMG is selected as a base composition [17]. Considering that the early transition element Zr/ Mo has a positive heat of mixing with the base element La (+ 13/ + 31 kJ mol−1) and a negative heat of mixing with Al and C (− 44/ − 5 and − 131/− 67 kJ mol− 1) [18], 0.5 at.% element Zr/Mo is added through replacing element La. 2. Material and methods Alloy ingots with a nominal composition of La57Al32.5C10Zr0.5 (at.%) and La57Al32.5C10Mo0.5 (at.%) were prepared by arc-melting the elements of high purity in a Ti-gettered argon atmosphere. From the ingots, cylindrical rods with a diameter of 1 mm and length of ∼40 mm were obtained by copper mold casting method. The structure was evaluated by X-ray diffraction (XRD) using Dmax2500PC X-ray diffractometer with CuKα radiation, and the detailed microstructure was investigated on JEM 2100 transmission electron microscope (TEM). For TEM and high-resolution TEM observation, the specimen was mechanically polished and then ion milled until perforation. The thermal stability of the specimen was characterized by NETZSCH 404C differential scanning calorimeter (DSC) under an Ar gas atmosphere at a heating rate of 0.33 K/s. Uniaxial compressive testing was carried out on 1 mm diameter specimen with an aspect ratio of 2:1 at a strain rate of 8 × 10−4 s−1 using SANS 5504 testing machine at room temperature. Five specimens were tested to ensure the reproducibility of the results. Morphologies of the deformed and fractured specimens were observed by CamScan 3400 scanning electron microscope (SEM). The density was measured by Archimedes's method using a Sartorius balance. The simulation of the structure in the present amorphous alloys was conducted via ab initio molecular dynamics (AIMD) calculations based on density functional theory using the Vienna ab initio simulation package. The generalized gradient approximation derived from Perdew-Wang was used
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to the calculate exchange-correlation potentials [19]. The simulation was carried out on a 200-atom cubic supercell with period boundary condition. The size of the box is determined from the experimental density at room temperature. The alloys were first melted at 1500 K and kept for 4000 steps to equilibrate (each timestep = 2 fs), and then quenched to 300 K at a rate of 5 × 1014 K/s. In order to obtain reliable statistical information of pair distribution function (PDF), a 3000 steps simulation was carried out at 300 K. 3. Results Fig. 1(a) shows the XRD patterns of as-cast Zr/Mo microalloyed specimens with diameter of 1 mm. No crystalline Bragg peaks were detected on the broad diffraction hump, indicating an amorphous structure within the resolution of XRD. Fig. 1(b) displays the DSC curves of the microalloyed specimens exhibiting a distinct endothermic peak followed by an exothermic peak corresponding to the glass transition and crystallization, respectively. It can be seen that as compared to the La57.5Al32.5C10 BMG the minor addition of element Zr/Mo does not significantly change the thermal behavior [17]. In order to further ensure the fully amorphous nature of the specimens, the detailed microstructure of the Zr-containing specimen was investigated by TEM. The homogeneous contrast in the bright-field image, the broad halo ring in the corresponding selected-area electron diffraction and the maze-like pattern in the HRTEM image (Fig. 1(c, d)) demonstrate that no phase separation and nanocrystalline exist. For comparison, engineering stress-strain curves of 1 mm diameter specimens with and without addition of element Zr/Mo are both presented in Fig. 2. Obviously, the base specimen (La57.5Al32.5C10) fails catastrophically without any plastic strain and the stress rapidly drops to zero. On the other hand, the 0.5 at.% Zr/Mo-containing specimens exhibit an apparent yielding plateau accompanied by numerous serrations which correspond to the intermittent shear-band operation, and the plateau of the Zr-containing specimen is greater than that of the Mocontaining specimen. Besides, after yielding there exists obvious
Fig. 2. Engineering stress-strain curves of the La57.5Al32.5C10 and La57Al32.5C10(Zr/Mo)0.5 BMGs.
increase in flow stress although the yielding strength is slightly decreased. When deformed about 4% and 1% for the Zr-containing and the Mo-containing specimens, respectively, the catastrophic failure does not occur, but instead the stress gradually decreases. Generally, the onset of the stress drop indicates the start of the specimen fracture, i.e., the formation of a crack [2]. Therefore, the Zr/Mo-containing specimen demonstrates a stable fracture process. 4. Discussion In order to explain the phenomenon, the fractured base specimen and incompletely fractured Zr-containing specimen were observed by SEM. It was found that both the specimens deform along the shear plane which deflects about 45° angle to the direction of the applied loading. However, the base specimen ruptures along the shear plane and breaks into pieces (Fig. 3 (a)). In contrast, the Zr-containing specimen is divided into two segments by the shear plane, and the segments move relatively along the shear plane (Fig. 3 (d)). Therefore, the Zr-
Fig. 1. XRD pattern (a), DSC curve (b), TEM bright-field image with selected area electron diffraction patterns (c) and high-resolution image (d) of the La57Al32.5C10(Zr/Mo)0.5 BMG.
Y. Cheng et al. / Journal of Non-Crystalline Solids 447 (2016) 55–58
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Fig. 3. Morphologies of lateral and fracture surface of the La57.5Al32.5C10 (a, b, c) and La57Al32.5C10Zr0.5 (d, e, f) BMGs.
containing specimen shows a stable fracture process. In addition, for the base specimen only limited shear bands are visible on the surface, while for the Zr-containing specimen with large plasticity numerous multiple and intersecting shear bands are formed. This is in agreement with the previous studies that the formation of profuse shear bands has a significant contribution to the plasticity of BMGs [20,21]. After the complete failure, the fracture morphology of the base specimen consists of veinlike patterns (Fig. 3 (b, c)), yet for the Zr-containing one the fracture morphology is constituted by dimples and a zigzag crack (Fig. 3 (e, f)). It has been reported that the fracture surface morphology is closely related to the velocity of the crack propagation, and the low crack propagation speed can generate smooth fracture surface [2,22]. In this work, it is reasonable to deduce that after the minor addition of Zr, the stable fracture process corresponds with the lower crack propagation speed, thus producing the smooth dimple-like pattern. A considerable amount of research suggests a close relationship between the elastic constants and the mechanical properties, and demonstrates that the BMG possessing low ratio of shear modulus to bulk modulus and high Poisson's ratio can exhibit outstanding plasticity [1, 10,12–14,23]. Accordingly, the positive effect of the microalloying on the plasticity is discussed in detail in terms of the change in elastic constants [10,12–14,23]. However, in this work, there is no pronounced difference in elastic constants between the base alloy and the Zr/Mocontaining alloy. To clarify the mechanism of enhanced plasticity, the La-Al, La-C and Al-C partial PDFs of the amorphous alloys with and without the addition of element Zr were analyzed via AIMD (Fig. 4). Although only 200 atoms are used, the bulk moduli of the La57.5Al32.5C10
and La57Al32.5C10Zr0.5 BMGs calculated by the simulation (41.2 and 40.1 GPa) are very close to the experimental values (43.2 and 42.1 GPa), which indicates the validity of the simulation. It can be seen that between the base alloy and the Zr-containing alloy the overall peak position, shape and intensity of the La-Al and La-C partial PDFs are almost identical, while the second maximum peak of Al-C partial PDF is split from two subpeaks into three subpeaks after the addition of Zr. It has been suggested that the splitting of the second maximum peak is derived from the unevenness of polyhedral connecting style [24,25]. Therefore, it is reasonable to infer that the added Zr/Mo may lead to the change in atomic bonding character. Recently, in order to underpin our understanding of the structural origin of mechanical properties in BMGs, the internal structure of BMGs is viewed as composed of clusters/superclusters where the atoms are tightly bonded and solvent-rich junctions with relatively loose bonding [26,27]. It is suggested that the strength and plasticity are mainly determined by the solvent elements [26–28]. From the experimental result, it should be expected that the base La57.5Al32.5C10 BMG consists of Al/C-centered clusters and La-rich junctions, and because the solvent element La exhibits a large negative heat of mixing with the solute elements Al and C (− 38 and − 116 kJ mol− 1) [18], some Al and C are likely to participate in the construction of the junction, thus weakening its compliant character. As a result, the strength of the base alloy is higher than that of other La-based BMGs, but it fails catastrophically without any plastic strain [29]. Considering that the junction may be relatively unstable compared to the clusters from a potential energy perspective [30,31] and the element Zr/Mo has a
Fig. 4. Simulative La-Al, La-C and Al-C partial PDFs of the La57.5Al32.5C10 and La57Al32·5C10Zr0.5 BMGs.
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large negative heat of mixing with the solute elements Al and C [18], the added Zr/Mo would be more prone to interact with the solute elements Al and C in the junctions, thus leaving behind weak and compliant junctions which tend to become preferential sites for shear transformation. In this scenario, the slightly decreased strength and significant enhancement of plasticity via the addition of Zr/Mo are ascribed to the change in configuration of the junction. On the other hand, considering that the Zr has more negative heat of mixing with Al and C (− 44 and −131 kJ mol−1) than that of Mo (−5 and −67 kJ mol−1) [18], the effect of the added Zr on weakening the junction is more significant leading to the larger plasticity. 5. Conclusions Extensive studies indicate that the mechanical properties of BMGs are mainly determined by the solvent elements [26–28]. Based on the model that the structure of BMGs is viewed as composed of solute-centered clusters and solvent-rich junctions and the affinity among constituent elements, we infer that the brittleness of La-Al-C BMG stems from the less compliant solvent-rich junctions, and after the addition of Zr/ Mo the junction becomes weak and compliant to serve as preferential sites for shear transformation, which facilitates the improvement of plasticity. Acknowledgements This work was supported by the Shanxi Provincial Science and Technology Major Project (Grant Nos. 322469 and 322402). References [1] C. Schuh, T. Hufnagel, U. Ramamurty, Mechanical behavior of amorphous alloys, Acta Mater. 55 (2007) 4067–4109. [2] B.A. Sun, J. Tan, S. Pauly, U. Kuhn, J. Eckert, Stable fracture of a malleable Zr-based bulk metallic glass, J. Appl. Phys. 112 (2012) 103533. [3] A.J. Cao, Y.Q. Cheng, E. Ma, Structural processes that initiate shear localization in metallic glass, Acta Mater. 57 (2009) 5146–5155. [4] A.L. Greer, Y.Q. Cheng, E. Ma, Shear bands in metallic glasses, Mater. Sci. Eng. R 74 (2013) 71–132. [5] O. Gendelman, A. Joy, P. Mishra, I. Procaccia, K. Samwer, On the effect of microalloying on the mechanical properties of metallic glasses, Acta Mater. 63 (2014) 209–215. [6] C.C. Yuan, J.F. Xiang, X.K. Xi, W.H. Wang, NMR signature of evolution of ductile-tobrittle transition in bulk metallic glasses, Phys. Rev. Lett. 107 (2011) 236403. [7] Y.J. Huang, J.C. Khong, T. Connolley, J. Mi, In situ study of the evolution of atomic strain of bulk metallic glass and its effects on shear band formation, Scr. Mater. 69 (2013) 207–210.
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