Effect of Ta particles on the fracture behavior of notched bulk metallic glass composites

Effect of Ta particles on the fracture behavior of notched bulk metallic glass composites

Intermetallics 106 (2019) 1–6 Contents lists available at ScienceDirect Intermetallics journal homepage: www.elsevier.com/locate/intermet Effect of ...

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Intermetallics 106 (2019) 1–6

Contents lists available at ScienceDirect

Intermetallics journal homepage: www.elsevier.com/locate/intermet

Effect of Ta particles on the fracture behavior of notched bulk metallic glass composites

T

J. Pan∗, Y. Lin, J. Zhang, W. Huang, Y. Li∗∗ Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Bulk metallic glass composite Fracture strength Triaxial stress state Dimples

The existence of second phase usually promotes the formation of multiple shear bands under uniaxial loading, leading to the apparent strain hardening and enhanced plasticity in bulk metallic glasses (BMGs). However, the effect of second phase on the fracture behavior of BMGs has been rarely investigated when shear banding is suppressed. In this work, the effect of Ta particles on the fracture behavior of notched bulk metallic glass composites (BMGCs) was investigated, where shear banding is suppressed due to the introduction of high triaxial stress state. With an increase in the volume fraction of Ta particles from 0 to 3.3%, the fracture strength significantly decreases from 2.70 to 1.20 GPa. Different from the vein-like patterns caused by shear banding, the fracture surface of notched BMGCs displays numerous equiaxed dimples and micro-cracks, indicating that voids nucleation and coalescence govern the fracture process. Due to the relatively weak bonding of the interface and the mechanical incompatibility between Ta particles and amorphous matrix, voids tend to nucleate at the interface during tensile deformation, giving rise to a reduction in the fracture strength of BMGCs.

1. Introduction Owing to the lack of long-range order and dislocation-like defects, plastic deformation of bulk metallic glasses (BMGs) tends to localize in a narrow region called shear band where extreme strain softening occurs [1–3]. The rapid propagation of the dominant shear band leads to the brittle fracture without any tensile plasticity, even though extensive plastic deformation can occur inside the shear band itself. Usually, there are two typical characteristics of this shear banding governed fracture. One is that the fracture occurs in a manner of shearing with no obvious plastic deformation and a fracture angle of ∼45°. Another is that the flat fracture surface shows a vein-like pattern associated with instability in a liquid-like layer with several microns in thickness [4–6]. Introduction of the second ductile phase in the amorphous matrix is a universal approach to enhance the plasticity of monolithic BMGs [7–13]. The second phase not only could act as the obstacles to impede the rapid propagation of shear bands, but also act as the precursors for the multiplication of shear bands, leading to the relatively homogeneous deformation and large plasticity. Hofmann et al. revealed that the large tensile elongation and obvious necking can be achieved in Zrand Ti-based bulk metallic glass composites (BMGCs) containing high volume fraction of body-centered cubic (bcc) dendrites through tailoring the alloy compositions [7,8]. Zhu et al. also reported that the Ta∗

particulate reinforced Zr–Ni–Cu–Al–Ta BMGC possesses a desirable plasticity of ∼23% under compression and a favorable elongation of ∼1.8% under tension without any sacrifice of strength [9]. Although the macroscopic plasticity of metallic glasses can be improved by tailoring the distributions of second phase and shear bands [7,11–13], the deformation mechanism governed by shear banding remains unchanged. Recently, the fracture resistance of BMGs was proposed to arise from a competition between shear banding and cavitation/voids nucleation [14–16]. The process of shear banding is widely recognized [17], but limited knowledge is known about the cavitation/voids nucleation governed deformation process of BMGs. Nano-scale cavitation in shear bands was reported in the previous work, which is usually associated with brittle fracture [16,18,19]. This is distinctly different from ductile crystalline metals wherein micro-scale cavities/voids nucleate and coalesce through necking, which mandates substantial energy dissipation and ductile fracture. Generally, the fracture behavior of metallic glasses is similar to that of glasses rather than metals, even the bonding is of primarily metallic character [2]. By suppressing shear banding in the notched BMGs, high strength, excellent ductility, as well as metal-like ductile facture were revealed in our previous work [15,20]. On this condition, the nucleation and subsequent growth and coalescence of cavies/voids dominate the initial

Corresponding author. Corresponding author. E-mail addresses: [email protected] (J. Pan), [email protected] (Y. Li).

∗∗

https://doi.org/10.1016/j.intermet.2018.12.005 Received 21 September 2018; Received in revised form 3 December 2018; Accepted 4 December 2018 0966-9795/ © 2018 Elsevier Ltd. All rights reserved.

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Fig. 1. XRD patterns of Zr62-xCu15.6Ni12·4Al10Tax (x = 4, 5, 6, 8) alloys with a diameter of 4 mm, which are labeled as Ta4, Ta5, Ta6, and Ta8, respectively.

failure process, showing the typical ductile fracture morphologies of cup-and-cone and micron-scale dimples. It is conjectured that voids nucleation in metallic glasses occurs at nano-scale since they are regarded as homogeneous in structure, resulting in the extra-high fracture and decohesion stresses. Unfortunately, this void nucleation process by decohesion in BMGs could not be observed, because the decohesion generally occurred at nano-scale weak zones. Considering the fact that cavity formation can be facilitated by the hard particles in engineering materials, it is natural to wonder what the fracture behavior of notched BMGs dispersed by a second phase is. In this study, we employed the notched Zr-based BMGCs dispersed by in situ Ta particles to investigate the effect of Ta particles on the

Fig. 3. (a) Volume fraction of Ta particles and (b) fracture strength of Ta4, Ta5, Ta6, and Ta8 alloys.

Fig. 2. Cross-sectional SEM images of as-cast (a) Ta4, (b) Ta5, (c) Ta6, and (d) Ta8 alloys, respectively. 2

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Fig. 4. Fracture morphologies of notched (a–c) Ta4, (d–f) Ta5, and (g–i) Ta8 alloys after tension, respectively.

fracture behavior in the absence of shear banding. It was found that voids nucleation and coalescence govern the fracture process in notched BMGCs, rather than shear banding. Compared with the monolithic BMG having a similar composition, the existence of Ta particles aids the nucleation and coalescence of voids, leading to a reduced fracture strength in the notched BMGCs.

temperature using an Instron 5982 instrument at a cross head speed of 0.03 mm/min. Fracture surface and the cross-sectional morphologies of notch region in the specimens after tension were examined by SEM to investigate the details of fracture process.

2. Experimental

Fig. 1 shows the XRD patterns of as-cast Zr62-xCu15.6Ni12·4Al10Tax (x = 4, 5, 6, 8) alloys. Ta4 alloy is fully amorphous, as indicated by a board diffraction hump with the absence of any detectable peaks. However, for Ta5, Ta6 and Ta8 alloys, the diffraction peaks corresponding to body-centered cubic (bcc) Ta appear which is superimposed over the amorphous maxima, implying that these alloys consist of two phases: bcc Ta and the amorphous matrix. The microstructures of Zr62-xCu15.6Ni12·4Al10Tax (x = 4, 5, 6, 8) alloys were also examined by SEM, as shown in Fig. 2. Ta4 alloy displays a featureless contrast (Fig. 2a), further confirming the monolithic amorphous structure. For Ta5 alloy, the particles with a size from several microns to 50 μm are homogeneously dispersed in the amorphous matrix, as shown in Fig. 2b. EDS analysis confirms that the second phase is pure Ta (not shown). With an increase in the Ta content, the size of Ta particles almost keeps unchanged, as shown in Fig. 2b–d. However, the volume fraction of Ta particles gradually increases from 0.4% in Ta5 alloy to 3.3% in Ta8

3. Results

Ingots with nominal compositions of Zr62-xCu15.6Ni12·4Al10Tax (x = 4, 5, 6, 8, labeled as Ta4, Ta5, Ta6 and Ta8, respectively) alloys were prepared by arc-melting the mixture of high-purity element metals Zr, Ta, Cu, Ni and Al in a Ti-gettered high-purity argon atmosphere. Rods with the diameter of 4 mm were produced using the copper mould injection casting method. The as-cast samples were characterized by Xray diffraction instrument (XRD; Bruker D2 phaser) with Cu-Kα, differential scanning calorimeter (DSC; TA Q2000) and scanning electron microscope (SEM; FEI Quanta 600 and Zeiss Supra 55). Compositions of the samples were determined by X-ray energy dispersive spectrometer (EDS) fixed on SEM. Notched tensile samples with the notch dimension (notch diameter d × notch height h) of 1.90 × 0.38 mm2 were produced by gentle grinding in a custom-made machine, followed by fine polishing. Quasi-static tensile tests were conducted at room 3

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Fig. 5. High magnification image of the dimple on fracture surface of notched Ta8 BMGC, as well as the corresponding Ta Lα map.

Fig. 6. Cross-section images of notched Ta8 alloy after fracture, where cracks initiate at the interface between Ta particles and amorphous matrix.

alloy, as shown in Fig. 3a. Subjected to tensile loading at room temperature, there is remarkable difference in fracture strength for the notched samples with various volume fractions of Ta particles, as shown in Fig. 3b. The notched Ta4 alloy shows an ultrahigh fracture strength of ∼2.70 GPa, almost 1.6 times larger than that of un-notched sample [9]. This value is also comparable to that of other notched monolithic BMGs [15,21–23]. However, the fracture strength significantly decreases with an increase in the volume fraction of Ta particles in the amorphous matrix. Ta5 alloy shows a fracture strength of 1.84 GPa even the volume fraction of Ta particle is only 0.4%. Ta8 alloy exhibits a fracture strength of only ∼1.20 GPa, which is even lower than that of the most un-notched Zrbased BMGs [6,24] and BMGCs [9,10,25]. To reveal the fracture process, the fracture morphologies of the notched Ta4, Ta5 and Ta8 alloys after tension were examined. Similar to the notched Zr64·13Cu15·75Ni10·12Al10 BMG [15,20], Ta4 alloy exhibits a rough fracture surface with cup-and-cone morphology (Fig. 4a), showing a typical ductile fracture. Numerous micro-cracks and equiaxed dimples can also be observed in Fig. 4b and c. The cracks are deep and long, indicating a stable crack propagation process before catastrophic failure. All these observations imply that the fracture of notched Ta4 alloy is dominated by the nucleation, subsequent stable growth and coalescence of cavies/voids. For the notched Ta5 and Ta8 alloys, they display the typical normal fracture with the relatively

rough fracture surface (Fig. 4d and g) compared with that of un-notched BMGs [4] or BMGCs [26]. There are plenty of micro-cracks and equiaxed dimples with the size of ∼17 μm on fracture surface (Fig. 4e, f, h and i), in contrast to the vein-like patterns caused by shear banding in BMGCs [25,27]. This phenomenon suggests that voids nucleation and coalescence also dominate the initial plastic failure process. However, the fracture surface of Ta5 and Ta8 alloys is smoother than that of Ta4 alloy. The fraction of dimples is much larger, and the cracks are shallow and short, suggesting an unstable crack propagation process. More interestingly, obvious protrusions can be observed at the center of each dimple in Ta5 and Ta8 alloys (Fig. 4f and i), which is also different from the relatively smooth dimples in the notched monolithic BMGs (Fig. 4c). Some particles are noticed to fracture at the plane normal to the tensile stress or debond the matrix, implying that the nucleation of micro-voids is likely through particle cracking or interfacial debonding with the matrix. In a particular dimple on the fracture surface of Ta8 alloy, as shown in Fig. 5, Ta Lα map was examined. Ignoring the effect of height, Ta element mainly concentrates at the center of dimple, matching well with the position of protrusions. As the center of dimple is the position of voids initiation or nucleation, thus, voids are reasonably deduced to nucleate at the interfaces between Ta particles and amorphous matrix. To more convincingly identify the position of the voids or cracks 4

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Fig. 7. Schematic diagram showing the deformation process of (a) un-notched BMG, (b) un-notched BMGC and (c) notched BMGC, respectively.

matrix would aid voids nucleation and coalescence (shown in Fig. 7c), which is mainly due to the following two reasons. (1) The interfacial strength between Ta particles and matrix should be much weaker than the atomic bonding in monolithic BMGs, resulting in a much lower decohesion stress in BMGC. (2) The incompatibility between Ta particles and amorphous matrix could cause stress concentration at the interface during deformation [36] (Young's modulus of Ta is 186 GPa, more than twice as that of amorphous matrix with the value of ∼81 GPa), which also facilitates the nucleation and coalescence of voids at the interface. Based on the analysis, it is speculated that the type and size of the particle, as well as the adhesion between particle and matrix would affect the fracture behavior, which needs to be further investigated in the future work.

initiation, the cross-sectional images of the Ta8 alloy after failure were investigated (Fig. 6). There is a typical cup-and-cone morphology with highly serrated crack plane, which is completely different from that associated with conventional shear banding. Besides the main crack, numerous micro-cracks can be observed in the notch region near the fracture surface. Almost all cracks are located around Ta particles, suggesting that the nucleation of voids/micro-cracks should be aided by the second phase. All these experimental observations confirm that fracture process of Ta8 specimen is governed by cavitation/void nucleation and growth rather than shear banding. Voids indeed nucleate via interface decohesion between Ta particles and amorphous matrix, and then link up with increasing stress/strain, leading to the final fracture [15]. In the monolithic BMGs, voids and cracks nucleate at the center of the notch, and then propagate toward the outer rim, as evidenced by the formation of many micro-cracks at the notch center of the surviving notch. To reveal initiation position of the BMGCs, the doubled notched Ta8 alloy was also tested. Unfortunately, there are no obvious cracks or voids can be observed, either in the notch center or notch root. It is speculated that there is no stable propagation process in the BMGCs, and the crack propagates rapidly once it nucleates, leaving no time to form micro-cracks in the surviving notch.

5. Conclusions In conclusion, the effect of second phase on the fracture behavior of the notched BMGCs dispersed by in situ Ta particles was studied. It is found that the fracture strength decreases significantly with the increase in volume fraction of Ta particles, from 2.70 GPa in the monoBMG to 1.20 GPa in the lithic Zr58Cu15·6Ni12·4Al10Ta4 Zr54Cu15·6Ni12·4Al10Ta8 alloy with 3.3% Ta particles. In contrast to the un-notched BMGs and BMGCs where shear banding dominates the fracture process, and the notched monolithic BMGs where the cavitation occurs at the nano-scale weak zones, the decohesion process occurs at the interface between Ta particles and amorphous matrix. Due to the relatively weak bonding between Ta particles and matrix, as well as the mechanical incompatibility, the notched BMGC exhibits a reduced fracture strength, comparing with that of monolithic BMGs with similar composition. The present study provides a new insight into the fracture behavior of BMGs with dispersed Ta particles by suppressing shear banding where voids nucleation dominates fracture process, which is of great importance to understand the deformation mechanism of metallic glasses.

4. Discussions In general, the deformation of BMGs at room temperature is carried out by shear banding, and the highly localization of shear banding leads to the brittle fracture with limited plasticity and vein-like patterns on the fracture surface, as displayed in Fig. 7a. By introducing a second phase in the amorphous matrix, such as Ta particles, the rapid propagation of main shear band could be inhibited, and multiple shear bands will be promoted, giving rise to the improved deformability (Fig. 7b) [9,10,25,28–32]. On the whole, shear banding still governs the deformation and fracture process of un-notched BMGs and BMGCs, as evidenced by the vein-like patterns on the fracture surface [25,28,33,34]. However, shear banding can be effectively suppressed in the notched BMGs and BMGCs under the triaxial stress state [35], and cavitation transits to the dominant deformation process at the initial stage of failure [15,20]. For the notched monolithic BMGs, decohesion occurs at nano-scale soft regions. This makes the decohesion difficult, leading to the high fracture strength and decohesion stress. For the notched BMGCs, the existence of interface between Ta particles and amorphous

Acknowledgments This work was financially supported by National Natural Science Foundation of China under Grant Nos.51871217, 51401220 and 51471165. J. Pan acknowledges the IMR foundation for “Young Merit Scholars”. 5

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