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Strain rate sensitivity and deformation behavior in a Ti-based bulk metallic glass composite
Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx
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Strain rate sensitivity and deformation behavior in a Ti-based bulk metallic glass composite Haimin Zhai, Yuhao Xu, Yin Du, Haifeng Wang⁎, Feng Liu⁎ State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an, Shaanxi 710072, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Bulk metallic glass composite Shear bands toughness Strain rate sensitivity Shear banding behavior Ti-based alloy
The Ti45Zr25Nb6Sn2Cu5Be17 bulk metallic glass composite was investigated by uniaxial quasi-static compression and tension tests under strain rates from 1 × 10− 4 s− 1 to 1 × 10− 2 s− 1 at the room temperature. The deformation behavior was characterized by analyzing the lateral and fracture surfaces. With the increase of strain rate, the yielding stress and plasticity decrease significantly, indicating negative strain rate sensitivity. At higher strain rates, the thermal softening effect is more prominent and the yielding stress is lower, while lower strain rates, the shear band toughness of the dendrites is higher than that of the glass matrix. Then the dislocations have sufficient time to be generated, multiplied and aggregated, thereby the shear banding can be retarded and hindered efficiently by dendrites. Furthermore, the primary shear bands are able to block more effectively the secondary ones, which results in more uniform distribution of plastic strain and thereby higher plasticity. The current work shows that the coordination of the shear band toughness of the dendrites and the glass matrix and the stable extension of the shear bands are of primary importance for obtaining excellent mechanical properties.
1. Introduction The remarkable properties of bulk metallic glasses (BMGs), such as ultrahigh strength, excellent hardness and large elastic limit, enable them to be potential candidates of structural materials in engineering [1–4]. Due to the highly localized plastic deformation within the shear bands, the monolithic BMGs, however, usually exhibit strain softening and poor plasticity at the room temperature, which seriously limits their potential applications [5–7]. To alleviate this deficiency, substantial studies have been carried out to enhance the plasticity: (1) designing BMGs by modulating their intrinsic properties, e.g. with a high Poisson's ratio [8,9]; (2) introducing nano-scale structural heterogeneity [10] or phase separation [11,12]; (3) inducing randomly distributed free volumes by pre-deformation [13–15]; (4) preparing ex-situ [16] or in-situ [17–23] BMG composites by introducing a second crystalline phase. Previous studies have shown that good plasticity could be achieved for in-situ formed BMG composites with a ductile crystalline phase such as the bcc (body-centered cubic) β dendrite phase [18–20] and the CuZr B2 phase [21–23]. The excellent plasticity is attributed to the ductile crystalline phase that acts as an obstacle to bifurcate or impede shear banding. The propagation of shear bands may bypass the barrier or be arrested by the crystalline phase. The intersection and branching of shear bands then lead to a relative
⁎
uniform distribution of plastic strain [18]. In fact, it is highly possible to show the capacity of materials to resist necking (that is closely associated with the plasticity) by evaluating the strain rate sensitivity (SRS) [23,24]. For crystalline materials, their SRS is normally positive, as is determined by the deformation modes of dislocations, twins and phase transformations [25]. In contrast, the deformation mode of BMGs is the initiation and propagation of shear bands, i.e. a shear-band based deformation mode [6,24]. Due to the different test conditions, different alloy compositions and microstructures, BMGs exhibit different deformation behaviors at different deformation strain rates and thus different SRSs [26]. All kinds of SRSs have been reported, i.e. positive [27], zero [28], and negative [29,30]. For the in-situ BMG composites, the rapid extension of shear bands can be effectively restrained by the dendrites strengthened by dislocations [18,20]. Therefore, it is generally believed that the SRS of BMG composites depends on the competition between the shear band based deformation mode in glass matrix and the dislocation based deformation mode in dendrites [31]. In addition, the extension of shear bands in BMG composites may be not only hindered and restricted by the dendrites, but also affected by the primary shear bands, that is to say, the extension mode of shear bands could be multiple [32]. In this sense, the SRS of BMG composites should be different from those of the BMGs and crystalline materials and should depend on the coordination
Corresponding authors. E-mail addresses: [email protected] (H. Wang), [email protected] (F. Liu).
http://dx.doi.org/10.1016/j.jnoncrysol.2017.05.032 Received 17 February 2017; Received in revised form 16 May 2017; Accepted 20 May 2017 0022-3093/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Zhai, H., Journal of Non-Crystalline Solids (2017), http://dx.doi.org/10.1016/j.jnoncrysol.2017.05.032
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645.0 K and 735.5 K, respectively. The rather large supercooled liquid region (ΔT = Tx1 − Tg) 90.5 K indicates that the current BMG composite has an excellent glass forming ability. A TEM bright field (BF) image of the BMG composite is shown Fig. 2a. One can see that the dendrites in white distribute homogeneously within the glass matrix in gray. The inserting selected area electron diffraction (SAED) pattern obtained from the dendrites represents the [001] zone axis of the bcc β-Ti phase while that obtained from the glass matrix is of typical diffuse holes of amorphous structure; see the insets in Fig. 2a. The HRTEM image (Fig. 2b) shows a regular arrangement of the lattice patterns in the dendrites without apparent lattice defects (e.g. dislocations), indicating that the as-cast sample is almost free of deformation [34].
between shear band toughness of the dendrites and the glass matrix, and on the extension mode of shear bands. In practical applications, a considerable change of loading rates caused by shaking and/or impacting may occur from time to time and results in variation of deformation modes in the dendrites and the glass matrix. For most BMGs, the effect of SRS on the mechanical properties might not be very significant considering their almost negligible plasticity at the loading rates in service. For BMG composites, however, the effect of SRS cannot be neglected anymore due to their complex deformation modes, such as the dislocation- and the shear-bands-based deformation mode. The studies of SRS not only can simulate the actual service process under different conditions but also may help to understand the deformation mechanisms of BMG composites. In view of this, it is noteworthy to study in detail the SRS and deformation behavior of BMG composites at different strain rates. In this paper, the Ti45Zr25Nb6Sn2Cu5Be17 (Sn2) BMG composite [33] was studied by the uniaxial quasi-static compressive and tensile tests under the strain rates from 1 × 10− 4 s− 1 to 1 × 10− 2 s− 1. On the basis of the stress-strain response at the room temperature, the SRS was analyzed to understand the deformation behaviors and the deformation mechanisms were investigated in detail by analyzing the lateral and fracture surfaces. Such a study is helpful for the applications of BMG composites in engineering.
3.2. Mechanical properties Typical compressive and tensile engineering stress-strain curves under the strain rates from 1 × 10− 4 s− 1 to 1 × 10− 2 s− 1 are shown in Fig. 3a and b, respectively. The detailed compression and tensile measurements are summarized in Table 1. It can be found that the mechanical properties of Sn2 BMG composite are significantly dependent on the loading strain rate under both compressive and tensile mode. One can see that as the strain rate increases, the compressive yielding strength (compressive fracture strength) decreases from about 1405 MPa (1929 MPa) to 1316 MPa (1706 MPa), and the compressive fracture strain also decreasing from about 30.8% to 15.1%. Similarly, as the strain rate increases, the tensile yield strength (ultimate tensile strength) decreases from about 927 MPa (1464 MPa) to 821 MPa (982 MPa), while the tensile plastic strain decreasing from 12.1% to 2.2%. All the experimental results indicating that the current Sn2 BMG composite has a negative SRS; see the discussion section for detail.
2. Experimental Ingots with a nominal composition of Ti45Zr25Nb6Sn2Cu5Be17 (at.%, denoted as Sn2 BMG composite) are prepared by arc-melting a mixture of constituent elements (Ti, Zr, Cu, Nb, Sn and Be metals) with purity better than 99.9% in a Ti-gettered Ar atmosphere. A detail of the preparation procedure is shown in our recent work [33]. Plate samples (5 × 20 × 60 mm) are prepared by casting into a water-cooled copper mold. The samples are characterized by X-ray diffraction (XRD: Bruker D8 with Co Kα radiation), scanning electron microscopy (SEM: TESCAN VEGA 3 LMU), and high-resolution transmission electron microscopy (HRTEM: Tecnai G2 F30, 300 kV). The TEM samples were first prepared by mechanical grinding to a 30 μm thick plate. In order to avoid the samples deformation caused by the pitting, the samples were twin-jet electropolished using a solution mixed in the ratio HNO3:CH4O = 1:3, and then the TEM samples were thinning again by ion milling. Thermal analyses are conducted by differential scanning calorimetry (DSC: PerkinElmer 8500) under the protection of high purity Ar gas at a constant heating rate of 40 K min− 1. The compressive samples (4 × 4 × 8 mm) and the tensile samples (1 × 2 × 10 mm) are machined from the as-cast plates, and then carefully polished. The roomtemperature quasi-static compressive and tensile tests are carried out on a testing machine (INSTRON 3382) under the strain rates from 1 × 10− 4 s− 1 to 1 × 10− 2 s− 1.
3.3. Fractography SEM images of the compressive and tensile deformation fractured BMG composite under the strain rates of 1 × 10− 4 s− 1 and 1 × 10− 2 s− 1 are shown in Figs. 4 and 5, respectively. SEM images of the compressive fractured BMG composite under the strain rates of 1 × 10− 4 s− 1 and 1 × 10− 2 s− 1 are shown in Fig. 4. At high strain rates (e.g. 1 × 10− 2 s− 1), a large number of different orientated shear bands (white arrows) and cracks (black arrows) are distributed on the lateral side near the fracture surface (Fig. 4a), indicating that the sample suffered serious distortion (blue cycles). In contrast, different oriented shear bands (white arrows in Fig. 4e) are more homogeneously distributed on the lateral surface at lower strain rates (e.g. 1 × 10− 4 s− 1), indicating that the sample should have excellent plasticity. Compared with the case of high strain rate 1 × 10− 2 s− 1 in which the sample fails at an inclination angle of 44.7° with respect to the loading axis under-static compression, a lower angle of 44.3° is found for the case of low strain rate 1 × 10− 4 s− 1; see the insets in Fig. 4a and b. The deviation of the maximum shear stress plane may give rise to a significant effect of the normal stress on the fracture behavior [35]. From the fracture surfaces at high and low strain rates (Fig. 4b and f), three different fracture patterns, i.e. droplet patterns (white ellipse), smooth patterns (red arrows) and shear steps (white arrows), can be found. The presence of a large number of droplet patterns is mainly due to the elastic energy stored upon the deformation process, which is released in a very short time when fracture happens and meanwhile the glass matrix is re-melted by the released heat [36]. Compared with the fracture surface for the high strain rate 1 × 10− 2 s− 1 that is mainly covered by melting layers (Fig. 4b), less melting layers can be found in the case of low strain rate 1 × 10− 4 s− 1 (Fig. 4f). Such a difference in the fracture surfaces implies that the temperature increase in the case of high strain rates should be higher than that in the case of low stain rates. Fig. 4c and g show the enlarged droplet patterns according to which one can find that the fracture
3. Results 3.1. Materials characterization The XRD pattern of the as-cast sample is shown Fig. 1a. One can see that several sharp Bragg diffraction peaks are superimposed on a broad hump. Similar to the other Ti-based BMG composites reinforced with bcc ductile dendrites [18,20], the sharp crystalline peaks correspond to the bcc β-Ti phase. For the as-cast sample (the inset in Fig. 1a), the dendritic phase in light gray is homogenously embedded in the glass matrix in dark gray. The volume fraction of dendrites is approximately 52 ± 2% and their average length is about 25 ± 4 μm. DSC measurements were performed to evaluate the Sn2 BMG composite (Fig. 1b). The DSC curve shows that a significant glass transition is followed by a crystallization process, confirming again the presence of amorphous phase. The glass-transition temperature (Tg) and onset crystallization temperature (Tx1) of the Sn2 BMG composite were measured to be 2
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Fig. 1. (a) XRD patterns and the inset SEM image, (b) DSC curve (heating rate: 40 K min− 1) of the as-cast Sn2 BMG composite.
Fig. 2. (a) TEM BF image and the insects the corresponding SAED patterns of glass matrix and dendrites, (b) HRTEM image for the interface between the glass matrix and dendrites in the as-cast Sn2 BMG composite.
Fig. 3. The quasi-static engineering stress-strain curves of the Sn2 BMG composite under different strain rates: (a) compression, (b) tension.
effectively stabilize the buckling of shear bands. Similar to the compressive deformation fracture surface, the tensile deformation fracture surface at high and low strain rates also exhibit significantly different morphologies, as shown in Fig. 5a and e. For the case of the high strain rate 1 × 10− 2 s− 1, the fracture surface has a brittle fracture features (“flat” cleavage-like), as shown in Fig. 5b. However, the magnification image (block region “c” in Fig. 5b) indicates that the fracture surface is not atomically flat, but has dendrite-like features on it, as shown in Fig. 5b. This result indicates that the dendrite does not effectively hinder the propagation of the shear bands and the subsequent crack [38]. Besides, due to the instantaneous increase in temperature at final fracture (adiabatic
surface in the case of high strain rates is much coarser than that in the case of low strain rates, indicating that more severe adiabatic shear banding occurs during deformation. In this sense, it might be reasonable to speculate that the thermal effect is more significant at high strain rates, thus leading to more shear softening [35] and smaller critical shear stress. By enlarging the rough patterns, some “rib-like” layers (marked by white arrows in Fig. 4d and h) can be found. The “rib-like” layers are considered to be generated via the stepwise sliding process during the initial evolution of large shear bands and induced by the absorbing and blocking functions that the dendrites worked on the shear bands [37]. Furthermore, more stepwise sliding appears on the fracture surface for low strain rates, indicating that the dendrites can 3
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[24,25] is introduced here:
Table 1 The compressive yielding strength (σy), compressive fracture strength (σf), compressive plastic strain (εf), tensile yielding strength (σy), tensile ultimate strength (σu), and the tensile ultimate plastic strain (εu) of the Sn2 BMG composite under different strain rates. The standard errors for the six different measurements are shown.
Compression
Tension
Strain rate (s− 1)
σy (MPa)
1 × 10− 4 5 × 10− 4 1 × 10− 3 5 × 10− 3 1 × 10− 2
1405 1387 1353 1334 1316
Strain rate (s− 1) 1 × 10− 4 5 × 10− 4 1 × 10− 3 5 × 10− 3 1 × 10− 2
σy (MPa) 927 ± 40 903 ± 25 880 ± 20 847 ± 35 821 ± 30
± ± ± ± ±
σf (MPa) 35 40 25 30 25
1929 1810 1786 1742 1706
± ± ± ± ±
m=
σu (MPa) 1464 ± 45 1328 ± 20 1223 ± 30 1068 ± 20 982 ± 30
30.8 25.2 20.8 18.1 15.1
± ± ± ± ±
(1)
where σ (MPa) is the flow stress at a selected strain and deformation temperature, dε/dt (s− 1) is the strain rate and ε is the strain. Based on the flow stress in the compressive and tensile stress-strain curves of the current Sn2 BMG composite, the SRS exponent m can be calculated at a constant strain using Eq. (1). Fig. 6a and b illustrates the SRS exponent m of the Sn2 BMG composite obtained from compression and tensile test data. It can be seen that the negative SRS is shown in both compression (m ≈ −0.027) and tensile deformations (m ≈ − 0.036) for the strain rates from 1 × 10− 4 s− 1 to 1 × 10− 2 s− 1. From the above experimental results (Figs. 4 and 5), it can be seen that the shear bands density and the fracture morphology have obvious differences in a wide loading strain rate range, indicating that the SRS of the Sn2 BMG composite is strongly affected by shear banding behavior. Upon loading deformation, the stress concentration will be generated in the interfaces between the dendrite and the glass matrix because of the differences in the intrinsic property and yield strength of the two-phases [18,39]. The soft dendrite can absorb most of elastic energy, and then relieve the stress concentration near the interface between the glass matrix and dendrite, thus delaying the formation of shear bands and cracks at the interface. As a result, more dislocations will be accumulated around the interface in order to accommodate the mechanical incompatibility across the interface, which produces the observed extra strain hardening, as shown in Fig. 7a (compression) and Fig. 8a (tension). For the BMG materials, when the applied stress increases largely enough to reach or exceed the atomic cohesive strength, the equilibrium of stress field nearby the tip would be destroyed; as a consequence, the shear bands will initiated and then cause a catastrophic fracture of the samples. However, in the BMG composite, the propagation of the shear bands can be hindered and stabilized by the dendrite, which results in a stable plastic flow in the BMG composites. Hence, the shear banding behavior of the BMG composite will be determined by the mismatch between the shear band toughness of the dendrite and the glass matrix [31]. Note that, it has been proposed that the shear band toughness can be used to quantitatively measure the inherent resistance capability of materials to the propagation of shear bands [40]. At high strain rates, the shear band toughness of the dendrites is smaller than that of the glass matrix, so that the rapid extension of shear
εf (%) 50 25 35 25 40
∂ln σ ∂ln dε dt
1.5 1.6 1.2 1.5 1.3
εu (%) 12.1 ± 1.8 9.8 ± 1.4 6.7 ± 2.1 4.1 ± 2.5 2.2 ± 1.8
heating), the magnification image of block region “d” in Fig. 5b also exhibits the microscale vein patterns. For the case of the low strain rate 1 × 10− 4 s− 1, one can see that profuse and dense shear bands are distributed on the lateral surface of tensile fracture sample, indicating that severe plastic deformation occurs in the glass matrix. Different from the fracture morphology at the high strain rate (Fig. 5f and g), the fracture surface at the low strain rate exhibits abundant ligament, indicating that a ductile fracture appear at the low strain rate. Moreover, one can see that some voids (marked by red arrows) which are a result of the pull-out of dendrite from matrix appears in the matrix, as shown in Fig. 5h. Such a pull-out of dendrites will enhance the toughness of the BMG composite, which is a possible reason for the significant work-hardening capability in the Sn2 BMG composite. 4. Discussions 4.1. Stress-strain responses As shown in Fig. 3, the current Sn2 BMG composite exhibits different compressive and tensile plasticity and yielding strength at different strain rates. Furthermore, the ductility and the strength decreases remarkably with the increase of strain rate; see Table 1. To show further the effect of strain rate on the deformation behavior and mechanical properties of the Sn2 BMG composite, the SRS exponent m
Fig. 4. SEM images of the fractography for the compressive fracture samples of the Sn2 BMG composite. At high strain rate (1 × 10− 2 s− 1): (a) lateral surface and the insets are corresponding to the photo of fracture samples, (b) fracture surface, (c) melting droplet patterns regions, (d) shear steps regions; at low strain rate (1 × 10− 4 s− 1): (e) lateral surface and the insets are corresponding to the image of fracture samples, (f) fracture surface, (g) melting droplet patterns regions, (h) shear steps regions. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5. SEM images of the fractography for the tensile fracture samples of the Sn2 BMG composite. At high strain rate (1 × 10− 2 s− 1): (a) lateral surface, (b) fracture surface, (c) melting droplet patterns regions, (d) vein patterns regions; At low strain rate (1 × 10− 4 s− 1): (e) lateral surface, (f) fracture surface, (g) ductile fracture features in the ligament, (h) holes created by dendrite pull-out.
Fig. 6. Determination of the exponent of strain rate sensitivity in the Sn2 BMG composite: (a) compression, (b) tension.
plastic deformation. According to many previous studies [18,20,39,41], the temperature increase within the shear bands leads to considerable thermal softening in the BMGs or BMG composites. Therefore, the yielding strength of the Sn2 BMG composite decreases with the increases of the strain rate and the current BMG composite exhibits negative SRS during the quasi-static compression and tension. Generally, the requirement of SRS is not the same for different applications of materials. For structural materials, a positive SRS corresponds to postpone of thinning and necking, i.e. a good service performance. In this sense, the current Sn2 BMG composite should not be a good candidate of structural materials if the loading rates change considerably in service. One should however keep in mind that in the forging, rolling, extrusion and other processing, the BMG composites with lower SRS can be processed under higher processing rate [25], indicating that the current Sn2 BMG composite with a negative SRS may take the advantage of reducing the energy consumption and increasing the processing efficiency.
bands cannot be hindered by the dendrite; see the red dotted line in Fig. 7b. However, since the samples show brittle fracture under tensile deformation, no shear bands were found in TEM image, but with only a small amount of dislocations, as shown in Fig. 8a. In contrast, at low strain rates, the shear band toughness of the dendrites is higher than that of the glass matrix. The dendrite therefore has sufficient time to generate a large number of dislocations, which can effectively alleviate the stress concentration at the interface and delay the shear softening caused by the initiation and extension of shear bands [31]. As a result of the multiplication and subsequent aggregation of dislocations at the interface, the initiation and propagation of shear bands can be retarded and hindered; see the red dotted line in Fig. 7c. Even within the dendrite, the shear bands can also be impeded, as shown by the points “B” to “G” in Fig. 7d. Similarly, in the tensile deformation of the sample, one can see that the extension of the shear band is also blocked by the two-phase interface, and ultimately stop within the dendrite. In other words, whether it is tensile or compression deformation, the dendrites not only can retard the rapid propagation of shear bands by the dislocation strengthening effect [18,20,41] but also may release efficiently the deformation energy by enlarging the width of shear bands. Furthermore, the thermal softening effect was suggested to be usually responsible for the reduction in the critical shear stress [31,35]. As shown in Fig. 4c and g (compression) and Fig. 5c and f (tension), the droplet patterns of the sample under high strain rates has much more rough molten traces than those under low strain rates, indicating a larger temperature increase within the shear bands during
4.2. Deformation behavior Usually, it has been recognized that the profuse multiplication shear bands are mainly associated with the large plasticity in the BMG composite [33]. For the Sn2 BMG composite, however, the negative SRS with the strain rate should be related not only to the shear band toughness of the dendrites and the glass matrix, but also to the propagation modes of shear bands. As shown in Fig. 9a and c (compression), it can be seen that 5
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Fig. 7. TEM BF images of the current Sn2 BMG composite for compressive deformation samples. At the strain rate of 1 × 10− 2 s− 1: (a) multiple dislocations in the deformed dendrite, (b) a shear band passing through the dendrite and forming a shear step at the interface between the dendrite and the glass matrix; at the strain rate of 1 × 10− 4 s− 1: (c) a shear band passing through the dendrite I and stopped by the dendrite II, (d) the extended mode of one shear band within the dendrite. The propagation direction of this shear band is not only changed by five times due to the barrier effect of dislocations in the dendrite, but also its width is increased from 45 nm in the glass matrix to 150 nm in the dendrite. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. TEM BF images of the current Sn2 BMG composite for tensile deformation samples. (a) At the strain rate of 1 × 10− 2 s− 1; (b) at the strain rate of 1 × 1042 s− 1. The propagation direction of this shear band is not only changed by the interface of the two-phase, but also its width is increased from 25 nm in the glass matrix to 78 nm in the dendrite.
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Fig. 9. SEM images of the Sn2 BMG composite after compressive deformation samples. At the strain rate of 1 × 10− 2 s− 1: (a) the primary shear bands are confined locally as marked by the blue circles, and a lot of cracks appear on the lateral surface, (b) the longer secondary shear bands interact with the primary shear bands and results in minor shear steps; At the strain rate of 1 × 10− 4 s− 1: (c) the primary shear bands are confined locally as marked by the blue circles, and the secondary shear bands are affected by the primary shear bands, (d) the enlarged images of Fig. 7b shows clearly that the extension mode of shear bands, i.e. the secondary shear bands are hindered by the primary ones. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
show obviously a decrease tendency of the local shear offsets, i.e. the width decreasing from 2.09 μm to 0.83 μm with the increase of the distance from the original site for the interaction between the primary and secondary shear bands. Similarly, as shown in Fig. 10a and c (tension), multi-directional shear bands also distributed on the lateral surface in the cases of both high and low strain rates. For the case of the high strain rate, it can be seen that the shear bands have not only cut the dendrites, but has also evolved into cracks, as shown in Fig. 10b. This result indicates that the primary shear bands are not hindered by the dendrite so as to cause the unstable propagation, then resulting in brittle fracture. Comparing with the case of high strain rate (1 × 10− 2 s− 1), the agminated primary shear bands (e.g. marked by blue ellipses) in the case of low strain rate (1 × 10− 4 s− 1) occupy more regions on the lateral surface; see Fig. 10c. Besides, it is also shown that dense intersections between the primary shear bands demonstrate secondary shear bands with spacing of 2–3 μm, suggesting that the sample undergoes more severe plastic deformation at the low strain rate; see Fig. 10c. Moreover, it can be seen that the shear bands cuts through the dendritic I and is then obstructed by the dendrites II, III, and IV, and then been bifurcated both in the two-phase interface or the dendrites and eventually stops in the glass matrix or in the dendrite V; see Fig. 10d. In other words, the primary shear band is able to block effectively the secondary shear band at low strain rates, and then shear strain with deformation is accommodated by the formation and stable extension of multiple shear bands, which leads to the enhanced macroscopic compressive and tensile plasticity; see Fig. 3 and Table 1. Moreover, the dislocations in the high temperature region will usually annihilate to lead a smooth interface. Nevertheless, after cooling to low temperature, the elastic residual stresses will be generated in the interface due to the difference the thermal expansion
intensive and multi-directional primary and secondary shear bands have a distinctly different distribution pattern on the lateral surface in the cases of high and low strain rates. In the early stages of deformation, the primary shear bands cannot easily and quickly propagate through the dendrites due to the blocking effect of dendrites (e.g. dislocations strengthening). Hence, almost all of those parallel primary shear bands are confined to local regions (e.g. marked by blue ellipses). Compared with the case of high strain rates, the agminated primary shear bands occupy large regions and their spacing is smaller for low strain rates (e.g. 5–9 μm for 1 × 10− 2 s− 1 vs 4–6 μm for 1 × 10− 4 s− 1). As the deformation proceeds, the plastic strain will not be able to accommodate continuously by the primary shear bands in glass matrix and the dislocations in dendrites. In this case, a large number of secondary shear bands with an average length in hundred microns are generated in the glass matrix to dissipate the deformation energy; see the red dashed lines in Fig. 9b and c. The parallel distribution of the secondary shear bands with a spacing of about 15–30 μm is similar to that of the primary shear bands, but with a different direction. When the secondary shear bands pass through the interface between the dendrites and the glass matrix or cut through small dendrite arms, their direction of propagation can be significantly altered, which leads to shear steps at the interface [34,41]; see Fig. 7b. In contrast to other BMG systems [42], when the longer secondary shear bands pass through the primary shear bands, they were blocked by several times. At high strain rates, the secondary shear bands traverse rapidly the primary shear bands and generate only few shear offsets (white arrows), indicating that the secondary shear bands cannot be effectively blocked. In contrast, the propagation direction of secondary shear bands is gradually changed at low strain rates, as shown in Fig. 9d. And meanwhile, the secondary shear band generates a new shear offset (white arrows) at each intersection point. The measured widths of the shear steps in Fig. 9d 7
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Fig. 10. SEM images of the Sn2 BMG composite after tensile deformation samples. At the strain rate of 1 × 10− 2 s− 1: (a) the primary shear bands are confined locally as marked by the blue circles, (b) a lot of cracks appear and interact with the primary shear bands and results in minor shear steps; At the strain rate of 1 × 10− 4 s− 1: (c) the primary shear bands are confined locally as marked by the blue circles, and the secondary shear bands are affected by the primary shear bands, (d) the extension mode of shear bands, i.e. the secondary shear bands are hindered or stopped by the dendrites. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
5. Conclusions
coefficient between the two-phases. Although the interface between dendrite and glass matrix has a smooth boundary (Fig. 2b) and the dendrite shows a regular arrangement of lattice patterns without obvious lattice defects (e.g. dislocations) [43], the residual stresses should exist within both the dendrite and the glass matrix. Moreover, because of lower Young's modulus and yielding strength, the elastic residual stresses will further promote the yielding and deformation of the dendrite, and thereby result in a “core-shell” structure in the dendrite [44]. The unique “core-shell” structure will improve the hardness of dendrite and restrain the cutting of shear bands, then leading to a stabilized plastic deformation in the present BMG composite. Based on the SEM and TEM studies of the deformed microstructures under different strain rates, it can be concluded that the stress-strain response of the Sn2 BMG composite depends on not only the shear band toughness of the dendrites and the glass matrix, but also the propagation mode of shear band. At high strain rates, the deformation of the BMG composite is controlled by the propagation mode of the shear bands in the glass matrix. Due to the reduced shear band toughness of the dendrites, the plasticity and yielding strength of the BMG composites are obviously reduced in the case of high strain rates. At low strain rates, the glass matrix and the dendrite have sufficient time to accommodate the plastic strain by shear bands multiplication and dislocations proliferation. Due to the hindrance of the dislocations in the dendrites and the primary shear bands, the secondary shear bands are stopped within the dendrites or in the regions of primary shear bands, and the shear strain are accommodated by the multiple shear bands which are stably extended. All the results suggest that the coordination of the shear band toughness of the dendrites and the glass matrix and the stable extension of shear bands should lead to excellent mechanical properties in BMG composites.
In the current work, the SSR and deformation behavior of the Sn2 BMG composite at room temperature have been studied in detail by the uniaxial compression and tension tests under different strain rates. The main conclusions are summarized as follows: (1) The mechanical properties of the Sn2 BMG composite depend strongly on the strain rate. With the increase of strain rate, the compressive or tensile yield strength of the Sn2 BMG composite, as well as its compressive and tensile plasticity, indicates negative SRS. (2) Compared with the fracture surface for the high strain rate that is mainly covered by melting layers, less melting layers can be found in the case of low strain rate. The fracture surface in the case of high strain rates is much coarser than that in the case of low strain rates, indicating that more severe adiabatic shear banding occurs during deformation. The thermal effect therefore is more significant at high strain rates, thus leading to more shear softening and smaller critical shear stress. (3) At lower strain rates, the shear band toughness of the dendrites is higher than that of the glass matrix. Dislocations have sufficient time to be generated, multiplied and aggregated, and shear banding can be retarded and hindered efficiently by dendrites. Furthermore, the primary shear bands are able to block more effectively the secondary ones, which results in more uniform distribution of plastic strains and higher plasticity. (4) The coordination of shear band toughness of the dendrites and the glass matrix and the stable extension of shear bands are suggested to lead to excellent mechanical properties of BMG composites. Since the BMG composites with lower SRS can be processed under higher processing rate, the current Sn2 BMG composite with a negative 8
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26–31. [20] D.C. Hofmann, J.Y. Suh, A. Wiest, G. Duan, M.L. Lind, M.D. Demetriou, W.L. Johnson, Designing metallic glass matrix composites with high toughness and tensile ductility, Nature 451 (2008) 1085–1089. [21] B.A. Sun, K.K. Song, S. Pauly, P. Gargarella, J. Yi, G. Wang, C.T. Liu, J. Eckert, Y. Yang, Transformation-mediated plasticity in CuZr based metallic glass composites: a quantitative mechanistic understanding, Int. J. Plast. 85 (2016) 34–51. [22] S. Pauly, S. Gorantla, G. Wang, U. Kühn, J. Eckert, Transformation-mediated ductility in CuZr-based bulk metallic glasses, Nat. Mater. 9 (2010) 473–477. [23] R. We, S. Yang, C.J. Zhang, L. He, Strain rate dependence of mechanical behavior in a CuZr-based bulk metallic glass composite containing B2-CuZr phase, Mater. Sci. Eng. A 606 (2014) 268–275. [24] Z.F. Yao, J.C. Qiao, J.M. Pelletier, Y. Yao, High temperature deformation behaviors of the Zr63.36Cu14.52Ni10.12Al12 bulk metallic glass, J. Mater. Sci. 51 (2016) 4079–4087. [25] J. Luo, M.Q. Li, W.X. Yu, H. Li, The variation of strain rate sensitivity exponent and strain hardening exponent in isothermal compression of Ti-6Al-4V alloy, Mater. Des. 31 (2010) 741–748. [26] J. Xu, L.L. Ma, Y.F. Xue, Z.H. Nie, Y.H. Long, L. Wang, Y.B. Deng, Work-hardening behavior, strain rate sensitivity, and failure behavior of in situ CuZr-based metallic glass matrix composite, J. Mater. Sci. 51 (2016) 5992–6001. [27] B.P. Wang, L. Wang, Y.F. Xue, Y.W. Wang, L.J. Zhu, H.F. Zhang, H.M. Fu, Mechanical response of Ti-based bulk metallic glass under static and dynamic indentations, J. Non-Cryst. Solids 422 (2015) 32–38. [28] S. González, G.Q. Xie, D.V. Louzguine-Luzgin, J.H. Perepezko, A. Inoue, Deformation and strain rate sensitivity of a Zr-Cu-Fe-Al metallic glass, Mater. Sci. Eng. A 528 (2011) 3506–3512. [29] Y.F. Xue, L. Wang, X.W. Chen, F.C. Wang, H.W. Cheng, H.F. Zhang, A.M. Wang, Strain rate dependent plastic mutation in a bulk metallic glass under compression, Mater. Des. 36 (2012) 284–288. [30] Z.F. Yao, J.C. Qiao, J.M. Pelletier, Y. Yao, High temperature deformation behaviors of the Zr63.36Cu14.52Ni10.12Al12 bulk metallic glass, J. Mater. Sci. 51 (2016) 4079–4087. [31] J.H. Chen, M.Q. Jiang, Y. Chen, L.H. Dai, Strain rate dependent shear banding behavior of a Zr-based bulk metallic glass composite, Mater. Sci. Eng. A 576 (2013) 134–139. [32] D.Q. Ma, W.T. Jiao, Y.F. Zhang, B.A. Wang, J. Li, X.Y. Zhang, M.Z. Ma, R.P. Liu, Strong work-hardening behavior induced by the solid solution strengthening of dendrites in TiZr-based bulk metallic glass matrix composites, J. Alloys Compd. 624 (2015) 9–16. [33] H.M. Zhai, H.F. Wang, F. Liu, Effects of Sn addition on mechanical properties of Tibased bulk metallic glass composites, Mater. Des. 110 (2016) 782–789. [34] J.W. Qiao, In-situ dendrite/metallic glass matrix composites: a review, J. Mater. Sci. Technol. 29 (2013) 685–701. [35] J. Bai, J.S. Li, J. Wang, J. Cui, L.Y. Li, H.C. Kou, P.K. Liaw, Strain-rate-dependent deformation behavior in a Ti-based bulk metallic glass composite upon dynamic deformation, J. Alloys Compd. 639 (2015) 131–138. [36] J.J. Lewandowski, A.L. Greer, Temperature rise at shear bands in metallic glasses, Nat. Mater. 5 (2006) 15–18. [37] J.L. Cheng, G. Chen, F. Xu, Y.L. Du, Y.S. Li, C.T. Liu, Correlation of the microstructure and mechanical properties of Zr-based in-situ bulk metallic glass matrix composites, Intermetallics 18 (2010) 2425–2430. [38] R.L. Narayan, P.S. Singh, D.C. Hofmann, N. Hutchinson, K.M. Flores, U. Ramamurty, On the microstructure–tensile property correlations in bulk metallic glass matrix composites with crystalline dendrites, Acta Mater. 60 (2012) 5089–5100. [39] H.M. Zhai, H.F. Wang, F. Liu, A strategy for designing bulk metallic glass composites with excellent work-hardening and large tensile ductility, J. Alloys Compd. 685 (2016) 322–330. [40] M.Q. Jiang, L.H. Dai, Shear-band toughness of bulk metallic glasses, Acta Mater. 59 (2011) 4525–4537. [41] J.W. Qiao, Y. Zhang, P.K. Liaw, G.L. Chen, Micromechanisms of plastic deformation of a dendrite/Zr-based bulk-metallic-glass composite, Scr. Mater. 61 (2009) 1087–1090. [42] R.T. Qu, Z.Q. Liu, G. Wang, Z.F. Zhang, Progressive shear band propagation in metallic glasses under compression, Acta Mater. 91 (2015) 19–33. [43] J.W. Qiao, A.C. Sun, E.W. Huang, Y. Zhang, P.K. Liaw, C.P. Chuang, Tensile deformation micromechanisms for bulk metallic glass matrix composites: From work-hardening to softening [J], Acta Mater. 59 (2011) 4126–4137. [44] E.W. Huang, J.W. Qiao, B. Winiarski, W.J. Lee, M. Scheel, C.P. Chuang, P.K. Liaw, Y.C. Lo, Y. Zhang, M.D. Michiel, Microyielding of core-shell crystal dendrites in a bulk metallic glass matrix composite, Sci. Rep. 4 (2014) 4394.
SRS might take the advantage of reducing the energy consumption and increasing the processing efficiency. Acknowledgments Haifeng Wang would like to thank the Natural Science Foundation of China (No. 51371149), the Huo Yingdong Young Teacher Fund (No. 151048), the Aeronautical Science Foundation of China (No. 2015ZF53066), the Free Research Fund of State Key Lab. of Solidification Processing (No. 92-QZ-2014) and the project of Shaanxi Young Stars of Science and Technology (No. 2015KJXX-10). Feng Liu is grateful to the National Basic Research Program of China (973 Program, No. 2011CB610403), the National Science Funds for Distinguished Young Scientists (No. 51125002), the Natural Science Foundation of China (Nos. 51431008 and 51134011), the Research Fund of the State Key Lab-oratory of Solidification Processing (117-TZ-2015), and the Fundamental Research Funds for the Central Universities (3102015BJ (II) CG005). References [1] A. Inoue, A. Takeuchi, Recent development and application products of bulk glassy alloys, Acta Mater. 59 (2011) 2243–2267. [2] T.C. Hufnagel, C.A. Schuh, M.L. Falk, Deformation of metallic glasses: recent developments in theory, simulations, and experiments, Acta Mater. 109 (2016) 375–393. [3] N. Nishiyama, K. Amiya, A. Inoue, Novel applications of bulk metallic glass for industrial products, J. Non-Cryst. Solids 353 (2007) 3615–3621. [4] C.A. Schuh, T.C. Hufnagel, U. Ramamurty, Mechanical behavior of amorphous alloys, Acta Mater. 55 (2007) 4067–4109. [5] A.L. Greer, Y.Q. Cheng, E. Ma, Shear bands in metallic glasses, Mater. Sci. Eng. R 74 (2013) 71–132. [6] W.H. Wang, C. Dong, C.H. Shek, Bulk metallic glasses, Mater. Sci. Eng. R 44 (2004) 45–89. [7] P. Thurnheer, R. Maaß, K.J. Laws, S. Pogatscher, J.F. Löffler, Dynamic properties of major shear bands in Zr-Cu-Al bulk metallic glasses, Acta Mater. 96 (2015) 428–436. [8] Y.H. Liu, G. Wang, R.J. Wang, D.Q. Zhao, M.X. Pan, W.H. Wang, Super plastic bulk metallic glasses at room temperature, Science 315 (2007) 1385–1388. [9] J.J. Lewandowski, W.H. Wang, A.L. Greer, Intrinsic plasticity or brittleness of metallic glasses, Philos. Mag. Lett. 85 (2005) 77–87. [10] B. Gu, F. Liu, Characterization of structural inhomogeneity in Al88Ce8Co4 metallic glass, Acta Mater. 112 (2016) 94–104. [11] K. Sarlar, I. Kucuk, Phase separation and glass forming ability of (Fe0.72Mo0.04B0.24)100 − xGdx (x = 4, 8) bulk metallic glasses, J. Non-Cryst. Solids 447 (2016) 198–201. [12] S.S. Chen, J.X. Tu, J.J. Wu, Q. Hu, S.H. Xie, J.Z. Zou, X.R. Zeng, Phase separation and significant plastic strain in a Zr-Cu-Ni-Al-Fe bulk metallic glass, Mater. Sci. Eng. A 656 (2016) 84–89. [13] O. Haruyama, K. Kisara, A. Yamashita, K. Kogure, Y. Yokoyama, K. Sugiyama, Characterization of free volume in cold-rolled Zr55Cu30Ni5Al10 bulk metallic glasses, Acta Mater. 61 (2013) 3224–3232. [14] J.S. Park, H.K. Lim, J.H. Kim, J.M. Park, W.T. Kim, D.H. Kim, Shear band formation and mechanical properties of cold-rolled bulk metallic glass and metallic glass matrix composite, J. Mater. Sci. 40 (2005) 1937–1941. [15] Y. Zhang, W.H. Wang, A.L. Greer, Making metallic glasses plastic by control of residual stress, Nat. Mater. 5 (2006) 857–860. [16] X.Q. Zhang, L.L. Ma, Y.F. Xue, Q.B. Fan, Z.H. Nie, L. Wang, J.M. Yin, H.F. Zhang, H.M. Fu, Temperature dependence of micro-deformation behavior of the porous tungsten/Zr-based metallic glass composite, J. Non-Cryst. Solids 436 (2016) 9–17. [17] C.C. Hays, C.P. Kim, W.L. Johnson, Microstructure controlled shear band pattern formation and enhanced plasticity of bulk metallic glasses containing in situ formed ductile phase dendrite dispersions, Phys. Rev. Lett. 84 (2000) 2901–2904. [18] J.W. Qiao, H.L. Jia, P.K. Liaw, Metallic glass matrix composites, Mater. Sci. Eng. R 100 (2016) 1–69. [19] Y.P. Jiang, L.G. Sun, Q.Q. Wu, K. Qiu, Enhanced tensile ductility of metallic glass matrix composites with novel microstructure, J. Non-Cryst. Solids 459 (2017)
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Strain rate sensitivity and deformation behavior in a Ti-based bulk metallic glass composite Haimin Zhai, Yuhao Xu, Yin Du, Haifeng Wang⁎, Feng Liu⁎ State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an, Shaanxi 710072, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Bulk metallic glass composite Shear bands toughness Strain rate sensitivity Shear banding behavior Ti-based alloy
The Ti45Zr25Nb6Sn2Cu5Be17 bulk metallic glass composite was investigated by uniaxial quasi-static compression and tension tests under strain rates from 1 × 10− 4 s− 1 to 1 × 10− 2 s− 1 at the room temperature. The deformation behavior was characterized by analyzing the lateral and fracture surfaces. With the increase of strain rate, the yielding stress and plasticity decrease significantly, indicating negative strain rate sensitivity. At higher strain rates, the thermal softening effect is more prominent and the yielding stress is lower, while lower strain rates, the shear band toughness of the dendrites is higher than that of the glass matrix. Then the dislocations have sufficient time to be generated, multiplied and aggregated, thereby the shear banding can be retarded and hindered efficiently by dendrites. Furthermore, the primary shear bands are able to block more effectively the secondary ones, which results in more uniform distribution of plastic strain and thereby higher plasticity. The current work shows that the coordination of the shear band toughness of the dendrites and the glass matrix and the stable extension of the shear bands are of primary importance for obtaining excellent mechanical properties.
1. Introduction The remarkable properties of bulk metallic glasses (BMGs), such as ultrahigh strength, excellent hardness and large elastic limit, enable them to be potential candidates of structural materials in engineering [1–4]. Due to the highly localized plastic deformation within the shear bands, the monolithic BMGs, however, usually exhibit strain softening and poor plasticity at the room temperature, which seriously limits their potential applications [5–7]. To alleviate this deficiency, substantial studies have been carried out to enhance the plasticity: (1) designing BMGs by modulating their intrinsic properties, e.g. with a high Poisson's ratio [8,9]; (2) introducing nano-scale structural heterogeneity [10] or phase separation [11,12]; (3) inducing randomly distributed free volumes by pre-deformation [13–15]; (4) preparing ex-situ [16] or in-situ [17–23] BMG composites by introducing a second crystalline phase. Previous studies have shown that good plasticity could be achieved for in-situ formed BMG composites with a ductile crystalline phase such as the bcc (body-centered cubic) β dendrite phase [18–20] and the CuZr B2 phase [21–23]. The excellent plasticity is attributed to the ductile crystalline phase that acts as an obstacle to bifurcate or impede shear banding. The propagation of shear bands may bypass the barrier or be arrested by the crystalline phase. The intersection and branching of shear bands then lead to a relative
⁎
uniform distribution of plastic strain [18]. In fact, it is highly possible to show the capacity of materials to resist necking (that is closely associated with the plasticity) by evaluating the strain rate sensitivity (SRS) [23,24]. For crystalline materials, their SRS is normally positive, as is determined by the deformation modes of dislocations, twins and phase transformations [25]. In contrast, the deformation mode of BMGs is the initiation and propagation of shear bands, i.e. a shear-band based deformation mode [6,24]. Due to the different test conditions, different alloy compositions and microstructures, BMGs exhibit different deformation behaviors at different deformation strain rates and thus different SRSs [26]. All kinds of SRSs have been reported, i.e. positive [27], zero [28], and negative [29,30]. For the in-situ BMG composites, the rapid extension of shear bands can be effectively restrained by the dendrites strengthened by dislocations [18,20]. Therefore, it is generally believed that the SRS of BMG composites depends on the competition between the shear band based deformation mode in glass matrix and the dislocation based deformation mode in dendrites [31]. In addition, the extension of shear bands in BMG composites may be not only hindered and restricted by the dendrites, but also affected by the primary shear bands, that is to say, the extension mode of shear bands could be multiple [32]. In this sense, the SRS of BMG composites should be different from those of the BMGs and crystalline materials and should depend on the coordination
Corresponding authors. E-mail addresses: [email protected] (H. Wang), [email protected] (F. Liu).
http://dx.doi.org/10.1016/j.jnoncrysol.2017.05.032 Received 17 February 2017; Received in revised form 16 May 2017; Accepted 20 May 2017 0022-3093/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Zhai, H., Journal of Non-Crystalline Solids (2017), http://dx.doi.org/10.1016/j.jnoncrysol.2017.05.032
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645.0 K and 735.5 K, respectively. The rather large supercooled liquid region (ΔT = Tx1 − Tg) 90.5 K indicates that the current BMG composite has an excellent glass forming ability. A TEM bright field (BF) image of the BMG composite is shown Fig. 2a. One can see that the dendrites in white distribute homogeneously within the glass matrix in gray. The inserting selected area electron diffraction (SAED) pattern obtained from the dendrites represents the [001] zone axis of the bcc β-Ti phase while that obtained from the glass matrix is of typical diffuse holes of amorphous structure; see the insets in Fig. 2a. The HRTEM image (Fig. 2b) shows a regular arrangement of the lattice patterns in the dendrites without apparent lattice defects (e.g. dislocations), indicating that the as-cast sample is almost free of deformation [34].
between shear band toughness of the dendrites and the glass matrix, and on the extension mode of shear bands. In practical applications, a considerable change of loading rates caused by shaking and/or impacting may occur from time to time and results in variation of deformation modes in the dendrites and the glass matrix. For most BMGs, the effect of SRS on the mechanical properties might not be very significant considering their almost negligible plasticity at the loading rates in service. For BMG composites, however, the effect of SRS cannot be neglected anymore due to their complex deformation modes, such as the dislocation- and the shear-bands-based deformation mode. The studies of SRS not only can simulate the actual service process under different conditions but also may help to understand the deformation mechanisms of BMG composites. In view of this, it is noteworthy to study in detail the SRS and deformation behavior of BMG composites at different strain rates. In this paper, the Ti45Zr25Nb6Sn2Cu5Be17 (Sn2) BMG composite [33] was studied by the uniaxial quasi-static compressive and tensile tests under the strain rates from 1 × 10− 4 s− 1 to 1 × 10− 2 s− 1. On the basis of the stress-strain response at the room temperature, the SRS was analyzed to understand the deformation behaviors and the deformation mechanisms were investigated in detail by analyzing the lateral and fracture surfaces. Such a study is helpful for the applications of BMG composites in engineering.
3.2. Mechanical properties Typical compressive and tensile engineering stress-strain curves under the strain rates from 1 × 10− 4 s− 1 to 1 × 10− 2 s− 1 are shown in Fig. 3a and b, respectively. The detailed compression and tensile measurements are summarized in Table 1. It can be found that the mechanical properties of Sn2 BMG composite are significantly dependent on the loading strain rate under both compressive and tensile mode. One can see that as the strain rate increases, the compressive yielding strength (compressive fracture strength) decreases from about 1405 MPa (1929 MPa) to 1316 MPa (1706 MPa), and the compressive fracture strain also decreasing from about 30.8% to 15.1%. Similarly, as the strain rate increases, the tensile yield strength (ultimate tensile strength) decreases from about 927 MPa (1464 MPa) to 821 MPa (982 MPa), while the tensile plastic strain decreasing from 12.1% to 2.2%. All the experimental results indicating that the current Sn2 BMG composite has a negative SRS; see the discussion section for detail.
2. Experimental Ingots with a nominal composition of Ti45Zr25Nb6Sn2Cu5Be17 (at.%, denoted as Sn2 BMG composite) are prepared by arc-melting a mixture of constituent elements (Ti, Zr, Cu, Nb, Sn and Be metals) with purity better than 99.9% in a Ti-gettered Ar atmosphere. A detail of the preparation procedure is shown in our recent work [33]. Plate samples (5 × 20 × 60 mm) are prepared by casting into a water-cooled copper mold. The samples are characterized by X-ray diffraction (XRD: Bruker D8 with Co Kα radiation), scanning electron microscopy (SEM: TESCAN VEGA 3 LMU), and high-resolution transmission electron microscopy (HRTEM: Tecnai G2 F30, 300 kV). The TEM samples were first prepared by mechanical grinding to a 30 μm thick plate. In order to avoid the samples deformation caused by the pitting, the samples were twin-jet electropolished using a solution mixed in the ratio HNO3:CH4O = 1:3, and then the TEM samples were thinning again by ion milling. Thermal analyses are conducted by differential scanning calorimetry (DSC: PerkinElmer 8500) under the protection of high purity Ar gas at a constant heating rate of 40 K min− 1. The compressive samples (4 × 4 × 8 mm) and the tensile samples (1 × 2 × 10 mm) are machined from the as-cast plates, and then carefully polished. The roomtemperature quasi-static compressive and tensile tests are carried out on a testing machine (INSTRON 3382) under the strain rates from 1 × 10− 4 s− 1 to 1 × 10− 2 s− 1.
3.3. Fractography SEM images of the compressive and tensile deformation fractured BMG composite under the strain rates of 1 × 10− 4 s− 1 and 1 × 10− 2 s− 1 are shown in Figs. 4 and 5, respectively. SEM images of the compressive fractured BMG composite under the strain rates of 1 × 10− 4 s− 1 and 1 × 10− 2 s− 1 are shown in Fig. 4. At high strain rates (e.g. 1 × 10− 2 s− 1), a large number of different orientated shear bands (white arrows) and cracks (black arrows) are distributed on the lateral side near the fracture surface (Fig. 4a), indicating that the sample suffered serious distortion (blue cycles). In contrast, different oriented shear bands (white arrows in Fig. 4e) are more homogeneously distributed on the lateral surface at lower strain rates (e.g. 1 × 10− 4 s− 1), indicating that the sample should have excellent plasticity. Compared with the case of high strain rate 1 × 10− 2 s− 1 in which the sample fails at an inclination angle of 44.7° with respect to the loading axis under-static compression, a lower angle of 44.3° is found for the case of low strain rate 1 × 10− 4 s− 1; see the insets in Fig. 4a and b. The deviation of the maximum shear stress plane may give rise to a significant effect of the normal stress on the fracture behavior [35]. From the fracture surfaces at high and low strain rates (Fig. 4b and f), three different fracture patterns, i.e. droplet patterns (white ellipse), smooth patterns (red arrows) and shear steps (white arrows), can be found. The presence of a large number of droplet patterns is mainly due to the elastic energy stored upon the deformation process, which is released in a very short time when fracture happens and meanwhile the glass matrix is re-melted by the released heat [36]. Compared with the fracture surface for the high strain rate 1 × 10− 2 s− 1 that is mainly covered by melting layers (Fig. 4b), less melting layers can be found in the case of low strain rate 1 × 10− 4 s− 1 (Fig. 4f). Such a difference in the fracture surfaces implies that the temperature increase in the case of high strain rates should be higher than that in the case of low stain rates. Fig. 4c and g show the enlarged droplet patterns according to which one can find that the fracture
3. Results 3.1. Materials characterization The XRD pattern of the as-cast sample is shown Fig. 1a. One can see that several sharp Bragg diffraction peaks are superimposed on a broad hump. Similar to the other Ti-based BMG composites reinforced with bcc ductile dendrites [18,20], the sharp crystalline peaks correspond to the bcc β-Ti phase. For the as-cast sample (the inset in Fig. 1a), the dendritic phase in light gray is homogenously embedded in the glass matrix in dark gray. The volume fraction of dendrites is approximately 52 ± 2% and their average length is about 25 ± 4 μm. DSC measurements were performed to evaluate the Sn2 BMG composite (Fig. 1b). The DSC curve shows that a significant glass transition is followed by a crystallization process, confirming again the presence of amorphous phase. The glass-transition temperature (Tg) and onset crystallization temperature (Tx1) of the Sn2 BMG composite were measured to be 2
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Fig. 1. (a) XRD patterns and the inset SEM image, (b) DSC curve (heating rate: 40 K min− 1) of the as-cast Sn2 BMG composite.
Fig. 2. (a) TEM BF image and the insects the corresponding SAED patterns of glass matrix and dendrites, (b) HRTEM image for the interface between the glass matrix and dendrites in the as-cast Sn2 BMG composite.
Fig. 3. The quasi-static engineering stress-strain curves of the Sn2 BMG composite under different strain rates: (a) compression, (b) tension.
effectively stabilize the buckling of shear bands. Similar to the compressive deformation fracture surface, the tensile deformation fracture surface at high and low strain rates also exhibit significantly different morphologies, as shown in Fig. 5a and e. For the case of the high strain rate 1 × 10− 2 s− 1, the fracture surface has a brittle fracture features (“flat” cleavage-like), as shown in Fig. 5b. However, the magnification image (block region “c” in Fig. 5b) indicates that the fracture surface is not atomically flat, but has dendrite-like features on it, as shown in Fig. 5b. This result indicates that the dendrite does not effectively hinder the propagation of the shear bands and the subsequent crack [38]. Besides, due to the instantaneous increase in temperature at final fracture (adiabatic
surface in the case of high strain rates is much coarser than that in the case of low strain rates, indicating that more severe adiabatic shear banding occurs during deformation. In this sense, it might be reasonable to speculate that the thermal effect is more significant at high strain rates, thus leading to more shear softening [35] and smaller critical shear stress. By enlarging the rough patterns, some “rib-like” layers (marked by white arrows in Fig. 4d and h) can be found. The “rib-like” layers are considered to be generated via the stepwise sliding process during the initial evolution of large shear bands and induced by the absorbing and blocking functions that the dendrites worked on the shear bands [37]. Furthermore, more stepwise sliding appears on the fracture surface for low strain rates, indicating that the dendrites can 3
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[24,25] is introduced here:
Table 1 The compressive yielding strength (σy), compressive fracture strength (σf), compressive plastic strain (εf), tensile yielding strength (σy), tensile ultimate strength (σu), and the tensile ultimate plastic strain (εu) of the Sn2 BMG composite under different strain rates. The standard errors for the six different measurements are shown.
Compression
Tension
Strain rate (s− 1)
σy (MPa)
1 × 10− 4 5 × 10− 4 1 × 10− 3 5 × 10− 3 1 × 10− 2
1405 1387 1353 1334 1316
Strain rate (s− 1) 1 × 10− 4 5 × 10− 4 1 × 10− 3 5 × 10− 3 1 × 10− 2
σy (MPa) 927 ± 40 903 ± 25 880 ± 20 847 ± 35 821 ± 30
± ± ± ± ±
σf (MPa) 35 40 25 30 25
1929 1810 1786 1742 1706
± ± ± ± ±
m=
σu (MPa) 1464 ± 45 1328 ± 20 1223 ± 30 1068 ± 20 982 ± 30
30.8 25.2 20.8 18.1 15.1
± ± ± ± ±
(1)
where σ (MPa) is the flow stress at a selected strain and deformation temperature, dε/dt (s− 1) is the strain rate and ε is the strain. Based on the flow stress in the compressive and tensile stress-strain curves of the current Sn2 BMG composite, the SRS exponent m can be calculated at a constant strain using Eq. (1). Fig. 6a and b illustrates the SRS exponent m of the Sn2 BMG composite obtained from compression and tensile test data. It can be seen that the negative SRS is shown in both compression (m ≈ −0.027) and tensile deformations (m ≈ − 0.036) for the strain rates from 1 × 10− 4 s− 1 to 1 × 10− 2 s− 1. From the above experimental results (Figs. 4 and 5), it can be seen that the shear bands density and the fracture morphology have obvious differences in a wide loading strain rate range, indicating that the SRS of the Sn2 BMG composite is strongly affected by shear banding behavior. Upon loading deformation, the stress concentration will be generated in the interfaces between the dendrite and the glass matrix because of the differences in the intrinsic property and yield strength of the two-phases [18,39]. The soft dendrite can absorb most of elastic energy, and then relieve the stress concentration near the interface between the glass matrix and dendrite, thus delaying the formation of shear bands and cracks at the interface. As a result, more dislocations will be accumulated around the interface in order to accommodate the mechanical incompatibility across the interface, which produces the observed extra strain hardening, as shown in Fig. 7a (compression) and Fig. 8a (tension). For the BMG materials, when the applied stress increases largely enough to reach or exceed the atomic cohesive strength, the equilibrium of stress field nearby the tip would be destroyed; as a consequence, the shear bands will initiated and then cause a catastrophic fracture of the samples. However, in the BMG composite, the propagation of the shear bands can be hindered and stabilized by the dendrite, which results in a stable plastic flow in the BMG composites. Hence, the shear banding behavior of the BMG composite will be determined by the mismatch between the shear band toughness of the dendrite and the glass matrix [31]. Note that, it has been proposed that the shear band toughness can be used to quantitatively measure the inherent resistance capability of materials to the propagation of shear bands [40]. At high strain rates, the shear band toughness of the dendrites is smaller than that of the glass matrix, so that the rapid extension of shear
εf (%) 50 25 35 25 40
∂ln σ ∂ln dε dt
1.5 1.6 1.2 1.5 1.3
εu (%) 12.1 ± 1.8 9.8 ± 1.4 6.7 ± 2.1 4.1 ± 2.5 2.2 ± 1.8
heating), the magnification image of block region “d” in Fig. 5b also exhibits the microscale vein patterns. For the case of the low strain rate 1 × 10− 4 s− 1, one can see that profuse and dense shear bands are distributed on the lateral surface of tensile fracture sample, indicating that severe plastic deformation occurs in the glass matrix. Different from the fracture morphology at the high strain rate (Fig. 5f and g), the fracture surface at the low strain rate exhibits abundant ligament, indicating that a ductile fracture appear at the low strain rate. Moreover, one can see that some voids (marked by red arrows) which are a result of the pull-out of dendrite from matrix appears in the matrix, as shown in Fig. 5h. Such a pull-out of dendrites will enhance the toughness of the BMG composite, which is a possible reason for the significant work-hardening capability in the Sn2 BMG composite. 4. Discussions 4.1. Stress-strain responses As shown in Fig. 3, the current Sn2 BMG composite exhibits different compressive and tensile plasticity and yielding strength at different strain rates. Furthermore, the ductility and the strength decreases remarkably with the increase of strain rate; see Table 1. To show further the effect of strain rate on the deformation behavior and mechanical properties of the Sn2 BMG composite, the SRS exponent m
Fig. 4. SEM images of the fractography for the compressive fracture samples of the Sn2 BMG composite. At high strain rate (1 × 10− 2 s− 1): (a) lateral surface and the insets are corresponding to the photo of fracture samples, (b) fracture surface, (c) melting droplet patterns regions, (d) shear steps regions; at low strain rate (1 × 10− 4 s− 1): (e) lateral surface and the insets are corresponding to the image of fracture samples, (f) fracture surface, (g) melting droplet patterns regions, (h) shear steps regions. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5. SEM images of the fractography for the tensile fracture samples of the Sn2 BMG composite. At high strain rate (1 × 10− 2 s− 1): (a) lateral surface, (b) fracture surface, (c) melting droplet patterns regions, (d) vein patterns regions; At low strain rate (1 × 10− 4 s− 1): (e) lateral surface, (f) fracture surface, (g) ductile fracture features in the ligament, (h) holes created by dendrite pull-out.
Fig. 6. Determination of the exponent of strain rate sensitivity in the Sn2 BMG composite: (a) compression, (b) tension.
plastic deformation. According to many previous studies [18,20,39,41], the temperature increase within the shear bands leads to considerable thermal softening in the BMGs or BMG composites. Therefore, the yielding strength of the Sn2 BMG composite decreases with the increases of the strain rate and the current BMG composite exhibits negative SRS during the quasi-static compression and tension. Generally, the requirement of SRS is not the same for different applications of materials. For structural materials, a positive SRS corresponds to postpone of thinning and necking, i.e. a good service performance. In this sense, the current Sn2 BMG composite should not be a good candidate of structural materials if the loading rates change considerably in service. One should however keep in mind that in the forging, rolling, extrusion and other processing, the BMG composites with lower SRS can be processed under higher processing rate [25], indicating that the current Sn2 BMG composite with a negative SRS may take the advantage of reducing the energy consumption and increasing the processing efficiency.
bands cannot be hindered by the dendrite; see the red dotted line in Fig. 7b. However, since the samples show brittle fracture under tensile deformation, no shear bands were found in TEM image, but with only a small amount of dislocations, as shown in Fig. 8a. In contrast, at low strain rates, the shear band toughness of the dendrites is higher than that of the glass matrix. The dendrite therefore has sufficient time to generate a large number of dislocations, which can effectively alleviate the stress concentration at the interface and delay the shear softening caused by the initiation and extension of shear bands [31]. As a result of the multiplication and subsequent aggregation of dislocations at the interface, the initiation and propagation of shear bands can be retarded and hindered; see the red dotted line in Fig. 7c. Even within the dendrite, the shear bands can also be impeded, as shown by the points “B” to “G” in Fig. 7d. Similarly, in the tensile deformation of the sample, one can see that the extension of the shear band is also blocked by the two-phase interface, and ultimately stop within the dendrite. In other words, whether it is tensile or compression deformation, the dendrites not only can retard the rapid propagation of shear bands by the dislocation strengthening effect [18,20,41] but also may release efficiently the deformation energy by enlarging the width of shear bands. Furthermore, the thermal softening effect was suggested to be usually responsible for the reduction in the critical shear stress [31,35]. As shown in Fig. 4c and g (compression) and Fig. 5c and f (tension), the droplet patterns of the sample under high strain rates has much more rough molten traces than those under low strain rates, indicating a larger temperature increase within the shear bands during
4.2. Deformation behavior Usually, it has been recognized that the profuse multiplication shear bands are mainly associated with the large plasticity in the BMG composite [33]. For the Sn2 BMG composite, however, the negative SRS with the strain rate should be related not only to the shear band toughness of the dendrites and the glass matrix, but also to the propagation modes of shear bands. As shown in Fig. 9a and c (compression), it can be seen that 5
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Fig. 7. TEM BF images of the current Sn2 BMG composite for compressive deformation samples. At the strain rate of 1 × 10− 2 s− 1: (a) multiple dislocations in the deformed dendrite, (b) a shear band passing through the dendrite and forming a shear step at the interface between the dendrite and the glass matrix; at the strain rate of 1 × 10− 4 s− 1: (c) a shear band passing through the dendrite I and stopped by the dendrite II, (d) the extended mode of one shear band within the dendrite. The propagation direction of this shear band is not only changed by five times due to the barrier effect of dislocations in the dendrite, but also its width is increased from 45 nm in the glass matrix to 150 nm in the dendrite. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. TEM BF images of the current Sn2 BMG composite for tensile deformation samples. (a) At the strain rate of 1 × 10− 2 s− 1; (b) at the strain rate of 1 × 1042 s− 1. The propagation direction of this shear band is not only changed by the interface of the two-phase, but also its width is increased from 25 nm in the glass matrix to 78 nm in the dendrite.
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Fig. 9. SEM images of the Sn2 BMG composite after compressive deformation samples. At the strain rate of 1 × 10− 2 s− 1: (a) the primary shear bands are confined locally as marked by the blue circles, and a lot of cracks appear on the lateral surface, (b) the longer secondary shear bands interact with the primary shear bands and results in minor shear steps; At the strain rate of 1 × 10− 4 s− 1: (c) the primary shear bands are confined locally as marked by the blue circles, and the secondary shear bands are affected by the primary shear bands, (d) the enlarged images of Fig. 7b shows clearly that the extension mode of shear bands, i.e. the secondary shear bands are hindered by the primary ones. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
show obviously a decrease tendency of the local shear offsets, i.e. the width decreasing from 2.09 μm to 0.83 μm with the increase of the distance from the original site for the interaction between the primary and secondary shear bands. Similarly, as shown in Fig. 10a and c (tension), multi-directional shear bands also distributed on the lateral surface in the cases of both high and low strain rates. For the case of the high strain rate, it can be seen that the shear bands have not only cut the dendrites, but has also evolved into cracks, as shown in Fig. 10b. This result indicates that the primary shear bands are not hindered by the dendrite so as to cause the unstable propagation, then resulting in brittle fracture. Comparing with the case of high strain rate (1 × 10− 2 s− 1), the agminated primary shear bands (e.g. marked by blue ellipses) in the case of low strain rate (1 × 10− 4 s− 1) occupy more regions on the lateral surface; see Fig. 10c. Besides, it is also shown that dense intersections between the primary shear bands demonstrate secondary shear bands with spacing of 2–3 μm, suggesting that the sample undergoes more severe plastic deformation at the low strain rate; see Fig. 10c. Moreover, it can be seen that the shear bands cuts through the dendritic I and is then obstructed by the dendrites II, III, and IV, and then been bifurcated both in the two-phase interface or the dendrites and eventually stops in the glass matrix or in the dendrite V; see Fig. 10d. In other words, the primary shear band is able to block effectively the secondary shear band at low strain rates, and then shear strain with deformation is accommodated by the formation and stable extension of multiple shear bands, which leads to the enhanced macroscopic compressive and tensile plasticity; see Fig. 3 and Table 1. Moreover, the dislocations in the high temperature region will usually annihilate to lead a smooth interface. Nevertheless, after cooling to low temperature, the elastic residual stresses will be generated in the interface due to the difference the thermal expansion
intensive and multi-directional primary and secondary shear bands have a distinctly different distribution pattern on the lateral surface in the cases of high and low strain rates. In the early stages of deformation, the primary shear bands cannot easily and quickly propagate through the dendrites due to the blocking effect of dendrites (e.g. dislocations strengthening). Hence, almost all of those parallel primary shear bands are confined to local regions (e.g. marked by blue ellipses). Compared with the case of high strain rates, the agminated primary shear bands occupy large regions and their spacing is smaller for low strain rates (e.g. 5–9 μm for 1 × 10− 2 s− 1 vs 4–6 μm for 1 × 10− 4 s− 1). As the deformation proceeds, the plastic strain will not be able to accommodate continuously by the primary shear bands in glass matrix and the dislocations in dendrites. In this case, a large number of secondary shear bands with an average length in hundred microns are generated in the glass matrix to dissipate the deformation energy; see the red dashed lines in Fig. 9b and c. The parallel distribution of the secondary shear bands with a spacing of about 15–30 μm is similar to that of the primary shear bands, but with a different direction. When the secondary shear bands pass through the interface between the dendrites and the glass matrix or cut through small dendrite arms, their direction of propagation can be significantly altered, which leads to shear steps at the interface [34,41]; see Fig. 7b. In contrast to other BMG systems [42], when the longer secondary shear bands pass through the primary shear bands, they were blocked by several times. At high strain rates, the secondary shear bands traverse rapidly the primary shear bands and generate only few shear offsets (white arrows), indicating that the secondary shear bands cannot be effectively blocked. In contrast, the propagation direction of secondary shear bands is gradually changed at low strain rates, as shown in Fig. 9d. And meanwhile, the secondary shear band generates a new shear offset (white arrows) at each intersection point. The measured widths of the shear steps in Fig. 9d 7
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Fig. 10. SEM images of the Sn2 BMG composite after tensile deformation samples. At the strain rate of 1 × 10− 2 s− 1: (a) the primary shear bands are confined locally as marked by the blue circles, (b) a lot of cracks appear and interact with the primary shear bands and results in minor shear steps; At the strain rate of 1 × 10− 4 s− 1: (c) the primary shear bands are confined locally as marked by the blue circles, and the secondary shear bands are affected by the primary shear bands, (d) the extension mode of shear bands, i.e. the secondary shear bands are hindered or stopped by the dendrites. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
5. Conclusions
coefficient between the two-phases. Although the interface between dendrite and glass matrix has a smooth boundary (Fig. 2b) and the dendrite shows a regular arrangement of lattice patterns without obvious lattice defects (e.g. dislocations) [43], the residual stresses should exist within both the dendrite and the glass matrix. Moreover, because of lower Young's modulus and yielding strength, the elastic residual stresses will further promote the yielding and deformation of the dendrite, and thereby result in a “core-shell” structure in the dendrite [44]. The unique “core-shell” structure will improve the hardness of dendrite and restrain the cutting of shear bands, then leading to a stabilized plastic deformation in the present BMG composite. Based on the SEM and TEM studies of the deformed microstructures under different strain rates, it can be concluded that the stress-strain response of the Sn2 BMG composite depends on not only the shear band toughness of the dendrites and the glass matrix, but also the propagation mode of shear band. At high strain rates, the deformation of the BMG composite is controlled by the propagation mode of the shear bands in the glass matrix. Due to the reduced shear band toughness of the dendrites, the plasticity and yielding strength of the BMG composites are obviously reduced in the case of high strain rates. At low strain rates, the glass matrix and the dendrite have sufficient time to accommodate the plastic strain by shear bands multiplication and dislocations proliferation. Due to the hindrance of the dislocations in the dendrites and the primary shear bands, the secondary shear bands are stopped within the dendrites or in the regions of primary shear bands, and the shear strain are accommodated by the multiple shear bands which are stably extended. All the results suggest that the coordination of the shear band toughness of the dendrites and the glass matrix and the stable extension of shear bands should lead to excellent mechanical properties in BMG composites.
In the current work, the SSR and deformation behavior of the Sn2 BMG composite at room temperature have been studied in detail by the uniaxial compression and tension tests under different strain rates. The main conclusions are summarized as follows: (1) The mechanical properties of the Sn2 BMG composite depend strongly on the strain rate. With the increase of strain rate, the compressive or tensile yield strength of the Sn2 BMG composite, as well as its compressive and tensile plasticity, indicates negative SRS. (2) Compared with the fracture surface for the high strain rate that is mainly covered by melting layers, less melting layers can be found in the case of low strain rate. The fracture surface in the case of high strain rates is much coarser than that in the case of low strain rates, indicating that more severe adiabatic shear banding occurs during deformation. The thermal effect therefore is more significant at high strain rates, thus leading to more shear softening and smaller critical shear stress. (3) At lower strain rates, the shear band toughness of the dendrites is higher than that of the glass matrix. Dislocations have sufficient time to be generated, multiplied and aggregated, and shear banding can be retarded and hindered efficiently by dendrites. Furthermore, the primary shear bands are able to block more effectively the secondary ones, which results in more uniform distribution of plastic strains and higher plasticity. (4) The coordination of shear band toughness of the dendrites and the glass matrix and the stable extension of shear bands are suggested to lead to excellent mechanical properties of BMG composites. Since the BMG composites with lower SRS can be processed under higher processing rate, the current Sn2 BMG composite with a negative 8
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SRS might take the advantage of reducing the energy consumption and increasing the processing efficiency. Acknowledgments Haifeng Wang would like to thank the Natural Science Foundation of China (No. 51371149), the Huo Yingdong Young Teacher Fund (No. 151048), the Aeronautical Science Foundation of China (No. 2015ZF53066), the Free Research Fund of State Key Lab. of Solidification Processing (No. 92-QZ-2014) and the project of Shaanxi Young Stars of Science and Technology (No. 2015KJXX-10). Feng Liu is grateful to the National Basic Research Program of China (973 Program, No. 2011CB610403), the National Science Funds for Distinguished Young Scientists (No. 51125002), the Natural Science Foundation of China (Nos. 51431008 and 51134011), the Research Fund of the State Key Lab-oratory of Solidification Processing (117-TZ-2015), and the Fundamental Research Funds for the Central Universities (3102015BJ (II) CG005). References [1] A. Inoue, A. Takeuchi, Recent development and application products of bulk glassy alloys, Acta Mater. 59 (2011) 2243–2267. [2] T.C. Hufnagel, C.A. Schuh, M.L. Falk, Deformation of metallic glasses: recent developments in theory, simulations, and experiments, Acta Mater. 109 (2016) 375–393. [3] N. Nishiyama, K. Amiya, A. Inoue, Novel applications of bulk metallic glass for industrial products, J. Non-Cryst. Solids 353 (2007) 3615–3621. [4] C.A. Schuh, T.C. Hufnagel, U. Ramamurty, Mechanical behavior of amorphous alloys, Acta Mater. 55 (2007) 4067–4109. [5] A.L. Greer, Y.Q. Cheng, E. Ma, Shear bands in metallic glasses, Mater. Sci. Eng. R 74 (2013) 71–132. [6] W.H. Wang, C. Dong, C.H. Shek, Bulk metallic glasses, Mater. Sci. Eng. R 44 (2004) 45–89. [7] P. Thurnheer, R. Maaß, K.J. Laws, S. Pogatscher, J.F. Löffler, Dynamic properties of major shear bands in Zr-Cu-Al bulk metallic glasses, Acta Mater. 96 (2015) 428–436. [8] Y.H. Liu, G. Wang, R.J. Wang, D.Q. Zhao, M.X. Pan, W.H. Wang, Super plastic bulk metallic glasses at room temperature, Science 315 (2007) 1385–1388. [9] J.J. Lewandowski, W.H. Wang, A.L. Greer, Intrinsic plasticity or brittleness of metallic glasses, Philos. Mag. Lett. 85 (2005) 77–87. [10] B. Gu, F. Liu, Characterization of structural inhomogeneity in Al88Ce8Co4 metallic glass, Acta Mater. 112 (2016) 94–104. [11] K. Sarlar, I. Kucuk, Phase separation and glass forming ability of (Fe0.72Mo0.04B0.24)100 − xGdx (x = 4, 8) bulk metallic glasses, J. Non-Cryst. Solids 447 (2016) 198–201. [12] S.S. Chen, J.X. Tu, J.J. Wu, Q. Hu, S.H. Xie, J.Z. Zou, X.R. Zeng, Phase separation and significant plastic strain in a Zr-Cu-Ni-Al-Fe bulk metallic glass, Mater. Sci. Eng. A 656 (2016) 84–89. [13] O. Haruyama, K. Kisara, A. Yamashita, K. Kogure, Y. Yokoyama, K. Sugiyama, Characterization of free volume in cold-rolled Zr55Cu30Ni5Al10 bulk metallic glasses, Acta Mater. 61 (2013) 3224–3232. [14] J.S. Park, H.K. Lim, J.H. Kim, J.M. Park, W.T. Kim, D.H. Kim, Shear band formation and mechanical properties of cold-rolled bulk metallic glass and metallic glass matrix composite, J. Mater. Sci. 40 (2005) 1937–1941. [15] Y. Zhang, W.H. Wang, A.L. Greer, Making metallic glasses plastic by control of residual stress, Nat. Mater. 5 (2006) 857–860. [16] X.Q. Zhang, L.L. Ma, Y.F. Xue, Q.B. Fan, Z.H. Nie, L. Wang, J.M. Yin, H.F. Zhang, H.M. Fu, Temperature dependence of micro-deformation behavior of the porous tungsten/Zr-based metallic glass composite, J. Non-Cryst. Solids 436 (2016) 9–17. [17] C.C. Hays, C.P. Kim, W.L. Johnson, Microstructure controlled shear band pattern formation and enhanced plasticity of bulk metallic glasses containing in situ formed ductile phase dendrite dispersions, Phys. Rev. Lett. 84 (2000) 2901–2904. [18] J.W. Qiao, H.L. Jia, P.K. Liaw, Metallic glass matrix composites, Mater. Sci. Eng. R 100 (2016) 1–69. [19] Y.P. Jiang, L.G. Sun, Q.Q. Wu, K. Qiu, Enhanced tensile ductility of metallic glass matrix composites with novel microstructure, J. Non-Cryst. Solids 459 (2017)
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