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Simultaneously enhancing strength and toughness of Zr-based bulk metallic glasses via minor Hf addition
Intermetallics 118 (2020) 106685
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Simultaneously enhancing strength and toughness of Zr-based bulk metallic glasses via minor Hf addition Y.H. Zhu a, b, Z.W. Zhu a, *, S. Chen a, H.M. Fu a, H.W. Zhang a, H. Li a, A.M. Wang a, H.F. Zhang a a b
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang, 110016, China School of Materials Science and Engineering, University of Science and Technology of China, Shenyang, 110016, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Bulk metallic glass Minor addition Compressive properties Impact toughness Fracture mechanisms Free volume
Zr65.5-xHfxNb3Cu13Ni11Al7.5 (x ¼ 0.0, 0.5, 1.0, 1.5 and 2.0 at.%) bulk metallic glasses (BMGs) were successfully prepared by copper mold casting. The effects of minor Hf addition on the mechanical properties and fracture mechanisms were investigated. The quasi-static compressive mechanical properties of Zr65.5-xHfxNb3Cu13 Ni11Al7.5 alloys were significantly improved by Hf addition. The optimum compressive fracture strength (σ f) and fracture strain (εf) are 1516 MPa and 2.6%, respectively, for the Zr64.5Hf1.0Nb3Cu13Ni11Al7.5 alloy. The impact toughness of the Zr64Hf1.5Nb3Cu13Ni11Al7.5 alloy (Ak ¼ 127 kJ/m2) is nearly four times than that of the Zr65.5Nb3Cu13Ni11Al7.5 alloy (Ak ¼ 33 kJ/m2). The increase of the fracture surface area and the ratio of vein-like patterns regions caused by Hf addition play an important role in fracture energy dissipation under dynamic loading. The dependence of mechanical properties on the Hf content is closely related to the variation of free volume in the current alloys. The relationship between the Hf addition and the free volume change is discussed in detail. These results may provide new insights into simultaneously enhancing the plasticity and toughness of BMGs with minor alloying.
1. Introduction Bulk metallic glasses (BMGs) exhibit unique properties, including high strength, large elastic strains of up to ~2% and high corrosion resistance due to their disorder microstructure [1–6]. Their intriguing mechanical properties have made them candidate materials in many potential structural applications [2]. A significant number of BMG sys tems have been investigated concerning the mechanical properties and fracture mechanisms [7–10]. Among the glass-forming alloy systems, Zr-based BMGs have attracted much more attention because of the perfect combinations of the super high glass-forming ability (GFA) and unique mechanical properties [8,9]. Of all the mechanical properties, the toughness is the most concerned because the catastrophic fracture occurs to BMGs under loading, which limits their widespread applica tion as structural engineering materials [10,11]. Chemical effects, i.e., minor addition or microalloying, etc., play an essential role in improving the GFA, thermal stability, and the toughness of BMGs [12,13]. Multicomponent systems could make the supercooled liquid stable, originating from higher degrees of densely packed atomic configurations in local regions [2]. In addition, the electronic in teractions and bond shortening could strongly influence the population
of the ordered clusters and their spatial connectivity [14]. In the Zr-TM (TM ¼ Ni, Co or Cu)–Al systems, a sufficient ductility of BMGs could be obtained by making a balance between the metallic bond nature and free volume [15]. In a word, the significant relationships between the in ternal structure and bond state may be the origin of the mechanical properties of BMGs. It is also reported that minor addition of some re fractory elements (including Mo, Ta or Nb, etc.) effectively improve the corrosion resistance and mechanical properties of some Cu- and Zr-based BMGs [16–19]. The addition of these elements is recognized to stabilize the icosahedral short-range ordering (ISRO) in the atomic-level structure, and the ISRO is the basic building unit in the BMGs [20–22]. Furthermore, the local distribution of free volume is recognized to be changed, leading to the enhancement of plastic deformation as well as the variation of thermodynamic properties [8,9,23,24]. Recent molec ular dynamics simulations show that chemical separation occurs in the process of cavitation in the tough BMGs, which elevates the energy barrier of the nucleation of micro-crack [25]. The substitution of alloying elements can promote structural and chemical heterogeneity [14]. As a result, the internal heterogeneity may be able to change the content of free volume at the atomic level, which is associated with the formation of shear bands. Meanwhile, multiple shear bands are
* Corresponding author. E-mail address: [email protected] (Z.W. Zhu). https://doi.org/10.1016/j.intermet.2019.106685 Received 9 October 2019; Received in revised form 26 November 2019; Accepted 23 December 2019 Available online 30 December 2019 0966-9795/© 2019 Elsevier Ltd. All rights reserved.
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generated to release energy storage and further balance the distribution of stress. In this way, the nucleation of micro-crack is inhibited, resulting in the significant enhancement of toughness for BMGs. From the perspective of fractography, the formation and propagation of shear bands, viscous deformation, and the transition of fracture patterns contribute to the improvement of impact toughness to some extent based on a model of energy dissipation mechanisms during failure [5,26,27]. Up to date, the fundamental fracture mechanisms under loading have not been well understood in BMGs. It is one of the main impediments to developing novel BMGs with superior plasticity and high toughness through alloy composition design [8,9,28]. In the present work, the impact toughness of Zr-based BMGs is focused because it is one of the most important mechanical properties for the materials served under the dynamic loading environments. Hafnium (Hf), the element of the same group in the Periodic Table of Elements and ordinarily symbiotic in practical with Zr, was minorly added into the Zr–Nb–Cu–Ni–Al alloy. It is found that Hf addition could not only enhance the compressive plasticity but also improve the impact toughness by about three to four times in the current Zr-based BMGs. The underlying mechanism is interpreted based on the theory of free volume and fractography. Our finds are not only important for studying the alloying effect on mechanical properties, but also open a new insight for understanding the atomic structure in multicomponent BMGs. 2. Experimental procedures The alloy ingots with nominal chemical compositions of Zr65.5-
xHfxNb3Cu13Ni11Al7.5 (x ¼ 0.0, 0.5, 1.0, 1.5 and 2.0 at.%) were prepared
by vacuum arc melting the mixture of pure metals of Zr, Nb, Cu, Ni, Al under a Ti-gettered argon atmosphere. Samples were denoted as Hf00, Hf05, Hf10, Hf15 and Hf20 based on the extra added Hf content (at.%). An intermediate Zr–Nb alloy was first melted. And then the other pure metals were mixed to prepare the master alloys, which were remelted at least three times to ensure chemical homogeneity. The as-cast rods were prepared by copper mold casting method in a purified argon atmo sphere. The structures of samples were analyzed by X-ray diffraction (XRD; D/max-2500PC) with Cu-Kα radiation. The thermal parameters (Tg, the glass transition temperature; Tx, onset crystallization tempera ture) and the enthalpy change (ΔH) were obtained using differential scanning calorimetry (DSC; Netzsch DSC204F) in a flow argon atmo sphere with a heating rate of 20 K/min. The compressive and impact fracture morphologies were observed using a scanning electron micro scope (SEM; FEI Quanta600) and laser scanning confocal microscope (LSCM; OLSM4000). The compressive samples with a dimension of 3 � 3 � 6 mm3 were cut from the central part of the as-cast rods. The uniaxial compressive tests were carried out using an electronic universal testing machine (INSTRON 5582). The impact tests were performed on rectangular samples (4 � 4 � 55 mm3) using a pendulum impact testing machine (ZBC2452-C, impact energy: 450 J, power supply: 1.5 kW). All samples were carefully polished to ensure parallel ends.
Fig. 1. (a) XRD patterns of all the as-cast rods with different critical diameters; (b) DSC curves for all the as-cast samples (heat rate of 20 K/min).
confirmed by DSC measurements. In the temperature range from 600 K to 900 K, some evident glass transition characteristics are observed (Tg and Tx are marked with the arrows in Fig. 1b). The glass transition and supercooled liquid region are visible in a temperature interval, Tx-Tg ~50 K. There are two exothermic peaks at the experimental temperature range for all the as-cast samples. The first exothermic event corresponds to the precipitation of the icosahedral phase (I-phase), and the second exothermic peak could reflect the formation of the stable crystalline phase. Fig. 2a shows the typical compressive stress-strain curves. All the ascast samples first yielded at the stress higher than 1400 MPa after an obvious linear elastic deformation stage, followed by some plastic deformation before ultimate fracture. It can be found that the ultimate fracture strain (εf) reaches the maximum value of about 2.6% when the Hf addition is 1.0 at.%. Fig. 2a and b offer a direct observation of the apparent variation of yielding strength (σ s), ultimate fracture strength (σf) and especially ultimate fracture strain (εf) with Hf addition. The changes of σ s and σ f are not shown due to they are similar to fracture strain in Fig. 2b. The changes of σs, σf and εf increase first and then decrease with the increase of Hf addition. The critical value of Hf addition with maximum compressive plasticity is about 1.0 at.%. Accordingly, the Hf10 alloy presents the maximum σ f and εf, which are approximately 1516 MPa and 2.6%, respectively. The variation ranges
3. Results and discussions Fig. 1 shows some details on the critical diameters and microstruc tures of the current BMGs. For the Zr65.5-xHfxNb3Cu13Ni11Al7.5 (x ¼ 0.0, 0.5, 1.0, 1.5 and 2.0 at.%) BMGs, the upper limit for amorphous phase formation is above 10 mm. As shown in Fig. 1a, the XRD patterns of the as-cast rods with critical diameters only exhibit broad diffraction peaks in the 2θ range of 30–40� . However, some sharp diffraction peaks cor responding to the crystalline phase are detectable when the diameters rods exceed the critical values. The structural characteristic of the ascast rods with 5 mm and 6 mm diameters is similar to Fig. 1a. There fore, XRD patterns of samples cut from as-cast rods were not shown in the next section for mechanical tests. Fig. 1b shows that the existence of the amorphous phase for all the as-cast samples could be further 2
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Fig. 2. (a) Room-temperature uniaxial compressive stress-strain curves of the as-cast samples; (b) The plastic strain of the samples with different Hf addition; (c) SEM images showing localized shear bands on the lateral surface, the inset image shows the shear angle and the local site; (d) SEM images showing the vein-like patterns in detail, the inset image shows the overview of the smooth shear region on the fracture surface.
of σ s, σ f and εf are slight for all the as-cast samples. However, the strength and plasticity could be simultaneously enhanced by an appropriate amount of Hf addition at the level of about 1.0 at.%. Fig. 2c and d shows that the shear fracture is the dominant mode for all the as-cast samples due to localized shear bands. For the Hf10 alloy, the shear angle is about 42.1� , which is consistent with the results re ported in other BMGs [29,30]. Fig. 2c shows that only a few localized shear bands (marked by white arrows) generated near the fracture surface region, which agrees well with the limited plastic deformation, as shown in Fig. 2a. Fig. 2d shows that the well-developed vein-like patterns, as well as some droplets, can be observed on the fracture surfaces. The results suggest that softening and melting happened inside the major shear band during the fracture process because of the release of considerable elastic strain energy instantaneously [31,32]. Generally, the alloying effects of the minor additions on the internal structure state of BMGs are often involved. There is also a correlation between the nature of disordered structure and mechanical properties [12,33,34]. The free volume model is a prevailing model to reveal the
local changes in the excess volume due to irregular atomic arrangements of constituent elements [10]. Based on the simplifying assumptions, the free volume content, which is a scalar parameter, plays a crucial and different role in mechanical properties under quasi-static loading and dynamic loading [35]. The relationship between free volume content and mechanical properties under two different loading conditions are discussed in the next section. The impact toughness, Ak, is a significant parameter to evaluate the capacity to resist the fracture under dynamic loading. The value is usually calculated by the equation of Ak ¼ AU/AN, where AU is the impact fracture energy and AN is the cross-sectional area of the tested samples. The impact toughness was measured for all the as-cast samples, as shown in Fig. 3a. In the present work, the glass-forming ability is limited for all the current BMGs, so the non-standard tested samples without U-shape notch were prepared and tested in the measurements. However, this work aims to understand the relative change on impact toughness via minor Hf addition, so great attention was paid to ensure the impact samples tested were identical in the dimension to compare
Fig. 3. (a) Relative changes of the impact toughness (Ak) and the enthalpy release (ΔH) during heating; (b) DSC curves with a heating rate of 20 K/min show structural relaxation in the glass transition range. 3
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the results in a simple way. The obtained Ak is rationalized to compare the variation of impact toughness among the BMGs with minor Hf addition. Fig. 3a shows that the values of impact toughness obviously increase when the additional Hf content is lower than 1.0 at.% and then keep almost stable when the Hf content increases to 2.0 at.%. For the Hf00 alloy, the impact toughness is measured to be approximately 33 kJ/m2. However, it is surprising that the impact toughness of Hf15 alloy reaches to about 127 kJ/m2, which is about four times than that of Hf00 alloy. The impact toughness value for Hf15 alloy is equivalent to that of typical Zr-based and Pd-based BMGs (i.e., Pd40Cu30Ni10P20) with the high glass-forming ability [11,36]. Based on the experimental results, it is clearly shown that the minor Hf addition results in the simultaneous enhancement of plasticity and toughness in the present BMGs. As discussed in previous sections, the local distribution of free volume content related to structure relaxation is tuned by Hf addition. It can also be speculated that the nanoscale atomic structure should be responsible for the improvement of me chanical properties [37,38]. Therefore, we first examined the variation of free volume content with minor Hf alloying for the present BMGs. Slipenyuk et al. [23] found that a linear dependence is illustrated be tween relaxation enthalpy change (ΔH) and free volume reduction (Δvf). Δvf is determined by measuring the difference in the density of the thermally treated Zr55Cu30Al10Ni5 alloy, which can be expressed as ΔH ¼ β⋅Δvf, where β is a constant. Fig. 3b shows the DSC curves of the exothermic structure relaxation for all the as-cast samples. The area corresponding to the enthalpy change due to structural relaxation in creases firstly and decreases slightly with the increasing of the Hf con tent, as illustrated in Fig. 3a. With the increase of Hf addition, ΔH rises by about four times from 2.0 J/g of the Hf00 alloy to 8.2 J/g of the Hf15 alloy. It is reasonable to infer that the content of free volume signifi cantly increases due to minor Hf addition by examining the variation of ΔH. In multicomponent alloys, the atomic radii among constituent ele ments are involved. The alloying effects of the minor Hf addition would change the nanoscale atomic structure. However, we stress on the minor Hf addition of 0–2 at.% into the Zr–Nb–Cu–Ni–Al metallic glass, which would not change the main basic building block in the amorphous phase. In this work, it is rationalized that the replacement of Zr by Hf in these basic building blocks, i.e., the icosahedral short-range ordering, increases the free volume because of the atomic size of Hf (0.156 nm) a little smaller than that of Zr (0.160 nm). In addition, it can also be concluded that the dependence of ΔH on the Hf content is consistent with that of Ak on the Hf content in Fig. 3a. To some extent, the increase of the free volume content is recognized to contribute to the enhance ment of mechanical properties in the current case via minor Hf addition due to the change of local structural order. The variation of impact toughness is also associated with distinct fracture morphologies. Fracture modes of failed BMGs could be gained by visually examining the fracture surfaces, which is also an excellent way to understand the intrinsical nature of materials. Therefore, we attempted to correlate these fractographic characteristics with the values of impact toughness. The morphologies of the impact fracture surface were observed to explain the energy dissipation mechanisms under dynamic loading [7,9,27]. The characteristic regions consist of initial impact region, vein-like patterns region and dimple-like patterns region [11,26]. Fig. 4a and c are the three-dimensional (3D) fracture surface morphologies of the Hf00 alloy and Hf15 alloy with the mini mum and maximum impact toughness, respectively. The analysis using the LSCM suggests that the formation of ductile vein-like patterns in the BMGs is the main energy dissipation. A higher surface area of vein-like patterns indicates an optimum impact toughness. In addition, the frac ture surfaces of the vein-like patterns are almost parallel to the loading axis, not like that surface of shear fracture in Fig. 2c. It was already explained in other literature that the fracture surfaces were likely to be formed by tensile stress rather than the shear stress under dynamic loading [39]. The characteristics of crack initiation and propagation and ultimate fracture are observed in different regions. Compared with
Fig. 4. (a)–(d) 3D morphologies on the fracture surfaces of Hf00 alloy and Hf15 alloy using an LSCM; (a) and (c) Fracture surfaces of the whole height views; (b) and (d) The vein-like patterns regions, located in height2 (h2) corresponding to (a) and (c), respectively; (e) SEM image of the fracture surface of Hf00 alloy; (f)–(h) Details of different regions marked in (e) at a higher magnification.
Fig. 4a and c, the fracture surface of Hf15 alloy is more uneven than that of Hf00 alloy. Meanwhile, the area of the vein-like patterns region of Hf15 alloy is larger than that of Hf00 alloy. The experimental results provided powerful insights to speculate that Hf15 alloy consumed more energy in this dynamic process, which determined the high impact toughness. Fig. 4b and d shows the surface areas of the vein-like patterns region where the energy consumed can be estimated in detail. The surface area and the ratio of vein-like patterns region for Hf00 alloy and Hf15 alloy are summarized in Table 1. The specific value of the surface area for Hf15 alloy is about three to four times higher than that for Hf00 alloy. A dramatically increasing trend is demonstrated, which is also in Table 1 The surface area and the ratio of vein-like patterns region.
4
Samples
Surface area (mm2)
Ratio (%)
Hf00 Hf15
139.66 446.71
43.5 65.4
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to the formation of densely packed icosahedron-like clusters and loosely packed defective regions. As a result, the mobility of atoms is very different and no longer exhibit heterogeneous distribution in space. The defective regions have a lower coordination number than that those in the densely packed icosahedron-like clusters, so the local atom rear rangements in the defective regions are easier to happen by shear stresses [35,37,43]. The alloying Hf element and matrix Zr element have very similar chemical characteristics, and they are ordinarily symbiotic in practical. The alloying Hf element into Zr-based BMGs substitutes Zr atom as the nearest neighbor. The strong interaction between the central atom and the alloying Hf atom makes the bond shorter [14,44,45]. Hence, the nanoscale atomic units are more stable, and it is also tempting to speculate that the bond shortening promotes more efficient atomic packing. Due to the bond shortening in the densely packed icosahedron-like clusters, atoms appear to be caged and immobile, so it is a primary step to reduce the Gibbs free energy and reach a local equilibrium state. As a result, the free volume content (V2) in the sites between the densely packed icosahedron-like clusters and the matrix would continuously increase. In contrast, atoms in the loosely packed defective regions are mobile and move more easily via Hf addition, which also promotes the change of free volume content (V1). So far, there is not a definite conclusion on the relationship between the nanoscale structure motifs and the dynamics [35]. The increase of atomic packing clusters would reduce the free volume content from a general structural description. However, we have constructed a struc tural model to elucidate the enhancement mechanisms of plasticity and toughness via minor Hf addition from an inhomogeneous standpoint in the local environment. Our structural insight indicates that the varia tions of V1 and V2 are associated with improvements in mechanical properties. On the one hand, the atomic rearrangement and the flow behaviors in the loosely packed defective regions happened easily under quasi-static loading, so the V1 is tuned to favor plasticity. On the other hand, icosahedral short-range order is a blocking unit in BMGs based on the theoretical and experimental effects, so V2 would continuously change via Hf addition. However, it is difficult to form the shear bands in the regions between clusters and matrix, so the effect of V2 on compressive plasticity is negligible due to the densely packed icosahedron-like clusters acting as obstacles for the movement of atoms. We have speculated that the stress state is more complex under dynamic loading. In ideal situations, obstacles can be easily overcome. Therefore, not only V1 but also V2 plays an essential role in the enhancement of impact toughness. Fig. 6 shows the relative changes of V1, V2 and V. V10 and V20 have lower content without Hf addition. The substitution of
accordance with the variation trend of the impact toughness by minor Hf addition, as shown in Fig. 3a. Besides, the fraction of the surface area of the vein-like patterns region in the two-dimensional (2D) projection plane for Hf15 alloy is larger than that for Hf00 alloy. The experimental data elucidate the underlying fracture mechanism under dynamic loading from another perspective. Fig. 4e shows that micro-cracks generated in the initial impact region and then propagated instantaneously toward the interior of as-cast samples under dynamic loading. The ratio of initial impact region is smaller than those for other regions, corresponding to lower energy consumed, As shown in Fig. 4f. With the formation and propagation of cracks, the fracture surface (Fig. 4g) shows that the typical vein-like patterns become rough. Furthermore, it would be reasonable to as sume that the fracture energy was mainly consumed to generate liga ments due to the massive flow of softened materials. The high impact toughness of the Hf15 alloy could attribute to the formation and stretching of ligaments. The traveling mode of cracks (Fig. 4h) changes and consequently, the dimple-like patterns region appears, which is relatively smoother. The dimple-like patterns consumed less fracture energy under dynamic loading due to the instantaneously adiabatic heating at the final fracture process. Under the framework of free volume theory of plastic deformation in the BMGs proposed by Spaepen [40,41], the zone with rich free volume favors the rearrangement of the atoms, further triggering the nucleation of shear transformation zones (STZs) and the formation of the shear bands [9]. The improvement of compressive mechanical properties and impact toughness are prominent without considering the experimental error in the present study. Fig. 5a shows an underlying schematic on bond shortening to explain the enhancement of mechanical properties because of minor Hf addi tion. The experimental results show that minor Hf addition could in crease the amount of free volume. However, when the content of additional alloying Hf elements reaches a critical level, the free volume content decreased. The complexity of the atomic size of constituent el ements favors a more efficiently dense random packing structure ac cording to the Greer’s confusion principle [42]. When the nearest neighboring Zr atom was substituted by Hf atom, an empty space is generated. The fundamental knowledge on the structure of BMGs has been reported by utilizing some state-of-the-art tools, and extensive ef fects show that the short-to-medium range order is the overwhelming structural feature [14,37,43]. In multicomponent BMGs, the spatial heterogeneity gives direct evidence of the evolution of disorder atoms to nanoscale atomic units [43]. The inefficient local atomic packing leads
Fig. 5. An underlying schematic on the bond shortening and the change of free volume content due to minor Hf addition. 5
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alloying Hf atoms for the matrix Zr atoms would promote the increase of V1. The alloy Hf10 possesses the largest value of V1, corresponding to the optimal compressive plasticity. When Hf addition exceeds a critical level, the extra free volume could accommodate more Hf atoms and V1 decreased. In this case, it can be speculated that the alloying Hf element increases the chance to restrict the movement of atoms in the loosely packed regions under quasi-static loading. As a result, the formation of the shear bands is more difficult, and the limited number of shear bands may deteriorate the compressive plasticity. As aforementioned, the ISRO is expected to be pronounced in BMGs [35]. V2 would continuously increase and eventually reach a stable state due to minor Hf atoms are quite easy to occupy extra V1 in the loosely packed regions. Therefore, V (¼V1þV2) mainly determines the enhancement of impact toughness by the impact energy absorption. Vmax could be obtained at Hf15 alloy due to a rapid decrease of V1 and sluggish increase of V2. On the one hand, the formation of multiple shear bands correlating with V1 in loosely packed regions is much easier, which improves the impact toughness of BMGs. In this case, the shear bands are preferentially initiated and propagated in the loosely packed region under dynamic loading. Once the tips of shear bands meet at the densely packed clusters during its propagation, the stress on the tips would be released by nucleating and multiplying shear bands. On the other hand, the nanoscale atomic motifs are unstable due to dynamic loading. The sites between the densely packed icosahedron-like clusters and the matrix with more V2 would further promote the formation of the shear bands. As a result, the interaction between the inhomogeneous structure and shear bands can enhance toughness under dynamic loading.
Fig. 6. The changes of different free volume content with the increasing of Hf addition.
Zhang: Conceptualization, Methodology, Writing - review & editing. Acknowledgments This work was supported by the National Key Research and Devel opment Program (2018YFB0703402), National Natural Science Foun dation of China (51790484 and 51531005), Liao Ning Revitalization Program (XLYC1807062 and XLYC1802078) and Shenyang Amorphous Metal Manufacturing Co.
4. Conclusions In summary, the uniaxial compressive mechanical properties and the impact toughness for all the as-cast samples of BMGs were improved by minor Hf addition. The Zr64.5Hf1.0Nb3Cu13Ni11Al7.5 alloy has the optimal compressive fracture strength and fracture strain, reaching 1516 MPa and 2.6%, respectively. The impact toughness is also dramatically improved from 33 kJ/m2 for Zr65.5Nb3Cu13Ni11Al7.5 alloy to 127 kJ/m2 for Zr64Hf1.5Nb3Cu13Ni11Al7.5 alloy. The initiation and propagation of the local shear bands lead to catastrophic fracture under quasi-static loading. Not only the high fraction of surface area but also the high fraction of the ratio of vein-like patterns regions should be responsible for the optimum impact toughness. A structural model is constructed to understand the enhancement mechanism. We found that the Hf addition can not only change the nanoscale atomic structure but also simultaneously increase the free volume content. The bond short ening is expected to play an essential role in the enhancement of impact toughness due to the increase of V2 in the sites between densely packed icosahedron-like clusters and the matrix. This work may provide a basis for elucidating the relationship between the alloying effects and the macroscopic mechanical properties. An intuitive concept of spatial heterogeneity is further developed and employed.
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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Y.H. Zhu: Conceptualization, Methodology, Formal analysis, Writing - original draft, Writing - review & editing. Z.W. Zhu: Conceptualization, Methodology, Formal analysis, Writing - original draft, Writing - review & editing. S. Chen: Formal analysis, Writing review & editing. H.M. Fu: Formal analysis, Writing - review & editing. H.W. Zhang: Formal analysis. H. Li: Formal analysis. A.M. Wang: Conceptualization, Methodology, Writing - review & editing. H.F. 6
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Intermetallics 118 (2020) 106685
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Intermetallics journal homepage: http://www.elsevier.com/locate/intermet
Simultaneously enhancing strength and toughness of Zr-based bulk metallic glasses via minor Hf addition Y.H. Zhu a, b, Z.W. Zhu a, *, S. Chen a, H.M. Fu a, H.W. Zhang a, H. Li a, A.M. Wang a, H.F. Zhang a a b
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang, 110016, China School of Materials Science and Engineering, University of Science and Technology of China, Shenyang, 110016, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Bulk metallic glass Minor addition Compressive properties Impact toughness Fracture mechanisms Free volume
Zr65.5-xHfxNb3Cu13Ni11Al7.5 (x ¼ 0.0, 0.5, 1.0, 1.5 and 2.0 at.%) bulk metallic glasses (BMGs) were successfully prepared by copper mold casting. The effects of minor Hf addition on the mechanical properties and fracture mechanisms were investigated. The quasi-static compressive mechanical properties of Zr65.5-xHfxNb3Cu13 Ni11Al7.5 alloys were significantly improved by Hf addition. The optimum compressive fracture strength (σ f) and fracture strain (εf) are 1516 MPa and 2.6%, respectively, for the Zr64.5Hf1.0Nb3Cu13Ni11Al7.5 alloy. The impact toughness of the Zr64Hf1.5Nb3Cu13Ni11Al7.5 alloy (Ak ¼ 127 kJ/m2) is nearly four times than that of the Zr65.5Nb3Cu13Ni11Al7.5 alloy (Ak ¼ 33 kJ/m2). The increase of the fracture surface area and the ratio of vein-like patterns regions caused by Hf addition play an important role in fracture energy dissipation under dynamic loading. The dependence of mechanical properties on the Hf content is closely related to the variation of free volume in the current alloys. The relationship between the Hf addition and the free volume change is discussed in detail. These results may provide new insights into simultaneously enhancing the plasticity and toughness of BMGs with minor alloying.
1. Introduction Bulk metallic glasses (BMGs) exhibit unique properties, including high strength, large elastic strains of up to ~2% and high corrosion resistance due to their disorder microstructure [1–6]. Their intriguing mechanical properties have made them candidate materials in many potential structural applications [2]. A significant number of BMG sys tems have been investigated concerning the mechanical properties and fracture mechanisms [7–10]. Among the glass-forming alloy systems, Zr-based BMGs have attracted much more attention because of the perfect combinations of the super high glass-forming ability (GFA) and unique mechanical properties [8,9]. Of all the mechanical properties, the toughness is the most concerned because the catastrophic fracture occurs to BMGs under loading, which limits their widespread applica tion as structural engineering materials [10,11]. Chemical effects, i.e., minor addition or microalloying, etc., play an essential role in improving the GFA, thermal stability, and the toughness of BMGs [12,13]. Multicomponent systems could make the supercooled liquid stable, originating from higher degrees of densely packed atomic configurations in local regions [2]. In addition, the electronic in teractions and bond shortening could strongly influence the population
of the ordered clusters and their spatial connectivity [14]. In the Zr-TM (TM ¼ Ni, Co or Cu)–Al systems, a sufficient ductility of BMGs could be obtained by making a balance between the metallic bond nature and free volume [15]. In a word, the significant relationships between the in ternal structure and bond state may be the origin of the mechanical properties of BMGs. It is also reported that minor addition of some re fractory elements (including Mo, Ta or Nb, etc.) effectively improve the corrosion resistance and mechanical properties of some Cu- and Zr-based BMGs [16–19]. The addition of these elements is recognized to stabilize the icosahedral short-range ordering (ISRO) in the atomic-level structure, and the ISRO is the basic building unit in the BMGs [20–22]. Furthermore, the local distribution of free volume is recognized to be changed, leading to the enhancement of plastic deformation as well as the variation of thermodynamic properties [8,9,23,24]. Recent molec ular dynamics simulations show that chemical separation occurs in the process of cavitation in the tough BMGs, which elevates the energy barrier of the nucleation of micro-crack [25]. The substitution of alloying elements can promote structural and chemical heterogeneity [14]. As a result, the internal heterogeneity may be able to change the content of free volume at the atomic level, which is associated with the formation of shear bands. Meanwhile, multiple shear bands are
* Corresponding author. E-mail address: [email protected] (Z.W. Zhu). https://doi.org/10.1016/j.intermet.2019.106685 Received 9 October 2019; Received in revised form 26 November 2019; Accepted 23 December 2019 Available online 30 December 2019 0966-9795/© 2019 Elsevier Ltd. All rights reserved.
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generated to release energy storage and further balance the distribution of stress. In this way, the nucleation of micro-crack is inhibited, resulting in the significant enhancement of toughness for BMGs. From the perspective of fractography, the formation and propagation of shear bands, viscous deformation, and the transition of fracture patterns contribute to the improvement of impact toughness to some extent based on a model of energy dissipation mechanisms during failure [5,26,27]. Up to date, the fundamental fracture mechanisms under loading have not been well understood in BMGs. It is one of the main impediments to developing novel BMGs with superior plasticity and high toughness through alloy composition design [8,9,28]. In the present work, the impact toughness of Zr-based BMGs is focused because it is one of the most important mechanical properties for the materials served under the dynamic loading environments. Hafnium (Hf), the element of the same group in the Periodic Table of Elements and ordinarily symbiotic in practical with Zr, was minorly added into the Zr–Nb–Cu–Ni–Al alloy. It is found that Hf addition could not only enhance the compressive plasticity but also improve the impact toughness by about three to four times in the current Zr-based BMGs. The underlying mechanism is interpreted based on the theory of free volume and fractography. Our finds are not only important for studying the alloying effect on mechanical properties, but also open a new insight for understanding the atomic structure in multicomponent BMGs. 2. Experimental procedures The alloy ingots with nominal chemical compositions of Zr65.5-
xHfxNb3Cu13Ni11Al7.5 (x ¼ 0.0, 0.5, 1.0, 1.5 and 2.0 at.%) were prepared
by vacuum arc melting the mixture of pure metals of Zr, Nb, Cu, Ni, Al under a Ti-gettered argon atmosphere. Samples were denoted as Hf00, Hf05, Hf10, Hf15 and Hf20 based on the extra added Hf content (at.%). An intermediate Zr–Nb alloy was first melted. And then the other pure metals were mixed to prepare the master alloys, which were remelted at least three times to ensure chemical homogeneity. The as-cast rods were prepared by copper mold casting method in a purified argon atmo sphere. The structures of samples were analyzed by X-ray diffraction (XRD; D/max-2500PC) with Cu-Kα radiation. The thermal parameters (Tg, the glass transition temperature; Tx, onset crystallization tempera ture) and the enthalpy change (ΔH) were obtained using differential scanning calorimetry (DSC; Netzsch DSC204F) in a flow argon atmo sphere with a heating rate of 20 K/min. The compressive and impact fracture morphologies were observed using a scanning electron micro scope (SEM; FEI Quanta600) and laser scanning confocal microscope (LSCM; OLSM4000). The compressive samples with a dimension of 3 � 3 � 6 mm3 were cut from the central part of the as-cast rods. The uniaxial compressive tests were carried out using an electronic universal testing machine (INSTRON 5582). The impact tests were performed on rectangular samples (4 � 4 � 55 mm3) using a pendulum impact testing machine (ZBC2452-C, impact energy: 450 J, power supply: 1.5 kW). All samples were carefully polished to ensure parallel ends.
Fig. 1. (a) XRD patterns of all the as-cast rods with different critical diameters; (b) DSC curves for all the as-cast samples (heat rate of 20 K/min).
confirmed by DSC measurements. In the temperature range from 600 K to 900 K, some evident glass transition characteristics are observed (Tg and Tx are marked with the arrows in Fig. 1b). The glass transition and supercooled liquid region are visible in a temperature interval, Tx-Tg ~50 K. There are two exothermic peaks at the experimental temperature range for all the as-cast samples. The first exothermic event corresponds to the precipitation of the icosahedral phase (I-phase), and the second exothermic peak could reflect the formation of the stable crystalline phase. Fig. 2a shows the typical compressive stress-strain curves. All the ascast samples first yielded at the stress higher than 1400 MPa after an obvious linear elastic deformation stage, followed by some plastic deformation before ultimate fracture. It can be found that the ultimate fracture strain (εf) reaches the maximum value of about 2.6% when the Hf addition is 1.0 at.%. Fig. 2a and b offer a direct observation of the apparent variation of yielding strength (σ s), ultimate fracture strength (σf) and especially ultimate fracture strain (εf) with Hf addition. The changes of σ s and σ f are not shown due to they are similar to fracture strain in Fig. 2b. The changes of σs, σf and εf increase first and then decrease with the increase of Hf addition. The critical value of Hf addition with maximum compressive plasticity is about 1.0 at.%. Accordingly, the Hf10 alloy presents the maximum σ f and εf, which are approximately 1516 MPa and 2.6%, respectively. The variation ranges
3. Results and discussions Fig. 1 shows some details on the critical diameters and microstruc tures of the current BMGs. For the Zr65.5-xHfxNb3Cu13Ni11Al7.5 (x ¼ 0.0, 0.5, 1.0, 1.5 and 2.0 at.%) BMGs, the upper limit for amorphous phase formation is above 10 mm. As shown in Fig. 1a, the XRD patterns of the as-cast rods with critical diameters only exhibit broad diffraction peaks in the 2θ range of 30–40� . However, some sharp diffraction peaks cor responding to the crystalline phase are detectable when the diameters rods exceed the critical values. The structural characteristic of the ascast rods with 5 mm and 6 mm diameters is similar to Fig. 1a. There fore, XRD patterns of samples cut from as-cast rods were not shown in the next section for mechanical tests. Fig. 1b shows that the existence of the amorphous phase for all the as-cast samples could be further 2
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Fig. 2. (a) Room-temperature uniaxial compressive stress-strain curves of the as-cast samples; (b) The plastic strain of the samples with different Hf addition; (c) SEM images showing localized shear bands on the lateral surface, the inset image shows the shear angle and the local site; (d) SEM images showing the vein-like patterns in detail, the inset image shows the overview of the smooth shear region on the fracture surface.
of σ s, σ f and εf are slight for all the as-cast samples. However, the strength and plasticity could be simultaneously enhanced by an appropriate amount of Hf addition at the level of about 1.0 at.%. Fig. 2c and d shows that the shear fracture is the dominant mode for all the as-cast samples due to localized shear bands. For the Hf10 alloy, the shear angle is about 42.1� , which is consistent with the results re ported in other BMGs [29,30]. Fig. 2c shows that only a few localized shear bands (marked by white arrows) generated near the fracture surface region, which agrees well with the limited plastic deformation, as shown in Fig. 2a. Fig. 2d shows that the well-developed vein-like patterns, as well as some droplets, can be observed on the fracture surfaces. The results suggest that softening and melting happened inside the major shear band during the fracture process because of the release of considerable elastic strain energy instantaneously [31,32]. Generally, the alloying effects of the minor additions on the internal structure state of BMGs are often involved. There is also a correlation between the nature of disordered structure and mechanical properties [12,33,34]. The free volume model is a prevailing model to reveal the
local changes in the excess volume due to irregular atomic arrangements of constituent elements [10]. Based on the simplifying assumptions, the free volume content, which is a scalar parameter, plays a crucial and different role in mechanical properties under quasi-static loading and dynamic loading [35]. The relationship between free volume content and mechanical properties under two different loading conditions are discussed in the next section. The impact toughness, Ak, is a significant parameter to evaluate the capacity to resist the fracture under dynamic loading. The value is usually calculated by the equation of Ak ¼ AU/AN, where AU is the impact fracture energy and AN is the cross-sectional area of the tested samples. The impact toughness was measured for all the as-cast samples, as shown in Fig. 3a. In the present work, the glass-forming ability is limited for all the current BMGs, so the non-standard tested samples without U-shape notch were prepared and tested in the measurements. However, this work aims to understand the relative change on impact toughness via minor Hf addition, so great attention was paid to ensure the impact samples tested were identical in the dimension to compare
Fig. 3. (a) Relative changes of the impact toughness (Ak) and the enthalpy release (ΔH) during heating; (b) DSC curves with a heating rate of 20 K/min show structural relaxation in the glass transition range. 3
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the results in a simple way. The obtained Ak is rationalized to compare the variation of impact toughness among the BMGs with minor Hf addition. Fig. 3a shows that the values of impact toughness obviously increase when the additional Hf content is lower than 1.0 at.% and then keep almost stable when the Hf content increases to 2.0 at.%. For the Hf00 alloy, the impact toughness is measured to be approximately 33 kJ/m2. However, it is surprising that the impact toughness of Hf15 alloy reaches to about 127 kJ/m2, which is about four times than that of Hf00 alloy. The impact toughness value for Hf15 alloy is equivalent to that of typical Zr-based and Pd-based BMGs (i.e., Pd40Cu30Ni10P20) with the high glass-forming ability [11,36]. Based on the experimental results, it is clearly shown that the minor Hf addition results in the simultaneous enhancement of plasticity and toughness in the present BMGs. As discussed in previous sections, the local distribution of free volume content related to structure relaxation is tuned by Hf addition. It can also be speculated that the nanoscale atomic structure should be responsible for the improvement of me chanical properties [37,38]. Therefore, we first examined the variation of free volume content with minor Hf alloying for the present BMGs. Slipenyuk et al. [23] found that a linear dependence is illustrated be tween relaxation enthalpy change (ΔH) and free volume reduction (Δvf). Δvf is determined by measuring the difference in the density of the thermally treated Zr55Cu30Al10Ni5 alloy, which can be expressed as ΔH ¼ β⋅Δvf, where β is a constant. Fig. 3b shows the DSC curves of the exothermic structure relaxation for all the as-cast samples. The area corresponding to the enthalpy change due to structural relaxation in creases firstly and decreases slightly with the increasing of the Hf con tent, as illustrated in Fig. 3a. With the increase of Hf addition, ΔH rises by about four times from 2.0 J/g of the Hf00 alloy to 8.2 J/g of the Hf15 alloy. It is reasonable to infer that the content of free volume signifi cantly increases due to minor Hf addition by examining the variation of ΔH. In multicomponent alloys, the atomic radii among constituent ele ments are involved. The alloying effects of the minor Hf addition would change the nanoscale atomic structure. However, we stress on the minor Hf addition of 0–2 at.% into the Zr–Nb–Cu–Ni–Al metallic glass, which would not change the main basic building block in the amorphous phase. In this work, it is rationalized that the replacement of Zr by Hf in these basic building blocks, i.e., the icosahedral short-range ordering, increases the free volume because of the atomic size of Hf (0.156 nm) a little smaller than that of Zr (0.160 nm). In addition, it can also be concluded that the dependence of ΔH on the Hf content is consistent with that of Ak on the Hf content in Fig. 3a. To some extent, the increase of the free volume content is recognized to contribute to the enhance ment of mechanical properties in the current case via minor Hf addition due to the change of local structural order. The variation of impact toughness is also associated with distinct fracture morphologies. Fracture modes of failed BMGs could be gained by visually examining the fracture surfaces, which is also an excellent way to understand the intrinsical nature of materials. Therefore, we attempted to correlate these fractographic characteristics with the values of impact toughness. The morphologies of the impact fracture surface were observed to explain the energy dissipation mechanisms under dynamic loading [7,9,27]. The characteristic regions consist of initial impact region, vein-like patterns region and dimple-like patterns region [11,26]. Fig. 4a and c are the three-dimensional (3D) fracture surface morphologies of the Hf00 alloy and Hf15 alloy with the mini mum and maximum impact toughness, respectively. The analysis using the LSCM suggests that the formation of ductile vein-like patterns in the BMGs is the main energy dissipation. A higher surface area of vein-like patterns indicates an optimum impact toughness. In addition, the frac ture surfaces of the vein-like patterns are almost parallel to the loading axis, not like that surface of shear fracture in Fig. 2c. It was already explained in other literature that the fracture surfaces were likely to be formed by tensile stress rather than the shear stress under dynamic loading [39]. The characteristics of crack initiation and propagation and ultimate fracture are observed in different regions. Compared with
Fig. 4. (a)–(d) 3D morphologies on the fracture surfaces of Hf00 alloy and Hf15 alloy using an LSCM; (a) and (c) Fracture surfaces of the whole height views; (b) and (d) The vein-like patterns regions, located in height2 (h2) corresponding to (a) and (c), respectively; (e) SEM image of the fracture surface of Hf00 alloy; (f)–(h) Details of different regions marked in (e) at a higher magnification.
Fig. 4a and c, the fracture surface of Hf15 alloy is more uneven than that of Hf00 alloy. Meanwhile, the area of the vein-like patterns region of Hf15 alloy is larger than that of Hf00 alloy. The experimental results provided powerful insights to speculate that Hf15 alloy consumed more energy in this dynamic process, which determined the high impact toughness. Fig. 4b and d shows the surface areas of the vein-like patterns region where the energy consumed can be estimated in detail. The surface area and the ratio of vein-like patterns region for Hf00 alloy and Hf15 alloy are summarized in Table 1. The specific value of the surface area for Hf15 alloy is about three to four times higher than that for Hf00 alloy. A dramatically increasing trend is demonstrated, which is also in Table 1 The surface area and the ratio of vein-like patterns region.
4
Samples
Surface area (mm2)
Ratio (%)
Hf00 Hf15
139.66 446.71
43.5 65.4
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to the formation of densely packed icosahedron-like clusters and loosely packed defective regions. As a result, the mobility of atoms is very different and no longer exhibit heterogeneous distribution in space. The defective regions have a lower coordination number than that those in the densely packed icosahedron-like clusters, so the local atom rear rangements in the defective regions are easier to happen by shear stresses [35,37,43]. The alloying Hf element and matrix Zr element have very similar chemical characteristics, and they are ordinarily symbiotic in practical. The alloying Hf element into Zr-based BMGs substitutes Zr atom as the nearest neighbor. The strong interaction between the central atom and the alloying Hf atom makes the bond shorter [14,44,45]. Hence, the nanoscale atomic units are more stable, and it is also tempting to speculate that the bond shortening promotes more efficient atomic packing. Due to the bond shortening in the densely packed icosahedron-like clusters, atoms appear to be caged and immobile, so it is a primary step to reduce the Gibbs free energy and reach a local equilibrium state. As a result, the free volume content (V2) in the sites between the densely packed icosahedron-like clusters and the matrix would continuously increase. In contrast, atoms in the loosely packed defective regions are mobile and move more easily via Hf addition, which also promotes the change of free volume content (V1). So far, there is not a definite conclusion on the relationship between the nanoscale structure motifs and the dynamics [35]. The increase of atomic packing clusters would reduce the free volume content from a general structural description. However, we have constructed a struc tural model to elucidate the enhancement mechanisms of plasticity and toughness via minor Hf addition from an inhomogeneous standpoint in the local environment. Our structural insight indicates that the varia tions of V1 and V2 are associated with improvements in mechanical properties. On the one hand, the atomic rearrangement and the flow behaviors in the loosely packed defective regions happened easily under quasi-static loading, so the V1 is tuned to favor plasticity. On the other hand, icosahedral short-range order is a blocking unit in BMGs based on the theoretical and experimental effects, so V2 would continuously change via Hf addition. However, it is difficult to form the shear bands in the regions between clusters and matrix, so the effect of V2 on compressive plasticity is negligible due to the densely packed icosahedron-like clusters acting as obstacles for the movement of atoms. We have speculated that the stress state is more complex under dynamic loading. In ideal situations, obstacles can be easily overcome. Therefore, not only V1 but also V2 plays an essential role in the enhancement of impact toughness. Fig. 6 shows the relative changes of V1, V2 and V. V10 and V20 have lower content without Hf addition. The substitution of
accordance with the variation trend of the impact toughness by minor Hf addition, as shown in Fig. 3a. Besides, the fraction of the surface area of the vein-like patterns region in the two-dimensional (2D) projection plane for Hf15 alloy is larger than that for Hf00 alloy. The experimental data elucidate the underlying fracture mechanism under dynamic loading from another perspective. Fig. 4e shows that micro-cracks generated in the initial impact region and then propagated instantaneously toward the interior of as-cast samples under dynamic loading. The ratio of initial impact region is smaller than those for other regions, corresponding to lower energy consumed, As shown in Fig. 4f. With the formation and propagation of cracks, the fracture surface (Fig. 4g) shows that the typical vein-like patterns become rough. Furthermore, it would be reasonable to as sume that the fracture energy was mainly consumed to generate liga ments due to the massive flow of softened materials. The high impact toughness of the Hf15 alloy could attribute to the formation and stretching of ligaments. The traveling mode of cracks (Fig. 4h) changes and consequently, the dimple-like patterns region appears, which is relatively smoother. The dimple-like patterns consumed less fracture energy under dynamic loading due to the instantaneously adiabatic heating at the final fracture process. Under the framework of free volume theory of plastic deformation in the BMGs proposed by Spaepen [40,41], the zone with rich free volume favors the rearrangement of the atoms, further triggering the nucleation of shear transformation zones (STZs) and the formation of the shear bands [9]. The improvement of compressive mechanical properties and impact toughness are prominent without considering the experimental error in the present study. Fig. 5a shows an underlying schematic on bond shortening to explain the enhancement of mechanical properties because of minor Hf addi tion. The experimental results show that minor Hf addition could in crease the amount of free volume. However, when the content of additional alloying Hf elements reaches a critical level, the free volume content decreased. The complexity of the atomic size of constituent el ements favors a more efficiently dense random packing structure ac cording to the Greer’s confusion principle [42]. When the nearest neighboring Zr atom was substituted by Hf atom, an empty space is generated. The fundamental knowledge on the structure of BMGs has been reported by utilizing some state-of-the-art tools, and extensive ef fects show that the short-to-medium range order is the overwhelming structural feature [14,37,43]. In multicomponent BMGs, the spatial heterogeneity gives direct evidence of the evolution of disorder atoms to nanoscale atomic units [43]. The inefficient local atomic packing leads
Fig. 5. An underlying schematic on the bond shortening and the change of free volume content due to minor Hf addition. 5
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alloying Hf atoms for the matrix Zr atoms would promote the increase of V1. The alloy Hf10 possesses the largest value of V1, corresponding to the optimal compressive plasticity. When Hf addition exceeds a critical level, the extra free volume could accommodate more Hf atoms and V1 decreased. In this case, it can be speculated that the alloying Hf element increases the chance to restrict the movement of atoms in the loosely packed regions under quasi-static loading. As a result, the formation of the shear bands is more difficult, and the limited number of shear bands may deteriorate the compressive plasticity. As aforementioned, the ISRO is expected to be pronounced in BMGs [35]. V2 would continuously increase and eventually reach a stable state due to minor Hf atoms are quite easy to occupy extra V1 in the loosely packed regions. Therefore, V (¼V1þV2) mainly determines the enhancement of impact toughness by the impact energy absorption. Vmax could be obtained at Hf15 alloy due to a rapid decrease of V1 and sluggish increase of V2. On the one hand, the formation of multiple shear bands correlating with V1 in loosely packed regions is much easier, which improves the impact toughness of BMGs. In this case, the shear bands are preferentially initiated and propagated in the loosely packed region under dynamic loading. Once the tips of shear bands meet at the densely packed clusters during its propagation, the stress on the tips would be released by nucleating and multiplying shear bands. On the other hand, the nanoscale atomic motifs are unstable due to dynamic loading. The sites between the densely packed icosahedron-like clusters and the matrix with more V2 would further promote the formation of the shear bands. As a result, the interaction between the inhomogeneous structure and shear bands can enhance toughness under dynamic loading.
Fig. 6. The changes of different free volume content with the increasing of Hf addition.
Zhang: Conceptualization, Methodology, Writing - review & editing. Acknowledgments This work was supported by the National Key Research and Devel opment Program (2018YFB0703402), National Natural Science Foun dation of China (51790484 and 51531005), Liao Ning Revitalization Program (XLYC1807062 and XLYC1802078) and Shenyang Amorphous Metal Manufacturing Co.
4. Conclusions In summary, the uniaxial compressive mechanical properties and the impact toughness for all the as-cast samples of BMGs were improved by minor Hf addition. The Zr64.5Hf1.0Nb3Cu13Ni11Al7.5 alloy has the optimal compressive fracture strength and fracture strain, reaching 1516 MPa and 2.6%, respectively. The impact toughness is also dramatically improved from 33 kJ/m2 for Zr65.5Nb3Cu13Ni11Al7.5 alloy to 127 kJ/m2 for Zr64Hf1.5Nb3Cu13Ni11Al7.5 alloy. The initiation and propagation of the local shear bands lead to catastrophic fracture under quasi-static loading. Not only the high fraction of surface area but also the high fraction of the ratio of vein-like patterns regions should be responsible for the optimum impact toughness. A structural model is constructed to understand the enhancement mechanism. We found that the Hf addition can not only change the nanoscale atomic structure but also simultaneously increase the free volume content. The bond short ening is expected to play an essential role in the enhancement of impact toughness due to the increase of V2 in the sites between densely packed icosahedron-like clusters and the matrix. This work may provide a basis for elucidating the relationship between the alloying effects and the macroscopic mechanical properties. An intuitive concept of spatial heterogeneity is further developed and employed.
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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Y.H. Zhu: Conceptualization, Methodology, Formal analysis, Writing - original draft, Writing - review & editing. Z.W. Zhu: Conceptualization, Methodology, Formal analysis, Writing - original draft, Writing - review & editing. S. Chen: Formal analysis, Writing review & editing. H.M. Fu: Formal analysis, Writing - review & editing. H.W. Zhang: Formal analysis. H. Li: Formal analysis. A.M. Wang: Conceptualization, Methodology, Writing - review & editing. H.F. 6
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