Effect of TaW particles on the microstructure and mechanical properties of metastable Cu47.5Zr47.5Al5 alloys

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Effect of TaW particles on the microstructure and mechanical properties of metastable Cu47.5Zr47.5Al5 alloys K.K. Song, S. Pauly, Z. Wang, U. Kühn, J. Eckert

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Received date: 22 May 2013 Revised date: 31 July 2013 Accepted date: 1 September 2013 Cite this article as: K.K. Song, S. Pauly, Z. Wang, U. Kühn, J. Eckert, Effect of TaW particles on the microstructure and mechanical properties of metastable Cu47.5Zr47.5Al5 alloys, Materials Science & Engineering A, http://dx.doi.org/10.1016/j. msea.2013.09.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effect of TaW particles on the microstructure and mechanical properties of metastable Cu47.5Zr47.5Al5 alloys K.K. Song 1, 2, *, S. Pauly 1, 3, *, Z. Wang 1, 2, U. Kühn 1, and J. Eckert 1, 2 1

IFW Dresden, Institut für Komplexe Materialien, Helmholtzstraße 20, D-01069

Dresden, Germany. 2

Institut für Werkstoffwissenschaft, TU Dresden, D-01062 Dresden, Germany.

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Present address: Department of Materials Science & Metallurgy, University of

Cambridge, Cambridge CB2 3QZ, UK.

*

Corresponding authors:

Address: IFW Dresden, Helmholtzstraße 20, D-01069 Dresden, Germany. Tel.: +49 351 4659 532; Fax: +49 351 4659 452. E-mail addresses: [email protected] (K. K. Song); s.pauly@ ifw-dresden.de (S. Pauly).

Abstract By introducing TaW particles into the Cu47.5Zr47.5Al5 melt, it was found that B2 CuZr dendrites nucleate and grow around the TaW particles, implying that these refractory particles can be effectively used to modify the microstructures of metastable CuZr-based alloys. At constant TaW addition of 2 at.% a decreasing particle size (i.e. higher density of nucleation sites) clearly promotes the nucleation of the B2 phase, but deteriorates the glass-forming ability (GFA) of the glassy matrix. All investigated samples exhibit compressive plasticity and obvious work hardening during deformation and the corresponding deformation mechanisms are discussed.

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Our studies may provide a strategy to control the formation of B2 CuZr phase in CuZr-based BMG composites.

Keywords: Bulk amorphous alloys; Composites; Martensitic transformation; Shear bands; Plasticity;

1. Introduction Over the past decades, bulk metallic glasses (BMGs) have been the subject of intense research owing to their extreme strength, high resilience and functional properties [1-6]. However, their use as new high-strength structural materials is severely limited due to their poor ductility at room temperature arising from shear localization during deformation [2, 5]. Yet, by ex-situ or in-situ introduction of crystalline phases into the glassy matrix, the intrinsic brittleness of BMGs can be overcome [7-12]. For example, the ductility or toughness of monoclinic BMGs can be effectively improved by introducing ex-situ different particles such as Ta, W, Nb, Mo, WC, or SiC into the glassy matrix [13-27], which can be linked to the restriction of shear band propagation and the generation of multiple shear bands in the glassy matrix, and the severe deformation of the reinforcing particles. By using this method, one can control the volume fraction, the size and the distribution of the reinforcement particles [13-27]. However, in most cases, it is difficult to obtain good tensile plasticity and work hardening for these materials due to the residual stress arising from the thermal expansion mismatch and the elastic and plastic incompatibilities between the reinforcing particles and the glassy matrix [18, 21, 28-30], and the rather limited efficiency of dislocation mechanisms in the reinforced particles [13-15].

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By an alternative in-situ approach, CuZr-based BMG composites containing a metastable B2 CuZr phase have been recently successfully fabricated [31-44]. Not only good tensile ductility but also obvious work hardening during deformation can be achieved owing to a martensitic transformation within the shape memory B2 CuZr crystals and the formation of multiple shear bands in the glassy matrix [31-44]. The investigations regarding the physical mechanisms determining the yielding and subsequent plastic deformation indicate that when the volume fraction of the B2 CuZr phase lies between 20 - 80 vol.%, satisfying mechanical properties of the CuZr-based BMG composites can be obtained [31-49]. Besides, the size and the distribution of the in-situ formed B2 CuZr phase can effectively change the stress distribution in the glassy matrix and release stress concentrations at the crystal/glassy matrix interface [40], which is vital for the formation of abundant shear bands during deformation. So it becomes essential to control the formation, i.e. the volume fraction, the size and the distribution of the B2 phase for the development of metastable CuZr-based alloys with improved failure tolerance [31-50]. Until now several methods have been developed to control the formation of B2 CuZr crystals: (1) controlling the casting temperature or melting current/time during casting [33-35, 51]; (2) adjusting the cooling rate [32]; (3) re-melting the ingot before casting [52]; and (4) introducing oxygen or boron nitride [34]. Nevertheless, it is still difficult to control the volume fraction, the distribution and the size of the B2 CuZr phase in the glassy matrix. Recently, relatively finely dispersed B2 CuZr precipitates have been obtained in a CuZr-based alloy after Ta was added to the alloy composition [50]. Even though the presence of Ta seems to have a positively affect the microstructures of CuZr-based BMG composites, it is not possible to artificially control the formation and distribution of the B2 CuZr phase in the glass matrix. As an alternative approach [53],

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the powder metallurgy technology is a viable method to control the size and the distribution of the reinforced crystals. However, it has not been tried to add particles based on refractory metals to the CuZr-based alloy melt by the ex-situ method. In this work, ball-milled WTa particles were added to a CuZr-based glass-forming alloy in order to control the size and distribution of the B2 CuZr phase. Rods with different diameters and volume fractions of the B2 phase in the glassy matrix were fabricated. The relation between the resulting microstructures and the mechanical properties of these BMG composites were investigated.

2. Experimental Pre-alloys with a nominal composition of Cu47.5Zr47.5Al5 (at.%) were prepared by arc-melting appropriate amounts of the constituting elements (purity ≥ 99.9%) and the samples were re-melted for at least three times under a Ti-gettered argon atmosphere to ensure chemical homogeneity. The Cu47.5Zr47.5Al5 ingots were broken into pieces and mixed with 2 at.% Ta50W50 (TaW) particles with different sizes (Table 1) obtained by the powder-metallurgical route. Firstly, a pre-TaW ingot was prepared using arc melting, crushed manually by hammering, and then milled for 6 hours in a ball milling machine with a rotation velocity of 150 rpm under the protection of the argon atmosphere. Secondly, differently sized TaW particles can be selected using different screens, respectively. Finally, Cu47.5Zr47.5Al5 and TaW particles were mixed and condensed into a bulk sample, and then re-melted and solidified in copper molds by suction casting. The diameters of the core cavity of the copper molds are 2 mm and 3 mm, respectively. The detailed casting parameters are listed in Table 1. During casting, the melting current and the melting time were 280 ± 2 μm and 18 ± 2 μm, respectively (Table 1). For comparison, Cu47.5Zr47.5Al5 ribbons were also fabricated

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by a melt spinning device at a wheel speed of 29 m·s-1 under argon atmosphere. The crystallization temperatures of the as-quenched ribbons and the as-cast rods were measured by differential scanning calorimetry (DSC, Netzsch 404C) at a heating rate of 20 K/min. From these measurements the volume fraction of amorphous can be determined based on the ratio of the crystallization enthalpies of the as-cast specimens and a fully amorphous ribbon from the DSC curves. Room temperature compression tests were performed and repeated for at least 2 times at room temperature using an Instron 5869 testing machine at an initial strain rate of 2.5×10-4 s-1. The phases and the microstructures of the samples before and after deformation were characterized using an X-ray diffractometer (XRD, STOE STADI P) with Mo-Kα1 radiation (λ = 0.07093 nm) in transmission geometry, a Zeiss Axiophot optical microscope (OM) and a scanning electron microscope (SEM, Gemini 1530), respectively.

3. Results and discussion 3.1 Formation and microstructure of CuZr-based BMG composites The composites containing 2 at.% TaW particles with sizes of 2 - 20 μm were cast into 2 mm and 3 mm, respectively, which are designated as S1, and S2. Additionally, 2 at.% TaW particles with sizes of 70 - 500 μm were also chosen, and the corresponding as-cast rods with diameters of 2 mm and 3 mm were named as S3 and S4, respectively. The XRD patterns of these as-cast rods with different sizes are shown in Fig. 1. The broad diffraction maximum together with sharp crystalline peaks in the XRD patterns at around 2θ = 18° proves the partially glassy structure of the S1 and S2 samples (Fig. 1(a)). For the S3 and S4 samples, sharp crystalline peaks prove that the samples contain a significant amount of crystals. For all the as-cast specimens, the B2 CuZr phase and a TaW solid solution can be identified, while other reflections

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belong to martensitic CuZr crystals and Cu10Zr7 (Fig. 1(a)). It is well known that Ta50W50 is a solid solution of bcc Ta and bcc W, and its calculated lattice constant is 0.32316 nm [54], being nearly identical to that of the B2 CuZr phase (lattice constant: 0.3262 nm [38]), so that the reflections of the B2 phase and the TaW solid solution overlap with each other. In order to further clarify the existence of the amorphous phase, the DSC measurements were conducted as shown in Fig. 1(b). It can be seen that for the S1, S2, S3, and S4 samples, an endothermic peak occurs during heating, which corresponds with the crystallization of the amorphous phase. Even for the S4 sample, a small crystallization peak is observed (see inset in Fig. 1(b)), confirming the presence of amorphous phase. Based on the crystallization enthalpies of the as-cast samples and the corresponding BMG rod (not show), the volume fraction of amorphous phase for the S1, S2, S3, and S4 samples (Table 1) were determined to be 87 ± 5 vol.%, 55 ± 4 vol.%, 11 ± 2 vol.%, and 2 ± 1 vol.%, respectively. As listed in Table 1 and shown in Fig. 1, the changes of both the diameters of the as-cast rods and the sizes of the TaW solid solution induce variations of the volume fraction of amorphous phase. An increasing rod diameter corresponds to the decrease of the cooling rate under the same casting conditions during quenching [55, 56]. As shown in Fig. 1 and listed in Table 1, with similarly sized TaW particles being added to the glassy matrix, the volume fractions of the B2 CuZr phase and the amorphous phase increase and decrease, respectively, as the cooling rate decreases gradually, i.e. as the diameter of the rods increases from 2 mm to 3 mm. It can be seen that the volume fraction of the amorphous phase for the S1 and S3 samples decreases from 87 ± 5 vol.% to 55 ± 4 vol.% with their diameter decreasing from 2 mm to 3mm. Similarly, for the S3 and S4 samples after the addition of larger TaW particles, the volume fraction of the

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amorphous phase is reduced from 11 ± 2 vol.% to 2 ± 1 vol.%. On the other hand, as listed in Table 1, the volume fraction of the amorphous phase of the S1 and S3 samples obtained under the same cooling rate decreases from 87 ± 5 vol.% to 11 ± 2 vol.% as the size of the TaW particles decreases from 70 - 500 μm to 2 - 20 μm, respectively (Fig. 1). For the S2 and S4 sample, the volume fraction of the amorphous phase decreases from 55 ± 4 vol.% to 2 ± 1 vol.%. Therefore, it can be concluded that the ex-situ addition of TaW particles is favorable for the formation of the B2 CuZr phase for metastable CuZr-based alloys. The OM images of the as-cast S1, S2, S3, and S4 rods containing TaW particles are shown in Figs. 2 and 3, respectively. For the S1 and S2 samples with the addition of large-sized TaW particles, the B2 phase forms around the TaW particles (Figs. 2(ab)), shows a similar compared to the TaW particles (insets in Figs. 2(a-b)), and extends farther into the matrix. As shown in Figs. 2(c-d), for the S1 and S2 samples containing larger TaW particles (70 - 500 μm), B2 dendrites (region B in Figs. 2(c-d)) preferably nucleate and grow around the TaW particles (region A in Figs. 2(c-d)). The dendrite spacing gradually becomes smaller farther away from the TaW particles, and finally the dendrites disappear in the glassy matrix (region C in Figs. 2(c-d)). For the S3 and S4 samples, when the size of TaW particles decreases to 2 - 20 μm, only a small volume fraction of amorphous phase can be obtained at the edge of the rods for the as-cast S3 and S4 samples (Figs. 3(a-b)). This agrees well with the results from Xray diffraction and calorimetry (Fig. 1). As shown in Figs. 3(c-d), B2 CuZr dendrites nucleate around smaller TaW particles and subsequently grow in preferential crystallographic directions for both the as-cast S3 and S4 samples. With a decreasing size of the TaW particles, the volume fraction of B2 dendrites becomes higher and the dendrite arms become clearly developed (Fig. 3(c)), and intersect with each other.

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Even at the edge of the rods, where a higher cooling rate can be obtained (the cooling rate as a function of the casting thickness can be given to be dT/dt=A/Z, A is a fitness constant, and Z is the casting thickness or diameter [56]), some smaller B2 CuZr precipitates still nucleate around the smaller TaW particles (inset in Fig. 3(d)). Furthermore, apart from the B2 phase around the TaW particles, some isolated B2 CuZr crystals also form in the glassy matrix at various places (see arrows in Figs. 2(ab)). Previous work has shown that the critical casting thickness of the fully amorphous Cu47.5Zr47.5Al5 alloy is about 3 mm [57, 58], which implies that the addition of TaW particles may also result to some extent in a decrease of the glass-forming ability (GFA) of the glassy matrix. However, systematic research has demonstrated that the thermal stability and the GFA of the glassy matrix do not deteriorate upon ex-situ addition of particles in Zr-based BMG composites [13-27], which is incompatible with our observation. In order to better understand the formation of the B2 CuZr phase and the amorphous phase after the addition of TaW particles, SEM and EDX measurements were conducted in the regions A, B, and C (Fig. 2(c)) for the S1 sample, and the according elemental maps are shown in Figs. 4(a-e). As listed in Table 2, a small amount of Cu (about 8.25 at.%) and Zr (about 0.89 at.%) elements are dissolved into the TaW solid solution compared with the glassy matrix and the B2 CuZr phase (Figs. 4(a-b)), while a lot of Al can be detected within the TaW particles (Fig. 4(c) and Table 2). Meanwhile, Ta and W appear to be hardly dissolved in the B2 CuZr phase and the glassy matrix (Fig. 4(d-e) and Table 2). The EDX results (Fig. 4(f)) of the S3 sample (see inset in Fig. 4(f)) containing small-sized TaW particles show an almost similar element distribution compared to the S1 samples. The relatively low solubility of Ta and W in the B2 phase and the glassy matrix may be due to the low casting

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temperature/time compared to the high melting point of TaW particles (around 3450 K) [59] and the positive heats of mixing between Ta, W and the main elements Cu and Zr, respectively (Table 3 [60]). The low solubility of Ta and W in the melt and the high solidus temperature are the reasons why most of the TaW particles are preserved during the sample synthesis. Since the TaW solid solution has a similar lattice constant as the B2 CuZr phase, the B2 CuZr phase may preferentially nucleate around the micrometer-scaled TaW particles. Furthermore, previous results have shown that as the Al content decreases from 6 at.% to 0 at.% [61-63] in CuZr-based glass-forming alloys, their GFA gradually decreases. Thus, in the present case, the diffusion of Al into the TaW particles may induce a decreasing Al in the glassy matrix, leading the decrease of the GFA of the glassy matrix. Therefore, some isolated B2 CuZr crystals start to precipitate in the glassy matrix. Besides, a small amount of the TaW solid solution with sub-nanometer sizes may also act to serve as nuclei for the B2 CuZr phase [50], which may be also associated with the formation of isolated B2 CuZr crystals. In the case of the S3 and S4 samples, the total content of TaW addition remains constant (2 at.%), but the number of TaW particles becomes higher due to their smaller size, which leads to a sharp increase of nucleation sites for the B2 phase, and thus leads to the formation of a larger amount of the B2 CuZr phase in the glassy matrix. Besides, it can be seen that the external surfaces of the TaW particles become porous (inset in Fig. 2(c)) for all the as-cast samples, which may result from surface re-melting. This re-crystallization behavior may support the nucleation of the B2 CuZr phase around the TaW particles. Conversely, this reaction may also explain the fact that a little Ta and W are found in the glassy matrix.

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Altogether, it can be concluded that the TaW particles added ex-situ to Cu47.5Zr47.5Al5 can be used as nucleation agent to modify the precipitation of the B2 CuZr phase in CuZr-based BMG composites. The proper size, density and volume fraction of the dopants are very important to modify the nucleation of the B2 CuZr phase by controlling the size of the TaW particles. Yet, further studies need to be done to optimize the milling and casting processes to guarantee the formation of CuZr-based BMG composites with optimized microstructures.

3.2 Mechanical properties of the CuZr-based BMG composites Fig. 5 displays the engineering stress - strain curves of the investigated BMG composites, revealing macroscopic ductility and obvious work hardening. When the volume fraction of the amorphous phase decreases from 87 ± 5 vol.% to 2 ± 1 vol.%, the yield strength of the CuZr-based BMG composites gradually deceases [20, 21]. As shown in Fig. 5, the S1 sample with 87 ± 5 vol.% amorphous phase exhibits one clear yielding at a strain of about 2% and a stress of 1812 ± 60 MPa. When the volume fraction of the amorphous phase decreases to 55 ± 4 vol.%, this yielding behavior still exists but the yield stress decreases to 1508 ± 60 MPa. It is worth noting that at a strain of about 0.95%, there is a deflection in the compressive curve for the S2 sample. As the volume fraction of the amorphous phase decreases to 11 ± 2 vol.% (sample S3), this deflection become more obvious, and seems to become a “yielding”. The corresponding strain and stress are 0.73 ± 0.5 % and 614 ± 60 MPa (see inset in Fig. 5). With further increasing stress, the S3 sample yields again at a stress of 1193 ± 60 MPa. However, for the S4 sample with a crystalline volume fraction of 98 ± 1 vol.%, the “first yielding” is more obvious while the “second yielding” starts to disappear (Fig. 5). As shown in Fig. 5, this “yielding” behavior separates the whole deformation

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process in CuZr-based BMG composites into three different stages. At the same time, all the samples show a relatively large plasticity (Table 4). With decreasing volume fraction of amorphous phase, the plastic strain gradually increases from 1.8 ± 0.5% to 6.5 ± 0.5%, and then decreases to 4.4 ± 0.5%. Besides large plasticity and different yielding behavior, obvious work hardening is observed for all samples. Furthermore, the fracture strength of the CuZr-based BMG composites gradually increases, reaches a maximum for 55 ± 2 vol.% crystalline phase precipitated in the glassy matrix, and then decreases again to 1630 ± 60 MPa. In previous work [35], we have already reported on the different “yielding” mechanisms: (1) the ”first yielding” results from the yielding of the B2 CuZr phase and the initiation of its martensitic transformation; (2) the “second yielding” originates from the combined action of the further development of the martensitic transformation and the multiplication of shear bands. Previous data for Cu47.5Zr47.5Al5 BMG composites have shown that there is a maximum in the plastic/fracture strain [35, 38], which can be described well by the percolation theory [35, 38, 50]. The optimum crystalline volume fraction with respect to the fracture strength is determined to lie between 20 vol.% and 80 vol.% [31-44, 49, 50], which corresponds well with the present observations. As shown in Figs. 6(a-d), multiple shear bands in the S1 and S2 samples which were induced around the B2 crystals and martensitic plates can be found within the B2 phase after deformation (insets in Figs. 6(a-b)). The martensite within the B2 CuZr phase for the S1 and S2 samples exhibits a typical twinned structure. When the content of the B2 CuZr phase increases, the features related to the formation of multiple shear bands and martensite become more evident. However, the surface morphologies of the deformed S3 sample are different compared with the S1 and S2 samples (Figs. 6(c-d)). As shown in Fig. 3(d), the amorphous areas impinge and are

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mainly located at the surface of the samples because of the higher cooling rate near the copper mold [56]. Multiple shear bands can be observed on the surface of the deformed sample (Fig. 6(c)). Meanwhile, accumulative CuZr crystals with twinned structures (Fig. 6(d)) are also observed after deformation. At even lower amorphous volume fractions (2 ± 1 vol.%), the surfaces of the deformed S4 sample consist of two typical structures (Fig. 6(e)): an isolated amorphous phase with few shear bands (region A, inset in Fig. 6(e)), and a vast stretch of CuZr crystals with a significant amount of martensitic plates (region B in Fig. 6(f)). All these deformation characteristics further confirm the responsible deformation mechanisms for the present observations. Namely, both the martensitic transformation within the B2 phase and the multiplication of shear bands contribute to the relatively high deformability of the present BMG composites. Based on previous results [64, 65], the occurrence of the phase transformation together with twinning is responsible for the obvious work hardening during deformation. It is worth mentioning here that as the content of the amorphous phase decreases to 11 ± 2 vol.% (sample S3), the observed yielding is somewhat different compared to reports which have shown an obvious “third yielding” originated from the partial detwining and dislocations with a high density [35, 65] for normal Cu47.5Zr47.5Al5 BMG composites with about 40 - 80 vol.% of crystals. Usually, this yielding behavior can be associated with the yielding of extensive martensites, i.e. the formation of dislocations with a high density and detwinning. In the present case, the addition of TaW particles changes this yielding behavior which usually occurs at a higher stress [35, 65]. Even though the strength of TaW intermetallics is very high (the ideal strengths is up to 20 GPa [66]), the internal stress at the interface of the TaW particles and the B2 CuZr phase may lead to the preferred formation and rapid propagation of

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micro-cracks, and early fracture of the sample [45, 67-70], which may result in the disappearance of the “third” yielding. In order to further ascertain the related deformation mechanisms, the fracture and lateral surfaces of the S2 samples with different deformation strains were systematically investigated. As shown in Fig. 7(a), no shear bands can be observed in the glassy matrix as the applied strain reaches 0.95 ± 0.5 %. However, martensitic plates appear within the B2 phase (see arrow in Fig. 7(a)), implying the occurrence of the martensitic transformation within the B2 CuZr phase [31-41]. When the applied strain is close to 2%, slip bands were observed within the TaW particles (see arrow in Fig. 7(b)) together with some shear bands in the glassy matrix in the vicinity of the B2 CuZr phase (see arrow in Fig. 7(c)), implying that both the TaW particles and the glassy matrix yield. Even though the B2 CuZr phase might be treated to be a buffer between the glassy matrix and the TaW particles, the different yield strength and different mismatch of the thermal expansion coefficients of the B2 CuZr phase and the TaW particles still can induce stress concentrations around these particles [18, 21, 28-30], which can result in the early failure of the B2 CuZr phase and the TaW particles at higher stresses. Close to fracture, some cracks form around the softer B2 CuZr and TaW crystals (see arrow in Fig. 7(d)). As shown in Fig. 7 (d), crack tips may be induced within the soft B2 CuZr phase, cross the soft B2 CuZr phase and the relatively brittle TaW particles, and are then stopped by the stronger glassy matrix. Catastrophic fracture finally occurs when the applied stress increases further. Fig. 8 displays the fracture surfaces of the S2 samples. As shown in Fig. 8(a), typical brittle BMG fracture patterns, i.e. vein-like and river-like patterns, can be observed [5, 71]. However, the fracture patterns around the B2 CuZr phase and the TaW particles (Figs. 8(b-d)) are different than those of monolithic BMGs [5, 71]. After deformation, the

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TaW particles exhibit a brittle cleavage fracture [72] (see arrow in Fig. 8(c)), and the B2 crystals around the TaW particles exhibit a lot of ductile dimple-like features (see arrow in Fig. 8(d)). Besides, around the B2 phase, a large amount of granulated structures and dimple within the vein-like patterns can be found (inset in Fig. 8(d)), which implies that the B2 crystals act as effective barriers for the rapid formation of shear bands. Similar fracture patterns have also been observed for other studied samples, which indicate that the addition of TaW particles indeed has a strong influence on the mechanical properties of CuZr-based BMG composites.

4. Conclusions By introducing ball-milled WTa particles into glass-forming Cu47.5Zr47.5Al5 alloys, BMG composites contain different volume fractions and distribution of the shape memory B2 CuZr phase were obtained. During the quenching process, TaW particles act as nucleation sites for B2 CuZr dendrites and deteriorate of the GFA of the glassy matrix. With decreasing volume fraction of amorphous phase one single-“double”single yielding transition can be observed. Furthermore, as the crystalline volume fraction increases from 13 ± 5 vol.% to 2 ± 1 vol.%, the corresponding plastic strain and fracture strength first increase, reach a maximum, and then decrease. The different deformability together with the obvious work-hardening effect of the present CuZr-based BMG composites are related to the martensitic transformation within the B2 phase, the multiplication of shear bands, the formation of twins, and the plastic deformation of the Ta50W50 intermetallic particles.

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Acknowledgments The authors are grateful to B. Bartusch, B. Opitz, D. Lohse, J. Thomas, L. B. Bruno, M. Frey, and S. Donath for technical assistance and helpful discussions. This work was supported by the Chinese Scholarship Council, the National Basic Research Program of China (973 Program 2007CB613901), the National Natural Science Foundation of China (50831003 and 50631010), the Excellent Youth Project of the Natural Science Foundation of Shandong Province (JQ201012), and the German Science Foundation under the Leibniz Program (grant EC 111/26-1).

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Figure Captions FIG. 1. (a) XRD patterns and (b) DSC curves (heating rate 20 K/min) for the as-cast rods S1 (Diameter: 2 mm, TaW particle size: 2 - 20 μm), S2 (Diameter: 3 mm, TaW particle size: 2 - 20 μm), S3 (Diameter: 2 mm, TaW particle size: 70 - 500 μm), and S4 (Diameter: 3 mm, TaW particle size: 70 - 500 μm) rods with the addition of 2 at.% TaW particles. Inset in Fig. 1(b) is the magnified crystallization peak of the S4 sample during heating.

FIG. 2. Optical micrographs (OM) of the as-cast (a) S1 and (b) S2 rod (Insets: magnified OMs of the B2 CuZr phase around the TaW particles), (c) SEM image of the B2 CuZr phase (region B) around the TaW particles (region A) in the glassy matrix (region C) (Inset: magnified SEM image of the outer surface of the TaW particles in region A), and (d) magnified substructures of the B2 CuZr phase around the TaW particles.

FIG. 3. Optical micrographs of the as-cast (a) S3 and (b) S4 rod, (c) SEM image of the B2 CuZr phase and the TaW particles in the S3 rod, and (d) SEM image of the B2 CuZr phase, TaW particles in the glassy matrix near the edge of the S4 rods (Inset: magnified SEM image of the B2 CuZr phase around a TaW particle at the edge).

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FIG. 4. EDX elemental maps of (a) Cu, (b) Zr, (c) Al, (d) Ta, and (e) W for the as-cast S1 sample shown in Fig. 2(c), and (f) the EDX results of the as-cast S3 sample (inset: SEM image of the very area).

FIG. 5. Room temperature engineering stress - strain curves of the investigated BMG composites (Inset: the magnification of the “first yielding” for the samples). The different “yielding” behaviors separate the whole deformation process into three different stages.

FIG. 6. Surface morphologies of the deformed (a) S1 and (b) S2 samples (Insets: twinned martensitic plates), (c) multiple shear bands and (d) twinned martensitic plates for the as-cast S3 sample, (e) the lateral surface (Inset: multiple shear bands on the surface) and (f) twinned martensitic plates for the as-cast S4 sample.

FIG. 7. Surface morphologies of (a) the B2 CuZr phase (see arrow) and (b) the TaW particles for the as-cast S2 sample strained to 0.95%, (c) surface morphology of the S2 sample strained to 2%, and (d) surface morphology for the as-cast S2 sample just before fracture.

FIG. 8. (a) Typical brittle fracture patterns of amorphous phase, (b) the fracture morphology around the TaW particles, (c) the fracture surface of the TaW particles, and (e) the dimple structures (Inset: the granulated structures distributed within each vein-like structure).

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Tables Table 1 Crystalline volume fraction and size of the TaW particles, the volume fraction of amorphous phase and the casting parameters of the as-cast CuZr-based rods containing 2 at.% TaW particles.

Table 2 Contents of Cu, Zr, Al, Ta, and W in Regions A, B, and C, respectively.

Table 3 Mixing enthalpies of Cu, Zr, Al, Ta and W [60].

Table 4 Elastic-plastic properties of all the samples during deformation.

Table 1. Crystalline volume fractions, sizes of the TaW particles, the volume fraction of amorphous phase and the casting parameters of the as-cast Cu47.5Zr47.5Al5 rods containing 2 at.% TaW particles. Sample

Diameter (mm)

Glassy volume fraction (vol.%)

S1

2

87 ± 5

S2

3

55 ± 4

S3

2

11 ± 2

S4

3

2±1

Size of TaW particles (μm)

Melting Current (A) / time (s)

70 - 500 280 ± 2 / 18 ± 2 2 - 20

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Table 2 Contents of Cu, Zr, Al, Ta, and W in Regions A, B, and C, respectively. Region

Cu (at.%)

Zr (at.%)

Al (at.%)

Ta (at.%)

W (at.%)

A

8.25 ± 2

0.89 ± 1

4.23 ± 2

44.51 ± 2

42.12 ± 2

B

42.70 ± 2

52.26 ± 2

4.47 ± 2

0.17 ± 0.06

0.41 ± 0.06

C

45.86 ± 2

50.06 ± 2

3.48 ± 2

0.15 ± 0.06

0.43 ± 0.06

Table 3 Mixing enthalpies of Cu, Zr, Al, Ta and W [60]. Element Cu Zr Al Ta W

Cu -23 -1 2 22

Mixing enthalpy of the elements (kJ/mol) Zr Al Ta -23 -1 2 -44 3 -44 -19 3 -19 -9 -2 -7

W 22 -9 -2 -7 -

Table 4 Elastic and plastic properties of all the samples during deformation.

Samples

"1st yielding" strain (%)

S1

-

S2

0.95 ± 0.5

S3 S4

"1st yielding" stress (MPa)

"2nd yielding" "2nd yielding" Fracture strength Plastic strain strain (%) stress (MPa) (MPa) (%) 1.9 ± 0.5

1812 ± 60

1867 ± 60

1.8 ± 0.5

850 ± 60

1.96 ± 0.5

1508 ± 60

1869 ± 60

3.2 ± 0.5

0.73 ± 0.5

614 ± 60

2.05 ± 0.5

1193 ± 60

2267 ± 60

6.5 ± 0.5

0.6 ± 0.5

445 ± 60

-

-

1856 ± 60

4.4 ± 0.5

21

Fig. 1.

22

Fig. 2.

23

Fig. 3.

24

Fig. 4.

25

Fig. 5.

26

Fig. 6.

27

Fig. 7.

28

Fig. 8.

29