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Lightweight Ti–Zr–Be–Al bulk metallic glasses with improved glass-forming ability and compressive plasticity
Journal of Non-Crystalline Solids 358 (2012) 2620–2625
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Lightweight Ti–Zr–Be–Al bulk metallic glasses with improved glass-forming ability and compressive plasticity P. Gong, K.F. Yao ⁎, Y. Shao Department of Mechanical Engineering, Tsinghua University Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 12 April 2012 Received in revised form 3 June 2012 Available online 27 June 2012 Keywords: Metallic glasses; Glass-forming ability; Mechanical properties
a b s t r a c t A series of lightweight Ti–Zr–Be–Al bulk metallic glasses (BMGs) have been developed through the addition of Al to Ti–Zr–Be ternary glassy alloy. By replacing Be with Al, the critical size of the glassy rod has been increased from 5 mm for Ti41Zr25Be34 alloy to 7 mm for Ti41Zr25Be29Al5 alloy, while the yield strength of Ti41Zr25Be34 − xAlx (x= 2–10) has been greatly enhanced, resulting in a significant increase of the specific strength which is defined as yield strength/density. Among these newly developed Ti–Zr–Be–Al BMGs, Ti41Zr25Be26Al8 glassy alloy exhibits a high specific strength of 4.33× 105 Nm/kg and a very large compressive plastic strain of 47.0%, which are much larger than those (3.69 × 105 Nm/kg and 2.9%, respectively) for Ti41Zr25Be34 glassy alloy. The present results show that Al is an effective alloying element for improving the glass-forming ability (GFA) and mechanical properties of Ti-Zr-Be glassy alloy. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Bulk metallic glasses (BMGs) have received extensive interest among scientists and engineers due to their superior properties [1,2]. Among all the developed glassy alloy systems, Ti-based BMGs have been paid much attention due to their high specific strength, good corrosion and abundant reserves of titanium mineral [3,4]. So far, a number of Ti-based bulk metallic glasses have been synthesized by the copper casting method and some of them exhibit relatively large glass-forming ability (GFA) [5–8]. However, almost every reported Tibased glassy alloy has its limits which prevent form becoming more useful engineering materials. For example, the Ti40Zr10Cu34Pd14Sn2 alloy with diameter of 10 mm has been prepared by copper mould casting can this alloy exhibits good plasticity of 3.5% and a yield strength of ~2000 MPa, but the addition of expensive Pd element raises the cost and decreases the specific strength of the alloy [9]. The low-cost (Ti36.1Zr33.2Ni5.8Be24.9)91Cu9 BMG with over 50 mm in diameter has been successfully developed by water-quenching and this alloy exhibits maybe the best glass-forming ability (GFA) in Ti-based glassy systems till now. Unfortunately, the fatal problem is that this alloy fails just after elastic deformation and lacks of room temperature plasticity [10]. Therefore more research work is needed to develop a new Ti-based BMG with the minimum shortcomings. Ti–Zr–Be ternary alloys exhibit good GFA, low density and certain plasticity and have arosen widely attention of scholars [11,12]. The existence of toxic element Be may restrict their biomedical applications,
⁎ Corresponding author. Tel.: + 86 10 62772292. E-mail address: [email protected] (K.F. Yao). 0022-3093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2012.06.011
However, the Be-doped alloys (such as beryllium bronze and beryllium aluminium alloy) always possess good properties and have been widely used in aerospace and other construction material fields. Therefore Ti–Zr–Be glassy alloys still have wide prospect although they possess some biological toxicity. A typical Ti–Zr–Be glassy composition is Ti41Zr25Be34 [11], possesses a critical size of 5 mm. In order to further improve the GFA and mechanical properties of Ti–Zr–Be glassy alloys, alloying method, which has been widely used in improvement of GFA and mechanical properties for BMGs is adopted [13–16]. In this paper, we developed a new Ti–Zr–Be–Al glassy system with the addition of Al based on the reported Ti41Zr25Be34 glassy alloy and a series of metallic glasses with good GFA and mechanical properties have been obtained. The related mechanisms on the effects of Al addition on the GFA and mechanical properties of Ti-Zr-Be alloy have also been investigated. 2. Experimental procedure The master alloy ingots with nominal compositions of Ti41Zr25Be34-x Alx (x= 0, 2, 4, 5, 6, 8, 10) were prepared by arc melting the mixtures of each pure Ti (99.4 mass%), Zr (99.7 mass%), Be (99.99 mass%), Al (99.999 mass%) in a Ti-gettered high-purity Ar atmosphere with electromagnetic stirring. Each ingot was remelted for several times to ensure compositional homogeneity. Cylindrical rods with different diameters were prepared from the ingots by suction casting method. Thin crosscut pieces of the as-cast rods were cut down in the middle of the rods and then examined by X-ray diffraction (XRD) and transmission electron microscope (TEM) analysis. The thin foil specimens for TEM observation were prepared by a standard twin-jet electrochemical polishing with a solution of 8% HClO4 and 92% C2H5OH at −30 °C cooled
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by liquid nitrogen. Thermal stability of the glassy samples was evaluated by differential scanning calorimeter (DSC) at a heating rate of 20 K/min using Ø2 mm rods. The unidirectional compression tests for the BMGs were carried out on WDW-100 testing machine under a stain rate of 4 × 10− 4 s− 1 at room temperature. The test samples had a diameter of 2 mm and a height of 4 mm. The fracture surface and side views of the fractured samples were examined by the scanning electron microscope (SEM) to study the deformation mechanism. 3. Results Fig. 1(a) presents X-ray diffraction patterns of the as-cast Ti41Zr25 Be34-xAlx samples (x = 0, 2, 4, 5, 6, 8) with different diameters. The XRD spectrum of Ti41Zr25Be34 alloy shows only broad peaks for the rod with a diameter of 5 mm, indicating a single glassy phase at least at the resolution limit of XRD diffractometer. But for the rod with a diameter of 6 mm, obvious crystalline peaks can be observed in its XRD spectrum (not shown here). It indicates that the critical size of the Ti41Zr25Be34 BMG is about 5 mm, agreed with the reported result [11]. As the content of Al increases to 2 at.%, the XRD spectrum of the sample with 6 mm in diameter shows amorphous structure resulted by maximum broad diffraction and only a weak sharp diffraction peak, which has been indexed as Be2Zr, is observed, indicating that the glass-forming ability (GFA) of the Ti–Zr–Be alloy has been enhanced slightly by replacing Be with Al. But for ϕ6 mm rod
Fig. 1. (a) XRD patterns of as-cast rods of Ti41Zr25Be34-xAlx (x= 0, 2, 4, 5, 6, 8). (b) HRTEM bright-field image of φ7 mm Ti41Zr25Be29Al5 glassy rod with the inset showing the selectarea diffraction pattern.
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Table 1 Thermal parameters (Tg, Tx, and ΔTx) and maximum diameters (Dmax, mm) of Ti41Zr25 Be34-xAlx (x = 0, 2, 4, 5, 6, 8, 10) BMGs. The error of the temperature values are ± 1 K. Composition (at. %)
Tg (K)
Tx (K)
ΔTx (K)
Dmax
Ti41Zr25Be34 Ti41Zr25Be32Al2 Ti41Zr25Be30Al4 Ti41Zr25Be29Al5 Ti41Zr25Be28Al6 Ti41Zr25Be26Al8 Ti41Zr25Be24Al10
599 610 622 619 624 628 633
656 673 683 689 691 694 693
57 63 61 70 67 66 60
5 5 6 7 7 3 2
samples of the alloys with Al content of 4 at. %, the XRD spectrum consists of only a broad diffraction halo without any sharp Bragg peak (Fig. 1(a)), suggesting the alloy possess full amorphous structure. While for the rod sample with 7 mm in diameter, the Ti41Zr25Be29Al5 alloy exhibits full amorphous characteristics but for the Ti41Zr25Be28Al6 alloy very weak peak could be observed in its XRD spectrum (see Fig. 1(a)). In order to reveal whether the sample possesses full amorphous structure or not, the microstructure of the sample has been examined with TEM. During TEM observation, no crystalline phase has been found in the sample. In addition, high-solution TEM (HRTEM) images of the as-cast Ti41Zr25Be29Al5 sample with a diameter of 7 mm have been analyzed and a typical one is shown in Fig. 1(b). No nanocrystalline grain has been observed, except for some short-range or medium-range ordered clusters. The corresponding select-area diffraction pattern (SADP) is shown in the insert of Fig. 1(b). Except a halo diffraction, no sharp rings or spots resulted from crystalline phase could be observed. It implies the sample is of full amorphous structure. As shown in Fig. 1(a), further increasing Al content up to 8 at. %, the XRD spectrum of casting rod with 6 mm in diameter exhibits clear diffraction peaks resulted from crystalline phases. Through detail experimental work it has been found that for Ti41Zr25Be26Al8 alloy the critical size for full glassy rod is about 3 mm while for Ti41Zr25Be24Al10 alloy it is only 2 mm. The obtained maximum diameters (Dmax) of Ti–Zr–Be–Al full amorphous rods are listed in Table 1. The result shows that Al addition could improve the GFA of the Ti–Zr–Be alloy. When the Al content is about 5 at.%, the Ti41Zr25Be29Al5 alloy exhibits best GFA and a critical size of 7 mm. When the content of Al is up to 8 at.%, the GFA of the alloy is getting even worse than that without Al addition. Fig. 2 shows DSC curves of Ti41Zr25Be34-xAlx alloys (x = 0, 2, 4, 5, 6, 8, 10) and the glass transition (Tg, extrapolated onset temperature Tig is actually adopted in this work), crystallization onset temperature (Tx) are marked by arrows in the DSC traces. The values of Tg,
Fig. 2. DSC curves for Ti41Zr25Be34-xAlx (x = 0, 2, 4, 5, 6, 8, 10) glassy alloys.
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Tx and supercooled liquid region (ΔTx, defined as Tx–Tg) are summarized in Table 1 and the variations of Tg, Tx andΔTx with Al content in Ti41Zr25Be34-xAlx (x = 0, 2, 4, 5, 6, 8, 10) BMGs are shown in Fig. 3. From Fig. 3 and Table 1, it can be seen that Tg, Tx, and ΔTx vary with the content of Al, further confirming the critical role of Al addition in the improvement of the GFA. The Tg of Ti41Zr25Be34-xAlx alloys increases from 599 K at x = 0 to 633 K at x = 10, only when the content of Al increases from 4 at.% to 5 at.% has a slight reduction. The Tx firstly increases from 656 K for Ti41Zr25Be34 alloy to 694 K for Ti41Zr25Be26Al8 alloy and then slightly decreases to 693 K for Ti41Zr25Be24Al10 alloy. It is known that ΔTx is suggested as a measure of the thermal stability based on the consideration of supercooled liquid stability against crystallization [17]. Larger ΔTx is usually corresponding to larger GFA of the alloys despite that there exist some disagreement results [18]. According to Fig. 3 and Table 1, for x=1–10, The ΔTx is enhanced by addition of Al, and the Ti41Zr25Be29Al5 glassy alloy possesses the largest ΔTx value among these alloys. It suggests that this alloy may possess the best GFA in the alloy series, agreed well with the XRD results (Fig. 1(a)). Fig. 4 shows the compressive stress-strain curves of Ti41Zr25Be34-x Alx samples (x = 0, 2, 4, 5, 6, 8, 10) at room temperature. The compression test results for Ti41Zr25Be34-xAlx alloys derived from Fig. 4 are also listed in Table 2. The alloy without Al addition exhibits a yield strength σ0.2 of 1755 MPa, a maximum compressive strength σmax of 1914 MPa, and a plastic strain εp of 2.9%. They are slightly below the reported result [11], and this difference may be caused by the difference of the preparation. After adding Al element, the yield strength, the maximum strength and the plastic strain of the glassy alloys are significantly improved (as shown in Table 2 and Fig. 4). Especially for the Ti41Zr25Be26Al8 glassy alloy, the εp is as large as 47.0% before final fracture, much larger than the reported value (such as εp = 5.0% for Ti40Zr25Cu12Ni3Be20 glassy alloy [5]; εp =2.24% for Ti43.15Zr9.59Cu36.24 Ni9.06Sn1.96 glassy alloy [8] and εp =3.5% for Ti40Zr10Cu34Pd14Sn2 glassy alloy [9]). According to our knowledge, it is the largest compressive plastic
Fig. 4. Compressive stress-strain curves at room temperature for glassy Ti41Zr25Be34-xAlx (x= 0, 2, 4, 5, 6, 8, 10) samples.
strain reported in Ti-based glassy alloys. The present results indicate that Al addition could greatly affect mechanical properties of Ti–Zr–Be glassy alloys. It is known that alloys with low density and high specific strength are very attractive to engineering application. In present work, although the addition of Al slightly enhances the density of Ti–Zr–Be alloy (Table 2), the specific strength (defined as yield strength/density) of Ti41Zr25Be34-xAlx (x= 2, 4, 5, 6, 8, 10) BMGs has been significantly increased. They are larger than 4.0 × 105 Nm/kg, obviously larger than 3.69 × 105 Nm/kg for Ti41Zr25Be34 glassy alloy. Among these alloys, Ti41Zr25Be26Al8 BMG possesses a specific strength of 4.33 × 105 Nm/kg, 17% larger than that of Ti41Zr25Be34 glassy alloy. It shows that the specific strength of present glassy is not only much larger than that of most other glassy alloys [5–12], but also much larger that of crystalline alloys, such about 2 times of high strength Ti–6Al–4V alloy. Fig. 5(a)–(c) are SEM images of the side view and fracture surface of the deformed Ti41Zr25Be34 sample. It shows that the sample under compression fractured in a shear mode with a shear fracture angle of about 42 º, which is similar to results observed in other plastic BMGs [19]. A few shear bands can be seen on the lateral surface and typical vein patterns can be observed on the fractured surface. For comparison, Fig. 5(d)–(f) are SEM images of fractured specimen in low magnification (Fig. 5(d)), the lateral surface (Fig. 5(e)) and the fracture surface (Fig. 5(f)) of a Ti41Zr25Be26Al8 sample. A large number of shear bands can be observed on the whole lateral surface, indicating that the large plasticity of the sample is resulted by activation and propagation of multiple shear bands. Because of the large deformation during the compression, the two parts of the sample separated by the major shear band would touch the platens of the compression clip, and the shearing process is confined. With the confinement of the geometry constraints, the propagation of primary shear band cannot lead to an early fracture and the localized shear deformation are greatly relaxed. Then abundant secondary shear bands generate
Table 2 Densities and mechanical properties of Ti41Zr25Be34-xAlx (x = 0, 2, 4, 5, 6, 8, 10 at. %) BMGs.
Fig. 3. Variations of Tg, Tx and ΔTx with Al content in Ti41Zr25Be34-xAlx (x= 0, 2, 4, 5, 6, 8, 10) BMGs.
Composition (at. %)
Density (g/cm3)
σ0.2 (MPa)
σmax (MPa)
εp (%)
Specific strength (Nm/kg)
Ti41Zr25Be34 Ti41Zr25Be32Al2 Ti41Zr25Be30Al4 Ti41Zr25Be29Al5 Ti41Zr25Be28Al6 Ti41Zr25Be26Al8 Ti41Zr25Be24Al10
4.76 4.78 4.79 4.80 4.80 4.81 4.81
1755 1948 1990 1938 1996 2084 2050
1914 2217 2125 2130 2190 3820 2373
2.9 ± 0.1 14.3 ± 0.1 4.3 ± 0.1 6.0 ± 0.1 16.3 ± 0.1 47.0 ± 0.1 17.2 ± 0.1
3.69 × 105 4.07 × 105 4.15 × 105 4.03 × 105 4.16 × 105 4.33 × 105 4.26 × 105
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Fig. 5. SEM images of lateral and fracture surfaces of the deformed samples for (a, b, and c) Ti41Zr25Be34 and (d, e, and f) Ti41Zr25Be26Al8 samples respectively.
along with the plastic deformation of the specimen. As a result of large plastic strain, the shear planes deviate from the original orientation [20], and the shear fracture angle increases to 49°. The fracture surface also reveals vein-like pattern and melt drops. In addition, some flower-like type veins (as shown in Fig. 5(f)), which are frequently observed on fractography of tensile specimens of BMGs [19], are observed. However, the arrangement of fishbone-like veins is disorderd and this indicates multiple shear bands have been activated during the deformation process with different propagation directions. The similar morphology can also be found in some other BMGs with good plasticity [21,22]. 4. Discussions It shows that with Al addition, the GFA and the mechanical properties of Ti–Zr–Be glassy alloys have been signicantly improves. Then, understanding the affecting mechanism of Al on the GFA and properties of the alloy is very important. It is known heats of mixing between Ti–Al and Zr–Al is −30 kJ/mol and −44 kJ/mol, respectively [23]. The absolute values are relatively large. Then, with Al addition the strong chemical short-range order would be expected, which may stabilize the
liquid phase and reduce the atomic mobility that mediates crystallization. In addition, abundant chemical clusters of short-range order (SRO) and medium-range order (MRO) would have already existed in Ti–Zr–Be ternary glassy alloy, because of the large negative heats of mixing between Ti–Be (−30 kJ/mol) and Zr–Be (−43 kJ/mol). So with the addition of Al, more types of local ordering clusters (SRO or MRO) would been formed and dispersed in the melt, which would benefit to enhance the GFA of the alloys [24]. On the other hand, the electronegativity difference (Δx) and the atomic size difference parameter (δ), two classic parameters related with the GFA of the glassy alloy, have also been adopted to examine the effect of Al addition on the glass-forming ability of Ti–Zr–Be glassy alloy. According to Fang et al's models [25], Δx and δ of Ti–Zr–Be–Al glassy alloys can be calculated and summarized in Table 3. (the Pauling electronegativity: Ti (1.54), Zr (1.33), Be (1.57) and Al (1.61); the atomic radius: Ti (1.46 Å), Zr (1.58 Å), Be (1.12 Å) and Al (1.43 Å)) As shown in Fig. 6, because Al possesses larger Pauling electrogenativity than Ti, Zr, and Be, the addition of Al would increase Δx value of the alloy, which is beneficial to the improvement of the GFA. However, the value of δ would decrease as the content of Al is increasing, resulting in a detrimental effect to the GFA [25]. Based on a combination of the
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Table 3 The electronegativity difference (Δx) and the atomic size parameter (δ) of Ti41Zr25Be34-x Alx (x= 0, 2, 4, 5, 6, 8, 10 at.%) glassy alloys. Composition (at. %)
Δx
δ
Ti41Zr25Be34 Ti41Zr25Be32Al2 Ti41Zr25Be30Al4 Ti41Zr25Be29Al5 Ti41Zr25Be28Al6 Ti41Zr25Be26Al8 Ti41Zr25Be24Al10
0.0977 0.0984 0.0992 0.0995 0.0999 0.1006 0.1013
0.1372 0.1345 0.1309 0.1292 0.1275 0.1239 0.1200
effects of the two parameters, a local maximum beneficial effect on GFA might be obtained. This may be the reason why Ti41Zr25Be29Al5 alloy possesses the best GFA. With the appropriate amount of Al addition, the Ti–Zr–Be-based BMGs show enhanced strength and plasticity and the mechanism for the improvement is discussed. According to Yang et al.'s work [26], higher Tg always implies higher yield strength. From Tables 1 and 2, there is a really positive correlation between σ0.2 and Tg for the Ti–Zr–Be–Al alloys. The improvement of strength is interpreted as a result of bonding between the constituent elements, especially expected from the large negative mixing enthalpy values (−30 kJ/mol, −44 kJ/mol for the Ti–Al, Zr–Al pairs respectively). It was generally recognized that the plasticity for BMGs are originated from chemical or structural inhomogeneity [21,27–30]; nanocrystallization [31,32]; high Poisson's ratio [33–35] and so on. From the HRTEM results (Fig. 1(b)), no nano-crystal but some local ordering clusters can been observed. In Ti–Zr–Be glassy system, abundant chemical clusters of short-range
Table 4 Elastic properties of Al and Be alloying elements. Elements
E (GPa)
G (GPa)
K (GPa)
υ
G/K
Al Be
70 287
26 132
76 130
0.35 0.032
0.37 1.02
order (SRO) and medium-range order (MRO) have already existed because of the large negative heats of mixing between Ti and Be (−30 kJ/mol), Zr and Be (−43 kJ/mol). With the addition of Al, more types of local ordering clusters have been formed and disperse in the melt, then a reinforcing effect is induced and the compressive plasticity of the alloy is improved. Moreover, recently it has been reported that the elastic modulus of glassy alloy is closely related with the elastic modulus of the constituent elements [34,35]. So that elastic modulus of the glassy alloy can be modified by adjusting the composition of alloy. Compared with Be element, the Al element has both lower shear modulus (G) to bulk modulus (K) ratio and higher Poisson's ratio ν (see Table 4 [36]). Then, replacing Be with Al, the shear modulus G of the alloy would be reduced and Poisson's ratio ν of the alloy would be enhanced. A glassy alloy possessed a low shear modulus and a high Poisson's ratio could favor to activate multiple shear bands, retard the shear localization and improve the plasticity of BMGs [35]. In this work, the enhancement in the plasticity of Ti-based BMG alloys is also attributed to the high value of Poisson's ratio of Al. However, more research work is still needed to find out the exact reason. According to reported results, Ti–Zr–Be alloys are the best glass former of Ti-based ternary glassy alloys. Recently, there are also other reports about Ti-based quaternary glassy alloy based on Ti–Zr–Be alloy systems through V or Cr alloying. Zhang et al. [11] reported a Ti41Zr25Be30V4 alloy with a critical diameter of 8 mm without providing concrete data of mechanical properties. Johnson et al. [12] reported a Ti40Zr25Be30Cr5 alloy, which possesses a critical size of 8 mm. Using 3 mm amorphous rods, compression tests indicated Ti40Zr25Be30Cr5 yields at ~1720 MPa, and finally fractures at a strength of ~1900 MPa, with a plastic strain of ~3.5%. By comparison, our Al-doped glassy alloys possess much higher specific strength and larger plastic strain than the reported alloys although the glass-forming ability is slightly inferior. Moreover, Al element is both cheaper and more plentiful than Cr and V elements. Through comparison, it has been perceived that Al element has a competitive advantage as an alloying element to improving the GFA especially mechanical properties of Ti–Zr–Be ternary glassy alloys.
5. Conclusions
Fig. 6. The electronegativity difference (Δx) and the atomic size parameter (δ) of Ti41Zr25Be34 − xAlx (x = 0, 2, 4, 5, 6, 8, 10 at.%) glassy alloys.
In summary, it has been found that Al addition could significantly improve the glass-forming ability and mechanical property of the Ti–Zr–Be glassy alloys. By replacing Be with Al, the critical diameter of glassy rods has been increased from 5 mm for Ti41Zr25Be34 alloy to 7 mm for Ti41Zr25Be29Al5 alloy. The yield strength of the Ti-Zr-Be glassy alloy has been greatly enhanced with Al addition. As a result the specific strength, which is defined as yield strength/density, has been greatly enhanced. For Ti41Zr25Be26Al8 glassy alloy, the yield strength and specific strength are about 2084 MPa and 4.33 × 105 Nm/kg respectively, which are significantly larger than those of 1755 MPa and 3.69× 105 Nm/kg respectively for Ti41Zr25Be34 glassy alloy. Especially, the specific strength is much larger than that of most glassy alloys and crystalline alloys, including the typical low density high strength Ti-6Al-4 V alloy (about 2.1 × 105 Nm/kg). In addition, the compressive plasticity of the Ti-Zr-Be glassy alloys has also been enhanced by Al addition. Ti41Zr25Be26Al8 BMG exhibits a compressive plastic strain as large as 47.0%, which is much larger than the plastic strain of 2.9% for Ti41Zr25Be34 glassy alloy, and that for most other glassy alloys. The present results show that the developed Ti-Zr-Be-Al bulk metallic glasses are high performance lightweight glassy alloys.
P. Gong et al. / Journal of Non-Crystalline Solids 358 (2012) 2620–2625
Acknowledgements This work is supported by National Basic Research Program of China (Grant No. 2007CB613905) and National Natural Science Foundation of China (Grant No. 50971073). The authors acknowledge the support from the National Center of Nano-Science and Nano-Technology of China and the Analysis Center of Tsinghua University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Lightweight Ti–Zr–Be–Al bulk metallic glasses with improved glass-forming ability and compressive plasticity P. Gong, K.F. Yao ⁎, Y. Shao Department of Mechanical Engineering, Tsinghua University Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 12 April 2012 Received in revised form 3 June 2012 Available online 27 June 2012 Keywords: Metallic glasses; Glass-forming ability; Mechanical properties
a b s t r a c t A series of lightweight Ti–Zr–Be–Al bulk metallic glasses (BMGs) have been developed through the addition of Al to Ti–Zr–Be ternary glassy alloy. By replacing Be with Al, the critical size of the glassy rod has been increased from 5 mm for Ti41Zr25Be34 alloy to 7 mm for Ti41Zr25Be29Al5 alloy, while the yield strength of Ti41Zr25Be34 − xAlx (x= 2–10) has been greatly enhanced, resulting in a significant increase of the specific strength which is defined as yield strength/density. Among these newly developed Ti–Zr–Be–Al BMGs, Ti41Zr25Be26Al8 glassy alloy exhibits a high specific strength of 4.33× 105 Nm/kg and a very large compressive plastic strain of 47.0%, which are much larger than those (3.69 × 105 Nm/kg and 2.9%, respectively) for Ti41Zr25Be34 glassy alloy. The present results show that Al is an effective alloying element for improving the glass-forming ability (GFA) and mechanical properties of Ti-Zr-Be glassy alloy. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Bulk metallic glasses (BMGs) have received extensive interest among scientists and engineers due to their superior properties [1,2]. Among all the developed glassy alloy systems, Ti-based BMGs have been paid much attention due to their high specific strength, good corrosion and abundant reserves of titanium mineral [3,4]. So far, a number of Ti-based bulk metallic glasses have been synthesized by the copper casting method and some of them exhibit relatively large glass-forming ability (GFA) [5–8]. However, almost every reported Tibased glassy alloy has its limits which prevent form becoming more useful engineering materials. For example, the Ti40Zr10Cu34Pd14Sn2 alloy with diameter of 10 mm has been prepared by copper mould casting can this alloy exhibits good plasticity of 3.5% and a yield strength of ~2000 MPa, but the addition of expensive Pd element raises the cost and decreases the specific strength of the alloy [9]. The low-cost (Ti36.1Zr33.2Ni5.8Be24.9)91Cu9 BMG with over 50 mm in diameter has been successfully developed by water-quenching and this alloy exhibits maybe the best glass-forming ability (GFA) in Ti-based glassy systems till now. Unfortunately, the fatal problem is that this alloy fails just after elastic deformation and lacks of room temperature plasticity [10]. Therefore more research work is needed to develop a new Ti-based BMG with the minimum shortcomings. Ti–Zr–Be ternary alloys exhibit good GFA, low density and certain plasticity and have arosen widely attention of scholars [11,12]. The existence of toxic element Be may restrict their biomedical applications,
⁎ Corresponding author. Tel.: + 86 10 62772292. E-mail address: [email protected] (K.F. Yao). 0022-3093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2012.06.011
However, the Be-doped alloys (such as beryllium bronze and beryllium aluminium alloy) always possess good properties and have been widely used in aerospace and other construction material fields. Therefore Ti–Zr–Be glassy alloys still have wide prospect although they possess some biological toxicity. A typical Ti–Zr–Be glassy composition is Ti41Zr25Be34 [11], possesses a critical size of 5 mm. In order to further improve the GFA and mechanical properties of Ti–Zr–Be glassy alloys, alloying method, which has been widely used in improvement of GFA and mechanical properties for BMGs is adopted [13–16]. In this paper, we developed a new Ti–Zr–Be–Al glassy system with the addition of Al based on the reported Ti41Zr25Be34 glassy alloy and a series of metallic glasses with good GFA and mechanical properties have been obtained. The related mechanisms on the effects of Al addition on the GFA and mechanical properties of Ti-Zr-Be alloy have also been investigated. 2. Experimental procedure The master alloy ingots with nominal compositions of Ti41Zr25Be34-x Alx (x= 0, 2, 4, 5, 6, 8, 10) were prepared by arc melting the mixtures of each pure Ti (99.4 mass%), Zr (99.7 mass%), Be (99.99 mass%), Al (99.999 mass%) in a Ti-gettered high-purity Ar atmosphere with electromagnetic stirring. Each ingot was remelted for several times to ensure compositional homogeneity. Cylindrical rods with different diameters were prepared from the ingots by suction casting method. Thin crosscut pieces of the as-cast rods were cut down in the middle of the rods and then examined by X-ray diffraction (XRD) and transmission electron microscope (TEM) analysis. The thin foil specimens for TEM observation were prepared by a standard twin-jet electrochemical polishing with a solution of 8% HClO4 and 92% C2H5OH at −30 °C cooled
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by liquid nitrogen. Thermal stability of the glassy samples was evaluated by differential scanning calorimeter (DSC) at a heating rate of 20 K/min using Ø2 mm rods. The unidirectional compression tests for the BMGs were carried out on WDW-100 testing machine under a stain rate of 4 × 10− 4 s− 1 at room temperature. The test samples had a diameter of 2 mm and a height of 4 mm. The fracture surface and side views of the fractured samples were examined by the scanning electron microscope (SEM) to study the deformation mechanism. 3. Results Fig. 1(a) presents X-ray diffraction patterns of the as-cast Ti41Zr25 Be34-xAlx samples (x = 0, 2, 4, 5, 6, 8) with different diameters. The XRD spectrum of Ti41Zr25Be34 alloy shows only broad peaks for the rod with a diameter of 5 mm, indicating a single glassy phase at least at the resolution limit of XRD diffractometer. But for the rod with a diameter of 6 mm, obvious crystalline peaks can be observed in its XRD spectrum (not shown here). It indicates that the critical size of the Ti41Zr25Be34 BMG is about 5 mm, agreed with the reported result [11]. As the content of Al increases to 2 at.%, the XRD spectrum of the sample with 6 mm in diameter shows amorphous structure resulted by maximum broad diffraction and only a weak sharp diffraction peak, which has been indexed as Be2Zr, is observed, indicating that the glass-forming ability (GFA) of the Ti–Zr–Be alloy has been enhanced slightly by replacing Be with Al. But for ϕ6 mm rod
Fig. 1. (a) XRD patterns of as-cast rods of Ti41Zr25Be34-xAlx (x= 0, 2, 4, 5, 6, 8). (b) HRTEM bright-field image of φ7 mm Ti41Zr25Be29Al5 glassy rod with the inset showing the selectarea diffraction pattern.
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Table 1 Thermal parameters (Tg, Tx, and ΔTx) and maximum diameters (Dmax, mm) of Ti41Zr25 Be34-xAlx (x = 0, 2, 4, 5, 6, 8, 10) BMGs. The error of the temperature values are ± 1 K. Composition (at. %)
Tg (K)
Tx (K)
ΔTx (K)
Dmax
Ti41Zr25Be34 Ti41Zr25Be32Al2 Ti41Zr25Be30Al4 Ti41Zr25Be29Al5 Ti41Zr25Be28Al6 Ti41Zr25Be26Al8 Ti41Zr25Be24Al10
599 610 622 619 624 628 633
656 673 683 689 691 694 693
57 63 61 70 67 66 60
5 5 6 7 7 3 2
samples of the alloys with Al content of 4 at. %, the XRD spectrum consists of only a broad diffraction halo without any sharp Bragg peak (Fig. 1(a)), suggesting the alloy possess full amorphous structure. While for the rod sample with 7 mm in diameter, the Ti41Zr25Be29Al5 alloy exhibits full amorphous characteristics but for the Ti41Zr25Be28Al6 alloy very weak peak could be observed in its XRD spectrum (see Fig. 1(a)). In order to reveal whether the sample possesses full amorphous structure or not, the microstructure of the sample has been examined with TEM. During TEM observation, no crystalline phase has been found in the sample. In addition, high-solution TEM (HRTEM) images of the as-cast Ti41Zr25Be29Al5 sample with a diameter of 7 mm have been analyzed and a typical one is shown in Fig. 1(b). No nanocrystalline grain has been observed, except for some short-range or medium-range ordered clusters. The corresponding select-area diffraction pattern (SADP) is shown in the insert of Fig. 1(b). Except a halo diffraction, no sharp rings or spots resulted from crystalline phase could be observed. It implies the sample is of full amorphous structure. As shown in Fig. 1(a), further increasing Al content up to 8 at. %, the XRD spectrum of casting rod with 6 mm in diameter exhibits clear diffraction peaks resulted from crystalline phases. Through detail experimental work it has been found that for Ti41Zr25Be26Al8 alloy the critical size for full glassy rod is about 3 mm while for Ti41Zr25Be24Al10 alloy it is only 2 mm. The obtained maximum diameters (Dmax) of Ti–Zr–Be–Al full amorphous rods are listed in Table 1. The result shows that Al addition could improve the GFA of the Ti–Zr–Be alloy. When the Al content is about 5 at.%, the Ti41Zr25Be29Al5 alloy exhibits best GFA and a critical size of 7 mm. When the content of Al is up to 8 at.%, the GFA of the alloy is getting even worse than that without Al addition. Fig. 2 shows DSC curves of Ti41Zr25Be34-xAlx alloys (x = 0, 2, 4, 5, 6, 8, 10) and the glass transition (Tg, extrapolated onset temperature Tig is actually adopted in this work), crystallization onset temperature (Tx) are marked by arrows in the DSC traces. The values of Tg,
Fig. 2. DSC curves for Ti41Zr25Be34-xAlx (x = 0, 2, 4, 5, 6, 8, 10) glassy alloys.
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Tx and supercooled liquid region (ΔTx, defined as Tx–Tg) are summarized in Table 1 and the variations of Tg, Tx andΔTx with Al content in Ti41Zr25Be34-xAlx (x = 0, 2, 4, 5, 6, 8, 10) BMGs are shown in Fig. 3. From Fig. 3 and Table 1, it can be seen that Tg, Tx, and ΔTx vary with the content of Al, further confirming the critical role of Al addition in the improvement of the GFA. The Tg of Ti41Zr25Be34-xAlx alloys increases from 599 K at x = 0 to 633 K at x = 10, only when the content of Al increases from 4 at.% to 5 at.% has a slight reduction. The Tx firstly increases from 656 K for Ti41Zr25Be34 alloy to 694 K for Ti41Zr25Be26Al8 alloy and then slightly decreases to 693 K for Ti41Zr25Be24Al10 alloy. It is known that ΔTx is suggested as a measure of the thermal stability based on the consideration of supercooled liquid stability against crystallization [17]. Larger ΔTx is usually corresponding to larger GFA of the alloys despite that there exist some disagreement results [18]. According to Fig. 3 and Table 1, for x=1–10, The ΔTx is enhanced by addition of Al, and the Ti41Zr25Be29Al5 glassy alloy possesses the largest ΔTx value among these alloys. It suggests that this alloy may possess the best GFA in the alloy series, agreed well with the XRD results (Fig. 1(a)). Fig. 4 shows the compressive stress-strain curves of Ti41Zr25Be34-x Alx samples (x = 0, 2, 4, 5, 6, 8, 10) at room temperature. The compression test results for Ti41Zr25Be34-xAlx alloys derived from Fig. 4 are also listed in Table 2. The alloy without Al addition exhibits a yield strength σ0.2 of 1755 MPa, a maximum compressive strength σmax of 1914 MPa, and a plastic strain εp of 2.9%. They are slightly below the reported result [11], and this difference may be caused by the difference of the preparation. After adding Al element, the yield strength, the maximum strength and the plastic strain of the glassy alloys are significantly improved (as shown in Table 2 and Fig. 4). Especially for the Ti41Zr25Be26Al8 glassy alloy, the εp is as large as 47.0% before final fracture, much larger than the reported value (such as εp = 5.0% for Ti40Zr25Cu12Ni3Be20 glassy alloy [5]; εp =2.24% for Ti43.15Zr9.59Cu36.24 Ni9.06Sn1.96 glassy alloy [8] and εp =3.5% for Ti40Zr10Cu34Pd14Sn2 glassy alloy [9]). According to our knowledge, it is the largest compressive plastic
Fig. 4. Compressive stress-strain curves at room temperature for glassy Ti41Zr25Be34-xAlx (x= 0, 2, 4, 5, 6, 8, 10) samples.
strain reported in Ti-based glassy alloys. The present results indicate that Al addition could greatly affect mechanical properties of Ti–Zr–Be glassy alloys. It is known that alloys with low density and high specific strength are very attractive to engineering application. In present work, although the addition of Al slightly enhances the density of Ti–Zr–Be alloy (Table 2), the specific strength (defined as yield strength/density) of Ti41Zr25Be34-xAlx (x= 2, 4, 5, 6, 8, 10) BMGs has been significantly increased. They are larger than 4.0 × 105 Nm/kg, obviously larger than 3.69 × 105 Nm/kg for Ti41Zr25Be34 glassy alloy. Among these alloys, Ti41Zr25Be26Al8 BMG possesses a specific strength of 4.33 × 105 Nm/kg, 17% larger than that of Ti41Zr25Be34 glassy alloy. It shows that the specific strength of present glassy is not only much larger than that of most other glassy alloys [5–12], but also much larger that of crystalline alloys, such about 2 times of high strength Ti–6Al–4V alloy. Fig. 5(a)–(c) are SEM images of the side view and fracture surface of the deformed Ti41Zr25Be34 sample. It shows that the sample under compression fractured in a shear mode with a shear fracture angle of about 42 º, which is similar to results observed in other plastic BMGs [19]. A few shear bands can be seen on the lateral surface and typical vein patterns can be observed on the fractured surface. For comparison, Fig. 5(d)–(f) are SEM images of fractured specimen in low magnification (Fig. 5(d)), the lateral surface (Fig. 5(e)) and the fracture surface (Fig. 5(f)) of a Ti41Zr25Be26Al8 sample. A large number of shear bands can be observed on the whole lateral surface, indicating that the large plasticity of the sample is resulted by activation and propagation of multiple shear bands. Because of the large deformation during the compression, the two parts of the sample separated by the major shear band would touch the platens of the compression clip, and the shearing process is confined. With the confinement of the geometry constraints, the propagation of primary shear band cannot lead to an early fracture and the localized shear deformation are greatly relaxed. Then abundant secondary shear bands generate
Table 2 Densities and mechanical properties of Ti41Zr25Be34-xAlx (x = 0, 2, 4, 5, 6, 8, 10 at. %) BMGs.
Fig. 3. Variations of Tg, Tx and ΔTx with Al content in Ti41Zr25Be34-xAlx (x= 0, 2, 4, 5, 6, 8, 10) BMGs.
Composition (at. %)
Density (g/cm3)
σ0.2 (MPa)
σmax (MPa)
εp (%)
Specific strength (Nm/kg)
Ti41Zr25Be34 Ti41Zr25Be32Al2 Ti41Zr25Be30Al4 Ti41Zr25Be29Al5 Ti41Zr25Be28Al6 Ti41Zr25Be26Al8 Ti41Zr25Be24Al10
4.76 4.78 4.79 4.80 4.80 4.81 4.81
1755 1948 1990 1938 1996 2084 2050
1914 2217 2125 2130 2190 3820 2373
2.9 ± 0.1 14.3 ± 0.1 4.3 ± 0.1 6.0 ± 0.1 16.3 ± 0.1 47.0 ± 0.1 17.2 ± 0.1
3.69 × 105 4.07 × 105 4.15 × 105 4.03 × 105 4.16 × 105 4.33 × 105 4.26 × 105
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Fig. 5. SEM images of lateral and fracture surfaces of the deformed samples for (a, b, and c) Ti41Zr25Be34 and (d, e, and f) Ti41Zr25Be26Al8 samples respectively.
along with the plastic deformation of the specimen. As a result of large plastic strain, the shear planes deviate from the original orientation [20], and the shear fracture angle increases to 49°. The fracture surface also reveals vein-like pattern and melt drops. In addition, some flower-like type veins (as shown in Fig. 5(f)), which are frequently observed on fractography of tensile specimens of BMGs [19], are observed. However, the arrangement of fishbone-like veins is disorderd and this indicates multiple shear bands have been activated during the deformation process with different propagation directions. The similar morphology can also be found in some other BMGs with good plasticity [21,22]. 4. Discussions It shows that with Al addition, the GFA and the mechanical properties of Ti–Zr–Be glassy alloys have been signicantly improves. Then, understanding the affecting mechanism of Al on the GFA and properties of the alloy is very important. It is known heats of mixing between Ti–Al and Zr–Al is −30 kJ/mol and −44 kJ/mol, respectively [23]. The absolute values are relatively large. Then, with Al addition the strong chemical short-range order would be expected, which may stabilize the
liquid phase and reduce the atomic mobility that mediates crystallization. In addition, abundant chemical clusters of short-range order (SRO) and medium-range order (MRO) would have already existed in Ti–Zr–Be ternary glassy alloy, because of the large negative heats of mixing between Ti–Be (−30 kJ/mol) and Zr–Be (−43 kJ/mol). So with the addition of Al, more types of local ordering clusters (SRO or MRO) would been formed and dispersed in the melt, which would benefit to enhance the GFA of the alloys [24]. On the other hand, the electronegativity difference (Δx) and the atomic size difference parameter (δ), two classic parameters related with the GFA of the glassy alloy, have also been adopted to examine the effect of Al addition on the glass-forming ability of Ti–Zr–Be glassy alloy. According to Fang et al's models [25], Δx and δ of Ti–Zr–Be–Al glassy alloys can be calculated and summarized in Table 3. (the Pauling electronegativity: Ti (1.54), Zr (1.33), Be (1.57) and Al (1.61); the atomic radius: Ti (1.46 Å), Zr (1.58 Å), Be (1.12 Å) and Al (1.43 Å)) As shown in Fig. 6, because Al possesses larger Pauling electrogenativity than Ti, Zr, and Be, the addition of Al would increase Δx value of the alloy, which is beneficial to the improvement of the GFA. However, the value of δ would decrease as the content of Al is increasing, resulting in a detrimental effect to the GFA [25]. Based on a combination of the
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Table 3 The electronegativity difference (Δx) and the atomic size parameter (δ) of Ti41Zr25Be34-x Alx (x= 0, 2, 4, 5, 6, 8, 10 at.%) glassy alloys. Composition (at. %)
Δx
δ
Ti41Zr25Be34 Ti41Zr25Be32Al2 Ti41Zr25Be30Al4 Ti41Zr25Be29Al5 Ti41Zr25Be28Al6 Ti41Zr25Be26Al8 Ti41Zr25Be24Al10
0.0977 0.0984 0.0992 0.0995 0.0999 0.1006 0.1013
0.1372 0.1345 0.1309 0.1292 0.1275 0.1239 0.1200
effects of the two parameters, a local maximum beneficial effect on GFA might be obtained. This may be the reason why Ti41Zr25Be29Al5 alloy possesses the best GFA. With the appropriate amount of Al addition, the Ti–Zr–Be-based BMGs show enhanced strength and plasticity and the mechanism for the improvement is discussed. According to Yang et al.'s work [26], higher Tg always implies higher yield strength. From Tables 1 and 2, there is a really positive correlation between σ0.2 and Tg for the Ti–Zr–Be–Al alloys. The improvement of strength is interpreted as a result of bonding between the constituent elements, especially expected from the large negative mixing enthalpy values (−30 kJ/mol, −44 kJ/mol for the Ti–Al, Zr–Al pairs respectively). It was generally recognized that the plasticity for BMGs are originated from chemical or structural inhomogeneity [21,27–30]; nanocrystallization [31,32]; high Poisson's ratio [33–35] and so on. From the HRTEM results (Fig. 1(b)), no nano-crystal but some local ordering clusters can been observed. In Ti–Zr–Be glassy system, abundant chemical clusters of short-range
Table 4 Elastic properties of Al and Be alloying elements. Elements
E (GPa)
G (GPa)
K (GPa)
υ
G/K
Al Be
70 287
26 132
76 130
0.35 0.032
0.37 1.02
order (SRO) and medium-range order (MRO) have already existed because of the large negative heats of mixing between Ti and Be (−30 kJ/mol), Zr and Be (−43 kJ/mol). With the addition of Al, more types of local ordering clusters have been formed and disperse in the melt, then a reinforcing effect is induced and the compressive plasticity of the alloy is improved. Moreover, recently it has been reported that the elastic modulus of glassy alloy is closely related with the elastic modulus of the constituent elements [34,35]. So that elastic modulus of the glassy alloy can be modified by adjusting the composition of alloy. Compared with Be element, the Al element has both lower shear modulus (G) to bulk modulus (K) ratio and higher Poisson's ratio ν (see Table 4 [36]). Then, replacing Be with Al, the shear modulus G of the alloy would be reduced and Poisson's ratio ν of the alloy would be enhanced. A glassy alloy possessed a low shear modulus and a high Poisson's ratio could favor to activate multiple shear bands, retard the shear localization and improve the plasticity of BMGs [35]. In this work, the enhancement in the plasticity of Ti-based BMG alloys is also attributed to the high value of Poisson's ratio of Al. However, more research work is still needed to find out the exact reason. According to reported results, Ti–Zr–Be alloys are the best glass former of Ti-based ternary glassy alloys. Recently, there are also other reports about Ti-based quaternary glassy alloy based on Ti–Zr–Be alloy systems through V or Cr alloying. Zhang et al. [11] reported a Ti41Zr25Be30V4 alloy with a critical diameter of 8 mm without providing concrete data of mechanical properties. Johnson et al. [12] reported a Ti40Zr25Be30Cr5 alloy, which possesses a critical size of 8 mm. Using 3 mm amorphous rods, compression tests indicated Ti40Zr25Be30Cr5 yields at ~1720 MPa, and finally fractures at a strength of ~1900 MPa, with a plastic strain of ~3.5%. By comparison, our Al-doped glassy alloys possess much higher specific strength and larger plastic strain than the reported alloys although the glass-forming ability is slightly inferior. Moreover, Al element is both cheaper and more plentiful than Cr and V elements. Through comparison, it has been perceived that Al element has a competitive advantage as an alloying element to improving the GFA especially mechanical properties of Ti–Zr–Be ternary glassy alloys.
5. Conclusions
Fig. 6. The electronegativity difference (Δx) and the atomic size parameter (δ) of Ti41Zr25Be34 − xAlx (x = 0, 2, 4, 5, 6, 8, 10 at.%) glassy alloys.
In summary, it has been found that Al addition could significantly improve the glass-forming ability and mechanical property of the Ti–Zr–Be glassy alloys. By replacing Be with Al, the critical diameter of glassy rods has been increased from 5 mm for Ti41Zr25Be34 alloy to 7 mm for Ti41Zr25Be29Al5 alloy. The yield strength of the Ti-Zr-Be glassy alloy has been greatly enhanced with Al addition. As a result the specific strength, which is defined as yield strength/density, has been greatly enhanced. For Ti41Zr25Be26Al8 glassy alloy, the yield strength and specific strength are about 2084 MPa and 4.33 × 105 Nm/kg respectively, which are significantly larger than those of 1755 MPa and 3.69× 105 Nm/kg respectively for Ti41Zr25Be34 glassy alloy. Especially, the specific strength is much larger than that of most glassy alloys and crystalline alloys, including the typical low density high strength Ti-6Al-4 V alloy (about 2.1 × 105 Nm/kg). In addition, the compressive plasticity of the Ti-Zr-Be glassy alloys has also been enhanced by Al addition. Ti41Zr25Be26Al8 BMG exhibits a compressive plastic strain as large as 47.0%, which is much larger than the plastic strain of 2.9% for Ti41Zr25Be34 glassy alloy, and that for most other glassy alloys. The present results show that the developed Ti-Zr-Be-Al bulk metallic glasses are high performance lightweight glassy alloys.
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Acknowledgements This work is supported by National Basic Research Program of China (Grant No. 2007CB613905) and National Natural Science Foundation of China (Grant No. 50971073). The authors acknowledge the support from the National Center of Nano-Science and Nano-Technology of China and the Analysis Center of Tsinghua University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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