# A Centimeter-Sized Quaternary Ti-Zr-Be-Ag Bulk Metallic Glass

Authors: S. F. Zhao; Y. Shao; P. Gong; K. F. Yao
Journal: Advances in Materials Science and Engineering
 (2014)

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## Abstract

A novel centimeter-sized Ti-based bulk metallic glass (BMG) was developed by the addition of Ag in the ternary Ti41Zr25Be34 glassy alloy. By replacing Be with Ag, the glass forming ability (GFA), the yield strength, and the supercooled liquid temperature of the quaternary Ti41Zr25Be34−xAgx (x=2, 4, 6, 8 at.%) glassy alloys have been obviously enhanced. Among the developed Ti-Zr-Be-Ag alloy systems, the Ti41Zr25Be28Ag6 alloy possesses the largest critical diameter (Dmax) of 10 mm, while the yield strength is also enhanced to 1961 MPa, which is much larger than that of Ti41Zr25Be34 (1755 MPa) alloy. The experimental results show that Ag is an effective element for improving the GFA and the yield strength of Ti-Zr-Be glassy alloy.


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## Body

## 1. Introduction

Ti-based BMGs have been under intense investigation for many years, owing to their excellent properties, such as low density, high strength, high specific strength, low elastic modulus, and strong corrosion resistance [1–4]. Moreover, the low cost makes the Ti-based BMGs a profound application prospect. Up to date, a number of Ti-based BMGs have been synthesized by the copper mold casting method [5–7]. However, compared with other alloy systems, the GFA of most Ti-based BMGs is relatively low [8, 9]. Therefore, it should be of scientific and technological interest to develop Ti-based BMGs with large GFA, together with good mechanical properties. Furthermore, introducing new elements, or so-called “alloying,” is proved to be an effective method to improve the GFA of alloys [8, 9], which makes the developing of Ti-based BMGs with better GFA and less components more challenging.It is known that Ti41Zr25Be34 ternary BMG possesses a critical size of 5 mm which is larger than other Ti-Zr-Be ternary alloys [6, 10]. In the previous work, it shows that its GFA and mechanical properties could be improved through alloying with suitable elements [11, 12]. In this paper, Ag element has been selected as an addition element in the Ti-Zr-Be alloy system. By replacing Be with Ag, a series of BMGs with the composition of Ti41Zr25Be34−xAgx (x = 2, 4, 6, 8), which have improved GFA and mechanical properties, have been obtained.


## 2. Experimental Procedure

The master alloy ingots with nominal compositions of Ti41Zr25Be34−xAgx (x = 2, 4, 6, 8 at.%) were prepared by arc-melting the mixtures of high-purity Ti, Zr, Be, and Ag metals in a Ti-gettered high-purity Ar atmosphere. The purity of Be and Ag metals is over 99.99% in weight, while that of Ti and Zr metals is 99.4% and 99.7% in weight, respectively. Each ingot was flipped and remelted four times to ensure the homogeneity. Cylindrical rods with different diameters were prepared by copper mold casting method.The structure of the as-prepared samples was examined by X-ray diffraction (XRD) using Cu Kα radiation. The thermal stability of the glassy samples was evaluated by differential scanning calorimeter (DSC) at a heating rate of 20 K/min. Compression tests were carried out on a WDW-100 testing machine under a stain rate of 4.2 × 10−4 s−1. The test samples were cut out from the as-cast Φ2 mm rods with gage aspect ratio of 2 : 1. For the compression tests, at least 3 samples of each glassy alloy were tested. The density ρ of each glassy alloy was measured by Archimedes’ principle in the deionized water.


## 3. Results

Figure1 presents X-ray diffraction spectra of the as-cast Ti41Zr25Be34−xAgx BMG samples (x = 2, 4, 6, 8 at.%) with the critical diameters. The typical broad halo patterns for the amorphous phases were observed in each XRD spectrum, and no sharp diffraction peaks corresponding to the crystalline phases could be observed. Figure 1 indicates that, with the proper addition of Ag, the GFA of the Ti41Zr25Be34 alloy has been obviously improved. Meanwhile, the optimized addition content of Ag is about 6 at.%, since its critical diameter for forming fully amorphous structure is 10 mm. As the content of Ag increased to 8 at.%, the critical diameter of the Ti41Zr25Be26Ag8 alloy returns to 5 mm, which is the same as that of Ti41Zr25Be34 alloy [6]. The experimental results indicate that Ag is an effective alloying element for improving the GFA of Ti-Zr-Be alloys. In present work, a new centimeter scale quaternary BMG with the nominal composition Ti41Zr25Be28Ag6 has been developed. According to some reported results [13, 14], this is the second quaternary centimeter-diameter Ti-based BMG.Figure 1
XRD patterns for the Ti41Zr25Be34−xAgx (x = 2, 4, 6, 8 at.%) glassy alloy system with their critical diameters.Figure2 shows the DSC curves of the sample cut out from the as-cast fully glassy Ti41Zr25Be34−xAgx (x = 2, 4, 6, 8 at.%) rods with a diameter of 2 mm. Thermodynamic parameters were measured from the DSC scans, while the glass transition temperature Tg, initial crystallization temperature Tx, and liquidus temperature Tl were marked with arrows in Figure 2. In addition, for evaluating the GFA of the Ti41Zr25Be34−xAgx alloys, the supercooled liquid region ΔTx (defined as Tx-Tg), γ parameter (defined as Tx/(Tg+Tl)), and reduced glass transition temperature Trg (defined as Tg/Tl) [15] were calculated as listed in Table 1.Table 1
Thermal parameters of Ti41Zr25Be34-xAgx (x=2, 4, 6, 8 at.%) BMGs. The error of the temperature values is ±1 K.
Composition (at.%)
T
g (K)
T
x (K)
Δ
T
x (K)
T
l (K)
T
rg = Tg/Tl
γ
D
max
⁡ (mm)
Ti41Zr25Be34 [6]
607
656
49
1123
0.5405
0.3792
Φ
5
Ti41Zr25Be32Ag2
589
651
62
1148
0.5131
0.3748
Φ
5
Ti41Zr25Be30Ag4
589
670
81
1107
0.5321
0.3950
Φ
6
Ti41Zr25Be28Ag6
597
655
58
1118
0.5340
0.3819
Φ
10
Ti41Zr25Be26Ag8
593
654
61
1153
0.5143
0.3746
Φ
5Figure 2
DSC curves of the Ti41Zr25Be34−xAgx (x = 2, 4, 6, 8 at.%) glassy alloys.From Figure2, it can be found that, with the addition of Ag, Tg decreases from 607 K for Ti41Zr25Be34 [6] alloy to 589 K for Ti41Zr25Be30Ag2 and Ti41Zr25Be30Ag4 alloy and then slightly increases to 597 K for Ti41Zr25Be28Ag6 alloy and 593 K for Ti41Zr25Be26Ag8 alloy, respectively. Tx increases from 656 K for Ti41Zr25Be34 [6] alloy to 670 K for Ti41Zr25Be28Ag6 alloy and then decreases to 654 K for Ti41Zr25Be26Ag8 alloy. It should be noted that, with Ag addition, the value of ΔTx has been obviously enlarged; especially, Ti41Zr25Be30Ag4 glass alloy has the largest supercooled liquid region of 81 K in the Ti-Zr-Be-Ag alloy system. ΔTx is considered as a measure to evaluate the thermal stability related to supercooled liquid stability against crystallization [16]; thus Ag addition can effectively improve the thermal stability of the Ti-Zr-Be-Ag glassy alloy. Moreover, the variation tendency of Trg and γ with the Ag content in the alloy is roughly the same. The value of Trg for Ti41Zr25Be28Ag6 alloy is the largest among all the Ti-Zr-Be-Ag alloys, and Ti41Zr25Be30Ag4 alloy possesses the largest γ value and the lowest Tl value. It is suggested that these two alloys may possess relatively good GFA [16], which is in accordance with the experimental results.Figure3 shows the compressive stress-strain curves of Ti41Zr25Be34−xAgx (x = 2, 4, 6, 8 at.%) at room temperature. The yield strength σ0.2, the maximum compression stress σmax⁡, and the plastic strain εp of the Ti41Zr25Be34−xAgx BMGs were listed in Table 2. In the present work, the addition of Ag enhances the density of Ti-Zr-Be alloy, while the value of the specific strength (defined as yield strength/density) of Ti41Zr25Be34−xAg (x = 2, 4, 5, 6, 8) BMGs does not change a lot. According to the reported results, the Ag-free alloy 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% [6]. As shown in Figure 3, Ag addition can greatly increase the yield strength of the BMGs.Table 2
Densities and mechanical properties of Ti41Zr25Be34-xAgx (x=2, 4, 6, 8 at.%) BMGs.
Composition (at.%)
Density (g/cm3)
σ
0.2 (MPa)
σ
max
⁡ (MPa)
ε
p (%)
Specfic strength (Nm/Kg)
Ti41Zr25Be34 [6]
4.76
1755
1914
2.9 ± 0.1
3.69 × 105
Ti41Zr25Be32Ag2
5.01
1808
1882
2.4 ± 0.1
3.61 × 105
Ti41Zr25Be30Ag4
5.13
1931
1995
2.9 ± 0.1
3.76 × 105
Ti41Zr25Be28Ag6
5.27
1961
2013
2.3 ± 0.1
3.73 × 105
Ti41Zr25Be26Ag8
5.42
1946
2054
4.8 ± 0.1
3.59 × 105Figure 3
Compressive stress-strain curves at room temperature for Ti41Zr25Be34-xAgx (x = 2, 4, 6, 8 at.%) glassy samples.For the glassy alloy with optimum Ag content of 6 at.%, the yield strengthσ0.2 is 1964 MPa, while with 8 at.% of Ag, the maximum compression stress σmax⁡ and plastic strain εp are 2054 MPa and 4.8%, respectively. The present results indicate that Ag addition could effectively improve the mechanical properties of Ti-Zr-Be glassy alloys.It shows that, among the quaternary Ti-Zr-Be-Ag alloy system, Ti41Zr25Be28Ag6 glassy alloy possesses not only the largest GFA, but also high strength and good compressive plastic strain.


## 4. Discussion

It is known that the mixing enthalpiesΔHmix between Ti-Ag, Ti-Zr, Zr-Ag, Ti-Be, Ag-Be, and Zr-Be are −2 kJ/mol, 0 kJ/mol, −20 kJ/mol, −30 kJ/mol, 2 kJ/mol, and −43 kJ/mol, respectively [17]. Thus, in the Ti-Zr-Be-Ag alloy system, the strong chemical short-range order clusters or medium-range order clusters would be expected [18], which may restrain the diffusion of the atoms, and could suppress crystallization during the solidification. Meanwhile, the addition of Ag increases the number of the components in the alloy, which could generate more types of local ordering clusters and stabilize the liquid phase [18].In addition, the electronegativity differenceΔx and the atomic size difference parameter δ, the two parameters that related to the GFA of the glassy alloy, have been applied to evaluate the effect of Ag addition on the GFA of Ti-Zr-Be glassy alloy [19]. δ is defined as δ=∑i=1nci(1-ri/r-), Δx is defined as Δx=∑i=1nci×(xi-x-)2, where r-=∑i=1Nciri, x-=∑i=1Ncixi,ci is the atomic fraction, ri and xi are atomic radius and electronegativity of ith element, and N is the number of allying elements [20, 21]. Δx and δ of Ti-Zr-Be-Ag glassy alloys were calculated and summarized in Figure 4.The atomic size parameterδ (a) and the electronegativity difference Δx (b) of Ti41Zr25Be34−xAgx (x = 2, 4, 6, 8 at.%) glassy alloys.
(a)
(b)According to the Hume-Rothery rules and Inoue’s three empirical rules [20, 21], the alloys with larger value of δ and Δx could form amorphous phase readily. As shown in Figure 4, because Ag possesses larger Pauling electronegativity (1.91) than Ti (1.54), Zr (1.33), and Be (1.57), the addition of Ag would increase the value of Δx in the Ti-Zr-Be alloys, which effectively enhance the GFA. However, the value of δ would decrease as the content of Ag increase, which is not beneficial to improve the GFA [22]. When the content of Ag is relatively low, the beneficial effect of Δx dominates the alloying effect on GFA. So the critical size increases with Ag content and reached the maximum value of 10 mm at 6 at.%. When increasing Ag content again, the effect from δ would significantly reduce the beneficial effect from Δx, resulting in the decrease of GFA. Similar effects have also been observed in Ti-Zr-Be-Al [8] and Ti-Zr-Be-Fe [6] quaternary BMGs, too. Due to the efforts of these two factors, there would exist an optimized Ag content in the Ti-Zr-Be alloy system, which is 6 at.%.


## 5. Conclusion

In summary, Ag addition could significantly improve the GFA, thermal stability, and mechanical properties of the Ti-Zr-Be glassy alloys. By replacing Be with Ag, it has been found that the developed Ti41Zr25Be28Ag6 alloy possesses much better GFA; the critical diameter of the quaternary BMG has been increased to 10 mm, while that of ternary Ti41Zr25Be34 [6] alloy is only 5 mm. This alloy also exhibits a yield strength of 1961 MPa, 10% higher than that of Ti41Zr25Be34 BMG [6]. Furthermore, The Ti-Zr-Be-Ag glassy alloys have a wider supercooled liquid temperature range than that of the Ti41Zr25Be34 glassy alloys, indicating a higher thermal stability of the glassy alloys. The enhanced GFA is supposed to be related to the improved atomic packing efficiency and high electronegativity difference, which can retard the atomic diffusion due to the addition of Ag.



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*Source: 192187-2014-04-28.xml*