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Advances in Materials Science and Engineering

Volume 2014 (2014), Article ID 192187, 5 pages

http://dx.doi.org/10.1155/2014/192187

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

School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

Received 17 March 2014; Accepted 11 April 2014; Published 28 April 2014

Academic Editor: Na Chen

Copyright © 2014 S. F. Zhao et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

A novel centimeter-sized Ti-based bulk metallic glass (BMG) was developed by the addition of Ag in the ternary Ti_{41}Zr_{25}Be_{34} glassy alloy. By replacing Be with Ag, the glass forming ability (GFA), the yield strength, and the supercooled liquid temperature of the quaternary (, 4, 6, 8 at.%) glassy alloys have been obviously enhanced. Among the developed Ti-Zr-Be-Ag alloy systems, the Ti_{41}Zr_{25}Be_{28}Ag_{6} alloy possesses the largest critical diameter () of 10 mm, while the yield strength is also enhanced to 1961 MPa, which is much larger than that of Ti_{41}Zr_{25}Be_{34} (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.

#### 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 Ti_{41}Zr_{25}Be_{34} 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 Ti_{41}Zr_{25}Be_{34−x}Ag_{x} (*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 Ti_{41}Zr_{25}Be_{34−x}Ag_{x} (*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 K*α* 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 of each glassy alloy was measured by Archimedes’ principle in the deionized water.

#### 3. Results

Figure 1 presents X-ray diffraction spectra of the as-cast Ti_{41}Zr_{25}Be_{34−x}Ag_{x} 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 Ti_{41}Zr_{25}Be_{34} 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 Ti_{41}Zr_{25}Be_{26}Ag_{8} alloy returns to 5 mm, which is the same as that of Ti_{41}Zr_{25}Be_{34} 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 Ti_{41}Zr_{25}Be_{28}Ag_{6} has been developed. According to some reported results [13, 14], this is the second quaternary centimeter-diameter Ti-based BMG.

Figure 2 shows the DSC curves of the sample cut out from the as-cast fully glassy Ti_{41}Zr_{25}Be_{34−x}Ag_{x} (*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 , initial crystallization temperature , and liquidus temperature were marked with arrows in Figure 2. In addition, for evaluating the GFA of the Ti_{41}Zr_{25}Be_{34−x}Ag_{x} alloys, the supercooled liquid region (defined as ), * γ* parameter (defined as ), and reduced glass transition temperature (defined as ) [15] were calculated as listed in Table 1.

From Figure 2, it can be found that, with the addition of Ag, decreases from 607 K for Ti_{41}Zr_{25}Be_{34} [6] alloy to 589 K for Ti_{41}Zr_{25}Be_{30}Ag_{2} and Ti_{41}Zr_{25}Be_{30}Ag_{4} alloy and then slightly increases to 597 K for Ti_{41}Zr_{25}Be_{28}Ag_{6} alloy and 593 K for Ti_{41}Zr_{25}Be_{26}Ag_{8} alloy, respectively. increases from 656 K for Ti_{41}Zr_{25}Be_{34} [6] alloy to 670 K for Ti_{41}Zr_{25}Be_{28}Ag_{6} alloy and then decreases to 654 K for Ti_{41}Zr_{25}Be_{26}Ag_{8} alloy. It should be noted that, with Ag addition, the value of has been obviously enlarged; especially, Ti_{41}Zr_{25}Be_{30}Ag_{4} glass alloy has the largest supercooled liquid region of 81 K in the Ti-Zr-Be-Ag alloy system. 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 and with the Ag content in the alloy is roughly the same. The value of for Ti_{41}Zr_{25}Be_{28}Ag_{6} alloy is the largest among all the Ti-Zr-Be-Ag alloys, and Ti_{41}Zr_{25}Be_{30}Ag_{4} alloy possesses the largest value and the lowest value. It is suggested that these two alloys may possess relatively good GFA [16], which is in accordance with the experimental results.

Figure 3 shows the compressive stress-strain curves of Ti_{41}Zr_{25}Be_{34−x}Ag_{x} (*x* = 2, 4, 6, 8 at.%) at room temperature. The yield strength , the maximum compression stress , and the plastic strain of the Ti_{41}Zr_{25}Be_{34−x}Ag_{x} 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 Ti_{41}Zr_{25}Be_{34−x}Ag (*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 of 1755 MPa, a maximum compressive strength of 1914 MPa, and a plastic strain of 2.9% [6]. As shown in Figure 3, Ag addition can greatly increase the yield strength of the BMGs.

For the glassy alloy with optimum Ag content of 6 at.%, the yield strength is 1964 MPa, while with 8 at.% of Ag, the maximum compression stress and plastic strain 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, Ti_{41}Zr_{25}Be_{28}Ag_{6} 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 enthalpies 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 difference and the atomic size difference parameter , 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]. is defined as , is defined as , where , , is the atomic fraction, and are atomic radius and electronegativity of th element, and is the number of allying elements [20, 21]. and of Ti-Zr-Be-Ag glassy alloys were calculated and summarized in Figure 4.

According to the Hume-Rothery rules and Inoue’s three empirical rules [20, 21], the alloys with larger value of and 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 in the Ti-Zr-Be alloys, which effectively enhance the GFA. However, the value of 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 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 would significantly reduce the beneficial effect from , 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 Ti_{41}Zr_{25}Be_{28}Ag_{6} alloy possesses much better GFA; the critical diameter of the quaternary BMG has been increased to 10 mm, while that of ternary Ti_{41}Zr_{25}Be_{34} [6] alloy is only 5 mm. This alloy also exhibits a yield strength of 1961 MPa, 10% higher than that of Ti_{41}Zr_{25}Be_{34} BMG [6]. Furthermore, The Ti-Zr-Be-Ag glassy alloys have a wider supercooled liquid temperature range than that of the Ti_{41}Zr_{25}Be_{34} 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.

#### Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

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