A Centimeter-Sized Quaternary Ti-Zr-BeAg Bulk Metallic Glass

A novel centimeter-sized Ti-based bulkmetallic 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 Ti41Zr25Be34−xAgx (x = 2, 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 (Dmax) of 10mm, while the yield strength is also enhanced to 1961MPa, which is much larger than that of Ti 41 Zr 25 Be 34 (1755MPa) 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.


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][2][3][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][6][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.

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 arcmelting 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 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.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.

Results
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 Δ  (defined as   −   ),  parameter (defined as   /(  +   )), and reduced glass transition temperature  rg (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]   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  rg and  with the Ag content in the alloy is roughly the same.The value of  rg 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  0.2 , the maximum compression stress  max , 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  0.2 of 1755 MPa, a maximum compressive strength  max 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  0.2 is 1964 MPa, while with 8 at.% of Ag, the maximum compression stress  max 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.

Discussion
It is known that the mixing enthalpies Δ mix between Ti-Ag, Ti-Zr, Zr-Ag, Ti-Be, Ag-Be, and Zr-Be are −2 kJ/mol, Engineering strain (%) 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 Δ 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  = √∑  =1   (1 −   /), Δ is defined as Δ = √ ∑  =1   × (  − ) 2 , where  = ∑  =1     ,  = ∑  =1     ,   is the atomic fraction,   and   are atomic radius and electronegativity of th element, and  is the number of allying elements [20,21].Δ 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 Δ 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 Δ 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 Δ 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 Δ, 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.%.

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.
[6]oy 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.increasesfrom 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 Δ  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.Δ