Effect of Ag addition on the thermal stability and glass-forming ability of Zr 35 Ti 30 Cu 7.5 Be 27.5 bulk metallic glass

The thermal stability and glass forming ability (GFA) of Zr 35 − x Ti 30 Cu 7.5 Be 27.5 Ag x ( x = 0–10) alloys were studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and ultrasonic techniques. We found that the addition of 1 at.% Ag can con-siderably enhance the GFA as indicated by an increase in the critical glass dimension from 15 mm in the Zr 35 Ti 30 Cu 7.5 Be 27.5 alloy to 20 mm in the Zr 34 Ti 30 Cu 7.5 Be 27.5 Ag 1 alloy. However, with the addition of more Ag the supercooled liquid region (  T x ) and  parameter (defined as T x /( T g + T l )) drastically decreased from 155 K and 0.436 to 76 K and 0.363, respectively, resulting in a de-crease in the GFA. Additionally, the elastic constant (the ratio of shear modulus to bulk modulus or Poisson’s ratio) was also used as a gauge to evaluate the GFA in Zr 35 − x Ti 30 Cu 7.5

Over the past two decades, bulk metallic glasses (BMGs) have attracted an increasing amount of attention because of their superb properties and potential as new structural materials [1][2][3]. A considerable number of studies related to the structural relaxation [4][5][6][7][8], fluidity and molding ability [9], corrosion behavior [10,11] and mechanical properties [12][13][14][15] of BMGs have been carried out, which contributes to the application of BMGs. However, the potential applications of BMGs are hindered by their limited glass forming ability (GFA). As a result, a great deal of efforts has been put into the development of BMGs and numerous multi-component alloys [16,17] capable of solidifying into bulk glass at relatively low cooling rates have been found in either noble metal Pd-based alloys or Zr-based alloys [18,19].
Minor alloying addition or microalloying technology has proven to be an effective approach in promoting glass formation, enhancing thermal stability and improving the plas-ticity of BMGs [20][21][22]. It has been found that microalloying with proper alloying elements can dramatically improve the GFA of various BMGs [23][24][25]. Previous studies showed that the GFA of Cu-, Mg-and ZrCu-based bulk metallic glasses were significantly enhanced by adding Ag [26][27][28][29]. In this work, Ag was also chosen as the doping element in the Zr 35 Ti 30 Cu 7.5 Be 27.5 system, which had a critical diameter of about 15 mm and a wide supercooled liquid region of 165.1 K [30]. To investigate the elastic properties of the resultant ZrTi-based BMGs their acoustic velocities were measured at room temperature by a pulse echo overlap method.
Zr 35-x Ti 30 Cu 7.5 Be 27.5 Ag x (x = 0, 1 at.%, 2 at.%, 3 at.%, 5 at.%, 7 at.%, 10 at.%) ingots were prepared by arc melting high purity ( 99.9 wt.%) constituents under a titanium gettered argon atmosphere. Each ingot was remelted at least four times to ensure chemical homogeneity. Cylindrical alloy rods with diameters of 3, 4, 15 and 20 mm were prepared by injection casting the remelted ingots into a copper mould under an argon atmosphere. The samples were then brought to you by CORE View metadata, citation and similar papers at core.ac.uk provided by Springer -Publisher Connector cut into small pieces with a low speed diamond saw and the transverse cross-section of the specimens was examined by X-ray diffraction (XRD) with Cu K radiation to examine the structure of the samples. The thermal properties associated with the glass transition, crystallization and melting behavior of the alloys were measured using a Netzsch STA 449C differential scanning calorimeter (DSC) under continuous argon flow at a heating rate of 20 K/min. The acoustic velocities were measured at room temperature by a pulse echo overlap method using a RITEC RAM-5000 ultrasonic system with a measuring sensitivity of 0.5 ns and a carry frequency of 10 MHz at room temperature. The samples used for ultrasonic measurements were 4 mm in diameter and 10 mm in length. The density was measured by the Archimedean principle and the accuracy was within 0.5%. The bulk modulus B, Young's modulus E, shear modulus G and Poisson's ratio ν of the BMGs were derived from the acoustic velocities and density [31]. Figure 1 shows XRD patterns of the as-cast Zr 35−x Ti 30 -Cu 7.5 Be 27.5 Ag x (x = 0, 1 at.%, 2 at.%, 3 at.%, 5 at.%, 7 at.%, 10 at.%) alloys with diameters of 15 and 20 mm. As shown in Figure 1(a), the patterns of the samples that contain 0-3 at.% Ag only contain a broad diffraction hump without any detectable crystalline peaks indicating their amorphous structure. However, for the alloy with 5 at.%, 7at.% and 10 at.% Ag sharp Bragg peaks superimposed over the amorphous maxima were observed implying that the alloys were partially crystallized. The diffraction peaks become more pronounced as Ag increases from 5 at.% to 10 at.%. From Figure 1(b), the best GFA was achieved for x = 1, which has a critical diameter of 20 mm. The enhanced GFA can probably be attributed to a denser local atomic packing and smaller differences in the Gibbs free energy between the amorphous and crystalline phases [29]. Therefore, the GFA of the Zr 35−x Ti 30 Cu 7.5 Be 27.5 Ag x (x=0-10) are x = 1x = 0x = 2x = 3x = 5x = 7x = 10. Figure 2 shows DSC curves for the as-cast Zr 35−x Ti 30 -Cu 7.5 Be 27.5 Ag x (x = 0-10) glassy alloys at a heating rate of 20 K/min. These glassy alloys have a distinct glass transi-tion followed by a supercooled liquid region and then an exothermic reaction because of crystallization. The specimens for the DSC measurement were cut from the 3 mm glassy rods. The glass transition temperature (T g ), onset crystallization temperature (T x ), onset melting temperature (T m ), liquidus temperature (T l ) and other thermodynamic parameters of the Zr 35−x Ti 30 Cu 7.5 Be 27.5 Ag x (x = 0-10) BMGs are listed in Table 1. From these DSC curves and Table 1, the supercooled liquid region (T x ) increases slightly from 153 to 155 K with 1 at.% addition and then steadily decreases to 76 K with an increase in Ag content from 1 at.% to 10 at.%. A large T x value may indicate that the supercooled liquid can remain stable over a wide temperature range without crystallization and that it has high resistance to the nucleation and growth of crystalline phases. In this study, T x , the reduced glass transition temperature T rg and the  parameter were selected as the thermal criteria to evaluate the GFA in the Zr 35−x Ti 30 Cu 7.5 Be 27.5 Ag x (x=0- 10) alloys. The composition dependence of , ∆T x and T rg (shown in the inset) in the Zr 35x Ti 30 Cu 7.5 Be 27.5 Ag x BMGs is shown in Figure 3. Combined with the XRD results, we found that T x and  proved to be more effective than T rg in evaluating the GFA of the Zr 35−x Ti 30 Cu 7.5 Be 27.5 Ag x glassy alloys with a variation in the Ag content x from 0 to 10.
Recent work has emphasized the glass transition dependence of elastic properties such as the shear modulus G and the Poisson ratio ν, and the elastic criteria is developed by using G/B or the Poisson ratio to judge the GFA [32,33]. The ratio of shear modulus G to bulk modulus B or Poisson's ratio was reported to correlate with the critical cooling rate (R c ) on the basis of experimental observations where glass with high G/B values (or a low Poisson's ratio) roughly correspond to a high GFA for the system [34]. Generally, a small ν means that atoms or molecules can hardly rearrange themselves as a result of shear strains without a drastic disturbance in bonding configurations and a large v indicates an easier atomic rearrangement. Therefore, glass-forming systems with relatively low ν may exhibit a high GFA.
Longitudinal and transverse velocities were determined    Samples and the elastic constants of interest were calculated. Table 2