The Effect of Ag Addition on the Enhancement of the Thermal and Mechanical Properties of CuZrAl Bulk Metallic Glasses

In this study, the thermal and mechanical properties of Cu50−xZr43Al7Agx (x = 0, 3, 4, 5, 6) bulk metallic glasses (BMGs) are investigated by using an X-ray diffractometer (XRD), a differential scanning calorimeter (DSC), differential thermal analysis (DTA), a Vickers hardness tester, a material test system (MTS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Cu50−xZr43Al7Agx (x = 0, 3, 4, 5, 6) BMGs were made by arc-melting and an injection casting process. The results revealed that the glass transition temperature (Tg) and the crystallization temperature (Tx) of CuZrAl alloy decreased with the Ag addition. Hence, the supercooled liquid region and γ of Cu45Zr43Al7Ag5 alloy increased to 76 K and 0.42, respectively. The thermal stability and glass forming ability of CuZrAlAg BMG alloys were enhanced by the microalloyed Ag content. The room temperature compressive fracture strength and strain measured of Cu47Zr43Al7Ag3 were about 2200 MPa and 2.1%, respectively. The distribution of vein patterns and the formation of nanocrystalline phases on the fracture surface of Cu47Zr43Al7Ag3 alloy can be observed by SEM and TEM to be significant, indicating a typical ductile fracture behavior and an improved plasticity of alloys with the addition of microalloyed Ag from 0 to 6 atom %.


Introduction
Cu-Zr-Al bulk metallic glasses (BMGs) with low density have attracted increasing attention due to their high specific strength and density ratio and excellent thermal properties [1][2][3][4][5][6], as compared with the conventional alloys.For example, the Cu 45 Zr 48 Al 7 BMGs [7] exhibits high fracture stress near 1890 MPa.Hence, the development of Cu-based bulk amorphous alloys in structural parts and the transportation vehicle applications is steadily growing.However, the low workability at room temperature of Cu-based BMGs has limited their applications [7].Therefore, the enhancement on the plasticity of BMGs with high glass forming ability (GFA) has been strongly requested.
In this study, the Cu 50 Zr 43 Al 7 [25] with high GFA and high thermal stability are used as base alloys, and the structural transformations and the shear deformation process of BMGs are studied BMGs made with microalloyed Ag content (from 0 to 6 atom %) are discussed in conjunction with the analysis of XRD, DSC, DTA, micro-hardness, and compression stress-strain curves of measured BMGs.

Materials and Methods
The composition of Cu 50−x Zr 43 Al 7 Ag x (x = 0, 3, 4, 5, and 6 in the following denoted as Ag0, Ag3, Ag4, Ag5, and Ag6) were selected for preparing the bulk metallic glasses (BMGs).High purity elements (>99.9 wt.% or higher) were pre-alloyed into an alloy ingot by an arc melting process in a Ti-gettered argon atmosphere.Alloy ingots were remelted five times to ensure chemical homogeneity.BMG rod specimens with diameters of 2-3 mm were fabricated by suction casting into a water-cooled Cu mold under an argon atmosphere.Differential scanning calorimeter (TA Instruments DSC 2920, TA Instruments Inc., New Castle, DE, USA) and differential thermal analysis (Perkin Elmer DTA-7, PerkinElmer Inc., Waltham, MA, USA) were used for thermal analyses of the specimens at a heating rate of 20 K/min.Hardness measurements and compression tests were performed with a Vickers hardness tester (Akashi AVK-C2, Akashi Corporation, Kanagawa, Japan) and the MTS 810 material test system at room temperature (at a strain rate of 1 × 10 −4 ).Cylindrical samples with the height-to-diameter ratio of 2:1 were cut from as-cast rods.Structural characterization was examined with an X-ray diffractometer (XRD, Sintage X-4000, with monochromatic Cu-Kα radiation), scanning electron microscopy (SEM, Hitachi S-3400, Hitachi High-Technologies Corporation, Tokyo, Japan), and transmission electron microscopy (TEM, FEI, Tecnai G2 20 S-Twin, FEI company, FEI company, OR, USA).

Phase Identification
The XRD patterns of Cu 50−x Zr 43 Al 7 Ag x (x = 0, 3, 4, 5, 6) alloys are shown in Figure 1.Within the limit of XRD resolution, the XRD spectra reveal broad diffraction humps without any characteristic Bragg peaks corresponding to crystalline phases.The verification of Cu 50−x Zr 43 Al 7 Ag x (x = 0, 3, 4, 5, 6) alloys with the microalloyed Ag addition was achieved.In this study, the Cu50Zr43Al7 [25] with high GFA and high thermal stability are used as base alloys, and the structural transformations and the shear deformation process of BMGs are studied by microalloyed Ag content.Variations in thermal and mechanical properties of Cu50−xZr43Al7Agx BMGs made with microalloyed Ag content (from 0 to 6 atom %) are discussed in conjunction with the analysis of XRD, DSC, DTA, micro-hardness, and compression stress-strain curves of measured BMGs.

Materials and Methods
The composition of Cu50−xZr43Al7Agx (x = 0, 3, 4, 5, and 6 in the following denoted as Ag0, Ag3, Ag4, Ag5, and Ag6) were selected for preparing the bulk metallic glasses (BMGs).High purity elements (>99.9 wt.% or higher) were pre-alloyed into an alloy ingot by an arc melting process in a Ti-gettered argon atmosphere.Alloy ingots were remelted five times to ensure chemical homogeneity.BMG rod specimens with diameters of 2-3 mm were fabricated by suction casting into a water-cooled Cu mold under an argon atmosphere.Differential scanning calorimeter (TA Instruments DSC 2920, TA Instruments Inc., New Castle, DE, USA) and differential thermal analysis (Perkin Elmer DTA-7, PerkinElmer Inc., Waltham, MA, USA) were used for thermal analyses of the specimens at a heating rate of 20 K/min.Hardness measurements and compression tests were performed with a Vickers hardness tester (Akashi AVK-C2, Akashi Corporation, Kanagawa, Japan) and the MTS 810 material test system at room temperature (at a strain rate of 1 × 10 −4 ).Cylindrical samples with the height-to-diameter ratio of 2:1 were cut from as-cast rods.Structural characterization was examined with an X-ray diffractometer (XRD, Sintage X-4000, with monochromatic Cu-Kα radiation), scanning electron microscopy (SEM, Hitachi S-3400, Hitachi High-Technologies Corporation, Tokyo, Japan), and transmission electron microscopy (TEM, FEI, Tecnai G2 20 S-Twin, FEI company, FEI company, OR, USA).

Thermal Analysis
Figure 2 illustrates the DSC and the DTA curves of Cu 50−x Zr 43 Al 7 Ag x (x = 0, 3, 4, 5, 6) BMGs.Their glass transition temperature (T g ), crystallization onset temperature (T x ), liquidus temperature (T l ), and related thermal parameters are summarized in Table 1.No obvious variation of the crystallization temperature of the alloys can be found, but both of the glass transition temperatures and the liquidus temperature of the alloys decreases with increasing Ag content.Hence, the reduced glass transition temperature (T rg = T g /T l ) and the γ parameter (γ = T x /(T g + T l )) increase as the content of microalloyed Ag increases, indicating an increment of the glass forming ability (GFA) of Cu 50−x Zr 43 Al 7 Ag x (x = 0, 3, 4, 5, 6) BMGs.The supercooled liquid region (∆T x = T x − T g ) are extended from 54 to 76 K with a 5-atom % Ag addition; however, this region is reduced to 62 K as the microalloyed Ag further increases to 6 atom %.

Compression Tests and Microstructure Observation
The room temperature compressive stress-strain curves of the Cu 50−x Zr 43 Al 7 Ag x rods are shown in Figure 3.The size of the rod samples used for the compression tests was 1 mm in diameter and 2 mm in height.As can be seen, the Ag-free base alloy with composition of Cu 50 Zr 43 Al 7 exhibits less than 1% plastic strain.Vein-like patterns (as shown in Figure 4a) are displayed on the fracture surface of Cu 50 Zr 43 Al 7 alloys, indicating a slightly plastic fracture behavior of this specimen.Moreover, the results presented in Figure 3 show that the alloys with Ag contents of 3 and 5 atom % exhibit compressive plasticity and that the compressive strain increased to 2.1% and 1.5%, respectively.However, we found that the plastic strain of CuZrAlAg alloys is lower than that of the base alloy when the microalloyed Ag content exceeds 4 atom %.In addition, the fracture stress of the CuZrAlAg alloy increased to 2280 MPa with a 3-atom % Ag addition; however, it decreased to ~1900 MPa upon further increases in the Ag content.It is observed that the strength of the Cu 50−x Zr 43 Al 7 Ag x (x = 0, 3, 4, 5, 6) BMG alloys does not improve with the addition of microalloyed Ag more than 3 atom %.The Cu 47 Zr 43 Al 7 Ag 3 BMG alloy exhibits the highest ductility.The variation tendency of hardness (Figure 5) is similar to that of the fracture stress of the Cu 50−x Zr 43 Al 7 Ag x (x = 0, 3, 4, 5, 6) alloys.Figure 6 shows SEM images of the shear bands that occur on the lateral surface of the Cu 50−x Zr 43 Al 7 Ag x alloys after compressive testing.It is evident that the primary and secondary shear bands (indicated using a dashed-line arrow) are formed after the room temperature compression.In Figure 6b-d, the secondary shear bands interact with the primary ones and subsequently retard the propagation of the primary shear bands.However, in Figure 6e, no secondary shear bands are visible and only one set of the primary shear bands near the edge of fracture surface is formed.Therefore, the Cu 47 Zr 43 Al 7 Ag 6 alloy was excluded from the calculation of shear band density.The statistical shear band density of Cu 50−x Zr 43 Al 7 Ag x (x = 0, 3, 4, 5) BMGs with different Ag contents are as follows.The results can be divided into two parts: a rapid increase from 4.1 × 10 −3 µm −1 (Ag0) to 11.6 × 10 −3 µm −1 (Ag3), followed by a decrease to 9.2 × 10 −3 µm −1 (Ag4) and 7.2 × 10 −3 µm −1 (Ag5).This suggests that the dense distribution of shear bands is related to the enhanced ductility and strength of BMGs.
The Cu 47 Zr 43 Al 7 Ag 3 BMG alloys possessed the largest fracture strength, hardness, and plastic strain.Therefore, they were selected for further study.SEM micrographs of the outer surface of Cu 47 Zr 43 Al 7 Ag 3 rods after compressive testing showed the formation and propagation of the secondary shear bands on the lateral surface of the Cu 47 Zr 43 Al 7 Ag 3 alloy, which leads to an increase in plastic strain.This result is confirmed via TEM observation.In contrast to the TEM images of Cu 47 Zr 43 Al 7 Ag 3 (Figure 7a,b) and Cu 47 Zr 43 Al 7 Ag 6 (Figure 7c,d) alloys shown in Figure 7, the nano-sized crystalline phase is evident from the TEM images of the Cu 47 Zr 43 Al 7 Ag 3 amorphous matrix.Ag has a positive enthalpy of mixing with Cu (Cu-Ag: +5 kJ/mol), which results in the separation of the nanocrystalline phase from the liquid state of the Cu 47 Zr 43 Al 7 Ag 3 alloy.The formation of a nanocrystalline phase in the BMG matrix leads to the discontinuous propagation and branching of the primary shear bands during the shear deformation [26].Therefore, the strength, plastic strain, and hardness of the Cu 47 Zr 43 Al 7 Ag 3 alloys are enhanced by the presence of small-sized and homogeneously scattered nanocrystals, as observed in the SEM images and in the room temperature compressive stress-strain curves of the Cu 47 Zr 43 Al 7 Ag 3 alloy.
separation of the nanocrystalline phase from the liquid state of the Cu47Zr43Al7Ag3 alloy.The formation of a nanocrystalline phase in the BMG matrix leads to the discontinuous propagation and branching of the primary shear bands during the shear deformation [26].Therefore, the strength, plastic strain, and hardness of the Cu47Zr43Al7Ag3 alloys are enhanced by the presence of small-sized and homogeneously scattered nanocrystals, as observed in the SEM images and in the room temperature compressive stress-strain curves of the Cu47Zr43Al7Ag3 alloy.
In contrast, the Cu47Zr43Al7Ag6 alloy had fewer nanocrystalline phases in the BMG matrix, which were also of larger size.Hence, the large-sized nanocrystalline phase in Cu47Zr43Al7Ag6 alloy is inefficient for the retarding of the shear band propagation and led to the reduction in hardness, compressive stress, and strain.

Conclusion
In conclusion, the structural, thermal, and mechanical properties of Cu50−xZr43Al7Agx amorphous alloys with microalloyed Ag addition (from 0 to 6 atom %) were studied by XRD, thermal analyses, a micro-hardness test, and a compression test.Results indicated that the glass forming ability (γ up to 0.42) and the thermal stability (ΔTx up to 76 K) can be improved by partial substitution of Cu by Ag element.The higher mechanical properties with compressive fracture stress, plastic strain, and hardness near to the values of 2280 MPa, 2.1%, and 510 Hv, respectively, are displayed in the Cu47Zr43Al7Ag3 alloy.In addition, from SEM and TEM observations, the crystallization of nano-sized phase aids to disrupt the propagation of shear deformation, to branch the primary shear bands, and to form the secondary shear bands of alloys during plastic deformation.

Figure 2 Figure 2 .
Figure 2 illustrates the DSC and the DTA curves of Cu50−xZr43Al7Agx (x = 0, 3, 4, 5, 6) BMGs.Their glass transition temperature (Tg), crystallization onset temperature (Tx), liquidus temperature (Tl), and related thermal parameters are summarized in Table1.No obvious variation of the crystallization temperature of the alloys can be found, but both of the glass transition temperatures and the liquidus temperature of the alloys decreases with increasing Ag content.Hence, the reduced glass transition temperature (Trg = Tg/Tl) and the γ parameter (γ = Tx/(Tg + Tl)) increase as the content of microalloyed Ag increases, indicating an increment of the glass forming ability (GFA) of Cu50−xZr43Al7Agx (x = 0, 3, 4, 5, 6) BMGs.The supercooled liquid region (ΔTx = Tx − Tg) are extended from 54 to 76 K with a 5-atom % Ag addition; however, this region is reduced to 62 K as the microalloyed Ag further increases to 6 atom %.

Figure 5 .Figure 5 .
Figure 5.The hardness of the Cu50−xZr43Al7Agx alloys with increasing microalloyed Ag content.Figure 5.The hardness of the Cu 50−x Zr 43 Al 7 Ag x alloys with increasing microalloyed Ag content.

Figure 6 .
Figure 6.SEM images of the multiple shear bands on the lateral surface of compressive Cu50−xZr43Al7Agx alloys: (a) x = 0, (b) x = 3, (c) x = 4, (d) x = 5, and (e) x = 6.The two arrows indicated the primary shear bands and the secondary shear bands (the arrow with dashed line).

Figure 6 .
Figure 6.SEM images of the multiple shear bands on the lateral surface of compressive Cu 50−x Zr 43 Al 7 Ag x alloys: (a) x = 0, (b) x = 3, (c) x = 4, (d) x = 5, and (e) x = 6.The two arrows indicated the primary shear bands and the secondary shear bands (the arrow with dashed line).

Figure 6 .Figure 7 .
Figure 6.SEM images of the multiple shear bands on the lateral surface of compressive Cu50−xZr43Al7Agx alloys: (a) x = 0, (b) x = 3, (c) x = 4, (d) x = 5, and (e) x = 6.The two arrows indicated the primary shear bands and the secondary shear bands (the arrow with dashed line).