Trans. Nonferrous Met. Soc. China 22(2012) 842-846

Effects of Y addition on structural and mechanical properties of CuZrAl bulk metallic glass

XU Hong-wei^{1, 2}, DU Yu-lei^{1, 2}, DENG Yu³

1. Engineering Research Center of Materials Behavior and Design of Ministry of Education,

School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China;

2. Jiangsu Institute of Advanced Materials, Danyang 212300, China;

3. National Laboratory of Solid State Microstructures, Center for Materials Analysis,Nanjing University, Nanjing 210093, China

Received 9 September 2011; accepted 12 January 2012

Abstract: The effects of Y addition on the structural and mechanical properties of CuZrAl bulk metallic glass (BMG) were studied. The results show that the glass forming ability of CuZrAl system is improved by the addition of Y and the fracture strength decreases with Y addition due to the reduction of binding energy induced by Y. The fracture surface is dominated by vein-like patterns in Cu₄₅Zr₄₈Al₇ bulk metallic glass, and changes to smooth regions in Cu₄₆Zr₄₂Al₇Y₅ BMG. TEM observation shows that Cu₄₅Zr₄₈Al₇ BMG has a composite microstructure of nanocrystalline phases dispersed in amorphous matrix. However, the Cu₄₆Zr₄₂Al₇Y₅ BMG shows a fully amorphous structure.

Key words: bulk metallic glass; Cu-based alloy; Y addition

1 Introduction

Bulk metallic glasses (BMGs) are considered new structural materials due to their unique mechanical, physical and chemical properties [1-5]. It is reported that the glass forming ability (GFA), physical, chemical and mechanical properties of BMGs can be greatly affected by the addition of rare earth metals [6, 7]. For example, it is found that the GFA of Fe-, Zr- and Ti-based BMGs can be enhanced by minor addition of Y [8-10]. The reason for the improvement is that the addition of Y can restrain the formation of crystalline phases and scavenge oxygen from the undercooled liquid [11]. Recently, the excellent enhancement in GFA by the addition of Y has been also found in Cu-based BMGs [12]. As reported, the fully amorphous Cu₄₆Zr_47-xAl₇Y_x (0≤x≤10) BMGs with diameter from 4 up to 10 mm could be easily prepared by an injection mold casting.

Compared to Zr-, Pd-, Fe- and Ti-based BMGs [1,2, 13-15], Cu-based BMGs with high GFA are more promising for industrial applications because of their relatively low cost and no toxic element like Be or Ni [16-20]. To promote their applications, the combination of high GFA and excellent mechanical properties of Cu-based BMGs is expected. However, in the previous work, it was found that the fracture strength of a Zr-based BMG decreased with increasing Y content [21]. Up to now, the effects of Y on the mechanical properties of Cu-based BMGs have not been clearly clarified. Therefore, it is valuable to reveal the effects of Y on the structural and mechanical properties of CuZrAl BMGs. In the present work, Cu₄₅Zr₄₈Al₇ and Cu₄₆Zr₄₂Al₇Y₅ BMGs were prepared. The effects of Y on the GFA, thermal stability and fracture strength were studied. The difference in the fracture surface and local-order microstructure induced by Y was also studied by SEM and HRTEM.

2 Experimental

The alloy ingots of Cu₄₅Zr₄₈Al₇ and Cu₄₆Zr₄₂Al₇Y₅ were prepared by arc melting the mixtures of Cu, Zr, Al and Y metals in a Ti-gettered argon atmosphere. All elements used are of >99.95% purity. Each ingot was melted at least five times to ensure the homogeneity. The alloy ingots were remelted in a fused glass tube under a vacuum level of about 5×10^-3 Pa and then injection cast with ultrahigh purity argon into a copper mold to prepare cylindrical rods with size of d3 mm×100 mm. X-ray diffraction (XRD) with Cu K_α radiation for phase identification was performed on the as-cast samples via θ-2θ scans. The thermal stability of the as-cast samples was examined by differential scanning calorimetry (DSC) at a constant heating rate of 20 K/min in argon atmosphere using a Netzsch STA 449C device. Room temperature compression tests were carried out with a WDW-200D machine under a maximum load of 200 kN at an engineering strain rate of 5×10^-4 s^-1. The cross-sectional surfaces of the Cu₄₅Zr₄₈Al₇ and Cu₄₆Zr₄₂Al₇Y₅ samples were examined using a S-3400N II scanning electron microscope (SEM). Specimens for transmission electron microscopy (TEM) were prepared by standard twin-jet electrolytic thinning with a HNO₃-CH₃OH electrolyte (volume ratio of 7:3). The microstructure was observed by TEM on a JEM-200CX instrument.

3 Results and discussion

Figure 1 shows the XRD patterns of the as-cast Cu₄₅Zr₄₈Al₇ and Cu₄₆Zr₄₂Al₇Y₅ alloys. It can be seen that the patterns of the two alloys exhibit only a broad diffraction maximum without any observable crystalline peaks, demonstrating the formation of a fully amorphous structure.

Fig. 1 XRD patterns of as-cast Cu₄₅Zr₄₈Al₇ and Cu₄₆Zr₄₂Al₇Y₅ alloys

The DSC curves of the as-cast Cu₄₅Zr₄₈Al₇ and Cu₄₆Zr₄₂Al₇Y₅ samples are shown in Fig. 2. During heating, both of the samples exhibit a distinct endothermic peak due to glass transition, followed by a supercooled liquid region, and then a sharp exothermic peak due to crystallization. The glass transition temperature (T_g) and the onset temperature of crystallization (T_x) are measured to be 700 and 761 K, 678 and 759 K for Cu₄₅Zr₄₈Al₇ and Cu₄₆Zr₄₂Al₇Y₅ alloys, respectively. The corresponding supercooled liquid region ΔT(T_x-T_g) are calculated to be 61 and 81 K, respectively. Obviously, the addition of Y expands the supercooled liquid region, indicating the improvement of the glass forming ability of CuZrAl bulk metallic glass. As shown in Fig. 2, the exothermic peaks tend to be broadened with Y addition, indicating a possible slow-down in the kinetics of crystal nucleation and growth. It is known that the glass forming ability can be strongly affected by the large negative heat of mixing among the constituent elements. The mixture heat of Y-Cu, Y-Al and Zr-Cu are -22, -31 and -23 kJ/mol, respectively [22]. Therefore, the formation of an inhomogeneous structure, such as short- or medium-range ordering clusters in the amorphous phase can be hindered by the addition of Y in CuZrAl system, which leads to the improvement of the glass forming ability of CuZrAl BMGs.

Fig. 2 DSC curves of as-cast Cu₄₅Zr₄₈Al₇ and Cu₄₆Zr₄₂Al₇Y₅ rods at heating rate of 20 K/min

Figure 3 illustrates the room temperature compressive engineering stress—strain curves of the as-cast Cu₄₅Zr₄₈Al₇ and Cu₄₆Zr₄₂Al₇Y₅ rods with dimensions of d3 mm×6 mm. As can be seen, both of the two samples exhibit brittle fracture without any macroscopic plastic deformation. It is seen that the ductility of CuZrAl BMGs is not improved by the addition of Y. The compressive fracture strength of Cu₄₅Zr₄₈Al₇ and Cu₄₆Zr₄₈Al₇Y₅ samples is measured to be about 1892 and 1465 MPa, respectively. Obviously, the strength of the CuZrAl bulk metallic glass decreases with the addition of Y. In previous work, a drop in ultimate fracture strength induced by the addition of Y was also found in a Zr-based bulk metallic glass. As known, the ultimate fracture strength of BMGs is related to the binding energy of constituent elements [23]. For BMGs, the binding energy is proportional to the glass transition temperature [24]. That is, the higher T_g is, the higher the binding energy is. As shown in Fig. 2, the value of T_g decreases greatly with the addition of Y. Thus, the decrease in the fracture strength of Cu₄₆Zr₄₂Al₇Y₅ BMG can be ascribed to the decrease of the binding energy induced by Y addition.

Fig. 3 Engineering stress—strain curves of Cu₄₅Zr₄₈Al₇ and Cu₄₆Zr₄₂Al₇Y₅ bulk metallic glasses under room temperature compression

Figure 4 shows the compressive fracture surface of Cu₄₅Zr₄₈Al₇ and Cu₄₆Zr₄₂Al₇Y₅ BMGs. It can be seen that the fracture surface is dominated by typical vein-like patterns, which indicates that local softening takes place. In addition, some intermittent smooth regions are also observed on the surface. However, as shown in Fig. 4(b), the fracture surface of Cu₄₆Zr₄₂Al₇Y₅ BMG is characterized by smooth featureless regions with larger spaced striations on the surface, which form during the fast crack propagation. Generally, the fracture surface of BMGs contains vein-like patterns, river-like patterns and smooth regions. The vein-like patterns are indicative of the ductile fracture in BMGs [25,26]. Based on the changes in the fracture morphology induced by Y, it can be concluded that the Cu₄₆Zr₄₂Al₇Y₅ BMG is more brittle than Cu₄₅Zr₄₈Al₇ BMG, which is also supported by the fact that the samples of Cu₄₆Zr₄₂Al₇Y₅ BMG shatter into dozens of pieces when compressed.

To understand the different brittle fracture behavior of Cu₄₅Zr₄₈Al₇ and Cu₄₆Zr₄₂Al₇Y₅ BMGs, TEM observation was carried out to reveal the difference in their microstructures. Figure 5(a) shows the HRTEM image of the Cu₄₅Zr₄₈Al₇ BMG exhibiting the microstructure of nanometer-sized crystalline phase embedded in the amorphous matrix. Moreover, the selected-area diffraction electron pattern (inset in     Fig. 5(a)) presents strong diffraction spots on the background of halo rings, indicating a composite microstructure of nanocrystalline phases dispersed in amorphous matrix. However, the TEM image of Cu₄₆Zr₄₂Al₇Y₅ BMG demonstrates a uniform amorphous structure within the resolution of the TEM adopted, as shown in Fig. 5(b). The corresponding selected-area electron diffraction inserted in Fig. 5(b) presents only halo diffraction intensity, which further supports the fully amorphous structure of Cu₄₆Zr₄₂Al₇Y₅ BMG. From the differences in microstructure of Cu₄₅Zr₄₈Al₇ and Cu₄₆Zr₄₂Al₇Y₅ BMGs, it can be concluded that the formation of nanometer-sized clusters in CuZrAl BMGs is hindered by the addition of Y, which leads to the improvement of GFA and the formation of fully amorphous structure. The more brittle nature of Cu₄₆Zr₄₂Al₇Y₅ BMGs may be related to the fully amorphous structure.

Fig. 4 SEM images of compressive fracture surface of Cu₄₅Zr₄₈Al₇ (a) and Cu₄₆Zr₄₂Al₇Y₅ (b) BMGs

Fig. 5 HRTEM images of Cu₄₅Zr₄₈Al₇ (a) and Cu₄₆Zr₄₂Al₇Y₅ (b) BMGs and corresponding selected-area electron diffraction patterns

4 Conclusions

1) Cu₄₅Zr₄₈Al₇ and Cu₄₆Zr₄₂Al₇Y₅ bulk metallic glasses are prepared by copper mold melting method. Compared to Cu₄₅Zr₄₈Al₇ bulk metallic glass, the T_g of Cu₄₆Zr₄₂Al₇Y₅ BMGs decreases and the supercooled liquid region widens, indicating the improvement of GFA due to Y addition.

2) The fracture strength decreases after Y addition due to the reduction in binding energy. The fracture surface is dominated by vein-like patterns in Cu₄₅Zr₄₈Al₇ BMG, however, it changes to smooth regions in Cu₄₆Zr₄₂Al₇Y₅ BMGs, indicating Cu₄₆Zr₄₂Al₇Y₅ BMG is more brittle than Y-free Cu₄₅Zr₄₈Al₇ BMG.

3) The TEM results show that Cu₄₅Zr₄₈Al₇ BMG has a composite microstructure of nanocrystalline phases dispersed in amorphous matrix. However, the Cu₄₆Zr₄₂Al₇Y₅ BMG exhibits a fully amorphous structure. It is suggested that the improvement in GFA and decrease of mechanical properties induced by Y addition in CuZrAl-based BMGs should be carefully balanced.

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Y元素添加对CuZrAl块体金属玻璃的结构和力学性能的影响

许宏伟^{1, 2}，杜宇雷^{1, 2}，邓  昱³

1. 南京理工大学 材料评价与设计教育部工程研究中心，南京 210094；

2. 江苏省(丹阳)高性能合金材料研究院，丹阳 212300；

3. 南京大学 固态微结构国家重点实验室，现代分析中心，南京 210093

摘  要：研究添加Y元素对CuZrAl块体金属玻璃的结构和力学性能的影响。结果表明，添加Y元素提高CuZrAl体系的玻璃形成能力，而且由于添加Y元素可以降低该体系的结合能，从而降低其断裂强度。Cu₄₅Zr₄₈Al₇块体金属玻璃的断裂表面主要呈脉状，而Cu₄₆Zr₄₂Al₇Y₅块体金属玻璃的断裂表面则很平滑。TEM观察表明，Cu₄₅Zr₄₈Al₇的微观结构为非晶基体中含有纳米相，然而Cu₄₆Zr₄₂Al₇Y₅块体金属玻璃为全非晶结构。

关键词：块体金属玻璃；Cu基合金；Y 添加

 (Edited by FANG Jing-hua)

Foundation item: Project (2010ZDJH10) supported by the NUST Research Funding; Project (BK2007213) supported by the Natural Science Foundation of Jiangsu Province, China

Corresponding author: DU Yu-lei; Tel: +86-25-84315159; Fax: +86-25-84315159; E-mail: yldu_njust@mail.njust.edu.cn

DOI: 10.1016/S1003-6326(11)61254-5