Trans. Nonferrous Met. Soc. China 26(2016) 2617-2632

Effects of aspect ratio and loading rate on room-temperature mechanical properties of Cu-based bulk metallic glasses

An-hui CAI^1,2,3, Yong LIU², Hong WU², Da-wei DING³, Wei-ke AN¹, Guo-jun ZHOU¹, Yun LUO¹, Yong-yi PENG⁴, Xiao-song LI¹

1. College of Mechanical Engineering, Hunan Institute of Science and Technology, Yueyang 414000, China;

2. State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China;

3. Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China;

4. School of Physics and Electronics, Central South University, Changsha 410083, China

Received 16 October 2015; accepted 2 May 2016

Abstract: Room-temperature mechanical properties of Cu₅₀Zr₄₀Ti_10-xNi_x (0≤x≤4, mole fraction, %) bulk metallic glasses (BMG) with aspect ratios in the range of 1:1-2.5:1 and loading rates in the range of 1×10^-5-1×10^-2 s^-1 were systematically investigated by room-temperature uniaxial compression test. In the condition of an aspect ratio of 1:1, the superplasticity can be clearly observed for Cu₅₀Zr₄₀Ti₁₀ BMG when the loading rate is 1×10^-4 s^-1, while for Cu₅₀Zr₄₀Ti_10-xNi_x (x=1-3, mole fraction, %) BMGs when the loading rate is 1×10^-2 s^-1. The plastic strain (ε_p), yielding strength (σ_y) and fracture strength (σ_f) of the studied Cu-based BMGs significantly depend on the aspect ratio and the loading rate. In addition, the σ_y of the studied Cu-based BMGs with an aspect ratio of 1:1 is close to the σ_f of those with the other aspect ratios when the loading rate is 1×10^-2 s^-1. The mechanism for the mechanical response to the loading rate and the aspect ratio was also discussed.

Key words: Cu-based bulk metallic glasses; aspect ratio; loading rate; plasticity; strength

1 Introduction

Cu-Zr-Ti ternary alloys are one of Cu-Zr-based glass forming alloys. Critical diameter (d_c) and plastic strain (ε_p) of Cu-Zr-Ti BMGs can reach up to 5 mm [1] and 7.4% [2], respectively. Recently, CAI et al [3-8] have found that structural, thermal and corrosive performances of Cu_60-xZr_30+xTi₁₀ (x=0, 5, 10, mole fraction, %) metallic glasses can be significantly changed after the tension/compression. Cu₅₀Zr₄₀Ti₁₀ metallic glass characterizes in good deformability [6,7] and low hardness [8] among Cu_60-xZr_30+xTi₁₀ (x=0, 5, 10, mole fraction, %) metallic glasses, but its critical dimension and plastic strain are only 2 mm and 1.5% [9], respectively. Interestingly, the glass forming ability, mechanical, electrical and thermal properties can be simultaneously improved for Cu-Zr-Ti glass forming alloys by Ni addition [10-12]. For example, WU et al [11] fabricated a monolithic Cu_54.5Zr₃₇Ti₈Ni_0.5 BMG whose plastic strain and fracture strength can reach up to 26% and 2471 MPa, respectively.

It is well-known that the mechanical properties of the BMG are related with two kinds of factors. One is intrinsic factors such as the composition and/or microstructure of the BMG [10-17]. For example, WU et al [12] designed a Cu₅₁Zr₃₇Ti₈Ni₄ BMG which displays remarkable plasticity of 10.5% together with the fracture strength of 2145 MPa through the compositional regulation. LIU et al [15] designed three Zr-based BMGs with room-temperature compressive superplasticity due to the structural heterogeneity. The other is external factors, including the size [18-20], aspect ratio H/D (H and D are the height and the diameter of samples, respectively) [21-23], loading/strain rate [24-33], geometry [34,35], stress/strain state [36,37], and other factors [38-40]. The aspect ratio and the loading/strain rate are two important factors significantly influencing the mechanical properties of the BMG. ZHANG et al [21] and JIANG et al [22] found that the plastic strain increased with decreasing aspect ratio and the yield strength almost maintained a constant value. However, BRUCK et al [23] investigated the effect of two aspect ratios (1:2 and 2:1) on the compressive properties and found a slight increase in the yield strength with decreasing aspect ratio. In addition, it was found that the compressive strength decreased with increasing strain rate for Zr₃₈Ti₁₇Cu_10.5Co₁₂Be_22.5 BMG [28], Zr₅₇Ti₅Cu₂₀Ni₈Al₁₀ BMG [29], and Pd₄₀Ni₄₀P₂₀ BMG [31], respectively. The fracture strength was independent of the strain rate for Zr_41.25Ti_13.75Cu_12.75Ni₁₀- Be_22.5 BMG in compression [32] and Pd₄₀Ni₄₀P₂₀ BMG in tension [30], respectively. However, the compressive strength was found to increase with increasing strain rate for Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈ BMG [20] and Nd₆₀Fe₂₀Co₁₀Al₁₀ BMG [27], respectively. In addition, the dependence of the plastic and/or fracture strain of the BMG on the loading rate and the aspect ratio is similar to that of the mechanical properties. For example, the plastic and fracture strain were found to decrease with increasing strain rate for Zr₃₈Ti₁₇Cu_10.5Co₁₂Be_22.5 BMG [28], Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈ BMG [20], Ti₄₅Zr₁₆Ni₉Cu₁₀Be₂₀ BMG [26], and Nd₆₀Fe₂₀Co₁₀Al₁₀ BMG [27] in compression, and increase with increasing strain rate for Zr_41.25Ti_13.75Cu_12.75Ni₁₀Be_22.5 BMG in tension [33], while they were independent of the strain rate for Pd₄₀Ni₄₀P₂₀ BMG in tension [31] and Zr_41.25Ti_13.75Cu_12.75Ni₁₀Be_22.5 BMG in compression [32], respectively. Interestingly, ZHANG et al [26] found little effect of the mechanical properties on the strain rate when the strain rate was below 1×10^-3 s^-1 and a positive strain rate dependence of yield strength when the strain rate was up to 1×10^-1 s^-1 for Ti₄₅Zr₁₆Ni₉Cu₁₀Be₂₀ BMG. In addition, both the strength and plasticity increased with increasing the strain rate up to a critical value, above which the strength and plasticity started to decrease for Zr₅₆Al_10.9Ni_4.6Cu_27.8-Nb_0.7 BMG [24] and SrCaYbMg(Li)Zn(Cu) BMGs [25], respectively. Nevertheless, no reports for these problems can be found for Cu-based BMGs.

In the present work, the effects of the aspect ratio and the loading rate on the mechanical properties of Cu₅₀Zr₄₀Ti_10-xNi_x (0≤x≤4, mole fraction, %) BMGs were investigated by room-temperature compressive tests. It is found that the yield strength, fracture strength and plasticity significantly depend on the aspect ratio and the loading rate for the studied Cu-based BMGs.

Table 1 Yielding strength σ_y, fracture strength σ_f, plastic strain ε_p, fracture strain ε_f, and ε_p/ε_f under different aspect ratios and loading rates for Cu₅₀Zr₄₀Ti_10-xNi_x (0≤x≤4) bulk metallic glasses

to be continued

Continued

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Continued

2 Experimental

Master ingots of Cu-Zr-Ti-(Ni) alloys with normal compositions (in mole fraction, %), as shown in Table 1, were prepared by arc melting the mixture of ultrasonically cleaned high purity Cu (99.99%), Zr (99.99%), Ti (99.99%) and Ni (99.99%) in a Ti-gettered argon atmosphere. Then, d2 mm samples were prepared by suction casting into a water-cooled Cu mold.

The glassy natures of the as-cast samples were characterized by X-ray diffraction (XRD) using an XD-3A diffractometer with Cu K_α. Room-temperature uniaxial compression tests were performed on the BMGs with aspect ratios in the range of 1:1-2.5:1 using an Instron 3369 testing machine at loading rates of 1×10^-5- 1×10^-2 s^-1, respectively. Two polished end surfaces of the samples for the compression tests were parallel to each other and vertical to the symmetry axis. It should be noted that at least three samples for all studied BMGs were examined in order to obtain the reliable results.

3 Results

The results of XRD and DSC indicate that Cu₅₀Zr₄₀Ti_10-xNi_x (0≤x≤4, mole fraction, %) alloys are all in amorphous states, as shown in Ref. [10]. Figures 1-8 present typical room-temperature uniaxial compression stress-strain curves of the studied Cu-based BMGs under differerent aspect ratios and loading rates, respectively. The corresponding mechanical properties are carefully examined and listed in Table 1.

3.1 Loading rate effect

Figure 1 presents the room temperature compression stress-strain curves of the studied Cu-based BMGs at different loading rates in condition of an aspect ratio of H/D=1:1. The superplasticity can be clearly observed for Cu₅₀Zr₄₀Ti₁₀ BMG at a loading rate of 1×10^-4s^-1 (see Fig. 1(a)) and Cu₅₀Zr₄₀Ti_10-xNi_x (1.0≤x≤3.0) BMGs at a loading rate of 1×10^-2 s^-1 (see Figs. 1(c)-(e)), while not for the other Cu-based BMGs at the studied loading rates. In addition, there is no plasticity for Cu₅₀Zr₄₀Ti₉Ni₁ BMG at a loading rate of 1×10^-5 s^-1 (see Fig. 1(c)). As shown in Table 1 and Fig. 1, there are maximum ε_p and ε_p/ε_f (ε_f is fracture strain) values for Cu₅₀Zr₄₀Ti_10-xNi_x (x=0 and 4) BMGs at a loading rate of 1×10^-4 s^-1 (see Figs. 1(a) and (f)), Cu₅₀Zr₄₀Ti_9.5Ni_0.5 BMG at a loading rate of 1×10^-3 s^-1 (see Fig. 1(b)), and Cu₅₀Zr₄₀Ti_10-xNi_x (1≤x≤3) BMGs at a loading rate of 1×10^-2 s^-1 (see Figs. 1(c)-(e)), respectively. In addition, the ε_p and ε_p/ε_f at high loading rates (≥1×10^-3 s^-1) are smaller than those for low loading rates (≤1×10^-4 s^-1) for Cu₅₀Zr₄₀Ti_10-xNi_x (x=0 and 4) BMGs. However, the ε_p and ε_p/ε_f at high loading rates (≥1×10^-4 s^-1) are larger than those at low loading rates (≤1×10^-5 s^-1) for Cu₅₀Zr₄₀Ti_9.5Ni_0.5 BMG. Interestingly, the ε_p and ε_p/ε_f increase with the increase of the loading rate for Cu₅₀Zr₄₀Ti_10-xNi_x (x=1 and 3) BMGs. As for Cu₅₀Zr₄₀Ti₈Ni₂ BMG, the ε_p and ε_p/ε_f at loading rates of 1×10^-3 and 1×10^-4 s^-1 are smaller than those at loading rates of 1×10^-2 and 1×10^-5 s^-1. On the other hand, there is a minimum σ_y for Cu₅₀Zr₄₀Ti_10-xNi_x (x=0 and 0.5) BMGs at a loading rate of 1×10^-3 s^-1, Cu₅₀Zr₄₀Ti_10-xNi_x (x=2 and 4) BMGs at a loading rate of 1×10^-4 s^-1, Cu₅₀Zr₄₀Ti₉Ni₁ BMG at a loading rate of 1×10^-4 s^-1, respectively. The σ_y of Cu₅₀Zr₄₀Ti₆Ni₄ BMG firstly increases with the decrease of the loading rate, and then decreases when the loading rate exceeds 1×10^-4 s^-1. However, the σ_y of Cu₅₀Zr₄₀Ti₇Ni₃ BMG increases with the decrease of the loading rate. There is a maximum σ_f for Cu₅₀Zr₄₀Ti_10-xNi_x (x=2 and 3) BMGs at a loading rate of 1×10^-2 s^-1, Cu₅₀Zr₄₀Ti_9.5Ni_0.5 BMG at a loading rate of 1×10^-3 s^-1, and Cu₅₀Zr₄₀Ti_10-xNi_x (x=0 and 4) BMGs at a loading rate of 1×10^-4 s^-1, respectively. As for Cu₅₀Zr₄₀Ti_10-xNi_x (x=0, 0.5 and 4) BMGs, the σ_f firstly increases with the decrease of the loading rate, and then decreases when the loading rate exceeds 1×10^-4 s^-1. However, the σ_f of Cu₅₀Zr₄₀Ti₉Ni₁ BMG increases with the decrease of the loading rate.

Fig. 1 Room temperature compression stress-strain curves of Cu₅₀Zr₄₀Ti_10-xNi_x (0≤x≤4) BMGs with aspect ratio of 1:1 at different loading rates

Figure 2 presents the room temperature compression stress-strain curves of the studied Cu-based BMGs at different loading rates in the condition of an aspect ratio of H/D=1.5:1. All Cu-based BMGs characterize in some plasticity at the studied loading rates except for Cu₅₀Zr₄₀Ti₉Ni₁ at a loading rate of 1×10^-2 s^-1. It is found from Fig. 2 and Table 1 that the ε_p and ε_p/ε_f of Cu₅₀Zr₄₀Ti₁₀ BMG at loading rates of 1×10^-2, 1×10^-4, and 1×10^-5 s^-1 are almost the same and larger than those at the other loading rates. Both ε_p and ε_p/ε_f reach up to a maximum value for Cu₅₀Zr₄₀Ti_10-xNi_x (x=1 and 3) BMGs at a loading rate of 1×10^-3 s^-1, Cu₅₀Zr₄₀Ti_9.5Ni_0.5 BMG at a loading rate of 1×10^-4 s^-1, and Cu₅₀Zr₄₀Ti_10-xNi_x (x=0 and 4) BMGs at a loading rate of 1×10^-5 s^-1, respectively. The ε_p and ε_p/ε_f of Cu₅₀Zr₄₀Ti₈Ni₂ BMG are the largest when the loading rates are 1×10^-3 and 1×10^-5 s^-1, respectively. In addition, the ε_p and ε_p/ε_f firstly decrease with the decrease of the loading rate and then increase when the loading rate exceeds 1×10^-3 s^-1 for Cu₅₀Zr₄₀Ti_10-xNi_x (x=0 and 4) BMGs. However, the ε_p and ε_p/ε_f firstly increase with the decrease of the loading rate and then decrease when the loading rate exceeds 1×10^-3 s^-1 for Cu₅₀Zr₄₀Ti₉Ni₁ BMG and 1×10^-4 s^-1 for Cu₅₀Zr₄₀Ti_9.5Ni_0.5 BMG. On the other hand, both σ_f and σ_y are the largest for Cu₅₀Zr₄₀Ti₁₀ BMG at a loading rate of 5×10^-4 s^-1, Cu₅₀Zr₄₀Ti_9.5Ni_0.5 BMG at a loading rate of 1×10^-2 s^-1, Cu₅₀Zr₄₀Ti₉Ni₁ BMG at a loading rate of 1×10^-3 s^-1, and Cu₅₀Zr₄₀Ti₇Ni₃ BMG at a loading rate of 1×10^-4 s^-1. The σ_y is the largest for Cu₅₀Zr₄₀Ti₈Ni₂ and Cu₅₀Zr₄₀Ti₆Ni₄ BMGs when the loading rates are 1×10^-4 and 1×10^-3 s^-1, respectively. The σ_f is the largest for Cu₅₀Zr₄₀Ti₈Ni₂ and Cu₅₀Zr₄₀Ti₆Ni₄ BMGs when the loading rates are 1×10^-5 and 1×10^-4 s^-1, respectively. In addition, the σ_y firstly increases with the decrease of the loading rate and then decreases when the loading rate exceeds 5×10^-4 s^-1 for Cu₅₀Zr₄₀Ti₁₀ BMG, 1×10^-4 s^-1 for Cu₅₀Zr₄₀Ti₈Ni₂ BMG, and 1×10^-3 s^-1 for Cu₅₀Zr₄₀Ti₆Ni₄ BMG, respectively. The σ_f firstly increases with the decrease of the loading rate and then decreases when the loading rate exceeds 5×10^-4 s^-1 for Cu₅₀Zr₄₀Ti₁₀ BMG, 1×10^-3 s^-1 for Cu₅₀Zr₄₀Ti₉Ni₁ BMG, and 1×10^-4 s^-1 for Cu₅₀Zr₄₀Ti₆Ni₄ BMG, respectively. However, the σ_f firstly decreases with the decrease of the loading rate and then increases when the loading rate exceeds 1×10^-3 s^-1 for Cu₅₀Zr₄₀Ti₉Ni₁ BMG.

Fig. 2 Room temperature compression stress-strain curves of Cu₅₀Zr₄₀Ti_10-xNi_x (0≤x≤4) BMGs with aspect ratio of 1.5:1 at different loading rates

Fig. 3 Room temperature compression stress-strain curves of Cu₅₀Zr₄₀Ti_10-xNi_x (0≤x≤4) BMGs with aspect ratio of 2:1 at different loading rates

Figure 3 presents the room temperature compression stress-strain curves of the studied Cu-based BMGs at different loading rates in the condition of an aspect ratio of H/D=2:1. It is found that the Cu-based BMGs all characterize in the plasticity. Both ε_p and ε_p/ε_f are the largest for Cu₅₀Zr₄₀Ti_10-xNi_x (x=0 and 3) BMGs at a loading rate of 1×10^-2 s^-1, Cu₅₀Zr₄₀Ti_10-xNi_x (x=1 and 4) BMGs at a loading rate of 1×10^-3 s^-1 and Cu₅₀Zr₄₀Ti_10-x- Ni_x (x=0.5 and 2) BMGs at a loading rate of 1×10^-4 s^-1, respectively. Both ε_p and ε_p/ε_f of Cu₅₀Zr₄₀Ti₁₀ BMG firstly decrease with the decrease of the loading rate and then increase when the loading rate exceeds 1×10^-3 s^-1. However, both ε_p and ε_p/ε_f firstly increase with the decrease of the loading rate and then decrease when the loading rate exceeds 1×10^-3 s^-1 for Cu₅₀Zr₄₀Ti₉Ni₁ BMG and 1×10^-4 s^-1 for Cu₅₀Zr₄₀Ti₈Ni₂ BMG. On the other hand, the σ_y is the largest for Cu₅₀Zr₄₀Ti_10-xNi_x (x=1 and 2), Cu₅₀Zr₄₀Ti₁₀, Cu₅₀Zr₄₀Ti_10-xNi_x (x=0.5 and 3), and Cu₅₀Zr₄₀Ti₆Ni₄ BMGs when the loading rates are 1×10^-3, 1×10^-4, 1×10^-2, and 1×10^-5 s^-1, respectively. The σ_y of Cu₅₀Zr₄₀Ti₁₀ BMG firstly increases with the decrease of the loading rate and then decreases when the loading rate exceeds 1×10^-3 s^-1. Nevertheless, the σ_y of Cu₅₀Zr₄₀Ti₆- Ni₄ BMG increases with the decrease of the loading rate. In addition, the σ_f is the largest for Cu₅₀Zr₄₀Ti_10-xNi_x (x=0 and 3) BMGs at a loading rate of 1×10^-2 s^-1, Cu₅₀Zr₄₀Ti₉Ni₁ BMG at a loading rate of 1×10^-3 s^-1, Cu₅₀Zr₄₀Ti_10-xNi_x (x=0.5 and 2) BMGs at a loading rate of 1×10^-4 s^-1, and Cu₅₀Zr₄₀Ti₆Ni₄ BMG at the loading rates of 1×10^-3 and 1×10^-5 s^-1, respectively. The σ_f of Cu₅₀Zr₄₀Ti₁₀ BMG firstly decreases with the decease of the loading rate and then increases when the loading rate exceeds 1×10^-3 s^-1.

Fig. 4 Room temperature compression stress-strain curves of Cu₅₀Zr₄₀Ti_10-xNi_x (0≤x≤4) BMGs with an aspect ratio of 2.5:1 at different loading rates

Figure 4 presents the room temperature compression stress-strain curves of the studied Cu-based BMGs at different loading rates in the condition of an aspect ratio of H/D=2.5:1. It is found that the plasticity cannot be observed for Cu₅₀Zr₄₀Ti_10-xNi_x (0≤x≤1) BMGs at a loading rate of 1×10^-2 s^-1 and Cu₅₀Zr₄₀Ti₆Ni₄ BMG at the loading rates of 1×10^-2 and 1×10^-3 s^-1. As shown in Table 1, the ε_p of Cu₅₀Zr₄₀Ti_10-xNi_x (x=0.5 and 4) BMGs and the ε_p/ε_f of Cu₅₀Zr₄₀Ti_10-xNi_x (x=1 and 4) BMGs increase with the decrease of the loading rate. The ε_p of Cu₅₀Zr₄₀Ti_10-xNi_x (x=0 and 1) BMGs and the ε_p/ε_f of Cu₅₀Zr₄₀Ti_10-xNi_x (x=0 and 0.5) BMGs firstly increase with the decrease of the loading rate and then decrease when the loading rate exceeds 1×10^-4 s^-1. The ε_p and ε_p/ε_f of Cu₅₀Zr₄₀Ti_10-xNi_x (x=2 and 3) BMGs at the loading rates of 1×10^-3 and 1×10^-5 s^-1 are almost the same and larger than those at the other loading rates. In addition, the σ_y of Cu₅₀Zr₄₀Ti_10-xNi_x (x=0, 0.5 and 4) BMGs and the σ_f of Cu₅₀Zr₄₀Ti_10-xNi_x (x=0.5, 2 and 4) BMGs increase with the decrease of the loading rate, while inversely for Cu₅₀Zr₄₀Ti₇Ni₃ BMG. The σ_f of Cu₅₀Zr₄₀Ti₁₀ BMG firstly increases with the decrease of the loading rate and then decreases when the loading rate exceeds 1×10^-4 s^-1. However, there is a complex dependence of the σ_y on the loading rate for Cu₅₀Zr₄₀Ti_10-xNi_x (x=1 and 2) BMGs. The σ_y is the largest for Cu₅₀Zr₄₀Ti₉Ni₁ and Cu₅₀Zr₄₀Ti₈Ni₂ BMGs when the loading rates are 1×10^-4 and 1×10^-5 s^-1, respectively.

Fig. 5 Room temperature compression stress-strain curves of Cu₅₀Zr₄₀Ti_10-xNi_x (0≤x≤4) BMGs with different aspect ratios at loading rate of 1×10^-2 s^-1

3.2 Aspect ratio effect

Figure 5 presents the room temperature compression stress-strain curves of the studied Cu-based BMGs at different aspect ratios under a loading rate of 1×10^-2 s^-1. It is found from Fig. 5 and Table 1 that the plasticity cannot be observed for Cu₅₀Zr₄₀Ti_10-xNi_x (x=0, 0.5 and 4) BMGs with an aspect ratio of 2.5:1 and Cu₅₀Zr₄₀Ti₇Ni₃ BMG with an aspect ratio of 1.5:1, while superplasticity is observed for Cu₅₀Zr₄₀Ti_10-xNi_x (1≤x≤3) BMGs with an aspect ratio of 1:1. The plasticity of Cu₅₀Zr₄₀Ti_10-xNi_x (0.5≤x≤4) BMGs with an aspect ratio of 1:1 is larger than that of the other aspect ratios. Both ε_p and ε_p/ε_f of Cu₅₀Zr₄₀Ti₁₀ BMG firstly increase and then decrease down to zero when the aspect ratio increases up to 2.5:1. The ε_p decreases with increasing aspect ratio for Cu₅₀Zr₄₀Ti_10-xNi_x (x=2 and 4) BMGs. In addition, both σ_y and σ_f of Cu₅₀Zr₄₀Ti_9.5Ni_0.5 BMG decrease with increasing aspect ratio. The σ_f of Cu₅₀Zr₄₀Ti₈Ni₂ BMG almost decreases with increasing aspect ratio. The σ_f of Cu₅₀Zr₄₀Ti₉Ni₁ BMG firstly decreases with increasing aspect ratio and then increases when the aspect ratio exceeds 1.5:1. However, there are complex dependences of the aspect ratio for the σ_y of Cu₅₀Zr₄₀Ti_10-xNi_x (x=0 and 1≤x≤4) BMGs and the σ_f of Cu₅₀Zr₄₀Ti_10-xNi_x (x=0, 3 and 4) BMGs. Both σ_y and σ_f of Cu₅₀Zr₄₀Ti_10-xNi_x (x=0 and 2) BMGs with the aspect ratios of 1:1 and 2:1 are larger than those with the aspect ratios of 1.5:1 and 2.5:1. The σ_y is the largest for Cu₅₀Zr₄₀Ti_10-xNi_x (x=0, 2 and 4) and Cu₅₀Zr₄₀Ti_10-xNi_x (x=1 and 3) BMGs when the aspect ratios are 1:1 and 2.5:1, respectively. The σ_f is the largest for Cu₅₀Zr₄₀Ti₁₀ and Cu₅₀Zr₄₀Ti_10-xNi_x (x=3 and 4) BMGs when the aspect ratios are 2:1 and 1:1, respectively. More interestingly, the σ_y of Cu₅₀Zr₄₀Ti_10-xNi_x (1≤x≤4) BMGs with an aspect ratio of 1:1 is close to the σ_f with the other aspect ratios.

Fig. 6 Room temperature compression stress-strain curves of Cu₅₀Zr₄₀Ti_10-xNi_x (0≤x≤4) BMGs with different aspect ratios at loading rate of 1×10^-3 s^-1

Figure 6 presents the room temperature compression stress-strain curves of the studied Cu-based BMGs at different aspect ratios under a loading rate of 1×10^-3 s^-1. As shown in Fig. 6 and Table 1, the ε_p firstly increases with increasing aspect ratio and then decreases for Cu₅₀Zr₄₀Ti₈Ni₂ BMG when the aspect ratio exceeds 1.5:1. The ε_p decreases with increasing aspect ratio for Cu₅₀Zr₄₀Ti₇Ni₃ BMG. However, there is a complex dependence of the ε_p on the aspect ratio for the other Cu-based BMGs. The ε_p is the largest for these Cu-based BMGs with an aspect ratio of 1:1. The ε_p is zero for Cu₅₀Zr₄₀Ti₆Ni₄ BMG with an aspect ratio of 2.5:1. The ε_p/ε_f firstly decreases with increasing aspect ratio and then increases for Cu₅₀Zr₄₀Ti_9.5Ni_0.5 BMG when the aspect ratio exceeds 1.5:1. The ε_p/ε_f is biggest for Cu₅₀Zr₄₀Ti_10-xNi_x (x=0.5, 3 and 4) BMGs with an aspect ratio of 1:1, Cu₅₀Zr₄₀Ti_10-xNi_x (x=0 and 1) BMGs with an aspect ratio of 2:1, and Cu₅₀Zr₄₀Ti₈Ni₂ BMG with an aspect ratio of 2.5:1, respectively. In addition, the σ_y decreases with increasing aspect ratio for Cu₅₀Zr₄₀Ti_10-xNi_x (x=0.5 and 2) BMGs. As for Cu₅₀Zr₄₀Ti_10-xNi_x (x=0 and 4) BMGs, the σ_y firstly increases with increasing aspect ratio and then decreases when the aspect ratio exceeds 1.5:1. However, the σ_y firstly decreases with increasing aspect ratio and then increases when the aspect ratio exceeds 2:1. The σ_f decreases with increasing aspect ratio for Cu₅₀Zr₄₀Ti_10-x- Ni_x (x=0.5, 2 and 3) BMGs. The σ_f of Cu₅₀Zr₄₀Ti₉Ni₁ BMG firstly increases with increasing aspect ratio and then decreases when the aspect ratio exceeds 2:1. The σ_f of Cu₅₀Zr₄₀Ti₁₀ BMG with the aspect ratio of 1:1-2:1 is almost the same and larger than that with an aspect ratio of 2.5:1. There is a complex dependence of the σ_y for Cu₅₀Zr₄₀Ti₇Ni₃ BMG and the σ_f for Cu₅₀Zr₄₀Ti₆Ni₄ BMG on the aspect ratio, but the σ_y is the largest for Cu₅₀Zr₄₀Ti₇Ni₃ BMG and σ_f is the largest for Cu₅₀Zr₄₀Ti₆Ni₄ BMG when the aspect ratio is 1:1. More interestingly, the σ_f of the studied Cu-based BMGs in the range of the aspect ratio of 1.5:1-2.5:1 is close to the σ_y with an aspect ratio of 1:1 except for Cu₅₀Zr₄₀Ti₉Ni₁ BMG.

Figure 7 presents the room temperature compression stress-strain curves of the studied Cu-based BMGs at different aspect ratios under a loading rate of 1×10^-4 s^-1. As shown in Fig. 7 and Table 1, Cu₅₀Zr₄₀Ti₁₀ BMG with an aspect ratio of 1:1 characterizes in superplasticity. One can also observe some plastic strain for Cu₅₀Zr₄₀Ti₁₀ BMG with the aspect ratios of 1.5:1-2.5:1 and Cu₅₀Zr₄₀Ti_10-xNi_x (0≤x≤4) BMGs with the aspect ratios of 1:1-2.5:1. The ε_p decreases with increasing aspect ratio for Cu₅₀Zr₄₀Ti₁₀ BMG. The ε_p firstly increases with increasing aspect ratio and then decreases when the aspect ratio exceeds 2:1 for Cu₅₀Zr₄₀Ti₉Ni₁ BMG, while inversely for Cu₅₀Zr₄₀Ti₇Ni₃ BMG. As for Cu₅₀Zr₄₀Ti_10-xNi_x (x=0 and 4) BMGs, the ε_p at the aspect ratios of 1.5:1-2.5:1 is almost same and smaller than that at the aspect ratio of 1:1. The ε_p/ε_f firstly increases with increasing aspect ratio and then decreases when the aspect ratio exceeds 2:1 for Cu₅₀Zr₄₀Ti_10-xNi_x (x=1 and 2) BMGs, while inversely for Cu₅₀Zr₄₀Ti_10-xNi_x (x=3 and 4) BMGs. The ε_p is the largest for Cu₅₀Zr₄₀Ti₈Ni₂ BMG with an aspect ratio of 2:1. The ε_p/ε_f is the largest for Cu₅₀Zr₄₀Ti_9.5Ni_0.5 BMG with an aspect ratio of 2:1 and Cu₅₀Zr₄₀Ti₁₀ BMG with an aspect ratio of 1:1. Both σ_y and σ_f decrease with increasing aspect ratio for Cu₅₀Zr₄₀Ti_10-xNi_x (x=2-4) BMGs. The σ_y of Cu₅₀Zr₄₀Ti_10-xNi_x (x=0 and 1) BMGs and the σ_f of Cu₅₀Zr₄₀Ti₁₀ BMG firstly decrease with increasing aspect ratio and then increase when the aspect ratio exceeds 2:1. The σ_y of Cu₅₀Zr₄₀Ti_9.5Ni_0.5 BMG decreases with increasing aspect ratio, while inversely for the σ_f of Cu₅₀Zr₄₀Ti₉Ni₁ BMG. More interestingly, the σ_f of the studied Cu-based BMGs with the aspect ratios of 1.5:1-2.5:1 is close to the σ_y of that with the aspect ratio of 1:1 except for Cu₅₀Zr₄₀Ti₉Ni₁ BMG.

Figure 8 presents the room temperature compression stress-strain curves of the studied Cu-based BMGs at different aspect ratios at a loading rate of 1×10^-5 s^-1. As shown in Fig. 8 and Table 1, all Cu-based BMGs characterize in the plasticity except for Cu₅₀Zr₄₀Ti₉Ni₁ BMG with an aspect ratio of 1:1. The ε_p decreases with increasing aspect ratio for Cu₅₀Zr₄₀Ti₁₀ BMG. Both ε_p and ε_p/ε_f firstly decrease with increasing aspect ratio and then increase when the aspect ratio exceeds 2:1 for Cu₅₀Zr₄₀Ti_10-xNi_x (x=2 and 4) BMGs, while inversely for Cu₅₀Zr₄₀Ti₉Ni₁ BMG. There are maximum values for the ε_p and the ε_p/ε_f of Cu₅₀Zr₄₀- Ti_10-xNi_x (x=0.5 and 3) BMGs with an aspect ratio of 2.5:1. However, the ε_p/ε_f is the largest for Cu₅₀Zr₄₀Ti₁₀ BMG with an aspect ratio of 1:1. On the other hand, both σ_y and σ_f decrease with increasing aspect ratio for Cu₅₀Zr₄₀Ti₇Ni₃ BMG. The σ_y of Cu₅₀Zr₄₀Ti_10-xNi_x (x=0, 0.5 and 2) BMGs and the σ_f of Cu₅₀Zr₄₀Ti_10-xNi_x (x=0.5 and 2) BMGs firstly decrease with increasing aspect ratio and then increase when the aspect ratio exceeds 2:1, while inversely for the σ_f of Cu₅₀Zr₄₀Ti₉Ni₁ BMG. The σ_y of Cu₅₀Zr₄₀Ti₉Ni₁ BMG firstly increases with increasing aspect ratio and then decreases when the aspect ratio exceeds 1.5:1, while inversely for the σ_f of Cu₅₀Zr₄₀Ti₁₀ BMG. The dependence of the σ_y and σ_f on the aspect ratio is complex for Cu₅₀Zr₄₀Ti₆Ni₄ BMG. There is a maximum σ_y at the aspect ratio of 2:1 and σ_f at the aspect ratio of 1:1, respectively. Interestingly, the σ_y at the aspect   ratio of 1:1 is larger than the σ_f at the aspect ratios of 1.5:1-2.5:1 for Cu₅₀Zr₄₀Ti₁₀ and Cu₅₀Zr₄₀Ti₇Ni₃ BMGs.

Fig. 7 Room temperature compression stress-strain curves of Cu₅₀Zr₄₀Ti_10-xNi_x (0≤x≤4) BMGs with different aspect ratios at loading rate of 1×10^-4 s^-1

4 Discussion

As shown in Figs. 1-8 and Table 1, the strength and the plasticity of the studied Cu-based BMGs significantly depend on the aspect ratio and the loading rate. For example, the strength of Cu₅₀Zr₄₀Ti₁₀ BMG with an aspect ratio of 1.5:1 and the plasticity of Cu₅₀Zr₄₀Ti₈Ni₂ BMG with an aspect ratio of 2:1 increase with  increasing the loading rate and then decrease when the loading rate reaches up to a critical value. It was also found in Zr₅₆Al_10.9Ni_4.6Cu_27.8Nb_0.7 BMG [24] and SrCaYbMg(Li)Zn(Cu) BMGs [25], respectively. The yield strength for Cu₅₀Zr₄₀Ti₉Ni₁ BMG with an aspect ratio of 1:1 at loading rates from 1×10^-2 to 1×10^-4 s^-1 and the plasticity for Cu₅₀Zr₄₀Ti₆Ni₄ BMG with an aspect ratio of 2:1 at loading rates from 1×10^-2 to 1×10^-5 s^-1 almost maintain a constant value, which is similar to the results in Refs. [30-32]. Both strength and strain decrease with increasing the loading rate for Cu₅₀Zr₄₀Ti₁₀ BMG with an aspect ratio of 2.5:1, which is similar to the results in Refs. [20,26-29,31]. However, the yield strength for Cu₅₀Zr₄₀Ti₇Ni₃ BMG with an aspect ratio of 2.5:1 and the strain for Cu₅₀Zr₄₀Ti₉Ni₁ BMG with an aspect ratio of 1:1 increase with increasing the loading rate, which was also found in Ti-based BMG [20], Nd-based BMG [27], and Zr-based BMG [33], respectively. Interestingly, in the condition of an aspect ratio of 2:1, the yield strength for Cu₅₀Zr₄₀Ti₉Ni₁ BMG and the plastic strain for Cu₅₀Zr₄₀Ti₇Ni₃ BMG decrease with increasing the loading rate and then decrease when the aspect ratio reaches up to a critical value, which is not reported in other BMGs up to date. On the other hand, the dependence of the strength and/or the plasticity on the aspect ratio and/or the loading rate varies with the alloy composition for the studied Cu-based BMGs. For instance, when the aspect ratio is 2:1, the yield strength firstly increases with decreasing the loading rate and then decreases for Cu₅₀Zr₄₀Ti₁₀ and Cu₅₀Zr₄₀Ti_9.5Ni_0.5 BMGs, while inversely for Cu₅₀Zr₄₀Ti₉Ni₁ and Cu₅₀Zr₄₀Ti₆Ni₄ BMGs. The plastic strain firstly decreases with decreasing the loading rate and then increases for Cu₅₀Zr₄₀Ti₁₀ and Cu₅₀Zr₄₀Ti₇Ni₃ BMGs, while inversely for Cu₅₀Zr₄₀Ti₉Ni₁ and Cu₅₀Zr₄₀Ti₈Ni₂ BMGs. Moreover, the superplasticity can be observed for Cu₅₀Zr₄₀Ti₁₀ at a loading rate of 1×10^-4 s^-1 and Cu₅₀Zr₄₀Ti_10-xNi_x (x=1-3) BMGs at a loading rate of 1×10^-2 s^-1 when the aspect ratio is 1:1. Different mechanical properties of the studied Cu-based BMGs would be resulted from the following factors. Firstly, it is well-known that there are atomic- and/or nano-, even micro-scale microstructures in the BMG [15,41-44]. Compositional difference, even minor addition/substitution would vary the magnitude, type and distribution of these microstructures, which influences the relaxation, diffusion and rearrangement of atoms, resulting in the change of the properties of glass forming alloys [10,43-47]. For example, the superplasticity of Zr-based BMGs developed by LIU et al [15] resulted from the nano-scale microstructure. Different properties of the studied Cu-based BMGs developed by CAI et al [10] would be due to different category, magnitude and distribution of the atomic-scale microstructures. Secondly, the strength and the plasticity of the BMG depend on the emission/propagation rate of the shear bands during deformation. If the emission/ propagation rate of the shear bands is consistent with the applied strain rates, shear bands will nucleate and propagate continuously during deformation, resulting in an enhanced strength and plasticity. Obviously, there is a critical loading rate suitable for the emission/propagation rate of the shear bands. SONG et al [24] investigated the effect of strain rate on the compressive behavior of Zr₅₆Al_10.9Ni_4.6Cu_27.8Nb_0.7 BMG and found that both strength and plasticity increase with increasing the strain rate up to 1×10^-5 s^-1, above which the strength and plasticity start to decrease. Similar results were also found in other BMGs [25]. In addition, the metallic glasses with different compositions were found to display different mechanical response to the loading  rate [20,24-33]. Thirdly, the larger the aspect ratio under the same sample size is, the more the atomic, even nano/ micro-scale microstructures in the metallic glass are. The shear band generally nucleates at weak sites. Small aspect ratio of the BMG would result in few shear bands due to few weak sites. However, the aspect ratio would result in the confining effect of the compressive sample [21]. The smaller the aspect ratio is, the stronger the confining effect is. It would result in the formation of multiple shear bands in the condition of small aspect ratio [21]. Finally, the plastic flow can be considered as the transition of the metallic glass to the supercooled liquid under external temperature or stress [48]. The external stress would lead to the increase of the temperature [48-50] and free volume [3-7]. The temperature and free volume increase would result in the increase of the atomic mobility and the decrease of the bonding strength among the atoms. The shear band velocity increases with increasing temperature [51]. In addition, CAI et al [6,7] found that the structural and thermal sensitivity of Cu-Zr-Ti metallic glasses to pressure/tension was related to the composition of the metallic glass. Therefore, different mechanical responses of the studied Cu-based BMGs to the loading rate and aspect ratio would be a comprehensive externalization of above-mentioned factors.

Fig. 8 Room temperature compression stress-strain curves of Cu₅₀Zr₄₀Ti_10-xNi_x (0≤x≤4) BMGs with different aspect ratios at loading rate of 1×10^-5 s^-1

5 Conclusions

1) The superplasticity can be clearly observed for Cu₅₀Zr₄₀Ti₁₀ BMG with an aspect ratio of 1:1 at a loading rate of 1×10^-4 s^-1 and Cu₅₀Zr₄₀Ti_10-xNi_x (x=1-3) BMGs with an aspect ratio of 1:1 at a loading rate of 1×10^-2 s^-1, while no plasticity for Cu₅₀Zr₄₀Ti_10-xNi_x (x=0, 0.5 and 4) BMGs with an aspect ratio of 2.5:1 at a loading rate of 1×10^-2 s^-1, Cu₅₀Zr₄₀Ti₆Ni₄ BMG with an aspect ratio of 2.5:1 at a loading rate of 1×10^-3 s^-1, Cu₅₀Zr₄₀Ti₉Ni₁ BMG with an aspect ratio of 1:1 at a loading rate of 1×10^-5 s^-1, respectively.

2) There are complex relationships between the mechanical properties and the aspect ratio and the loading rate for the studied Cu-based BMGs, which depend on the content of Ni, the aspect ratio, and the loading rate. The largest yielding strength σ_y can be up to 2106.5 MPa for Cu₅₀Zr₄₀Ti₉Ni₁ BMG with an aspect ratio of 1:1 at a loading rate of 1×10^-5 s^-1.

3) The σ_y at an aspect ratio of 1:1 is close to the σ_f of the other aspect ratios for Cu₅₀Zr₄₀Ti_10-xNi_x (1≤x≤4) BMGs at a loading rate of 1×10^-2 s^-1. The σ_f at the aspect ratios of 1.5:1-2.5:1 is close to the σ_y at an aspect ratio of 1:1 for all studied Cu-based BMG except for Cu₅₀Zr₄₀Ti₉Ni₁ BMG at loading rates of 1×10^-3 and 1×10^-4 s^-1. The σ_y at an aspect ratio of 1:1 is larger than the σ_f at the aspect ratios of 1.5:1-2.5:1 of Cu₅₀Zr₄₀Ti₁₀ and Cu₅₀Zr₄₀Ti₇Ni₃ BMGs at a loading rate of 1×10^-5 s^-1.

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长径比和加载速率对铜基块体金属玻璃室温力学性能的影响

蔡安辉^1,2,3，刘 咏²，吴 宏²，丁大伟³，安伟科¹，周果君¹，罗 云¹，彭勇宜⁴，李小松¹

1. 湖南理工学院 机械工程学院，岳阳 414000；

2. 中南大学 粉末冶金国家重点实验室，长沙 410083；

3. 中国科学院 物理研究所，北京 100190；

4. 中南大学 物理与电子学院，长沙 410083

摘  要：通过单向压缩实验在试样长径比(H/D)和加载速率分别为1:1~2.5:1 和1×10^-5~1×10^-2 s^-1的条件下对Cu₅₀Zr₄₀Ti_10-xNi_x (0≤x≤ 4，摩尔分数，%) 块体金属玻璃的室温力学性能进行了系统研究。在长径比为1:1的情况下，当加载速率为1×10^-4 s^-1时，Cu₅₀Zr₄₀Ti₁₀ 块体金属玻璃表现出超塑性；而Cu₅₀Zr₄₀Ti_10-xNi_x (x=1~3，摩尔分数，%) 块体金属玻璃在加载速度为1×10^-2 s^-1 的条件下出现超塑性；塑性应变(ε_p)、屈服强度(σ_y)和断裂强度(σ_f)显著地依赖于长径比和加载速率；当加载速率为1×10^-2 s^-1时，长径比为1:1的块体金属玻璃的屈服强度几乎与其他长径比的块体金属玻璃的断裂强度接近；另外，本文作者也探讨了铜基块体金属玻璃力学性能对加载速率和长径比的响应机理。

关键词：铜基块体金属玻璃；长径比；加载速率；塑性；强度

(Edited by Wei-ping CHEN)

Foundation item: Projects (50874045, 51301194) supported by the National Natural Science Foundation of China; Project (2144057) supported by the Beijing Natural Science Foundation, China

Corresponding author: An-hui CAI; Tel: +86-730-8648848; E-mail: cah1970@sohu.com

DOI: 10.1016/S1003-6326(16)64388-1