SCIENCE CHINA Physics, Mechanics & Astronomy • Letter •

August 2013 Vol.56 No.8: 1606–1610 doi: 10.1007/s11433-013-5155-9

Structural and thermal sensitivity of Cu-Zr-Ti amorphous alloys to tension CAI AnHui1,2*, XIONG Xiang2, LIU Yong2, AN WeiKe1, ZHOU GuoJun1, LUO Yun1, LI TieLin1 & LI XiaoSong1 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 Received November 26, 2012; accepted January 30, 2013; published online June 25, 2013

Structural and thermal sensitivity of Cu(60–x)Zr(30+x)Ti10 (x=0, 5, and 10 at%) amorphous alloys to the application of tension was investigated. The structural sensitivity to tension decreases with increasing Cu content. The crystallization enthalpy increases with increasing excess free volume. The characteristic temperatures of the tensile samples can surpass those of the as-cast ones under a critical heating rate which differs in the Cu content. The increase of the excess free volume significantly influences the glass transition and crystallization procedures. Cu-based amorphous alloy, tension, sensitivity PACS number(s): 61.43.Dq, 81.05.Kf, 61.66.Dk, 65.60.+a Citation:

Cai A H, Xiong X, Liu Y, et al. Structural and thermal sensitivity of Cu-Zr-Ti amorphous alloys to tension. Sci China-Phys Mech Astron, 2013, 56: 16061610, doi: 10.1007/s11433-013-5155-9

Cu-based amorphous alloys has been a focus of research interest because of their special properties, such as roomtemperature ductility [1–6], phase separation [7–14] and work-hardening [1,15]. In addition, they can be cast into 15 cm rod by copper mould [5] and possess better catalytic effects on alcohol [16]. These characteristics have potential engineering applications. In order to obtain required resulting products and/or improve the performance of the parts, the materials need to be processed by machining methods, such as shot-peening, rolling, forging, and high pressure torsion. Phase separation occurrs in Cu60Zr20Ti10 amorphous alloy rolled at cryogenic temperature [11] and in Cu60Zr30Ti10 amorphous alloy treated by high torsion pressure [14]. Pauly et al. [15] found that Cu-Zr-based amorphous alloys showed macroscopically detectable plastic strain and work hardening by tensile deformation. Song et al. [17] found that there was a signifi*Corresponding author (email: [email protected]) © Science China Press and Springer-Verlag Berlin Heidelberg 2013

cant tensile ductility by cold rolling Cu47.5Zr47.5Al5 bulk amorphous alloy. These treatments usually lead to changes in the inner and/or surface structures, resulting in altered thermal and chemical stability of these parts. Hóbor et al. [14] investigated the influence of the severe deformation on the thermal stability of, and found that the Tg and Tp of Cu60Zr30Ti10 amorphous alloy increased with increasing deformation. However, the crystallization enthalpy Hx changed with the deformation degree. Song et al. [17] found that the Tg and Tx of Cu47.5Zr47.5Al5 bulk amorphous alloy decreased slightly upon cold rolling. Park et al. [18] found that the Hx of Cu46Zr47Al7 amorphous alloy was not clearly affected by severe cold rolling. It can be observed from above results that there is different thermal behavior of the treatment on different alloy composition in Cu-based amorphous alloys. Thus it is important to determine the reason and mechanism of differing thermal behaviors. The aim of the present work is to investigate the structural and thermal sensitivity of Cu-based amorphous alloy phys.scichina.com

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to the tension based on Cu(60–x)Zr(30+x)Ti10 (x=0, 5, 10 at%) ternary amorphous alloys. Cu-Zr-Ti ternary alloy ingots were prepared from the mixture of pure metals by arc melting in an argon atmosphere. Ribbons with a thickness of 50 m were prepared by melt spinning at a wheel speed of 30 ms–1. Tensile test was performed on the ribbons along spinning direction by using an Instron 8802 test machine at primary strain rate of 5×10–2 s–1 until the samples were fractured. The glassy structures of the as-cast and tensile samples were confirmed by X-ray diffraction (XRD) using Cu K radiation, transmission electron microscope (TEM), and selected area electron diffraction (SAED), respectively. The thermal properties of the as-cast and tensile samples were investigated by differential scanning calorimetry (DSC) at scanning rate of 5, 10, 20, 40 and 80 K min1, respectively. It should be noted that the DSC samples of three kinds of Cu-based amorphous alloys were acquired near the fracture part. Five DSC samples for every Cu-based amorphous alloy were divided along with the tensile direction in order to almost maintain the same state for the five samples. Surface morphologies of the samples were observed by scanning electron microscope (SEM). Figures 1(a)–(c) show the morphologies of the tensile ribbons. The vein-like patterns can be observed on the fracture surfaces, indicating the brittleness of these alloys. The magnitude of the shear bands decreases with increasing Cu content. As shown in Figure 1(d), the ribbons exhibit a pronounced first broad diffuse diffraction followed by a second smaller one, indicating amorphous structures. The first broad diffraction peak shifts to a higher-degree with increasing Cu content, indicating that the atomic position and distribution differ from one another. However, the intensity and the full half width of the first broad diffraction peak are larger for the tensile ribbon than for the as-cast ribbon for all studied alloys. The intensity of the first broad diffraction peak increases by 20.6% for Cu60Zr30Ti10, 26.8% for Cu55Zr35Ti10, and 37.6% for Cu50Zr40Ti10, respectively. The full half width of the first broad diffraction peak increases by 1.9% for Cu60Zr30Ti10, 2.4% for Cu55Zr35Ti10, and 3.6% for Cu50Zr40Ti10, respectively. The position of the first broad diffraction peak of Cu50Zr40Ti10 ribbon clearly shifts to a lower 2θ after the application of tension. These results show that the degree of the change of inner and surface structure decreases with increasing Cu content after the application of tension. It indicates that the sensitivity of the structure of the studied Cu-based alloys on tension decreases with increasing Cu content, which would be a reason for different structure change of Cu-based amorphous alloys with different composition during the deformation [11,14, 19]. For example, the phase separation and/or the crystallization appears in Cu60Zr20Ti10 [11] and Cu60Zr30Ti10 [14], while not in Cu47.5Zr47.5Al5 [19]. It also indicates that the atomic mobility decreases with increasing Cu content, resulting in the increase of the deforming (Figures 1(a)–(c))

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and casting [20] capacity that might be a possible reason for the decrease of the width of the ribbons with increasing Cu content (not shown here) in the most ideal GFA of Cu60Zr30Ti10 alloy among Cu-Zr-Ti alloys [21]. Moreover, it can be seen from the SAED patterns (Figures 1(e) and (f)) that no nanocrystallization or nanometer-sized phase separation can be detected within the experimental uncertainty, further indicating the amorphous structure of these as-cast and tensile ribbons. Figure 2 shows the DSC scans for the as-cast and tensile ribbons. It is found that the characteristic temperatures, such as Tg, Tx, and Tp, all increase with increasing heating rates. As shown in Figures 2(a)–(c), the Tgs are larger for the as-cast ribbons than for the tensile ones in studied heating rates. However, the Tx and the Tp for the tensile Cu50Zr40Ti10 and Cu60Zr30Ti10 ribbons can overpass those for the corresponding as-cast ones when the heating rates exceed ~40 K/min for Cu50Zr40Ti10 and ~80 K min1 for Cu60Zr30Ti10, respectively. As shown in Figure 2(d), the characteristic temperatures increase with increasing Cu content for the as-cast ribbons. However, the characteristic temperatures of Cu50Zr40Ti10 surpass those of Cu55Zr35Ti10 for the tensile ribbons when the heating rate reaches up to 80 K min1, as shown in Figure 2(e). These results indicate that the sensitivity of the characteristic temperatures of Cu50Zr40Ti10 is the most ideal among the studied Cu-based amorphous alloys. As shown in Figure 3(a), the relaxation enthalpy Hr increases with increasing Cu content for the as-cast ribbons, while decreases for the tensile ones. However, the Hr increases after the ribbons are subjected to tension, namely by 66.5% for Cu50Zr40Ti10, 25.7% for Cu55Zr35Ti10, and 20.9% for Cu60Zr30Ti10, respectively. It has been demonstrated that the relaxation enthalpy is proportional to the amount of excess free volume frozen within the amorphous structure of the glass [22]. Thus the magnitude of the excess free volume of the studied Cu-based amorphous alloys increases with increasing Cu content for the as-cast ribbons. As mentioned above, the cast making ability of the studied Cubased amorphous alloys decreases with increasing Cu content, that is, the flow rate decreases with increasing Cu content. This indicates that the similarity of the structure of the studied Cu-based amorphous alloys to that of the corresponding liquid increases with increasing Cu content, resulting in that the frozen free volume of the as-cast samples increases with increasing Cu content. In addition, the amount of excess free volume is required more for the tensile samples than for the as-cast samples. The increased percent of the excess free volume decreases with increasing Cu content. This indicates that the sensitivity of the change of the excess free volume on the tension decreases with increasing Cu content. The change tendency of the Hx is similar to that of the Hr, as shown in Figure 3(b). The H xs of the studied amorphous alloys increase by 6.7% for Cu50Zr40Ti10, 3.9% for Cu55Zr35Ti10, and 0.8% for Cu60Zr30-

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Figure 1 SEM morphologies of the tensile ribbons: Cu50Zr40Ti10 (a), Cu55Zr35Ti10 (b), Cu60Zr30Ti10 (c), XRD patterns (d), and SAED patterns for the as-cast ribbon (e) and for the tensile ribbon (f), respectively.

Figure 2

DSC curves of Cu50Zr40Ti10 (a), Cu55Zr35Ti10 (b), Cu60Zr30Ti10 (c), three as-cast ribbons (d), and three tensile ribbons (e), respectively.

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Ti10 after the tension, respectively. It indicates that the sensitivity of the Hx on the tension decreases with increasing Cu content. Moreover, it is seen from Figure 3(c) and (d) that the Hx increases with increasing Hr, indicating that the excess free volume remains in the sample has undergone a supercooled liquid phase thereby significantly affecting the Hx of the amorphous alloy because the annihilation of the excess free volume is exothermal. Thus these results indicate that the sensitivity of the amorphous alloy to the tension differs from the composition and the heating rate, which may be the reason for the different thermal behavior of Cu-based amorphous alloys to deformation [14,17,18]. Conversely, the activation energies for the Tg, Tx, and Tp (noted as Eg, Ex, and Ep, respectively) are calculated by Kissinger equation [23] and are presented in Figure 4(a). The activation energies all decrease after the application of tension. The Egs of the as-cast and tensile ribbons increase with increasing Cu content. However, Ep and Ex for the as-cast and tensile Cu55Zr35Ti10 alloys are the smallest among the studied amorphous alloys. As shown in Figure 4(b), the decreased percent of the Eg decreases with increasing Cu content. The increasing sequence of the decreased percent of the Ex is Cu60Zr30Ti10, Cu50Zr40Ti10, and Cu55Zr35Ti10, respectively. This would be the likely reason for the phase separation of Cu60Zr30Ti10 amorphous alloy during deformation [14]. The increasing sequence of the decreased percent for the Ep is Cu50Zr40Ti10, Cu60Zr30Ti10, and Cu55Zr35Ti10, respectively. The sensitivity of the activation

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energies on the application of tension is identical with that of the decreased percent of the activation energies. In addition, it can be seen from Figure 4(c) that the Eg for the as-cast ribbon increases with increasing Hr, while inversely increasing for the tensile samples. Nevertheless, the decreased percent of the Eg increases with the increase of the increased percent of the Hr, as shown in Figure 4(d). It is known that glass transition is a relaxation procedure. With the addition of free volume, the decrease of Eg results. Thus it indicates that the sensitivity of Eg depends on not the magnitude of the excess free volume but the increased percent of the excess free volume. It can also be deduced from Figure 4(d) that the sensitivity of Ex and Ep depends on other factors besides the increased percent of excess free volume. Thermal and structural sensitivity of Cu(60–x)Zr(30+x)Ti10 (x=0, 5, and 10 at%) amorphous alloys to the application of tension was investigated. The structural sensitivity of studied Cu-based amorphous alloys decreases with increasing Cu content. With the increase of the excess free volume, the more increase in crystallization enthalpy. The characteristic temperatures of the tensile samples can surpass those of the as-cast ones under a critical heating rate which differs from the Cu content. The characteristic temperatures and the activation energies decrease after the application of tension. The sensitivity of Eg depends on not the magnitude of the excess free volume but the increased percent of the excess free volume. The sensitivity of Ex and Ep depends on not

Figure 3 (Color online) Hr (a) and Hx (b) for the as-cast and tensile ribbons, relationships between Hr and Hx (c), and relationship between the increased percent of Hr and the increased percent of Hx (d), respectively.

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Figure 4 (Color online) Activation energies of the as-cast and tensile Cu-based amorphous alloys (a), the decreased percentage of the activation energies of Cu-based amorphous alloys after the tension (b), relationships between Hr and Eg (c), and relationships between decreased percentage of the activation energies and increased percent of Hr (d), respectively.

only the increased percent of the relaxation enthalpy but also other factors. This work was supported by the National Natural Science Foundation (Grant No. 50874045), and the Scientific Research Fund of the Hunan Provincial Education Department (Grant No. 10A044). The authors would like to thank Prof. Wang W H and his co-workers for their help.

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