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Synthesis and nanoindentation behaviors of binary CuTi nanoglass films
Physica B 521 (2017) 28–31
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Synthesis and nanoindentation behaviors of binary CuTi nanoglass films
MARK
⁎
Qingzhuo Hu, Jili Wu, Bo Zhang
Institute of Amorphous Matter Science, School of Materials Science and Engineering & Anhui Provincial Key Lab of Advanced Functional Materials and Devices, Hefei University of Technology, Hefei 230009, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: Nanoglass film Nanoindentation Creep Strain rate sensitivity Free volume
In this paper, we prepared the binary Cu-Ti nanoglass films by composite deposition technique. The X-ray diffraction and atomic force microscopy indicate that as-synthesized films are consisted of nanoglass with apparent interfaces. The room temperature creep behavior and strain rate sensitivity of as-synthesized nanoglass films were investigated with nanoindetation and explored that Cu58Ti42 has a better creep-resistance property at room temperature but Cu42Ti58 has a more relaxed structure. The as-synthesized nanoglass films exhibit anomalously negative strain rate sensitivity.
1. Introduction Amorphous alloys, such as bulk metallic glasses (BMGs) [1,2], nanoglassy alloys [3] and glassy thin films/nanolaminates [4,5] etc., are the interesting and attractive model materials in the physical metallurgy and condense physics areas. They possess good mechanical properties, such as high strength, high hardness, high elastic energy storage capacity and excellent plastic performance in micromechanical systems, etc. [6,7]. Undeniably, a major weakness of “bulk”-size amorphous alloys, especially for BMGs, is failure to open up macroscopically ductile/plastic deformation behavior before fracture due to highly localized shear deformation procedures [6,8], but “small”-size amorphous alloys show good ductility/plasticity [9]. To unveil the mechanism of plastic deformation (and understanding the brittle fracture) of amorphous alloys, both theoretical (modeling and simulation, etc.) [10,11] and experimental (in-situ mechanical examination and nanoindentation, etc.) [8,12,13] investigations have been implemented in the past decades, many valuable conclusions have been reached. In the aspect of nanoindentation of amorphous alloys, micro-pillar compression [13,14], indentation creep [15,16] and crack initiation [12,17] etc., have drawn fruitful models and assumptions for understanding their deformation behavior. Yang et al. reported the loaddisplacement curve was shown to be insensitive to the crack initiation but sensitive to subsequent crack propagation and demonstrated that shear transformation zone (STZ) volume of Cu-Zr-Al glassy films increases with the film thickness increasing [17]. Gu et al. disclosed that inhomogeneous plastic flow at nano-size scale can evolve in a wellcontrolled manner without further developing of shear bands, under-
⁎
Corresponding author. E-mail address: [email protected] (B. Zhang).
http://dx.doi.org/10.1016/j.physb.2017.06.053 Received 20 April 2017; Received in revised form 17 June 2017; Accepted 19 June 2017 Available online 20 June 2017 0921-4526/ © 2017 Published by Elsevier B.V.
going an elastoplastic transition, and embryonic shear localization propagates with a very slow velocity of order of similar to 1 nm/s without the formation of a hot matured shear band [12]. Chen's works found that minor Ti addition can strongly promote the plasticity of CuZr-Al BMGs [18], and Ma et al. explored that the minor Ti addition effectively induces excess free volume through nanoindentation creep measurements [19]. Obviously, nanoindentation is a very useful tool to study the mechanism of plastic deformation of amorphous alloys. Nanoglassy alloy (simply called nanoglasses) are consisted of nanometer-sized glassy regions connected by glass/glass interfaces with an amorphous structure [3,20], which are of excellent physical properties, remarkable biocompatibility and good mechanical properties. In this paper, binary Cu-Ti films were synthesized, and characterizations indicated that the as-synthesized films are of nanoglass nature. By utilizing nanoindentation, we uncovered the creep behaviors and strain rate sensitivity of as-synthesized nanoglass films and found that the nanoglass films show anomalously negative strain rate sensitivity. 2. Experiments 2.1. Preparation of samples The binary CuTi nanoglass films were prepared by electron beam evaporation of pure Ti and resistance thermal evaporation of pure Cu (purity not less than 99.9%) in a high vacuum chamber with a base pressure of about 5×10−4 Pa. The compositions of thin films were empirically controlled through altering the current of evaporator powers. The metal Cu and Ti were evaporation deposited on quartz glass substrates, and the thickness of films was controlled about
Physica B 521 (2017) 28–31
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400 nm. The thickness of films was monitored with quartz crystal oscillator (Taiyao vacuum, Shanghai FTM-V). 2.2. Characterization The structures of specimens were detected using X-ray diffraction (the X′Pert Pro MPD). Compositions of the specimens were examined by energy dispersive spectroscopy (EDS) in a field-emission scanning electron microscopy (Japan-Hitachi Su8020) (The specific compositions of as-synthesized films are shown in Section S1 in Supplemental materials. It identifies the compositions of films are close to Cu42Ti58 and Cu58Ti42, respectively.). The surface morphology was examined by atomic force microscope (AFM, Bruker Dimension). The nanoindentation experiments were conducted by a commercial depth-sensing instrument (MTS Nano Indenters XP) with a Berkovich diamond indenter at room temperature. Nanoindentation creep measurements were performed at the holding load of 5 mN and holding time is 60 s at room temperature; the loading rate and unloading rate were kept at 20 mN/s. The strain rate sensitivity was examined with 5 strain rates ranging from 0.001 to 0.1 s−1. Thermal drift for all nanoindentation measurements is controlled at 0.01 nm/s. 3. Results and discussion Fig. 1 shows the XRD patterns of the two specimens Cu42Ti58 and Cu58Ti42. Each diffraction pattern indicates a single broad hump-like peak, suggesting the as-synthesized film is of monolithic amorphous nature. Fig. 2 shows the surface morphology of as-synthesized Cu42Ti58 film. From Fig. 2a, it clearly displays that the nano-size (about 20– 40 nm) spheres form in the film. Fig. 2b indicates the corresponding depth information which strongly demonstrates the formation of interfaced nanoglasses. Correspondingly, the Cu58Ti42 thin film also shows the same morphology (Section S2 in Supplemental materials). The morphology of Cu-Ti amorphous thin film, in the current work, is similar to those of the Zr-Cu [21], Fe-Sc [22] and Ni50Ti45Cu5 [23] nanoglasses, being of the typical features of nanoglasses, such as nanosize grains and apparent interfaces. Generally, during the nanoindentation creep measurement at the maximum load process, the indenter displacement into the specimen gradually increases with the time. As shown in Fig. 3, compared with Cu42Ti58, Cu58Ti42 is indented lower referring to the smaller creep displacement, suggesting that it has a better creep-resistance property at room temperature. On the other hand, it also indicates that, in hardness, Cu58Ti42 (∼3.67 GPa) is larger than Cu42Ti58 (∼3.45 GPa) according to Oliver-Pharr method [24] (displacement-load curves are presented in Section S3 in Supplemental materials). It is worth noticing that the hardness of as-synthesized films is about 3–4 GPa that is much
Fig. 2. AFM images of as-synthesized Cu42Ti58 nanoglass film. (a) 2D image; (b) depth profile.
smaller than that of reported BMGs with as hardness of 6–10 GPa [25,26]. It is may be attributed to the accelerated diffusion [27,28], that is referred to the response to the stress, in the amorphous films. In general, the viscoelastic deformation dominates the holding stage during indentation process of amorphous alloys [29,30], which can be described by Kelvin model indicated with a series of dashpots and linear springs. For the nanoindentation creep, a generalized Kelvin model, with two exponential terms [31], was utilized to fit the indentation creep (displacement-time) curves, in the current work. It is described as n
h (t ) = he +
∑ hi (1 − e−t / τi ) + i =1
t (i = 1 and 2). μ0
(1)
where he is the indentation depth at the first spring, hi represents the indentation depth at the ith Kelvin element, μ0 is a constant related to the viscosity coefficient of the last dashpot and τi is the retardation time for the ith element. The fitting curves of creep displacement-time of Cu42Ti58 and Cu58Ti42 derived from Eq. (1) are shown in Fig. 3a, and the fitting parameters are listed in Table 1. The retardation times (τ1 and τ2) are referred to different time responses, which reflect the effects of the amorphous structure on the indentation load.
Fig. 1. XRD patterns of as-synthesized films.
29
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Fig. 4. Double logarithmic patterns of hardness vs. strain rate of as-synthesized Cu42Ti58 and Cu58Ti42 nanoglass films.
lated with the population of the corresponding void “defects”. And thus, the lower of intensity and the shorter retardation time suggest that the activation of “defects” become more difficult as structural relaxation proceeds [35]. Fig. 3b clearly shows that the relatively sharp peak and longer retardation time occur in the Cu42Ti58 specimen, and thus Cu42Ti58 has a more relaxed state than Cu58Ti42. Fig. 4 shows the double logarithmic plots of hardness versus indentation strain rate of Cu42Ti58 and Cu58Ti42 nanoglass films. The slope equals the strain rate sensitivity exponent (m) [36] that reflects the hardness (H) of material along with the change of strain rate (ξ), and usually reveals the deformation mechanism of material. It can be written as
Fig. 3. Nanoindentation displacement-time curves (a) and retardation spectra (b) of assynthesized Cu42Ti58 and Cu58Ti42 nanoglass films.
m= Table 1 fitting parameters by Eq. (1) of as-synthesized nanoglass films. Specimen
he
h1
τ1
h2
τ2
η
R2
Cu42Ti58 Cu58Ti42
282.48 215.14
17.47 6.05
1.19 0.31
30.20 14.25
13.37 7.13
1.48 5.07
0.9996 0.9991
(3)
Interestingly, both two fitting lines exhibit the anomalous slope (negative; −0.03553 and −0.06575 for Cu42Ti58 and Cu58Ti42, respectively) and are not agreed with the previous works that reported positive slope with fitting line of H vs. ξ [36–38]. In general, the indenter penetrating into the film will cause a localized temperature rise due to the application of energy of stress. At a high strain rate, there is insufficient time for the heat to be transferred to surrounding regions, and it results in a rapid, large temperature rise occurring in these deformed areas. This in turn induces a localized atomic dilation (free volume) and viscosity decrease, promoting a local softening and the hardness decreases [39]. In contrast, more time is provided for heat conduction at a low strain rate and no local softening phenomenon occurs, thus a high hardness can be reached to produce a negative m. Alternatively, the two straight lines are intersected at the point of 0.006 s−1. Specifically, when the strain rate is faster than 0.006 s−1, the hardness value of Cu42Ti58 is larger than Cu58Ti42; whereas indicate a contrary trend at less than 0.006 s−1. It may be also referred to motion of free volume, in the current work, the free volume spreading to the free surface is accompanying with indenter pressed into films. when strain rate is very small (≤ 0.006 s−1), the area where the indenter contact with films will produce more free volume aggregation and annihilation accompanying with stress energy dissipation, combining with Cu42Ti58 has a more relaxed structure, causing a weaker hardness of Cu42Ti58 film. Nevertheless, when strain rate is fast (≥ 0.006 s−1), the free volume may have no enough time spread into the interface between indenter and thin film; not only this, indentation creep is under the condition of constant force, whereas the applied stress actually decreases because the volume of materials is involved increasingly. Larger strain rate may easily initiate the intersection of free
Retardation spectrum (L(τ)) is an effective way for evaluation of structure-related properties in amorphous solid [32,33], which can be derived from the creep compliance spectrum (J(t) = h(t)·A0/(P0·hin), where A0 and P0 are the contact area and the applied load corresponding to the virtual length (hin), which equals to the depth after loading.). The retardation spectrum is described as,
L (τ ) = dJ (t )/ d ln t − d 2J (t )/ d (ln t )2 |t =2τ .
∂ ln H . ∂ ln ξ
(2)
Fig. 3b shows the retardation spectra for as-synthesized nanoglass films. Clearly, both two creep retardation spectra are consisted of two separated peaks with well-defined relaxation times, which imply two kinds of relaxation processes during indentation creep at room temperature. Alternatively, both two peaks of Cu42Ti58 show a shift toward larger relaxation time and a stronger intensity with regard to Cu58Ti42. As investigated before, metallic glasses are characterized by two types of traps where positrons are annihilated [34,35], i.e. small intrinsic voids, similar to Bernal's interstitial sites, referring to the shorter position life time, and larger voids, referring to the longer positron life time. Theoretically, the voids are generally assumed as free volume, liquid-like region or flow units, etc. On this point, the intensity and retardation time corresponding to creep behaviors can be corre30
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volume forming the collision of separated redundant “free volume”, like dislocation pile-up in deformation of crystalline metals, but these redundant free volumes were not broken their “shell”, unlike the aggregation of free volume into a “bigger” free volume that usually loosen the structure under the indenter and thus decrease the hardness [40]. Consequently, at a higher strain rate, the more relaxed Cu42Ti58 can indicate a higher hardness than Cu58Ti42.
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Mater. 63 (2010) 1205–1208. [17] Y.J. Yang, J. Luo, L.P. Huang, G.L. Hu, K.D. Vargheese, Y.F. Shi, J.C. Mauro, Crack initiation in metallic glasses under nanoindentation, Acta Mater. 115 (2016) 413–422. [18] L.Y. Chen, Z.D. Fu, G.Q. Zhang, X.P. Hao, Q.K. Jiang, X.D. Wang, Q.P. Cao, H. Franz, Y.G. Liu, H.S. Xie, S.L. Zhang, B.Y. Wang, Y.W. Zeng, J.Z. Jiang, New class of plastic bulk metallic glass, Phys. Rev. Lett. 100 (2008) 075501. [19] Y. Ma, G.J. Peng, Y.H. Feng, T.H. Zhang, Nanoindentation investigation on the creep mechanism in metallic glassy films, Mater. Sci. Eng. A-Struct. Mater. Prop. Microstruct. Process. 651 (2016) 548–555. [20] D. Danilov, H. Hahn, H. Gleiter, W. Wenzel, Mechanisms of nanoglass ultrastability, ACS Nano 10 (2016) 3241–3247. [21] I. Bataev, N.T. Panagiotopoulos, F. Charlot, A.M. Jorge, M. Pons, G.A. Evangelakis, A.R. Yavari, Structure and deformation behavior of Zr-Cu thin films deposited on Kapton substrates, Surf. Coat. Technol. 239 (2014) 171–176. [22] J.X. Fang, U. Vainio, W. Puff, R. Wurschum, X.L. Wang, D. Wang, M. Ghafari, F. Jiang, J. Sun, H. Hahn, H. Gleiter, Atomic structure and structural stability of Sc75Fe25 nanoglasses, Nano Lett. 12 (2012) 458–463. [23] Z. Sniadecki, D. Wang, Y. Ivanisenko, V.S.K. Chakravadhanula, C. Kubel, H. Hahn, H. Gleiter, Nanoscale morphology of Ni50Ti45Cu5 nanoglass, Mater. Charact. 113 (2016) 26–33. [24] W.C. Oliver, G.M. Pharr, Measurement of hardness and elastic modulus by instrumented indentation: advances in understanding and refinements to methodology, J. Mater. Res. 19 (2004) 3–20. [25] J.L. Wu, Y. Pan, J.H. Pi, Nanoindentation mechanical properties of indium-alloyed Cu-based bulk metallic glasses, J. Mater. Eng. Perform. 23 (2014) 486–492. [26] C.A. Schuh, T.G. Nieh, A survey of instrumented indentation studies on metallic glasses, J. Mater. Res. 19 (2004) 46–57. [27] C.R. Cao, Y.M. Lu, H.Y. Bai, W.H. Wang, High surface mobility and fast surface enhanced crystallization of metallic glass, Appl. Phys. Lett. 107 (2015) 141606. [28] L. Chen, C.R. Cao, J.A. Shi, Z. Lu, Y.T. Sun, P. Luo, L. Gu, H.Y. Bai, M.X. Pan, W.H. Wang, Fast surface dynamics of metallic glass enable superlattice like nanostructure growth, Phys. Rev. Lett. 118 (2017) 016101. [29] C. Wang, Q.P. Cao, X.D. Wang, D.X. Zhang, S.X. Qu, J.Z. Jiang, Stress-dependent shear transformation zone in Ni-Nb thin film metallic glass and its correlation with deformation mode transition, Scr. Mater. 122 (2016) 59–63. [30] M. Hasan, G. Kumar, High strain rate thermoplastic demolding of metallic glasses, Scr. Mater. 123 (2016) 140–143. [31] S. Yang, Y.W. Zhang, K.Y. Zeng, Analysis of nanoindentation creep for polymeric materials, J. Appl. Phys. 95 (2004) 3655–3666. [32] Y.D. Sun, Z.Q. Li, J.S. Liu, M.Q. Cong, J.Y. Qin, Indentation creep behaviors of Mg61Cu28Gd11 and (Mg61Cu28Gd11)99.5Sb0.5 bulk metallic glasses at room temperature, J. Rare Earths 29 (2011) 253–258. [33] W.H. Li, K. Shin, C.G. Lee, B.C. Wei, T.H. Zhang, Y.Z. He, The characterization of creep and time-dependent properties of bulk metallic glasses using nanoindentation, Mater. Sci. Eng. A-Struct. Mater. Prop. Microstruct. Process. 478 (2008) 371–375. [34] D. Suh, R.H. Dauskardt, P. Asoka-Kumar, P.A. Sterne, R.H. Howell, Temperature dependence of positron annihilation in a Zr-Ti-Ni-Cu-Be bulk metallic glass, J. Mater. Res. 18 (2003) 2021–2024. [35] A. Castellero, B. Moser, D.I. Uhlenhaut, F.H. Dalla Torre, J.F. Loeffler, Roomtemperature creep and structural relaxation of Mg-Cu-Y metallic glasses, Acta Mater. 56 (2008) 3777–3785. [36] R. Limbach, B.P. Rodrigues, L. Wondraczek, Strain-rate sensitivity of glasses, J. Non-Cryst. Solids 404 (2014) 124–134. [37] Y. Ma, G.J. Peng, W.F. Jiang, H. Chen, T.H. Zhang, Nanoindentation study on shear transformation zone in a Cu-Zr-Al metallic glassy film with different thickness, J. Non-Cryst. Solids 442 (2016) 67–72. [38] P.F. Yu, S.D. Feng, G.S. Xu, X.L. Guo, Y.Y. Wang, W. Zhao, L. Qi, G. Li, P.K. Liaw, R.P. Liu, Room-temperature creep resistance of Co-based metallic glasses, Scr. Mater. 90–91 (2014) 45–48. [39] Y.Q. Wang, J.Y. Zhang, X.Q. Liang, K. Wu, G. Liu, J. Sun, Size- and constituentdependent deformation mechanisms and strain rate sensitivity in nanolaminated crystalline Cu/amorphous Cu-Zr films, Acta Mater. 95 (2015) 132–144. [40] P. Li, J.Y. Hao, J.L. Tan, Q. Wang, C. Dong, L. Li, Effect of temperature on elastic property and microstructure of Cu58.1Zr35.9Al6 bulk metallic glasses, Rare Metal. Mater. Eng. 38 (2009) 1835–1838.
4. Conclusion In summary, we prepared the binary Cu-Ti nanoglass films via composite deposition technique by electron beam evaporation of pure Ti and resistance thermal evaporation of pure Cu. The XRD and AFM characterizations of as-synthesized nanoglass films clearly show that nano-sized glassy particles with apparent interfaces make up the films. The room temperature creep behaviors and strain rate sensitivity of as-synthesized nanoglass films were investigated with nanoindetation. It discloses that Cu58Ti42 has a better creep-resistance property at room temperature but Cu42Ti58 has a more relaxed structure. The assynthesized nanoglass films exhibit anomalously negative strain rate sensitivity, may be caused by temperature rise that affect the dilation of free volumes. And the fitting lines of H vs. ξ intersect at 0.006 s−1 indicating different roles of free volume during indentation with different strain rates. At higher strain rate, it may easily initiate the intersection of free volume forming the collision of separated redundant free volume. Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 51322103, 51571079 and 51601050), National Key Technologies R & D Programs (Nos. 2015CB856800 and 2016YFB0300500) and the Fundamental Research Funds for the Central Universities (Nos. JZ2016HGBZ0772 and JZ2016HGPB0671). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.physb.2017.06.053. References [1] C.J. Byrne, M. Eldrup, Materials science - bulk metallic glasses, Science 321 (2008) 502–503. [2] J.J. Kruzic, Bulk metallic glasses as structural materials: a review, Adv. Eng. Mater. 18 (2016) 1308–1331. [3] J. Jing, A. Kramer, R. Birringer, H. Gleiter, U. Gonser, Modified atomic-structure in a Pd-Fe-Si nanoglass - a Mossbauer study, J. Non-Cryst. Solids 113 (1989) 167–170. [4] G.A. Almyras, G.M. Matenoglou, P. Komninou, C. Kosmidis, P. Patsalas, G.A. Evangelakis, On the deposition mechanisms and the formation of glassy Cu-Zr thin films, J. Appl. Phys. 107 (2010) 084313. [5] W. Guo, E. Jagle, J.H. Yao, V. Maier, S. Korte-Kerzel, J.M. Schneider, D. Raabe, Intrinsic and extrinsic size effects in the deformation of amorphous CuZr/ nanocrystalline Cu nanolaminates, Acta Mater. 80 (2014) 94–106. [6] C.A. Schuh, T.C. Hufnagel, U. Ramamurty, Mechanical behavior of amorphous alloys, Acta Mater. 55 (2007) 4067–4109. [7] M.M. Trexler, N.N. Thadhani, Mechanical properties of bulk metallic glasses, Prog. Mater. Sci. 55 (2010) 759–839. [8] B.A. Sun, W.H. Wang, The fracture of bulk metallic glasses, Prog. Mater. Sci. 74 (2015) 211–307. [9] G. Kumar, A. Desai, J. Schroers, Bulk metallic glass: the smaller the better, Adv. Mater. 23 (2011) 461–476. [10] H.F. Zhou, C. Zhong, Q.P. Cao, S.X. Qu, X.D. Wang, W. Yang, J.Z. Jiang, Nonlocalized deformation in metallic alloys with amorphous structure, Acta Mater. 68 (2014) 32–41. [11] L. Tian, Y.Q. Cheng, Z.W. Shan, J. Li, C.C. Wang, X.D. Han, J. Sun, E. Ma, Approaching the ideal elastic limit of metallic glasses, Nat. Commun. 3 (2012) 609.
31
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Synthesis and nanoindentation behaviors of binary CuTi nanoglass films
MARK
⁎
Qingzhuo Hu, Jili Wu, Bo Zhang
Institute of Amorphous Matter Science, School of Materials Science and Engineering & Anhui Provincial Key Lab of Advanced Functional Materials and Devices, Hefei University of Technology, Hefei 230009, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: Nanoglass film Nanoindentation Creep Strain rate sensitivity Free volume
In this paper, we prepared the binary Cu-Ti nanoglass films by composite deposition technique. The X-ray diffraction and atomic force microscopy indicate that as-synthesized films are consisted of nanoglass with apparent interfaces. The room temperature creep behavior and strain rate sensitivity of as-synthesized nanoglass films were investigated with nanoindetation and explored that Cu58Ti42 has a better creep-resistance property at room temperature but Cu42Ti58 has a more relaxed structure. The as-synthesized nanoglass films exhibit anomalously negative strain rate sensitivity.
1. Introduction Amorphous alloys, such as bulk metallic glasses (BMGs) [1,2], nanoglassy alloys [3] and glassy thin films/nanolaminates [4,5] etc., are the interesting and attractive model materials in the physical metallurgy and condense physics areas. They possess good mechanical properties, such as high strength, high hardness, high elastic energy storage capacity and excellent plastic performance in micromechanical systems, etc. [6,7]. Undeniably, a major weakness of “bulk”-size amorphous alloys, especially for BMGs, is failure to open up macroscopically ductile/plastic deformation behavior before fracture due to highly localized shear deformation procedures [6,8], but “small”-size amorphous alloys show good ductility/plasticity [9]. To unveil the mechanism of plastic deformation (and understanding the brittle fracture) of amorphous alloys, both theoretical (modeling and simulation, etc.) [10,11] and experimental (in-situ mechanical examination and nanoindentation, etc.) [8,12,13] investigations have been implemented in the past decades, many valuable conclusions have been reached. In the aspect of nanoindentation of amorphous alloys, micro-pillar compression [13,14], indentation creep [15,16] and crack initiation [12,17] etc., have drawn fruitful models and assumptions for understanding their deformation behavior. Yang et al. reported the loaddisplacement curve was shown to be insensitive to the crack initiation but sensitive to subsequent crack propagation and demonstrated that shear transformation zone (STZ) volume of Cu-Zr-Al glassy films increases with the film thickness increasing [17]. Gu et al. disclosed that inhomogeneous plastic flow at nano-size scale can evolve in a wellcontrolled manner without further developing of shear bands, under-
⁎
Corresponding author. E-mail address: [email protected] (B. Zhang).
http://dx.doi.org/10.1016/j.physb.2017.06.053 Received 20 April 2017; Received in revised form 17 June 2017; Accepted 19 June 2017 Available online 20 June 2017 0921-4526/ © 2017 Published by Elsevier B.V.
going an elastoplastic transition, and embryonic shear localization propagates with a very slow velocity of order of similar to 1 nm/s without the formation of a hot matured shear band [12]. Chen's works found that minor Ti addition can strongly promote the plasticity of CuZr-Al BMGs [18], and Ma et al. explored that the minor Ti addition effectively induces excess free volume through nanoindentation creep measurements [19]. Obviously, nanoindentation is a very useful tool to study the mechanism of plastic deformation of amorphous alloys. Nanoglassy alloy (simply called nanoglasses) are consisted of nanometer-sized glassy regions connected by glass/glass interfaces with an amorphous structure [3,20], which are of excellent physical properties, remarkable biocompatibility and good mechanical properties. In this paper, binary Cu-Ti films were synthesized, and characterizations indicated that the as-synthesized films are of nanoglass nature. By utilizing nanoindentation, we uncovered the creep behaviors and strain rate sensitivity of as-synthesized nanoglass films and found that the nanoglass films show anomalously negative strain rate sensitivity. 2. Experiments 2.1. Preparation of samples The binary CuTi nanoglass films were prepared by electron beam evaporation of pure Ti and resistance thermal evaporation of pure Cu (purity not less than 99.9%) in a high vacuum chamber with a base pressure of about 5×10−4 Pa. The compositions of thin films were empirically controlled through altering the current of evaporator powers. The metal Cu and Ti were evaporation deposited on quartz glass substrates, and the thickness of films was controlled about
Physica B 521 (2017) 28–31
Q. Hu et al.
400 nm. The thickness of films was monitored with quartz crystal oscillator (Taiyao vacuum, Shanghai FTM-V). 2.2. Characterization The structures of specimens were detected using X-ray diffraction (the X′Pert Pro MPD). Compositions of the specimens were examined by energy dispersive spectroscopy (EDS) in a field-emission scanning electron microscopy (Japan-Hitachi Su8020) (The specific compositions of as-synthesized films are shown in Section S1 in Supplemental materials. It identifies the compositions of films are close to Cu42Ti58 and Cu58Ti42, respectively.). The surface morphology was examined by atomic force microscope (AFM, Bruker Dimension). The nanoindentation experiments were conducted by a commercial depth-sensing instrument (MTS Nano Indenters XP) with a Berkovich diamond indenter at room temperature. Nanoindentation creep measurements were performed at the holding load of 5 mN and holding time is 60 s at room temperature; the loading rate and unloading rate were kept at 20 mN/s. The strain rate sensitivity was examined with 5 strain rates ranging from 0.001 to 0.1 s−1. Thermal drift for all nanoindentation measurements is controlled at 0.01 nm/s. 3. Results and discussion Fig. 1 shows the XRD patterns of the two specimens Cu42Ti58 and Cu58Ti42. Each diffraction pattern indicates a single broad hump-like peak, suggesting the as-synthesized film is of monolithic amorphous nature. Fig. 2 shows the surface morphology of as-synthesized Cu42Ti58 film. From Fig. 2a, it clearly displays that the nano-size (about 20– 40 nm) spheres form in the film. Fig. 2b indicates the corresponding depth information which strongly demonstrates the formation of interfaced nanoglasses. Correspondingly, the Cu58Ti42 thin film also shows the same morphology (Section S2 in Supplemental materials). The morphology of Cu-Ti amorphous thin film, in the current work, is similar to those of the Zr-Cu [21], Fe-Sc [22] and Ni50Ti45Cu5 [23] nanoglasses, being of the typical features of nanoglasses, such as nanosize grains and apparent interfaces. Generally, during the nanoindentation creep measurement at the maximum load process, the indenter displacement into the specimen gradually increases with the time. As shown in Fig. 3, compared with Cu42Ti58, Cu58Ti42 is indented lower referring to the smaller creep displacement, suggesting that it has a better creep-resistance property at room temperature. On the other hand, it also indicates that, in hardness, Cu58Ti42 (∼3.67 GPa) is larger than Cu42Ti58 (∼3.45 GPa) according to Oliver-Pharr method [24] (displacement-load curves are presented in Section S3 in Supplemental materials). It is worth noticing that the hardness of as-synthesized films is about 3–4 GPa that is much
Fig. 2. AFM images of as-synthesized Cu42Ti58 nanoglass film. (a) 2D image; (b) depth profile.
smaller than that of reported BMGs with as hardness of 6–10 GPa [25,26]. It is may be attributed to the accelerated diffusion [27,28], that is referred to the response to the stress, in the amorphous films. In general, the viscoelastic deformation dominates the holding stage during indentation process of amorphous alloys [29,30], which can be described by Kelvin model indicated with a series of dashpots and linear springs. For the nanoindentation creep, a generalized Kelvin model, with two exponential terms [31], was utilized to fit the indentation creep (displacement-time) curves, in the current work. It is described as n
h (t ) = he +
∑ hi (1 − e−t / τi ) + i =1
t (i = 1 and 2). μ0
(1)
where he is the indentation depth at the first spring, hi represents the indentation depth at the ith Kelvin element, μ0 is a constant related to the viscosity coefficient of the last dashpot and τi is the retardation time for the ith element. The fitting curves of creep displacement-time of Cu42Ti58 and Cu58Ti42 derived from Eq. (1) are shown in Fig. 3a, and the fitting parameters are listed in Table 1. The retardation times (τ1 and τ2) are referred to different time responses, which reflect the effects of the amorphous structure on the indentation load.
Fig. 1. XRD patterns of as-synthesized films.
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Fig. 4. Double logarithmic patterns of hardness vs. strain rate of as-synthesized Cu42Ti58 and Cu58Ti42 nanoglass films.
lated with the population of the corresponding void “defects”. And thus, the lower of intensity and the shorter retardation time suggest that the activation of “defects” become more difficult as structural relaxation proceeds [35]. Fig. 3b clearly shows that the relatively sharp peak and longer retardation time occur in the Cu42Ti58 specimen, and thus Cu42Ti58 has a more relaxed state than Cu58Ti42. Fig. 4 shows the double logarithmic plots of hardness versus indentation strain rate of Cu42Ti58 and Cu58Ti42 nanoglass films. The slope equals the strain rate sensitivity exponent (m) [36] that reflects the hardness (H) of material along with the change of strain rate (ξ), and usually reveals the deformation mechanism of material. It can be written as
Fig. 3. Nanoindentation displacement-time curves (a) and retardation spectra (b) of assynthesized Cu42Ti58 and Cu58Ti42 nanoglass films.
m= Table 1 fitting parameters by Eq. (1) of as-synthesized nanoglass films. Specimen
he
h1
τ1
h2
τ2
η
R2
Cu42Ti58 Cu58Ti42
282.48 215.14
17.47 6.05
1.19 0.31
30.20 14.25
13.37 7.13
1.48 5.07
0.9996 0.9991
(3)
Interestingly, both two fitting lines exhibit the anomalous slope (negative; −0.03553 and −0.06575 for Cu42Ti58 and Cu58Ti42, respectively) and are not agreed with the previous works that reported positive slope with fitting line of H vs. ξ [36–38]. In general, the indenter penetrating into the film will cause a localized temperature rise due to the application of energy of stress. At a high strain rate, there is insufficient time for the heat to be transferred to surrounding regions, and it results in a rapid, large temperature rise occurring in these deformed areas. This in turn induces a localized atomic dilation (free volume) and viscosity decrease, promoting a local softening and the hardness decreases [39]. In contrast, more time is provided for heat conduction at a low strain rate and no local softening phenomenon occurs, thus a high hardness can be reached to produce a negative m. Alternatively, the two straight lines are intersected at the point of 0.006 s−1. Specifically, when the strain rate is faster than 0.006 s−1, the hardness value of Cu42Ti58 is larger than Cu58Ti42; whereas indicate a contrary trend at less than 0.006 s−1. It may be also referred to motion of free volume, in the current work, the free volume spreading to the free surface is accompanying with indenter pressed into films. when strain rate is very small (≤ 0.006 s−1), the area where the indenter contact with films will produce more free volume aggregation and annihilation accompanying with stress energy dissipation, combining with Cu42Ti58 has a more relaxed structure, causing a weaker hardness of Cu42Ti58 film. Nevertheless, when strain rate is fast (≥ 0.006 s−1), the free volume may have no enough time spread into the interface between indenter and thin film; not only this, indentation creep is under the condition of constant force, whereas the applied stress actually decreases because the volume of materials is involved increasingly. Larger strain rate may easily initiate the intersection of free
Retardation spectrum (L(τ)) is an effective way for evaluation of structure-related properties in amorphous solid [32,33], which can be derived from the creep compliance spectrum (J(t) = h(t)·A0/(P0·hin), where A0 and P0 are the contact area and the applied load corresponding to the virtual length (hin), which equals to the depth after loading.). The retardation spectrum is described as,
L (τ ) = dJ (t )/ d ln t − d 2J (t )/ d (ln t )2 |t =2τ .
∂ ln H . ∂ ln ξ
(2)
Fig. 3b shows the retardation spectra for as-synthesized nanoglass films. Clearly, both two creep retardation spectra are consisted of two separated peaks with well-defined relaxation times, which imply two kinds of relaxation processes during indentation creep at room temperature. Alternatively, both two peaks of Cu42Ti58 show a shift toward larger relaxation time and a stronger intensity with regard to Cu58Ti42. As investigated before, metallic glasses are characterized by two types of traps where positrons are annihilated [34,35], i.e. small intrinsic voids, similar to Bernal's interstitial sites, referring to the shorter position life time, and larger voids, referring to the longer positron life time. Theoretically, the voids are generally assumed as free volume, liquid-like region or flow units, etc. On this point, the intensity and retardation time corresponding to creep behaviors can be corre30
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volume forming the collision of separated redundant “free volume”, like dislocation pile-up in deformation of crystalline metals, but these redundant free volumes were not broken their “shell”, unlike the aggregation of free volume into a “bigger” free volume that usually loosen the structure under the indenter and thus decrease the hardness [40]. Consequently, at a higher strain rate, the more relaxed Cu42Ti58 can indicate a higher hardness than Cu58Ti42.
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4. Conclusion In summary, we prepared the binary Cu-Ti nanoglass films via composite deposition technique by electron beam evaporation of pure Ti and resistance thermal evaporation of pure Cu. The XRD and AFM characterizations of as-synthesized nanoglass films clearly show that nano-sized glassy particles with apparent interfaces make up the films. The room temperature creep behaviors and strain rate sensitivity of as-synthesized nanoglass films were investigated with nanoindetation. It discloses that Cu58Ti42 has a better creep-resistance property at room temperature but Cu42Ti58 has a more relaxed structure. The assynthesized nanoglass films exhibit anomalously negative strain rate sensitivity, may be caused by temperature rise that affect the dilation of free volumes. And the fitting lines of H vs. ξ intersect at 0.006 s−1 indicating different roles of free volume during indentation with different strain rates. At higher strain rate, it may easily initiate the intersection of free volume forming the collision of separated redundant free volume. Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 51322103, 51571079 and 51601050), National Key Technologies R & D Programs (Nos. 2015CB856800 and 2016YFB0300500) and the Fundamental Research Funds for the Central Universities (Nos. JZ2016HGBZ0772 and JZ2016HGPB0671). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.physb.2017.06.053. References [1] C.J. Byrne, M. Eldrup, Materials science - bulk metallic glasses, Science 321 (2008) 502–503. [2] J.J. Kruzic, Bulk metallic glasses as structural materials: a review, Adv. Eng. Mater. 18 (2016) 1308–1331. [3] J. Jing, A. Kramer, R. Birringer, H. Gleiter, U. Gonser, Modified atomic-structure in a Pd-Fe-Si nanoglass - a Mossbauer study, J. Non-Cryst. Solids 113 (1989) 167–170. [4] G.A. Almyras, G.M. Matenoglou, P. Komninou, C. Kosmidis, P. Patsalas, G.A. Evangelakis, On the deposition mechanisms and the formation of glassy Cu-Zr thin films, J. Appl. Phys. 107 (2010) 084313. [5] W. Guo, E. Jagle, J.H. Yao, V. Maier, S. Korte-Kerzel, J.M. Schneider, D. Raabe, Intrinsic and extrinsic size effects in the deformation of amorphous CuZr/ nanocrystalline Cu nanolaminates, Acta Mater. 80 (2014) 94–106. [6] C.A. Schuh, T.C. Hufnagel, U. Ramamurty, Mechanical behavior of amorphous alloys, Acta Mater. 55 (2007) 4067–4109. [7] M.M. Trexler, N.N. Thadhani, Mechanical properties of bulk metallic glasses, Prog. Mater. Sci. 55 (2010) 759–839. [8] B.A. Sun, W.H. Wang, The fracture of bulk metallic glasses, Prog. Mater. Sci. 74 (2015) 211–307. [9] G. Kumar, A. Desai, J. Schroers, Bulk metallic glass: the smaller the better, Adv. Mater. 23 (2011) 461–476. [10] H.F. Zhou, C. Zhong, Q.P. Cao, S.X. Qu, X.D. Wang, W. Yang, J.Z. Jiang, Nonlocalized deformation in metallic alloys with amorphous structure, Acta Mater. 68 (2014) 32–41. [11] L. Tian, Y.Q. Cheng, Z.W. Shan, J. Li, C.C. Wang, X.D. Han, J. Sun, E. Ma, Approaching the ideal elastic limit of metallic glasses, Nat. Commun. 3 (2012) 609.
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