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Creep behaviors of Ta-alloyed Cuzr-Based metallic glass composite
Journal of Non-Crystalline Solids 534 (2020) 119950
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Creep behaviors of Ta-alloyed Cuzr-Based metallic glass composite a
a
a
a
a,⁎
b,⁎
Jili Wu , Ziyi Zhou , Zijie Tang , Li Wang , Xiangfeng Liang , Jinhong Pi a b
T
School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China School of Materials Engineering, Nanjing Institute of Technology, Nanjing 211167, China
ARTICLE INFO
ABSTRACT
Keywords: Ta alloying Composite Precipitate Tensile creep Activation volume
In the current work, the tensile creep behaviors of a (Cu0.5Zr0.5)92Ta8 metallic glass composite were investigated. The precipitates and the metallic glass matrix in the as-spun ribbons have nearly same compositions. The addition of Ta element enhances the thermal stability of binary Cu50Zr50 metallic glass. The apparent activation energies of tensile creep increase with the applied stress, and the values of strain rate sensitivity decrease from 0.39 to 0.16 with the increasing applied temperatures. The correlationship of activation energy and activation volume suggests that the largest activation energy corresponds to the smallest activation volume, and vice versa. The values of strain rate sensitivity are noticeably smaller than reported values, which is related to the softening of (Cu0.5Zr0.5)92Ta8 ribbons with the increasing creep temperatures.
1. Introduction It is well known that Cu-Zr is a good glass-forming system, and numerous CuZr-based bulk metallic glasses (BMGs), with large critical size (i.e. diameter for rod, or thickness for plate) have been developed [1, 2]. In the past decades, much attention has been attracted to the compositional design and synthesis of Cu-Zr-Ti [3, 4], Cu-Zr-Hf [5,6] and Cu-Zr-Al [7, 8]. BMGs. Alloying is one effective approach for exploring new glass-forming CuZr-based alloys, and Ni, Nb, Si, Sn, refractory metals and rare earth elements etc. are frequently utilized as alloying elements in the CuZr-based MGs [9, 10]. Cu-Ta alloys are of binary immiscible nature, and thus Ta atoms easily precipitate from the melt during the solidification in the form of nanometer-scale clusters which is coherent with the Cu matrix [11]. Therefore, Cu-Ta alloys were usually used for the development of bulk nanocrystalline Cu alloys with extraordinary structural stability and high strength at high temperatures [12-14]. In the previous works, Ta element was usually utilized as alloying element to construct the composite structures in the Cu- and Zr-based MGs [15, 16]. It has been demonstrated that the addition of Ta element benefits the precipitation of B2 CuZr phase which can constrain the propagation of shear bands and thus improve the plasticity of monolithic CuZr-based BMGs [15, 17]. The explanations for the roles of Ta element on the microstructural tailoring of B2 CuZr phase embedded
⁎
BMG composites were concluded as the nucleation catalyst and growth suppression of B2 phase, homogenizing B2 CuZr phase and enhancing the glass-forming ability of glassy matrix [18-20]. Obviously, alloying with Ta element can facilitate the development of CuZr-based BMGs or BMG composites. Due to the high melting temperature and thermal stability of Ta metal, particulate, fibrous or wiry Ta were used as reinforcements in the CuZr-based BMG composites, and these composites have shown the good plasticity under the compression [21, 22]. On the fabrication procedures, Okazaki et al. reported that Ta dendrite is the primary phase during the solidification and synthesized Ta dendrite reinforced Cu40Zr50Al10 BMG composite which has the 2180 MPa of fracture strength and 15.9% of plastic strain [23]. Adding Ta metal can also optimize interfacial bonding via chemical reaction to improve the mechanical properties of BMG composites [24]. For the mechanical properties of Ta-alloyed BMGs and composites, the properties were focused on the tension and compression measurements to uncover the deformation mechanism under the room temperature. In recent years, at high temperature (e.g. at the temperatures of glass transition vicinity), the time-dependent mechanical properties of BMGs and composites, such as creep and stress relaxation etc., have attracted an increasing attention to understand the origin of plasticity in kinetics [25, 26]. Generally, the mechanical response of materials is a combined response of the elastic component, the viscoelastic compo-
Corresponding author. E-mail addresses: [email protected] (J. Wu), [email protected] (X. Liang), [email protected] (J. Pi).
https://doi.org/10.1016/j.jnoncrysol.2020.119950 Received 20 October 2019; Received in revised form 23 January 2020; Accepted 28 January 2020 0022-3093/ © 2020 Elsevier B.V. All rights reserved.
Journal of Non-Crystalline Solids 534 (2020) 119950
J. Wu, et al.
Fig. 1. XRD pattern of as-spun CZT ribbon.
nent and the viscoplastic component [27]. The activation of viscoplastic component is a key factor for disclosing the origin of plastic deformation of BMGs and composites, and the tensile creep provides the large time window to evaluate the plastic deformation behaviors at the fixed constant stress and can also be as a global experiment for understanding the physical meaning for a viscoplastic phenomenon [25]. In the current work, the 8 at.% Ta-alloyed Cu50Zr50 MG composite was investigated by tensile creep. The reason for the selection of this MG as the model material can be attributed to enough quantity of Ta element for the enhancement of thermal stability. 2. Experiments 2.1. preparation and characterization of materials In the preparation, the master ingot with the nominal composition of (Cu0.5Zr0.5)92Ta8 (CZT) (in at.%) was fabricated by arc-melting pure Cu, Zr metals and Ta powders (diameter: 10~30 μm) in a Ti-gettered pre-pumped vacuum furnace. For the compositional homogeneity, each ingot was flipped and re-melted at least 4 times. The ribbons (thickness: ca. 50 μm; width: ca. 0.8 mm) were synthesized by the melt-spinning technique in an inert argon atmosphere. The linear speed of copper roller was fixed at 25 m/s. The microstructural nature was examined by X-ray diffraction (XRD, D8, Bruker AXS) with equipped with Cu Kα radiation. The thermal properties and relaxation behavior of the as-spun CTZ ribbons were characterized by differential scanning calorimetry (DSC, DSC-7, Perkin Elmer) at a specific heating rate of 20 K/min. Aluminum pans were used as sample holders. The microstructural morphology of as-spun CTZ ribbons were ascertained by Scanning electron microscopy (SEM, NovaNano 450, FEI). The specimens for SEM were carefully polished with diamond polishing reagent.
Fig. 2. The SEM image (a) and EDS spectrum (b) (line scanning) of as-spun CTZ ribbon. (The line in (a) is the scanning trace for EDS).
For the reproducibility, each specimen was tested 3 times. The error analysis is based on the meaning values of measured results. 3. Results and discussion Fig. 1 indicates a typical XRD pattern of as-spun CTZ ribbon. Clearly, a diffraction hump which can be assigned to amorphous phase and two sharp peaks which can be indexed to B2 Cu(Zr, Ta) phase. Therefore, the composite structures of as-spun CTZ ribbon have been synthesized. On the other hand, for the peaks, it seems to be assigned to (110) and (200) planes of Ta metal (JCPDF No. 01–1182) (Fig. S1 in supplementary material). However, the element analysis reveals that there is no noticeable difference in the distribution from the glassy matrix to crystalline precipitates (Fig. 2). The uniform distribution of Cu, Zr and Ta elements can be also observed in Fig. 3, and it validates
2.2. creep experiments Tensile creep tests were performed on a platform (DMA Q800, TA Instruments) using its film tensile method under argon flux. The applied stress was fixed at 200 MPa, 225 MPa, 250 MPa and 275 MPa, respectively. The testing temperature ranged from 340 °C to 380 °C with an interval of 10 °C, and the duration time was set as 54,000 s (15 h).
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Journal of Non-Crystalline Solids 534 (2020) 119950
J. Wu, et al.
[28]. However, it has been validated that there is a metastable liquidliquid miscibility gap referring to the solidification of Cu-Ta binary alloys. On this point, Cu and Ta are difficult to form binary phase or intermetallics [29]. Nevertheless, Ta atoms can be solid-solved into Zr sites in B2 CuZr phases, similar with Ti atom into B2 CuZr phase [30] and Al atom into B2 ZrCo phase [31]. The synthetic effect of highly miscible Zr-Ta and immiscible Cu-Ta easily causes the formation of different phases, amorphous phase and crystalline phase. Fig. 4 presents the DSC curve of as-spun CTZ ribbon. It clearly indicates that the noticeable glass transition at the temperature of 708 K (Tg) and an onset crystallization temperature of 763 K (Tx). Evidently, the supercooled liquid region (ΔTx), the temperature interval between Tx and Tg, has a width of ca. 55 K. Compared with previous works, the value of ΔTx is very close to the reported values of as-cast Cu50Zr50 binary MG [2, 32]. However, both Tg and Tx of as-spun CZT ribbon in the current work are remarkably higher than that of Cu50Zr50 binary MG. Obviously, the addition of Ta element enhances thermal stability of binary Cu50Zr50 MG. The selection of temperatures for the creep experiments is in accordance with temperature-dependent loss modulus spectrum (Fig. S2 in supplementary material). For CuZr-based MGs, it usually displays a shoulder-like β relaxation event, and after as-defined peak temperature of event, β relaxation will be gradually merged into α relaxation event [33]. On the other hand, β relaxation is the response of small localized atomic motion, α relaxation, however, is responsible for atomic motion within the large region in the MG matrix [34]. Therefore, at the selected temperature range, the atomic motion stands at the β relaxation margin and localized motion attempts to break the restriction and evolves into α relaxation event. Fig. 5 displays the tensile creep curves of as-spun CZT ribbons under different conditions. Under the applied temperatures of 340 °C and 350 °C, each creep curve was retained to its creep time of 5.4 × 104 s under the applied stresses of 200 MPa ~ 275 MPa. When the temperature was increased to 360 °C ~ 380 °C, the high applied stress causes the early fracture due to the temperature-induced softening. In this temperature range, the creep strain under the applied stress of 275 MPa is remarkably larger than other two cases, implying that, in this temperature range, a good coordination of applied stress and temperature-induced softening, i.e. softening with proper applied stress, can be caused large creep strain. By considering the experimental results, the dependence of steady state creep rate on applied stress can be expressed as
Fig. 3. EDS spectrum (map scanning) of as-spun CTZ ribbon. (a) The selected area; (b) the overall distribution of Cu (yellow), Zr (red) and Ta (green) elements); (c-d) the distribution of Cu, Zr and Ta elements, respectively.
=A
Qa n e RT
(1)
where A is a dimensionless constant, n is stress exponent, R and T are referred to their own meanings and Qa is the apparent activation energy for creep; denotes the creep stain rate. For a common used method to determine the Qa, the steady state creep strain rate ss (In the current work, it is defined as the strain rate at the last stage of creep curve) was taken into Eq. (1) which was derived into Eq. (2),
Fig. 4. DSC curve of as-spun ribbon. (The shaded area is the creep temperature range.).
Qa ss=Ae RT
(2)
where A, T and Qa have the same meanings in the Eq. (1). By plotting 1 ln ss ln( RT ) , the Qa can be calculated to be the slop. As shown in Fig. 6, it clearly shows that the values of Qa approximately increase from 134.3 kJ/mol to 401.8 kJ/mol with the applied stress. For the understanding of creep from the viewpoint of microstructure, the strain rate sensitivity m has been taken into account to explain the dependence of strain rate on the applied stress σ under a fixed temperature, T. The m is usually defined as
that no remarkable segregation of compositions in the as-spun CTZ ribbon. However, it is worth noticing that this negligible difference of compositions between the precipitates and glassy matrix may be caused by the limitation of X-ray technique. As aforementioned in the introduction, the addition of Ta element favors the formation of B2 CuZr phase in the CuZr-based BMGs [15, 16]. In the current work, nevertheless, the precipitates have nearly same compositions with glassy matrix. It can be interpreted from binary phase diagrams of Zr-Ta and Cu-Ta. Zr and Ta are miscible at high temperature, and the solidification temperature range is very narrow (less than 20 K) in a large composition range to 20 at.% of Ta element
m=
3
ln ln ss
T
(3)
Journal of Non-Crystalline Solids 534 (2020) 119950
J. Wu, et al.
Fig. 5. the tensile creep curves of as-spun CTZ ribbons under different condition.
While the parameter, n in the Eq. (1), generally defined as
n=
ln ss ln
T
plasticity of materials which is related to the diffusion-dominated creep mechanism, and under the case of low strain rate sensitivity, the deformation of materials is governed by the dislocation-mediated plasticity mechanism [36, 37]. Summarily, for most of materials, the values of m are in the range of 0.2 < m < 1 [37]. As shown in Fig. 7, the values of m decrease from 0.39 to 0.16 (2.51 < n < 6.12) with the increasing applied temperatures. Notably, for the applied temperatures blew 360 °C (633 K), the values of m, in the current work, are approximately located in the reported range, but above this temperature the values are obviously smaller than the reported values. For the latter case, this is because of that, the strain rate
(4)
Clearly, both m and n represent the isothermal relationship between stress and strain rate, but show a reciprocal relationship [35]. Fig. 7 indicates the trend of m and n with the increasing temperatures. It confirms the reciprocal relationship of m and n. Generally, the high strain rate sensitivity is usually expected that the materials can well resolve the localized deformation and facilitate the ductility or super-
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Journal of Non-Crystalline Solids 534 (2020) 119950
J. Wu, et al.
p
=
G 0 e kB T
(5)
where G = G0 scribed as [37, 45]
V = 3
G
=
3
kB T
V A general partial differential of Eq. (5) is de-
ln
0
1 m
(6)
where 0 is the pre-factor, kB is the Boltzmann constant, and ΔG denotes the apparent activation energy. Fig. 8 clearly shows that the start point is nearly equal, suggesting that the 0 is independent with applied stress. The relationship between strain rate sensitivity m and the activation volume V was as follows:
V 1 = kBT , orequivalently V = m 3
3 kB T m
(7)
Fig. 9 shows activation volume under different testing conditions. It illustrates the correlations among the values of activation volume, creep stress and creep temperature. As shown, the smallest activation volume appears with the creep temperature of 613 K and the applied stress of 270 MPa, however, the largest activation volume comes into sight on the 653 K and 200 MPa of creep temperature and applied stress, respectively. Regarding to apparent activation energy, the smallest V is referred to the case at 280 MPa and 613 K, while the largest V is for the experimental condition at 200 MPa and 653 K. Obviously, the largest Qa corresponds to the smallest V, and vice versa. From Eq. (7), it clearly indicates that the strain rate sensitivity (m) has reciprocal relationship with activation volume (V), that is, the larger m, smaller V. As summarized, during creep deformation in the supercooling region, Pd-, Zr- and Ti-based MGs normally have the 0.4~1.0 values of m and La-based MGs have a constant value of m (m = 1) [46]. Alternatively, the activation energy (Qa) is collaboratively transformed with strain rate sensitivity (m). Eq. (3) suggests that m is a parameter describing strain rate-stress correlation. In this sense, the change of m is suggested to the deformation of MGs under different strain rates. At higher strain rates during the deformation in the supercooling region, there may emerge a partially inhomogeneous flow that is accompanied with the homogeneous flow (m = 1) to reach a linear deviation of strain rate-stress relation. Totally, the overall values of m, as summarized in Ref. [46], are smaller than 1, and bounded at 0.4 of m. In the current work, nevertheless, the values of m are noticeably lower than summarized values, it can be interpreted by that the consistently increase of strain rate with applied stress, i.e. softening of CTZ ribbons.
Fig. 6. Apparent activation energies of as-spun CZT ribbons at different creep temperatures.
Fig. 7. the trend of m and n with the temperatures during the creep.
4. Conclusion
sensitivity is a response from multiple factors, such as the creep temperature and the external stress, etc. [36]. On the understanding of crystalline materials, for example, in Si-C-O ceramics derived polymers, Gan et al. supposed that the small m may be due to dislocation-mediated creep and volumetric densification [38]. On the other hand, the microstructures, especially the grain size, are also the important factors that affect the strain rate sensitivity [39-43]. However, the MGs are free from the microstructures of crystalline materials, such as the features of lattice, grain size and boundary etc. The strain rate sensitivity of MGs may be strongly related with their internal states, i.e. flow unit and free volume, etc. [44]. Athigher temperature, more flow units or free volumes will be easily activated and thus soften the MGs[34]. According to Eq. (3), the decrease of σ and the increase of ss will derive a relatively lower value of m. Due to the softening of CZT ribbons, the ss will increases and thus decreases the values of m. It has been established that the correlationship between the plastic strain rate p and the creep temperature can be is expressed by the Arrhenius equation
In the current work, the creep behaviors of a (Cu0.5Zr0.5)92Ta8 MG composite ribbons were investigated by tensile creep. The phase nature and microstructures of as-spun ribbons were determined by XRD, SEM and EDS. The Qa, m, n and V of as-spun ribbons during creep are derived and studied. The main results of this work can be concluded as follows. (1) The B2 Cu(Zr, Ta) precipitates have a nearly same compositions with the metallic glass matrix. The formation of B2 Cu(Zr, Ta) precipitates and glassy matrix may be caused by the synthetic effect of highly miscible Zr-Ta and immiscible Cu-Ta systems. (2) The Tg and Tx of as-spun CTZ ribbons are remarkably higher than that of binary Cu50Zr50 MG, and thus the addition of Ta element enhances thermal stability of Cu50Zr50 MG. The values of Qa approximately increase with the applied stress. (3) For the applied temperatures above 633 K, the values of m are obviously smaller than the reported values. the values of m decrease
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Journal of Non-Crystalline Solids 534 (2020) 119950
J. Wu, et al.
Fig. 8. the strain rate-strain curves of creep.
Declaration of Competing Interest
from 0.39 to 0.16 with the increasing applied temperatures due to softening of as-spun CTZ ribbons. the largest Qa is corresponding to the smallest V, and vice versa.
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.
CRediT authorship contribution statement Jili Wu: Conceptualization, Methodology, Writing - original draft. Ziyi Zhou: Validation, Investigation. Zijie Tang: Resources, Data curation. Li Wang: Data curation. Xiangfeng Liang: Supervision. Jinhong Pi: Writing - review & editing.
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Fig. 9. the correlations among activation volume, creep stress and creep temperature.
Acknowledgements This work was supported by the National Natural Science Foundation of China (grant numbers 51601050 and 51601089), Jiangsu Key Laboratory for Advanced Metallic Materials (grant number BM2007204) and Natural Science Research of Jiangsu Higher Education Institutions of China (grant number 19KJB590001). Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jnoncrysol.2020.119950. References [1] P. Yu, H.Y. Bai, M.B. Tang, W.L. Wang, Excellent glass-forming ability in simple Cu50Zr50-based alloys, j. Non-Cryst. Solids 351 (2005) 1328–1332. [2] O.J. Kwon, Y.C. Kim, K.B. Kim, Y.K. Lee, E. Fleury, Formation of amorphous phase in the binary cu-zr alloy system, Met. Mater. Int 12 (2006) 207–212. [3] A. Inoue, W. Zhang, T. Zhang, K. Kurosaka, High-strength cu-based bulk glassy alloys in cu-zr-ti and cu-hf-ti ternary systems, Acta Mater 49 (2001) 2645–2652. [4] Y. Pan, Y.Q. Zeng, L.J. Jing, L. Zhang, J.H. Pi, Composition design and mechanical properties of ternary cu-zr-ti bulk metallic glasses, Mater. Des 55 (2014) 773–777. [5] S.Y. Luo, Y.Y. Cui, Y. Dai, J.H. Li, B.X. Liu, Interatomic potential to predict the favored and optimized compositions for ternary cu-zr-hf metallic glasses, J. Appl. Phys. 112 (2012) 103518. [6] S.Y. Luo, J.H. Li, Y.Y. Cui, B.X. Liu, Glass-formation and atomic structures of cux (Zr0.22Hf0.78)1-x and (Cu0.61Hf0.39)1-x Zr- x alloys investigated by monte carlo simulation, Mater. Lett 100 (2013) 130–132. [7] P. Yu, H.Y. Bai, M.B. Tang, W.L. Wang, W.H. Wang, CuZr-based bulk metallic glasses with good glass-forming ability prepared by al addition, Acta Phys. Sin 54 (2005) 3284–3289. [8] N.S. Barekar, S. Pauly, R.B. Kumar, U. Kuhn, B.K. Dhindaw, J. Eckert, Structureproperty relations in bulk metallic cu-zr-al alloys, Mater. Sci. Eng. A 527 (2010) 5867–5872. [9] Z.P. Lu, C.T. Liu, Role of minor alloying additions in formation of bulk metallic glasses: a review, J. Mater. Sci. 39 (2004) 3965–3974. [10] W.H. Wang, Roles of minor additions in formation and properties of bulk metallic glasses, Prog. Mater. Sci 52 (2007) 540–596. [11] M. Rajagopalan, K. Darling, S. Turnage, R.K. Koju, B. Hornbuckle, Y. Mishin, K.N. Solanki, Microstructural evolution in a nanocrystalline cu-ta alloy: a combined in-situ tem and atomistic study, Mater. Des 113 (2017) 178–185. [12] T. Venugopal, K.P. Rao, B.S. Murty, Mechanical and electrical properties of cu-ta nanocomposites prepared by high-energy ball milling, Acta Mater 55 (2007) 4439–4445. [13] T. Frolov, K.A. Darling, L.J. Kecskes, Y. Mishin, Stabilization and strengthening of nanocrystalline copper by alloying with tantalum, Acta Mater 60 (2012) 2158–2168.
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Creep behaviors of Ta-alloyed Cuzr-Based metallic glass composite a
a
a
a
a,⁎
b,⁎
Jili Wu , Ziyi Zhou , Zijie Tang , Li Wang , Xiangfeng Liang , Jinhong Pi a b
T
School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China School of Materials Engineering, Nanjing Institute of Technology, Nanjing 211167, China
ARTICLE INFO
ABSTRACT
Keywords: Ta alloying Composite Precipitate Tensile creep Activation volume
In the current work, the tensile creep behaviors of a (Cu0.5Zr0.5)92Ta8 metallic glass composite were investigated. The precipitates and the metallic glass matrix in the as-spun ribbons have nearly same compositions. The addition of Ta element enhances the thermal stability of binary Cu50Zr50 metallic glass. The apparent activation energies of tensile creep increase with the applied stress, and the values of strain rate sensitivity decrease from 0.39 to 0.16 with the increasing applied temperatures. The correlationship of activation energy and activation volume suggests that the largest activation energy corresponds to the smallest activation volume, and vice versa. The values of strain rate sensitivity are noticeably smaller than reported values, which is related to the softening of (Cu0.5Zr0.5)92Ta8 ribbons with the increasing creep temperatures.
1. Introduction It is well known that Cu-Zr is a good glass-forming system, and numerous CuZr-based bulk metallic glasses (BMGs), with large critical size (i.e. diameter for rod, or thickness for plate) have been developed [1, 2]. In the past decades, much attention has been attracted to the compositional design and synthesis of Cu-Zr-Ti [3, 4], Cu-Zr-Hf [5,6] and Cu-Zr-Al [7, 8]. BMGs. Alloying is one effective approach for exploring new glass-forming CuZr-based alloys, and Ni, Nb, Si, Sn, refractory metals and rare earth elements etc. are frequently utilized as alloying elements in the CuZr-based MGs [9, 10]. Cu-Ta alloys are of binary immiscible nature, and thus Ta atoms easily precipitate from the melt during the solidification in the form of nanometer-scale clusters which is coherent with the Cu matrix [11]. Therefore, Cu-Ta alloys were usually used for the development of bulk nanocrystalline Cu alloys with extraordinary structural stability and high strength at high temperatures [12-14]. In the previous works, Ta element was usually utilized as alloying element to construct the composite structures in the Cu- and Zr-based MGs [15, 16]. It has been demonstrated that the addition of Ta element benefits the precipitation of B2 CuZr phase which can constrain the propagation of shear bands and thus improve the plasticity of monolithic CuZr-based BMGs [15, 17]. The explanations for the roles of Ta element on the microstructural tailoring of B2 CuZr phase embedded
⁎
BMG composites were concluded as the nucleation catalyst and growth suppression of B2 phase, homogenizing B2 CuZr phase and enhancing the glass-forming ability of glassy matrix [18-20]. Obviously, alloying with Ta element can facilitate the development of CuZr-based BMGs or BMG composites. Due to the high melting temperature and thermal stability of Ta metal, particulate, fibrous or wiry Ta were used as reinforcements in the CuZr-based BMG composites, and these composites have shown the good plasticity under the compression [21, 22]. On the fabrication procedures, Okazaki et al. reported that Ta dendrite is the primary phase during the solidification and synthesized Ta dendrite reinforced Cu40Zr50Al10 BMG composite which has the 2180 MPa of fracture strength and 15.9% of plastic strain [23]. Adding Ta metal can also optimize interfacial bonding via chemical reaction to improve the mechanical properties of BMG composites [24]. For the mechanical properties of Ta-alloyed BMGs and composites, the properties were focused on the tension and compression measurements to uncover the deformation mechanism under the room temperature. In recent years, at high temperature (e.g. at the temperatures of glass transition vicinity), the time-dependent mechanical properties of BMGs and composites, such as creep and stress relaxation etc., have attracted an increasing attention to understand the origin of plasticity in kinetics [25, 26]. Generally, the mechanical response of materials is a combined response of the elastic component, the viscoelastic compo-
Corresponding author. E-mail addresses: [email protected] (J. Wu), [email protected] (X. Liang), [email protected] (J. Pi).
https://doi.org/10.1016/j.jnoncrysol.2020.119950 Received 20 October 2019; Received in revised form 23 January 2020; Accepted 28 January 2020 0022-3093/ © 2020 Elsevier B.V. All rights reserved.
Journal of Non-Crystalline Solids 534 (2020) 119950
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Fig. 1. XRD pattern of as-spun CZT ribbon.
nent and the viscoplastic component [27]. The activation of viscoplastic component is a key factor for disclosing the origin of plastic deformation of BMGs and composites, and the tensile creep provides the large time window to evaluate the plastic deformation behaviors at the fixed constant stress and can also be as a global experiment for understanding the physical meaning for a viscoplastic phenomenon [25]. In the current work, the 8 at.% Ta-alloyed Cu50Zr50 MG composite was investigated by tensile creep. The reason for the selection of this MG as the model material can be attributed to enough quantity of Ta element for the enhancement of thermal stability. 2. Experiments 2.1. preparation and characterization of materials In the preparation, the master ingot with the nominal composition of (Cu0.5Zr0.5)92Ta8 (CZT) (in at.%) was fabricated by arc-melting pure Cu, Zr metals and Ta powders (diameter: 10~30 μm) in a Ti-gettered pre-pumped vacuum furnace. For the compositional homogeneity, each ingot was flipped and re-melted at least 4 times. The ribbons (thickness: ca. 50 μm; width: ca. 0.8 mm) were synthesized by the melt-spinning technique in an inert argon atmosphere. The linear speed of copper roller was fixed at 25 m/s. The microstructural nature was examined by X-ray diffraction (XRD, D8, Bruker AXS) with equipped with Cu Kα radiation. The thermal properties and relaxation behavior of the as-spun CTZ ribbons were characterized by differential scanning calorimetry (DSC, DSC-7, Perkin Elmer) at a specific heating rate of 20 K/min. Aluminum pans were used as sample holders. The microstructural morphology of as-spun CTZ ribbons were ascertained by Scanning electron microscopy (SEM, NovaNano 450, FEI). The specimens for SEM were carefully polished with diamond polishing reagent.
Fig. 2. The SEM image (a) and EDS spectrum (b) (line scanning) of as-spun CTZ ribbon. (The line in (a) is the scanning trace for EDS).
For the reproducibility, each specimen was tested 3 times. The error analysis is based on the meaning values of measured results. 3. Results and discussion Fig. 1 indicates a typical XRD pattern of as-spun CTZ ribbon. Clearly, a diffraction hump which can be assigned to amorphous phase and two sharp peaks which can be indexed to B2 Cu(Zr, Ta) phase. Therefore, the composite structures of as-spun CTZ ribbon have been synthesized. On the other hand, for the peaks, it seems to be assigned to (110) and (200) planes of Ta metal (JCPDF No. 01–1182) (Fig. S1 in supplementary material). However, the element analysis reveals that there is no noticeable difference in the distribution from the glassy matrix to crystalline precipitates (Fig. 2). The uniform distribution of Cu, Zr and Ta elements can be also observed in Fig. 3, and it validates
2.2. creep experiments Tensile creep tests were performed on a platform (DMA Q800, TA Instruments) using its film tensile method under argon flux. The applied stress was fixed at 200 MPa, 225 MPa, 250 MPa and 275 MPa, respectively. The testing temperature ranged from 340 °C to 380 °C with an interval of 10 °C, and the duration time was set as 54,000 s (15 h).
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Journal of Non-Crystalline Solids 534 (2020) 119950
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[28]. However, it has been validated that there is a metastable liquidliquid miscibility gap referring to the solidification of Cu-Ta binary alloys. On this point, Cu and Ta are difficult to form binary phase or intermetallics [29]. Nevertheless, Ta atoms can be solid-solved into Zr sites in B2 CuZr phases, similar with Ti atom into B2 CuZr phase [30] and Al atom into B2 ZrCo phase [31]. The synthetic effect of highly miscible Zr-Ta and immiscible Cu-Ta easily causes the formation of different phases, amorphous phase and crystalline phase. Fig. 4 presents the DSC curve of as-spun CTZ ribbon. It clearly indicates that the noticeable glass transition at the temperature of 708 K (Tg) and an onset crystallization temperature of 763 K (Tx). Evidently, the supercooled liquid region (ΔTx), the temperature interval between Tx and Tg, has a width of ca. 55 K. Compared with previous works, the value of ΔTx is very close to the reported values of as-cast Cu50Zr50 binary MG [2, 32]. However, both Tg and Tx of as-spun CZT ribbon in the current work are remarkably higher than that of Cu50Zr50 binary MG. Obviously, the addition of Ta element enhances thermal stability of binary Cu50Zr50 MG. The selection of temperatures for the creep experiments is in accordance with temperature-dependent loss modulus spectrum (Fig. S2 in supplementary material). For CuZr-based MGs, it usually displays a shoulder-like β relaxation event, and after as-defined peak temperature of event, β relaxation will be gradually merged into α relaxation event [33]. On the other hand, β relaxation is the response of small localized atomic motion, α relaxation, however, is responsible for atomic motion within the large region in the MG matrix [34]. Therefore, at the selected temperature range, the atomic motion stands at the β relaxation margin and localized motion attempts to break the restriction and evolves into α relaxation event. Fig. 5 displays the tensile creep curves of as-spun CZT ribbons under different conditions. Under the applied temperatures of 340 °C and 350 °C, each creep curve was retained to its creep time of 5.4 × 104 s under the applied stresses of 200 MPa ~ 275 MPa. When the temperature was increased to 360 °C ~ 380 °C, the high applied stress causes the early fracture due to the temperature-induced softening. In this temperature range, the creep strain under the applied stress of 275 MPa is remarkably larger than other two cases, implying that, in this temperature range, a good coordination of applied stress and temperature-induced softening, i.e. softening with proper applied stress, can be caused large creep strain. By considering the experimental results, the dependence of steady state creep rate on applied stress can be expressed as
Fig. 3. EDS spectrum (map scanning) of as-spun CTZ ribbon. (a) The selected area; (b) the overall distribution of Cu (yellow), Zr (red) and Ta (green) elements); (c-d) the distribution of Cu, Zr and Ta elements, respectively.
=A
Qa n e RT
(1)
where A is a dimensionless constant, n is stress exponent, R and T are referred to their own meanings and Qa is the apparent activation energy for creep; denotes the creep stain rate. For a common used method to determine the Qa, the steady state creep strain rate ss (In the current work, it is defined as the strain rate at the last stage of creep curve) was taken into Eq. (1) which was derived into Eq. (2),
Fig. 4. DSC curve of as-spun ribbon. (The shaded area is the creep temperature range.).
Qa ss=Ae RT
(2)
where A, T and Qa have the same meanings in the Eq. (1). By plotting 1 ln ss ln( RT ) , the Qa can be calculated to be the slop. As shown in Fig. 6, it clearly shows that the values of Qa approximately increase from 134.3 kJ/mol to 401.8 kJ/mol with the applied stress. For the understanding of creep from the viewpoint of microstructure, the strain rate sensitivity m has been taken into account to explain the dependence of strain rate on the applied stress σ under a fixed temperature, T. The m is usually defined as
that no remarkable segregation of compositions in the as-spun CTZ ribbon. However, it is worth noticing that this negligible difference of compositions between the precipitates and glassy matrix may be caused by the limitation of X-ray technique. As aforementioned in the introduction, the addition of Ta element favors the formation of B2 CuZr phase in the CuZr-based BMGs [15, 16]. In the current work, nevertheless, the precipitates have nearly same compositions with glassy matrix. It can be interpreted from binary phase diagrams of Zr-Ta and Cu-Ta. Zr and Ta are miscible at high temperature, and the solidification temperature range is very narrow (less than 20 K) in a large composition range to 20 at.% of Ta element
m=
3
ln ln ss
T
(3)
Journal of Non-Crystalline Solids 534 (2020) 119950
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Fig. 5. the tensile creep curves of as-spun CTZ ribbons under different condition.
While the parameter, n in the Eq. (1), generally defined as
n=
ln ss ln
T
plasticity of materials which is related to the diffusion-dominated creep mechanism, and under the case of low strain rate sensitivity, the deformation of materials is governed by the dislocation-mediated plasticity mechanism [36, 37]. Summarily, for most of materials, the values of m are in the range of 0.2 < m < 1 [37]. As shown in Fig. 7, the values of m decrease from 0.39 to 0.16 (2.51 < n < 6.12) with the increasing applied temperatures. Notably, for the applied temperatures blew 360 °C (633 K), the values of m, in the current work, are approximately located in the reported range, but above this temperature the values are obviously smaller than the reported values. For the latter case, this is because of that, the strain rate
(4)
Clearly, both m and n represent the isothermal relationship between stress and strain rate, but show a reciprocal relationship [35]. Fig. 7 indicates the trend of m and n with the increasing temperatures. It confirms the reciprocal relationship of m and n. Generally, the high strain rate sensitivity is usually expected that the materials can well resolve the localized deformation and facilitate the ductility or super-
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Journal of Non-Crystalline Solids 534 (2020) 119950
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p
=
G 0 e kB T
(5)
where G = G0 scribed as [37, 45]
V = 3
G
=
3
kB T
V A general partial differential of Eq. (5) is de-
ln
0
1 m
(6)
where 0 is the pre-factor, kB is the Boltzmann constant, and ΔG denotes the apparent activation energy. Fig. 8 clearly shows that the start point is nearly equal, suggesting that the 0 is independent with applied stress. The relationship between strain rate sensitivity m and the activation volume V was as follows:
V 1 = kBT , orequivalently V = m 3
3 kB T m
(7)
Fig. 9 shows activation volume under different testing conditions. It illustrates the correlations among the values of activation volume, creep stress and creep temperature. As shown, the smallest activation volume appears with the creep temperature of 613 K and the applied stress of 270 MPa, however, the largest activation volume comes into sight on the 653 K and 200 MPa of creep temperature and applied stress, respectively. Regarding to apparent activation energy, the smallest V is referred to the case at 280 MPa and 613 K, while the largest V is for the experimental condition at 200 MPa and 653 K. Obviously, the largest Qa corresponds to the smallest V, and vice versa. From Eq. (7), it clearly indicates that the strain rate sensitivity (m) has reciprocal relationship with activation volume (V), that is, the larger m, smaller V. As summarized, during creep deformation in the supercooling region, Pd-, Zr- and Ti-based MGs normally have the 0.4~1.0 values of m and La-based MGs have a constant value of m (m = 1) [46]. Alternatively, the activation energy (Qa) is collaboratively transformed with strain rate sensitivity (m). Eq. (3) suggests that m is a parameter describing strain rate-stress correlation. In this sense, the change of m is suggested to the deformation of MGs under different strain rates. At higher strain rates during the deformation in the supercooling region, there may emerge a partially inhomogeneous flow that is accompanied with the homogeneous flow (m = 1) to reach a linear deviation of strain rate-stress relation. Totally, the overall values of m, as summarized in Ref. [46], are smaller than 1, and bounded at 0.4 of m. In the current work, nevertheless, the values of m are noticeably lower than summarized values, it can be interpreted by that the consistently increase of strain rate with applied stress, i.e. softening of CTZ ribbons.
Fig. 6. Apparent activation energies of as-spun CZT ribbons at different creep temperatures.
Fig. 7. the trend of m and n with the temperatures during the creep.
4. Conclusion
sensitivity is a response from multiple factors, such as the creep temperature and the external stress, etc. [36]. On the understanding of crystalline materials, for example, in Si-C-O ceramics derived polymers, Gan et al. supposed that the small m may be due to dislocation-mediated creep and volumetric densification [38]. On the other hand, the microstructures, especially the grain size, are also the important factors that affect the strain rate sensitivity [39-43]. However, the MGs are free from the microstructures of crystalline materials, such as the features of lattice, grain size and boundary etc. The strain rate sensitivity of MGs may be strongly related with their internal states, i.e. flow unit and free volume, etc. [44]. Athigher temperature, more flow units or free volumes will be easily activated and thus soften the MGs[34]. According to Eq. (3), the decrease of σ and the increase of ss will derive a relatively lower value of m. Due to the softening of CZT ribbons, the ss will increases and thus decreases the values of m. It has been established that the correlationship between the plastic strain rate p and the creep temperature can be is expressed by the Arrhenius equation
In the current work, the creep behaviors of a (Cu0.5Zr0.5)92Ta8 MG composite ribbons were investigated by tensile creep. The phase nature and microstructures of as-spun ribbons were determined by XRD, SEM and EDS. The Qa, m, n and V of as-spun ribbons during creep are derived and studied. The main results of this work can be concluded as follows. (1) The B2 Cu(Zr, Ta) precipitates have a nearly same compositions with the metallic glass matrix. The formation of B2 Cu(Zr, Ta) precipitates and glassy matrix may be caused by the synthetic effect of highly miscible Zr-Ta and immiscible Cu-Ta systems. (2) The Tg and Tx of as-spun CTZ ribbons are remarkably higher than that of binary Cu50Zr50 MG, and thus the addition of Ta element enhances thermal stability of Cu50Zr50 MG. The values of Qa approximately increase with the applied stress. (3) For the applied temperatures above 633 K, the values of m are obviously smaller than the reported values. the values of m decrease
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Fig. 8. the strain rate-strain curves of creep.
Declaration of Competing Interest
from 0.39 to 0.16 with the increasing applied temperatures due to softening of as-spun CTZ ribbons. the largest Qa is corresponding to the smallest V, and vice versa.
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.
CRediT authorship contribution statement Jili Wu: Conceptualization, Methodology, Writing - original draft. Ziyi Zhou: Validation, Investigation. Zijie Tang: Resources, Data curation. Li Wang: Data curation. Xiangfeng Liang: Supervision. Jinhong Pi: Writing - review & editing.
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Fig. 9. the correlations among activation volume, creep stress and creep temperature.
Acknowledgements This work was supported by the National Natural Science Foundation of China (grant numbers 51601050 and 51601089), Jiangsu Key Laboratory for Advanced Metallic Materials (grant number BM2007204) and Natural Science Research of Jiangsu Higher Education Institutions of China (grant number 19KJB590001). Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jnoncrysol.2020.119950. References [1] P. Yu, H.Y. Bai, M.B. Tang, W.L. Wang, Excellent glass-forming ability in simple Cu50Zr50-based alloys, j. Non-Cryst. Solids 351 (2005) 1328–1332. [2] O.J. Kwon, Y.C. Kim, K.B. Kim, Y.K. Lee, E. Fleury, Formation of amorphous phase in the binary cu-zr alloy system, Met. Mater. Int 12 (2006) 207–212. [3] A. Inoue, W. Zhang, T. Zhang, K. Kurosaka, High-strength cu-based bulk glassy alloys in cu-zr-ti and cu-hf-ti ternary systems, Acta Mater 49 (2001) 2645–2652. [4] Y. Pan, Y.Q. Zeng, L.J. Jing, L. Zhang, J.H. Pi, Composition design and mechanical properties of ternary cu-zr-ti bulk metallic glasses, Mater. Des 55 (2014) 773–777. [5] S.Y. Luo, Y.Y. Cui, Y. Dai, J.H. Li, B.X. Liu, Interatomic potential to predict the favored and optimized compositions for ternary cu-zr-hf metallic glasses, J. Appl. Phys. 112 (2012) 103518. [6] S.Y. Luo, J.H. Li, Y.Y. Cui, B.X. Liu, Glass-formation and atomic structures of cux (Zr0.22Hf0.78)1-x and (Cu0.61Hf0.39)1-x Zr- x alloys investigated by monte carlo simulation, Mater. Lett 100 (2013) 130–132. [7] P. Yu, H.Y. Bai, M.B. Tang, W.L. Wang, W.H. Wang, CuZr-based bulk metallic glasses with good glass-forming ability prepared by al addition, Acta Phys. Sin 54 (2005) 3284–3289. [8] N.S. Barekar, S. Pauly, R.B. Kumar, U. Kuhn, B.K. Dhindaw, J. Eckert, Structureproperty relations in bulk metallic cu-zr-al alloys, Mater. Sci. Eng. A 527 (2010) 5867–5872. [9] Z.P. Lu, C.T. Liu, Role of minor alloying additions in formation of bulk metallic glasses: a review, J. Mater. Sci. 39 (2004) 3965–3974. [10] W.H. Wang, Roles of minor additions in formation and properties of bulk metallic glasses, Prog. Mater. Sci 52 (2007) 540–596. [11] M. Rajagopalan, K. Darling, S. Turnage, R.K. Koju, B. Hornbuckle, Y. Mishin, K.N. Solanki, Microstructural evolution in a nanocrystalline cu-ta alloy: a combined in-situ tem and atomistic study, Mater. Des 113 (2017) 178–185. [12] T. Venugopal, K.P. Rao, B.S. Murty, Mechanical and electrical properties of cu-ta nanocomposites prepared by high-energy ball milling, Acta Mater 55 (2007) 4439–4445. [13] T. Frolov, K.A. Darling, L.J. Kecskes, Y. Mishin, Stabilization and strengthening of nanocrystalline copper by alloying with tantalum, Acta Mater 60 (2012) 2158–2168.
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