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Pseudo-quinary Ti20Zr20Hf20Be20(Cu20 -xNix) high entropy bulk metallic glasses with large glass forming ability
Materials and Design 87 (2015) 625–631
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Pseudo-quinary Ti20Zr20Hf20Be20(Cu20 -xNix) high entropy bulk metallic glasses with large glass forming ability S.F. Zhao, Y. Shao, X. Liu, N. Chen, H.Y. Ding, K.F. Yao ⁎ School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 5 May 2015 Received in revised form 2 August 2015 Accepted 16 August 2015 Available online 22 August 2015 Keywords: Amorphous alloy Glass-forming ability Mechanical properties Thermodynamic stability
a b s t r a c t A series of pseudo-quinary Ti20Zr20Hf20Be20(Cu20-xNix) (x = 2.5, 5, 7.5, 10, 12.5, 15, 17.5 and 20 at.%) high entropy bulk metallic glasses (HE-BMGs) with large glass forming ability (GFA) were successfully prepared by copper mold tilt-casing. The critical diameters (Dc) of these HE-BMGs are all above 12 mm. In particular, the developed Ti20Zr20Hf20Be20(Cu7.5Ni12.5) and Ti20Zr20Hf20Be20Ni20 high entropy alloys (HEAs) can be produced in the amorphous state with diameters up to 30 mm and 15 mm, respectively, which are the largest HE-BMG and quinary HE-BMG hitherto. The two HE-BMGs also exhibit high yield strength, together with the plastic strain values of (3.0 ± 1.1) % and (4.0 ± 0.9) %, respectively. With increasing Ni additions in the pseudo-quinary HEAs, the crystallization growth resistance and thermal stability have been improved, which is apparently due to the sluggish diffusion of the atoms in the HEAs. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Emerging as advanced materials, both high entropy alloys (HEAs) [1,2] and bulk metallic glasses (BMGs) [3–5] have independently evolved into important research topics in metallic materials science, due to their specific features in alloy design and microstructures. Although they contain at least 5 components in equiatomic or nearequiatomic ratio, HEAs usually show very simple structures such as face-centered cubic (FCC) and/or body-centered cubic (BCC) solid solutions instead of complex intermetallics [1]. Meanwhile, they possess unique properties, including high strength, hardness, and abrasive resistance [6]. On the other hand, BMGs also exhibit superior properties over their crystalline counterparts [3,4] and therefore hold promises for a variety of applications. Given the unique characteristics and excellent properties of the two kinds of materials mentioned above, formation of the HEAs with amorphous structures, which is termed as high entropy bulk metallic glasses (HE-BMGs) [7–10], provides new possibilities in developing alloys with the advantages of both HEAs and BMGs. It is known that the HEAs with 5 or more elements in equiatomic ratio possess higher mixing entropy than the traditional ones, corresponding to a higher degree of confusion. This is supposed to favor glass formation according to the confusion principle [11]. However, it has been found that most of the reported HEAs cannot be prepared in amorphous structure, which indicates that the glass-forming ability (GFA) of these HEAs is quite low. The apparent contradict causes a debate on the correlation between the GFA and mixing entropy. Thus, it is of great importance to explore this ⁎ Corresponding author. E-mail address: [email protected] (K.F. Yao).
http://dx.doi.org/10.1016/j.matdes.2015.08.067 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
question from experimental side and prepare HEAs with large GFA. Until now, several HEAs have been prepared in BMGs. Among the developed HE-BMGs, the Pd20Pt20Cu20Ni20P20 [7] alloy has relatively large GFA in the quinary HEAs with a 10 mm critical diameter of fully amorphous structure. The composition development of Pd20Pt20Cu20Ni20P20 HE-BMG is based on the Pd40Cu30Ni10P20 [12] BMG that has an extremely large GFA and its critical diameter is about 72 mm. Since Pd/Pt and Cu/Ni are the neighboring elements in the periodic table and have similar atomic radius and electronegativity, Pd40Cu30Ni10P20 alloy could be regarded as (PdPt)40(CuNi)40P20 alloy, which yields formation of the quinary Pd20Pt20Cu20Ni20P20 HE-BMG. The result indicates that partially replacing constituent elements with similar atomic radius or electronegativity in an alloy with good GFA is an effective way to search for new HE-BMG systems. In addition, the senary Ti16.7Zr16.7Hf16.7Be16.7Cu16.7Ni16.7 [13] HE-BMG is reported to have the largest critical diameter of 15 mm among all HEBMGs, which indicates that the Ti–Zr–Hf–Be–Cu–Ni alloy system possesses excellent GFA. In particular, the typical Zr41.2Ti13.8Be22.5Cu12.5Ni10 [14] and Ti32.85Zr30.21Be22.66Ni5.28Cu9 [15] alloys have large GFA with critical diameters over 50 mm. From the constituents of the above three alloys, Ti, Zr and Hf are in the same IVB subgroup of the periodic table, which could be regarded as one constituent in some cases. Note that the total content of Ti and Zr in each of the Zr41.2Ti13.8Be22.5Cu12.5Ni10 [14] and Ti32.85Zr30.21Be22.66Ni5.28Cu9 [15] alloy is about 55 at.%–63 at.%, while that of Cu and Ni is about 15 at.%–23 at.%. As a result, the newly (TiZrHf)60Be20(CuNi)20 alloy, namely, Ti20Zr20Hf20Be20(CuNi)20 HEA was developed. The senary Ti–Zr–Hf–Be–Cu–Ni alloy system seems a pseudo-quinary Ti–Zr–Hf–Be–(Cu, Ni) system, in which the effect of alloying Ni on the GFA and the related properties can be studied. Alloying indeed proves to be an effective method in alloy design [16–19], which
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could drastically improve the GFA and promote formation of centimeterscale HE-BMGs. As a result, replacing partial Cu with Ni may be beneficial to improve the GFA. Consequently, the pseudo-quinary HAEs with compositions of Ti20Zr20Hf20Be20(Cu20-xNix) (x = 2.5, 5, 7.5, 10, 12.5, 15, 17.5 and 20 at.%) were developed in the present study. 2. Experimental procedure The alloy ingots of the Ti20Zr20Hf20Be20(Cu20-xNix) (x = 0 [19], 2.5, 5, 7.5, 10, 12.5, 15, 17.5 and 20 at.%) were prepared by arc melting the mixtures of the metals in a Ti-gettered pure argon atmosphere. The purities of Ti, Cu, Ni, Be metals are over 99.9 mass%, and those of Hf and Zr metals are over 99.7 mass%. Each ingot was flipped and re-melted at least four times to ensure the compositional homogeneity. Through re-melting the ingots, the HEA rods were prepared by copper mold tilt-pour casting method. The microstructures of the as-prepared samples were determined by X-ray diffraction (XRD) technique using a Japan Rigaku D/max-RB XRD spectrometry with Cu Kα radiation (λ = 0.15406 nm) and a transmission electron microscope (TEM, JOEL-2011, 200 kV). All the samples for XRD examinations are the cross sections cut from the top of the as-cast HEA rods. The TEM foil sample was prepared by means of a standard twin-jet electro-chemical polishing with an electrolyte of 8% perchloric acid and 92% methanol at − 20 °C. Thermal parameters of the HE-BMGs were investigated by a Shimadzu DSC-60 differential scanning calorimeter (DSC) instrument with a heating rate of 20 K min−1. The values of onset glass transition temperatures Tg, onset temperatures of the crystallization Tx, and liquidus temperatures Tl were examined from the thermal analysis curves. Compression tests of the HE-BMGs were performed on WDW-100 testing machine under a stain rate of 4.2 × 10−4 s−1 at room temperature. The samples were cut out from the as-cast ϕ2 mm rods with gage aspect ratio of 2:1. For the compression tests, at least 3 samples of each glassy alloy were tested. Elastic constants (bulk modulus K, shear modulus G, Young's modulus E and Poisson's ratio υ) of the HE-BMGs were measured by a Quasar RUSpec 4000 ultrasonic resonance spectrometer. 3. Results Fig. 1 shows the XRD spectra of the as-cast Ti 20 Zr20 Hf 20 Be20 (Cu20 -xNix) rods (x = 0 [19], 2.5, 5, 7.5, 10, 12.5, 15, 17.5 and 20 at.%) with their critical diameters for forming fully amorphous structures. No sharp diffraction peak corresponding to crystalline phase but typical broad halo patterns resulted from the amorphous phase
Fig. 1. XRD patterns for the as-cast Ti20Zr20Hf20Be20(CuxNi20-x) rods with their critical diameters.
are observed on the XRD spectra. It indicates that the as-cast Ti20Zr20Hf20Be20(Cu20-xNix) HEAs are all of full amorphous structure. As shown in Fig. 1, the critical diameter of the Ti20Zr20Hf20Be20(Cu17.5Ni2.5) HE-BMG is 12 mm, which is the same as that of the quinary Ti20Zr20Hf20Be20Cu20 HEA [19]. Through replacing Cu with Ni, the critical diameters of the Ti20Zr20Hf20Be20(Cu20-xNix) HE-BMGs increase obviously. As the Ni addition is increased to 12.5 at.%, the critical diameter of the pseudo-quinary Ti20Zr20Hf20Be20(Cu7.5Ni12.5) HE-BMG reaches a maximum value of 30 mm, which is the largest size among the reported HE-BMG hitherto [7–9,13]. However, with further increasing the Ni content, the GFA of the Ti20Zr20Hf20Be20(Cu20-xNix) HEAs becomes lower gradually. As the Cu is completely replaced by Ni (x = 20 at.%), the critical diameter of the quinary Ti20Zr20Hf20Be20Ni20 HE-BMG is 15 mm, which is the largest size in the reported quinary HE-BMGs to date [13]. Fig. 2(a) shows the shape of the as-cast Ti20Zr20Hf20Be20(Cu7.5Ni12.5) rod with 30 mm in diameter and 55 mm in length. The sample exhibits good metallic luster. The microstructure of the Ti20Zr20Hf20Be20(Cu7.5Ni12.5) rod has been examined with a highresolution transmission electron microscope (HRTEM), and no crystalline phase has been observed. A typical HRTEM image is shown in Fig. 2(b), revealing maze-like microstructures, which is the characteristic pattern of amorphous. Except for a typical diffraction halo resulted by amorphous structure, no sharp diffraction ring or diffraction spot could be found in the corresponding selected area electron diffraction (SAED) pattern, as shown in the inset of Fig. 2(b). The HRTEM image and SAED pattern further confirm that the as-cast rod is of full amorphous structure. Fig. 3 shows the DSC curves of the Ti20Zr20Hf20Be20(Cu20-xNix) HEBMGs. All the curves exhibit an endothermic glass transition, followed by three exothermic peaks related to the crystallization, and subsequent endothermic peaks resulted from the melting process. Thermodynamic parameters have been extracted from the DSC curves. The onset glass transition temperatures Tg, onset crystallization temperatures Tx, peak values of the first exothermic peak Tp1, melting temperatures T m and liquidus temperatures T l were marked with arrows in Fig. 2 and listed in Table 1. To evaluate the GFA of the Ti20Zr20Hf20Be20(Cu20 -xNix) alloys, the parameters of the supercooled liquid region ΔT (ΔT = Tx - Tg), γ parameter (γ = Tx/(Tg + Tl)), and reduced glass transition temperature Trg (Trg = Tg/Tl) [16], together with the enthalpy of fusion ΔHm were calculated and listed in Table 1. According to the values in Table 1, it is found that increasing Ni additions will lead to reducing the Tl values of the pseudo-quinary Ti– Zr–Be–Hf–(CuNi) HE-BMGs. As the Ni content is increased to 12.5 at.%, the Ti20Zr20Hf20Be20(Cu7.5Ni12.5) HE-BMG shows a relatively small value of Tg (632 K), Tx (684 K) and Tl (1040 K), resulting in a relatively large value of γ (0.409) and Trg (0.610). This result implies that the Ti20Zr20Hf20Be20(Cu7.5Ni12.5) HE-BMG possesses a relative high GFA among the as-prepared HE-BMGs. Then, the further addition of Ni reduces the values of thermodynamic parameters γ and Trg, which is also in accordance with the experimental results of their critical diameters. Uniaxial compression tests were performed using ϕ2 mm × 4 mm rod samples. The compressive stress–strain curves are shown in Fig. 4. The mechanical properties (yield strength σ0.2, maximum compressive strength σmax and plastic strain ε p) were obtained from the compressive stress–strain curves (see Fig. 4), and elastic constants including bulk modulus K, shear modulus G, Young's modulus E and Poisson's ratio υ of the pseudo-quinary Ti20Zr20Hf20Be20(Cu20 -xNix) HE-BMGs were measured and listed in Table 2. As shown in Table 2, all the HE-BMGs exhibit obvious plastic strains, which promise them potential applications as structural materials. In particular, the values of σ0.2 and σmax increase with increasing Ni content. σ0.2, σmax and εp values of the Ti20Zr20Hf20Be20(Cu7.5Ni12.5) HE-BMG with relatively large GFA were measured as 2067 MPa, 2124 MPa and (3.0 ± 1.1) %, respectively. Especially, the quinary
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Fig. 2. Outer morphology and surface appearance of Ti20Zr20Hf20Be20(Cu7.5Ni12.5) alloy rod with the diameter of 30 mm (a), and a typical high resolution TEM image for the 30 mm rod, the inset is the corresponding selected area electron diffraction pattern (b).
Ti20Zr20Hf20Be20Ni20 HE-BMG exhibits the best mechanical properties among the developed Ti20Zr20Hf20Be20(Cu20-xNix) HE-BMGs, with the σ0.2, σmax and εp values of 2142 MPa, 2276 MPa and (4.0 ± 0.9) %, respectively. Note that the difference in their plasticity is not large. In addition, the “work-hardenable” phenomena at the early stage of plastic deformation are supposed to result from the nanoscale heterogeneities usually present in Ti-based BMGs [20]. Compared with Ti20Zr20Hf20Be20Cu20 HE-BMG, replacing Cu with Ni makes the HEBMGs to have higher values of yield strength. Accompanied by the enhanced strength, partially replacing Cu with Ni firstly reduces the plasticity ad then increases it. Although the strength and plasticity are mutually exclusive particularly in the metal-metalloid BMG alloy systems [21], the mechanical properties are sensitive to their compositions [22,23]. The continuous increase in both strength and plasticity can be also observed in metal-metal type Cu–Zr–Ag BMGs [24]. This may originate from their local structural difference including the structural stability and the short-range-ordered/medium-range-ordered clusters associated with continuous Ni additions. The Ti20Zr20Hf20Be20(Cu7.5Ni12.5) HE-BMG sample exhibits a typical shear fracture morphology (see Fig. 5(a)). Multiple shear bands carrying the plastic deformation are clearly observed on the side view of the fractured sample. As shown in Fig. 5(b), the facture surface is characteristic of vein patterns stemming from the viscous flow during shear fracture, indicating a ductile fracture mode of the HE-BMG.
Fig. 3. DSC curves of the Ti20Zr20Hf20Be20(Cu20-xNix) HE-BMGs.
It has been found that with the increase of Ni content, the Young's modulus E, shear modulus G and bulk modulus K increase slightly, but reach their maximum values at different Ni content, as shown in Table 2. The Poisson's ratio υ of the Ti20Zr20Hf20Be20(Cu20 -xNix) HE-BMGs ranges from 0.349 to 0.355, which is higher than the critical value (0.31–0.32) for a ductile–brittle-transition mode of BMGs [25]. When Ni content is higher than 10 at.%, the Poisson's ratio υ of the BMGs is slightly larger than 0.351. It seems the higher Ni content in the alloy, the larger values of the plastic stain εp and the Poisson's ratio υ the HE-BMGs possess. Formation of amorphous state is a competition process to crystallization during cooling of a liquid. The crystallization behavior of the HE-BMGs is thus important to reveal the origin for their excellent GFA. To explore the resistance to crystallization and the corresponding thermal stability, the Ti20Zr20Hf20Be20(Cu20-xNix) HE-BMGs (x = 0, 5, 10, 15 and 20 at.%) were firstly annealed at the temperature of their respective (Tx − 5) K for 40 min, and then experienced a second annealing at the temperature of their respective (Tp1 + 5) K for 40 min. The structures of the Ti20Zr20Hf20Be20(Cu20-xNix) HE-BMGs after each annealing process were investigated by XRD, and the corresponding XRD spectra are shown in Fig. 6. It was noticed in Fig. 6(a) that in the Ti20Zr20Be20Hf20Cu20 HE-BMG an FCC (face-centered cubic) phase and a BCC (body-centered cubic) phase formed after the first annealing. The crystallization peaks are quite sharp, indicative of almost full devitrification. Difference from that of the Ti20Zr20Be20Hf20Cu20 HE-BMG, only weak peaks appear on the original broad diffraction peaks of the other Ni-containing HEBMGs. With increasing Ni additions, the crystal size reduces. The second-step annealing process greatly promotes the crystallization, as shown in Fig. 6(b). Despite this, the crystallization of other three HEAs with higher Ni additions were not completed yet. The morphology of the Ti20Zr20Hf20Be20(Cu10Ni10) HE-BMG subjected to twice annealing process has been examined through TEM. As shown in Fig. 7(a), some bright dendrite-like areas with size around 100 nm could be observed on the matrix. By examining dendrite-like areas at a higher magnification (shown in Fig. 7(b)), it can be found that those dendrite-like areas in Fig. 7(a) are small holes, which may be due to the crystallites (about 100 nm) that were etched off during the preparation of the TEM samples. Through the SAED from this dendrite-like area (see the inset of Fig. 7(b)), some diffraction spots were noticed, which indicates the precipitation of some crystallized phases around the dendrite-like area. Moreover, in Fig. 7(c), the matrix shown in Fig. 7(a) is consisting of amorphous matrix and small crystalline grains with the size about 5 nm. Small crystalline grains formation inside the amorphous matrix can also be identified from the SAED shown in the inset of
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Table 1 Thermodynamic parameters of the Ti20Zr20Hf20Be20(Cu20 −xNix) HE-BMGs. x
Tg
Tx
Tp1
Tm
Tl
ΔHm
ΔT
(at.%)
(K)
(K)
(K)
(K)
(K)
(J/g-atom)
(K)
0 [19] 2.5 5 7.5 10 12.5 15 17.5 20
630 641 638 644 643 632 644 641 646
708 693 694 692 695 684 696 692 701
719 710 706 703 706 703 711 709 719
935 973 926 925 948 951 962 972 969
1164 1156 1102 1090 1066 1040 1109 1114 1108
8485 5962 7264 7693 7087 6763 7025 6608 7883
78 52 56 48 52 50 52 51 55
Fig. 7(c). Based on the experimental results, the crystallization of the Ti20Zr20Hf20Be20(Cu10Ni10) HE-BMG may experience a two-step process: the precipitation of nano-scale crystallites as shown in Fig. 7(c) and the growth and aggregation of the small crystallites. The finally formed large dendritic crystallites are around 100 nm based on Fig. 7(a) and (b). As a result, the developed HE-BMG exhibits a relatively strong crystallization growth resistance and thermal stability due to the sluggish diffusion of the atoms in the HEAs. 4. Discussion BMGs are usually formed based on one or two major metals with some other elemental additions to improve their GFA [11]. The properties are thus dependent, to a large degree, on the base metals [26,27]. For example, both Ti-based and Zr-based BMGs of quinary Ti–Zr–Cu– Ni–Be system are strong with high yielding strength [14,15]. However, they show no apparent plasticity similar to most other BMGs. HE alloys contain multiple elements in equal or near equal atomic ratio. They exhibit unique properties, i.e., high strength, high hardness and relatively large plasticity, which are determined by all the constituents and their interplay [28,29]. HE-BMGs may sustain the intrinsic properties of the two kinds of materials. Developed from the pseudo-quinary Ti–Zr–Hf– Be–(CuNi) alloy system, the present HE-BMGs indeed exhibit high strength and enhanced plasticity (Fig. 4). In addition, HEAs have high thermal stability due to their high melting temperature and maximum configurational entropy. Correspondingly, the HE-BMGs developed in the present study exhibit very slow crystallization kinetics, indicative of high thermal stability compared with the other Ti-based BMGs [15, 30–32]. In the case of Ti20Zr20Hf20Be20(Cu10Ni10) HE-BMG, the 5 nmsized ultrafine nanocrystals are observed in the amorphous matrix even after a two-step annealing for 40 min of each treatment (Fig. 7). Whereas the devitrification of a regular Ti-based BMG is normally
γ
Trg
0.395 0.386 0.388 0.399 0.407 0.409 0.397 0.394 0.400
0.541 0.554 0.579 0.591 0.603 0.608 0.581 0.575 0.583
Dc (mm) 12 12 15 20 25 30 20 15 15
completed within 15 min and the crystal size is much bigger under the similar annealing condition [30,31]. It is suggested that the developed HE-BMG exhibits a strong resistance to growth of crystalline phases due to the sluggish atomic diffusion associated with the “confusion principle” in the HEAs [11]. Such thermodynamics stability of the HE-BMG is believed to be one advantage over the conventional BMGs. This indicates that using HE-BMGs as precursors may offer a new way to prepare nanocomposites with unique properties. Meanwhile, the high thermal stability of these HE-BMGs can be utilized for their thermoplastic forming of small sized parts in Micro-electro-Mechanical System (MEMS) devices [33]. In MEMS devices, the integration of optical, electrical and magnetic components requires the joining of BMGs with the other materials [34]. Therefore, the nice resistance to crystallization is also advantageous in applications of the HE-BMGs in MEMS devices. In Ti20Zr20Hf20Be20(Cu20-xNix) alloy system, HE-BMGs with excellent GFA are successfully prepared with critical diameters all over 12 mm. To evaluate and investigate the origin of the GFA from the as-prepared HE-BMGs, five different parameters are usually implemented. The five parameters are the atomic size difference δ, electronegativity difference Δx [35,36], enthalpy of mixing ΔHmix [37], entropy of mixing ΔSmix and the entropy of mismatch normalized with Boltzmann constant Sσ/kB [38,39]. To evaluate the GFA, the five parameters mentioned above were calculated and listed in Table 3. For convenience, all the values of δ are amplified by 100 times. Here, all the values of the atomic radii are taken from Guo's work [37]. From Table 3, it can be found that the δ and Δx increase gradually as Ni addition increases. The parameter δ and Δx reflect the atomic size discrepancy and the electronegativity difference among the constituent elements in the alloy system. According to the three empirical rules summarized by Inoue [40], alloys with large values of δ and Δx could have large GFA, which is also in accordance with the rules proposed by Y. Zhang et al. [41] and C.T. Liu et al. [37,42]. Furthermore, due to the sufficient atomic-level stress, large value of δ could lead to the topological destabilization of the solid solution phases, thus stabilize the amorphous phases [43–45], but if the values of δ and Δx are too large, solid solution phases or intermetallics would form. The parameter ΔHmix shows the weight averaged enthalpy of mixing of arbitrary two elements of the alloy system, that is, the overall Table 2 Mechanical properties and elastic constants of the Ti20Zr20Hf20Be20(Cu20−xNix) HE-BMGs.
Fig. 4. Compressive stress-strain curves for the Ti20Zr20Hf20Be20(Cu20-xNix) HE-BMGs.
x
σ0.2
σmax
εp
K
G
E
(at.%)
(MPa)
(MPa)
(%)
(GPa)
(GPa)
(GPa)
0 [19] 2.5 5 7.5 10 12.5 15 17.5 20
1889 1943 1992 2005 2019 2067 2088 2094 2142
1995 2001 2012 2047 2101 2124 2143 2226 2276
2.3 ± 0.4 0.7 ± 0.3 0.6 ± 0.4 1.6 ± 0.3 1.5 ± 0.4 3.0 ± 1.1 1.7 ± 0.8 2.7 ± 1.0 4.0 ± 0.9
106.9 113.3 113.5 117.8 116.7 120.2 119.9 118.8 118.9
36.1 37.5 38.1 39.4 38.6 38.7 38.4 39.1 38.7
97.3 101.3 102.8 106.3 104.6 104.9 104.0 105.6 104.7
υ
0.348 0.351 0.349 0.350 0.351 0.354 0.355 0.352 0.353
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Fig. 5. SEM images of lateral and fracture surfaces of the deformed Ti20Zr20Hf20Be20(Cu7.5Ni12.5) HE-BMG sample.
chemical attraction in the alloy system. A negative ΔHmix is beneficial for the formation of chemically short-range ordering atomic clusters, which hinders interatomic diffusion in a long range and favors glass formation [46,47]. In the present study, the ΔHmix values of the Ti20Zr20Hf20Be20(Cu20 -xNix) HE-BMGs are more and more negative with the increase of the Ni content. This implies that the GFA of as-prepared HEAs is improved as the increased Ni addition. However, if the value of ΔHmix is too negative, intermetallic phases would form instead of the amorphous phase. Thus, the optimum composition of Ni addition at 12.5 at.% with highest GFA is achieved in the Ti20Zr20Hf20Be20(Cu20-xNix) HE-BMG, and possesses a critical diameter up to 30 mm. The parameter ΔSmix reflects the complexity of an alloy system. As is shown in Table 3, the ΔSmix values increase from 13.38 J/(mol K) (x = 0 at.%) to 14.53 J/(mol K) (x = 10 at.%), and then decrease to 13.38 J/(mol K) (x = 20 at.%). According to the Gibbs free energy formula (ΔGmix = ΔHmix - TΔSmix), a high value of ΔSmix might lower the Gibbs free energy of the solid solution phases at elevated temperatures. Thus alloys with large entropy of mixing usually show large GFA. The change of the ΔSmix values in the Ti20Zr20Hf20Be20(Cu20 -xNix) HEBMGs with different Ni contents is according with the experimental results of critical diameters. However, ΔSmix is only associated with the number and composition of the alloys, but unrelated to the kinds of the constituents that may also affect the entropy change. Whereas, the parameter Sσ/kB is resulting from the mismatch of atomic size, which can more properly represent the change of the entropy caused by different constituents and their concentrations. A large Sσ/kB value generally indicates large atomic size difference in an alloy system, which has a beneficial influence on the GFA. The Sσ/kB values are listed
in Table 3, and their change tendency with Ni content is also consistent with the GFA evaluated from experimental results of the critical diameters. For BMGs, a high stability of their supercooled liquids generally favors a better GFA. The difference of Gibbs free energy (ΔG) [48] between crystals and a liquid, which provides the driving force for nucleation and growth of crystalline phases, is a key parameter to determin the GFA of an alloy system. For an alloy system with a lower value of ΔG throughout their supercooled liquid temperature region, the atoms are easier to pack into a disorder structure, which is inherited from their high temperature liquid structure. For such situation, the supercooled liquid can have a large undercooling temperature region before it finally transforms into a glass, and ordering phases are difficult to form. The ΔG values of Ti20Zr20Hf20Be20(Cu20 -x Nix) (x = 0 [19], 5, 10, 12.5, 15 and 20 at.%) HE-GMGs were calculated from the DSC data (listed in Table 1) and were shown in Fig. 8. It is found that the tendency of ΔG value of Ti20Zr20Hf20Be20(Cu20 -xNix) system is excellently consistent with GFA estimated from the experimental results of the critical diameters, i.e., the lower the ΔG value is, the better the GFA will be. The best GFA with Ni content of 12.5 at.% has the lowest ΔG among all other compositions throughout the whole temperature range we studied. According to the discussion above on the generally used five parameters and ΔG on the GFA, the pseudo-quinary Ti20Zr20Hf20Be20 (Cu20 -xNix) with x = 12.5 at.% has the largest GFA among the Ti20Zr20Hf20Be20(Cu20-xNix) HE-BMGs. The origin of the better GFA for the x = 12.5 at.% sample, can be attributed to an optimum combination of relatively large values of atomic size difference δ, electronegativity difference Δx and entropy of mixing ΔSmix, entropy of mismatch Sσ/kB,
Fig. 6. XRD spectra of the annealed Ti20Zr20Be20Hf20(Cu20-xNix) (x = 0, 5, 10, 15 and 20 at.%) HE-BMGs at the temperatures of their respective (Tx − 5) K for 40 min (a) and at the temperatures of their respective (Tp1 + 5) K for 40 min again (b).
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Fig. 7. The images of Ti20Zr20Hf20Be20(Cu10Ni10) HE-BMG after the second annealing (a), a typical high resolution TEM image for the white area in red frame with the inset of the corresponding SAED, and a typical high resolution TEM image for the area in blue frame with the inset of the corresponding SAED (c). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
a proper negative enthalpy of mixing ΔHmix value and relatively small difference of Gibbs free energies ΔG. 5. Conclusion Pseudo-quinary Ti20Zr20Hf20Be 20(Cu20 -x Nix ) (x = 0.25, 5, 7.5, 10, 12.5, 15, 17.5 and 20 at.%) HEAs possess excellent GFA. Among them, the critical diameters of the pseudo-quinary Ti 20 Zr 20 Hf 20 Be 20 (Cu 7.5 Ni 12.5 ) and quinary Ti 20 Zr 20 Hf 20 Be 20 Ni 20 HEAs are as large as 30 mm and 15 mm, respectively, which are the largest size ever in the HE-BMGs and the largest size in the quinary HE-BMGs hitherto. Moreover, the two Ti20Zr20Hf20Be20(Cu20 -xNix)
HEAs exhibit high yield strength and show relatively large plastic strain values of (3.0 ± 1.1) % and (4.0 ± 0.9) %, respectively. The experimental results indicate that Ni is a beneficial element for enhancing the thermal stability and the GFA of the Ti20Zr20Hf20Be20(Cu20-xNix) HE-BMGs. The origins of their excellent GFA are discussed from the aspect of atomic size difference δ, electronegativity difference Δx, entropy of mixing
Table 3 The atomic size difference δ, electronegativity difference Δx, enthalpy of mixing ΔH mix , entropy of mixing ΔS mix and the entropy of mismatch S σ /k B values of the Ti20Zr20Hf20Be20(Cu20−xNix) HEAs. x
δ (×100)
Δx
(at.%) 0 [19] 2.5 5 7.5 10 12.5 15 17.5 20
12.89 12.94 13.00 13.05 13.10 13.15 13.20 13.26 13.31
0.22 0.38 0.42 0.46 0.50 0.54 0.59 0.63 0.68
ΔHmix
ΔSmix
(kJ/mol)
(J/(mol K))
−25.44 −26.99 −28.56 −30.15 −31.76 −33.39 −35.04 −36.71 −38.40
13.38 14.01 14.32 14.48 14.53 14.48 14.32 14.01 13.38
Sσ/kB
Dc (mm)
0.341 0.344 0.347 0.350 0.352 0.355 0.358 0.361 0.364
12 12 15 20 25 30 20 15 15
Fig. 8. The difference of Gibbs free energies of the typical Ti20Zr20Hf20Be20(Cu20-xNix) (x = 0, 5, 10, 12.5, 15 and 20 at.%) HE-BMGs for crystallization in the supercooled state below Tm.
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Pseudo-quinary Ti20Zr20Hf20Be20(Cu20 -xNix) high entropy bulk metallic glasses with large glass forming ability S.F. Zhao, Y. Shao, X. Liu, N. Chen, H.Y. Ding, K.F. Yao ⁎ School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 5 May 2015 Received in revised form 2 August 2015 Accepted 16 August 2015 Available online 22 August 2015 Keywords: Amorphous alloy Glass-forming ability Mechanical properties Thermodynamic stability
a b s t r a c t A series of pseudo-quinary Ti20Zr20Hf20Be20(Cu20-xNix) (x = 2.5, 5, 7.5, 10, 12.5, 15, 17.5 and 20 at.%) high entropy bulk metallic glasses (HE-BMGs) with large glass forming ability (GFA) were successfully prepared by copper mold tilt-casing. The critical diameters (Dc) of these HE-BMGs are all above 12 mm. In particular, the developed Ti20Zr20Hf20Be20(Cu7.5Ni12.5) and Ti20Zr20Hf20Be20Ni20 high entropy alloys (HEAs) can be produced in the amorphous state with diameters up to 30 mm and 15 mm, respectively, which are the largest HE-BMG and quinary HE-BMG hitherto. The two HE-BMGs also exhibit high yield strength, together with the plastic strain values of (3.0 ± 1.1) % and (4.0 ± 0.9) %, respectively. With increasing Ni additions in the pseudo-quinary HEAs, the crystallization growth resistance and thermal stability have been improved, which is apparently due to the sluggish diffusion of the atoms in the HEAs. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Emerging as advanced materials, both high entropy alloys (HEAs) [1,2] and bulk metallic glasses (BMGs) [3–5] have independently evolved into important research topics in metallic materials science, due to their specific features in alloy design and microstructures. Although they contain at least 5 components in equiatomic or nearequiatomic ratio, HEAs usually show very simple structures such as face-centered cubic (FCC) and/or body-centered cubic (BCC) solid solutions instead of complex intermetallics [1]. Meanwhile, they possess unique properties, including high strength, hardness, and abrasive resistance [6]. On the other hand, BMGs also exhibit superior properties over their crystalline counterparts [3,4] and therefore hold promises for a variety of applications. Given the unique characteristics and excellent properties of the two kinds of materials mentioned above, formation of the HEAs with amorphous structures, which is termed as high entropy bulk metallic glasses (HE-BMGs) [7–10], provides new possibilities in developing alloys with the advantages of both HEAs and BMGs. It is known that the HEAs with 5 or more elements in equiatomic ratio possess higher mixing entropy than the traditional ones, corresponding to a higher degree of confusion. This is supposed to favor glass formation according to the confusion principle [11]. However, it has been found that most of the reported HEAs cannot be prepared in amorphous structure, which indicates that the glass-forming ability (GFA) of these HEAs is quite low. The apparent contradict causes a debate on the correlation between the GFA and mixing entropy. Thus, it is of great importance to explore this ⁎ Corresponding author. E-mail address: [email protected] (K.F. Yao).
http://dx.doi.org/10.1016/j.matdes.2015.08.067 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
question from experimental side and prepare HEAs with large GFA. Until now, several HEAs have been prepared in BMGs. Among the developed HE-BMGs, the Pd20Pt20Cu20Ni20P20 [7] alloy has relatively large GFA in the quinary HEAs with a 10 mm critical diameter of fully amorphous structure. The composition development of Pd20Pt20Cu20Ni20P20 HE-BMG is based on the Pd40Cu30Ni10P20 [12] BMG that has an extremely large GFA and its critical diameter is about 72 mm. Since Pd/Pt and Cu/Ni are the neighboring elements in the periodic table and have similar atomic radius and electronegativity, Pd40Cu30Ni10P20 alloy could be regarded as (PdPt)40(CuNi)40P20 alloy, which yields formation of the quinary Pd20Pt20Cu20Ni20P20 HE-BMG. The result indicates that partially replacing constituent elements with similar atomic radius or electronegativity in an alloy with good GFA is an effective way to search for new HE-BMG systems. In addition, the senary Ti16.7Zr16.7Hf16.7Be16.7Cu16.7Ni16.7 [13] HE-BMG is reported to have the largest critical diameter of 15 mm among all HEBMGs, which indicates that the Ti–Zr–Hf–Be–Cu–Ni alloy system possesses excellent GFA. In particular, the typical Zr41.2Ti13.8Be22.5Cu12.5Ni10 [14] and Ti32.85Zr30.21Be22.66Ni5.28Cu9 [15] alloys have large GFA with critical diameters over 50 mm. From the constituents of the above three alloys, Ti, Zr and Hf are in the same IVB subgroup of the periodic table, which could be regarded as one constituent in some cases. Note that the total content of Ti and Zr in each of the Zr41.2Ti13.8Be22.5Cu12.5Ni10 [14] and Ti32.85Zr30.21Be22.66Ni5.28Cu9 [15] alloy is about 55 at.%–63 at.%, while that of Cu and Ni is about 15 at.%–23 at.%. As a result, the newly (TiZrHf)60Be20(CuNi)20 alloy, namely, Ti20Zr20Hf20Be20(CuNi)20 HEA was developed. The senary Ti–Zr–Hf–Be–Cu–Ni alloy system seems a pseudo-quinary Ti–Zr–Hf–Be–(Cu, Ni) system, in which the effect of alloying Ni on the GFA and the related properties can be studied. Alloying indeed proves to be an effective method in alloy design [16–19], which
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could drastically improve the GFA and promote formation of centimeterscale HE-BMGs. As a result, replacing partial Cu with Ni may be beneficial to improve the GFA. Consequently, the pseudo-quinary HAEs with compositions of Ti20Zr20Hf20Be20(Cu20-xNix) (x = 2.5, 5, 7.5, 10, 12.5, 15, 17.5 and 20 at.%) were developed in the present study. 2. Experimental procedure The alloy ingots of the Ti20Zr20Hf20Be20(Cu20-xNix) (x = 0 [19], 2.5, 5, 7.5, 10, 12.5, 15, 17.5 and 20 at.%) were prepared by arc melting the mixtures of the metals in a Ti-gettered pure argon atmosphere. The purities of Ti, Cu, Ni, Be metals are over 99.9 mass%, and those of Hf and Zr metals are over 99.7 mass%. Each ingot was flipped and re-melted at least four times to ensure the compositional homogeneity. Through re-melting the ingots, the HEA rods were prepared by copper mold tilt-pour casting method. The microstructures of the as-prepared samples were determined by X-ray diffraction (XRD) technique using a Japan Rigaku D/max-RB XRD spectrometry with Cu Kα radiation (λ = 0.15406 nm) and a transmission electron microscope (TEM, JOEL-2011, 200 kV). All the samples for XRD examinations are the cross sections cut from the top of the as-cast HEA rods. The TEM foil sample was prepared by means of a standard twin-jet electro-chemical polishing with an electrolyte of 8% perchloric acid and 92% methanol at − 20 °C. Thermal parameters of the HE-BMGs were investigated by a Shimadzu DSC-60 differential scanning calorimeter (DSC) instrument with a heating rate of 20 K min−1. The values of onset glass transition temperatures Tg, onset temperatures of the crystallization Tx, and liquidus temperatures Tl were examined from the thermal analysis curves. Compression tests of the HE-BMGs were performed on WDW-100 testing machine under a stain rate of 4.2 × 10−4 s−1 at room temperature. The samples were cut out from the as-cast ϕ2 mm rods with gage aspect ratio of 2:1. For the compression tests, at least 3 samples of each glassy alloy were tested. Elastic constants (bulk modulus K, shear modulus G, Young's modulus E and Poisson's ratio υ) of the HE-BMGs were measured by a Quasar RUSpec 4000 ultrasonic resonance spectrometer. 3. Results Fig. 1 shows the XRD spectra of the as-cast Ti 20 Zr20 Hf 20 Be20 (Cu20 -xNix) rods (x = 0 [19], 2.5, 5, 7.5, 10, 12.5, 15, 17.5 and 20 at.%) with their critical diameters for forming fully amorphous structures. No sharp diffraction peak corresponding to crystalline phase but typical broad halo patterns resulted from the amorphous phase
Fig. 1. XRD patterns for the as-cast Ti20Zr20Hf20Be20(CuxNi20-x) rods with their critical diameters.
are observed on the XRD spectra. It indicates that the as-cast Ti20Zr20Hf20Be20(Cu20-xNix) HEAs are all of full amorphous structure. As shown in Fig. 1, the critical diameter of the Ti20Zr20Hf20Be20(Cu17.5Ni2.5) HE-BMG is 12 mm, which is the same as that of the quinary Ti20Zr20Hf20Be20Cu20 HEA [19]. Through replacing Cu with Ni, the critical diameters of the Ti20Zr20Hf20Be20(Cu20-xNix) HE-BMGs increase obviously. As the Ni addition is increased to 12.5 at.%, the critical diameter of the pseudo-quinary Ti20Zr20Hf20Be20(Cu7.5Ni12.5) HE-BMG reaches a maximum value of 30 mm, which is the largest size among the reported HE-BMG hitherto [7–9,13]. However, with further increasing the Ni content, the GFA of the Ti20Zr20Hf20Be20(Cu20-xNix) HEAs becomes lower gradually. As the Cu is completely replaced by Ni (x = 20 at.%), the critical diameter of the quinary Ti20Zr20Hf20Be20Ni20 HE-BMG is 15 mm, which is the largest size in the reported quinary HE-BMGs to date [13]. Fig. 2(a) shows the shape of the as-cast Ti20Zr20Hf20Be20(Cu7.5Ni12.5) rod with 30 mm in diameter and 55 mm in length. The sample exhibits good metallic luster. The microstructure of the Ti20Zr20Hf20Be20(Cu7.5Ni12.5) rod has been examined with a highresolution transmission electron microscope (HRTEM), and no crystalline phase has been observed. A typical HRTEM image is shown in Fig. 2(b), revealing maze-like microstructures, which is the characteristic pattern of amorphous. Except for a typical diffraction halo resulted by amorphous structure, no sharp diffraction ring or diffraction spot could be found in the corresponding selected area electron diffraction (SAED) pattern, as shown in the inset of Fig. 2(b). The HRTEM image and SAED pattern further confirm that the as-cast rod is of full amorphous structure. Fig. 3 shows the DSC curves of the Ti20Zr20Hf20Be20(Cu20-xNix) HEBMGs. All the curves exhibit an endothermic glass transition, followed by three exothermic peaks related to the crystallization, and subsequent endothermic peaks resulted from the melting process. Thermodynamic parameters have been extracted from the DSC curves. The onset glass transition temperatures Tg, onset crystallization temperatures Tx, peak values of the first exothermic peak Tp1, melting temperatures T m and liquidus temperatures T l were marked with arrows in Fig. 2 and listed in Table 1. To evaluate the GFA of the Ti20Zr20Hf20Be20(Cu20 -xNix) alloys, the parameters of the supercooled liquid region ΔT (ΔT = Tx - Tg), γ parameter (γ = Tx/(Tg + Tl)), and reduced glass transition temperature Trg (Trg = Tg/Tl) [16], together with the enthalpy of fusion ΔHm were calculated and listed in Table 1. According to the values in Table 1, it is found that increasing Ni additions will lead to reducing the Tl values of the pseudo-quinary Ti– Zr–Be–Hf–(CuNi) HE-BMGs. As the Ni content is increased to 12.5 at.%, the Ti20Zr20Hf20Be20(Cu7.5Ni12.5) HE-BMG shows a relatively small value of Tg (632 K), Tx (684 K) and Tl (1040 K), resulting in a relatively large value of γ (0.409) and Trg (0.610). This result implies that the Ti20Zr20Hf20Be20(Cu7.5Ni12.5) HE-BMG possesses a relative high GFA among the as-prepared HE-BMGs. Then, the further addition of Ni reduces the values of thermodynamic parameters γ and Trg, which is also in accordance with the experimental results of their critical diameters. Uniaxial compression tests were performed using ϕ2 mm × 4 mm rod samples. The compressive stress–strain curves are shown in Fig. 4. The mechanical properties (yield strength σ0.2, maximum compressive strength σmax and plastic strain ε p) were obtained from the compressive stress–strain curves (see Fig. 4), and elastic constants including bulk modulus K, shear modulus G, Young's modulus E and Poisson's ratio υ of the pseudo-quinary Ti20Zr20Hf20Be20(Cu20 -xNix) HE-BMGs were measured and listed in Table 2. As shown in Table 2, all the HE-BMGs exhibit obvious plastic strains, which promise them potential applications as structural materials. In particular, the values of σ0.2 and σmax increase with increasing Ni content. σ0.2, σmax and εp values of the Ti20Zr20Hf20Be20(Cu7.5Ni12.5) HE-BMG with relatively large GFA were measured as 2067 MPa, 2124 MPa and (3.0 ± 1.1) %, respectively. Especially, the quinary
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Fig. 2. Outer morphology and surface appearance of Ti20Zr20Hf20Be20(Cu7.5Ni12.5) alloy rod with the diameter of 30 mm (a), and a typical high resolution TEM image for the 30 mm rod, the inset is the corresponding selected area electron diffraction pattern (b).
Ti20Zr20Hf20Be20Ni20 HE-BMG exhibits the best mechanical properties among the developed Ti20Zr20Hf20Be20(Cu20-xNix) HE-BMGs, with the σ0.2, σmax and εp values of 2142 MPa, 2276 MPa and (4.0 ± 0.9) %, respectively. Note that the difference in their plasticity is not large. In addition, the “work-hardenable” phenomena at the early stage of plastic deformation are supposed to result from the nanoscale heterogeneities usually present in Ti-based BMGs [20]. Compared with Ti20Zr20Hf20Be20Cu20 HE-BMG, replacing Cu with Ni makes the HEBMGs to have higher values of yield strength. Accompanied by the enhanced strength, partially replacing Cu with Ni firstly reduces the plasticity ad then increases it. Although the strength and plasticity are mutually exclusive particularly in the metal-metalloid BMG alloy systems [21], the mechanical properties are sensitive to their compositions [22,23]. The continuous increase in both strength and plasticity can be also observed in metal-metal type Cu–Zr–Ag BMGs [24]. This may originate from their local structural difference including the structural stability and the short-range-ordered/medium-range-ordered clusters associated with continuous Ni additions. The Ti20Zr20Hf20Be20(Cu7.5Ni12.5) HE-BMG sample exhibits a typical shear fracture morphology (see Fig. 5(a)). Multiple shear bands carrying the plastic deformation are clearly observed on the side view of the fractured sample. As shown in Fig. 5(b), the facture surface is characteristic of vein patterns stemming from the viscous flow during shear fracture, indicating a ductile fracture mode of the HE-BMG.
Fig. 3. DSC curves of the Ti20Zr20Hf20Be20(Cu20-xNix) HE-BMGs.
It has been found that with the increase of Ni content, the Young's modulus E, shear modulus G and bulk modulus K increase slightly, but reach their maximum values at different Ni content, as shown in Table 2. The Poisson's ratio υ of the Ti20Zr20Hf20Be20(Cu20 -xNix) HE-BMGs ranges from 0.349 to 0.355, which is higher than the critical value (0.31–0.32) for a ductile–brittle-transition mode of BMGs [25]. When Ni content is higher than 10 at.%, the Poisson's ratio υ of the BMGs is slightly larger than 0.351. It seems the higher Ni content in the alloy, the larger values of the plastic stain εp and the Poisson's ratio υ the HE-BMGs possess. Formation of amorphous state is a competition process to crystallization during cooling of a liquid. The crystallization behavior of the HE-BMGs is thus important to reveal the origin for their excellent GFA. To explore the resistance to crystallization and the corresponding thermal stability, the Ti20Zr20Hf20Be20(Cu20-xNix) HE-BMGs (x = 0, 5, 10, 15 and 20 at.%) were firstly annealed at the temperature of their respective (Tx − 5) K for 40 min, and then experienced a second annealing at the temperature of their respective (Tp1 + 5) K for 40 min. The structures of the Ti20Zr20Hf20Be20(Cu20-xNix) HE-BMGs after each annealing process were investigated by XRD, and the corresponding XRD spectra are shown in Fig. 6. It was noticed in Fig. 6(a) that in the Ti20Zr20Be20Hf20Cu20 HE-BMG an FCC (face-centered cubic) phase and a BCC (body-centered cubic) phase formed after the first annealing. The crystallization peaks are quite sharp, indicative of almost full devitrification. Difference from that of the Ti20Zr20Be20Hf20Cu20 HE-BMG, only weak peaks appear on the original broad diffraction peaks of the other Ni-containing HEBMGs. With increasing Ni additions, the crystal size reduces. The second-step annealing process greatly promotes the crystallization, as shown in Fig. 6(b). Despite this, the crystallization of other three HEAs with higher Ni additions were not completed yet. The morphology of the Ti20Zr20Hf20Be20(Cu10Ni10) HE-BMG subjected to twice annealing process has been examined through TEM. As shown in Fig. 7(a), some bright dendrite-like areas with size around 100 nm could be observed on the matrix. By examining dendrite-like areas at a higher magnification (shown in Fig. 7(b)), it can be found that those dendrite-like areas in Fig. 7(a) are small holes, which may be due to the crystallites (about 100 nm) that were etched off during the preparation of the TEM samples. Through the SAED from this dendrite-like area (see the inset of Fig. 7(b)), some diffraction spots were noticed, which indicates the precipitation of some crystallized phases around the dendrite-like area. Moreover, in Fig. 7(c), the matrix shown in Fig. 7(a) is consisting of amorphous matrix and small crystalline grains with the size about 5 nm. Small crystalline grains formation inside the amorphous matrix can also be identified from the SAED shown in the inset of
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Table 1 Thermodynamic parameters of the Ti20Zr20Hf20Be20(Cu20 −xNix) HE-BMGs. x
Tg
Tx
Tp1
Tm
Tl
ΔHm
ΔT
(at.%)
(K)
(K)
(K)
(K)
(K)
(J/g-atom)
(K)
0 [19] 2.5 5 7.5 10 12.5 15 17.5 20
630 641 638 644 643 632 644 641 646
708 693 694 692 695 684 696 692 701
719 710 706 703 706 703 711 709 719
935 973 926 925 948 951 962 972 969
1164 1156 1102 1090 1066 1040 1109 1114 1108
8485 5962 7264 7693 7087 6763 7025 6608 7883
78 52 56 48 52 50 52 51 55
Fig. 7(c). Based on the experimental results, the crystallization of the Ti20Zr20Hf20Be20(Cu10Ni10) HE-BMG may experience a two-step process: the precipitation of nano-scale crystallites as shown in Fig. 7(c) and the growth and aggregation of the small crystallites. The finally formed large dendritic crystallites are around 100 nm based on Fig. 7(a) and (b). As a result, the developed HE-BMG exhibits a relatively strong crystallization growth resistance and thermal stability due to the sluggish diffusion of the atoms in the HEAs. 4. Discussion BMGs are usually formed based on one or two major metals with some other elemental additions to improve their GFA [11]. The properties are thus dependent, to a large degree, on the base metals [26,27]. For example, both Ti-based and Zr-based BMGs of quinary Ti–Zr–Cu– Ni–Be system are strong with high yielding strength [14,15]. However, they show no apparent plasticity similar to most other BMGs. HE alloys contain multiple elements in equal or near equal atomic ratio. They exhibit unique properties, i.e., high strength, high hardness and relatively large plasticity, which are determined by all the constituents and their interplay [28,29]. HE-BMGs may sustain the intrinsic properties of the two kinds of materials. Developed from the pseudo-quinary Ti–Zr–Hf– Be–(CuNi) alloy system, the present HE-BMGs indeed exhibit high strength and enhanced plasticity (Fig. 4). In addition, HEAs have high thermal stability due to their high melting temperature and maximum configurational entropy. Correspondingly, the HE-BMGs developed in the present study exhibit very slow crystallization kinetics, indicative of high thermal stability compared with the other Ti-based BMGs [15, 30–32]. In the case of Ti20Zr20Hf20Be20(Cu10Ni10) HE-BMG, the 5 nmsized ultrafine nanocrystals are observed in the amorphous matrix even after a two-step annealing for 40 min of each treatment (Fig. 7). Whereas the devitrification of a regular Ti-based BMG is normally
γ
Trg
0.395 0.386 0.388 0.399 0.407 0.409 0.397 0.394 0.400
0.541 0.554 0.579 0.591 0.603 0.608 0.581 0.575 0.583
Dc (mm) 12 12 15 20 25 30 20 15 15
completed within 15 min and the crystal size is much bigger under the similar annealing condition [30,31]. It is suggested that the developed HE-BMG exhibits a strong resistance to growth of crystalline phases due to the sluggish atomic diffusion associated with the “confusion principle” in the HEAs [11]. Such thermodynamics stability of the HE-BMG is believed to be one advantage over the conventional BMGs. This indicates that using HE-BMGs as precursors may offer a new way to prepare nanocomposites with unique properties. Meanwhile, the high thermal stability of these HE-BMGs can be utilized for their thermoplastic forming of small sized parts in Micro-electro-Mechanical System (MEMS) devices [33]. In MEMS devices, the integration of optical, electrical and magnetic components requires the joining of BMGs with the other materials [34]. Therefore, the nice resistance to crystallization is also advantageous in applications of the HE-BMGs in MEMS devices. In Ti20Zr20Hf20Be20(Cu20-xNix) alloy system, HE-BMGs with excellent GFA are successfully prepared with critical diameters all over 12 mm. To evaluate and investigate the origin of the GFA from the as-prepared HE-BMGs, five different parameters are usually implemented. The five parameters are the atomic size difference δ, electronegativity difference Δx [35,36], enthalpy of mixing ΔHmix [37], entropy of mixing ΔSmix and the entropy of mismatch normalized with Boltzmann constant Sσ/kB [38,39]. To evaluate the GFA, the five parameters mentioned above were calculated and listed in Table 3. For convenience, all the values of δ are amplified by 100 times. Here, all the values of the atomic radii are taken from Guo's work [37]. From Table 3, it can be found that the δ and Δx increase gradually as Ni addition increases. The parameter δ and Δx reflect the atomic size discrepancy and the electronegativity difference among the constituent elements in the alloy system. According to the three empirical rules summarized by Inoue [40], alloys with large values of δ and Δx could have large GFA, which is also in accordance with the rules proposed by Y. Zhang et al. [41] and C.T. Liu et al. [37,42]. Furthermore, due to the sufficient atomic-level stress, large value of δ could lead to the topological destabilization of the solid solution phases, thus stabilize the amorphous phases [43–45], but if the values of δ and Δx are too large, solid solution phases or intermetallics would form. The parameter ΔHmix shows the weight averaged enthalpy of mixing of arbitrary two elements of the alloy system, that is, the overall Table 2 Mechanical properties and elastic constants of the Ti20Zr20Hf20Be20(Cu20−xNix) HE-BMGs.
Fig. 4. Compressive stress-strain curves for the Ti20Zr20Hf20Be20(Cu20-xNix) HE-BMGs.
x
σ0.2
σmax
εp
K
G
E
(at.%)
(MPa)
(MPa)
(%)
(GPa)
(GPa)
(GPa)
0 [19] 2.5 5 7.5 10 12.5 15 17.5 20
1889 1943 1992 2005 2019 2067 2088 2094 2142
1995 2001 2012 2047 2101 2124 2143 2226 2276
2.3 ± 0.4 0.7 ± 0.3 0.6 ± 0.4 1.6 ± 0.3 1.5 ± 0.4 3.0 ± 1.1 1.7 ± 0.8 2.7 ± 1.0 4.0 ± 0.9
106.9 113.3 113.5 117.8 116.7 120.2 119.9 118.8 118.9
36.1 37.5 38.1 39.4 38.6 38.7 38.4 39.1 38.7
97.3 101.3 102.8 106.3 104.6 104.9 104.0 105.6 104.7
υ
0.348 0.351 0.349 0.350 0.351 0.354 0.355 0.352 0.353
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Fig. 5. SEM images of lateral and fracture surfaces of the deformed Ti20Zr20Hf20Be20(Cu7.5Ni12.5) HE-BMG sample.
chemical attraction in the alloy system. A negative ΔHmix is beneficial for the formation of chemically short-range ordering atomic clusters, which hinders interatomic diffusion in a long range and favors glass formation [46,47]. In the present study, the ΔHmix values of the Ti20Zr20Hf20Be20(Cu20 -xNix) HE-BMGs are more and more negative with the increase of the Ni content. This implies that the GFA of as-prepared HEAs is improved as the increased Ni addition. However, if the value of ΔHmix is too negative, intermetallic phases would form instead of the amorphous phase. Thus, the optimum composition of Ni addition at 12.5 at.% with highest GFA is achieved in the Ti20Zr20Hf20Be20(Cu20-xNix) HE-BMG, and possesses a critical diameter up to 30 mm. The parameter ΔSmix reflects the complexity of an alloy system. As is shown in Table 3, the ΔSmix values increase from 13.38 J/(mol K) (x = 0 at.%) to 14.53 J/(mol K) (x = 10 at.%), and then decrease to 13.38 J/(mol K) (x = 20 at.%). According to the Gibbs free energy formula (ΔGmix = ΔHmix - TΔSmix), a high value of ΔSmix might lower the Gibbs free energy of the solid solution phases at elevated temperatures. Thus alloys with large entropy of mixing usually show large GFA. The change of the ΔSmix values in the Ti20Zr20Hf20Be20(Cu20 -xNix) HEBMGs with different Ni contents is according with the experimental results of critical diameters. However, ΔSmix is only associated with the number and composition of the alloys, but unrelated to the kinds of the constituents that may also affect the entropy change. Whereas, the parameter Sσ/kB is resulting from the mismatch of atomic size, which can more properly represent the change of the entropy caused by different constituents and their concentrations. A large Sσ/kB value generally indicates large atomic size difference in an alloy system, which has a beneficial influence on the GFA. The Sσ/kB values are listed
in Table 3, and their change tendency with Ni content is also consistent with the GFA evaluated from experimental results of the critical diameters. For BMGs, a high stability of their supercooled liquids generally favors a better GFA. The difference of Gibbs free energy (ΔG) [48] between crystals and a liquid, which provides the driving force for nucleation and growth of crystalline phases, is a key parameter to determin the GFA of an alloy system. For an alloy system with a lower value of ΔG throughout their supercooled liquid temperature region, the atoms are easier to pack into a disorder structure, which is inherited from their high temperature liquid structure. For such situation, the supercooled liquid can have a large undercooling temperature region before it finally transforms into a glass, and ordering phases are difficult to form. The ΔG values of Ti20Zr20Hf20Be20(Cu20 -x Nix) (x = 0 [19], 5, 10, 12.5, 15 and 20 at.%) HE-GMGs were calculated from the DSC data (listed in Table 1) and were shown in Fig. 8. It is found that the tendency of ΔG value of Ti20Zr20Hf20Be20(Cu20 -xNix) system is excellently consistent with GFA estimated from the experimental results of the critical diameters, i.e., the lower the ΔG value is, the better the GFA will be. The best GFA with Ni content of 12.5 at.% has the lowest ΔG among all other compositions throughout the whole temperature range we studied. According to the discussion above on the generally used five parameters and ΔG on the GFA, the pseudo-quinary Ti20Zr20Hf20Be20 (Cu20 -xNix) with x = 12.5 at.% has the largest GFA among the Ti20Zr20Hf20Be20(Cu20-xNix) HE-BMGs. The origin of the better GFA for the x = 12.5 at.% sample, can be attributed to an optimum combination of relatively large values of atomic size difference δ, electronegativity difference Δx and entropy of mixing ΔSmix, entropy of mismatch Sσ/kB,
Fig. 6. XRD spectra of the annealed Ti20Zr20Be20Hf20(Cu20-xNix) (x = 0, 5, 10, 15 and 20 at.%) HE-BMGs at the temperatures of their respective (Tx − 5) K for 40 min (a) and at the temperatures of their respective (Tp1 + 5) K for 40 min again (b).
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Fig. 7. The images of Ti20Zr20Hf20Be20(Cu10Ni10) HE-BMG after the second annealing (a), a typical high resolution TEM image for the white area in red frame with the inset of the corresponding SAED, and a typical high resolution TEM image for the area in blue frame with the inset of the corresponding SAED (c). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
a proper negative enthalpy of mixing ΔHmix value and relatively small difference of Gibbs free energies ΔG. 5. Conclusion Pseudo-quinary Ti20Zr20Hf20Be 20(Cu20 -x Nix ) (x = 0.25, 5, 7.5, 10, 12.5, 15, 17.5 and 20 at.%) HEAs possess excellent GFA. Among them, the critical diameters of the pseudo-quinary Ti 20 Zr 20 Hf 20 Be 20 (Cu 7.5 Ni 12.5 ) and quinary Ti 20 Zr 20 Hf 20 Be 20 Ni 20 HEAs are as large as 30 mm and 15 mm, respectively, which are the largest size ever in the HE-BMGs and the largest size in the quinary HE-BMGs hitherto. Moreover, the two Ti20Zr20Hf20Be20(Cu20 -xNix)
HEAs exhibit high yield strength and show relatively large plastic strain values of (3.0 ± 1.1) % and (4.0 ± 0.9) %, respectively. The experimental results indicate that Ni is a beneficial element for enhancing the thermal stability and the GFA of the Ti20Zr20Hf20Be20(Cu20-xNix) HE-BMGs. The origins of their excellent GFA are discussed from the aspect of atomic size difference δ, electronegativity difference Δx, entropy of mixing
Table 3 The atomic size difference δ, electronegativity difference Δx, enthalpy of mixing ΔH mix , entropy of mixing ΔS mix and the entropy of mismatch S σ /k B values of the Ti20Zr20Hf20Be20(Cu20−xNix) HEAs. x
δ (×100)
Δx
(at.%) 0 [19] 2.5 5 7.5 10 12.5 15 17.5 20
12.89 12.94 13.00 13.05 13.10 13.15 13.20 13.26 13.31
0.22 0.38 0.42 0.46 0.50 0.54 0.59 0.63 0.68
ΔHmix
ΔSmix
(kJ/mol)
(J/(mol K))
−25.44 −26.99 −28.56 −30.15 −31.76 −33.39 −35.04 −36.71 −38.40
13.38 14.01 14.32 14.48 14.53 14.48 14.32 14.01 13.38
Sσ/kB
Dc (mm)
0.341 0.344 0.347 0.350 0.352 0.355 0.358 0.361 0.364
12 12 15 20 25 30 20 15 15
Fig. 8. The difference of Gibbs free energies of the typical Ti20Zr20Hf20Be20(Cu20-xNix) (x = 0, 5, 10, 12.5, 15 and 20 at.%) HE-BMGs for crystallization in the supercooled state below Tm.
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