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Improved plasticity of bulk metallic glasses by electrodeposition
Materials Science & Engineering A 615 (2014) 240–246
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Improved plasticity of bulk metallic glasses by electrodeposition Mengmeng Meng a,b, Zhipeng Gao c, Liwei Ren a,b, Huijun Yang a,d, Shengguo Ma c, Zhihua Wang c, Junwei Qiao a,b,n a
Laboratory of Applied Physics and Mechanics of Advanced Materials, College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China b Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, China c Institute of Applied Mechanics and Biomedical Engineering, Taiyuan University of Technology, Taiyuan 030024, China d Research Institute of Surface Engineering, Taiyuan University of Technology, Taiyuan 030024, China
art ic l e i nf o
a b s t r a c t
Article history: Received 21 March 2014 Received in revised form 12 June 2014 Accepted 11 July 2014 Available online 30 July 2014
Zr52.5Cu17.9Ni14.6Al10Ti5 (Vit 105) bulk metallic glasses (BMGs) are coated with Ni films, costing different time for coating. The plasticity of coated Vit 105 BMGs is greatly improved. Experimental results reveal that coating thickness of Ni film is responsible for modifying the mechanical properties of BMG. For compressive tests, the corresponding plastic deformation of 3 hours-coating sample is 9%, which improves 7% than the uncoated sample. However, the compressive plasticity is reduced by 6 hourscoating with the thicker (100 μm) film. For bending tests, bending strength and fracture deflection are increasingly improved with the coating thickness of Ni-layer. The deformation mechanisms are investigated in detail. Crown Copyright & 2014 Published by Elsevier B.V. All rights reserved.
Keywords: Mechanical properties Shear bands Neutral layer Plasticity
1. Introduction Bulk metallic glasses (BMGs) have a potential for advanced structural applications due to their unique mechanical properties including high strength and hardness, large elastic limits, and excellent corrosion and wear resistance [1–3]. However, their inhomogeneous plastic deformation at room temperature is the major weakness and limitation in practical application. This poor plasticity is manifested in serrated plastic flows on the stress–strain curves, which is characterized by repeating cycles of a sudden stress drop followed by elastic reloading [4]. The corresponding images of scanning electron microscopy (SEM) after fracture have demonstrated that the serrated flow is correlated with shear bands. Specially, under compression, at the yielding point a shear band is initiated and propagates across the sample, resulting in a serrated flow pattern until the sample becomes quite brittle and fails catastrophically due to uninhibited propagation of the shear bands [1]. Therefore, lack of ductility in monolithic BMGs is mainly attributed to the fast propagation of localized shear bands. Generally speaking, the main approach to solve this problem is to produce a composite structure and relying on the second phases to delay its catastrophic rupture. Recently, Qiao et al. [5,6] reported that the fabrication of in-situ BMG matrix composites is a valid
n
Corresponding author. E-mail address: [email protected] (J. Qiao).
http://dx.doi.org/10.1016/j.msea.2014.07.033 0921-5093/Crown Copyright & 2014 Published by Elsevier B.V. All rights reserved.
way to improve the plasticity of BMGs. Besides, it is also reported that coating a deposited surface layer on the monolithic BMGs, which is much thinner than BMGs, is a successful approach of surface modification to improve the corresponding ductility at room temperature [7,8]. It has been demonstrated that depositing a thin-film coating can significantly improve the bending ductility of BMGs [7]. Recently, Chen et al. [9] reported that the compressive plasticity of BMGs could be improved by coating a Cu layer deposited electrolytically, but the corresponding strength was reduced due to the soft characteristic of coating layer. Although the plasticity is achieved by coating, the thickness of coating and the deformation mechanism have yet to be investigated so far. The present study gives an investigation on the thickness effect of the coating layer, which is controlled by the electrodeposition time. Zr52.5Cu17.9Ni14.6Al10Ti5 (Vit 105) BMG was selected as the substrate and processed into rob samples and plate samples. The uniaxial compressive test was utilized to determine the mechanical properties of rob samples, while that of plate ones was measured using the threepoint bending test, which is a widely useful protocol for a bending experiment [10–12]. The corresponding results show that the bending strength is increasingly improved with the coating time. However, compared with the sample without coating, the compressive plasticity of the sample with the shorter coating time (3 h) is improved, but that with the longer time (6 h) is reduced. This phenomenon suggests coating thickness plays an important role in modifying the mechanical properties of BMGs.
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2. Experimental Ingots with the nominal compositions of Zr52.5Cu17.9Ni14.6Al10Ti5 (Vit 105) were prepared by arc-melting the mixture of Cu, Zr, and Al with purity higher than 99.9 wt% (wt%) in a Tigetted high-purity-argon atmosphere. The BMGs samples were produced by suction-casting into a water-cooled copper mold to form 2-mm-in-diameter and 60-mm-long rods. Compressive samples, whose ratios of height/diameter were approximately 2/ 1, were made into the form of cylinder rods. Bending samples with dimension of 30 5 1.5 mm3 (length width height) were cut from as-cast plates of 60 10 1.5 mm3 (length width height). Prior to casting, each substrate was well polished. Ni films were grown on the Zr-based BMG substrates by electroplating in a solution with 280 g/L NiSO4, 40 g/L NiCl2, and 40 g/L H3BO3 at 40 1C, as schematically shown in Fig. 1. The reactions of cathode and anode are Cathode: Ni2 þ þ2e-Ni Anode: Ni 2e-Ni2 þ
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coating. The corresponding plastic deformation of uncoated sample, 3 hours-coating sample, and 6 hours-coating sample is 2%, 9%, and 2.5%, respectively. Serrated flows are the characteristics of plastic deformation of BMGs. While the fracture strain (21%) of the 3 hours' coating sample improves by 16% than the uncoated substrate (5%), that (12%) of the 6 hours' coating sample increases by 7% than the uncoated sample. Therefore, it is not a law that coating film can improve the plasticity of BMGs, at least in this Table 1 The geometrics of samples. Sample-coating time (h)
Dimension (mm)
Ni-layer thickness (μm)
Cylinder-0 h Cylinder-3 h Cylinder-6 h Plate-0 h Plate-3 h Plate-5 h Plate-8 h
42 42 4.05 2 30 4.39 1.49 30 4.3 1.49 30 4.61 1.50 30 4.32 1.46
0 60 100 0 70 85 100
and the corresponding voltages were 1.33 V and 1.00 V for the cylinder and plate samples, respectively. Both compressive and bending experiments were performed at room temperature. The uniaxial compressive experiment was performed using an Instron 5969 testing machine. The samples were tested until fracture at a constant strain rate of 2 10 4 s 1. The three-point bending experiment was performed using an Instron 5544 tester with a load cell of 2 kN. 2–5 Samples were prepared for each coating thickness, as the repeatability is very good, only the typical sample was presented here. The corresponding rupture characteristics were analyzed using a MIRA-3 scanning electron microscope (SEM) at an accelerating voltage of 10 kV. The fractured morphology of the compressive and bending samples was observed and the neutral layers of bending samples were determined.
3. Results The geometrics of samples used for the compressive test and the bending test were measured and summarized in Table 1. The compressive engineering stress–strain curves are shown in Fig. 2. It reveals that reductions of yielding stresses happen in the coating samples compared to the monolithic BMGs without Ni-layer coating. The compressive properties of the uncoated samples and coated samples are summarized in Table 2. The yielding stress of monolithic BMG (Vit 105) without coating is 2239 MPa, 26% and 32% higher than that (1771 MPa) of the sample coated Ni film with 6 h and that (1689 MPa) of the sample with 3 h, respectively. This is evidence in the softening effect of Ni-layer
Fig. 2. Experimental results of the compressive samples.
Table 2 The room temperature compressive properties of the uncoated and coated Vit 105 sample. Sample-coating time (h) Yield strength (MPa) Plastic strain (%) Fracture strain (%)
Fig. 1. Schematic illustration for electrolytic deposition.
0 2239 2 5
3 1689 9 21
6 1771 2.5 12
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work, the thickness of Ni film is a responsible factor, i.e., the sample with the thicker (100 μm) Ni-layer reduces the plastic strain of BMG substrate. According to the inset in Fig. 2, serrations of the sample which is coated for 3 h are more denser and smaller than the other two samples, which also proved that 3 hourscoating sample possessed better plasticity; this result is consistent with the view of Chen et al. [9]. The corresponding SEM images of samples after compression are shown in Fig. 3. The BMG substrate and Ni coating separate into two segments and there are some humps on the surface of the coating, as shown in Fig. 3a. Fig. 3b and c reveals the lateral morphology of substrates of Ni-BMG with 3 and 6 hours' coating, respectively. Multiple shear bands, closely related with the serrated plastic flow as shown in the inset of Fig. 2, can be clearly observed from the above two coated samples. Fig. 3d shows the outside coating surface of 3 hours-coating sample, some shear bands penetrate the coating film and expose outside (as indicated by the red arrow), which does not happen on the surface of coated samples with 6 h. This interesting finding indicates that an improved plasticity is obtained for 3 hours' coating samples in contrast to 6 hours' coating samples, in accordance with Fig. 2. The analogous phenomenon has been found in dendrite/BMG composites, and there exist shear bands within the dendrites [13]. Three-point bending results shown in Fig. 4 suggest that both bending strength and fracture deflection are increasingly improved with the thickness of coating Ni film. Interestingly, the corresponding mechanical behavior of monolithic BMG substrate has no plastic stage under bending; while the plasticity increasingly appear in the samples with Ni-layer, especially in the sample with 8 hours' coating, obvious serrated flows occur. The bending properties of the uncoated and coated samples upon bending are summarized in Table 3. The fractured load of BMG substrate is 637 N, which is improved by 17%, 59%, and 95% through coating Ni-layer with 70, 85, and 100 μm, respectively. The corresponding fracture deflection of BMG substrate is 1.14 mm, which is improved by 20%, 80%, and 153% through coating Ni-layer with 3, 5, and 8 h, respectively.
In addition, attention should be taken of the load values on elastic limit of the coating samples, which are 745 N, 858 N, and 1018 N, respectively. After coating, the cross sectional area of the samples has been changed, so it is not accurate to describe the strength with the load value directly, but due to the coating thickness is very small
Fig. 4. Experimental results of the bending samples.
Table 3 The bending properties of the uncoated and coated samples. Sample-coating time (h) Coating thickness (μm) Fracture deflection (mm) Fracture load value (N) Elastic limit load value (N)
0 0 1.14 637 637
3 70 1.37 745 745
5 85 2.05 1013 858
8 100 2.89 1240 1018
Fig. 3. SEM images showing the surfaces of compressive samples: (a) the integral morphology of the BMG substrate and Ni coating, (b) lateral morphology of substrates of Ni-BMG with 3 hours-coating, (c) lateral morphology of substrates of Ni-BMG with 6 hours-coating and (d) coating surface of 3 hours-coating sample. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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relative to the BMG substrate, so the trend of improvement of the strength can be considered to be the same. Interestingly, Schaufler et al. [14] have reported that the fracture toughness of two hydrogenated amorphous carbon coatings (a-C:H) with Cradhesion layers on steel substrate under microcantilever bending experiments is 2.18 MPa m0.5, which has even exceeded that of pure a-C:H. SEM images of the samples after bending are exhibited in Fig. 5. The graphs a, b, c, and d demonstrate the geometric characteristics of monolithic BMG substrate, coated samples with 0 h, 3 h, 5 h, and 8 h, respectively. The lateral surface of the monolithic BMG sample is smooth, and no shear band is found, as revealed in Fig. 5a, which is in agreement with brittle failure. For the sample with 3 hours' coating, obvious cracks occur in the tensile side, shown as the white arrow in Fig. 5b, but there are few cracks in the compressive side. A slight bending can be observed in the vicinity of fracture. For samples with 5 hours' coating, cracks also appear in the tensile side, shown as the white arrow in Fig. 5c, while some macro-cracks occur in the compressive side, as shown in the inset in Fig. 5c. Compared with the samples with 3 hours' coating, its bending trend is more obvious. For the sample with 8 hours' coating, it has the obvious bending trend and many macro-cracks appear on the tensile side. Remarkably, profuse shear bands (black arrow in Fig. 5d) appear in the tensile side and a serious crack (white arrow in Fig. 5d) also occurs in the compressive side. Intuitively, the bending trend of samples is correlated with the thickness of coating film, i.e., a BMG with the thicker Ni-layer has the larger bending deflection, as revealed in Figs. 4 and 5.
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4. Discussion The uniaxial compressive test and the three-point bending test were used to determine the corresponding mechanical properties. SEM images were used to analyze the fractured morphology of samples. The main limitations of this work were the heterogeneity of coating film and the bonding quality between BMG and Nilayer.
4.1. Compressive behavior Under the uniaxial compressive test, the yielding point of Nicoated BMG samples appear earlier than that of monolithic BMG (Fig. 2), which induces the reduction of plastic strain and strength. This result coincides with the previous investigation [9]. Composite mechanics can give an interpretation to the present result. As the coating and the substrate undergo the same deformation, the respective strain is equal. Therefore, the yield strength can be theoretically predicted by the mixing rule [15], σ y ¼ σ A V A þ σ B V B , in which σA and σB are the yielding strengths of monolithic Vit 105 and Ni coating, respectively; VA and VB are the volume fractions of monolithic Vit 105 and Ni coating, respectively. Accordingly, the strength (1000 MPa [16]) of Ni is much lower than that of BMG substrate, thus the yield decline is acceptable. It must be noted that the yielding stress obtained from the rule of mixture is actually the upper bound of the predicted overall yield stress, this is because that the prediction from rule of mixture could be
Fig. 5. SEM images showing the profiles of bending samples (a) 0 h, (b) 3 h, (c) 5 h and (d) 8 h.
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accurate only if the Ni coating layer and the BMG substrate yield simultaneously. Here we simplified the rule of mixture and considered that the Ni coating layer and the BMG substrate yield simultaneously. Generally, the stress-drops during the serrated flows correspond to the propagation of shear bands [9,17,18], as shown in the inset of Fig. 2. In this sense, if one serration is regarded as a shear band emission, the shear band density or amount on the deformed sample can be roughly estimated using the number of serrations [19,20]. In other words, it is possible to deduce the plasticity of BMGs following the amount of discernible serrations, since the plastic strain is in accordance with the localized shear bands. Actually, in this work, the number of serrations of the three compressive samples agree well with their different levels of plasticity. Many research groups have reported that the plasticity of BMGs can be improved by depositing Cu film [9,21] and Ni–Fe alloy layer [22]. Analogous improvement has been achieved in the current work, i.e., BMG (Vit 105) coated Ni film with 60 μm improves compressive plasticity by 7% and fracture strain by 16% (Fig. 2). The reason of this plasticity increase was suggested to result from the following three factors. Firstly, friction between the compressive punch and the ends of sample was changed with deformation. Secondly, the lateral restrained stress was imposed by coating film. Thirdly, macroscopic deformation induced the structural variation of the heterogeneous microscopic morphology [8]. However, plasticity reduction is observed in the sample with Ni-layer of 100 μm (Fig. 2). The reason of this contradictoriness may be found from the microstructure. Penetration of shear bands can be observed from the SEM images of the lateral surface of BMG coated Ni film with 60 μm, while this phenomenon does not appear in the sample with Nilayer of 100 μm (see Fig. 3d). In the microscopic level, these observations suggest that the bonding quality might be influenced by the coating thickness. The thinner (60 μm) Ni film was well bonded with the BMG substrate and then both composite materials together provided contribution to oppose the increasingly compressive load. However, it was possible that there were more defects such as dislocations in the crystalline Ni-layer during growth. Upon compression, an early failure is accompanied. The present study gives a clue to control the effective coating thickness to improve the plasticity of BMGs.
Fig. 6. Schematic illustration for a sandwich plate under bending.
Fig. 7. Schematic illustration for the equivalent model. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
inertia moment of neutral layer for this structure: 3
Iz ¼
2
Bending is an inherently stable deformation method for metallic glasses; the stress to drive shear bands diminishes when the shear band approaches the neutral axis [23,24]. Bending ductility of Vit 105 BMG is significantly improved, without strength loss, by depositing Ni film, which can be evidenced from both the bending experimental results and the corresponding analysis of SEM images. A similar result is also obtained by depositing a bilayer coating film [7]; the bending ductility of 3 mm thick Zr50 Cu30Al10Ni10 BMGs can be dramatically enhanced from 0% to 13.7% by the deposition of a thin bilayer film (consisting of a 25 nm thick Ti adhesive layer with a 200 nm thick metallic glass (MG) overlayer) on the tensile side of the BMG sample. In addition, Huang et al. [25] have shown BMGs to exhibit excellent bending ductility at much high temperatures (350 and 360 1C). This work focused on the coating thickness effect at room temperature. For simplification, the lateral Ni films are omitted, and then the cross section of a bending sample is changed into a sandwich plate, as shown in Fig. 6. According to the parallel axis theorem, the following equation can be used to describe the corresponding
ð1Þ
where h2 is the thickness of Ni film, and b and h1 are the width and the height of BMG substrate, respectively. Substituting b¼ 5 mm and h1 ¼1.5 mm into Eq. (1), the inertia moment Iz can be simplified into the function equation (2) of the coating thickness. 3
4.2. Bending behavior
3
2bh2 bh1 h2 bh1 2 þbh1 h2 þ þ ; 3 2 12
2
I z ¼ 3:33h2 þ 7:5h2 þ 5:625h2 þ 1:406:
ð2Þ
The Tresca yield criterion in planar-stressed state, i.e., the equivalent stress σr of a point of the cross sectional plane can be formulated as Eq. (3), is selected to describe the corresponding failure strength. pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi σ r ¼ σ 2 þ 4τ2 : ð3Þ From the view of materials mechanics, 8 < σ ¼ My Iz : τ ¼ 2IF s ðy2c y2 Þ
ð4Þ
z
where M and Fs represent the bending moment and the shear force, respectively. Substituting Eq. (4) into Eq. (3) the following equation is obtained:
σr ¼
1 Iz
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðMyÞ2 þ F 2s ðy2c y2 Þ2 :
ð5Þ
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Both Eqs. (2) and (5) reveal that the coating thickness is responsible for the yielding strength of the composite BMG plate with Ni film. It is obtained that the thicker Ni-layer sharply improves the yielding limit, in agreement with Fig. 4. In addition, the maximum shear stress appears on the neutral layer; since the neutral layer passes through the geometric center, thus the maximum tensile stress (on the bottom surface) is equal to the maximum compressive stress (on the top surface). However, the actual SEM image in Fig. 7 of a fractured sample reveals a void, as indicated by the pink arrow, which is thicker than coating Ni-layer, occurs between the top coating film and BMG substrate. This void is caused by the different elastic moduli of Ni and Vit 105, which are 207 GPa [26] and 88.6 GPa [27], respectively. Different moduli induce different flexural rigidities, which are a major reason for the appearance of the void. Additionally, this void can raise the position of neutral layer, and then varies the values of maximum normal and shear stresses. The actual neutral layer of the fractured samples can be determined using an elasticity equivalent model.
Fig. 8. Neutral layer of bending samples.
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Assuming a homogeneous structure as shown in Fig. 8 under elastic bending, theoretically, this composite plate with different materials is equivalent to a structure composed with only one material through varying their cross sections. This theory is valid under the assumption that variation does not happen in the flexural rigidity of each cross section. The neutral layer of the elasticity equivalent model as shown in Fig. 8 can be determined using Eq. (6), which is derived from its geometrics (b¼5 mm, h1 ¼1.5 mm, and n E2.3). 2
yc ¼
23h2 þ ð24:75 þ 11:5tÞh2 þ 5:625 ; 23h2 þ 7:5
ð6Þ
where t is the void height. The average values of t for the three bending samples with Ni film, i.e., Ni-BMG with 3, 5 and 8 hours' coating, are 93.51 μm, 97.31 μm, and 105.26 μm, respectively. Substituting the corresponding values of h2 and t into Eq. (6), the actual neutral layer of each bending sample is determined and is shown in Fig. 9. The calculation results as shown in Fig. 9 reveal that the neutral layer increasingly rises with the thickness of Ni film, which is also observed from the corresponding analysis of SEM images (Fig. 9). As shown in Fig. 9, the neutral layer (red line) increasingly migrates towards the compressive side with coating thickness. This can be identified through the roughness difference between the tensile side and the compressive side, i.e., the tensile side has smoother cross section. The error between calculation and observation is caused by the fact that BMG is inhomogeneous, while the theoretical model is established with the homogeneous assumption. Fig. 9 SEM images show the cross sections of bending samples; the red lines denote the neutral layer observed from SEM images, the blue lines denote the neutral layer calculated from Eq. (6), and the red arrows denote the fractures induced by the maximum shear stress. For the bending failure mechanism, the responsibility of shear stress is increasingly reinforced with the coating thickness, as shown in Fig. 9. Deriving from Eq. (4), the maximum shear stress τmax ¼ f s y2c =2I z appears in the neutral layer, and there is not any normal stress appearing in the neutral layer. Accordingly, the increment of neutral layer yc will sharply improve the corresponding maximum
Fig. 9. SEM images showing the cross sections of bending samples; note that, the red lines denote the neutral layer observed from SEM images, the blue lines denote the neutral layer calculated from Eq. (6), and the red arrows denote the fractures induced by the maximum shear stress. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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shear stress. This is a major reason for the fractures of BMG substrate with the thicker (85 and 100 μm) Ni-layers (red arrows in Fig. 9), which is evidenced by the appearance of shear bands. As the corresponding SEM images shown in Fig. 5, shear bands can be obviously observed in the graphs of c and d, while there are not shear bands occurring in the graphs of a and b. This also interpreted the bending experimental results shown in Fig. 4, in which the serrated flow appears along the plasticity of BMG with the thicker (85 and 100 μm) Ni-layer. In addition, the raised-neutral layer reduces the corresponding maximum compressive stress (Fig. 9), σ c; max ¼ Mðh1 yc Þ=I z , which is also derived from Eq. (4). This is the reason for the fractured phenomenon of the sample with the thickest (100 μm) Ni film. 5. Summary The mechanical properties of BMG of Zr52.5Cu17.9Ni14.6Al10Ti5 (Vit 105) coated by Ni films with different thicknesses were determined using compressive and bending tests. Coating thickness has different effects on the Vit 105 BMG under bending and compression. For bending tests, bending strength and fracture deflection are increasingly improved with the coating thickness of Ni-layer. That is to say, after coating, the plasticity has been improved, and thicker coating corresponds to better plasticity. In addition, the stress state has been changed with the Ni film, which resulted in the neutral layer moving to the compressive side. For compressive tests, the corresponding plastic deformation of 3 hours-coating sample is 9%, which improves 7% than the uncoated sample. However, the compressive plasticity is reduced by 6 hours-coating with the thicker (100 μm) film. Acknowledgments J.W.Q. would like to acknowledge the financial support of National Natural Science Foundation of China (Nos. 51101110 and 51371122), the Technology Foundation for Selected Overseas Chinese Scholar, Ministry of Human Resources and Social Security of China, and the Program for the Outstanding Innovative Teams of
Higher Learning Institutions of Shanxi (2013). H.J.Y. would like to acknowledge the financial support from the National Natural Science Foundation of China (No. 51341006), the State Key Lab of Advanced Metals and Materials (No. 2013-Z03), and the Youth Science Foundation of Shanxi Province, China (No. 2014021017-3). Z.H.W. would like to acknowledge the National Natural Science Foundation of China (No. 11390362).
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Improved plasticity of bulk metallic glasses by electrodeposition Mengmeng Meng a,b, Zhipeng Gao c, Liwei Ren a,b, Huijun Yang a,d, Shengguo Ma c, Zhihua Wang c, Junwei Qiao a,b,n a
Laboratory of Applied Physics and Mechanics of Advanced Materials, College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China b Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, China c Institute of Applied Mechanics and Biomedical Engineering, Taiyuan University of Technology, Taiyuan 030024, China d Research Institute of Surface Engineering, Taiyuan University of Technology, Taiyuan 030024, China
art ic l e i nf o
a b s t r a c t
Article history: Received 21 March 2014 Received in revised form 12 June 2014 Accepted 11 July 2014 Available online 30 July 2014
Zr52.5Cu17.9Ni14.6Al10Ti5 (Vit 105) bulk metallic glasses (BMGs) are coated with Ni films, costing different time for coating. The plasticity of coated Vit 105 BMGs is greatly improved. Experimental results reveal that coating thickness of Ni film is responsible for modifying the mechanical properties of BMG. For compressive tests, the corresponding plastic deformation of 3 hours-coating sample is 9%, which improves 7% than the uncoated sample. However, the compressive plasticity is reduced by 6 hourscoating with the thicker (100 μm) film. For bending tests, bending strength and fracture deflection are increasingly improved with the coating thickness of Ni-layer. The deformation mechanisms are investigated in detail. Crown Copyright & 2014 Published by Elsevier B.V. All rights reserved.
Keywords: Mechanical properties Shear bands Neutral layer Plasticity
1. Introduction Bulk metallic glasses (BMGs) have a potential for advanced structural applications due to their unique mechanical properties including high strength and hardness, large elastic limits, and excellent corrosion and wear resistance [1–3]. However, their inhomogeneous plastic deformation at room temperature is the major weakness and limitation in practical application. This poor plasticity is manifested in serrated plastic flows on the stress–strain curves, which is characterized by repeating cycles of a sudden stress drop followed by elastic reloading [4]. The corresponding images of scanning electron microscopy (SEM) after fracture have demonstrated that the serrated flow is correlated with shear bands. Specially, under compression, at the yielding point a shear band is initiated and propagates across the sample, resulting in a serrated flow pattern until the sample becomes quite brittle and fails catastrophically due to uninhibited propagation of the shear bands [1]. Therefore, lack of ductility in monolithic BMGs is mainly attributed to the fast propagation of localized shear bands. Generally speaking, the main approach to solve this problem is to produce a composite structure and relying on the second phases to delay its catastrophic rupture. Recently, Qiao et al. [5,6] reported that the fabrication of in-situ BMG matrix composites is a valid
n
Corresponding author. E-mail address: [email protected] (J. Qiao).
http://dx.doi.org/10.1016/j.msea.2014.07.033 0921-5093/Crown Copyright & 2014 Published by Elsevier B.V. All rights reserved.
way to improve the plasticity of BMGs. Besides, it is also reported that coating a deposited surface layer on the monolithic BMGs, which is much thinner than BMGs, is a successful approach of surface modification to improve the corresponding ductility at room temperature [7,8]. It has been demonstrated that depositing a thin-film coating can significantly improve the bending ductility of BMGs [7]. Recently, Chen et al. [9] reported that the compressive plasticity of BMGs could be improved by coating a Cu layer deposited electrolytically, but the corresponding strength was reduced due to the soft characteristic of coating layer. Although the plasticity is achieved by coating, the thickness of coating and the deformation mechanism have yet to be investigated so far. The present study gives an investigation on the thickness effect of the coating layer, which is controlled by the electrodeposition time. Zr52.5Cu17.9Ni14.6Al10Ti5 (Vit 105) BMG was selected as the substrate and processed into rob samples and plate samples. The uniaxial compressive test was utilized to determine the mechanical properties of rob samples, while that of plate ones was measured using the threepoint bending test, which is a widely useful protocol for a bending experiment [10–12]. The corresponding results show that the bending strength is increasingly improved with the coating time. However, compared with the sample without coating, the compressive plasticity of the sample with the shorter coating time (3 h) is improved, but that with the longer time (6 h) is reduced. This phenomenon suggests coating thickness plays an important role in modifying the mechanical properties of BMGs.
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2. Experimental Ingots with the nominal compositions of Zr52.5Cu17.9Ni14.6Al10Ti5 (Vit 105) were prepared by arc-melting the mixture of Cu, Zr, and Al with purity higher than 99.9 wt% (wt%) in a Tigetted high-purity-argon atmosphere. The BMGs samples were produced by suction-casting into a water-cooled copper mold to form 2-mm-in-diameter and 60-mm-long rods. Compressive samples, whose ratios of height/diameter were approximately 2/ 1, were made into the form of cylinder rods. Bending samples with dimension of 30 5 1.5 mm3 (length width height) were cut from as-cast plates of 60 10 1.5 mm3 (length width height). Prior to casting, each substrate was well polished. Ni films were grown on the Zr-based BMG substrates by electroplating in a solution with 280 g/L NiSO4, 40 g/L NiCl2, and 40 g/L H3BO3 at 40 1C, as schematically shown in Fig. 1. The reactions of cathode and anode are Cathode: Ni2 þ þ2e-Ni Anode: Ni 2e-Ni2 þ
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coating. The corresponding plastic deformation of uncoated sample, 3 hours-coating sample, and 6 hours-coating sample is 2%, 9%, and 2.5%, respectively. Serrated flows are the characteristics of plastic deformation of BMGs. While the fracture strain (21%) of the 3 hours' coating sample improves by 16% than the uncoated substrate (5%), that (12%) of the 6 hours' coating sample increases by 7% than the uncoated sample. Therefore, it is not a law that coating film can improve the plasticity of BMGs, at least in this Table 1 The geometrics of samples. Sample-coating time (h)
Dimension (mm)
Ni-layer thickness (μm)
Cylinder-0 h Cylinder-3 h Cylinder-6 h Plate-0 h Plate-3 h Plate-5 h Plate-8 h
42 42 4.05 2 30 4.39 1.49 30 4.3 1.49 30 4.61 1.50 30 4.32 1.46
0 60 100 0 70 85 100
and the corresponding voltages were 1.33 V and 1.00 V for the cylinder and plate samples, respectively. Both compressive and bending experiments were performed at room temperature. The uniaxial compressive experiment was performed using an Instron 5969 testing machine. The samples were tested until fracture at a constant strain rate of 2 10 4 s 1. The three-point bending experiment was performed using an Instron 5544 tester with a load cell of 2 kN. 2–5 Samples were prepared for each coating thickness, as the repeatability is very good, only the typical sample was presented here. The corresponding rupture characteristics were analyzed using a MIRA-3 scanning electron microscope (SEM) at an accelerating voltage of 10 kV. The fractured morphology of the compressive and bending samples was observed and the neutral layers of bending samples were determined.
3. Results The geometrics of samples used for the compressive test and the bending test were measured and summarized in Table 1. The compressive engineering stress–strain curves are shown in Fig. 2. It reveals that reductions of yielding stresses happen in the coating samples compared to the monolithic BMGs without Ni-layer coating. The compressive properties of the uncoated samples and coated samples are summarized in Table 2. The yielding stress of monolithic BMG (Vit 105) without coating is 2239 MPa, 26% and 32% higher than that (1771 MPa) of the sample coated Ni film with 6 h and that (1689 MPa) of the sample with 3 h, respectively. This is evidence in the softening effect of Ni-layer
Fig. 2. Experimental results of the compressive samples.
Table 2 The room temperature compressive properties of the uncoated and coated Vit 105 sample. Sample-coating time (h) Yield strength (MPa) Plastic strain (%) Fracture strain (%)
Fig. 1. Schematic illustration for electrolytic deposition.
0 2239 2 5
3 1689 9 21
6 1771 2.5 12
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work, the thickness of Ni film is a responsible factor, i.e., the sample with the thicker (100 μm) Ni-layer reduces the plastic strain of BMG substrate. According to the inset in Fig. 2, serrations of the sample which is coated for 3 h are more denser and smaller than the other two samples, which also proved that 3 hourscoating sample possessed better plasticity; this result is consistent with the view of Chen et al. [9]. The corresponding SEM images of samples after compression are shown in Fig. 3. The BMG substrate and Ni coating separate into two segments and there are some humps on the surface of the coating, as shown in Fig. 3a. Fig. 3b and c reveals the lateral morphology of substrates of Ni-BMG with 3 and 6 hours' coating, respectively. Multiple shear bands, closely related with the serrated plastic flow as shown in the inset of Fig. 2, can be clearly observed from the above two coated samples. Fig. 3d shows the outside coating surface of 3 hours-coating sample, some shear bands penetrate the coating film and expose outside (as indicated by the red arrow), which does not happen on the surface of coated samples with 6 h. This interesting finding indicates that an improved plasticity is obtained for 3 hours' coating samples in contrast to 6 hours' coating samples, in accordance with Fig. 2. The analogous phenomenon has been found in dendrite/BMG composites, and there exist shear bands within the dendrites [13]. Three-point bending results shown in Fig. 4 suggest that both bending strength and fracture deflection are increasingly improved with the thickness of coating Ni film. Interestingly, the corresponding mechanical behavior of monolithic BMG substrate has no plastic stage under bending; while the plasticity increasingly appear in the samples with Ni-layer, especially in the sample with 8 hours' coating, obvious serrated flows occur. The bending properties of the uncoated and coated samples upon bending are summarized in Table 3. The fractured load of BMG substrate is 637 N, which is improved by 17%, 59%, and 95% through coating Ni-layer with 70, 85, and 100 μm, respectively. The corresponding fracture deflection of BMG substrate is 1.14 mm, which is improved by 20%, 80%, and 153% through coating Ni-layer with 3, 5, and 8 h, respectively.
In addition, attention should be taken of the load values on elastic limit of the coating samples, which are 745 N, 858 N, and 1018 N, respectively. After coating, the cross sectional area of the samples has been changed, so it is not accurate to describe the strength with the load value directly, but due to the coating thickness is very small
Fig. 4. Experimental results of the bending samples.
Table 3 The bending properties of the uncoated and coated samples. Sample-coating time (h) Coating thickness (μm) Fracture deflection (mm) Fracture load value (N) Elastic limit load value (N)
0 0 1.14 637 637
3 70 1.37 745 745
5 85 2.05 1013 858
8 100 2.89 1240 1018
Fig. 3. SEM images showing the surfaces of compressive samples: (a) the integral morphology of the BMG substrate and Ni coating, (b) lateral morphology of substrates of Ni-BMG with 3 hours-coating, (c) lateral morphology of substrates of Ni-BMG with 6 hours-coating and (d) coating surface of 3 hours-coating sample. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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relative to the BMG substrate, so the trend of improvement of the strength can be considered to be the same. Interestingly, Schaufler et al. [14] have reported that the fracture toughness of two hydrogenated amorphous carbon coatings (a-C:H) with Cradhesion layers on steel substrate under microcantilever bending experiments is 2.18 MPa m0.5, which has even exceeded that of pure a-C:H. SEM images of the samples after bending are exhibited in Fig. 5. The graphs a, b, c, and d demonstrate the geometric characteristics of monolithic BMG substrate, coated samples with 0 h, 3 h, 5 h, and 8 h, respectively. The lateral surface of the monolithic BMG sample is smooth, and no shear band is found, as revealed in Fig. 5a, which is in agreement with brittle failure. For the sample with 3 hours' coating, obvious cracks occur in the tensile side, shown as the white arrow in Fig. 5b, but there are few cracks in the compressive side. A slight bending can be observed in the vicinity of fracture. For samples with 5 hours' coating, cracks also appear in the tensile side, shown as the white arrow in Fig. 5c, while some macro-cracks occur in the compressive side, as shown in the inset in Fig. 5c. Compared with the samples with 3 hours' coating, its bending trend is more obvious. For the sample with 8 hours' coating, it has the obvious bending trend and many macro-cracks appear on the tensile side. Remarkably, profuse shear bands (black arrow in Fig. 5d) appear in the tensile side and a serious crack (white arrow in Fig. 5d) also occurs in the compressive side. Intuitively, the bending trend of samples is correlated with the thickness of coating film, i.e., a BMG with the thicker Ni-layer has the larger bending deflection, as revealed in Figs. 4 and 5.
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4. Discussion The uniaxial compressive test and the three-point bending test were used to determine the corresponding mechanical properties. SEM images were used to analyze the fractured morphology of samples. The main limitations of this work were the heterogeneity of coating film and the bonding quality between BMG and Nilayer.
4.1. Compressive behavior Under the uniaxial compressive test, the yielding point of Nicoated BMG samples appear earlier than that of monolithic BMG (Fig. 2), which induces the reduction of plastic strain and strength. This result coincides with the previous investigation [9]. Composite mechanics can give an interpretation to the present result. As the coating and the substrate undergo the same deformation, the respective strain is equal. Therefore, the yield strength can be theoretically predicted by the mixing rule [15], σ y ¼ σ A V A þ σ B V B , in which σA and σB are the yielding strengths of monolithic Vit 105 and Ni coating, respectively; VA and VB are the volume fractions of monolithic Vit 105 and Ni coating, respectively. Accordingly, the strength (1000 MPa [16]) of Ni is much lower than that of BMG substrate, thus the yield decline is acceptable. It must be noted that the yielding stress obtained from the rule of mixture is actually the upper bound of the predicted overall yield stress, this is because that the prediction from rule of mixture could be
Fig. 5. SEM images showing the profiles of bending samples (a) 0 h, (b) 3 h, (c) 5 h and (d) 8 h.
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accurate only if the Ni coating layer and the BMG substrate yield simultaneously. Here we simplified the rule of mixture and considered that the Ni coating layer and the BMG substrate yield simultaneously. Generally, the stress-drops during the serrated flows correspond to the propagation of shear bands [9,17,18], as shown in the inset of Fig. 2. In this sense, if one serration is regarded as a shear band emission, the shear band density or amount on the deformed sample can be roughly estimated using the number of serrations [19,20]. In other words, it is possible to deduce the plasticity of BMGs following the amount of discernible serrations, since the plastic strain is in accordance with the localized shear bands. Actually, in this work, the number of serrations of the three compressive samples agree well with their different levels of plasticity. Many research groups have reported that the plasticity of BMGs can be improved by depositing Cu film [9,21] and Ni–Fe alloy layer [22]. Analogous improvement has been achieved in the current work, i.e., BMG (Vit 105) coated Ni film with 60 μm improves compressive plasticity by 7% and fracture strain by 16% (Fig. 2). The reason of this plasticity increase was suggested to result from the following three factors. Firstly, friction between the compressive punch and the ends of sample was changed with deformation. Secondly, the lateral restrained stress was imposed by coating film. Thirdly, macroscopic deformation induced the structural variation of the heterogeneous microscopic morphology [8]. However, plasticity reduction is observed in the sample with Ni-layer of 100 μm (Fig. 2). The reason of this contradictoriness may be found from the microstructure. Penetration of shear bands can be observed from the SEM images of the lateral surface of BMG coated Ni film with 60 μm, while this phenomenon does not appear in the sample with Nilayer of 100 μm (see Fig. 3d). In the microscopic level, these observations suggest that the bonding quality might be influenced by the coating thickness. The thinner (60 μm) Ni film was well bonded with the BMG substrate and then both composite materials together provided contribution to oppose the increasingly compressive load. However, it was possible that there were more defects such as dislocations in the crystalline Ni-layer during growth. Upon compression, an early failure is accompanied. The present study gives a clue to control the effective coating thickness to improve the plasticity of BMGs.
Fig. 6. Schematic illustration for a sandwich plate under bending.
Fig. 7. Schematic illustration for the equivalent model. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
inertia moment of neutral layer for this structure: 3
Iz ¼
2
Bending is an inherently stable deformation method for metallic glasses; the stress to drive shear bands diminishes when the shear band approaches the neutral axis [23,24]. Bending ductility of Vit 105 BMG is significantly improved, without strength loss, by depositing Ni film, which can be evidenced from both the bending experimental results and the corresponding analysis of SEM images. A similar result is also obtained by depositing a bilayer coating film [7]; the bending ductility of 3 mm thick Zr50 Cu30Al10Ni10 BMGs can be dramatically enhanced from 0% to 13.7% by the deposition of a thin bilayer film (consisting of a 25 nm thick Ti adhesive layer with a 200 nm thick metallic glass (MG) overlayer) on the tensile side of the BMG sample. In addition, Huang et al. [25] have shown BMGs to exhibit excellent bending ductility at much high temperatures (350 and 360 1C). This work focused on the coating thickness effect at room temperature. For simplification, the lateral Ni films are omitted, and then the cross section of a bending sample is changed into a sandwich plate, as shown in Fig. 6. According to the parallel axis theorem, the following equation can be used to describe the corresponding
ð1Þ
where h2 is the thickness of Ni film, and b and h1 are the width and the height of BMG substrate, respectively. Substituting b¼ 5 mm and h1 ¼1.5 mm into Eq. (1), the inertia moment Iz can be simplified into the function equation (2) of the coating thickness. 3
4.2. Bending behavior
3
2bh2 bh1 h2 bh1 2 þbh1 h2 þ þ ; 3 2 12
2
I z ¼ 3:33h2 þ 7:5h2 þ 5:625h2 þ 1:406:
ð2Þ
The Tresca yield criterion in planar-stressed state, i.e., the equivalent stress σr of a point of the cross sectional plane can be formulated as Eq. (3), is selected to describe the corresponding failure strength. pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi σ r ¼ σ 2 þ 4τ2 : ð3Þ From the view of materials mechanics, 8 < σ ¼ My Iz : τ ¼ 2IF s ðy2c y2 Þ
ð4Þ
z
where M and Fs represent the bending moment and the shear force, respectively. Substituting Eq. (4) into Eq. (3) the following equation is obtained:
σr ¼
1 Iz
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðMyÞ2 þ F 2s ðy2c y2 Þ2 :
ð5Þ
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Both Eqs. (2) and (5) reveal that the coating thickness is responsible for the yielding strength of the composite BMG plate with Ni film. It is obtained that the thicker Ni-layer sharply improves the yielding limit, in agreement with Fig. 4. In addition, the maximum shear stress appears on the neutral layer; since the neutral layer passes through the geometric center, thus the maximum tensile stress (on the bottom surface) is equal to the maximum compressive stress (on the top surface). However, the actual SEM image in Fig. 7 of a fractured sample reveals a void, as indicated by the pink arrow, which is thicker than coating Ni-layer, occurs between the top coating film and BMG substrate. This void is caused by the different elastic moduli of Ni and Vit 105, which are 207 GPa [26] and 88.6 GPa [27], respectively. Different moduli induce different flexural rigidities, which are a major reason for the appearance of the void. Additionally, this void can raise the position of neutral layer, and then varies the values of maximum normal and shear stresses. The actual neutral layer of the fractured samples can be determined using an elasticity equivalent model.
Fig. 8. Neutral layer of bending samples.
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Assuming a homogeneous structure as shown in Fig. 8 under elastic bending, theoretically, this composite plate with different materials is equivalent to a structure composed with only one material through varying their cross sections. This theory is valid under the assumption that variation does not happen in the flexural rigidity of each cross section. The neutral layer of the elasticity equivalent model as shown in Fig. 8 can be determined using Eq. (6), which is derived from its geometrics (b¼5 mm, h1 ¼1.5 mm, and n E2.3). 2
yc ¼
23h2 þ ð24:75 þ 11:5tÞh2 þ 5:625 ; 23h2 þ 7:5
ð6Þ
where t is the void height. The average values of t for the three bending samples with Ni film, i.e., Ni-BMG with 3, 5 and 8 hours' coating, are 93.51 μm, 97.31 μm, and 105.26 μm, respectively. Substituting the corresponding values of h2 and t into Eq. (6), the actual neutral layer of each bending sample is determined and is shown in Fig. 9. The calculation results as shown in Fig. 9 reveal that the neutral layer increasingly rises with the thickness of Ni film, which is also observed from the corresponding analysis of SEM images (Fig. 9). As shown in Fig. 9, the neutral layer (red line) increasingly migrates towards the compressive side with coating thickness. This can be identified through the roughness difference between the tensile side and the compressive side, i.e., the tensile side has smoother cross section. The error between calculation and observation is caused by the fact that BMG is inhomogeneous, while the theoretical model is established with the homogeneous assumption. Fig. 9 SEM images show the cross sections of bending samples; the red lines denote the neutral layer observed from SEM images, the blue lines denote the neutral layer calculated from Eq. (6), and the red arrows denote the fractures induced by the maximum shear stress. For the bending failure mechanism, the responsibility of shear stress is increasingly reinforced with the coating thickness, as shown in Fig. 9. Deriving from Eq. (4), the maximum shear stress τmax ¼ f s y2c =2I z appears in the neutral layer, and there is not any normal stress appearing in the neutral layer. Accordingly, the increment of neutral layer yc will sharply improve the corresponding maximum
Fig. 9. SEM images showing the cross sections of bending samples; note that, the red lines denote the neutral layer observed from SEM images, the blue lines denote the neutral layer calculated from Eq. (6), and the red arrows denote the fractures induced by the maximum shear stress. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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shear stress. This is a major reason for the fractures of BMG substrate with the thicker (85 and 100 μm) Ni-layers (red arrows in Fig. 9), which is evidenced by the appearance of shear bands. As the corresponding SEM images shown in Fig. 5, shear bands can be obviously observed in the graphs of c and d, while there are not shear bands occurring in the graphs of a and b. This also interpreted the bending experimental results shown in Fig. 4, in which the serrated flow appears along the plasticity of BMG with the thicker (85 and 100 μm) Ni-layer. In addition, the raised-neutral layer reduces the corresponding maximum compressive stress (Fig. 9), σ c; max ¼ Mðh1 yc Þ=I z , which is also derived from Eq. (4). This is the reason for the fractured phenomenon of the sample with the thickest (100 μm) Ni film. 5. Summary The mechanical properties of BMG of Zr52.5Cu17.9Ni14.6Al10Ti5 (Vit 105) coated by Ni films with different thicknesses were determined using compressive and bending tests. Coating thickness has different effects on the Vit 105 BMG under bending and compression. For bending tests, bending strength and fracture deflection are increasingly improved with the coating thickness of Ni-layer. That is to say, after coating, the plasticity has been improved, and thicker coating corresponds to better plasticity. In addition, the stress state has been changed with the Ni film, which resulted in the neutral layer moving to the compressive side. For compressive tests, the corresponding plastic deformation of 3 hours-coating sample is 9%, which improves 7% than the uncoated sample. However, the compressive plasticity is reduced by 6 hours-coating with the thicker (100 μm) film. Acknowledgments J.W.Q. would like to acknowledge the financial support of National Natural Science Foundation of China (Nos. 51101110 and 51371122), the Technology Foundation for Selected Overseas Chinese Scholar, Ministry of Human Resources and Social Security of China, and the Program for the Outstanding Innovative Teams of
Higher Learning Institutions of Shanxi (2013). H.J.Y. would like to acknowledge the financial support from the National Natural Science Foundation of China (No. 51341006), the State Key Lab of Advanced Metals and Materials (No. 2013-Z03), and the Youth Science Foundation of Shanxi Province, China (No. 2014021017-3). Z.H.W. would like to acknowledge the National Natural Science Foundation of China (No. 11390362).
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