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Dynamic shear punching of metallic glass matrix composites
Intermetallics 36 (2013) 31e35
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Intermetallics journal homepage: www.elsevier.com/locate/intermet
Dynamic shear punching of metallic glass matrix composites J.W. Qiao a, b, *, H.Y. Ye a, H.J. Yang c, **, W. Liang a, b, B.S. Xu a, b, P.K. Liaw d, M.W. Chen e a
College of Materials Science and Engineering, Taiyuan University of Technology, Yingze West 30, Shanxi, Taiyuan 030024, China Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, China c Research Institute of Surface Engineering, Taiyuan University of Technology, Taiyuan 030024, China d Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996-2200, USA e WPI Advanced Institute of Materials Research, Tohoku University, Sendai 980-8577, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 August 2012 Received in revised form 14 November 2012 Accepted 31 December 2012 Available online 30 January 2013
Dynamic shear punching is employed to process Zr36.6Ti31.4Nb7Cu5.9Be19.1 metallic glass matrix composites. A brittle fracture dominates during punchforming, and no macroscopic cracks or ruffles can be found, indicative of a good surface finish. The softening instability for metallic glasses happens once the yielding is available, avoiding the existence of shear bands and achieving smooth surfaces. No crystallization is accompanied in the heavy deformation zones during dynamic shear punching. The thickness of punched sheets for various BMG systems is highly dependent on the shear strengths. Furthermore, it is deduced that larger glass-transition temperatures of BMGs correspond to the thinner punched sheets, which provides a theoretical guidance to rapidly process metallic glass sheets. Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
Keywords: B. Glasses, metallic B. Plastic deformation mechanism C. Plastic forming, cold E. Mechanical properties, theory
1. Introduction Bulk metallic glasses (BMGs) have attracted a great deal of interest over past decades due to their high yielding strength, good corrosion resistance, and large elastic limits as compared with their crystalline counterparts [1]. However, a catastrophic failure usually happens once BMGs yield upon loading at room temperature, accompanied by the fast propagation of localized shear bands. This brittle fracture behavior without appropriate fracture resistance hinders their future applications as structural materials. The attainment of both strength and toughness is a vital requirement for most structural materials [2]. Therefore, a series of in-situ ductile dendrite-reinforced metallic glass matrix (MGM) composites, with the simple casting, high glass-forming ability of matrices, and improved tensile ductility, etc, have been widely developed to solve the conflict between strength and toughness [3e5]. The promising properties, combining the high strength and toughness, make
MGM composites potential candidates as structural engineering applications. For actual applications, processing is one of the most important procedures into final products. Up to date, almost all the processing for metallic glasses is at a temperature within the supercooled liquid region, namely, between the glass-transition and crystallization temperatures, since the rheology of the viscous glass or glass matrix within the supercooled liquid is greatly improved at high temperatures [6e8].With a viscous state, BMGs easily undergo a homogeneous deformation, instead of the availability of “defects” - localized shear bands. However, at room temperature, the machining processing usually results in the presence of shear bands and voids on the surface rather than a smooth surface [9,10]. In this letter, punchforming is employed to process the metallic-glass sheets, which avoids profuse shear bands in the deformation zone, and mechanical behavior is investigated.
2. Experimental
* Corresponding author. College of Materials Science and Engineering, Taiyuan University of Technology, Yingze West 30, Shanxi, Taiyuan 030024, China. Tel.: þ86 351 6018051. ** Corresponding author. E-mail addresses: [email protected] (J.W. Qiao), [email protected] (H.J. Yang).
Ingots of a nominal composition (in atomic percent, at %), Zr36.6Ti31.4Nb7Cu5.9Be19.1 [3], was prepared by arc melting the mixture of Zr, Ti, Nb, Cu, and Be with a purity higher than 99.9% (weight percent) under a Ti-gettered argon atmosphere. The liquid alloys were suctioned into a water-cooled copper mold with a plate shape, and the as-cast sample is 60 mm (length) 30 (width)
0966-9795/$ e see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.intermet.2012.12.016
32
J.W. Qiao et al. / Intermetallics 36 (2013) 31e35
mm 2 (thickness) mm plates. A thin sheet with a thickness of 0.50 mm was obtained by wire cutting from the as-cast samples and well polished with a final thickness of about 0.48 mm and with mirror surfaces. High-speed dynamic punchforming was carried out to process metallic-glass sheets by a crank press and die cutting machine with the maximum punching velocities of 440 and 140 mm/s, respectively. The microstructure of the as-cast plate was observed using an optical microscope. The fracture and lateral surfaces of the samples after punchforming were investigated by scanning-electron microscopy (SEM) to identify the fracture mechanisms. The synchrotron X-ray studies were carried out on the specimens near the edges of the punched part, and the detailed experimental setup was found in Ref. [5].
3. Results and discussion Fig. 1 shows the microstructure of as-cast samples. The flowery dendrites, b-Zr(Ti) solid solutions with a volume fraction of dendrites about 40%, are homogeneously distributed in the featureless glass matrix. The elemental Nb acts as a stabilizer for the precipitation of the b-Zr(Ti) solid solution in the ZreTieNbeCueBe system, and yield high volume fractions. An individual dendrite tree, which has been cross-sectioned near its nucleation site, has an estimate of the spanning length of w15 mm. The diameter of the primary dendrite arms is w3 mm. Fig. 2(a) displays the illustration of the punching process by a crank press (left picture) and die cutting machine (right picture). Upon loading with a direction indicated by the arrows in Fig. 2(a), a round pore or a straight edge is easily obtained in a sheet sample through dynamic shear punching, as illustrated in Fig. 2(b). Fig. 2(c) shows many punched sheet samples of present metallic glass matrix composites and 304 stainless steels for reference, indicated by arrows in Fig. 2(c). No macroscopic cracks or ruffles can be found, regardless of present composite or steel samples, indicating a successful way to process the metallic glass matrix composites. It has been demonstrated that dynamic shear punching was a convenient method to process the crystalline alloys, such as 1018 steel, 6061-T6 aluminum, and Ti 6% Al - 4% V alloys [11]. During punching, adiabatic shearing happens for these alloys based on dislocation theories with a width of shear bands of tens of microns. What will happen for metallic glasses lacking of dislocations during dynamic punching? Previously, Wu et al. [12] conducted a small
Fig. 1. The microstructure of as-cast metallic glass matrix composites.
Fig. 2. Illustration of the punching process by crank press (left picture) and die cutting machine (right picture) (a), illustration of punched sheets (b), and many punched sheet samples of present metallic glass matrix composites and 304 stainless steels after dynamic shear punching (c).
punching test on a Zr-based metallic glass sheet with a thickness of 0.3e0.5 mm under a loading rate of 0.001 mm/s, and found that the metallic glass could be controlled to create regularly arrayed fine multiple shear bands under multiaxial loading. Guduru et al. [13] have found that a distinct plasticity could be obtained for a Zrbased BMG with quasi-static shear punching velocities of about 0.424e84.7 mm/s. It is well known that a brittle fracture dominates upon high-speed dynamic compression for BMGs or metallic glass matrix composites [14,15], though some plasticity could be gained under a quasi-static compressive loading. In order to give a detailed investigation, the further observation of fractographs near the edges of samples after dynamic punching is conducted, since the surface quality is a key factor subjected to heavy deformation. Fig. 3(a) shows an SEM image of ring samples, and the inner and outer edges have undergone shear punching. Roughly smooth surfaces instead of macroscopic cracks or ruffles exist, consistent with Fig. 2(c). After close inspections to the area near the inner boundary of the ring, indicated by arrow in Fig. 3(a), few deformation zones with an availability of very localized shear bands along the edges can be found, as shown in Fig. 3(b), and the span of the deformation zone is less than 20 mm with a space between shear bands of several hundred nanometers. The shear plane along the thickness of sheets is exhibited in Fig. 3(c), and vein patterns dominate, together with obvious initial shear zones between two lines, indicated by white and black lines. Locally enlarged initial shear zones are displayed in the inset of Fig. 3(c), and the arrows indicate the sliding traces. Usually, the final facture corresponds to a pure shear, similar to the shear offset region during compression, accompanied by the light and sound emissions. These pure shearing regions experience rapid heat dissipation at the moment of fracture. Magnified vein patterns are shown in Fig. 3(d), in agreement with patterns on the fracture surfaces after dynamic compression [15]. Besides, liquid droplets can be found on the fracture surfaces, indicating the occurrence of local melting during fracture [16]. Zener and Holloman [17] proposed the adiabatic shear localization criteria: when the shear stress, s, decreases with an increase x, the deformation at this point in shear strain, g, at a material point, ~ is unstable, and the instability occurs as follows:
J.W. Qiao et al. / Intermetallics 36 (2013) 31e35
33
Fig. 3. SEM image of ring samples shown in (a), very localized shear bands along the edges found (b), the shear plane along the thickness of sheets exhibited in (c), and locally enlarged initial shear zones displayed in the inset of (c), and magnified vein patterns shown in (d).
ds dg
~ x
0
(1)
By assuming that the flow stress is a function of plastic strain, plastic strain rate, and temperature,
s ¼ f ðg; g_ ; TÞ
(2)
Considering that the plastic work converting into heat upon adiabatic conditions, Eq. (1) can be expressed in the following form [18]:
bs vs vs dg_ =dt vs þ 0 þ vg g_ ;T vg_ g;T dg=dt vT g_ ;g rcp
(3)
The partial derivatives in the first, second, and third terms on the right-hand side represent strain hardening, strain-rate hardening, and thermal softening, respectively. b is the ratio of the storage energy converted to heat; r is the density of materials, and Cp is the heat capacity. Careful examination of Eq. (3) indicates that the criterion may be satisfied if the thermal softening term is greater than the strain and strain rate hardening terms. The above analysis is based on the shear deformation of crystalline alloys. For monolithic BMGs, the structural softening instead of thermal softening dominates upon shear punching. Therefore, it is not possible to apply Eq. (3) to the study of monolithic BMGs. For the amorphous alloys, Spaepen [19] has proposed the softening mechanism during inhomogeneous deformation: if there is to be a lowing of viscosity in the shear bands, there must be an increase of the free volume. Based on Spaepen’s free volume theory, Yang [20] has developed the flow model.
g_ ¼ cexp
aV * Vf
!
DGm slU sinh exp RT 2RT
(4)
where g_ is the shear strain rate, c is a factor associated with the amount of the flow units, a is a constant between 1 and 1/2, V* is the effective hard-sphere size of atoms, Vf is the average free volume of an atom, DGm is the thermal activation energy, R is the gas constant, T is the absolute temperature, and U is the molar atomic volume. The value of l is in the range of 0e1. From Eq. (4), it is simply deduced that the higher shear strain rate, g_ , may lead to the more creation of free volumes, Vfwithin the shear plane, which facilitates the propagation of shear bands. As a consequence, single mature shear bands rather than multiple shear bands are available during high-speed dynamic shear punching, evidenced by smooth surfaces shown in Fig. 3(a). As far as the dual-phase composites are concerned, the highvolume fractioned crystalline dendrites may experience a certain extent thermal softening during shear punching. Due to the lack of dislocations in the glass matrix and pile-ups of dislocations within the dendrites upon dynamic shear punching [15], very little strain hardening is accompanied, and softening dominates with a premier failure after yielding [14,15]. Thus, it is simply assumed that strain hardening and strain-rate hardening can be ignored in the present composites. During dynamic shear punching of present metallic glass matrix composites, softening instability occurs once the yielding is available. On the assumption that the capability of further deformation upon yielding be popular, the interaction between the propagating shear bands and dendrites would lead to the presence of localized deformation zones full of shear bands. For example, distinguished work hardening is gained upon quasi-static compression, accompanied by profuse shear bands on the lateral surfaces of samples [4,15]. In contrast, multiplication of shear bands is absent upon dynamic loading. Therefore, it is beneficial to employ high-speed dynamic shear punching to process the present composites, avoiding the existence of shear bands and achieving smooth surfaces. Besides, dynamic shear punching is demonstrated to be an advancement in rapid processing. It should be noted that although electrical-discharge machining could process BMGs [21],
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J.W. Qiao et al. / Intermetallics 36 (2013) 31e35
it is a time-consuming method in contrast to dynamic shear punching. Dynamic shear punching results in a sharp temperature rise within thin shear bands, which may facilitate the crystallization within and near shear bands. Although few shear bands are accompanied, special care is taken to investigate whether crystallization happens. Here, the highly accurate synchrotron X-ray is employed to check the deformation zone full of shear bands, in superposition with Fig. 3(b). As shown in Fig. 4, the high-energy Xray pattern suggests the body-centered-cubic (bcc) b-Zr solidsolution diffraction peaks are superimposed on the broad diffuse scattering maxima from the amorphous matrix. Except for the solid solutions, no other crystallized phases can be determined. Fully continuous diffraction rings in the inset of Fig. 4, corresponding to the X-ray line profile, are ascribed to the homogeneouslydistributed dendrites. Therefore, no crystallization occurs in the heavy deformation zones during dynamic shear punching, indicative of an efficient way to process metallic-glass sheets with a good surface finish. Usually, the maximum blanking force, which is needed to break the sheet samples, can be calculated as follows:
F ¼ KLts ss
(5)
where F is the maximum blanking force (N), K is a factor with a value of about 1.3, L is the perimeter of punched samples (mm), ts is the thickness of samples (mm), and ss is the maximum shear stress (MPa). Here, we consider the maximum diameter of punched rings to be 7 mm for the present composites, and the thickness to be 0.48 mm. Regardless of quasi-static or dynamic loading, the shear fracture angle is approximately 45 [15]. As a result, ss is simply taken to be half of the ultimate strength, with a value of 756 MPa [3]. Thus, F ¼ 10,369 N. Supposing that the maximum blanking force be fixed for the current crank press, i.e., F ¼ 10,369 N, and the diameter of punched samples be constant with a value of 7 mm, the dependence of the thickness of punched BMG sheets on shear strengths is shown in Fig. 5. It should be noted that shear strengths for various bulk metallic glasses are half of the ultimate strengths, since it is simply to consider a shear fracture angle of 45 and the data in Fig. 5 are summarized in supporting materials. Obviously, Co-, Ta-, Fe-, and Ni-based BMGs exhibit superhigh shear strengths. As a result, very thin sheets can be sheared. For example, only 129 mm thickness for the Co55Ta10B35 BMG with a shear strength of 2815 MPa is obtained. Contrarily, La-, Nd-, and Ce-based BMGs possess very low shear strengths, which could be shear punched with thicker sheets. The Ce70Al10Ni10Cu10 BMG can be shear punched as thick as 1815 mm, much thicker than
Fig. 5. The dependence of maximum shear punched thickness on shear strengths for various BMGs. The punching process illustrated in the inset. “C” denoting present composites.
other BMGs since it has a shear strength of only 200 MPa. Zr-, Ti-, Mg, and Ti-based BMGs have medium shear thicknesses, as displayed in Fig. 5. Yang et al. [22] have found that the ultimate strengths of BMGs are highly dependent on the glass-transition temperatures. The lager the glass-transition temperatures, the higher the ultimate strengths of BMGs are. Thus, it is deduced that larger glass-transition temperatures of BMGs correspond to the thinner punched sheets in this study. Conformably, the Co55Ta10B35 BMG with a largest glasstransition temperature of 975 K in all investigated BMG systems has a thinnest shear punched thickness. It should be noted that the maximum punched thickness of BMG sheets is determined by the strengths of materials regardless of the microstructures of materials in the current analysis. Actually, the other factors, such as Poisson’s ratio associated with intrinsic plasticity, may be the main factor, since brittle BMGs easily enable the fast propagation of shear bands, leading to a surface crack, instead of a smooth or mirror surface. Besides, the composites containing ductile dendrites can be sheared favorably compared to the monolithic BMGs, since the availability of ductile dendrites make the frontier of open cracks blunt during shear punching. The more affecting factors are not fully understood, which exceeds the scope of the present study. 4. Conclusion In summary, a method of dynamic shear punching through a crank press and die cutting machine is employed to process Zr36.6Ti31.4Nb7Cu5.9Be19.1 metallic glass matrix composites. The composite sheets are shear punched with brittle behavior, and no macroscopic cracks or ruffles can be found, suggesting a good surface finish. During dynamic shear punching of present metallic glass matrix composites, softening instability occurs once the yielding is available, avoiding the existence of shear bands and achieving smooth surfaces. Even undergoing a heavy deformation within or near shear bands, i.e., a high temperature rise, no crystallization occurs at the deformation zones. Supposing that the maximum blanking force and the diameter of punched sheets be fixed for current crank press, the dependence of the thickness of punched sheets on the shear strengths for various BMG systems is investigated in detail. It is deduced that larger glass-transition temperatures of BMGs correspond to the thinner punched sheets, which provides a theoretical foundation to process metallic glass sheets. Acknowledgment
Fig. 4. Synchrotron X-ray of composite sheets after dynamic shear punching.
J.W.Q. would like to acknowledge the financial support of the National Natural Science Foundation of China (No.51101110), the
J.W. Qiao et al. / Intermetallics 36 (2013) 31e35
Youth Science Foundation of Shanxi Province, China (No.2012021018-1), and the Research Project Supported by Shanxi Scholarship Council of China (No.2012-032). P.K.L. appreciates the support of National Science Foundations (DMR-0909037, CMMI0900271, and CMMI-1100080) with Drs. A. Ardell and C.V. Cooper as program directors. References [1] Miller MK, Liaw PK. Bulk metallic glasses. New York: Springer; 2007. [2] Ritchie RO. Nat Mater 2011;10:817. [3] Hofmann DC, Suh JY, Wiest A, Duan G, Lind ML, Demetriou MD, et al. Nature 2008;451:1085. [4] Qiao JW, Wang S, Zhang Y, Liaw PK, Chen GL. Appl Phys Lett 2009;94:151905. [5] Qiao JW, Sun AC, Huang EW, Zhang Y, Liaw PK, Chuang CP. Acta Mater 2011; 59:4126. [6] Schroers J. Adv Mater 2010;22:1566.
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[7] Hofmann DC, Kozachkov H, Khalifa HE, Schramm JP, Demetriou MD, Vecchio KS, et al. JOM 2009;61:11. [8] Qiao JW, Zhang Y, Jia HL, Yang HJ, Liaw PK, Xu BS. Appl Phys Lett 2012;100:121902. [9] Bakkal M, Shih AJ, Scattergood RO, Liu CT. Scr Mater 2004;50:583. [10] Jiang MQ, Dai LH. Acta Mater 2009;57:2730. [11] Roessig KM, Mason JJ. Int J Plasticity 1999;15:241. [12] Wu FF, Zhang ZF, Shen J, Mao SX. Acta Mater 2008;56:894. [13] Guduru RK, Darling KA, Scattergood RO, Koch CC, Murty KL, Bakkal M, et al. Intermetallics 2006;14:1411. [14] Lee D-J, Kim YG, Lee S, Kim NJ. Metall Mater Trans A 2006;37:2893. [15] Qiao JW, Feng P, Zhang Y, Zhang QM, Liaw PK, Chen GL. J Mater Res 2010;25: 2264. [16] Liu CT, Heatherly L, Easton DS, Carmichael CA, Schneibel JH, Chen CH, et al. Metall Mater Trans A 1998;29:1811. [17] Zener C, Hollomon JH. J Appl Phys 1944;15:22. [18] Roessig KM, Mason JJ. Int J Plasticity 1999;15:263. [19] Spaepen F. Acta Metall 1977;25:407. [20] Yang F. Appl Phys Lett 2007;91:051922. [21] Liu Z, Li R, Huang L, Lu X, Zhang T. J Non-Crysts Solids 2011;357:3190. [22] Yang B, Liu CT, Nieh TG. Appl Phys Lett 2006;88:221911.
Contents lists available at SciVerse ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Dynamic shear punching of metallic glass matrix composites J.W. Qiao a, b, *, H.Y. Ye a, H.J. Yang c, **, W. Liang a, b, B.S. Xu a, b, P.K. Liaw d, M.W. Chen e a
College of Materials Science and Engineering, Taiyuan University of Technology, Yingze West 30, Shanxi, Taiyuan 030024, China Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, China c Research Institute of Surface Engineering, Taiyuan University of Technology, Taiyuan 030024, China d Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996-2200, USA e WPI Advanced Institute of Materials Research, Tohoku University, Sendai 980-8577, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 August 2012 Received in revised form 14 November 2012 Accepted 31 December 2012 Available online 30 January 2013
Dynamic shear punching is employed to process Zr36.6Ti31.4Nb7Cu5.9Be19.1 metallic glass matrix composites. A brittle fracture dominates during punchforming, and no macroscopic cracks or ruffles can be found, indicative of a good surface finish. The softening instability for metallic glasses happens once the yielding is available, avoiding the existence of shear bands and achieving smooth surfaces. No crystallization is accompanied in the heavy deformation zones during dynamic shear punching. The thickness of punched sheets for various BMG systems is highly dependent on the shear strengths. Furthermore, it is deduced that larger glass-transition temperatures of BMGs correspond to the thinner punched sheets, which provides a theoretical guidance to rapidly process metallic glass sheets. Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
Keywords: B. Glasses, metallic B. Plastic deformation mechanism C. Plastic forming, cold E. Mechanical properties, theory
1. Introduction Bulk metallic glasses (BMGs) have attracted a great deal of interest over past decades due to their high yielding strength, good corrosion resistance, and large elastic limits as compared with their crystalline counterparts [1]. However, a catastrophic failure usually happens once BMGs yield upon loading at room temperature, accompanied by the fast propagation of localized shear bands. This brittle fracture behavior without appropriate fracture resistance hinders their future applications as structural materials. The attainment of both strength and toughness is a vital requirement for most structural materials [2]. Therefore, a series of in-situ ductile dendrite-reinforced metallic glass matrix (MGM) composites, with the simple casting, high glass-forming ability of matrices, and improved tensile ductility, etc, have been widely developed to solve the conflict between strength and toughness [3e5]. The promising properties, combining the high strength and toughness, make
MGM composites potential candidates as structural engineering applications. For actual applications, processing is one of the most important procedures into final products. Up to date, almost all the processing for metallic glasses is at a temperature within the supercooled liquid region, namely, between the glass-transition and crystallization temperatures, since the rheology of the viscous glass or glass matrix within the supercooled liquid is greatly improved at high temperatures [6e8].With a viscous state, BMGs easily undergo a homogeneous deformation, instead of the availability of “defects” - localized shear bands. However, at room temperature, the machining processing usually results in the presence of shear bands and voids on the surface rather than a smooth surface [9,10]. In this letter, punchforming is employed to process the metallic-glass sheets, which avoids profuse shear bands in the deformation zone, and mechanical behavior is investigated.
2. Experimental
* Corresponding author. College of Materials Science and Engineering, Taiyuan University of Technology, Yingze West 30, Shanxi, Taiyuan 030024, China. Tel.: þ86 351 6018051. ** Corresponding author. E-mail addresses: [email protected] (J.W. Qiao), [email protected] (H.J. Yang).
Ingots of a nominal composition (in atomic percent, at %), Zr36.6Ti31.4Nb7Cu5.9Be19.1 [3], was prepared by arc melting the mixture of Zr, Ti, Nb, Cu, and Be with a purity higher than 99.9% (weight percent) under a Ti-gettered argon atmosphere. The liquid alloys were suctioned into a water-cooled copper mold with a plate shape, and the as-cast sample is 60 mm (length) 30 (width)
0966-9795/$ e see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.intermet.2012.12.016
32
J.W. Qiao et al. / Intermetallics 36 (2013) 31e35
mm 2 (thickness) mm plates. A thin sheet with a thickness of 0.50 mm was obtained by wire cutting from the as-cast samples and well polished with a final thickness of about 0.48 mm and with mirror surfaces. High-speed dynamic punchforming was carried out to process metallic-glass sheets by a crank press and die cutting machine with the maximum punching velocities of 440 and 140 mm/s, respectively. The microstructure of the as-cast plate was observed using an optical microscope. The fracture and lateral surfaces of the samples after punchforming were investigated by scanning-electron microscopy (SEM) to identify the fracture mechanisms. The synchrotron X-ray studies were carried out on the specimens near the edges of the punched part, and the detailed experimental setup was found in Ref. [5].
3. Results and discussion Fig. 1 shows the microstructure of as-cast samples. The flowery dendrites, b-Zr(Ti) solid solutions with a volume fraction of dendrites about 40%, are homogeneously distributed in the featureless glass matrix. The elemental Nb acts as a stabilizer for the precipitation of the b-Zr(Ti) solid solution in the ZreTieNbeCueBe system, and yield high volume fractions. An individual dendrite tree, which has been cross-sectioned near its nucleation site, has an estimate of the spanning length of w15 mm. The diameter of the primary dendrite arms is w3 mm. Fig. 2(a) displays the illustration of the punching process by a crank press (left picture) and die cutting machine (right picture). Upon loading with a direction indicated by the arrows in Fig. 2(a), a round pore or a straight edge is easily obtained in a sheet sample through dynamic shear punching, as illustrated in Fig. 2(b). Fig. 2(c) shows many punched sheet samples of present metallic glass matrix composites and 304 stainless steels for reference, indicated by arrows in Fig. 2(c). No macroscopic cracks or ruffles can be found, regardless of present composite or steel samples, indicating a successful way to process the metallic glass matrix composites. It has been demonstrated that dynamic shear punching was a convenient method to process the crystalline alloys, such as 1018 steel, 6061-T6 aluminum, and Ti 6% Al - 4% V alloys [11]. During punching, adiabatic shearing happens for these alloys based on dislocation theories with a width of shear bands of tens of microns. What will happen for metallic glasses lacking of dislocations during dynamic punching? Previously, Wu et al. [12] conducted a small
Fig. 1. The microstructure of as-cast metallic glass matrix composites.
Fig. 2. Illustration of the punching process by crank press (left picture) and die cutting machine (right picture) (a), illustration of punched sheets (b), and many punched sheet samples of present metallic glass matrix composites and 304 stainless steels after dynamic shear punching (c).
punching test on a Zr-based metallic glass sheet with a thickness of 0.3e0.5 mm under a loading rate of 0.001 mm/s, and found that the metallic glass could be controlled to create regularly arrayed fine multiple shear bands under multiaxial loading. Guduru et al. [13] have found that a distinct plasticity could be obtained for a Zrbased BMG with quasi-static shear punching velocities of about 0.424e84.7 mm/s. It is well known that a brittle fracture dominates upon high-speed dynamic compression for BMGs or metallic glass matrix composites [14,15], though some plasticity could be gained under a quasi-static compressive loading. In order to give a detailed investigation, the further observation of fractographs near the edges of samples after dynamic punching is conducted, since the surface quality is a key factor subjected to heavy deformation. Fig. 3(a) shows an SEM image of ring samples, and the inner and outer edges have undergone shear punching. Roughly smooth surfaces instead of macroscopic cracks or ruffles exist, consistent with Fig. 2(c). After close inspections to the area near the inner boundary of the ring, indicated by arrow in Fig. 3(a), few deformation zones with an availability of very localized shear bands along the edges can be found, as shown in Fig. 3(b), and the span of the deformation zone is less than 20 mm with a space between shear bands of several hundred nanometers. The shear plane along the thickness of sheets is exhibited in Fig. 3(c), and vein patterns dominate, together with obvious initial shear zones between two lines, indicated by white and black lines. Locally enlarged initial shear zones are displayed in the inset of Fig. 3(c), and the arrows indicate the sliding traces. Usually, the final facture corresponds to a pure shear, similar to the shear offset region during compression, accompanied by the light and sound emissions. These pure shearing regions experience rapid heat dissipation at the moment of fracture. Magnified vein patterns are shown in Fig. 3(d), in agreement with patterns on the fracture surfaces after dynamic compression [15]. Besides, liquid droplets can be found on the fracture surfaces, indicating the occurrence of local melting during fracture [16]. Zener and Holloman [17] proposed the adiabatic shear localization criteria: when the shear stress, s, decreases with an increase x, the deformation at this point in shear strain, g, at a material point, ~ is unstable, and the instability occurs as follows:
J.W. Qiao et al. / Intermetallics 36 (2013) 31e35
33
Fig. 3. SEM image of ring samples shown in (a), very localized shear bands along the edges found (b), the shear plane along the thickness of sheets exhibited in (c), and locally enlarged initial shear zones displayed in the inset of (c), and magnified vein patterns shown in (d).
ds dg
~ x
0
(1)
By assuming that the flow stress is a function of plastic strain, plastic strain rate, and temperature,
s ¼ f ðg; g_ ; TÞ
(2)
Considering that the plastic work converting into heat upon adiabatic conditions, Eq. (1) can be expressed in the following form [18]:
bs vs vs dg_ =dt vs þ 0 þ vg g_ ;T vg_ g;T dg=dt vT g_ ;g rcp
(3)
The partial derivatives in the first, second, and third terms on the right-hand side represent strain hardening, strain-rate hardening, and thermal softening, respectively. b is the ratio of the storage energy converted to heat; r is the density of materials, and Cp is the heat capacity. Careful examination of Eq. (3) indicates that the criterion may be satisfied if the thermal softening term is greater than the strain and strain rate hardening terms. The above analysis is based on the shear deformation of crystalline alloys. For monolithic BMGs, the structural softening instead of thermal softening dominates upon shear punching. Therefore, it is not possible to apply Eq. (3) to the study of monolithic BMGs. For the amorphous alloys, Spaepen [19] has proposed the softening mechanism during inhomogeneous deformation: if there is to be a lowing of viscosity in the shear bands, there must be an increase of the free volume. Based on Spaepen’s free volume theory, Yang [20] has developed the flow model.
g_ ¼ cexp
aV * Vf
!
DGm slU sinh exp RT 2RT
(4)
where g_ is the shear strain rate, c is a factor associated with the amount of the flow units, a is a constant between 1 and 1/2, V* is the effective hard-sphere size of atoms, Vf is the average free volume of an atom, DGm is the thermal activation energy, R is the gas constant, T is the absolute temperature, and U is the molar atomic volume. The value of l is in the range of 0e1. From Eq. (4), it is simply deduced that the higher shear strain rate, g_ , may lead to the more creation of free volumes, Vfwithin the shear plane, which facilitates the propagation of shear bands. As a consequence, single mature shear bands rather than multiple shear bands are available during high-speed dynamic shear punching, evidenced by smooth surfaces shown in Fig. 3(a). As far as the dual-phase composites are concerned, the highvolume fractioned crystalline dendrites may experience a certain extent thermal softening during shear punching. Due to the lack of dislocations in the glass matrix and pile-ups of dislocations within the dendrites upon dynamic shear punching [15], very little strain hardening is accompanied, and softening dominates with a premier failure after yielding [14,15]. Thus, it is simply assumed that strain hardening and strain-rate hardening can be ignored in the present composites. During dynamic shear punching of present metallic glass matrix composites, softening instability occurs once the yielding is available. On the assumption that the capability of further deformation upon yielding be popular, the interaction between the propagating shear bands and dendrites would lead to the presence of localized deformation zones full of shear bands. For example, distinguished work hardening is gained upon quasi-static compression, accompanied by profuse shear bands on the lateral surfaces of samples [4,15]. In contrast, multiplication of shear bands is absent upon dynamic loading. Therefore, it is beneficial to employ high-speed dynamic shear punching to process the present composites, avoiding the existence of shear bands and achieving smooth surfaces. Besides, dynamic shear punching is demonstrated to be an advancement in rapid processing. It should be noted that although electrical-discharge machining could process BMGs [21],
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it is a time-consuming method in contrast to dynamic shear punching. Dynamic shear punching results in a sharp temperature rise within thin shear bands, which may facilitate the crystallization within and near shear bands. Although few shear bands are accompanied, special care is taken to investigate whether crystallization happens. Here, the highly accurate synchrotron X-ray is employed to check the deformation zone full of shear bands, in superposition with Fig. 3(b). As shown in Fig. 4, the high-energy Xray pattern suggests the body-centered-cubic (bcc) b-Zr solidsolution diffraction peaks are superimposed on the broad diffuse scattering maxima from the amorphous matrix. Except for the solid solutions, no other crystallized phases can be determined. Fully continuous diffraction rings in the inset of Fig. 4, corresponding to the X-ray line profile, are ascribed to the homogeneouslydistributed dendrites. Therefore, no crystallization occurs in the heavy deformation zones during dynamic shear punching, indicative of an efficient way to process metallic-glass sheets with a good surface finish. Usually, the maximum blanking force, which is needed to break the sheet samples, can be calculated as follows:
F ¼ KLts ss
(5)
where F is the maximum blanking force (N), K is a factor with a value of about 1.3, L is the perimeter of punched samples (mm), ts is the thickness of samples (mm), and ss is the maximum shear stress (MPa). Here, we consider the maximum diameter of punched rings to be 7 mm for the present composites, and the thickness to be 0.48 mm. Regardless of quasi-static or dynamic loading, the shear fracture angle is approximately 45 [15]. As a result, ss is simply taken to be half of the ultimate strength, with a value of 756 MPa [3]. Thus, F ¼ 10,369 N. Supposing that the maximum blanking force be fixed for the current crank press, i.e., F ¼ 10,369 N, and the diameter of punched samples be constant with a value of 7 mm, the dependence of the thickness of punched BMG sheets on shear strengths is shown in Fig. 5. It should be noted that shear strengths for various bulk metallic glasses are half of the ultimate strengths, since it is simply to consider a shear fracture angle of 45 and the data in Fig. 5 are summarized in supporting materials. Obviously, Co-, Ta-, Fe-, and Ni-based BMGs exhibit superhigh shear strengths. As a result, very thin sheets can be sheared. For example, only 129 mm thickness for the Co55Ta10B35 BMG with a shear strength of 2815 MPa is obtained. Contrarily, La-, Nd-, and Ce-based BMGs possess very low shear strengths, which could be shear punched with thicker sheets. The Ce70Al10Ni10Cu10 BMG can be shear punched as thick as 1815 mm, much thicker than
Fig. 5. The dependence of maximum shear punched thickness on shear strengths for various BMGs. The punching process illustrated in the inset. “C” denoting present composites.
other BMGs since it has a shear strength of only 200 MPa. Zr-, Ti-, Mg, and Ti-based BMGs have medium shear thicknesses, as displayed in Fig. 5. Yang et al. [22] have found that the ultimate strengths of BMGs are highly dependent on the glass-transition temperatures. The lager the glass-transition temperatures, the higher the ultimate strengths of BMGs are. Thus, it is deduced that larger glass-transition temperatures of BMGs correspond to the thinner punched sheets in this study. Conformably, the Co55Ta10B35 BMG with a largest glasstransition temperature of 975 K in all investigated BMG systems has a thinnest shear punched thickness. It should be noted that the maximum punched thickness of BMG sheets is determined by the strengths of materials regardless of the microstructures of materials in the current analysis. Actually, the other factors, such as Poisson’s ratio associated with intrinsic plasticity, may be the main factor, since brittle BMGs easily enable the fast propagation of shear bands, leading to a surface crack, instead of a smooth or mirror surface. Besides, the composites containing ductile dendrites can be sheared favorably compared to the monolithic BMGs, since the availability of ductile dendrites make the frontier of open cracks blunt during shear punching. The more affecting factors are not fully understood, which exceeds the scope of the present study. 4. Conclusion In summary, a method of dynamic shear punching through a crank press and die cutting machine is employed to process Zr36.6Ti31.4Nb7Cu5.9Be19.1 metallic glass matrix composites. The composite sheets are shear punched with brittle behavior, and no macroscopic cracks or ruffles can be found, suggesting a good surface finish. During dynamic shear punching of present metallic glass matrix composites, softening instability occurs once the yielding is available, avoiding the existence of shear bands and achieving smooth surfaces. Even undergoing a heavy deformation within or near shear bands, i.e., a high temperature rise, no crystallization occurs at the deformation zones. Supposing that the maximum blanking force and the diameter of punched sheets be fixed for current crank press, the dependence of the thickness of punched sheets on the shear strengths for various BMG systems is investigated in detail. It is deduced that larger glass-transition temperatures of BMGs correspond to the thinner punched sheets, which provides a theoretical foundation to process metallic glass sheets. Acknowledgment
Fig. 4. Synchrotron X-ray of composite sheets after dynamic shear punching.
J.W.Q. would like to acknowledge the financial support of the National Natural Science Foundation of China (No.51101110), the
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Youth Science Foundation of Shanxi Province, China (No.2012021018-1), and the Research Project Supported by Shanxi Scholarship Council of China (No.2012-032). P.K.L. appreciates the support of National Science Foundations (DMR-0909037, CMMI0900271, and CMMI-1100080) with Drs. A. Ardell and C.V. Cooper as program directors. References [1] Miller MK, Liaw PK. Bulk metallic glasses. New York: Springer; 2007. [2] Ritchie RO. Nat Mater 2011;10:817. [3] Hofmann DC, Suh JY, Wiest A, Duan G, Lind ML, Demetriou MD, et al. Nature 2008;451:1085. [4] Qiao JW, Wang S, Zhang Y, Liaw PK, Chen GL. Appl Phys Lett 2009;94:151905. [5] Qiao JW, Sun AC, Huang EW, Zhang Y, Liaw PK, Chuang CP. Acta Mater 2011; 59:4126. [6] Schroers J. Adv Mater 2010;22:1566.
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