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Significant tensile ductility induced by cold rolling in Cu47.5Zr47.5Al5 bulk metallic glass
Intermetallics 19 (2011) 1394e1398
Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Significant tensile ductility induced by cold rolling in Cu47.5Zr47.5Al5 bulk metallic glass K.K. Song a, b, *, S. Pauly a, Y. Zhang a, S. Scudino a, P. Gargarella a, K.B. Surreddi a, U. Kühn a, J. Eckert a, b a b
IFW Dresden, Institut für Komplexe Materialien, Helmholtzstraße 20, D-01069 Dresden, Germany TU Dresden, Institut für Werkstoffwissenschaft, D-01062 Dresden, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 March 2011 Received in revised form 28 April 2011 Accepted 2 May 2011 Available online 15 June 2011
Significant tensile plasticity up to 0.7 0.1% together with work-hardening and larger fracture strength was obtained in Cu47.5Zr47.5Al5 bulk metallic glass (BMG) upon cold rolling with only 2.9 0.3% thickness reduction. The good deformability could be attributed to the multiple pre-existing shear bands and structural inhomogeneity induced by rolling. The distributions of shear bands upon rolling can be predicted by a simplified rolling model. The underlying mechanism for the tensile plasticity was further discussed in the frame of potential energy landscape theory (PEL). Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Ternary alloy systems B. Glasses, metallic B. Mechanical properties at ambient temperature C. Rolling
1. Introduction Bulk metallic glasses (BMGs) are of commercial interests due to their outstanding properties such as high strength and large elastic strain combined with excellent glass-forming ability (GFA), making them potentially useful for a variety of structural and functional applications [1e5]. Recently, attention has been paid to improve their plasticity due to BMGs usually failing in an apparently brittle manner under unconstrained loading conditions [1e5]. The low ductility/toughness and poor fatigue resistance severely restricted their use as new-generation structural engineering materials. Therefore, it has become one of the most important issues to improve the ductility of BMGs. In order to improve the plasticity of BMGs at room temperature, in situ formed ductile phases or ex situ secondary particles with different length scales were introduced into the amorphous matrix to hinder the instantaneous propagation of shear bands [6e14]. Tensile plasticity combined with work-hardening has been achieved in BMG composites with nano- [13,15] or micro-scaled [14,16] ductile phases by using such strategies. Unfortunately, the resultant
* Corresponding author. IFW Dresden, Institut für Komplexe Materialien, Helmholtzstraße 20, D-01069 Dresden, Germany. Tel.: þ49 351 4659 532; fax: þ49 351 4659 452. E-mail address: [email protected] (K.K. Song). 0966-9795/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2011.05.001
composites with high volume fractions of crystalline phases usually sacrifice the high yield strength of the monolithic BMGs. It has been demonstrated that the pre-straining and cold rolling methods can effectively promote the plasticity of BMGs [17e20]. A promising advantage of cold rolling is that the yielding and fracture strengths of monolithic BMGs are not reduced but are reported to be even increased after rolling in some BMGs [21,22]. However, the improvement of the tensile plasticity usually requires a large thickness reduction after the cold rolling. For example, tensile plasticity of about 0.5% and 0.27% in Zr50Cu30Ni10Al10 and Zr64.13Cu15.75Ni10.12Al10 BMGs can be achieved by cold-rolling with thickness reductions of 10% and 50%, respectively. Moreover, the mechanism leading to the increased plasticity upon cold rolling is still under debate. In this letter, we report that an obvious tensile plasticity of 0.7 0.1% with work-hardening in a monolithic Cu47.5Zr47.5Al5 BMG can be attained with a very small thickness reduction of only 2.9 0.3% by cold rolling. A simplified model is proposed to interpret the formation of the multiple shear bands during cold rolling. The underlying mechanism for the tensile ductility improvement can be understood in the frame of potential energy landscape theory (PEL). 2. Experimental procedures The pre-alloys were prepared by arc melting appropriate amounts of the constituting elements (purity 99.9%) and were re-
K.K. Song et al. / Intermetallics 19 (2011) 1394e1398
melted three times in order to ensure chemical homogeneity under a Ti-gettered argon atmosphere. The samples were finally cast into 1.5t 10w 60l mm3 plates by an in situ suction casting facility attached to the arc melter. The casting temperature of CuZr-based alloys during casting was controlled by applying a certain melting current, namely 280 A, i.e. 70% of the maximum power which can be delivered by the power generator, for approximately 17 s. The elastic properties were measured using an Olympus Panametrics-NDT 5900PR ultrasonic testing device. The as-cast plates were cut into dog bone shape specimens with a gauge section of about 1.5t 2w 12l mm3 for the tensile tests. The specimens were rolled in one direction (1st-rolling, i.e. continuous rolling without changing direction) and two directions (2nd-rolling, i.e. rotating 180 in the rolling (horizontal) plane after each rolling pass), respectively, until the desired reduction thickness was obtained. The thickness reduction was calculated by E ¼ (h0 - h)/h0, where h0 and h represent the specimen thicknesses before and after rolling, respectively. Tensile tests were performed on an Instron 5869 testing machine (equipped with a Fiedler laser extensometer) at an initial strain rate of 2.5 104 s1. The structures of both ascast and cold-rolled specimens were ascertained by X-ray diffraction (XRD) (STOE STADI, Mo, Ka radiation l ¼ 0.07093187 nm). The glass transition, crystallization temperatures and the change of the enthalpy were measured using a PerkineElmer Pyris diamond differential scanning calorimeter (DSC) under argon atmosphere at the heating rate of 40 K/min. A thermomechanical analyzer (TMA) was used for viscosity measurements for as-cast and as-rolled specimens under argon atmosphere at a heating rate of 40 K/min, respectively. The microhardness of the as-cast and cold-rolled specimens were measured after grinding and polishing the surfaces of the specimens, respectively, with 1000e1200 indents for each specimen. The evolutions of shear bands and the fracture morphology were examined using a scanning electron microscope (SEM, Gemini 1530).
3. Results and discussion Fig. 1 shows the true stress-strain curves of the as-cast and asrolled specimens under tensile loading. All the specimens exhibit an elastic strain limit of less than 2%, which is a typical feature of monolithic BMGs. No obvious yielding is observed in the as-cast specimens and the fracture stress, sFT, is 1600 40 MPa, which is similar to previous results [13,15]. The as-rolled specimens exhibit an obvious yielding (sFT ¼ 1400 40 MPa) and a work-hardening behavior. The decrease of the yielding strength is believed to
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result from the pre-existing shear bands (Fig. 3(cee)) induced by the cold rolling before the tensile tests. Compared to the results reported in Refs. [21,22], it can be seen that the as-rolled specimens exhibit a much larger plasticity of 0.7 0.1% with a much smaller thickness reduction of only 2.9 0.3%. A pronounced workhardening behavior of the as-rolled specimens can be observed with the fracture strength of 1625 40 MPa, which may be due to the suppression of crack propagation in the as-rolled specimen by the deformation-induced residual stress [23]. As depicted in the inset of Fig. 1, XRD patterns of the as-cast and as-rolled specimens both show broad diffraction humps with no detectable crystalline Bragg peak, which indicates that cold rolling did not induce any crystallization in the specimens as reported in the previous works [19e22]. SEM observations reveal the significant difference in the morphologies of shear bands and fracture surfaces of the as-cast and as-rolled specimens after the tensile test. As shown in the inset of Fig. 2(a), the formation and propagation of one major shear band in the as-cast specimens dominate the fracture process. The tensile fracture angle, qT, between the tensile axis and the fracture plane is equal to 54 , which deviates from the angle of the maximum shear stress plane (45 ). It indicated that the von Mises yielding criterion cannot describe the tensile deformation behavior of the Cu47.5Zr47.5Al5 BMG in accordance with previous results [24]. A vein-like pattern is visible on the smooth fracture surface (Fig. 2(a) and (b)) together with some round cores of different diameters are on the whole surface for the as-cast specimens. Zhang et al. [24] suggested that the tensile fractures of BMGs should first originate from these cores induced by normal tension stress on the plane, and then catastrophically propagate towards outside of the cores driven by the shear stress. Moreover, there are only several tiny shear bands, which can be seen in the vicinity of the fracture part, implying that shear bands generated under tension are rather few and hence cannot contribute significantly to the overall tensile plasticity for the as-cast specimens. However, the asrolled specimens fail at an angle of 45 with respect to the loading axis (inset of Fig. 2(c)). It indicates that the occurrence of shear fracture under unconstrained loading conditions, which is in accord with the previous results [22]. Besides the above mentioned morphologies of the fracture surface and the whole surface of the as-cast specimens, some network-like structures (Fig. 2(c)) can be observed on the edge of the fracture surface of the as-rolled specimens after tensile tests. Such network-like structure is
1750 0.2%
True Stress (MPa)
1500 1250 Intensity (a.u.)
1000 750 500 250
As-cast
As-rolled
15
0 0.0
20
25
3.0
3.5
2 (Degree)
0.5
1.0
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True strain (%) Fig. 1. The true tensile stressestrain curves of as-cast (black curves) and as-rolled (red curves) specimens, respectively. Inset: XRD patterns of the as-cast and as-rolled specimens after tensile test.
Fig. 2. SEM images of (a,b) the as-cast specimens and (c,d) the as-rolled specimens after tensile test, respectively. Insets: the fractures of the as-cast and as-rolled specimens after tensile tests, respectively.
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Fig. 3. (a) The simple schematic illustration of the loading conditions; F from the rollers can be decomposed into two different forces in the vertical and the horizontal directions (Fv and Fp), respectively, (b) the diagrammatic sketch of evolutions of shear bands upon cold rolling, (c, d) the shear band distributions on the side surface after cold rolling and (e) the shear band distributions on the top surface after cold rolling and tensile test.
significantly different from the surface morphology of the as-cast specimens, which should originate from the typical wavy-curved shear band morphology [22]. Meanwhile, there are some cracks crossing the plate vertically as shown in the inset of Fig. 2(d). The surface micrograph of the as-rolled specimen before tensile tests is shown in Fig. 3(c)e(e). It can be seen that a large number of pre-existing shear bands with different morphologies distribute on the surface of the as-rolled sample. Here, we propose a simplified model to predict the evolutions of shear bands based on the loading conditions of cold rolling to illustrate the formation and evolution of shear bands during cold rolling. As illustrated in Fig. 3(a), the ascast plates subjected to cold rolling are in an unrestrained state in the rolling direction and are in a state similar to the condition of compression tests being vertical to the rolling direction. Therefore, the whole plate can be treated to be composed of many identical units as schematically shown in Fig. 3(b). Each unit experiences a force (Fv) vertical to the rolling direction and can be thought as under the condition of compression tests, whereas there is a rolling force (Fp) along at the rolling direction. Therefore, the rolling process can be simplified as a continuous process where all the units are subjected to compression tests one by one. During first cold rolling, primary shear bands form almost parallel at an angle of about 45 with respect to the loading direction (red lines in Fig. 3(b)) on the side surface, which is in good agreement with the results of compression tests of Cu47.5Zr47.5Al5 BMGs [25]. However, primary shear bands, named as primary strips, on the top surface as shown in Fig. 3(b) exhibit a parallel strip-like pattern because of the small contact area between each unit and the rolling mill, and the cooperation of Fp and Fv. Upon repeated rolling, secondary shear bands introduced by the following rolling in the different directions (e.g. blue lines in Fig. 3(b)) intersect with the prior primary shear bands on the side surface, together with the further occurrence of some secondary shear bands (secondary strips seen in Fig. 3)(b) on the top surface. Moreover, Fig. 3(c) and (d) and (e) show the side surfaces and top surfaces of the as-rolled specimens, respectively. It can be seen that the actual distributions and morphologies of shear bands upon cold rolling correspond very well with our model prediction. As shown in Fig. 3(c) and (d), the average spacing for the primary and secondary shear bands on the side surface of the as-
rolled specimen is 200 40 mm and 100 40 mm, respectively. It can be seen from Fig. 3(e) that the average spacing for the primary and secondary strips on the top surface is 800 40 mm and 400 40 mm, respectively. However, the primary and secondary shear bands are not homogeneously distributed in the whole specimens (Fig. 3(c) and (d)) due to the unavoidable loading difference and contact roughness of the rolling apparatus. A small amount of shear bands being parallel to the rolling direction can also be observed on both sides of the top surfaces, which may be attributed to the expansion of the specimens along the rolling direction. As we know, good ductility in BMGs is usually related to formation of multiple shear bands generated during deformation [21,26]. The more generated shear bands, the larger plasticity the alloy exhibits. It should be noted that no significant change in the shear band density can be observed in the as-rolled specimens after tensile tests. According to Han et al. [27], the activation of preexisting shear bands is more predominate than the formation of new shear bands of BMGs under tension. In our case, the propagation of the highly intersected pre-existing shear bands introduced by cold rolling can be effectively suppressed (Fig. 2(d)). It also indicates that the pre-existing shear bands induced by cold rolling may considerably change the stress distribution of the as-rolled specimens upon tensile tests. According to the free volume model, the plasticity is closely related to the content of free volume of BMGs. It is believed that the formation of shear bands can introduce extra free volume within the shear bands region [28]. In our case, the density of the as-cast specimens decrease from 7.21 0.01 g/cm3 to 7.19 0.01 g/cm3 after cold rolling, indicating an increase in the free volume content of about 0.28%. However, as shown in Fig. 4, there is no obvious difference between the exothermic events on the DSC traces of the as-cast and as-rolled specimens. The glass transition temperature, Tg, and other characteristic temperatures are also found to decrease a little upon cold rolling (inset in Fig. 4). After calculating the fragility parameter of the as-cast and as-rolled specimens, m ¼ Fg/ RTg,20 defined by Ref [29], the m value of the as-cast specimen was found to increase slightly from 49 5 to 53 5 after cold rolling. Besides, we measure the viscosity of as-cast and as-rolled specimens, respectively. Being similar with the DSC results, the viscosity of
K.K. Song et al. / Intermetallics 19 (2011) 1394e1398
690
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Tg-onset log g
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Tx
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675
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750
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1000/T
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Exothermic heat flow (a.u.)
Temperature (K)
660
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Fig. 4. Temperature dependence of the viscosities (Left) and the DSC curves (Right) for the as-cast and as-rolled R5 specimens during heating process. Inset: enlarged views near to the glass transition temperature (Tg).
as-rolled specimen becomes smaller a little than that of as-cast specimen (Fig. 4). However, even though these measurements are not within the range of error, the fragility and viscosity results tend to change a little upon cold rolling, which indicates that the structure of the specimens may be changed a little in a nano- or micro-scaled level. Recently, Egami et al. [30] demonstrated that the shear stress has a similar influence on the temperature-dependent viscosity of CuZrAl MG. Therefore, the decrease in the viscosity of as-cast R5 specimen could attribute to the structural rearrangement during cold rolling, which corresponds very well with the recent results [19,31e33]. So the structural rearrangement during the rolling and tensile processes may play an important role on the plasticity. It has been shown that the deformation such as cold rolling can introduce both residual stresses and structural heterogeneities [19,21,31e33]. In order to evaluate the contribution of the residual stresses and the structural heterogeneities to the enhancement in the plasticity, we measured the microhardness of the as-cast and as-rolled specimens and the contour plot was shown in Fig. 5(a) and (b), respectively. An increase in the microhardness of the asrolled specimens can be clearly identified with a more heterogeneous distribution. The increase in microhardness of a similar composition is also found within a very thin pretreatment layer and can be associated with the residual stresses [33]. However, Jiang
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et al. [21] calculated the rolling-induced residual stress and found that the increase in the microhardness is less than 30 Hv with a thickness reduction of 50% in a Zr64.13Cu15.75Ni10.12Al10 BMG. In our case, the average values of microhardness of the as-cast and asrolled specimens with a thickness reduction of only 2.9 0.1% are 545 Hv and 600 Hv, respectively, after grinding and polishing the surface of the specimens. It can be seen that such difference in the microhardness of the as-cast and as-rolled specimens is mostly related with the structural heterogeneities. The deformationinduced structural heterogeneities in Cu47.5Zr47.5Al5 BMGs (e.g. nanocrystals) have already been demonstrated by our previous works [6,7,13,19,32]. Due to both the residual stresses and the structural heterogeneities, the Young’s modulus decreases from 87 3 GPa to 80 3 GPa according to the tensile test results whereas the Poisson’ ratio (n) of as-cast specimens was measured to increase a little from 0.3740 to 0.3755 after cold rolling based on ultrasonic measurements. Schroers [34] proposed that the deformability of any BMG increases with increasing Poisson’s ratio, which is consistent with our results. The formation of multiple shear bands and structural heterogeneities can be understood in the frame of potential energy landscape theory (PEL) from the thermodynamic point of view. According to the PEL theory [35,36], shear strain can cause the disappearance of local potential energy minima [37], rendering the system mechanically unstable and forcing the system to move to alternate local minima (mechanical instability) as shown in Fig. 6. It signifies that shear strain derived from cold rolling may reduce the local potential energy minima, which makes it possible for the structural arrangement from the thermodynamic point of view. According to the cooperative shearing model [35], the total barrier height, DF(T), for the initiation of structural inhomogeneity can be evaluated by the following equation:
DFðTÞhf0 U ¼
8g2c
p2
GU
(1)
where f0 is the amplitude of the potential energy function, G is the shear modulus and U is the size of the cooperatively rearranging zone, respectively. On the basis of the relation G ¼ E/2(1 þ n), the shear modulus and n changed by cold rolling result in the decrease of the shear modulus (G). The reduction of G induced by rolling reflects clearly the decrease in DF(T), which increases the mechanical instability, when the change in the size of the rearranging zone is neglectable. On the other hand, Egami et al. [30]
Fig. 5. Microhardness contour maps of the specimens (a) before and (b) after cold rolling.
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Acknowledgments We would like to acknowledge financial support from the Chinese scholarship Council (CSC), the National Basic Research Program of China (973 Program 2007CB613901), the National Natural Science Foundation of China (50831003) and the Excellent Youth Project of Natural Science Foundation of Shandong Province (JQ201012). The authors thank the colleges in IFW for technical discussions.
References
Fig. 6. Schematic representation of the mechanical instability [37].
found a simple scaling relationship between temperature and stress as they affect viscosity, which provides a new understanding of the microscopic mechanism of shear flow in the glassy state, based on the elastic energy of the applied stress modifying the local energy landscape. So the decrease of viscosity induced by the stress should result in the decrease of the cluster energy, which correlates strongly with the mobility of the corresponding atoms in the liquid [38]. These changes correspond very well to the decrease of the barrier separating local minima, DF(T) upon cold rolling. Both the decreasing cluster energy and DF(T) would lead to a state that is more convenient for the structural heterogeneities and the formation of multiple shear bands to occur, resulting in a significant enhancement in the tensile plasticity.
4. Conclusions In summary, a pronounced tensile plasticity of 0.7 0.1% with an obvious work-hardening behavior can be achieved in a monolithic Cu47.5Zr47.5Al5 BMG using the cold rolling method. The enhancement of the tensile plasticity cannot be fully interpreted using the free volume model. The residual stresses and structural heterogeneities are found to coexist in the as-rolled specimens. The structural heterogeneities are proposed to be a key factor that governs the flow behavior of BMGs under tensile tests. The mechanical instabilities discussed above based on the PEL theory and the formation of multiplied shear bands resulted from cold rolling, apparently modify the mechanical performances and may be used to explore ductile BMGs in the future.
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Greer AL. Science 1995;267:1947. Bruck HA, Christman T, Rosakis AJ, Johnson WL. Scr. Metall. Mater. 1994;30:429. Johnson WL. MRS Bull. 1999;24:42. Inoue A. Acta Mater. 2000;48:279. Wang WH, Dong C, Shek CH. Mater. Sci. Eng. R 2004;44:45. Eckert J, Das J, Pauly S, Duhame C. J. Mater. Res. 2007;22:285. Liu YH, Wang G, Wang RJ, Zhao DQ, Pan MX, Wang WH. Science 2007;315:1385. Hofmann DC, Suh J, Wiest A, Duan G, Lind M, Demetriou MD, et al. Nature (London) 2008;451:1085. [9] Szuecs F, Kim CP, Johnson WL. Acta Mater. 2001;49:1507. [10] Gu XJ, McDermott AG, Poon SJ, Shiflet GJ. Appl. Phys. Lett. 2006;88:211905. [11] Lewandowski JJ, Gu XJ, Nouri AS, Poon SJ, Shiflet GJ. Appl. Phys. Lett. 2008;92: 091918. [12] Ma H, Xu J, Ma E. Appl. Phys. Lett. 2003;83:2793. [13] Pauly S, Gorantla S, Wang G, Kühn U, Eckert J. Nat. Mater. 2010;9:473. [14] Wu Y, Xiao YH, Chen GL, Liu CT, Lu ZP. Adv. Mater. 2010;22:2770. [15] Pauly S. Doctoral thesis work, Technische Universität Dresden, D-01062 Dresden, Germany, 2010. [16] Kim KB, Das J, Lee MH, Yi S, Fleury E, Zhang ZF, et al. J. Mater. Res. 2008;23:6. [17] Zhang JL, Yu HB, Lu JX, Bai HY, Shek CH. Appl. Phys. Lett. 2009;95:071906. [18] Wu WF, Li Y, Schuh CA. Philos. Mag. 2008;88:71. [19] Lee MH, Lee KS, Das J, Thomas J, Kühn U, Eckert J. Scripta Mater. 2010;62:678. [20] Yao KF, Ruan F, Yang YQ, Chen N. Appl. Phys. Lett. 2006;88:122106. [21] Cao QP, Liu JW, Yang KJ, Xu F, Yao ZQ, Minkow A, et al. Acta Mater. 2010;58: 1276. [22] Yokoyama Y, Yamasaki T, Inoue A. Rev. Adv. Mater. Sci. 2008;18:131. [23] Zhang Y, Wang WH, Greer AL. Nat. Mater. 2006;5:857. [24] Zhang ZF, Eckert J, Schultz L. Acta Mater. 2003;51:1167. [25] Das J, Tang MB, Kim KB, Theissmann R, Baier F, Wang WH, et al. Phys. Rev. Lett. 2005;94:205501. [26] Bei H, Xie S, George EP. Phys. Rev. Lett. 2006;96:105503. [27] Han Z, Yang H, Wu WF, Li Y. Appl. Phys. Lett. 2008;93:231912. [28] Argon AS. Acta Metall. 1979;27:47. [29] Bruning R, Sutton M. J. Non-Cryst. Solids 1996;205e207:480. [30] Guan PF, Chen MW, Egami T. Phys. Rev. Lett. 2010;104:205701. [31 Yu HB, Hu J, Xia XX, Sun BA, Li XX, Wang WH, et al. Scripta Mater. 2009;61:640. [32] Dmowski W, Yokoyama Y, Chuang A, Ren Y, Umemoto M, Tsuchiya K, et al. Acta Mater. 2010;58:429. [33] Raghavan R, Ayer R, Jin HW, Marzinsky CN, Ramamurty U. Scripta Mater. 2008;59:167. [34] Schroers J. Acta Mater. 2008;56:471. [35] Johnson WL, Samwer K. Phys. Rev. Lett. 2005;95:195501. [36] Johnson WL, D.Demetriou M, Harmon JS, Lind ML, Samwer K. MRS Bull. 2007; 32:644. [37] Malandro DL, Lacks DJ. J. Chem. Phys. 1999;110:4593. [38] Wu SQ, Wang CZ, Hao SG, Zhu ZZ, Ho KM. Appl. Phys. Lett. 2010;97:021901.
Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Significant tensile ductility induced by cold rolling in Cu47.5Zr47.5Al5 bulk metallic glass K.K. Song a, b, *, S. Pauly a, Y. Zhang a, S. Scudino a, P. Gargarella a, K.B. Surreddi a, U. Kühn a, J. Eckert a, b a b
IFW Dresden, Institut für Komplexe Materialien, Helmholtzstraße 20, D-01069 Dresden, Germany TU Dresden, Institut für Werkstoffwissenschaft, D-01062 Dresden, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 March 2011 Received in revised form 28 April 2011 Accepted 2 May 2011 Available online 15 June 2011
Significant tensile plasticity up to 0.7 0.1% together with work-hardening and larger fracture strength was obtained in Cu47.5Zr47.5Al5 bulk metallic glass (BMG) upon cold rolling with only 2.9 0.3% thickness reduction. The good deformability could be attributed to the multiple pre-existing shear bands and structural inhomogeneity induced by rolling. The distributions of shear bands upon rolling can be predicted by a simplified rolling model. The underlying mechanism for the tensile plasticity was further discussed in the frame of potential energy landscape theory (PEL). Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Ternary alloy systems B. Glasses, metallic B. Mechanical properties at ambient temperature C. Rolling
1. Introduction Bulk metallic glasses (BMGs) are of commercial interests due to their outstanding properties such as high strength and large elastic strain combined with excellent glass-forming ability (GFA), making them potentially useful for a variety of structural and functional applications [1e5]. Recently, attention has been paid to improve their plasticity due to BMGs usually failing in an apparently brittle manner under unconstrained loading conditions [1e5]. The low ductility/toughness and poor fatigue resistance severely restricted their use as new-generation structural engineering materials. Therefore, it has become one of the most important issues to improve the ductility of BMGs. In order to improve the plasticity of BMGs at room temperature, in situ formed ductile phases or ex situ secondary particles with different length scales were introduced into the amorphous matrix to hinder the instantaneous propagation of shear bands [6e14]. Tensile plasticity combined with work-hardening has been achieved in BMG composites with nano- [13,15] or micro-scaled [14,16] ductile phases by using such strategies. Unfortunately, the resultant
* Corresponding author. IFW Dresden, Institut für Komplexe Materialien, Helmholtzstraße 20, D-01069 Dresden, Germany. Tel.: þ49 351 4659 532; fax: þ49 351 4659 452. E-mail address: [email protected] (K.K. Song). 0966-9795/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2011.05.001
composites with high volume fractions of crystalline phases usually sacrifice the high yield strength of the monolithic BMGs. It has been demonstrated that the pre-straining and cold rolling methods can effectively promote the plasticity of BMGs [17e20]. A promising advantage of cold rolling is that the yielding and fracture strengths of monolithic BMGs are not reduced but are reported to be even increased after rolling in some BMGs [21,22]. However, the improvement of the tensile plasticity usually requires a large thickness reduction after the cold rolling. For example, tensile plasticity of about 0.5% and 0.27% in Zr50Cu30Ni10Al10 and Zr64.13Cu15.75Ni10.12Al10 BMGs can be achieved by cold-rolling with thickness reductions of 10% and 50%, respectively. Moreover, the mechanism leading to the increased plasticity upon cold rolling is still under debate. In this letter, we report that an obvious tensile plasticity of 0.7 0.1% with work-hardening in a monolithic Cu47.5Zr47.5Al5 BMG can be attained with a very small thickness reduction of only 2.9 0.3% by cold rolling. A simplified model is proposed to interpret the formation of the multiple shear bands during cold rolling. The underlying mechanism for the tensile ductility improvement can be understood in the frame of potential energy landscape theory (PEL). 2. Experimental procedures The pre-alloys were prepared by arc melting appropriate amounts of the constituting elements (purity 99.9%) and were re-
K.K. Song et al. / Intermetallics 19 (2011) 1394e1398
melted three times in order to ensure chemical homogeneity under a Ti-gettered argon atmosphere. The samples were finally cast into 1.5t 10w 60l mm3 plates by an in situ suction casting facility attached to the arc melter. The casting temperature of CuZr-based alloys during casting was controlled by applying a certain melting current, namely 280 A, i.e. 70% of the maximum power which can be delivered by the power generator, for approximately 17 s. The elastic properties were measured using an Olympus Panametrics-NDT 5900PR ultrasonic testing device. The as-cast plates were cut into dog bone shape specimens with a gauge section of about 1.5t 2w 12l mm3 for the tensile tests. The specimens were rolled in one direction (1st-rolling, i.e. continuous rolling without changing direction) and two directions (2nd-rolling, i.e. rotating 180 in the rolling (horizontal) plane after each rolling pass), respectively, until the desired reduction thickness was obtained. The thickness reduction was calculated by E ¼ (h0 - h)/h0, where h0 and h represent the specimen thicknesses before and after rolling, respectively. Tensile tests were performed on an Instron 5869 testing machine (equipped with a Fiedler laser extensometer) at an initial strain rate of 2.5 104 s1. The structures of both ascast and cold-rolled specimens were ascertained by X-ray diffraction (XRD) (STOE STADI, Mo, Ka radiation l ¼ 0.07093187 nm). The glass transition, crystallization temperatures and the change of the enthalpy were measured using a PerkineElmer Pyris diamond differential scanning calorimeter (DSC) under argon atmosphere at the heating rate of 40 K/min. A thermomechanical analyzer (TMA) was used for viscosity measurements for as-cast and as-rolled specimens under argon atmosphere at a heating rate of 40 K/min, respectively. The microhardness of the as-cast and cold-rolled specimens were measured after grinding and polishing the surfaces of the specimens, respectively, with 1000e1200 indents for each specimen. The evolutions of shear bands and the fracture morphology were examined using a scanning electron microscope (SEM, Gemini 1530).
3. Results and discussion Fig. 1 shows the true stress-strain curves of the as-cast and asrolled specimens under tensile loading. All the specimens exhibit an elastic strain limit of less than 2%, which is a typical feature of monolithic BMGs. No obvious yielding is observed in the as-cast specimens and the fracture stress, sFT, is 1600 40 MPa, which is similar to previous results [13,15]. The as-rolled specimens exhibit an obvious yielding (sFT ¼ 1400 40 MPa) and a work-hardening behavior. The decrease of the yielding strength is believed to
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result from the pre-existing shear bands (Fig. 3(cee)) induced by the cold rolling before the tensile tests. Compared to the results reported in Refs. [21,22], it can be seen that the as-rolled specimens exhibit a much larger plasticity of 0.7 0.1% with a much smaller thickness reduction of only 2.9 0.3%. A pronounced workhardening behavior of the as-rolled specimens can be observed with the fracture strength of 1625 40 MPa, which may be due to the suppression of crack propagation in the as-rolled specimen by the deformation-induced residual stress [23]. As depicted in the inset of Fig. 1, XRD patterns of the as-cast and as-rolled specimens both show broad diffraction humps with no detectable crystalline Bragg peak, which indicates that cold rolling did not induce any crystallization in the specimens as reported in the previous works [19e22]. SEM observations reveal the significant difference in the morphologies of shear bands and fracture surfaces of the as-cast and as-rolled specimens after the tensile test. As shown in the inset of Fig. 2(a), the formation and propagation of one major shear band in the as-cast specimens dominate the fracture process. The tensile fracture angle, qT, between the tensile axis and the fracture plane is equal to 54 , which deviates from the angle of the maximum shear stress plane (45 ). It indicated that the von Mises yielding criterion cannot describe the tensile deformation behavior of the Cu47.5Zr47.5Al5 BMG in accordance with previous results [24]. A vein-like pattern is visible on the smooth fracture surface (Fig. 2(a) and (b)) together with some round cores of different diameters are on the whole surface for the as-cast specimens. Zhang et al. [24] suggested that the tensile fractures of BMGs should first originate from these cores induced by normal tension stress on the plane, and then catastrophically propagate towards outside of the cores driven by the shear stress. Moreover, there are only several tiny shear bands, which can be seen in the vicinity of the fracture part, implying that shear bands generated under tension are rather few and hence cannot contribute significantly to the overall tensile plasticity for the as-cast specimens. However, the asrolled specimens fail at an angle of 45 with respect to the loading axis (inset of Fig. 2(c)). It indicates that the occurrence of shear fracture under unconstrained loading conditions, which is in accord with the previous results [22]. Besides the above mentioned morphologies of the fracture surface and the whole surface of the as-cast specimens, some network-like structures (Fig. 2(c)) can be observed on the edge of the fracture surface of the as-rolled specimens after tensile tests. Such network-like structure is
1750 0.2%
True Stress (MPa)
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0 0.0
20
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True strain (%) Fig. 1. The true tensile stressestrain curves of as-cast (black curves) and as-rolled (red curves) specimens, respectively. Inset: XRD patterns of the as-cast and as-rolled specimens after tensile test.
Fig. 2. SEM images of (a,b) the as-cast specimens and (c,d) the as-rolled specimens after tensile test, respectively. Insets: the fractures of the as-cast and as-rolled specimens after tensile tests, respectively.
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Fig. 3. (a) The simple schematic illustration of the loading conditions; F from the rollers can be decomposed into two different forces in the vertical and the horizontal directions (Fv and Fp), respectively, (b) the diagrammatic sketch of evolutions of shear bands upon cold rolling, (c, d) the shear band distributions on the side surface after cold rolling and (e) the shear band distributions on the top surface after cold rolling and tensile test.
significantly different from the surface morphology of the as-cast specimens, which should originate from the typical wavy-curved shear band morphology [22]. Meanwhile, there are some cracks crossing the plate vertically as shown in the inset of Fig. 2(d). The surface micrograph of the as-rolled specimen before tensile tests is shown in Fig. 3(c)e(e). It can be seen that a large number of pre-existing shear bands with different morphologies distribute on the surface of the as-rolled sample. Here, we propose a simplified model to predict the evolutions of shear bands based on the loading conditions of cold rolling to illustrate the formation and evolution of shear bands during cold rolling. As illustrated in Fig. 3(a), the ascast plates subjected to cold rolling are in an unrestrained state in the rolling direction and are in a state similar to the condition of compression tests being vertical to the rolling direction. Therefore, the whole plate can be treated to be composed of many identical units as schematically shown in Fig. 3(b). Each unit experiences a force (Fv) vertical to the rolling direction and can be thought as under the condition of compression tests, whereas there is a rolling force (Fp) along at the rolling direction. Therefore, the rolling process can be simplified as a continuous process where all the units are subjected to compression tests one by one. During first cold rolling, primary shear bands form almost parallel at an angle of about 45 with respect to the loading direction (red lines in Fig. 3(b)) on the side surface, which is in good agreement with the results of compression tests of Cu47.5Zr47.5Al5 BMGs [25]. However, primary shear bands, named as primary strips, on the top surface as shown in Fig. 3(b) exhibit a parallel strip-like pattern because of the small contact area between each unit and the rolling mill, and the cooperation of Fp and Fv. Upon repeated rolling, secondary shear bands introduced by the following rolling in the different directions (e.g. blue lines in Fig. 3(b)) intersect with the prior primary shear bands on the side surface, together with the further occurrence of some secondary shear bands (secondary strips seen in Fig. 3)(b) on the top surface. Moreover, Fig. 3(c) and (d) and (e) show the side surfaces and top surfaces of the as-rolled specimens, respectively. It can be seen that the actual distributions and morphologies of shear bands upon cold rolling correspond very well with our model prediction. As shown in Fig. 3(c) and (d), the average spacing for the primary and secondary shear bands on the side surface of the as-
rolled specimen is 200 40 mm and 100 40 mm, respectively. It can be seen from Fig. 3(e) that the average spacing for the primary and secondary strips on the top surface is 800 40 mm and 400 40 mm, respectively. However, the primary and secondary shear bands are not homogeneously distributed in the whole specimens (Fig. 3(c) and (d)) due to the unavoidable loading difference and contact roughness of the rolling apparatus. A small amount of shear bands being parallel to the rolling direction can also be observed on both sides of the top surfaces, which may be attributed to the expansion of the specimens along the rolling direction. As we know, good ductility in BMGs is usually related to formation of multiple shear bands generated during deformation [21,26]. The more generated shear bands, the larger plasticity the alloy exhibits. It should be noted that no significant change in the shear band density can be observed in the as-rolled specimens after tensile tests. According to Han et al. [27], the activation of preexisting shear bands is more predominate than the formation of new shear bands of BMGs under tension. In our case, the propagation of the highly intersected pre-existing shear bands introduced by cold rolling can be effectively suppressed (Fig. 2(d)). It also indicates that the pre-existing shear bands induced by cold rolling may considerably change the stress distribution of the as-rolled specimens upon tensile tests. According to the free volume model, the plasticity is closely related to the content of free volume of BMGs. It is believed that the formation of shear bands can introduce extra free volume within the shear bands region [28]. In our case, the density of the as-cast specimens decrease from 7.21 0.01 g/cm3 to 7.19 0.01 g/cm3 after cold rolling, indicating an increase in the free volume content of about 0.28%. However, as shown in Fig. 4, there is no obvious difference between the exothermic events on the DSC traces of the as-cast and as-rolled specimens. The glass transition temperature, Tg, and other characteristic temperatures are also found to decrease a little upon cold rolling (inset in Fig. 4). After calculating the fragility parameter of the as-cast and as-rolled specimens, m ¼ Fg/ RTg,20 defined by Ref [29], the m value of the as-cast specimen was found to increase slightly from 49 5 to 53 5 after cold rolling. Besides, we measure the viscosity of as-cast and as-rolled specimens, respectively. Being similar with the DSC results, the viscosity of
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Exothermic heat flow (a.u.)
Temperature (K)
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Fig. 4. Temperature dependence of the viscosities (Left) and the DSC curves (Right) for the as-cast and as-rolled R5 specimens during heating process. Inset: enlarged views near to the glass transition temperature (Tg).
as-rolled specimen becomes smaller a little than that of as-cast specimen (Fig. 4). However, even though these measurements are not within the range of error, the fragility and viscosity results tend to change a little upon cold rolling, which indicates that the structure of the specimens may be changed a little in a nano- or micro-scaled level. Recently, Egami et al. [30] demonstrated that the shear stress has a similar influence on the temperature-dependent viscosity of CuZrAl MG. Therefore, the decrease in the viscosity of as-cast R5 specimen could attribute to the structural rearrangement during cold rolling, which corresponds very well with the recent results [19,31e33]. So the structural rearrangement during the rolling and tensile processes may play an important role on the plasticity. It has been shown that the deformation such as cold rolling can introduce both residual stresses and structural heterogeneities [19,21,31e33]. In order to evaluate the contribution of the residual stresses and the structural heterogeneities to the enhancement in the plasticity, we measured the microhardness of the as-cast and as-rolled specimens and the contour plot was shown in Fig. 5(a) and (b), respectively. An increase in the microhardness of the asrolled specimens can be clearly identified with a more heterogeneous distribution. The increase in microhardness of a similar composition is also found within a very thin pretreatment layer and can be associated with the residual stresses [33]. However, Jiang
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et al. [21] calculated the rolling-induced residual stress and found that the increase in the microhardness is less than 30 Hv with a thickness reduction of 50% in a Zr64.13Cu15.75Ni10.12Al10 BMG. In our case, the average values of microhardness of the as-cast and asrolled specimens with a thickness reduction of only 2.9 0.1% are 545 Hv and 600 Hv, respectively, after grinding and polishing the surface of the specimens. It can be seen that such difference in the microhardness of the as-cast and as-rolled specimens is mostly related with the structural heterogeneities. The deformationinduced structural heterogeneities in Cu47.5Zr47.5Al5 BMGs (e.g. nanocrystals) have already been demonstrated by our previous works [6,7,13,19,32]. Due to both the residual stresses and the structural heterogeneities, the Young’s modulus decreases from 87 3 GPa to 80 3 GPa according to the tensile test results whereas the Poisson’ ratio (n) of as-cast specimens was measured to increase a little from 0.3740 to 0.3755 after cold rolling based on ultrasonic measurements. Schroers [34] proposed that the deformability of any BMG increases with increasing Poisson’s ratio, which is consistent with our results. The formation of multiple shear bands and structural heterogeneities can be understood in the frame of potential energy landscape theory (PEL) from the thermodynamic point of view. According to the PEL theory [35,36], shear strain can cause the disappearance of local potential energy minima [37], rendering the system mechanically unstable and forcing the system to move to alternate local minima (mechanical instability) as shown in Fig. 6. It signifies that shear strain derived from cold rolling may reduce the local potential energy minima, which makes it possible for the structural arrangement from the thermodynamic point of view. According to the cooperative shearing model [35], the total barrier height, DF(T), for the initiation of structural inhomogeneity can be evaluated by the following equation:
DFðTÞhf0 U ¼
8g2c
p2
GU
(1)
where f0 is the amplitude of the potential energy function, G is the shear modulus and U is the size of the cooperatively rearranging zone, respectively. On the basis of the relation G ¼ E/2(1 þ n), the shear modulus and n changed by cold rolling result in the decrease of the shear modulus (G). The reduction of G induced by rolling reflects clearly the decrease in DF(T), which increases the mechanical instability, when the change in the size of the rearranging zone is neglectable. On the other hand, Egami et al. [30]
Fig. 5. Microhardness contour maps of the specimens (a) before and (b) after cold rolling.
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Acknowledgments We would like to acknowledge financial support from the Chinese scholarship Council (CSC), the National Basic Research Program of China (973 Program 2007CB613901), the National Natural Science Foundation of China (50831003) and the Excellent Youth Project of Natural Science Foundation of Shandong Province (JQ201012). The authors thank the colleges in IFW for technical discussions.
References
Fig. 6. Schematic representation of the mechanical instability [37].
found a simple scaling relationship between temperature and stress as they affect viscosity, which provides a new understanding of the microscopic mechanism of shear flow in the glassy state, based on the elastic energy of the applied stress modifying the local energy landscape. So the decrease of viscosity induced by the stress should result in the decrease of the cluster energy, which correlates strongly with the mobility of the corresponding atoms in the liquid [38]. These changes correspond very well to the decrease of the barrier separating local minima, DF(T) upon cold rolling. Both the decreasing cluster energy and DF(T) would lead to a state that is more convenient for the structural heterogeneities and the formation of multiple shear bands to occur, resulting in a significant enhancement in the tensile plasticity.
4. Conclusions In summary, a pronounced tensile plasticity of 0.7 0.1% with an obvious work-hardening behavior can be achieved in a monolithic Cu47.5Zr47.5Al5 BMG using the cold rolling method. The enhancement of the tensile plasticity cannot be fully interpreted using the free volume model. The residual stresses and structural heterogeneities are found to coexist in the as-rolled specimens. The structural heterogeneities are proposed to be a key factor that governs the flow behavior of BMGs under tensile tests. The mechanical instabilities discussed above based on the PEL theory and the formation of multiplied shear bands resulted from cold rolling, apparently modify the mechanical performances and may be used to explore ductile BMGs in the future.
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