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Effects of room-temperature rolling on microstructure and crystallization behavior of Zr55Cu40Al5 metallic glass
Intermetallics 18 (2010) 2039e2043
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Intermetallics journal homepage: www.elsevier.com/locate/intermet
Effects of room-temperature rolling on microstructure and crystallization behavior of Zr55Cu40Al5 metallic glass Yuanli Xu a, b, c, Horst Hahn b, c, *, Jiangong Li a, ** a
Institute of Materials Science and Engineering, MOE Key Laboratory for Magnetism and Magnetic Materials, Lanzhou University, Lanzhou 730000, PR China Institute for Nanotechnology, Forschungszentrum Karlsruhe, P. O. Box 3640, 76021 Karlsruhe, Germany c Joint Research Laboratory of Nanomaterials, Darmstadt University of Technology, Petersenstr. 23, 64287 Darmstadt, Germany b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 April 2010 Received in revised form 2 June 2010 Accepted 12 June 2010 Available online 10 July 2010
Using X-ray diffraction, transmission electron microscopy, and differential scanning calorimetry, the effects of room-temperature rolling and subsequent annealing treatment on the microstructure and crystallization behavior of the Zr55Cu40Al5 metallic glass ribbons have been investigated. High density of shear bands with spacings less than 100 nm and ordered clusters of 3e5 nm in size in the shear band regions were observed in the room-temperature rolled Zr55Cu40Al5 metallic glass ribbons. In addition, the rolling deformation changes the crystallization behavior of the Zr55Cu40Al5 metallic glass during the isothermal annealing treatment and results in the formation of a new crystalline phase. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: B. Glasses, metallic C. Rolling C. Heat treatment D. Microstructure F. Calorimetry
1. Introduction The plastic deformation of metallic glasses at temperatures below the glass transition temperature and high strain rates is known to be inhomogeneous and localized to shear bands [1]. According to the theoretical prediction [2,3] and experimental studies [4,5], a shear band, as a highly deformed region [6e8], has more free volume, i.e. a lower atomic density compared with the undeformed regions. Hence inhomogeneously distributed free volume is introduced into metallic glasses that endured inhomogeneous plastic deformation. In addition, phase separation or nanocrystallization was observed within shear bands for several metallic glasses that endured inhomogeneous deformation [9,10]. These structural changes will undoubtedly affect the properties of metallic glasses. For instance, the intrinsic plasticity of Zr44Ti11Cu9.8Ni10.2Be25 and Zr55Ti5Al10Cu20Ni10 bulk metallic glasses are significantly improved from 0.5% up to 15% plastic strain due to the introduction of microstructural inhomogeneities upon cold rolling [11]. Moreover, subsequent annealing treatment will cause reordering within shear bands and thus affect the structure and
* Corresponding author. Institute for Nanotechnology, Forschungszentrum Karlsruhe, P. O. Box 3640, 76021 Karlsruhe, Germany. Tel.: +49 7247 826350. ** Corresponding author. Institute of Materials Science and Engineering, Lanzhou University, Lanzhou 730000, PR China. Tel.: +86 931 8910364. E-mail addresses: [email protected] (H. Hahn), [email protected] (J. Li). 0966-9795/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2010.06.007
properties of the inhomogeneously deformed metallic glasses [12e14]. For example, the isothermal annealing increased the hardness of the room-temperature rolled Al86.8Ni3.7Y9.5 metallic glass [14]. The re-ordering within shear bands caused by subsequent annealing may change the local atomic arrangement and thus probably affect the crystallization behavior for metallic glasses that endured inhomogeneous deformation. However, the effects of inhomogeneous deformation on the crystallization behavior for metallic glasses have not been systematically studied so far. Therefore, it seems necessary to investigate the effects of inhomogeneous deformation on the microstructure and the crystallization behavior of metallic glasses. In this work, the effects of room-temperature rolling deformation on the microstructure and crystallization behavior of Zr55Cu40Al5 metallic glass were investigated. High density of shear bands with spacings less than 100 nm and ordered clusters of 3e5 nm in size in the shear band regions were observed in the room-temperature rolled samples. In addition, the rolling deformation changes the crystallization behavior of the samples during the isothermal annealing treatment and results in the formation of a new crystalline phase.
2. Experimental procedure Zr55Cu40Al5 (at. %) ingots were prepared by arc melting the high purity Zr (99.9%), Cu (99.5%), and Al (99.9%) under a Ti-gettered
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argon atmosphere. Melt-spun ribbons with a 60 mm thickness were produced using a melt spinner at a wheel speed of 20 m/s. The amorphous nature of the as-spun alloy ribbon was examined by Xray diffraction (XRD) on Panalytical X’pert X-ray diffractometer with monochromatic Cu Ka radiation (wavelength 1.5405 Å) and transmission electron microscopy (TEM) observations on Tecnai F20 electron microscope working with an acceleration voltage of 200 kV. The as-spun ribbons were cut into pieces with a length of about 10 mm. Several stacked ribbon pieces were placed between two flat steel plates with an original thickness of 1 mm and rolled in one direction at room temperature until a desired strain was obtained. During rolling, many small deformation rolling passes were repeated with a reducing gap between two rollers so that constant strain rates could be kept. The used strain rates were approximately 5.0 104 and 1.0 102 s1 in this work. The microstructure of the room-temperature rolled (RT-rolled) samples with different strains was examined by XRD and TEM. The differential scanning calorimetry (DSC) analysis was performed on the as-spun and RT-rolled samples by a PerkineElmer Pyris 1 calorimeter at 20 K/min under a flow of purified argon. In addition, unless specified otherwise, all the DSC measurements here were performed from room temperature to 830 K. Isothermal annealing treatments at 738 K on the as-spun and rolled samples were performed with this calorimeter. 3. Results and discussion The XRD pattern of the as-spun Zr55Cu40Al5 metallic glass ribbon sample is shown in Fig. 1(a). Besides the two broad diffraction halos around 2q ¼ 40 and 69 , no diffraction peaks
corresponding to any crystalline phases can be detected. To further confirm the amorphous nature of the as-spun sample, the TEM analyses were performed. A uniform contrast with no obvious lattice fringes can be found from the corresponding high resolution TEM (HRTEM) image shown in Fig. 1(b). The inset in Fig. 1(b) shows the selected area electron diffraction (SAED) pattern of the area corresponding to the TEM image in Fig. 1(b). It consists of a broad diffraction halo and a faint large one which are typical for amorphous materials. Therefore, the as-spun Zr55Cu40Al5 metallic glass ribbons are completely amorphous. A strain of 60% was achieved for the as-spun Zr55Cu40Al5 metallic glass ribbons by RT-rolling. To examine whether crystallization occurred in the RT-rolled samples, the XRD measurements were performed. The XRD patterns for the 60% RT-rolled samples deformed at strain rates of 5.0 104 and 1.0 102 s1 are also displayed in Fig. 1(a). The XRD patterns for these two RT-rolled samples exhibit two broad diffraction halos around 2q ¼ 40 and 69 . In the bright field TEM image for the 60% RT-rolled sample deformed at 1.0 102 s1 shown in Fig. 1(c), a large number of uniformly distributed bright “band-like” regions with a spacing of less than 100 nm can be observed. Obviously, these bright regions should be shear bands as they were usually observed with a bright contrast in the bright field TEM images because of the lower atomic density in the shear band region [15]. The corresponding SAED pattern in the inset of Fig. 1(c) consists of two broad diffraction halos without obvious diffraction spots. However, some ordered clusters of about 3e5 nm in size, as encircled in the HRTEM image in Fig. 1(d), form in the shear band regions [the dash line highlights roughly the boundary between a shear band (bright) and its neighboring undeformed matrix (dark)]. The ordered cluster can be
Fig. 1. (a) The XRD patterns of the as-spun Zr55Cu40Al5 metallic glass ribbons and the RT-rolled samples with a strain of 60% deformed at strain rates of 5.0 104 and 1.0 102 s1, (b) the HRTEM micrograph and the SAED pattern of the as-spun Zr55Cu40Al5 metallic glass ribbons, (c) the TEM (an SAED pattern in inset), and (d) the HRTEM micrographs of the 60% RT-rolled sample deformed at 1.0 102 s1.
Y. Xu et al. / Intermetallics 18 (2010) 2039e2043
clearly seen in the enlargement of an encircled area in the inset in Fig. 1(d). Hence, the RT-rolling deformation induces the formation of ordered clusters in the shear band regions in the Zr55Cu40Al5 metallic glass. The DSC curves obtained at a heating rate of 20 K/min for the asspun sample and the RT-rolled samples with a strain of 60% deformed at strain rates of 5.0 104 and 1.0 102 s1 are shown in Fig. 2(a). The inset shown in Fig. 2(a) displays the enlargement part of these DSC curves in the temperature range of 660e820 K. An endotherm, which can be clearly seen from the inset, occurs in the temperature range of about 720e755 K in the DSC curve for the asspun sample. This endotherm corresponds to the glass transition process. An exothermic peak centered at 788 K appears subsequently and corresponds to the crystallization process for the asspun sample. The DSC curves for the 60% RT-rolled samples deformed at 5.0 104 and 1.0 102 s1 are similar to that for the as-spun sample. The crystallization peak temperature remains unchanged compared with the as-spun sample. Therefore, the room-temperature rolling deformation does not significantly change the thermal stability of the Zr55Cu40Al5 metallic glass. To investigate further the effects of rolling deformation on thermal stability and crystallization behavior of the Zr55Cu40Al5 metallic glass ribbons, the as-spun and RT-rolled samples were
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isothermally annealed at 738 K, a temperature slightly higher than the glass transition temperature, for various times and analyzed by DSC measurements thereafter. Fig. 2(b)‑(d) shows the corresponding DSC curves for the as-spun sample, the 60% RT-rolled samples deformed at 5.0 104 and 1.0 102 s1, respectively. It can be clearly seen from Fig. 2(b) that both the crystallization peak temperature and the enthalpy of the crystallization exothermic event decrease with increasing annealing time for the as-spun sample. The crystallization peak temperature for the as-spun sample annealed at 738 K for 50 min is 767 K and is about 21 K lower compared with the as-spun sample without annealing. This indicates that the isothermal annealing at 738 K leads to significant crystallization, thus creating nuclei that grow at a lower temperature than that required for nucleation and growth in the as-spun sample without annealing. In addition, the exothermic event disappears completely when the annealing time reaches up to 70 min. For the 60% RT-rolled sample deformed at 5.0 104 s1 shown in Fig. 2(c), the crystallization peak temperature is 786 K after 10 min annealing treatment. However, when the annealing time reaches 30 min, the corresponding DSC curve exhibits two exothermic events. In these two exothermic events, one is centered at 780 K and almost the same as that for the as-spun sample annealed at 738 K for 30 min, and the other weak one is centered at
Fig. 2. (a) The DSC curves measured at 20 K/min for the as-spun Zr55Cu40Al5 metallic glass ribbons and the RT-rolled samples with a strain of 60% deformed at strain rates of 5.0 104 and 1.0 102 s1, (b) the DSC curves measured at 20 K/min for the as-spun Zr55Cu40Al5 metallic glass ribbons annealed at 738 K for various times, (c) the DSC curves measured at 20 K/min for the 60% RT-rolled samples deformed at 5.0 104 s1 and annealed at 738 K for various times, and (d) the DSC curves measured at 20 K/min for the 60% RT-rolled samples deformed at 1.0 102 s1 and annealed at 738 K for various times.
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about 798 K. As the annealing time increases further thereafter, both of the crystallization peak temperature and the enthalpy of the first crystallization exothermic event decrease. In addition, the first crystallization exothermic event still exists when annealed at 738 K for 70 min. For the 60% RT-rolled sample deformed at 1.0 102 s1 and annealed at 738 K for various times [Fig. 2(d)], the two exothermic events exist already when annealed at 738 K for 10 min, earlier than that for the sample deformed at 5.0 104 s1. Similarly, both the crystallization peak temperature and the enthalpy of the first crystallization exothermic event decrease with increasing annealing time. When annealed at 738 K for 70 min, the first crystallization exothermic event still exists. This indicates that for the 60% RT-rolled samples deformed at strains rates of 5.0 104 and 1.0 102 s1, a trace amount of amorphous phase still exists when annealed at 738 K for 70 min. Comparison of Fig. 2b with Figs. 2c, d indicates that the thermal stability of the amorphous phase against crystallization during isothermal annealing treatment does not deteriorate significantly after RT-rolling deformation for our Zr55Cu40Al5 metallic glass. However, the change of one crystallization exothermic event for the as-spun sample into two crystallization exothermic events for the samples that endured RTrolling deformation indicates that the crystallization behavior changes from one single to a double step process for the Zr55Cu40Al5 metallic glass ribbon after the RT-rolling deformation and subsequent annealing treatment. To identify the origin of the two crystallization exothermic events appear in the DSC curves for the deformed Zr55Cu40Al5 metallic glass ribbon samples annealed at 738 K, the XRD measurements were performed on the annealed samples after the DSC runs. Fig. 3(a) displays the XRD patterns of the as-spun sample run through the DSC measurement, the as-spun sample annealed at 738 K for 30 min and run through the DSC measurement, the 60% RT-rolled sample deformed at 1.0 102 s1 and run through the DSC measurement, the 60% RT-rolled samples deformed at 1.0 102 s1, annealed at 738 K for different times, and run through the DSC measurement. Fig. 3(b) is the enlargement part of Fig. 3(a) in the 2q range of 28e49 . As can be clearly seen from Fig. 3 (b), Cu8Zr3, Al3Zr, and unknown phases can be found in the as-spun sample after the DSC run, the 30 min annealed as-spun sample after the DSC run, and the 60% RT-rolled sample after the DSC run. In addition, after annealing at 738 K for different times, DSC measurements were performed on the 60% RT-rolled samples from room temperature to a temperature between the first crystallization peak temperature and the second one. Subsequently these samples were analyzed by XRD. The XRD pattern for the 60%
RT-rolled sample undergone 10 min annealing and subsequent DSC measurement performed from room temperature to 793 K [indicated as RT-60% (10 min, 793 K)] is shown in Fig. 3. Only Cu8Zr3, Al3Zr, and unknown phases can be found from the XRD pattern for this sample. Thus the crystallization exothermic event for the asspun sample and 60% RT-rolled samples and the first crystallization exothermic event for the 60% RT-rolled sample that endured annealing treatments correspond to the formation of the Cu8Zr3, Al3Zr, and unknown phases. The Cu10Zr7 phase as well as the Cu8Zr3, Al3Zr, and unknown phases can be observed in the XRD analysis for the 60% RT-rolled samples that endured 10, 30, and 50 min annealing treatments at 738 K and the DSC run. Obviously, the second crystallization exothermic event centered at about 798 K in Fig. 2(c) and (d) corresponds to the formation of Cu10Zr7 phase. Based on these results, the room-temperature rolling deformation changes the crystallization behavior of the Zr55Cu40Al5 metallic glass and leads to the formation of a new crystalline phase during the annealing treatment for the Zr55Cu40Al5 metallic glass. The inhomogeneous deformation leads to the formation of high density of shear bands in metallic glasses. It has been speculated that temperature plays a crucial rule in nanocrystallization in shear bands due to adiabatic heating produced by mechanical deformation [16]. However, for an Al90Fe5Gd5 metallic glass ribbon bent at 233 K, nanocrystals was still observed in shear bands [17]. Since the deformation heat could be easily dissipated and the temperature rise in the sample was very limited, it was proposed that the precipitation of nanocrystallites at shear bands was ascribed to the creation of excess free volume [17]. This was further supported by nanoindentation experiment on metallic glasses [10]. Considering the excess free volume in shear bands created by plastic deformation, the free energy of a glass will increase with increasing deformation [18]. On the other hand, the viscosity h in these regions will decrease based on the relationship between the free volume vf and h, i.e. h ¼ ðh=vm Þexpðbvm =vf Þ, where h, vm, and b represent the Planck’s constant, the critical free volume for atomic diffusion, and an overlap factor, respectively [19,20]. In addition, according to the classical theory of nucleation, the nucleation rate Iv, as the product of one kinetic term and one thermodynamic term, can be depicted as Iv ¼ ðAv =hÞexp½16ps3 =ð3kB T DG2 Þ [21,22], where Av is a constant of the order of 1036 Pa/m3, s the interfacial energy per area, kB the Boltzmann constant, DG the Gibbs free energy difference per volume between the new and old phases, respectively. Obviously, the reduction of the viscosity in the nucleation equation above will enhance the atomic diffusional mobility and thus enhances the fluctuation process leading to the
Fig. 3. (a) The XRD patterns of the as-spun Zr55Cu40Al5 metallic glass ribbons run through the DSC measurement, the as-spun sample annealed at 738 K for 30 min and run through the DSC measurement, the 60% RT-rolled sample deformed at 1.0 102 s1 and run through the DSC measurement, the 60% RT-rolled samples deformed at 1.0 102 s1, annealed at 738 K for different times, and run through the DSC measurement, the 60% RT-rolled sample deformed at 1.0 102 s1, annealed at 738 K for 10 min, and run through the DSC measurement performed from room temperature to 793 K [indicated as RT-60% (10 min, 793 K)] and (b) the enlargements of the same XRD patterns from 2q ¼ 28 to 49 .
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formation of nuclei in the shear band regions. Moreover, the increase of the Gibbs free energy offers a driving force for nucleation in the shear band regions in terms of the crystal nucleation. These probably lead to the formation of ordered clusters in shear bands in the RT-rolled Zr55Cu40Al5 metallic glass samples. The thermal stability of the amorphous phase against crystallization during isothermal annealing treatment has been reported to decrease for the inhomogeneously deformed Cu60Zr20Ti20 bulk metallic glass with nanocrystallization in shear bands [9]. However, no new crystallization exothermic event was observed from the corresponding DSC curves. This indicates that the crystallization behavior didn’t change for the inhomogeneously deformed Cu60Zr20Ti20 bulk metallic glass [9]. Hypothetically speaking, local short range order re-ordering may take place and regions with composition different from the original amorphous phase may form during isothermal annealing treatments in shear bands due to high atomic mobility. Thus the crystallization behavior, as observed in this letter for the Zr55Cu40Al5 metallic glass ribbons, may be changed and new crystalline phases form during the isothermal annealing. 4. Conclusion In summary, high density of shear bands with spacings less than 100 nm are introduced in the Zr55Cu40Al5 metallic glass ribbons that endured room-temperature rolling deformation. Ordered clusters of 3e5 nm in size form in the shear band regions for the 60% room-temperature rolled sample. The inhomogeneous deformation and subsequent annealing treatment at a temperature near the glass transition change the crystallization behavior from one single step which corresponds to the formation of Cu8Zr3, Al3Zr, and
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unknown phases, to a double step process which corresponds to the formation of the Cu8Zr3, Al3Zr, and unknown phases and the formation of the Cu10Zr7 phase. Acknowledgements This work was supported by the International S&T Cooperation Program (ISCP) of the Chinese Ministry of Science & Technology (MOST) under Grant No. 2008DFA50340 and the Deutsche Forschungsgemeinschaft (DFG) under contract HA1344/24-1. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
Spaepen F. Acta Metall 1977;25:407. Steif PS, Spaepen F, Hutchinson JW. Acta Metall 1982;30:447. Bailey NP, Schiøtz J, Jacobsen KW. Phys Rev B 2006;73:064108. Flores KM, Suh D, Dauskardt RH, Asoka-Kumar P, Sterne PA, Howell RH. J Mater Res 2002;17:1153. Hajlaoui K, Benameur T, Vaughan G, Yavari AR. Scripta Mater 2004;51:843. Bei H, Xie S, George EP. Phys Rev Lett 2006;96:105503. Argon AS. Acta Metall 1979;27:47. Chen H, He Y, Shiflet GJ, Poon SJ. Nature 1994;367:541. Cao QP, Li JF, Zhou YH, Horsewell A, Jiang JZ. Acta Mater 2006;54:4373. Kim JJ, Choi Y, Suresh S, Argon AS. Science 2002;295:654. Lee MH, Lee KS, Das J, Thomas J, Kühn U, Eckert J. Scripta Mater 2010;62:678. Krishnanand KD, Cahn RW. Scripta Metall 1975;9:1259. Pampillo CA. Scripta Metall 1972;6:915. Jiang WH, Pinkerton FE, Atzmon M. Acta Mater 2005;53:3469. Li J, Wang ZL, Hufnagel TC. Phys Rev B 2002;65:144201. Gao MC, Hackenberg RE, Shiflet GJ. Mater Trans JIM 2001;42:1741. Jiang WH, Pinkerton FE, Atzmon M. Scripta Mater 2003;48:1195. Wright WJ, Hufnagel TC, Nix WD. J Appl Phys 2003;93(3):1432. Masuhr A, Waniuk TA, Busch R, Johnson WL. Phys Rev Lett 1999;82:2290. van den Beukel A, Sietsma J. Acta Metall Mater 1990;38:383. Herlach DM. Mater Sci Eng R 1994;12:177. Mondal K, Murty BS. J Non-Cryst Solids 2006;352:5257.
Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Effects of room-temperature rolling on microstructure and crystallization behavior of Zr55Cu40Al5 metallic glass Yuanli Xu a, b, c, Horst Hahn b, c, *, Jiangong Li a, ** a
Institute of Materials Science and Engineering, MOE Key Laboratory for Magnetism and Magnetic Materials, Lanzhou University, Lanzhou 730000, PR China Institute for Nanotechnology, Forschungszentrum Karlsruhe, P. O. Box 3640, 76021 Karlsruhe, Germany c Joint Research Laboratory of Nanomaterials, Darmstadt University of Technology, Petersenstr. 23, 64287 Darmstadt, Germany b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 April 2010 Received in revised form 2 June 2010 Accepted 12 June 2010 Available online 10 July 2010
Using X-ray diffraction, transmission electron microscopy, and differential scanning calorimetry, the effects of room-temperature rolling and subsequent annealing treatment on the microstructure and crystallization behavior of the Zr55Cu40Al5 metallic glass ribbons have been investigated. High density of shear bands with spacings less than 100 nm and ordered clusters of 3e5 nm in size in the shear band regions were observed in the room-temperature rolled Zr55Cu40Al5 metallic glass ribbons. In addition, the rolling deformation changes the crystallization behavior of the Zr55Cu40Al5 metallic glass during the isothermal annealing treatment and results in the formation of a new crystalline phase. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: B. Glasses, metallic C. Rolling C. Heat treatment D. Microstructure F. Calorimetry
1. Introduction The plastic deformation of metallic glasses at temperatures below the glass transition temperature and high strain rates is known to be inhomogeneous and localized to shear bands [1]. According to the theoretical prediction [2,3] and experimental studies [4,5], a shear band, as a highly deformed region [6e8], has more free volume, i.e. a lower atomic density compared with the undeformed regions. Hence inhomogeneously distributed free volume is introduced into metallic glasses that endured inhomogeneous plastic deformation. In addition, phase separation or nanocrystallization was observed within shear bands for several metallic glasses that endured inhomogeneous deformation [9,10]. These structural changes will undoubtedly affect the properties of metallic glasses. For instance, the intrinsic plasticity of Zr44Ti11Cu9.8Ni10.2Be25 and Zr55Ti5Al10Cu20Ni10 bulk metallic glasses are significantly improved from 0.5% up to 15% plastic strain due to the introduction of microstructural inhomogeneities upon cold rolling [11]. Moreover, subsequent annealing treatment will cause reordering within shear bands and thus affect the structure and
* Corresponding author. Institute for Nanotechnology, Forschungszentrum Karlsruhe, P. O. Box 3640, 76021 Karlsruhe, Germany. Tel.: +49 7247 826350. ** Corresponding author. Institute of Materials Science and Engineering, Lanzhou University, Lanzhou 730000, PR China. Tel.: +86 931 8910364. E-mail addresses: [email protected] (H. Hahn), [email protected] (J. Li). 0966-9795/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2010.06.007
properties of the inhomogeneously deformed metallic glasses [12e14]. For example, the isothermal annealing increased the hardness of the room-temperature rolled Al86.8Ni3.7Y9.5 metallic glass [14]. The re-ordering within shear bands caused by subsequent annealing may change the local atomic arrangement and thus probably affect the crystallization behavior for metallic glasses that endured inhomogeneous deformation. However, the effects of inhomogeneous deformation on the crystallization behavior for metallic glasses have not been systematically studied so far. Therefore, it seems necessary to investigate the effects of inhomogeneous deformation on the microstructure and the crystallization behavior of metallic glasses. In this work, the effects of room-temperature rolling deformation on the microstructure and crystallization behavior of Zr55Cu40Al5 metallic glass were investigated. High density of shear bands with spacings less than 100 nm and ordered clusters of 3e5 nm in size in the shear band regions were observed in the room-temperature rolled samples. In addition, the rolling deformation changes the crystallization behavior of the samples during the isothermal annealing treatment and results in the formation of a new crystalline phase.
2. Experimental procedure Zr55Cu40Al5 (at. %) ingots were prepared by arc melting the high purity Zr (99.9%), Cu (99.5%), and Al (99.9%) under a Ti-gettered
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Y. Xu et al. / Intermetallics 18 (2010) 2039e2043
argon atmosphere. Melt-spun ribbons with a 60 mm thickness were produced using a melt spinner at a wheel speed of 20 m/s. The amorphous nature of the as-spun alloy ribbon was examined by Xray diffraction (XRD) on Panalytical X’pert X-ray diffractometer with monochromatic Cu Ka radiation (wavelength 1.5405 Å) and transmission electron microscopy (TEM) observations on Tecnai F20 electron microscope working with an acceleration voltage of 200 kV. The as-spun ribbons were cut into pieces with a length of about 10 mm. Several stacked ribbon pieces were placed between two flat steel plates with an original thickness of 1 mm and rolled in one direction at room temperature until a desired strain was obtained. During rolling, many small deformation rolling passes were repeated with a reducing gap between two rollers so that constant strain rates could be kept. The used strain rates were approximately 5.0 104 and 1.0 102 s1 in this work. The microstructure of the room-temperature rolled (RT-rolled) samples with different strains was examined by XRD and TEM. The differential scanning calorimetry (DSC) analysis was performed on the as-spun and RT-rolled samples by a PerkineElmer Pyris 1 calorimeter at 20 K/min under a flow of purified argon. In addition, unless specified otherwise, all the DSC measurements here were performed from room temperature to 830 K. Isothermal annealing treatments at 738 K on the as-spun and rolled samples were performed with this calorimeter. 3. Results and discussion The XRD pattern of the as-spun Zr55Cu40Al5 metallic glass ribbon sample is shown in Fig. 1(a). Besides the two broad diffraction halos around 2q ¼ 40 and 69 , no diffraction peaks
corresponding to any crystalline phases can be detected. To further confirm the amorphous nature of the as-spun sample, the TEM analyses were performed. A uniform contrast with no obvious lattice fringes can be found from the corresponding high resolution TEM (HRTEM) image shown in Fig. 1(b). The inset in Fig. 1(b) shows the selected area electron diffraction (SAED) pattern of the area corresponding to the TEM image in Fig. 1(b). It consists of a broad diffraction halo and a faint large one which are typical for amorphous materials. Therefore, the as-spun Zr55Cu40Al5 metallic glass ribbons are completely amorphous. A strain of 60% was achieved for the as-spun Zr55Cu40Al5 metallic glass ribbons by RT-rolling. To examine whether crystallization occurred in the RT-rolled samples, the XRD measurements were performed. The XRD patterns for the 60% RT-rolled samples deformed at strain rates of 5.0 104 and 1.0 102 s1 are also displayed in Fig. 1(a). The XRD patterns for these two RT-rolled samples exhibit two broad diffraction halos around 2q ¼ 40 and 69 . In the bright field TEM image for the 60% RT-rolled sample deformed at 1.0 102 s1 shown in Fig. 1(c), a large number of uniformly distributed bright “band-like” regions with a spacing of less than 100 nm can be observed. Obviously, these bright regions should be shear bands as they were usually observed with a bright contrast in the bright field TEM images because of the lower atomic density in the shear band region [15]. The corresponding SAED pattern in the inset of Fig. 1(c) consists of two broad diffraction halos without obvious diffraction spots. However, some ordered clusters of about 3e5 nm in size, as encircled in the HRTEM image in Fig. 1(d), form in the shear band regions [the dash line highlights roughly the boundary between a shear band (bright) and its neighboring undeformed matrix (dark)]. The ordered cluster can be
Fig. 1. (a) The XRD patterns of the as-spun Zr55Cu40Al5 metallic glass ribbons and the RT-rolled samples with a strain of 60% deformed at strain rates of 5.0 104 and 1.0 102 s1, (b) the HRTEM micrograph and the SAED pattern of the as-spun Zr55Cu40Al5 metallic glass ribbons, (c) the TEM (an SAED pattern in inset), and (d) the HRTEM micrographs of the 60% RT-rolled sample deformed at 1.0 102 s1.
Y. Xu et al. / Intermetallics 18 (2010) 2039e2043
clearly seen in the enlargement of an encircled area in the inset in Fig. 1(d). Hence, the RT-rolling deformation induces the formation of ordered clusters in the shear band regions in the Zr55Cu40Al5 metallic glass. The DSC curves obtained at a heating rate of 20 K/min for the asspun sample and the RT-rolled samples with a strain of 60% deformed at strain rates of 5.0 104 and 1.0 102 s1 are shown in Fig. 2(a). The inset shown in Fig. 2(a) displays the enlargement part of these DSC curves in the temperature range of 660e820 K. An endotherm, which can be clearly seen from the inset, occurs in the temperature range of about 720e755 K in the DSC curve for the asspun sample. This endotherm corresponds to the glass transition process. An exothermic peak centered at 788 K appears subsequently and corresponds to the crystallization process for the asspun sample. The DSC curves for the 60% RT-rolled samples deformed at 5.0 104 and 1.0 102 s1 are similar to that for the as-spun sample. The crystallization peak temperature remains unchanged compared with the as-spun sample. Therefore, the room-temperature rolling deformation does not significantly change the thermal stability of the Zr55Cu40Al5 metallic glass. To investigate further the effects of rolling deformation on thermal stability and crystallization behavior of the Zr55Cu40Al5 metallic glass ribbons, the as-spun and RT-rolled samples were
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isothermally annealed at 738 K, a temperature slightly higher than the glass transition temperature, for various times and analyzed by DSC measurements thereafter. Fig. 2(b)‑(d) shows the corresponding DSC curves for the as-spun sample, the 60% RT-rolled samples deformed at 5.0 104 and 1.0 102 s1, respectively. It can be clearly seen from Fig. 2(b) that both the crystallization peak temperature and the enthalpy of the crystallization exothermic event decrease with increasing annealing time for the as-spun sample. The crystallization peak temperature for the as-spun sample annealed at 738 K for 50 min is 767 K and is about 21 K lower compared with the as-spun sample without annealing. This indicates that the isothermal annealing at 738 K leads to significant crystallization, thus creating nuclei that grow at a lower temperature than that required for nucleation and growth in the as-spun sample without annealing. In addition, the exothermic event disappears completely when the annealing time reaches up to 70 min. For the 60% RT-rolled sample deformed at 5.0 104 s1 shown in Fig. 2(c), the crystallization peak temperature is 786 K after 10 min annealing treatment. However, when the annealing time reaches 30 min, the corresponding DSC curve exhibits two exothermic events. In these two exothermic events, one is centered at 780 K and almost the same as that for the as-spun sample annealed at 738 K for 30 min, and the other weak one is centered at
Fig. 2. (a) The DSC curves measured at 20 K/min for the as-spun Zr55Cu40Al5 metallic glass ribbons and the RT-rolled samples with a strain of 60% deformed at strain rates of 5.0 104 and 1.0 102 s1, (b) the DSC curves measured at 20 K/min for the as-spun Zr55Cu40Al5 metallic glass ribbons annealed at 738 K for various times, (c) the DSC curves measured at 20 K/min for the 60% RT-rolled samples deformed at 5.0 104 s1 and annealed at 738 K for various times, and (d) the DSC curves measured at 20 K/min for the 60% RT-rolled samples deformed at 1.0 102 s1 and annealed at 738 K for various times.
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about 798 K. As the annealing time increases further thereafter, both of the crystallization peak temperature and the enthalpy of the first crystallization exothermic event decrease. In addition, the first crystallization exothermic event still exists when annealed at 738 K for 70 min. For the 60% RT-rolled sample deformed at 1.0 102 s1 and annealed at 738 K for various times [Fig. 2(d)], the two exothermic events exist already when annealed at 738 K for 10 min, earlier than that for the sample deformed at 5.0 104 s1. Similarly, both the crystallization peak temperature and the enthalpy of the first crystallization exothermic event decrease with increasing annealing time. When annealed at 738 K for 70 min, the first crystallization exothermic event still exists. This indicates that for the 60% RT-rolled samples deformed at strains rates of 5.0 104 and 1.0 102 s1, a trace amount of amorphous phase still exists when annealed at 738 K for 70 min. Comparison of Fig. 2b with Figs. 2c, d indicates that the thermal stability of the amorphous phase against crystallization during isothermal annealing treatment does not deteriorate significantly after RT-rolling deformation for our Zr55Cu40Al5 metallic glass. However, the change of one crystallization exothermic event for the as-spun sample into two crystallization exothermic events for the samples that endured RTrolling deformation indicates that the crystallization behavior changes from one single to a double step process for the Zr55Cu40Al5 metallic glass ribbon after the RT-rolling deformation and subsequent annealing treatment. To identify the origin of the two crystallization exothermic events appear in the DSC curves for the deformed Zr55Cu40Al5 metallic glass ribbon samples annealed at 738 K, the XRD measurements were performed on the annealed samples after the DSC runs. Fig. 3(a) displays the XRD patterns of the as-spun sample run through the DSC measurement, the as-spun sample annealed at 738 K for 30 min and run through the DSC measurement, the 60% RT-rolled sample deformed at 1.0 102 s1 and run through the DSC measurement, the 60% RT-rolled samples deformed at 1.0 102 s1, annealed at 738 K for different times, and run through the DSC measurement. Fig. 3(b) is the enlargement part of Fig. 3(a) in the 2q range of 28e49 . As can be clearly seen from Fig. 3 (b), Cu8Zr3, Al3Zr, and unknown phases can be found in the as-spun sample after the DSC run, the 30 min annealed as-spun sample after the DSC run, and the 60% RT-rolled sample after the DSC run. In addition, after annealing at 738 K for different times, DSC measurements were performed on the 60% RT-rolled samples from room temperature to a temperature between the first crystallization peak temperature and the second one. Subsequently these samples were analyzed by XRD. The XRD pattern for the 60%
RT-rolled sample undergone 10 min annealing and subsequent DSC measurement performed from room temperature to 793 K [indicated as RT-60% (10 min, 793 K)] is shown in Fig. 3. Only Cu8Zr3, Al3Zr, and unknown phases can be found from the XRD pattern for this sample. Thus the crystallization exothermic event for the asspun sample and 60% RT-rolled samples and the first crystallization exothermic event for the 60% RT-rolled sample that endured annealing treatments correspond to the formation of the Cu8Zr3, Al3Zr, and unknown phases. The Cu10Zr7 phase as well as the Cu8Zr3, Al3Zr, and unknown phases can be observed in the XRD analysis for the 60% RT-rolled samples that endured 10, 30, and 50 min annealing treatments at 738 K and the DSC run. Obviously, the second crystallization exothermic event centered at about 798 K in Fig. 2(c) and (d) corresponds to the formation of Cu10Zr7 phase. Based on these results, the room-temperature rolling deformation changes the crystallization behavior of the Zr55Cu40Al5 metallic glass and leads to the formation of a new crystalline phase during the annealing treatment for the Zr55Cu40Al5 metallic glass. The inhomogeneous deformation leads to the formation of high density of shear bands in metallic glasses. It has been speculated that temperature plays a crucial rule in nanocrystallization in shear bands due to adiabatic heating produced by mechanical deformation [16]. However, for an Al90Fe5Gd5 metallic glass ribbon bent at 233 K, nanocrystals was still observed in shear bands [17]. Since the deformation heat could be easily dissipated and the temperature rise in the sample was very limited, it was proposed that the precipitation of nanocrystallites at shear bands was ascribed to the creation of excess free volume [17]. This was further supported by nanoindentation experiment on metallic glasses [10]. Considering the excess free volume in shear bands created by plastic deformation, the free energy of a glass will increase with increasing deformation [18]. On the other hand, the viscosity h in these regions will decrease based on the relationship between the free volume vf and h, i.e. h ¼ ðh=vm Þexpðbvm =vf Þ, where h, vm, and b represent the Planck’s constant, the critical free volume for atomic diffusion, and an overlap factor, respectively [19,20]. In addition, according to the classical theory of nucleation, the nucleation rate Iv, as the product of one kinetic term and one thermodynamic term, can be depicted as Iv ¼ ðAv =hÞexp½16ps3 =ð3kB T DG2 Þ [21,22], where Av is a constant of the order of 1036 Pa/m3, s the interfacial energy per area, kB the Boltzmann constant, DG the Gibbs free energy difference per volume between the new and old phases, respectively. Obviously, the reduction of the viscosity in the nucleation equation above will enhance the atomic diffusional mobility and thus enhances the fluctuation process leading to the
Fig. 3. (a) The XRD patterns of the as-spun Zr55Cu40Al5 metallic glass ribbons run through the DSC measurement, the as-spun sample annealed at 738 K for 30 min and run through the DSC measurement, the 60% RT-rolled sample deformed at 1.0 102 s1 and run through the DSC measurement, the 60% RT-rolled samples deformed at 1.0 102 s1, annealed at 738 K for different times, and run through the DSC measurement, the 60% RT-rolled sample deformed at 1.0 102 s1, annealed at 738 K for 10 min, and run through the DSC measurement performed from room temperature to 793 K [indicated as RT-60% (10 min, 793 K)] and (b) the enlargements of the same XRD patterns from 2q ¼ 28 to 49 .
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formation of nuclei in the shear band regions. Moreover, the increase of the Gibbs free energy offers a driving force for nucleation in the shear band regions in terms of the crystal nucleation. These probably lead to the formation of ordered clusters in shear bands in the RT-rolled Zr55Cu40Al5 metallic glass samples. The thermal stability of the amorphous phase against crystallization during isothermal annealing treatment has been reported to decrease for the inhomogeneously deformed Cu60Zr20Ti20 bulk metallic glass with nanocrystallization in shear bands [9]. However, no new crystallization exothermic event was observed from the corresponding DSC curves. This indicates that the crystallization behavior didn’t change for the inhomogeneously deformed Cu60Zr20Ti20 bulk metallic glass [9]. Hypothetically speaking, local short range order re-ordering may take place and regions with composition different from the original amorphous phase may form during isothermal annealing treatments in shear bands due to high atomic mobility. Thus the crystallization behavior, as observed in this letter for the Zr55Cu40Al5 metallic glass ribbons, may be changed and new crystalline phases form during the isothermal annealing. 4. Conclusion In summary, high density of shear bands with spacings less than 100 nm are introduced in the Zr55Cu40Al5 metallic glass ribbons that endured room-temperature rolling deformation. Ordered clusters of 3e5 nm in size form in the shear band regions for the 60% room-temperature rolled sample. The inhomogeneous deformation and subsequent annealing treatment at a temperature near the glass transition change the crystallization behavior from one single step which corresponds to the formation of Cu8Zr3, Al3Zr, and
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unknown phases, to a double step process which corresponds to the formation of the Cu8Zr3, Al3Zr, and unknown phases and the formation of the Cu10Zr7 phase. Acknowledgements This work was supported by the International S&T Cooperation Program (ISCP) of the Chinese Ministry of Science & Technology (MOST) under Grant No. 2008DFA50340 and the Deutsche Forschungsgemeinschaft (DFG) under contract HA1344/24-1. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
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