Characteristics of Shear Bands and Fracture Surfaces of Zr 65 Al 7:5 Ni 10 Pd 17:5 Bulk Metallic Glass

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1 Materials Transactions, Vol. 46, No. 12 (2005) pp to 2874 Special Issue on Materials Science of Bulk Metallic Glasses #2005 The Japan Institute of Metals Characteristics of Shear Bands and Fracture Surfaces of Zr 65 Al 7:5 Ni 10 Pd 17:5 Bulk Metallic Glass Kyosuke Yoshimi 1, Hidemi Kato 2, Junji Saida 3 and Akihisa Inoue 2 1 Department of Environmental Studies, Tohoku University, Sendai , Japan 2 Institute for Materials Research, Tohoku University, Sendai , Japan 3 Center for Interdisciplinary Research, Tohoku University, Sendai , Japan In this work, the characteristics of shear bands and fracture surfaces of Zr 65 Al 7:5 Ni 10 Pd 17:5 bulk metallic glass fractured by a tensile test was investigated. Zr 65 Al 7:5 Ni 10 Pd 17:5 bulk metallic glass shows a yield stress of approximately 1.3 GPa, a fracture stress of approximately 1.5 GPa and a tensile plastic strain of % irrespective of the applied strain. Wavy, meandering shear bands were observed in the relatively wide area of specimen surfaces around the point of failure, and typical vein patterns were observed on the fracture surfaces. Shear bands and fracture surfaces were further examined by confocal microscopy to obtain more precise information on their roughness. On the other hand, evidence of viscous flow due to crack propagation was also obtained around the edge at the point of failure on specimen surfaces by confocal microscopy. The deformability of Zr 65 Al 7:5 Ni 10 Pd 17:5 bulk metallic glass is discussed on the basis of the obtained results. (Received August 9, 2005; Accepted November 14, 2005; Published December 15, 2005) Keywords: bulk metallic glass, tensile test, fracture, confocal microscopy, shear band 1. Introduction Recently, Kato et al. 1) found that the increase in hydrostatic pressure during rapid solidification enhances the glass forming ability (GFA) of Zr 65 Al 7:5 Ni 10 Pd 17:5, whereas the GFA of Zr 65 Al 7:5 Ni 10 Cu 17:5 is suppressed under hydrostatic pressure. This difference in the dependence of GFA on hydrostatic pressure between Zr 65 Al 7:5 Ni 10 Pd 17:5 and Zr 65 Al 7:5 Ni 10 Cu 17:5 has been shown to be related to the difference in molar volume due to the primary crystallization of icosahedral quasi-crystalline phases. 2) Quite recently, Mukherjee et al. 3) reported that, among five bulk metallic glass-forming alloys, the better glass formers had higher viscosity at their liquidus temperature and showed a correspondingly smaller change in volume upon crystallization. GFA was clearly correlated with the volume change upon crystallization in their study. 3) Moreover, they found that, among Zr-based bulk metallic glass formers, the better glass formers had a lower crystal-melt interfacial tension. 4) They explained these seemingly competing properties by the icosahedral short-range order (ISRO) of undercooled liquids in their literature. 4) Thus, hydrostatic pressure may change the ISRO of Zr 65 Al 7:5 Ni 10 Pd 17:5 at the melting temperature. Interestingly, Zr 65 Al 7:5 Ni 10 Pd 17:5 exhibits large compressive plastic elongation at room temperature. 2) Although the mechanism of the good compressive deformability remains unclear, such a large compressive plasticity should be related to the GFA of Zr 65 Al 7:5 Ni 10 Pd 17:5 under hydrostatic pressure. In this work, the mechanical properties of Zr 65 Al 7:5 Ni 10 Pd 17:5 bulk metallic glass are investigated in tension at room temperature. Its morphological features on deformation and fracture are characterized by scanning electron microscopy and confocal microscopy. The deformability of Zr 65 Al 7:5 - Ni 10 Pd 17:5 bulk metallic glass is discussed based on the obtained results. 7.5 mol%al, 10 mol%ni and 17.5 mol%pd was produced by a conventional arc-melting technique in an Ar atmosphere. The alloy was re-melted in a quartz tube, and rapidly solidified into a tensile specimen mold in the Ar atmosphere of about 0.1 Pa. The dimensions of the tensile specimens were 11 mm in gage length, 2 mm in gage width, 2 mm in gage thickness and 2 mm in shoulder radius. A schematic diagram of the shape of the tensile specimen is shown in Fig. 1. Specimen surfaces were mechanically ground with silica papers up to #3000 so that the thickness of the tensile specimens became about 1 mm. The ground surfaces were further mechanically polished with 1 mm alumina slurry. Tensile tests were conducted using an Instron 8562-type machine in an ambient atmosphere at room temperature. An initial strain rate of 1: to 1: s 1 was applied, and load-displacement data were recorded in two ways with analogue and digital signals. After failure, specimen and fracture surfaces were observed using a Hitachi S-4300 scanning electron microscope and a Lasertec HD100D confocal microscope. 2. Experimental Procedure An alloy having a nominal composition of 65 mol%zr, Fig. 1 Schematic diagram of tensile specimens used in the present study.

2 Characteristics of Shear Bands and Fracture Surfaces of Zr 65 Al 7:5 Ni 10 Pd 17:5 Bulk Metallic Glass 2871 Stress, σ / MPa ε = 10-5 s s s -1 5 % Table 1 Tensile properties of Zr 65 Al 7:5 Ni 10 Pd 17:5 bulk metallic glass obtained at room temperature. Strain Rate (s 1 ) Yiels Stress (MPa) Fracture Stress (MPa) Plastic Elongation (%) Strain, ε (%) Fig. 2 Stress strain curves obtained by tensile tests at the initial strain rate of 1: to 10 3 s Results and Discussion Figure 2 shows the stress strain ( ") curves of Zr 65 Al 7:5 - Ni 10 Pd 17:5 metallic glass obtained by tensile tests at the initial strain rates of 1: to 10 3 s 1. A small amount of plastic elongation is observed after yielding. The yield stress and ultimate tensile, the latter of which was equal to the fracture stress in the present case, were about 1300 and 1500 MPa, respectively, and showed almost no change within the range of strain rates applied. The strain rate-dependence of the fracture stress of Zr 65 Al 7:5 Ni 10 Pd 17:5 metallic glass obtained in the present work was quite similar to that of Zr 65 Al 10 Ni 10 Cu 15 metallic glass ribbons obtained by Kawamura et al.. 5) The values of yield stress obtained by the tensile tests in the present work were MPa smaller than those by the compression tests. 2) This difference in yield stress depending on the deformation mode is in good agreement with that simulated by Lund and Schuh. 6) Zr 65 Al 7:5 Ni 10 Pd 17:5 metallic glass was plastically deformed under tension, as shown in Fig. 2. The plastic strains were quite small (less than 1%), whereas the Zr 65 Al 7:5 Ni 10 Pd 17:5 metallic glass showed a large plastic strain in compression mode in the previous work. 2) The values obtained in the tensile tests are shown in Table 1. Figure 3 shows secondary electron (SE) micrographs of the specimen surfaces around the location of the fracture. The applied strain rate was 10 4 s 1. As shown in Fig. 3(a), the fracture surfaces were inclined from the tensile axis. The inclination angle was larger than that of the maximum shear stress plane, 45, which has been often reported for bulk metallic glasses. 7 13) Though the tensile specimens were elastically strained up to about 5% before failure, the effect of elastic strain on the inclination angle was not large enough. Therefore, the larger inclination angle can be explained by taking account of not only the maximum shear stress but also the hydrostatic pressure or the normal stress acting on the shear plane, as Lund and Schuh pointed out. 6) Through most of the tensile tests, the fracture occurred preferentially in the vicinity of the end of the gage section. The specimens used in this work have shoulders for hooking over the specimen holders, as shown in Fig. 1, so that stress should be strongly concentrated at the boundaries between the shoulders and the gage section. In fact, shear bands were more frequently generated on the shoulder side rather than on the gate side. Fig. 3 SEM images of specimen surfaces around the location of fracture. (a) Side surface. (b) Plane-view of the surface. (c) Plane-view of the surface at a higher magnification. The propagation of shear bands is not simple: in addition to the main channels seen in Fig. 3(b), a high density of subchannels was generated, as seen in Fig. 3(c). These main and sub-channels of shear bands propagated in a zigzag manner. The zigzag propagation of shear bands should help to promote uniform plastic deformation. This might be the reason why Zr 65 Al 7:5 Ni 10 Pd 17:5 metallic glass exhibits a large

3 2872 K. Yoshimi, H. Kato, J. Saida and A. Inoue Fig. 5 SEM images of a fracture surface taken at (a) a low magnification and (b) a high magnification. Fig. 4 Confocal microscopic images of a specimen surface after straining. (a) 3-D image. (b) Corresponding color image. plastic strain in compression. Unfortunately, the stress concentration at the end of the gage section was measurably larger in this study. If the stress concentration could be suppressed in the tensile test, a larger plastic strain would be observed in Zr 65 Al 7:5 Ni 10 Pd 17:5 metallic glass. Height differences of specimen surfaces formed through shear band propagation were observed by confocal microscopy. Figure 4(a) is a 3-D image and Fig. 4(b) is a corresponding color-image of surface morphology showing the height differences among the different specimen surfaces. Drastic color changes are seen at the main channels of shear bands, indicating that higher steps were formed by shear band propagation. The largest difference in height in Fig. 4 is about 6 mm. Although the shear bands appeared to have a certain level of thickness by SEM, as seen in Fig. 3, this effect does not represent the actual thickness of the bands, but rather the height of the steps. Fractography was examined by SEM as shown in Fig. 5. Figure 5(a) is a lower-magnification SE image of the fracture surface of a specimen strained at 10 4 s 1. Many rough, vertical stripes consisting of ridges and valleys are observed, as well as fine vein patterns. This fracture morphology with ridges and valleys should make a good contribution to enhancing the fracture toughness. Figure 5(b) is an SE image of the fracture surface taken at a higher magnification. Typical vein patterns are observed in this picture. The vertical stripes observed in Fig. 5(a) are also seen faintly. To clarify the asperity of the fracture surfaces, the surface morphology of the above fracture surface was examined by confocal microscopy. Figure 6(a) is a 3-D image and Fig. 6(b) is a corresponding color-image of the fracture surface. Vein patterns are clearly observed in the 3-D image [Fig. 6(a)]. The height difference of veins, however, is not more than 1 mm, as shown in Fig. 6(b). On the other hand, the height difference between the ridges and valleys is much larger than that of the veins. Therefore, it is clear that the vein pattern formation is the phenomenon limited near surfaces. Confocal microscopy was further applied to the characterization of the edges of specimen surfaces at the point of failure. Figure 7(a) shows a 3-D image and Fig. 7(b) a corresponding color-image of the edge of the specimen surface. The fracture surface is faced diagonally forward right. A gradual color change corresponding to a continuous depression can be seen from the inside toward the edge of the specimen surface. The depression area spreads out up to more than 50 mm inside at some places. This depression morphology is obviously different from that of shear bands, e.g., as

4 Characteristics of Shear Bands and Fracture Surfaces of Zr 65 Al 7:5 Ni 10 Pd 17:5 Bulk Metallic Glass 2873 Fig. 6 Confocal microscopic images of a fracture surface. (a) 3-D image. (b) Corresponding color image. Fig. 7 Confocal microscopic images of a specimen surface around the edge at the point of fracture. (a) 3-D image. (b) Corresponding color image. seen in the middle of this image, suggesting that viscous flow deformation occurred through crack propagation in the Zr 65 Al 7:5 Ni 10 Pd 17:5 metallic glass. The occurrence of viscous flow deformation through crack propagation makes sense when we consider the large plane stress around the crack tip near the specimen surface and the evidence of vein pattern formation. As seen in Fig. 2, it is clear that shear bands act as the nucleation and propagation sites of a crack. Since passing shear bands meander considerably in Zr 65 Al 7:5 Ni 10 Pd 17:5 metallic glass, larger energy should be consumed against crack propagation, suggesting that the glass is severely deformed around the crack tip. The morphology of fracture surfaces consisting of ridges and valleys as shown in Figs. 5 and 6 would correspond to the zigzag shear band propagation. The mechanism of the viscous flow deformation around the crack tip and the zigzag shear band propagation should be clarified in the future in order to improve the deformability of metallic glasses under their glass transition temperatures. 4. Conclusions In this study, the tensile and fracture behavior of Zr 65 Al 7:5 Ni 10 Pd 17:5 bulk metallic glass was investigated at room temperature. The conclusions are as follows: (1) The yield stress and ultimate tensile stress obtained by the tensile tests were 1300 and 1500 MPa irrespective of the strain rate. The values of the yield stress were MPa smaller than those previously obtained by compression tests. 2) The values of the plastic strains are quite small less than 1%. (2) Wavy, meandering shear bands were observed on specimen surfaces around the point of failure. The sharp height differences at shear bands were confirmed by confocal microscopy. (3) Continuous depression suggesting viscous flow deformation was observed around the edge at the point of failure on specimen surfaces by confocal microscopy. The depression areas spread out up to several tens of microns inside from the edges into the interior of the specimen surfaces. (4) In fracture surfaces, many rough, vertical stripes consisting of ridges and valleys were observed as well as fine vein patterns. Height differences between the

5 2874 K. Yoshimi, H. Kato, J. Saida and A. Inoue individual ridges and valleys were in the range of several microns, while the height differences of the veins were not more than 1 mm. Acknowledgments This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan (No ). REFERENCES 1) H. Kato, A. Inoue and J. Saida: Appl. Phys. Lett. 85 (2004) ) H. Kato, J. Saida and A. Inoue: Scr. Mater. 51 (2004) ) S. Mukherjee, J. Schroers, Z. Zhou, W. L. Johnson and W.-K. Rhim: Acta Mater. 52 (2004) ) S. Mukherjee, J. Schroers, W. L. Johnson and W.-K. Rhim: Phys. Rev. Lett. 94 (2005) ) Y. Kawamura, T. Shibata, A. Inoue and T. Masumoto: Appl. Phys. Lett. 69 (1996) ) A. C. Lund and C. A. Schuh: Acta Mater. 51 (2003) ) T. Mukai, T. G. Nieh, Y. Kawamura, A. Ioue and K. Higashi: Intermetallics 10 (2002) ) C. T. Liu, L. Heatherly, D. S. Easton, C. A. Carmichael, J. H. Schneibel, C. H. Chen, J. L. Wright, M. H. Yoo, J. A. Horton and A. Inoue: Metall. Mater. Trans. A 29 (1998) ) P. E. Donovan: Acta Metall. 37 (1989) ) G. He, J. Lu, Z. Bian, D. Chen, G. Chen, G. Tu and G. Chen: Mater. Trans. 42 (2001) ) J. J. Lewandowski and P. Lowhaphandu: Philos. Mag. A 82 (2002) ) J. Megusar, A. S. Argon and N. J. Grant: Mater. Sci. Eng. 38 (1979) ) S. Takeyama: Scr. Metall. 13 (1979)