Nanometer-scale observations of metallic glass fracture surfaces

Materials Science and Engineering A176 (1994) 411-415

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Nanometer-scale observations of metallic glass fracture surfaces Yoshihisa Watanabe and Yoshikazu Nakamura Department of Materials Science and Engineering, National Defense Academy, 1-10-20 Hashirirnizu, Yokosuka, Kanagawa 239 (Japan)

J. T. Dickinson, D. M. Kulawansa* and S. C. Langford Department of Physics, Washington State UniversiO,, Pullman, WA 99164-2814 (USA) (Received June 2, 1993)

Abstract The effect of the structural change of metallic glasses on fracture is studied by observing the fracture surfaces of as-cast and annealed FeTsBl.~Si9 metallic glasses on the nanometer scale using atomic force microscopy (AFM). X-ray diffraction (XRD) was applied in studying the structural change. Metallic glass ribbons were fractured by the tensile test machine at room temperature. In the as-cast specimens, the fracture surface displays a characteristic vein pattern macroscopically. AFM observations of the fracture surface of the as-cast specimen show nanometer-scale steps and grooves, suggesting that the fracture has occurred associated with plastic deformation. However, in the fracture surfaces of the specimens annealed at and above 723 K, AFM observations reveal many pyramid-like features, although the fracture surface is featureless macroscopically. The pyramid-like features appear to be crystallites produced during heating, because the XRD indicates a crystalline structure. Since the pyramid-like features are not observed in the original annealed surface, they are found to be formed by fracture. This suggests that the fracture of the specimens annealed above 723 K has occurred along the crystallite facets, i.e. a kind of cleavage fracture.

1. Introduction

Metallic glasses, produced by rapid quenching from the molten state [1], have unique properties because of their amorphous and metallic structure. The fracture properties of metallic glasses have been widely studied [2] and observations of fracture topography (fractography) are reviewed in the literature [31. Typical features of the fracture surface of metallic glasses are vein patterns, suggesting that the fracture has occurred associated with localized catastrophic plastic deformation [3]. Scanning electron microscopy (SEM) has made it possible to observe fracture surfaces on the micrometer scale but it is desirable to shrink down to fractography of metallic glasses to the nanometer scale, in order to study the localized behavior of fracture. Both scanning tunneling microscopy (STM) [41 and atomic force microscopy (AFM) [5] have potential uses to characterize the surface from the atomic level to the nanometer scale. Previous STM and A F M observations of metallic glasses have mainly focused on the *Present address: Department of Physics, Texas Tech University, Lubbock, TX, USA. 0921-5093/94/$7.00 ,%SDI (1921-5093(93)02532-8

crystallization process on the surface during heat treatment [6-8]. Recently, the present authors have started to apply STM and A F M in observing fracture surfaces of metallic glasses [9, 10]. In our previous paper, STM observations of the fracture surfaces of as-cast metallic glasses have been reported and the origin of the vein and groove features on the nanometer scale have been discussed in detail [10]. In general, many properties of metallic glasses deteriorate as a result of crystallization during heat treatment [1 1], and the fracture mode is supposed to change when the structure changes from amorphous to crystalline during the heat treatment. In the Fe:~,P~,C4Ai~B metallic glasses, it has been reported that, at and below room temperature, the specimens shatter into minute fragments. However, above room temperature, fragmentation does not occur but specimens are found with their fracture tips sharply curled up [21. To study the change in fracture mode associated with the structural change in metallic glasses, it is useful to observe the change in the fracture surface of metallic glasses during heat treatment, using STM or AFM as well as SEM. The present paper describes the results of AFM observations of the nanometer-scale change in © 1994 Elsevier Sequoia. All rights reserved

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Metallic glass fracture surfaces

the fracture surface of iron-based metallic glasses, as associated with the structural change.

F e B 3Si9

(a) cLs-cast

2. Experimental details

The specimens used in this study were iron-based metallic glasses of Fe78B 135i9 (Metglas TCA), produced by Allied-Signal Metglas Products. The ribbon was 25 /~m thick, and was cut into a dog-bone shape with gauge dimensions of 6 mm × 35 mm and loaded in tension until it fractured at room temperature. The effect of the structural change on fracture was studied by comparing the fracture surfaces of the ascast and annealed specimens. The annealing was performed in a vacuum furnace, evacuated by a diffusion pump, by keeping the specimens at elevated temperatures for 2 h and then cooling the specimens to room temperature in the vacuum furnace. To determine the annealing temperatures, differential thermal analysis (DTA) was applied to measure the crystallization temperature. The DTA results showed that the exotheraml reaction peak starts at about 753 K and ends at about 783 K. Thus, the annealing temperatures were set to be 573 K (well below the crystallization temperature), 723 K (near the crystallization temperature) and 873 K (above the crystallization temperature). The structural change resulting from the heat treatment was studied by X-ray diffraction (XRD) with a copper target X-ray tube. The fracture surfaces were examined by SEM (Hitachi, S-2100A) to confirm the fracture behavior on the micrometer scale. A F M observations of the fracture surface were made in air with a NanoScope III (Digital Instruments), equipped with Si3N 4 tips with tip radii of about 40 nm. The topographic data were acquired at 512 × 512 points per frame. The A F M images shown in this paper are the profiles corrected with subtraction of the background slope.

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I (b) annealed at 573 K

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I (c) annealed at 723 Ko

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I o ¢ - Fe solid solution

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(d)

i

annealed

I

I o ~ - Fe solid

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at873K

20

I

:°tuti°n

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I

I

40

60

80

100

2e (deg) Fig. 1. XRD patterns of Fe78Bl3Si9 metallic glasses: (a) as-cast specimen; (b) specimen annealed at 573 K for 2 h; (c) specimen annealed at 723 K for 2 h; (d) specimen annealed at 873 K for 2h.

3. Results and discussion

The XRD results of Fe7sBl3Si9 metallic glasses ascast and annealed at 573 K, 723 K and 873 K are shown in Figs. l(a), l(b), l(c) and l(d) respectively. From these figures it can be seen that the structural changes from amorphous to crystalline at about 723 K, which is consistent with the DTA results. The peaks in Figs. l(c) and l(d) are due to a-Fe solid solutions. A typical SEM image of the fracture surface of the as-cast specimen is shown in Fig. 2. The characteristic vein pattern is clearly observed on the fracture surface. Figures 3(a) and 3(b) show the A F M images of the fracture surface of the as-cast specimen, which were

Fig. 2. SEM photograph of an as-cast Fe7sBl3Si9 metallic glass fracture surface, showing the characteristic vein pattern.

obtained by scanning the zone of fracture initiation. Figure 3(a) shows many parallel ridges originating from the valley seen in the left-hand side, and the opposite surface of the valley is found to be smooth. The height

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Metallic glass fracture su~wes

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1.0

nm

m

0 (a/

~lll

1.0

U

Fig. 4. SEM photograph of a n surface annealed at 873 K.

Fe78Bi3Si 9

metallic glass fracture

1.0 1.0 5.0 pm (b)

I.O

Fig. 3. AFM images of an as-cast Fe7sB~3Si~metallic glass fracture surface, showing (a) the parallel ridges and (b) the grooves. ]~lll

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5.O of the steps varies from approximately 10 to 50 nm. This image suggests that the crack originated from the valley. Figure 3(b) shows several parallel grooves on the smooth surface. The depth of the groove is approximately 20 nm. In our previous paper [10], we already discussed the mechanism of the vein and groove pattern formation: we have proposed that (1) the vein features are attributed to the interaction of adjacent crack fingers in which the material between adjacent fingers fails in plane stress, and (2) the origin of the grooves may be due to a shear instability near the tips of the crack fingers. However, an SEM image of the fracture surfaces of the specimen annealed at 873 K is shown in Fig. 4. Compared with the SEM image of the as-cast specimen (Fig. 2), it is found that the fracture surfaces change macroscopically from the vein to the featureless pattern, with a structural change from amorphous to crystalline. Although the fracture mode in the specimen annealed at 873 K is obviously different from that in the as-cast specimens, because of the limited resolution of SEM, it is difficult to determine the microscopic origin of the mode change from the SEM images only. Figure 5 shows an A F M image of the fracture surface of the specimen annealed at 573 K. The parallel ridges also can be observed in the image,

Fig. 5. AFM image of a n Fe78Bi3Si 9 metallic glass fracture surface annealed at 573 K, showing similar ridge pattern to that of the as-cast specimen.

although the step height is much smaller. The similarity in the fracture surface of this specimen to that of the ascast specimen implies that the fracture mode is almost unchanged during heat treatment up to 573 K. In contrast the A F M image of the fracture surface of the specimen annealed at 723 K is quite different from those for the as-cast specimen and that annealed at 573 K. Many pyramid-like features are observed in the fracture surface, as shown in Fig. 6, although the SEM images does not reveal this kind of feature. When the annealing temperature increases, pyramid-like features become more prominent, as can be seen in the A F M image of the fracture surface of the specimen annealed at 873 K shown in Fig. 7. These pyramid-like features may be crystallites produced during the heat treatment, because the XRD results indicate the crystalline structure. A high magnification A F M image of the pyramidlike crystallites is shown in Fig. 8. From this figure, several steps are observed on the surface of pyramidlike crystallites. These steps may be explained by assuming that the materials in the opposite fracture surface moved slightly before separation.

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Metallic" glass fracture surfaces

5.0 2.0

5.0

A

°°

2.5 5.0

Fig. 6. AFM image of an Fe7sB,.~Si9 metallic glass fracture surface annealed at 723 K, showing the layered pyramid-like features.

0

2.0 0

~ ~

0 5.0

2.5 X (pm)

5.0

Fig. 9. Top-view A F M image of an Fe7sBt3Si,) metallic glass

fracture surface annealed at 873 K (Fig. 7). The pyramid-like features are found to face each other along the two characteristic lines.

~m 1.0

F%B~3S~

5.O

annealed at 873K

Fig. 7. A F M image of an FeT~Bl.~Si~ metallic glass fracture surface annealed at 873 K. The pyramid-like features become

.•

more prominent.

1,4

0.5

0

|

! 2.0 v

Fig. 8. High magnificiation A F M image of the pyramid-like feature observed in Fig. 7. Several steps are detected on the surface of the pyramid-like feature.

The top-view image of Fig. 7 is shown in Fig. 9. In this image, we can see two characteristic lineage structures, along which the surfaces of many crystallites face to face with each other. The cross-section along the line A-A' in Fig. 9 is shown in Fig. 10. It should be noted that the shape of the indent marked by the arrow is similar to the shape of the pyramid-like protrusion, and

I 2.5 X (lain)

5.0

Fig. 10. Cross-section of the pryamid-like feature formed on the fracture surface of Fe7sB]3SL) metallic glass annealed at 873 K. This figure is obtained by cutting the AFM image along the line A-A' in Fig. 9.

the facets of the crystallites are almost parallel to each other. This suggests that the valley was formed by removing the pyramid-like crystallite during fracture. To confirm whether or not the pyramid-like crystallites are unique in the fracture surface, the surface of the specimen annealed at 873 K was observed by A F M . The A F M image obtained is shown in Fig. 11. No pyramid-like crystallites are observed but many bumps can be seen uniformly on the original surface, which agrees with our previous A F M observations of the surface of Fe78B138i9 metallic glasses annealed at 773 K [7]. From a series of A F M observations, it is concluded that, in the fracture surface of the as-cast specimen, the steps and grooves are observed. I n contrast, in the

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Metallic glass,lmcture surfiu'es

600

rim

600

u

Fig. 11. AFM image of an FeTsB~3Si~ metallic glass surface annealed at 873 K, showing no pyramid-like features but many bumps formed uniformly.

specimens annealed above 723 K, the pyramid-like protrusions, presumably precipitated crystallites, are formed by fracture. We attribute the fracture of FeTsB~3Si9 metallic glasses annealed above 723 K as being the result of the separation along the facets of the crystallites, i.e. a kind of cleavage fracture.

4. Conclusions T h e nanometer-scale changes in the fracture surfaces of FeTsBj3Si9 metallic glasses, as associated with the structural change, are observed by A F M . In the fracture surface of the as-cast specimen, the characteristic vein patterns are observed by SEM, and A F M observations reveal nanometer-scale ridges and grooves. These results support the hypothesis that the fracture has occurred associated with the localized plastic deformation, because many ridges and grooves are considered to be formed after plastic deformation. In the fracture surface of the specimen annealed at 573 K, no drastic changes are detected by AFM. However, the fracture surfaces of the specimens annealed at 723 and 873 K display many pyramid-like features, although the corresponding features are not observed by SEM. These pyramid-like features may be crystallites produced during the heat treatment, because the XRD indicates the crystalline structure of

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the specimen. Since the pyramid-like crystaUites are not observed in the surface of the annealed specimen, these features are considered to be formed during fracture. Thus, we propose that the fracture has occurred along the facets of the crystallites, i.e. as a kind of cleavage fracture. Finally, it can be concluded that the A F M observations reveal the nanometer-scale changes in the fracture surfaces of FeTsBl3Si~) metallic glasses, as associated with the structural change. They also give valuable information in the study of the fracture mode of this material, which is hardly possible from the SEM images only.

Acknowledgment T h e authors would like to thank Dr. H. H. Libermann, Allied-Signal Metglas Products, for providing the metallic ribbon used in this work.

References 1 W. Klement, Jr., R. H. Willens and P. Duwez, Nature, 187 (1960) 869. 2 C.A. Pampillo, J. Mater. Sci., 10 ( 1975) 1194. 3 R.W. Rice, in J. R. Varner and V. D. Frechette (eds.), Fractography of Glasses and Ceramics, American Ceramic Society, 1988, p. 36. 4 G. Binnig, H. Rhorer, Ch. Gerber and E. Weibel, Phys. Rev. Lett., 49(1982) 57. 5 G. Binnig, C. F. Quare and Ch. Gerber, Phys. Rev. Lea., 56 (1986) 930. 6 Y. Watanabe, T. Kubozoe and Y. Nakamura, J. Mater. Res., 7 (1992) 1396. 7 Y. Watanabe and Y. Nakamura, J. Muter. Sci. Lett., 11 (1992) 1534. 8 Y. Watanabe and Y. Nakamura, J. Mater. Res., 7(1992) 2126 and references cited therein. 9 Y. Watanabe, J. T. Dickinson, D. M. Kulawansa and S. C. Langford, Memoirs Nat. Defense Acad., Jpn., 31 (1992) 53. 10 D. M. Kulawansa, J. T. Dickinson, S. C. Langford and Y. Watanabe, J. Mater. Res., 8 (1993) 2543. 11 H. H. kibermann, J. Marti, R. J. Martis and C. R Wong, Metall. 7?ans. A, 20 (1989) 63.