Scanning tunneling microscope observations of metallic glass fracture surfaces
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Yoshihisa Watanabe Department of Materials Science and Engineering, National Defense Academy, Hashirimizu, Yokosuka, Kanagawa 239, Japan (Received 28 September 1992; accepted 20 May 1993)
We report scanning tunneling microscope observations of fracture surfaces formed during catastrophic crack growth in three metallic glasses: Ni56Cr18Si22B4, Co69Fe4NiiMo2Bi2Sii2, and Fe78 Bi 3Si9. Macroscopically, the first two glasses fail along a slip band formed during loading and display a characteristic, /xm-scale pattern of vein-like ridges; in contrast, Fe78B13Si9 displays little slip prior to fracture, and its fracture surface shows a yu,m-scale chevron pattern of steps. STM observations of fracture surfaces of all three materials show nm-scale grooves. The grooves in Co69Fe4NiiMo2B12Sii2 are especially prominent and display stepped edges which we attribute to the intersection of shear bands with the surface. STM observations of the vein-like features on Ni56Cr18Si22B4 also show stepped edges. We attribute the vein features to the interaction of adjacent crack fingers in which the material between adjacent fingers fails in plane stress. The origin of the grooves is uncertain, but may be due to other shear instabilities along crack fingers.
I. INTRODUCTION Metallic glasses are amorphous alloys produced by extremely rapid quenching from the molten state.1 Their amorphous structure and metallic character are associated with unique deformation and fracture behavior. In particular, deformation by slip in these materials tends to be highly localized in shear bands on the order of 5 nm thick.2 The fracture of amorphous alloys has been widely studied and has been extensively reviewed.3-4 Observations of fracture surface topography (fractography) have long played an important role in studies of fracture behavior,5 especially on the //,m-distance scales accessible by scanning electron microscopy (SEM). However, nm-scale observations of metallic glass fracture surfaces are desirable, especially in view of the extremely localized nature of slip in these materials. Although metallic glasses are typically brittle on macroscopic scales, microscopically they can be extremely ductile. In many metallic glasses, fracture in tension is preceded by extensive but highly localized slip; crack growth in these materials generally proceeds along a slip band formed during loading and produces a characteristic veined morphology. Metallic glasses in which slip bands are not formed prior to fracture generally display dimpled surfaces, often associated with a chevron pattern of steps. Except at temperatures well below room temperature, virtually all metallic glasses display fracture features associated with localized plastic deformation. J. Mater. Res., Vol. 8, No. 10, Oct 1993 http://journals.cambridge.org
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Due to the extreme localization of slip, its role in fracture can be difficult to assess. The amorphous structure of metallic glasses does not allow for contrast mechanisms suitable for the observation of individual disloc
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