AFM observations of the surface morphology of metallic glasses Fe 78 B 13 Si 9 in the early stage of crystallization
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The change in the surface morphology of metallic glasses Fe78B13Si9 during the early stage of the crystallization process has been studied mainly by atomic force microscopy (AFM). Specimens of 50-yu,m thickness have been heated up to 323 K, 373 K, 423 K, and 523 K with a heating rate of 30 K/min in vacuum, and then the surface morphology of each specimen has been observed by AFM in air. In the surface image of the specimen heated to 323 K, many holes are observed and inside the holes single or plural protrusions can be observed. Clusters composed of aggregated protrusions are also found like islands in the amorphous sea. The specimen heated to 323 K has also been observed by a high-resolution transmission electron microscope and crystalline structures among the amorphous matrix have been detected. From the selected area diffraction study, the crystalline structure is found to be a— Fe crystallites. In contrast, in the AFM image of the surface of the specimen heated to 423 K, no holes are seen and many protrusions are found to extend above the surface and form several parallel lines. The spatial density of protrusions above the surface becomes much higher in the specimen heated to 523 K. We propose that this change in the surface morphology during heat treatments indicates the process of nucleation and growth of a-Fe crystallites in the surface of metallic glasses. It is also found that the stage of many protrusions extending above the surface corresponds with the beginning of exoelectron emission from the surface. This result suggests that exoelectron emission and surface crystallization connect with each other.
I. INTRODUCTION In recent years, metallic glasses produced by rapid quenching from the molten state of alloys are applied in the various fields of technology because of their outstanding properties, such as high tensile strength and hardness, high corrosion-resistance, and magnetic softness. However, many properties are deteriorated by crystallization and thus the study of the crystallization process has become an important subject. The surface crystallization process has lately attracted much attention, especially, because the surface plays an important role in determining the overall properties, and crystallization is generally found to start in the surface.1 Both scanning tunneling microscopy (STM)2 and atomic force microscopy (AFM) 3 are powerful tools for characterizing the surface in nanometer (nm) scale. STM observations of the metallic glass surfaces have been performed on Nd ]35 Fe8i 3 B5 2 , 4 Rh25Zr75,5 Pd84 5 Si 155 , 6 Fe 91 Zr 9 , 7 Fe 8 0 B 2 0 , 8 : i 0 Co66Fe4Ni1B14Si15,11-12 and Nb 40 Ni6().13 Among them, Slawaska-Waniewska et al.10 have reported the beginning of surface crystallization of Fe8OB2o alloys after annealing by current pulses, and Scandella et al.13 have observed nanocrystallites in the surface of Nb 40 Ni 60 alloys irradiated with a Nd: YAG laser. However, no systematic studies on the surface 2126
J. Mater. Res., Vol. 7, No. 8, Aug 1992
crystallization process by STM have b
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