Exoelectron emission from unexcited metallic glasses Fe 78 B 13 Si 9 during heat treatment
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Exoelectron emission from the surface of unexcited metallic glasses Fe78B13Si9 during heat treatment has been studied under ultra high vacuum condition. In the first heating cycle, exoelectrons are emitted from the as-cast ribbon in the temperature range from approximately 423 K to 773 K (150 °C to 500 °C), although the surface of the specimen is not excited by ionizing radiation, chemical processes, or mechanical treatments prior to measurements. In the second and subsequent heating cycles, however, there is no anomalous emission observed in the same temperature range. In order to elucidate the mechanism of emission, the surface of the specimen is observed by the atomic force microscope (AFM) before and after measurements. In the AFM image, many crystallites in the amorphous matrix can be found in the surface of the heated specimen. These experimental results show that exoelectrons are emitted in the same temperature range as the early stages of crystallization on the surface of metallic glasses. We hypothesize that the two effects are correlated.
I. INTRODUCTION Metallic glasses are amorphous alloys produced by quenching from the molten state. Since the discovery by Klement et al.,] many kinds of metallic glasses have been developed and 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, these prominent properties decrease during the crystallization process and thus elaborate studies of property change in various properties in heat treatment have been carried out.2 Exoelectron emission from amorphous materials was first detected by Gorecki for glassy selenium and boric anhydride glass in 1981, 3 and Gorecki and his collaborators have intensively studied exoelectron emission from metallic glass.4"9 They have observed photostimulated exoelectron emission (PSEE) associated with structural transformation in amorphous materials by means of an open-air point counter with saturated quenching vapor of ethanol, and investigated the relation between the temperature dependence of the intensity of PSEE and differential thermal analysis (DTA)3~5>7~9 and the temperature dependence of the intensity of PSEE and the contact potential difference (CPD) relative to platinum. 68 From the results of PSEE and DTA, they have shown the possibility of detecting early crystallization in the surface of metallic glasses by exoelectron emission phenomena.4"9 They have also indicated that the PSEE and CPD curves show anomalies corresponding ^Professor Emeritus, National Defense Academy.
1396 http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 6, Jun 1992
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to the structural transformations in the surface6'8 and they have proposed a difference in temperatures and activation energy between the volume and the surface crystallization in metallic glasses, assuming that PSEE reflects the process of surface crystallization and the DTA data show the bulk crystallization.7"9 On the othe
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