Effect of Compositional Short Range Order on Glass Formation in Binary Metallic System
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Effect of Compositional Short Range Order on Glass Formation in Binary Metallic System Hao Chen, Mahadevan Khantha and Takeshi Egami Department of Materials Science and Engineering, University of Pennsylvania Philadelphia, Pa 19104-6272, U.S.A. ABSTRACT Molecular Dynamics simulation was carried out to study the glass transition and crystallization in the metal-metalloid binary system with pair-wise potentials. The results show that a repulsive potential between metalloid (small) atoms increases the glass forming ability. The observation is consistent with the recent theory of bulk metallic glass formation through local glass transition and nano-glass formation. The theory predicted that the compositional short-range order (CSRO) prevents the small atom pairing so as to increase the glass forming ability (GFA). The present results demonstrate the important role of CSRO in bulk metallic glass formation. INTRODUCTION The recent development of bulk metallic glass has drawn tremendous interest in both technological applications and fundamental research of metallic glasses. Since Duwez [1] fabricated the first amorphous metallic alloy Au75Si25 by using the rapid solidification technique in 1959, the glass forming ability has been greatly improved, seen from the critical cooling rate decreasing from 106 K/s to 1 K/s. It has been found that various factors, such as the atomic size ratio, composition, and interatomic potential, affect the GFA of metallic systems, while the underlying microscopic mechanism of how to improve GFA remains a major challenge in theoretical research. Recently, Egami [2] proposed a theory of bulk metallic glass formation based on the idea of local glass transition. This theory suggests that GFA is improved if the glass transition does not take place at a single temperature for bulk metallic glasses and there is a series of local glass transitions, which are distributed over a wide temperature range. The GFA depends on the width of the temperature range, which can be enlarged by introducing large size difference ratio and CSRO. In this paper, we describe the results of molecular dynamics simulation which prove that the presence of CSRO improves the GFA of metallic systems. COMPUTATION PROCEDURE We performed constant number, pressure and temperature (NPT) molecular dynamics simulation on a binary AB system composed of 500 particles, 400 “A” and 100 “B” atoms representing a metal-metalloid binary system. We cooled the system from 1800 K to 600K at a cooling rate of 2 × 1013 K/s. The time step is 1 × 10-3 ps, and 5 × 104 steps are used for equilibrating the system and additional 5 × 104 steps for collecting the data of interest. Since the purpose of this study is to examine the general principles of the atomic size effect and not to simulate any particular alloy, simple model potentials were used to describe the system. The transition metal atom “A” has the atomic size and mass of iron atom, and the modified Johnson’s potential is used
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to describe its interaction. The metalloid ato
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