Local Atomic Structures and Plastic Deformation Modes in the Supercooled Liquid State of La 55 Al 25 Ni 20
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Local Atomic Structures and Plastic Deformation Modes in the Supercooled Liquid State of La55Al25Ni20 Tadakatsu Ohkubo, Yoshihiko Hirotsu and Akihisa Inoue1 Institute of Science and Industrial Research, Osaka University, Osaka, 567-0047, JAPAN 1 Institute of Materials Research, Tohoku University, Sendai, 980-8577, JAPAN ABSTRACT An in-situ electron diffraction structure analysis of La55Al25Ni20 in the supercooled liquid temperature range ( Tx) revealed a gradual structural change with a local phase separation due to strong atomic correlations especially of La-La and Al-Ni. On the basis of the structural data, structure models corresponding to the structures at temperatures between Tg and Tx were constructed using molecular dynamics (MD) simulation technique. These constructed structure models were then utilized for MD simulations to understand plastic deformation in the supercooled liquid state under a constant strain-rate. A larger elongation was obtained for the structure in the middle of the Tx range, but for the structure near Tx the elongation was relatively smaller which was supposed to be due to the local phase separation. These simulation results correspond well to the experimental elongation behaviors in the superplastic deformation that the elongation is enhanced in the middle of Tx but suppressed as the temperature is increased towards Tx.
INTRODUCTION The amorphous La55Al25Ni20 alloy has an excellent superplasticity in the stable supercooled liquid state between the glass transition (Tg) and the crystallization (Tx) temperatures [1,2]. We have recently performed an in-situ electron diffraction and HREM study of this alloy by annealing up to Tx using a specimen-heating stage in TEM [3,4]. Diffraction intensities were recorded on imaging plates. In order to avoid inelastic scattering, energy-filter was used. Any appreciable change was observed by HREM on heating across the temperature Tg. However, atomic radial distribution function analysis revealed that a clear structural change proceeds in the temperature range between Tg and Tx on an atomic level. The structural change is concerned with a local phase separation due to strong atomic correlations especially for La-La and Al-Ni. In our experiment of the superplasticity of this alloy in the supercooled liquid temperature range Tx (=Tx-Tg) [5], it was found that the elongation is enhanced in the middle of Tx but suppressed as the temperature increased towards Tx. This result must be closely related to the local structural change observed by the in-situ electron diffraction study in the range of Tx. In the present study, we have performed a molecular dynamics (MD) simulation of the plastic deformation process of this alloy in Tx by preparing suitable structure models. In this article we describe the results of structural study in the Tx range by in-situ electron diffraction and also of the plastic deformation experiments briefly, followed by the process of structure modeling and the MD simulation results with a discussion on the superplastic behavior and it
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