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Y. KOGURE, M. DOYAMA Teikyo University of Science & Technology, Uenohara, Yamanashi 409-01, Japan, kogure ntu.ac.jp

ABSTRACT Glass transition and effects of pressure and shear stress on atomic configuration are investigated by means of molecular dynamics simulation. An embedded atom method potential is used for the atomic interaction. A model crystal for Cu is melted and quenched to realize the glass state. Atomic configuration in the glass is examined through the radial distribution function. External stress is applied and displacement of atoms is monitored to investigate the mechanism of mechanical relaxation. Larger displacement of atoms is observed under a shear stress.

INTRODUCTION In a glass state, especially at lower temperatures, material shows anomalous behavior in mechanical and in thermal properties. The resonance absorption of sound at very low temperature (< 1K) and the mechanical relaxation at higher temperatures (10 - 30 K) are characteristic phenomena of glass. These are considered to be due to the motion of atoms through the potential barriers by the quantum mechanical and the thermal activation processes. The purpose of the present study is to investigate the atomic configuration responsible to the double well potential, which may be formed on the course of glass transition. As a model material copper is chosen because the atomic interaction is expressed by a simple function.

METHOD OF SIMULATION The atomic interaction in metals has non-central characters due to the existence of conduction electrons, and can be expressed by a n-body potential. Recently, the embedded atom method (EAM) potentials have been developed by many authors[1-3]. An EAM potential for copper atoms developed by the present authors [4,5] are used for the simulation. The potential function is expressed as

Et= Z F() + 1

(1) Zj

F(p)= Dplnp, p= Y',f(ri,). 279

Mat. Res. Soc. Symp. Proc. Vol. 455 ©1997 Materials Research Society

(2)

where, F(p) is the embedding energy for i-th atom, p is the electron density, and r,, is the distance between i-th and j-th atom. The functions 0(r) and f(r) are 1(r) = A(r, - r) 2 exp(-c1r),

(3)

f(r) = B(rc - r)2 exp(-c 2r),

(4)

where, rc is a cut off distance of the potential. These functions contain five parameters A, B, C1, C2, and D. They are determined by fitting the potential to experimental values of physical properties for copper. The determined values are A = 789.92[eV], B = 0.037794, C, = 7.8893r 0 , C2 = 0.026582, and D = 14.005eV, where r0 is the nearest neighbour distance.

As an initial configuration atoms are arranged in the fcc structure, and a velocity with random distribution is given to each atom to melt the crystal. The mean value of the velocity vm = v is related to reference temperature T* as mvm = 3kBT*,

(5)

where, m is the atomic mass and kB is the Boltzmann's constant. The time interval At for the molecular dynamics was chosen to be 5 x 10-'sec. An initial velocity corresponds to 2000 K was given to atoms in the crystal. Judging from the velocity distribution the