Calculation of Diffraction Patterns Associated with Electron Irradiation Induced Amorphization of CuTi.
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CALCULATION OF DIFFRACTION PATTERNS ASSOCIATED WITH ELECTRON IRRADIATION INDUCED AMORPHIZATION OF CuTi. AND R. DEVANATHAN', N. Q. LAM". M. J. SABOCHICK***, P. OKAMOTO** M.MESHII* *Dept. of Materials Science and Engineering, Northwestern University, Evanston, IL 60201 Argonne National Laboratory. Argonne IL 'Wright-Patterson Air Force Base, OH ABSTRACT A new approach that uses the multislice method in conjunction with molecular dynamics simulations to study electron irradiation induced amorphisation is presented. Diffraction patterns were calculated for CuTi and found to be more sensitive than the pair correlation function to the structural changes preceding amorphisation. The results from this approach and from a study of long range order are presented.
INTRODUCTION Crystalline materials have been rendered amorphous by a variety of solid-state techniques such as irradiation by energetic particles, hydrogenation, mechanical alloying and annealing of multilayer films [1]. Because of its simple polymorphous nature, the amorphization of intermetallic compounds by high energy electron beams has been extensively studied [2-8]. The damage produced by electrons is homogeneous and consists of two defect components: namely, isolated interstitials and vacancies (i.e. Frenkel pairs) and antisite defects (i.e. chemical disorder). One aspect of the transformation concerns the relative importance of Frenkel defects versus chemical disorder in inducing amorphization. This question was recently addressed by Sabochick and Lam [9], using the molecular dynamics (MD) simulation technique. The crystal-to-glass transformation of a model intermetallic compound was monitored via the changes in the calculated pair correlation function caused by the introduction of Frenkel pairs or by chemical disorder introduced by random atom pair interchanges. Although very informative, the calculated pair correlation functions are difficult to compare directly with experiments, in particular with the commonly observed, but poorly understood changes in single crystal diffraction patterns that often precede the onset of amorphization [10]. In order to bring the MD results in closer contact with experiments, the multislice method was used to calculate the single crystal diffraction patterns and high resolution lattice images of the atomic defect structures obtained from MD simulations.
THE MOLECULAR DYNAMICS SIMULATION The simulation technique used by Sabochick and Lam [9] was an isothermal isobaric molecular dynamics scheme using a modified version of the computer code DYNAMO [11]. The simulation cell consisted of 288 atoms each of copper and titanium arranged in a B 11 lattice as shown in Figure 1. The system was maintained at 160K and zero pressure. The interatomic potentials for Cu, Ti and CuTi were developed using the approach of Oh and Johnson [12] based on the embedded-atom method [13]. A perfect lattice was initially equilibrated for 500 time steps (dt = 2x10"1 s). Every 20 time steps a randomly chosen pair of copper and titanium atoms were interc
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