Atomistic Simulation of Radiation-Induced Amorphization of the B2 Ordered Intermetallic Compound NiTi
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ATOMISTIC SIMULATION OF RADIATION-INDUCED AMORPHIZATION OF THE B2 ORDERED INTERMETALLIC COMPOUND NiTi MICIL,\kEL J. SABOCIILCIKi and NGIII
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Ai," Force Institute of '1'echnology, Department of' Enginecring Physics, \Vright-l'attcrson Air Force Base, Ol ,15-133-6583 -Argonne National laboratory, Materials Science D)ivision, Argonne, IL 60-139 ABSTRACT Amorphization of the B2 intermetallic compound NiTi under electron irradiation has been investigated using molecular dynamics. The effect of irradiation was simulated using two processes: 1) Ni and Ti atoms were exchanged, resulting in chemical disorder, and 2) Frenkel pairs were introduced, leading to the formation of stable point defects and also chemical disorder upon mutual recombination of interstitials and exchanges per atom, the first process resulted in an energy vacancies. After -0. increase of approximately 0.11 eV/atom and a volume increase of 1.01%. On the other. hand, after introducing -0.5 Frenkel pairs per atom, the second process led to smaller increases of 0.092 eV/atom in energy and( 1.43% in volume. The calculated radial distribution functions (RDFs) were essentially identical to each other and to the calculated RI)v of a quenched liquid. The structure factor, however, showed that long-range order was still present after atom exchanges, while the introduction of Frenkel pairs resulted in the loss of long-range order. It was concluded that point defects are necessary for amorphization to occur in NiTi, although chemical disorder alone is capable of storing enough energy to make the transition possible. INTRODUCTION Comnp)uter simulation studies of amorphization induced by electron irradiation have concentrated on" the question or what key factors are essential to induce the crystalline-to-amorpl)hous (C-A) transition. In the earliest investigation, all of which were of pure systems, the introduction of point defects (either as single interstitials or as Frenkel pairs) resulted in amorphization [1-3]. Similar results were obtained upon the introduction of interstitials into a binary Lennard-Jones system [.1]. Although few pure metals have been made amorphous experimentally, these simulations supported the importance of point defects in the amorplhization process, in general agreement with point-defect theories of aniorphization [5-7]. In the first study of the amorphization of an intermctallic compound using realistic potentials, on the other hand, siml)ly exchanging Ni and Zr atoms in simulated NiZr. was sulficient to induce amorphization [8,91, sul)porting the theory of amorphization based on chemical disorder [10]. In recent work on CuM 1 'i, compounds, tile elfects of chemical disorder and point defect introduction were studied in the same material. It w'as found that chemical disorder alone was insufficient to drive the crystalline-to-amorphots (C-A) transition, which occurred only after point defects were introduced [11,121. WVereport here the results of the simulation or NiTi, a B2 compound which has been amorphized in experiments un
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