Simulation study of tracer self-diffusion mechanism in cobalt amorphous solids

The simulation finds a large number of vacancy-simplexes, which plays a role of diffusion vehicle for cobalt atom and varies with relaxation degree. A new diffusion mechanism for tracer diffusivity in cobalt amorphous solid is supposed as follows: The elemental atomic movement includes a jump of neighbouring atoms into the vacancy-simplexes and then collective displacement of a large number of atoms.
Nội dung trích xuất từ tài liệu:
Simulation study of tracer self-diffusion mechanism in cobalt amorphous solids JOURNAL OF SCIENCE OF HNUE Natural Sci., 2010, Vol. 55, No. 6, pp. 28-36 SIMULATION STUDY OF TRACER SELF-DIFFUSION MECHANISM IN COBALT AMORPHOUS SOLIDS Pham Huu Kien(∗) and Pham Khac Hung Hanoi University of Science and Technology Vu Van Hung Hanoi National University of Education (∗) E-mail: huukienpham@yahoo.com Abstract.The statistical relaxation (SR) simulation has been conducted to study the behavior of simplexes in cobalt amorphous solid containing 2×105 atoms. The simulation reveals that the fraction of 4-simplex increases and of N-simplex with N > 4 decreases upon relaxation degree. The simulation finds a large number of vacancy-simplexes, which plays a role of diffusion vehicle for cobalt atom and varies with relaxation degree. A new diffusion mechanism for tracer diffusivity in cobalt amorphous solid is supposed as follows: The elemental atomic movement includes a jump of neighbouring atoms into the vacancy-simplexes and then collective displacement of a large number of atoms. Keywords: Amorphous solid, Microscopic bubble, Vacancy-simplex, Diffu- sion mechanism, Statistical relaxation, Molecular dynamic.1. Introduction Metal has been known as an indispensable material in the lives of the wholeof mankind. So, it has attracted the attention of many researchers in all fields: ex-periments, theory and computer simulation [1-10]. In all of the nature of the metal,atom diffusion mechanism in amorphous metals (AM) is the topical area of highnature and should be particularly interesting to research. Many works have studiedthe atomic diffusion mechanisms in amorphous solids [1, 8, 9]. For example, the ex-perimental researches [4-7], said that in amorphous solids there exists quasi-vacancyand diffusion mechanism of the atom is indirect through quasi-vacancy. However,the definition of quasi-vacancy is still not well understood and is not a question to beanswered. On the other hand, computer simulation has proved the existence of thevacancy in amorphous solids [1, 2, 8], the number of vacancies level changes uponrelaxation degree. More comprehensive research of computer simulation is to detectthe existence of voids and simulation results have found a continuous spectrum ofthese voids in amorphous solid. Size of the voids found smaller radius of the atom,28 Simulation study of tracer self-diffusion mechanism in cobalt amorphous solidsbut the radius of the atoms is determined also containing a uniformity. Recently, VoVan Hoang and colleagues have discovered in models of liquid and amorphous Fe,the temperature dependence of the diffusion coefficient showing two properties: fromnot obeying the Arrhenius law at high temperature regions to obeying the Arrheniuslaw in a low temperature region. However, they have not yet determined exactly thediffusion coefficient of Fe atoms in these temperature regions [2]. Therefore, atomicdiffusion mechanisms in amorphous solids are still a question to be addressed. Fromthe evidence of existence of the vacancy-simplexes in Co amorphous solids, we havediscovered the role of the vacancy-simplexes for atomic diffusion in amorphous met-als. Therefore, in this paper, three models of amorphous Co have been built tostudy the existence of the vacancy-simplexes and their role for the atomic diffusionmechanism. The dependence on the relaxation degree of the vacancy-simplexes anddiffusion coefficient of Co atoms are also addressed and discussed in this article.2. Computation procedure The simulation has been conducted for the model consisting of 2 × 105 cobaltatoms in a cube box (size 132.7 × 132.7 × 132.7 ˚A) with periodic boundary condi-tions. We use the Pak - Doyama potential, it is given as follows: U(r) = −0.12812(r − 1.82709)4 + 1.15421(r − 2.50849)2 − 0.13448, (2.1)for 0 ≤ r ≤ 3.44 A ˚, which has a cut-off between the second and third nearest-neighbour distance of cobalt. From this pair interatomic potential was initialtyobtained from the data in [1]; here r is the interatomic distance in ˚ A and U(r) isin eV. The density is set to be the value of real amorphous cobalt (8.666g/cm3).Initial configuration is generated by randomly placing all atoms in the simulationbox. Then the sample is relaxed over several thousand steps until the system reachesthe equilibrium, using the statistic relaxation (SR) technique. The SR movementlength is equal to 0.002 ˚ A. The equilibrium model obtained corresponds only to oneamong the possible metastable states and a more stable state can be prepared bythe shaking proced ...