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29MILLION-ATOM MOLECULAR DYNAMICS SIMULATIONS OF EXPLOSIVE CRYSTALLIZATION IN AMORPHOUS CuTi THIN FILMS
O. Politano*", S. A. Rogachev0, F. Baras", and A. S. Rogachev0
^Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR 6303 CNRS-Université de Bourgogne Franche-Comté, Dijon, France.
bMerzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, Chernogolovka, Russia *e-mail: [email protected]
DOI: 10.24411/9999-0014A-2019-10126
Metallic glasses produced by fast quenching of melts possess very interesting physicochemical properties (e.g. high Young modulus, corrosion resistance, specific electric and magnetic properties), which make them attractive materials for various applications [1]. They are metastable at room temperature and can spontaneously transform into a crystalline solid by an exothermic process, named devitrification. During the transformation of a restricted amorphous domain, the local temperature increases and accelerates the crystallization process in adjacent regions. When the heat release is large enough, the local initiating amorphous-to-crystalline transformation becomes self-sustained and self-accelerating.
The very first observations of spontaneous self-propagating crystallization wave (i.e. "explosive crystallization") were done in layers of amorphous antimony [2]. In recent years, one can notice a renewed interest on this process due to its promising application for the production of thin film polycrystalline solar cells or optical data storage [3, 4]. Beyond semiconductors and dielectrics systems, explosive crystallization was also experimentally characterized in metallic Cu-Ti, Fe-B, and Fe-Si-B glasses [5-7]. Among them, the Cu50Ti50 glass is a perfect candidate to study theoretically the "explosive crystallization" process as it transforms into the crystalline phase CuTi of the same chemical composition, without the appearance of any other stable or metastable phase.
In this work, we explored the processes of devitrification and explosive crystallization in the CuTi metallic glass by molecular dynamic simulations using LAMMPS [8]. The simulation box contains one to two million atoms interacting through the EAM potential developed by Zhou et al. [9]. Two types of systems were considered [10]. The first one was obtained by first heating a y-CuTi single crystal above its melting temperature. Then, a fast quenching (cooling rate ~ 1013 K/s) of the metallic melt down to the ambient temperature allowed us to obtain a metastable metallic CuTi glass. This sample was then reheated to 675 K and the spontaneous formation of the crystalline phase was monitored (see Fig. 1). In such systems, it was shown that the nucleation of primary crystalline clusters occurs homogeneously due to spontaneous fluctuations of atomic structure. We observed that most of the clusters larger than 50-100 atoms (~ 8-10 nm in diameter) will grow into steady crystalline grains. The spontaneous nucleation and growth of many crystalline grains observed leads to a local increase of temperature, which is the initial step to observe a thermal explosion. A second type of simulations concerns self-propagating front of crystallization in very long sample (see Fig. 2). This geometry was inspired by our pioneering work on SHS into Ni-Al system by molecular dynamics [11]. The reaction was initiated locally by heating one extremity (10 nm) of the glassy sample. The devitrification starts and propagates across the whole sample accompanied by a thermal wave (see Fig. 3). Due to the exothermic devitrification, temperature rises in the reactive front up to 665-714 K. The thickness of the preheated zone is about 20 nm and the thickness of the amorphous-to-crystalline transition zone is about 1 nm. The velocity of the front was ~ 40 m/s.
Fig. 1. Spontaneous homogeneous nucleation and growth of crystallite (cross section of the simulated box). The color-bar represents an ordering parameter (W). Blue (dark grey) indicates atoms in an amorphous state, yellow (light grey) crystallized atoms; intermediate colors indicate partially ordered clusters.
Fig. 2. Schematic representation of MD simulated sample for studying self-propagating explosive crystallization.
Fig. 3. Temperatures profiles of propagating waves during explosive crystallization along the x coordinate at
successive times.
As we will show during the talk, molecular dynamics allowed us to model short-time atomic processes occurring during explosive crystallization. In particular, we studied the nucleation and growth of crystalline clusters and grains. which are difficult to characterize by experimental approaches. The simulated results will be compared with the available experimental data on macroscopic behavior of the explosive crystallization waves.
This work was financially supported by the Russian Science Foundation (grant no. 16-13-10431). The use of computational facilities at the Computing Center of the University of Bourgogne. PSIUN-CCUB. is gratefully acknowledged.
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