An overview of atomistic approaches in SHS processes Текст научной статьи по специальности «Физика»

ÏSHS2019

Moscow, Russia

AN OVERVIEW OF ATOMISTIC APPROACHES IN SHS PROCESSES

F. Baras*", O. Politano", A. Fourmont", S. Le Gallet", A. Nepapushev0, A. Sedegov0,

S. Vadchenkoc, and A. Rogachevôc

aLaboratoire ICB, CNRS/Université Bourgogne Franche Comté, Dijon, France bCenter of Functional Nano-Ceramics, National University of Science and Technology

MISiS, Moscow, 119049 Russia cMerzhanov Institute of Structural Macrokinetics and Material Science, Russian Academy of

Sciences, Chernogolovka, Moscow, 142432 Russia *e-mail: [email protected]

DOI: 10.24411/9999-0014A-2019-10018

Since the discovery of SHS by A. Merzhanov and I.P. Borovinskaya, the topic attracts interest both from the experimental and theoretical point of view. The "Concise Encyclopedia of SHS" [1] demonstrates the wide range of theoretical interests in SHS processes including combustion theory, thermal explosion, auto-oscillations, reaction-diffusion, dissolution-precipitation, grain growth, spin combustion, heterogeneous kinetics, phase transformations, and non-equilibrium systems. The understanding the SHS processes relies on the study of reactions kinetics, heat and mass transfer, as well as the dynamics of structural transformations. Besides the usual multi-physics description, the atomistic approach provided by Molecular Dynamics simulations (MDS) becomes a useful tool in describing SHS processes. MDS indeed give the basic atomistic steps leading to observed microstructure beyond any thermodynamic or kinetic modeling. MDS can be considered as a modeling that offers a counterpart to in-situ experiments to explore elemental mechanisms. In addition, MDS allow us to calculate most of the parameters that are used in numerical modeling at the macroscopic scale. This approach thus offers a self-consistent multi-scale modeling that provides a powerful tool for the interpretation of experimental results.

In the case of SHS in nanometric metallic multilayers, MDS prove to be a very appropriate method for numerical studies, as the accessible time and length scales are in the same range of magnitude as in "real" experiments. During this talk, we will briefly review the main features of SHS in nanofoils considering Ni/Al as a model system [2]. We will show how modeling and experiments are complementary approaches in order to detect intrinsic behaviors and reactive mechanisms (see Fig. 1).

10 nm

Fig. 1. Comparative study of MDS and experiments in the case of SHS in Ni/Al nanofoils.

XV International Symposium on Self-Propagating High-Temperature Synthesis

Next, an overview of recent achievements will be reported, including crystal growth, nucleation, and SHS propagation in complex systems.

Crystal growth in the case of self-propagating reactions in Ni-Al nanofoils was also investigated by means of MDS [3]. We studied the heteroepitaxial growth of NiAl on Ni during mixing and alloying at interfaces (see Fig. 2). The microstructure evolution was tracked along with grain orientation dynamics. For the different orientations of the Ni interface, a simple geometric construction based on the relationship between unit cells of Ni and NiAl explains crystal growth specificities. The nucleation process and growth kinetics were also investigated. This study proves that crystal growth varies considerably, in relation to Ni orientation. The influence of composition gradient on the crystal nucleation of the NiAl intermetallic was considered in [4].

Fig. 2. Snapshot of a (OOl)-slice across the NiAl phase, parallel to the interface, and corresponding virtual diffraction (SAED) pattern. The A1 and Ni atoms are shown as blue (dark grey) and yellow (light grey) spheres, respectively.

Thanks to the progress in computing facilities and the set up of large-scale MDS, more and more complex systems can be considered in order to understand the role of defects or intricate microstructure in nanostructured samples. For instance, the influence of defect concentration on the combustion of reactive Ni/Al nanofoils was considered in [5] as well as the grain size effects in [6]. As shown in Fig. 3, the SHS propagation can also be studied in the case of disordered nanostructured grains.

Fig. 3. Snapshot of SHS propagation in a sample with 80 disoriented 15 nm-grains. Each grain has a layered structure.

In the last part of the talk, we will focus on the case of reactive composite particles Ni/Al and Ti/Al. In parallel to experimental investigations for producing highly reactive particles for

■SHS 2019 Moscow, Russia

additive technologies, MD simulations [10] are developed in order to follow the elemental mechanisms governing the kinetics aspects at the microscopic level, such as friction between metallic particles, diffusion, creation of defects, local ordering, precipitation, local stress and reactive behavior due to laser initiation. Figure 4 shows the thermal response of a Ni/Al reactive particle pre-heated at 600K.

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Fig. 4. Thermal behavior of a reactive particle and initial configuration of the reactive particle (d = 24nm).

In summary, molecular dynamics approaches allow us to elucidate the mechanisms of nonisothermal processes such as phase transformations and self-propagation reactions. The atomistic-level understanding obtained from MDS can help interpret and guide experimental work in SHS.

Part of this work is supported by the International Russian-French PHC Kolmogorov "RECIPES" (no. 41144SG) the Ministry of Science and Higher Education of the Russian Federation in the framework of the Federal Target Program "Research and Development on Priority Directions of the Scientific and Production Complex of Russia for 2014-2020", agreement no. 14.587.21.0051, project RFMEFI58718X0051.

1. I.P. Borovinskaya, A.A. Gromov, E.A. Levachov, Y.M. Maksimov, A.S. Mukasyan, A.S. Rogachev, Concise Encyclopedia of Self-Propagating High-Temperature Synthesis: History, Theory, Technology, and Products, Elsevier Pbl., 2011.

2. F. Baras, V. Turlo, O. Politano, S.G. Vadchenko, A.S. Rogachev, A.S. Mukasyan, SHS in Ni/Al Nanofoils: A Review of Experiments and Molecular Dynamics Simulations, Adv. Eng. Mater., 2018, vol. 20, pp. 1-20.

3. F. Baras, O. Politano, Epitaxial growth of the intermetallic compound NiAl on low-index Ni surfaces in Ni/Al reactive multilayer nanofoils, Acta Mater., 2018, vol. 148, pp. 133-146.

4. P. Yi, M.L. Falk, T.P. Weihs, Suppression of homogeneous crystal nucleation of the NiAl intermetallic by a composition gradient: A molecular dynamics study, J. Appl. Phys., 2017, vol. 146, 184501.

5. B. Witbeck, J. Sink, D.E. Spearot, Influence of vacancy defect concentration on the combustion of reactive Ni/Al nanolaminates, J. Appl Phys., 2018, vol. 124, 045105-9.

6. B. Witbeck, D.E. Spearot, Grain size effects on Ni/Al nanolaminate combustion, J. Mater. Res., 2019, vol. 23, pp. 1-10.