Molecular dynamics simulation of the effect of dislocations on the martensitic transformations in a two-dimensional model

Investigation of the thermoelastic martensitic transformation is of high interest nowadays because of the numerous applications of the materials with such structural peculiarities. Thermodynamics, kinetics, structure, morphology of martensitic transformation still remain unclear in many respects. From this point of view, the effective way to study various properties of metallic crystals on atomistic level is molecular dynamics simulation, for which good qualitative agreement with the experiment can be achieved even with simple Morse or Lennard-Jones interatomic potentials. In this paper, the effect of dislocations on the direct and reverse martensitic transformation is studied by molecular dynamics simulation in a two-dimensional model of the ordered alloy with the AB stoichiometry. The three dimensional analog to this structure is B2 superstructure based on bcc lattice, which is characteristic for intermetallic NiTi alloy. It is found, that the dislocations can be considered as the nucleation centers for martensite phase, increasing the temperature of the direct martensitic transformation in comparison with the homogeneous martensitic transformation. The martensite domains found in the structure after transformation and the reverse martensitic transformation takes place in the presence of the domain boundaries, meaning that the austenite nucleates heterogeneously. At the reverse transformation, splitting of perfect dislocations into partials dislocations took place. Thus, it was established in the present study that, on the one hand, dislocations affect the direct martensitic transformation as the nucleation centers, and from the other hand, reverse martensitic transformation changes the dislocation structure of the modeled alloy. © 2017, Institute for Metals Superplasticity Problems of Russian Academy of Sciences. All rights reserved.MD simulation of direct and reverse MT in the framework of the 2D model of an ordered alloy shows that dislocations can be considered as the nucleation centers of martensite during direct MT. In the absence of dislocations, martensite nucleates homogeneously at lower temperatures. The reverse MT occurs in the presence of twinned martensite and domain walls in the system, which means that even in the absence of dislocations, the structure of the alloy is not ideal. During the reverse MT, the perfect dislocations split into partial dislocations. It should be noted that for selected model parameters MT is associated with large lattices deformations, therefore, temperature hysteresis is also large, which is not typical for SM alloys. However, the consideration of the Acta Mater. 95, 37 (2015). effects of SM and superelasticity is not prohibited in frame of 14. C. Ni, H. Ding, XJ.. Jin. J. Alloys Compd. 546, 1 (2013). this model, since the degree of coherence of the martensite 15. S. Kazanc, FA. . Celik, S. Ozgen. J. Phys. Chem. Solids. 74, and austenite lattices is quite high. One of the important 1836 (2013). problems for futher investigations is the studying of the MT 16. T.  Suzuki, M.  Shimno, K.  Otsuka, X.  Ren, A.  Saxena. as the detonation process [36]. J. Alloys Compd. 577S, S113 (2013). 17. B.  Wang, E.  Sak-Saracino, N.  Gunkelmann, Aknowledgements. S.V. D. acknowledges support from the H.M. Urbassek. Comp. Mater. Sci. 82, 399 (2014). Russian Foundation for Basic Research, grant No. 16-58-48001 18. C.  Tatar, S.  Kazanc. Current Applied Physics. 12, 98 IND_omi (design of the research, discussion of the results). (2012). J.A. B and V.G. P. are grateful for the financial support from 19. A.R.  Kuznetsov, Yu.N.  Gornostyrev, M.I.  Katsnelson, the Russian Science Foundation, grant No. 15-12-10014 A. V. Trefilov. Mater. Sci Eng. A309–311068 (2001). (numerical simulations and writing the paper). R.I. B. thanks 20. I.N.  Kar’kin, Yu.N.  Gornostyrev, L.E.  Kar’kina. Phys. financial support from the Russian Science Foundation, grant Solid State. 52, 431 (2010). No. 17-79-10410 (numerical simulations). The work of D. V. G. 21. Y. N. Gornostyrev, IN. . Kar’kin, LE. . Kar’kina. Phys. Solid was supported by the Saint-Petersburg State University, State. 53, 1388 (2011). research grant No. 6.65.43.2017 (discussion of the results). 22. J.A. Baimova, R.I. Babicheva, A.V. Lukyanov, V.G. Pushin

Elastic properties of fullerites and diamond-like phases

Diamond‐like structures, that include sp2 and sp3 hybridized carbon atoms, are of considerable interest nowadays. In the present work, various carbon auxetic structures are studied by the combination of molecular dynamics (MD) and analytical approach. Two fullerites based on the fullerene C60 and fullerene‐like molecule C48 are investigated as well as diamond‐like structures based on other fullerene‐like molecules (called fulleranes), carbon nanotubes (called tubulanes) and graphene sheets. MD is used to find the equilibrium states of the structures and calculate compliance and stiffness coefficients for stable configurations. Analytical methods are used to calculate the engineering elastic coefficients (Young's modulus, Poisson's ratio, shear modulus and bulk modulus), and to study their transformation under rotation of the coordinate system. All the considered structures are partial auxetics with the negative value of Poisson's ratio for properly chosen tensile directions. It is shown that some of these structures, in a particular tension direction, have a very high Young's modulus, that is, 1852 GPa for tubulane TA6

ELASTIC DAMPER BASED ON THE CARBON NANOTUBE BUNDLE

Mechanical response of the carbon nanotube bundle to uniaxial and biaxial lateral compression followed by unloading is modeled under plane strain conditions. The chain model with a reduced number of degrees of freedom is employed with high efficiency. During loading, two critical values of strain are detected. Firstly, period doubling is observed as a result of the second order phase transition, and at higher compressive strain, the first order phase transition takes place when carbon nanotubes start to collapse. The loading-unloading stress-strain curves exhibit a hysteresis loop and, upon unloading, the structure returns to its initial form with no residual strain. This behavior of the nanotube bundle can be employed for the design of an elastic damper

Molecular and genetic characterization of LEPTOSPIRA spp. collection strains from the St. Petersburg Pasteur institute based on 16S rRNA gene sequencing data

Leptospirosis is a zoonotic disease found virtually worldwide. Microscopic Agglutination Test with live leptospira (MAT) is the reference method for the serological diagnosis of leptospirosis. MAT is based on assessing serum potential to agglutinate live reference serovar Leptospira maintained at a reference laboratory. At some laboratories having own collections of isolated and reference Leptospira strains applicable for serological diagnosis, those microorganisms are maintained for many years by repeated subculturing, that increases markedly a chance of strain cross-contamination. The lack of adequate quality control for reference strains may affect data of epidemiological studies. Control of Leptospira spp. reference strains purity and stability of their antigenic composition is very important for diagnosis of leptospirosis. The study objective was to compare the 16S rRNA gene nucleotide sequences of some Leptospira strains from the collection of the St. Petersburg Pasteur Institute to with relevant sequences uploaded to GenBank. In this study, 38 Leptospira strains were investigated. Nucleotide sequences of 36 strains were deposited in the international GenBank database, inconsistencies were revealed in two strains. The study found that the control Leptospira strains from the collection of the St. Petersburg Pasteur Institute had minimal dissimilarities from international control strains. The analysis of the resultant 16S rRNA sequences has shown the presence of point mutations, transitions, deletions and insertions, regardless of the strain species. The open leptospira pan-genome demonstrates high genomic variability in species due to the capability of leptospira for lateral gene transfer in order to adapt to changing environmental conditions. The massive acquisition and loss of genes give rise to an increased species diversity. The 16S rRNA gene is suitable for screening diagnostics; however, high level of the fragment similarity and close phylogenetic relationship between different species put bounds to its use in genotyping. The presence of point nucleotide mutations is most likely associated with the evolutionary mechanisms of leptospira, their ability to horizontal gene transfer and crossing-over, including ribosomal genes, but this assumption necessitates additional research. For specimen genotyping it is necessary to select alternative genes with high specificity and sufficient level of nucleotide divergence. The study shows a need for genetic analysis of collection strains in order to control the purity of cultures

Molecular dynamics simulation of the effect of dislocations on the martensitic transformations in a two-dimensional model

Investigation of the thermoelastic martensitic transformation is of high interest nowadays because of the numerous applications of the materials with such structural peculiarities. Thermodynamics, kinetics, structure, morphology of martensitic transformation still remain unclear in many respects. From this point of view, the effective way to study various properties of metallic crystals on atomistic level is molecular dynamics simulation, for which good qualitative agreement with the experiment can be achieved even with simple Morse or Lennard-Jones interatomic potentials. In this paper, the effect of dislocations on the direct and reverse martensitic transformation is studied by molecular dynamics simulation in a two-dimensional model of the ordered alloy with the AB stoichiometry. The three dimensional analog to this structure is B2 superstructure based on bcc lattice, which is characteristic for intermetallic NiTi alloy. It is found, that the dislocations can be considered as the nucleation centers for martensite phase, increasing the temperature of the direct martensitic transformation in comparison with the homogeneous martensitic transformation. The martensite domains found in the structure after transformation and the reverse martensitic transformation takes place in the presence of the domain boundaries, meaning that the austenite nucleates heterogeneously. At the reverse transformation, splitting of perfect dislocations into partials dislocations took place. Thus, it was established in the present study that, on the one hand, dislocations affect the direct martensitic transformation as the nucleation centers, and from the other hand, reverse martensitic transformation changes the dislocation structure of the modeled alloy. © 2017, Institute for Metals Superplasticity Problems of Russian Academy of Sciences. All rights reserved.MD simulation of direct and reverse MT in the framework of the 2D model of an ordered alloy shows that dislocations can be considered as the nucleation centers of martensite during direct MT. In the absence of dislocations, martensite nucleates homogeneously at lower temperatures. The reverse MT occurs in the presence of twinned martensite and domain walls in the system, which means that even in the absence of dislocations, the structure of the alloy is not ideal. During the reverse MT, the perfect dislocations split into partial dislocations. It should be noted that for selected model parameters MT is associated with large lattices deformations, therefore, temperature hysteresis is also large, which is not typical for SM alloys. However, the consideration of the Acta Mater. 95, 37 (2015). effects of SM and superelasticity is not prohibited in frame of 14. C. Ni, H. Ding, XJ.. Jin. J. Alloys Compd. 546, 1 (2013). this model, since the degree of coherence of the martensite 15. S. Kazanc, FA. . Celik, S. Ozgen. J. Phys. Chem. Solids. 74, and austenite lattices is quite high. One of the important 1836 (2013). problems for futher investigations is the studying of the MT 16. T.  Suzuki, M.  Shimno, K.  Otsuka, X.  Ren, A.  Saxena. as the detonation process [36]. J. Alloys Compd. 577S, S113 (2013). 17. B.  Wang, E.  Sak-Saracino, N.  Gunkelmann, Aknowledgements. S.V. D. acknowledges support from the H.M. Urbassek. Comp. Mater. Sci. 82, 399 (2014). Russian Foundation for Basic Research, grant No. 16-58-48001 18. C.  Tatar, S.  Kazanc. Current Applied Physics. 12, 98 IND_omi (design of the research, discussion of the results). (2012). J.A. B and V.G. P. are grateful for the financial support from 19. A.R.  Kuznetsov, Yu.N.  Gornostyrev, M.I.  Katsnelson, the Russian Science Foundation, grant No. 15-12-10014 A. V. Trefilov. Mater. Sci Eng. A309–311068 (2001). (numerical simulations and writing the paper). R.I. B. thanks 20. I.N.  Kar’kin, Yu.N.  Gornostyrev, L.E.  Kar’kina. Phys. financial support from the Russian Science Foundation, grant Solid State. 52, 431 (2010). No. 17-79-10410 (numerical simulations). The work of D. V. G. 21. Y. N. Gornostyrev, IN. . Kar’kin, LE. . Kar’kina. Phys. Solid was supported by the Saint-Petersburg State University, State. 53, 1388 (2011). research grant No. 6.65.43.2017 (discussion of the results). 22. J.A. Baimova, R.I. Babicheva, A.V. Lukyanov, V.G. Pushin

Helium passage through homogeneous ultrafine hydrocarbon layers

The present paper deals with the problem of helium atoms and methane molecules moving through a hydrocarbon layer of evenly distributed energy sources. A computational technique for integrating the Schrödinger equation based on formulation of two fundamental numerical solutions to the problem of waves passing through a barrier is suggested. A linear combination of these solutions defines the required wave function, while cross-linking with asymptotic boundary conditions allows determining the coefficients of transmission and particle reflection from the potential layer barrier

Auxetics and other systems of “negative” characteristics

This is the eleventh issue of Physica Status Solidi (B) focussed on materials and models exhibiting negative Poisson’s ratio (PR), called auxetics, and other systems of “negative” characteristics [1]. It contains 22 papers, from which the first 17 papers concern auxetics, the following 4 are related to negative stiffness, and the last paper describes auxetic-like magneto-elastic effect.peer-reviewe

Discrete breathers in ϕ4\phi^4 and related models

We touch upon the wide topic of discrete breather formation with a special emphasis on the the $\phi^4$ model. We start by introducing the model and discussing some of the application areas/motivational aspects of exploring time periodic, spatially localized structures, such as the discrete breathers. Our main emphasis is on the existence, and especially on the stability features of such solutions. We explore their spectral stability numerically, as well as in special limits (such as the vicinity of the so-called anti-continuum limit of vanishing coupling) analytically. We also provide and explore a simple, yet powerful stability criterion involving the sign of the derivative of the energy vs. frequency dependence of such solutions. We then turn our attention to nonlinear stability, bringing forth the importance of a topological notion, namely the Krein signature. Furthermore, we briefly touch upon linearly and nonlinearly unstable dynamics of such states. Some special aspects/extensions of such structures are only touched upon, including moving breathers and dissipative variations of the model and some possibilities for future work are highlighted

Effect of the Structure Morphology on the Mechanical Properties of Crumpled Graphene Fiber

Crumpled graphene fiber is a promising structure to be a graphene precursor to enhance the production and mechanical properties of various carbon fibers. The primary goal of the present work is to study the crumpled graphene of different morphologies using molecular dynamics simulations to find the effect of the structural peculiarities on the mechanical properties, such as the tensile strength, elastic modulus, and deformation characteristics. Mono- and poly-disperse structures are considered under uniaxial tension along two different axes. As it is found, both structures are isotropic and stress–strain curves for tension along different directions are very similar. Young’s modulus of crumpled graphene is close, about 50 and 80 GPa; however, the strength of the polydisperse structure is bigger at the elastic regime. While a monodisperse structure can in-elastically deform until high tensile strength of 90 GPa, structure analysis showed that polydisperse crumpled graphene fiber pores appeared two times faster than the monodisperse ones