Institute of Metal Superplasticity Problems

Abstract One of the challenges of the modern photonics is to develop all‐optical devices enabling increased speed and energy efficiency for transmitting and processing information on an optical chip. It is believed that the recently suggested Parity‐Time (PT) symmetric photonic systems with alternating regions of gain and loss can bring novel functionalities. In such systems, losses are as important as gain and, depending on the structural parameters, gain compensates losses. Generally, PT systems demonstrate nontrivial non‐conservative wave interactions and phase transitions, which can be employed for signal filtering and switching, opening new prospects for active control of light. In this review, we discuss a broad range of problems involving nonlinear PT‐symmetric photonic systems with an intensity‐dependent refractive index. Nonlinearity in such PT symmetric systems provides a basis for many effects such as the formation of localized modes, nonlinearly‐induced PT‐symmetry breaking, and all‐optical switching. Nonlinear PT‐symmetric systems can serve as powerful building blocks for the development of novel photonic devices targeting an active light control. image

Microstructural evolution of commercial grade pure magnesium was studied during plastic deformation by torsion under high pressure at ambient temperature and by compression at temperatures ranging from 293 to 773 K and at a strain rate of 3×10−3 s−1. Grain refinement takes place by operation of dynamic recrystallization (DRX) at all examined temperatures. The mechanisms of DRX change with temperature and strain. As a result, unusual dependencies of recrystallized grain size against strain and recrystallized volume fraction against temperature are observed. In the temperature interval of 293–623 K the deformation twinning results in “twin” mechanism of DRX, which processes strain softening at an initial stage of deformation. At T≤423 K the other mechanism of low temperature DRX takes place at high strains. Such DRX is accompanied by strain hardening. In contrast, continuous DRX (CDRX) yielding a steady-state flow operates frequently at temperatures ranging from 523 to 773 K . CDRX occurs mainly in overall recrystallization process at elevated temperatures. Discontinuous DRX (DDRX) takes place by bulging of boundaries of coarse recrystallized grains evolved from twins at T=723 K . DDRX occurs repetitively, but gives an insignificant contribution into total recrystallization process. The present results suggest that the mechanisms of DRX and the deformation mechanisms are closely related.

Titanium near-alpha alloy Ti-6Аl-2Zr-1.2Мо-1.3V was used to manufacture a multilayer laminate by diffusion bonding. The laminate consisted of thirteen sheets stacked relative to each other in a way that they all had common rolling direction. The influence of cutting orientation of specimens on the mechanical behavior under impact loading of the laminate was determined. The results of quantitative assessment of the impact fracture characteristics for the studied specimens was analyzed. The transverse specimens had a higher impact strength value in comparison to the longitudinal specimens. The results of fractographic studies showed the presence of minor delaminations on the surfaces of fractured specimens.

van der Waals heterostructures, obtained by stacking layers of isolated two-dimensional atomic crystals like graphene (GE) and silicene (SE), are one of emerging nanomaterials for the development of future multifunctional devices. Thermal transport behaviors at the interface of these heterostructures play a pivotal role in determining their thermal properties and functional performance. Using molecular dynamics simulations, the interfacial thermal conductance G of an SE/GE bilayer heterostructure is studied. Simulations show that G of a pristine SE/GE bilayer at room temperature is 11.74 MW/m(2)K when heat transfers from GE to SE, and is 9.52 MW/m(2)K for a reverse heat transfer, showing apparent thermal rectification effects. In addition, G increases monotonically with both the temperature and the interface coupling strength. Furthermore, hydrogenation of GE is efficient in enhancing G if an optimum hydrogenation pattern is adopted. By changing the hydrogen coverage f, G can be controllably manipulated and maximized up to five times larger than that of pristine SE/GE. This study is helpful for understanding the interface thermal transport behaviors of novel van der Waals heterostructures and provides guidance for the design and control of their thermal properties.

We study nonlinear binary arrays composed of parity-time-symmetric optical waveguides with gain and loss. We demonstrate that such nonlinear binary lattices support stable discrete solitons, which can be adiabatically tuned and switched through nonlinear symmetry breaking by varying gain and loss parameters.

The poor structural stability of phosphorene in air was commonly ascribed to humidity and oxygen molecules. Recent exfoliation of phosphorene in deoxygenated water promotes the need to re-examine the role of H2O and O2 molecules. Considering the presence of high population of vacancies in phosphorene, we investigate the interaction of H2O and O2 molecules with vacancy-contained phosphorene using first-principles calculations. In contrast to the common notion that physisorbed molecules tend to have a stronger adsorption at vacancy sites, we show that H2O has nearly the same adsorption energy at the vacancy site as that at the perfect one. Charge transfer analysis shows that O2 is a strong electron scavenger, which transfers the lone-pair electrons of the phosphorus atoms to the 2{\pi}* antibonding orbital of O2. As a result, the barrier for the O-O bond splitting to form O-P bonds is reduced from 0.81 eV at the perfect site to 0.59 eV at the defect site, leading to an about 5000 faster oxidizing rate at the defect site than at the perfect site at room temperature. Hence, our work reveals that the vacancy in phosphorene shows a stronger oxygen affinity than the perfect phosphorene lattice site. Structural degradation of phosphorene due to oxidization may occur rapidly at edges and grain boundaries where vacancies tend to agglomerate.

A comparative investigation of mechanical properties of Ti–6Al–4V titanium alloy with microcrystalline and submicrocrystalline structures in the temperature range of 20–600°C has been carried out. The grain sizes under the submicrocrystalline and microcrystalline conditions are 0.4 and 10 μm, respectively. The alloy with the microcrystalline structure has been additionally subjected to a heat-strengthened treatment. The structure refinement of the alloy results in increase in both strength and fatigue limit at room temperature by about 20%. With increasing deformation temperature, the strength of the submicrocrystalline alloy is higher than that of the microcrystalline alloy up to 400°C. However, the creep strength of the submicrocrystalline alloy is slightly lower than that of the heat-strengthened microcrystalline alloy already at 250°C.

We show that parity-time- ($\mathcal{PT}$-) symmetric coupled optical waveguides with gain and loss support localized oscillatory structures similar to the breathers of the classical ${\ensuremath{\phi}}^{4}$ model. The power carried by the $\mathcal{P}\mathcal{T}$ breather oscillates periodically, switching back and forth between the waveguides, so that the gain and loss are compensated on the average. The breathers are found to coexist with solitons and to be prevalent in the products of the soliton collisions. We demonstrate that the evolution of a small-amplitude breather's envelope is governed by a system of two coupled nonlinear Schr\"odinger equations and employ this Hamiltonian system to show that small-amplitude $\mathcal{P}\mathcal{T}$ breathers are stable.

Shear stresses are shown to induce the alpha (hcp) to omega (simple hexagonal) plus beta (bcc) transformation in pure Zr at room temperature. The beta Zr thus fabricated is stable at 1 atm and room temperature. This phase has so far only been found to occur at high pressures (P>30 GPa) and/or at high temperatures (T>1135 K). This experimental observation provides new insights about the physical processes underlying allotropic transformations and opens the door to future investigations aiming to stabilize high temperature or pressure phases at ambient conditions.

Dynamics of a chain of interacting parity-time-invariant nonlinear dimers is investigated. A dimer is built as a pair of coupled elements with equal gain and loss. A relation between stationary soliton solutions of the model and solitons of the discrete nonlinear Schrödinger (DNLS) equation is demonstrated. Approximate solutions for solitons whose width is large in comparison to the lattice spacing are derived, using a continuum counterpart of the discrete equations. These solitons are mobile, featuring nearly elastic collisions. Stationary solutions for narrow solitons, which are immobile due to the pinning by the effective Peierls-Nabarro potential, are constructed numerically, starting from the anticontinuum limit. The solitons with the amplitude exceeding a certain critical value suffer an instability leading to blowup, which is a specific feature of the nonlinear parity-time-symmetric chain, making it dynamically different from DNLS lattices. A qualitative explanation of this feature is proposed. The instability threshold drops with the increase of the gain-loss coefficient, but it does not depend on the lattice coupling constant, nor on the soliton's velocity.

The in-plane and out-of-plane thermal transport properties of the graphene–MoS₂ bilayer are investigated with several influencing factors being considered.

Recently synthesized two-dimensional (2D) boron, borophene, exhibits a novel metallic behavior rooted in the s-p orbital hybridization, distinctively different from other 2D materials such as sulfides/selenides and semi-metallic graphene. This unique feature of borophene implies new routes for charge delocalization and band gap opening. Herein, using first-principles calculations, we explore the routes to localize the carriers and open the band gap of borophene via chemical functionalization, ribbon construction, and defect engineering. The metallicity of borophene is found to be remarkably robust against H- and F-functionalization and the presence of vacancies. Interestingly, a strong odd-even oscillation of the electronic structure with width is revealed for H-functionalized borophene nanoribbons, while an ultra-high work function (∼7.83 eV) is found for the F-functionalized borophene due to its strong charge transfer to the atomic adsorbates.

Water molecules form layered structures inside graphene bilayers and ultra-high pressure-driven flow rates can be observed.

Molecular-dynamics method is used to study the influence of mass ratio of anions and cations on the phonon spectrum of the crystal with NaCl structure and on the conditions of existence and properties of gap discrete breathers (DBs). We show that DBs can be easily excited when the mass ratio of light to heavy components is less than 0.2 and the gap in the phonon spectrum is sufficiently wide. Nonexistence of DBs for larger mass ratios is explained through the excitation of the harmonic with the frequency equal to half of the main DB frequency that interacts with phonons below the gap. For the mass ratio equal to 0.1 we could find at least three types of stable DBs that differ by the number of atoms with the largest amplitude and by polarization of oscillations. A zone-boundary phonon mode with sufficiently large amplitude was found to be modulationally unstable. Dynamics of such mode lead to spontaneous localization of energy in the form of large-amplitude DBs according to the anti-Fermi-Pasta-Ulam mechanism.

Abstract

The acceptor role of water impedes the interaction between water molecules and oxygen species on antimonene; this may be the underlying reason for its high stability.

The dynamic process of fine grain evolution as well as deformation behaviour under warm working conditions was studied in compression of a 304 type austenitic stainless steel. Multiple compression tests were carried out at a strain rate of 10-3 s-1 to produce high cumulative strains, with changing of the loading direction in 90º and decreasing temperature from 1223 to 873 K (0.7-0.5Tm) in each pass. The steel exhibits two types of deformation behaviours with different mechanical and structural characteristics. In the deformation region where flow stresses are below about 400 MPa, conventional dynamic recrystallization takes place accompanied mainly by bulging of serrated grain boundaries. The dynamic grain size evolved can be related to the high temperature flow stress through a power law function with a grain size exponent of -0.72. On the other hand, in the region of higher stresses above 400 MPa the flow stresses show small strain rate and temperature dependence, and so it is suggested to be in an athermal deformation region. The stress-strain curves show a steady state like flow without any strain softening, while the multiple deformation to high cumulative strains brings about the evolution of fine grained structures with grain sizes less than one micron. The relationship between the warm temperature flow stresses and the grain sizes evolved also can be expressed by a unique power law function of grain size with an exponent of -0.42. The interrelations between the mechanisms of plastic deformation and microstructure evolution at warm and high temperatures are analysed in detail and also the multiple compression method for obtaining ultra fine grained structure is discussed as a simple thermomechanical processing.

The structural changes are characterized by the development of deformation or microshear bands in coarse-grain interiors, followed by homogeneous evolution of new grains at high strains. The mechanism of fine grain production and the factors controlling grain refinement during hot multidirectional deformation are discussed in detail

Dynamic process of new grain formation was studied in a coarse-grained Mg-Zn-Zr alloy deformed at elevated temperatures

Effect of pass strain (Δε) on grain refinement was studied in multidirectional forging (MDF) of a coarse-grained 7475 Al alloy at 490°C under a strain rate of 3 × 10−4 s−1. Samples of rectangular shape were deformed up to accumulated strains of around 6 with subsequent changes in loading direction 90° from pass to pass. The pass strains in each compression (Δε) were 0.4 and 0.7. The cumulative flow curves integrated by each compression exhibit significant work softening just after yielding, followed by apparent steady state plastic flow at high strains. Structural changes were characterized by grain fragmentation due to frequent development of deformation and/or microshear bands followed by full evolution of new fine grains in the original grains. Increasing Δε accelerates significantly the kinetics of grain refinement, leading to more clear reduction of flow stresses at moderate to high strains. MDF of Δε = 0.7 results finally in formation of a finer grained structure with an average size of around 7.5 μm at strains of above 3.5, while, the processing with Δε = 0.4 develops a slightly coarser grain structure at higher strain of about 6. The effect of MDF on new grain evolution and the mechanisms of grain refinement are discussed in details.