Plastic flow between nanometer-spaced planar defects in nanostructured diamond and boron nitride

The mathematical analysis of plastic flow processes under uniform plane, axisymmetric and volumetric deformation is carried out. The analysis is based on the external shape change of the body, which determines the movement of material points. It is shown that the plastic flow of an isotropic rigid-plastic body under plane deformation obeys the hyperbolic law, and for axisymmetric and volumetric deformations – the inverse square law. Spatial-geometric expressions of these laws made it possible to reveal and explain in a new way the physical essence of plastic shear. It is proved that the stressed state of a body under uniform tension-compression deformation is complex and cannot be defined as “linear”. The normal stress, which coincides with the direction of the resulting deformation force, is not the main one, since in the areas perpendicular to this direction, the shear stresses are not equal to zero. Examples of solving technological problems are given: extrusion of cylindrical billets and wire drawing, rolling of a wide strip of rectangular profile. It is shown that the problems of determining the stress-strain state of isotropic rigid-plastic bodies along the known trajectories of movement of material points are statically definable.

Taylor-Quinney coefficient determination from simultaneous strain and temperature measurements of uniform and localized deformation in tensile tests

Considerable research has been directed towards discovering new engineering materials for various applications. As a superhard material, Cubic Boron Nitride (CBN) has been developed and applied to engineering for several tens of years. Due to its high specific strength and stiffness as well as good creep, fatigue and wear resistance at elevated temperatures, CBN has been widely used as cutting tool material in manufacturing industry. In this paper, the preparation and characteristics of CBN are introduced. As hard turning has been more and more employed in recent years as an advanced metal cutting technique, the application of CBN cutting tools in hard turning is presented based on the literature, and in particular, the main wear mechanisms of CBN tools in hard turning are summarized, owing to the significant influence of tool wear on the tool life and product quality.

Point-like defects in wide-bandgap materials are at the heart of a broad range of emerging applications including quantum information processing and metrology [1]. A well-known example is the nitrogen-vacancy (NV) defect in diamond, which can be used as a solid-state qubit to perform elaborate quantum information protocols [2] and highly sensitive magnetic field sensing [3]. These results motivate the search of new defects in other wide-bandgap materials, which would offer an expanded range of functionalities compared to NV defects in diamond. In that context, hexagonal boron nitride (hBN) appears as an appealing material. First, it has a 6-eV bandgap, which is ideally suited to host optically active defects with energy levels deeply buried between the valence band and the conduction band. Second, hBN is an electrical insulator with a two-dimensional (2D) honeycomb structure, which is a key element of Van der Waals heterostructures. Such "artificial" materials are currently attracting a great interest owing to their unique mechanical, electrical and optical properties [4]. Combining these properties with individual quantum systems would likely open new perspectives in quantum technologies. In this talk, I will report on the optical detection of individual defects hosted in a high-purity hBN crystal. Stable single photon emission is demonstrated under ambient conditions by means of photon correlation measurements [5]. A detailed analysis of the photophysical properties of the defect reveals a highly efficient radiative transition, leading to one of the brightest single photon source reported to date from a bulk, unpatterned, material. These results make a bridge between the physics of 2D materials and quantum technologies, and pave the way towards applications of van der Waals heterostructures in photonic-based quantum information science, metrology and optoelectronics.

ZnO nanoparticles and Cu-doped ZnO nanoparticles were prepared by co-precipitation method. Also, a part of the pure ZnO nanoparticles were annealed at 750 °C for 3, 6, and 9 h. X-ray diffraction studies were carried out and the lattice parameters, unit cell volume, interplanar spacing, and Young’s modulus were calculated for all the samples, and also the crystallite size was found using the Scherrer method. X-ray peak broadening analysis was used to estimate the crystallite sizes and the strain using the Williamson–Hall (W–H) method and the size–strain plot (SSP) method. Stress and the energy density were calculated using the W–H method assuming different models such as uniform deformation model, uniform strain deformation model, uniform deformation energy density model, and the SSP method. Optical absorption properties of the samples were understood from their UV–visible spectra. Photocatalytic activities of ZnO and 5 % Cu-doped ZnO were observed by the degradation of methylene blue dye in aqueous medium under the irradiation of 20-W compact fluorescent lamp for an hour.

Strain engineering shows distinct advantages in thermal management by tuning thermal resistance in a wide range. Till now, most of the relative studies were concentrated in uniform deformation, wherein the effects of the localized strain field are rarely exploited. Herein, by using non-equilibrium molecular dynamics simulations, we explore the local strain field engineering effects on the interfacial thermal resistance (ITR) of graphene nanoribbons (GNRs). The model of GNRs employed in this work contains extended drag threads, which are used to create a local strain field. Our simulation results show that the ITR has a quasi-linear relationship with the local tensile strain. GNRs are very sensitive to the local strain field in terms of ITR with a maximum enhancement factor of ∼1.5 at the strain of 10%. The ITR is found to depend linearly on the local strain. This phenomenon is thoroughly explained by micro-structure deformation, heat flux scattering, and phonon density of state overlapping. Our findings here offer a simple yet useful tool in modulating the thermal properties of graphene and other two-dimensional materials by using local strain engineering.

Diamond, cubic boron nitride (c-BN), silicon (Si), and germanium (Ge), as examples of typical strong covalent materials, have been extensively investigated in recent decades, owing to their fundamental importance in material science and industry. However, an in-depth analysis of the character of these materials' mechanical behaviors under harsh service environments, such as high pressure, has yet to be conducted. Based on several mechanical criteria, the effect of pressure on the mechanical properties of these materials is comprehensively investigated. It is demonstrated that, with respect to their intrinsic brittleness/ductile nature, all these materials exhibit ubiquitous pressure-enhanced ductility. By analyzing the strength variation under uniform deformation, together with the corresponding electronic structures, we reveal for the first time that the pressure-induced mechanical softening/weakening exhibits distinct characteristics between diamond and c-BN, owing to the differences in their abnormal charge-depletion evolution under applied strain, whereas a monotonous weakening phenomenon is observed in Si and Ge. Further investigation into dislocation-mediated plastic resistance indicates that the pressure-induced shuffle-set plane softening in diamond (c-BN), and weakening in Si (Ge), can be attributed to the reduction of antibonding states below the Fermi level, and an enhanced metallization, corresponding to the weakening of the bonds around the slipped plane with increasing pressure, respectively. These findings not only reveal the physical mechanism of pressure-induced softening/weakening in covalent materials, but also highlights the necessity of exploring strain-tunable electronic structures to emphasize the mechanical response in such covalent materials.

Coseismic rupture and aftershock development on a fault plane are complex and heterogeneous processes. The Mw 6.1 L’Aquila 2009 normal faulting earthquake is a perfect case to explore how fault geometry and rheology influence the rupture process and aftershocks distribution. In this study, we use for the first time a dense set of earthquake data to obtain enhanced images of the causative normal fault structure to the kilometer scale. The hypocenter of the emergent onset of the mainshock took place within a low Vp/Vs volume, while the large coseismic slip occurred a few kilometers above, as the rupture propagated through a high Vp and high Vp/Vs fluid‐filled rock volume. The increase of Vp/Vs in the fault hanging wall during the sequence suggests a strong dehydration in the earthquake asperity, with an upward fluid pressure migration along the fault toward the host rock volume. We propose that the localization of deformation on the fault plane is favored by high fluid pressure, while the spreading of aftershocks on a wide volume around the fault is driven by the depletion of fluids from the slipped portion of the fault plane and migration to small segments within the fault host rocks.

Multicomponent phase space contains extended regions of random or near-random multicomponent solid-solution single phases, stabilised by a relatively large configurational entropy of mixing that can often (though not always) suppress compound formation between the different atomic species. The present paper shows that there are very extensive variations of local nanostructure, local atomic clusters and associated local lattice strains within multicomponent high-entropy solid-solution single phases such as the fcc Cantor alloys, bcc Senkov alloys and rock-salt-structured Rost mono-oxides, even when there is no short-range ordering, i.e. even when the solid solution is completely random or ideal. There are, for instance, many billions of different local nanostructures and different local atomic clusters in equiatomic five-component fully random solid-solution single-phase materials such as the original fcc Cantor alloy CrMnFeCoNi and the original bcc Senkov alloy VNbMoTaW, extending over distances of many microns, with associated fluctuating hydrostatic and shear lattice strains of several percent. The number and extent of the variations in local nanostructure, atomic clusters and lattice strains increase dramatically to even higher values with increasing number of components in the material. The present paper also shows that there are similar variations in local nanostructure, local atomic clusters and associated local lattice strains surrounding point defects such as vacancies, line defects such as dislocations and planar defects such as grain boundaries and external surfaces, influencing many important material properties such as diffusion, plastic flow, recrystallisation, grain growth and catalysis. The number and extent of the variations in local nanostructure, atomic clusters and lattice strains make it difficult to have too much confidence in structures and properties of multicomponent high-entropy materials calculated using ab initio and other atomistic computer modelling techniques, since these techniques are restricted to relatively small numbers of atoms and are unable to sample effectively the full range of local structures and properties.

There are significant changes in metalcutting indexable inserts for the past 15 years. The use of ceramics (both alumina and silicon nitride/sialon-based), cermets base on titanium carbide/titanium nitride, and superhard tools using diamond or cubic boron nitride is increased. The traditional tungsten carbide tools are being further optimized, with greater use of very fine-grained grades proceeding in parallel with a continual replacement of uncoated by coated carbides. Chemical vapor deposited (CVD) coatings have been further developed, and carbide grades using physical vapor deposited (PVD) coating have been introduced. The range of insert geometries and chipbreaker styles has also broadened. This paper reviews these trends in terms of materials science developments of the insert materials, and also in the light of changes in machine tools and the nature of the workpieces being machined.

Formation and structure of Ag, Ge and SiC nanoparticles encapsulated in boron nitride and carbon nanocapsules

Vibrations of the Earth crust and variations in the physical fields of the Earth atmosphere and ionosphere are continuously monitored by a variety of techniques and specialized facilities across the world. Nevertheless, most catastrophic earthquakes even in this century have occurred in “incidental” or “unexpected” places in “unpredicted” time. Earthquake predictions have errors as the current knowledge of focal mechanisms of strong (M≥8) earthquakes is still insufficient. It is believed today that the most common source of earthquakes is movement of rock blocks along a fault/megafracture. Such movements take place in a stepwise pattern with high or reduced friction, depending on the presence of fluids, hitches on the fault planes and other factors. Modern seismic forecasting is based on the concept of precursors. The author considers geological and geophysical settings in areas of dynamic influence of faults, wherein 8>М>7.5 earthquakes took place. Based on earthquake recurrence curves constructed for such areas, four tectonic criteria for formation of strong earthquake sources are identified: structural (large seismically active faults), kinematic (large amplitudes of the fault wing’s displacements), rheological (physical properties of the fault infill material, such as low viscosity of the intra-fault medium) and dynamic (high rates of the fault wing’s displacements) criteria. These criteria should be in the focus of quantitative studies in order to provide a solid scientific basis for long-term forecasting of strong earthquakes. Curves constructed for the criteria can show changes in the physics of earthquake foci in case of strong seismic events. With account of the tectonophysical features of faults associated with strong seismic events, the following conclusions are drawn. (1) In the continental lithosphere, catastrophic earthquakes (M≥8) occur in areas of dynamic influence of the major faults in the lithosphere in case of relatively high amplitudes of displacements of boundary blocks (i.e. fault wings). (2) In the relatively stable stress field, high amplitude displacements take place in case of reduced viscosity/quasi-viscosity of the medium comprising the internal structure of faults. (3) Reduced viscosity of the intra-fault medium is related to the physical conditions of transition of rocks in fault zones (mainly along the fault planes) in the state of quasi-plastic or plastic flow (unilateral pressure in excess of hydrostatic pressure, and relatively decreasing strength properties of the intra-fault medium with increasing length of the fault wings). (4) Reduced viscosity of the fault zone leads to an increase in the displacement rate of the fault wings in the constant stress field. The latter factor is the main one, transforming seismically active faults with M≤7.5 seismic events into faults of similar characteristics, but with earthquakes of higher energy, M≥8. Focal mechanisms of such earthquakes are associated with conditions for a potential increase of the displacement amplitude regardless of the presence of fluids, hitches on the fault planes and other poorly predictable factors. In-depth studies of the internal structure of faults with M≥8 earthquakes, their foci, conditions of the temporal regime of the seismic process before and after strong seismic events can discover a key to understanding the origin of earthquake sources, the criteria of energy release, and the occurrence of earthquakes with maximum energy. Further steps to develop the geological and geophysical (including tectonic) criteria for prediction of strong earthquakes should be focused on more detailed research of seismic zones wherein strong earthquakes were recorded.

Deformation of silver-sheathed oxide superconducting tapes by trapped gases

This paper investigates the effect of spatially localized versus uniform strain on the performance of n-type InAs nanowire Tunnel FETs. To this purpose we make use of a simulator based on the NEGF formalism and employing an eight-band k·p Hamiltonian, describing the strain implicitly as a modification of the band-structure. Our results indicate that, when the uniform strain degrades the subthreshold slope because of an increased band-to-band-tunneling at the drain, a localized strain at the source side can achieve a better tradeoff between on-current and subthreshold slope than a uniform strain configuration.

Effects of hardness and grain size on wear resistance of polycrystalline cubic boron nitride