Local Internal Stresses Research Articles

Presentation of the dynamical core of neXtSIM, a new sea ice model

The dynamical core of a new sea ice model is presented. It is based on the Elasto–Brittle rheology, which is inspired by progressive damage models used for example in rock mechanics. The main idea is that each element can be damaged when the local internal stress exceeds a Mohr–Coulomb failure criterion. The model is implemented with a finite element method and a Lagrangian advection scheme. Simulations of 10 days are performed over the Arctic at a resolution of 7 km. The model, which has only a few parameters, generates discontinuous sea ice velocity fields and strongly localized deformation features that occupy a few percent of the total sea ice cover area but accommodate most of the deformation. For the first time, a sea ice model is shown to reproduce the multifractal scaling properties of sea ice deformation. The sensitivity to model parameters and initial conditions is presented, as well as the ability of the Lagrangian advection scheme at preserving discontinuous fields.

Nanodipoles of partial disclinations in the region of localized elastic distortions

Strain localization in the region of elastic distortions is revealed in nickel dynamic recrystallization grains during torsion in Bridgman anvils, which leads to the formation of reorientation nanobands with the elastic lattice curvature equal to hundreds of degrees per micron. It is shown that the nanobands are formed by the motion of partial disclination nanodipoles, i.e., zones of constrained elastic shear and rotations distinguished by extremely high local internal stress gradients.

Peculiar Features of Microstructure Formation and Microhardness Variations During Torsional Straining of Tantalum Specimens in Bridgman Anvils

Using the method of transmission electron microscopy, peculiar features of evolution of microstructure and variations in microhardness of Та are investigated under torsional loading in the Bridgman anvil as a function of plastic deformation at room temperature. A quantitative examination of grain and defect’s structure of the material under study and the values of local internal stresses is performed in different loading stages. The mechanisms of formation of submicrocrystalline and nanostructured states are analyzed and so is the microstructure variation as a function of the defect-structure characteristics, strain level, and spacing from the axis of torsion.

Internal and external stresses: In situ TEM compression of Cu bicrystals containing a twin boundary

Uniaxial compression experiments on single- and twinned bicrystalline Cu samples using conventional and scanning in situ transmission electron microscopy reveal no increase in flow stress for the bicrystals. Dislocation curvature and dislocation source size analysis combined with indenter force measurements show agreement between local internal stresses acting on the dislocations and external stresses imposed by the indenter, indicating no stress concentrations due to the twin boundary. Furthermore, the dislocation density evolution shows stochastic variations but never a complete dislocation starvation.

Effect of Polarization on Mechanical Properties of Lead Zirconate Titanate Ceramics

This study concerns two following commercial lead zirconate titanate (PZT) materials, PZT-5H and PZT-8 piezoelectric ceramics. Young's modulus and internal friction versus temperature at different vibration frequencies were investigated by means of Dynamic Mechanical Analysis (DMA) before and after polarization. The phase transition temperatures (Tc) of PZT-5H and PZT-8 ceramics were 438 K and 550 K. PZT-8 ceramic had higher Young's modulus than PZT-5H ceramic at ferroelectric phase, which resulted from the increase of internal local stress in PZT-8 ceramic. After polarization, the Young's modulus of both PZT ceramics increased, the influence of the piezo- electric effect was investigated. A relaxation peak emerges in internal friction curve of each PZT sample. Considering the hard ceramic, this peak is a pure relaxation mechanism controlled by oxygen vacancies diffusion. Considering the soft ceramic, the relaxation peak can be related to viscous motion of domain walls and the interaction between domain walls and point defects. The activation energy of the relaxation peak increased after polarization, which resulted from the increased local internal stress and directional arranged space charges caused by polarization.

Microstructure evolution and mechanical behavior of a high strength dual-phase steel under monotonic loading

Transmission electron microscopy (TEM) microstructures of a high-strength dual-phase steel DP800 have been examined after moderate plastic deformations in simple shear and uniaxial tension. Special attention has been paid to the effect of the intergranular hard phase (martensite) on the microstructure evolution in the near-grain boundary regions. Quantitative parameters of dislocation patterning have been determined and compared with the similar characteristics of previously examined single-phase steels. The dislocation patterning in the interiors of the ferrite grains in DP800 steel is found to be similar to that already observed in the single-phase IF (Interstitial Free) steel whereas the martensite-affected zones present a delay in patterning and display very high gradients of continuous (gradual) disorientations associated with local internal stresses. The above stresses are shown to control the work-hardening of dual-phase materials at moderate strains for monotonic loading and are assumed to influence their microstructure evolution and mechanical behavior under strain-path changes.

Temperature dependent mechanical properties and thermal activation plasticity of nanocrystalline and coarse grained Ni-18.75 at.% Fe alloy

Mechanical properties of Ni-18.75 at.% Fe in coarse grained (average grain size 15 gm) and nanocrystalline (average grain size 22 nm) states were studied in uniaxial compression in the temperature range 4.2-350 K. Temperature dependences of the flow stress, strain rate sensitivity and activation volume of plastic deformation were measured. The thermal activation analysis of the experimental data has been fulfilled for the the plastic deformation value of 2 %. It was shown that plastic deformation in temperature range from 35 to 350 K in both studied structural states has the thermally activated type. Comparative analysis of low temperature thermal activation plastic deformation was carried out for the alloy in coarse grained and nanocrystalline states. Empirical estimates of parameters of the dislocation interaction with local barriers and internal stress value estimates were obtained for the both studied structural states. Analysis of the results indicates that different mechanisms control the thermal activation plasticity of the Ni-18.75 at.% Fe alloy in coarse grained and nanocrystalline states. Possible mechanisms, which control plactisity of the studied states, are disscussed.

Inter-Granular Phase Formation during Reactive Diffusion of Gallium with Al Alloy

Peculiarities of formation of solid-state joining of the parts for bimetallic heat exchanger are considered. For solid-state activation of the surfaces, being joined, the layer-activator (liquid gallium) is used. Inter-granular segregation of an activator along the grain of the alloy-matrix is rapid and deep, which leads to polygonal structure in the diffusion zone. A negative fact for such joints is the brittleness of materials after intermetallic phase transformation of the chemical elements of alloy-matrix with gallium. The joint formed with an increasing of volume of the diffusion zone after rotation of newly formed phases at growth. Rotation of grains leads to emergence of high local internal stresses. Excess of an activator may be a reason for generation and distribution of main cracks. Indentation allows recording an abnormal decrease of the micro-hardness in grain boundary area. Obtained data of the Young's modulus, micro-hardness and coefficient of plasticity, with spectral mapping of chemical elements and phase analysis are allowed to exactly specify structural components that are the initiators of the materials stress state and are the reason for local cracks.

On relationship between annealing treatment and magnetostriction behavior of Fe–16Cr–2.5Mo damping alloy

Abstract The damping capacity of Fe–16Cr–2.5Mo damping alloy was analyzed using a cantilever device. The magnetostriction property was measured using an improved Michelson interferometer. The domain morphology was optically observed using a magneto-fluid. Magnetomechanical Hysteresis Effect (MMHE) was discussed by analyzing the relationship between the magnetostriction performance and the annealing process of the alloy. The results show that the variation of saturate magnetostriction coefficient is quite consistent with that of the damping capacity of the annealed samples. With the local internal stress widely distributing in the annealed alloy, both the domain’s mobility and their structure vary with the increasing annealing temperature and time. The local internal stress not only pins the domain mobility but also makes the domain structure disorder. The domain size and the number of 90° magnetic domains are responsible for the highest damping capacity of the sample annealed at 900 °C for 1 h. However, with the prolonged annealing time, the number of 90° domain decreases sharply and more heterogeneous domains appears in the alloy. The spike domains can also pin domain-walls.

Features of the modification of microstructure and properties for copper-doped titanium nitride coatings

Features of a defective microstructure and the mechanical properties of titanium nitride coatings having different copper content are investigated by means of transmission electron microscopy, X-ray structural analysis, microhardness measurement, and scratch tests. It is shown that, under a relatively low deposition temperature and ion-induced surface activation during the growth of a TiN-Cu coating, a highly defective single-phase crystalline state of titanium nitride is formed at a copper concentration of up to 12 at %. The noted state is characterized by a scale hierarchy of the lattice fragmentation from sizes of ∼10–15 to 100 nm, high bending-torsion of the crystal lattice (tens of degrees per micron), and a level of local internal stresses from ∼E/40 to ∼E/120 (E is the modulus of elasticity). The revealed high gradients, including those of a dipole character, for the bending-torsion on characteristic scales up to some tens of nanometers can be described within the framework of a model for the continuous distribution of dislocation-disclination ensembles. The effect of the copper content on an increase in the degree of dispersion of the subgrained structure, a decrease in the local bending-torsion of the crystal lattice within subgrains, the level of local internal stresses, and variation of the hardness and strength in scratch tests is revealed.

Applied Stress Affecting the Environmentally Assisted Cracking

Stress corrosion cracking (SCC) is affected by the mode of applied stress, i.e., tension, compression, or torsion. The cracking is measured in terms of initiation time to nucleate a crack or time to failure. In a simple uniaxial loading under tension or compression, it is observed that the initiation time can vary in orders of magnitude depending on the alloy and the environment. Fracture can be intergranular or transgranular or mixed mode. Factors that affect SCC are solubility of the metal into surrounding chemical solution, and diffusion rate (like hydrogen into a tensile region) of an aggressive element into the metal and liquid metallic elements in the grain boundaries. Strain hardening exponent that affects the local internal stresses and their gradients can affect the diffusion kinetics. We examine two environments (Ga and 3.5 pct NaCl) for the same alloy 7075-T651, under constant uniaxial tension and compression load. These two cases provide us application to two different governing mechanisms namely liquid metal embrittlement (7075-Ga) and hydrogen-assisted cracking (7075-NaCl). We note that, in spite of the differences in their mechanisms, both systems show similar behavior in the applied K vs crack initiation time plots. One common theme among them is the transport mechanism of a solute element to a tensile-stress region to initiate fracture.

Peculiarities of the formation of high-defect states in mechanocomposites and powders of niobium and aluminum under severe deformation in planetary ball mills

The microstructural peculiarities of Nb powder and Nb + Al powder mixtures after mechanical activation in a high-energy planetary ball mill were investigated by transmission electron microscopy. The materials revealed two-level structural states: nanograins of size from 50 to 100 nm that contain subgrains of size ∼20 nm and less with low-angle misorientation boundaries, elastic lattice curvature gCij≈100°…200°μm−1, curl or curvature gradient ∂χij/∂r > 100 μm−2 and high (up to 10E μm−1) local stress gradients (couples). An important factor in the formation of the above states is the capability of nano-objects to reach high elastic lattice curvature, high gradients of this curvature, and high local internal stress gradients at rather low absolute values of the internal stress.

Estimation of the strength and crack resistance of the metal of railway wheels after long-term operation

We describe the results of investigations of the state of the metal near the roll surfaces of solid-rolled railway wheels after operation. It is shown that the greatest changes in structure (phase components, grain, subgrain, and dislocation structures, phase precipitates, etc.) over the width of the working surface of wheels take place in the zone of passage to the flange and propagate to a depth of up to 200 to 300 μm (from the roll surface). The analytical estimate of changes in the strength and crack-resistance properties of the metal in different zones of railway wheels, based on the results of structural examinations, enabled us to establish that separate zones are characterized by a substantial strengthening of the near-surface layers, a dramatic increase in the local internal stresses up to the value of theoretical strength, and, correspondingly, a decrease in crack resistance.

Features of the formation of the submicrocrystalline structural state upon plastic deformation of a V-4Ti-4Cr alloy in Bridgman anvils

Results are presented of electron-microscopic investigations of the main features of the microstructure of an V-4Ti-4Cr alloy after severe plastic deformation by high-pressure torsion in Bridgman anvils. Parameters of the defect structure in the bulk and at the boundaries of grains have been studied using the method of dark-field analysis of discrete and continuous misorientations. Fields of local internal stresses and gradients of these stresses on the submicron level have been analyzed. Mechanisms of the formation of the submicrocrystalline structural state have been discussed.

Composite polymer protective coatings for aggressive media

The most important physicochemical processes that determine the performance of composite polymer protective coatings for operation in highly aggressive media and severe operating conditions are considered. It is shown that optimization of a multilayer structure can significantly decrease penetrability of highly aggressive media, decrease the overall level and local internal stresses in the internal coatings and support. The concept of multilayer coatings pays special attention to the adhesion layer that provides not only mechanical properties, but also to the required adhesion level under the conditions of selective penetration of the corrosive medium. This level is an ideal place to locate corrosion inhibitors and protective additives, while the system of the overlying layers hinders the leaching out of the active additives.

Nanodipoles of partial disclinations as quasi-ductile strain carriers responsible for nanocrystalline structure formation in metals and alloys under severe plastic deformation

Two-level nanostructural states were detected by transmission electron microscopy in V- and Mo-Re-based alloys and Ni under plastic deformation by high-pressure torsion at a true logarithmic strain e ≈3–6. The two-level nano structural states represent nanocrystals of size 50–100 nm with nanobands of width less than 10 nm and dipole and multipole misorientations. In nickel, the nanobands are bounded by nanodipoles of partial disclinations or dislocations of noncrystallographic shear with effective Burgers vectors several times smaller than Burgers vectors of lattice dislocations. In the vicinity of the nanodipoles, structural states with extraordinarily high (hundreds of degrees per micrometer) elastic crystal lattice curvature and local internal stress gradients up to ≈(10–20) E μm −1 are found. The mechanism proposed for the formation of the nanostructures is quasi-ductile motion of nanodipoles of partial disclinations (or dislocations of noncrystallographic shear) controlled by flows of nonequilibrium point defects in high local gradient fields of normal stress tensor components. In V-based alloy at e > 6, the nanobands are grouped in bundles of width 1–2 μm forming vortex deformation mesobands with dipole misorientations and high density of equiaxial nanocrystals of size several nanometers. It is supposed that these mesobands result from collective effects in the disclination substructure and attendant collective motion of nanodipoles of partial disclinations in inhomo-geneous couple stress fields.

Coking theory: Internal stress in the coal batch

The development of local internal stress in the coal batch is analyzed on the basis of recent theoretical and experimental data. Its influence on coke quality is demonstrated. The influence of mineralized rock particles and large hard-to-crush particles of highly metamorphic coal (including petrographically uniform coal) is considered. The results of appropriate preparation of coal batch for coking are described.

Effect of Grain Size Distribution on the Local Mechanical Behavior of Nanocrystalline Materials

A theoretical model based on self-consistent approximation is proposed to explore the effect of grain size distribution on the local mechanical response of nanocrystalline (nc) materials. The representative volume element (RVE) is composed of grains randomly distributed with a grain size distribution following a log-normal statistical function. The grain interior and grain boundary are taken as an integral object to sustain deformation mechanisms of grain-boundary sliding, grain-boundary diffusion and grain-interior plasticity. Local plastic strains and internal stresses, developing within the RVE, have been recorded and discussed.

Recent developments in the preparation, characterization and applications of glass- and glass–ceramic-to-metal seals and coatings

There has, in recent years, been a major revival of interest in glass- and glass–ceramic-to-metal seals and coatings for new applications. Experience dictates that many factors need to be taken into consideration in the successful design and manufacture of high-quality seals, particularly if an adequate component lifetime is to be achieved. For example, during their preparation, undesirable reactions may occur between diffusing metal species and glass constituents, and these can lead to the formation of highly localized internal stresses, which can initiate failure of a seal either during manufacture or, more seriously, whilst in service due to the influence of static fatigue. In the case of high-temperature systems, reactions under hostile operating conditions also need to be taken into consideration. In this review, the factors learnt from past experience that influence the formation and lifetime behaviour of glass and glass–ceramic/metal systems are briefly introduced, and their relevance to the newer applications including solid oxide fuel cell sealants and coatings on titanium for biomedical applications is discussed.

Microstructure of Mo – 47% Re – 0.4% Zr alloys after rolling at room temperature. I. Anisotropy of microband structure and peculiarities of internal structure of microbands

The results of an electron microscopy investigation of microstructure of a Mo – 47% Re – 0.4% Zr alloy after rolling deformation (e ≈ 90%) at room temperature are presented. A special focus is made on investigation of anisotropy of microband nanostructured states and high-energy defect substructures with high values of the crystal lattice curvature, dislocation density and local internal stresses. A disclination mechanism of reorientation as a mechanism of fragmentation of the internal microband structure is proposed.