Random Orientation Distribution Research Articles

Serendipitous relationship between discrete distribution models representing random orientations of fillers in composite materials and the golden ratio

The purpose of this study is to establish simple discrete distribution models capable of expressing the two and three-dimensional random orientation states of fillers in composite materials. Initially, a new method is proposed to divide the elastic constants into isotropic and anisotropic parts. Employing this method, for the composite material containing ellipsoidal fillers with various orientations in the material, the macroscopic elastic constants and thermal expansion coefficients of the material are derived based on the Mori–Tanaka method. Subsequently, the analysis of the macroscopic properties is performed for fillers oriented two and three-dimensional discrete distributions. As a result, it is found that when both angular pitches of the azimuth angle and the rotational angle of the fillers in a plane are at most 72°, the macroscopic elastic constants and the thermal expansion coefficients of the material exhibit in-plane isotropy, leading to the realization of a two-dimensional random state. Furthermore, if the angular distribution of the zenith angle and azimuth angle corresponds to the vertices of the regular dodecahedron and icosahedron, and the angular pitch of the rotational angle of the fillers is at most 72°, the macroscopic properties of the material become isotropic, resulting in a three-dimensional random state. Notably, the angle of 72° and the orientation angles of the vertices of the regular dodecahedron and icosahedron are associated with the golden ratio. Therefore, this analysis suggests a serendipitous relationship between the golden ratio and the angular pitch that simplifies the random orientation distribution of fillers.

Self-assembled Ti3C2Tx membrane driven by Marangoni effect for anti-corrosive and anti-wear applications

The recent rise of Ti3C2Tx has extended the applications in epoxy resin due to its large specific surface area and excellent mechanical properties. Whereas, the synergy between epoxy and Ti3C2Tx restricts properties enhancements when Ti3C2Tx is subjected to random orientation and discontinuous distribution. To optimize the barrier and lubrication characteristics of Ti3C2Tx, we have successfully prepared Ti3C2Tx membrane by using the Marangoni effect self-assembled method to move Ti3C2Tx nanosheets to the same latitude and deposit them on epoxy resin for the first time. Meanwhile, micro-arc oxidation technology was adopted to enhance the bonding strength of the substrate with epoxy. Due to the effective corrosion resistance offered by the Ti3C2Tx membrane, even after 28 days of immersion, the pure epoxy coating had completely failed, while the |Z|0.01Hz value of the multilayer coating still increased by 36.5 % compared to MAO/EP/EP. Furthermore, the wear rate and the friction coefficient reduced by 58.2 % and 21.4 % in comparison to the pure epoxy coating. This exceptional lubrication property stems from the interlayer shear sliding of the expansive Ti3C2Tx nanosheets, effectively preventing direct contact between the friction ball and the epoxy.

Epitaxial Growth of Monolayer SnP3 with High Mobility and Chiral Boundary Junctions.

Two-dimensional materials with layered structures, appropriate band gaps, and high carrier mobility have attracted tremendous interest for their potential applications. Here we report the growth of monolayer SnP3 on Au(111) surfaces by molecular beam epitaxy. The kinetic processes for the growth and the crystalline properties are studied by scanning tunneling microscopy. The weak interaction between SnP3 and its Au(111) substrate is signified by the random crystal orientation distributions of SnP3 nanosheets. The electronic structures exhibit a band gap of ∼0.25 eV and high charge carrier mobility comparable to that of black phosphorus engineered by compressive strain. Additionally, domain boundary junctions with opposite chirality are observed, resulting from the strained film in the epitaxial growth process. Our work provides a method to fabricate high-quality monolayer SnP3 and suggests that the monolayer SnP3 is a promising candidate for applications in nanoelectronics and optoelectronics.

Explosive blast loading effect on transient mechanical responses of aircraft panels with curvilinear fibers: 3D elasticity approach

In the present paper, for the first time, the dynamic loading effect on transient bending and stress distributions of aircraft nanocomposite sandwich panels including curvilinear fibers are presented and explained. Face sheets of the panel contain curvilinear fibers with an orientation angle varies linearly with respect to the horizontal coordinate. The core layer is made of polymer matrix reinforced by graphene platelets (GPLs) with uniform and random orientation distribution. 3-D elasticity theory is used to formulate the transient problem of the panel. The model of the panel is so general to include different time-dependent loads, especially explosive ones. A three-dimensional layerwise-differential quadrature method (LW-DQM) together with a non-uniform rational B-spline-based multi-step time integration scheme is employed to investigate the transient responses of the sandwich panel with variable stiffness composite laminated face sheets (VSCL-FS) and graphene platelets reinforced porous core (GPLR-PC) under dynamic loading. The convergence behavior of the method is examined numerically and to assure its accuracy, the results in the limit cases are compared with those available in the literature. Finally, through the parametric studies, the effects of core porosity distribution and amounts, GPLs weight fraction and boundary conditions on the transient responses of the sandwich panel with VSCL-FS and GPLR-PC subjected to explosive blast loading are investigated. The results show that the addition of GPLs to the core layer decreases the transverse displacement but increases the peak values of the stress components. Also, the core porosity increases the transverse displacement and the stress components. However, it has not significant effect on the period of oscillation of the field variables under explosive blast loading. On the other hand, the curvature of the fibers does not considerable effect on the plate response and on the period of oscillation of the field variables under explosive blast loading

The anisotropic grain size effect on the mechanical response of polycrystals: The role of columnar grain morphology in additively manufactured metals

Additively manufactured (AM) metals exhibit highly complex microstructures, particularly in terms of grain morphology which typically features heterogeneous grain size distribution, irregular and anisotropic grain shapes, and the so-called columnar grains. The conventional morphological descriptors based on grain shape idealization are generally inadequate for representing complex and anisotropic grain morphology of AM microstructures. The primary aspect of microstructural grain morphology is the state of grain boundary spacing or grain size whose effect on the mechanical response is known to be crucial. In this paper, we formally introduce the notion of axial grain size from which we derive mean axial grain size, effective grain size, and grain size anisotropy as robust morphological descriptors capable of effectively representing highly complex grain morphologies. We instantiated a discrete sample of polycrystalline aggregate as a representative volume element (RVE) featuring random crystallographic orientation and misorientation distributions. However, the instantiated RVE incorporates the typical morphological features of AM microstructures including distinctive grain size heterogeneity and anisotropic grain size owing to its pronounced columnar grain morphology. We ensured that any anisotropy observed in the macroscopic mechanical response of the instantiated sample primarily originates from its underlying anisotropic grain size. The RVE was then employed for mesoscale full-field crystal plasticity simulations corresponding to uniaxial tensile deformation along various axes via a spectral solver and a physics-based crystal plasticity constitutive model which was developed, calibrated, and validated in earlier studies. Through the numerical analyses, we isolated the contribution of anisotropic grain size to the anisotropy in the mechanical response of polycrystalline aggregates, particularly those with the characteristic complex grain morphology of AM metals. This contribution can be described by an inverse square relation.

Interaction between growing dendrite and rising bubble under convection

Prediction and control of gas porosity defect during solidification is challenging because of multiphase and multiphysics characteristics. In this work, a multiphase-field lattice Boltzmann method is employed to study the interaction between solidifying dendrite and rising bubble. The influencing factors are divided into two categories including external and internal factors, and the effects of the multiple factors are discussed and compared. The bubble changes the dendrite skeleton by hindering the solid growth, including entrapped in the dendrite root, tilting growth towards the neighboring bubble, forming the knot structure, and growing along the radial direction of the bubble. The bubble in the columnar dendrite array with random orientation distribution has larger effects on the average primary dendrite arm spacing. Based on the summary of different interaction results, a phase diagram is proposed to distinguish different interaction regimes. Further, the size of the closed liquid phase region is measured to evaluate the porosities tendency and guide how to reduce the porosities.

Effect of volume energy density on microstructure and mechanical properties of TC4 alloy by selective laser melting

Many factors can affect the formability of TC4 alloy in laser additive, and the effects of their interaction are complex. The essence lies in the energy input of the molten pool. In this study, the energy input of the molten pool was characterized by the laser volume energy density under the coupling of multiple process parameters. The effects of volume energy density on the microstructure evolution and mechanical properties of TC4 alloy were studied, and the fracture mechanism was discussed. The results showed that the microstructure of TC4 alloy prepared by selective laser melting (SLM) was composed of α phase, metastable α′ phase, and a small amount of equiaxed nano-β phase. When the TC4 powder was irradiated by low volume energy density, fine columnar β grain with low texture intensity and random grain orientation distribution appear. Meanwhile, a large number of fine acicular martensite α′ phases generated inside the β grains, but fewer nano-β phases precipitated. At the same volume energy density, compared with increasing laser power, reducing the scanning speed could lead to the coarsening of α′ martensite, and more nano-β phase precipitated at the martensite boundary. The volume energy density greatly influenced the elongation (EL) but had little effect on the ultimate tensile strength (UTS), and the UTS could reach up to 1507 MPa. The decreasing volume energy density was beneficial to improve the plasticity, mainly attributed to the increasing deformation coordination ability caused by the refinement of α/α′ martensite and the reduction of the nano-β phase. When the volume energy density was 36 J/mm3, the fine columnar β grain precipitated, and a ductile fracture displayed with high-density dimples shown in fracture morphology. The mapping relationship between process, microstructure and properties was established, which provided a theoretical basis for process, microstructure optimization and performance improvement.

Study on Microstructure Evolution Mechanism of Gradient Structure Surface of AA7075 Aluminum Alloy by Ultrasonic Surface Rolling Treatment.

The materials with grain size gradient variation on the surface, which were prepared with mechanical-induced severe plastic deformation, always show high resistance to high and low cycle fatigue and frictional wear because of their good strength-ductility synergy. The ultrasonic surface rolling treatment (USRT) has the advantages of high processing efficiency, good surface quality, and large residual compressive stress introduced to the surface after treatment. The USRT was used to prepare aluminum alloy (AA7075) samples with a surface gradient structure; meanwhile, the microstructural evolution mechanism of the deformation layers on the gradient structure was studied with XRD, SEM, and TEM. The microstructure with gradient distribution of grain size and dislocation density formed on the surface of AA7075 aluminum alloy after USRT. The surface layer consists of nanocrystals with random orientation distribution, and high-density dislocation cells and subgrains formed in some grains in the subsurface layer, while the center of the material is an undeformed coarse-grained matrix. The results show that the dislocation slip dominates the grain refinement process, following the continuous cutting and refinement of dislocation cells, subgrains, and fragmentation of the second precipitates. This study systematically clarified the mechanism of grain refinement and nanocrystallization on the surface of high-strength aluminum alloys and laid a theoretical foundation for further research on mechanical behavior and surface friction and wear properties of high-strength non-ferrous materials with gradient structure.

Texture components and magnetic properties of laser powder bed fusion fabricated near grain-oriented and near non-oriented silicon steel

Silicon steel is a widely used soft magnetic material that requires different texture components for different applications, typically classified as grain-oriented or non-oriented. However, the methods of fabricating such types of silicon steel via laser-powder bed fusion (LPBF) have not been fully investigated. In this study, near grain-oriented and near non-oriented Fe-3.5 wt.%Si silicon steel is fabricated using LPBF by controlling processing parameters. Different textures are investigated using electron backscatter diffraction (EBSD), and the morphology of the molten pool is characterized by optical microscopy (OM) and scanning electron microscopy (SEM). Magnetic properties are measured with alternating current (AC) method. The results show that reducing both the linear energy density (LED) and laser power leads to a change in the side morphology of the molten pool from large, flat, and well-overlapped to small, protuberant, and less-overlapped, resulting in an extremely strong θ-fiber texture or a random distribution of grain orientations, respectively. Additionally, reducing both the laser power and scanning speed causes the top morphology of the molten pool to change from teardrop to elliptical shape at the trailing edge, resulting in a shift in the angle between the 〈001〉 of grains in the θ-fiber texture and the scanning direction from 45° to 30°. Samples with fewer defects (i.e., larger grain size and fewer pores) and a larger area fraction of 〈001〉//H exhibit higher permeability, although this superiority is not so significant due to residual stress and high dislocation in the as-built samples. This study provides insight into the relationship between processing parameters, texture evolution, and magnetic properties in LPBFed silicon steel.

Evolution of microstructure, texture and mechanical properties of AZ80 Mg alloy tubular produced by rotating backward extrusion with modified open punch

Deformation behavior, microstructures and mechanical properties of AZ80 alloy with modified open punches (groove numbers of 0, 1, 4, 6 and 8) during rotating backward extrusion (RBE) were investigated. The results showed that increasing groove numbers effectively improved the accumulative strain and resulted in lower deformation load. When the groove number increased to 8, the bottom metal could be accelerated to flow towards the wall. Meanwhile, the grain refinement and DRX proportion could also be promoted by the increase of groove numbers. And the most homogeneous microstructure and severe deformation was obtained in six-groove tubular (G6). The accelerated metal flow in the eight-groove tubular (G8) preserved a relatively fine microstructure along the tubular wall, resulting in an excellent axial homogeneity. Moreover, increasing groove numbers could disrupted the tendency for developing an intense basal texture during extrusion, and the minimum texture intensity was formed in G6. It demonstrated that with the groove increasing, the comprehensive effects, random grain orientation distribution resulted from new nucleation and grown dynamic recrystallization (DRX) grains and the c-axis rotation of grains triggered by increased activity of pyramidal <c+a> slip, contributed to the texture weakening. The mechanical property was significantly improved with the groove increasing, and the microhardness of open punch was considerably improved in comparison with the non-open counterpart. Favored by the grain refinement and dislocation strengthening, the G6 and the four-groove sample (G4) achieved maximum hardness values of 98.8 HV and 98.5 HV at the tubular bottom and wall, respectively.

Effect of cooling rate on the microstructure and mechanical properties of high Ti-V microalloyed steel

High-strength low-alloy steels are known for their superior mechanical properties and these desired properties are achieved through controlled hot rolling. The cooling rate is one of the most important factors in controlled hot rolling, which plays a major role in the final microstructure and mechanical properties. In this work, the effect of the cooling rate on the final microstructure and mechanical properties of high Ti-V HSLA microalloyed steel was investigated using the Gleeble 1500™ thermomechanical processing simulator. The samples were cooled to room temperature after final pass deformation using two different cooling rates to simulate the 16 and 30 mm thick plate rolling patterns. Polygonal ferrite was found to be a dominant phase in the final microstructures; SEM-EBSD IPF maps also showed grains with random orientation distribution. Yield strength and ultimate tensile stress increased by approximately 14% and 10%, respectively, with increasing cooling rate.

A Census of Outflow to Magnetic Field Orientations in Nearby Molecular Clouds

We define a sample of 200 protostellar outflows showing blue- and redshifted CO emission in the nearby molecular clouds Ophiuchus, Taurus, Perseus, and Orion, to investigate the correlation between outflow orientations and local, but relatively large-scale, magnetic field directions traced by Planck 353 GHz dust polarization. At high significance (p ∼ 10−4), we exclude a random distribution of relative orientations and find that there is a preference for alignment of projected plane of sky outflow axes with magnetic field directions. The distribution of relative position angles peaks at ∼30° and exhibits a broad dispersion of ∼50°. These results indicate that magnetic fields have dynamical influence in regulating the launching and/or propagation directions of outflows. However, the significant dispersion around perfect alignment orientation implies that there are large measurement uncertainties and/or a high degree of intrinsic variation caused by other physical processes, such as turbulence or strong stellar dynamical interactions. Outflow to magnetic field alignment is expected to lead to a correlation in the directions of nearby outflow pairs, depending on the degree of order of the field. Analyzing this effect, we find limited correlation, except on relatively small scales ≲0.5 pc. Furthermore, we train a convolutional neural network to infer the inclination angle of outflows with respect to the line of sight and apply it to our outflow sample to estimate their full 3D orientations. We find that the angles between outflow pairs in 3D space also show evidence of small-scale alignment.

Modeling and characterization of dynamic recrystallization under variable deformation states

Flexible rolling with variable deformation states has brought new challenges to dynamic recrystallization (DRX) theory based on conventional constant deformation state. Modeling and characterization of DRX are investigated, inspired by the successful application of constitutive models and EBSD technique in the field of characterizing the recrystallization of metallic materials, combined with the characteristics of flow stress. Two constitutive models for constant and transient deformation states were considered. The dynamic recovery effect was integrated into the model for predicting the flow stress and DRX volume fraction. A hot processing map was established to predict the instability region. Various DRX synergistic regulation mechanisms and crystallographic orientation distribution under a transient deformation state were explored. The results suggest that DRX is the main softening characteristic under various deformation states. The predicted flow stress of the constant and transient models are in good agreement with the experimental values. The regulation mechanism of DRX transforms from continuous DRX to discontinuous DRX when the strain rate dynamic decreases. DRX grains display a random orientation distribution, and the crystallographic orientation is toward the 〈111〉 fiber parallel to the normal direction of the compression axis when the DRX volume fraction is high. These findings provide insight into the DRX constitutive description and microstructure evolution under variable deformation states.

Field-induced orientational switching produces vertically aligned Ti3C2Tx MXene nanosheets

Controlling the orientation of two-dimensional materials is essential to optimize or tune their functional properties. In particular, aligning MXene, a two-dimensional carbide and/or nitride material, has recently received much attention due to its high conductivity and high-density surface functional group properties that can easily vary based on its arranged directions. However, erecting 2D materials vertically can be challenging, given their thinness of few nanometres. Here, vertical alignment of Ti3C2Tx MXene sheets is achieved by applying an in-plane electric field, which is directly observed using polarised optical microscopy and scanning electron microscopy. The electric field-induced vertical alignment parallel to the applied alternating-current field is demonstrated to be reversible in the absence of a field, back to a random orientation distribution. Interdigitated electrodes with uniaxially aligned MXene nanosheets are demonstrated. These can be further modulated to achieve various patterns using diversified electrode substrates. Anisotropic electrical conductivity is also observed in the uniaxially aligned MXene nanosheet film, which is quite different from the randomly oriented ones. The proposed orientation-controlling technique demonstrates potential for many applications including sensors, membranes, polarisers, and general energy applications.

Two-state model for the SmAP_{R} phase with randomized layer polarization exhibited by some compounds with bent-core molecules.

One of the unusual liquid crystalline phases exhibited by some compounds with bent-core (BC) molecules is designated as SmAP_{R}, in which transverse polarization (P) of smectic layers with upright molecules has a random orientational distribution. Most of such compounds undergo a transition to the SmAP_{A} phase with antiferroelectric order of adjacent layers as the temperature is lowered. Second harmonic studies have shown that the medium consists of polarized domains with only a few hundred molecules, the number increasing at lower temperatures. This is in contrast to the random orientations of entire layers predicted by a few phenomenological models which have been proposed for the SmAP_{R} phase. In this paper we show that the two-state model, developed earlier by us to describe modulated phases found in compounds with BC molecules, can successfully account for all the experimental results on the SmAP_{R} phase. It is proposed that polarized domains which are made of ground state conformers, with a large bend of the aromatic cores, nucleate as circular disks in a first order transition from the isotropic phase. Excited state conformers (with a smaller bend) freely rotate about their long axes and surround the disks. The polarization in the disk has a spontaneous splay distortion which generates a texture with an expelled center of a partial disclination. The interdisk electrostatic energy is lower than the thermal energy and the disks are subject to significant rotational fluctuations, resulting in the uniaxial symmetry of the medium. Calculated equilibrium properties of the medium as functions of temperature and electric field reflect the trends seen in experiments. The model overestimates properties such as the radius of the disks as the transition to a lower temperature uniform phase is approached, and physical arguments are given to show that interlayer interactions ignored in the model become important in that regime, and should be included for an improved description of the medium.

In-situ synchrotron x-ray diffraction texture analysis of tensile deformation of nanocrystalline superelastic NiTi wire at various temperatures

According to the state-of-the-art view, superelastic deformation of NiTi wires at room temperature proceeds via stress induced martensitic transformation from B2 cubic austenite to B19′monoclinic martensite. With increasing test temperature, the stress induced martensitic transformation is substituted by plastic deformation of austenite at martensite desist temperature MD. However, there are many unsolved problems with this widely accepted view. What are the texture and martensite variant microstructure in stress induced martensite and do they depend on test temperature? Does the austenite transform to martensite completely within the transformation plateau range? How the superelasticity changes into plastic deformation of austenite with increasing temperature - is it stepwise or gradual change? How the wire deforms plastically at various temperatures? Does plastic deformation occur in austenite or in martensite, via dislocation slip or deformation twinning? Are the deformation/transformation processes in nanocrystalline NiTi wires same as in large grain polycrystals? We have addressed these long standing but unsolved questions by performing series of in-situ synchrotron x-ray diffraction experiments on superelastic nanocrystalline NiTi wire subjected to tensile tests at 20, 90 and 150 °C until fracture supplemented by post mortem TEM analysis of lattice defects created by the tensile deformation.It was found that, in case of conventional superelasticity at 20 °C, austenite transformed almost completely to stress induced martensite within the transformation plateau range. The stress induced martensite displayed a sharp two fibre texture reflecting its (001) compound twinned microstructure. Stress induced martensite transformed back to the parent austenite without leaving any significant unrecovered strains and lattice defects in the austenitic microstructure. When loaded further into the plastic deformation range, this martensite deformed via combination of (20-1) and (100) deformation twinning and kinking assisted by [100](001) dislocation slip in martensite. Recoverability of tensile strains on unloading and heating remained surprisingly large (∼10%) up to wire fracture at 62% strain.The superelasticity at elevated temperatures 90 °C (150 °C) was found to be very different. The austenite transformed into a mixture of phases containing 40% (10%) volume fraction of stress induced martensite within the transformation plateau range. The stress induced martensite displayed four fibre texture, which further evolved with increasing strain. The original <111> fibre texture of austenite evolved with increasing strain towards random orientation distribution. The recoverability of tensile strains on unloading and heating sharply decreased with increasing temperature. Two alternative deformation mechanisms are proposed to explain these changes. The first mechanism assumes that martensite stress induced at elevated temperatures appears in a form of thin internally twinned CVP martensite plates surrounded by austenite deforming via dislocation slip. Requirement for strain compatibility at habit plane interfaces affects selection of martensite variants under stress and texture. The second mechanism is based on the idea that martensite stress induced at elevated temperatures immediately deforms plastically and undergoes reverse martensitic transformation to austenite leaving behind unrecovered plastic strain, slip dislocations and {114} austenite twins in the austenitic microstructure of the wire. Since the second mechanism explains the experimental observations better, it is considered to be more realistic.

Revisiting the Hail Radar Reflectivity–Kinetic Energy Flux Relation by Combining T-Matrix and Discrete Dipole Approximation Calculations to Size Distribution Observations

Abstract The retrieval of hail kinetic energy with weather radars or its simulation in numerical models is challenging because of the shape complexity and variable density of hailstones. We combine 3D scans of individual hailstones with measurements of the particle size distributions (PSD) and T-matrix calculations to understand how hail reflectivity Z changes when approximating hailstones as spheroids, as compared to the realistic shapes obtained by 3D scanning technology. Additionally, recent terminal velocity relations are used to compare Z to the hail kinetic energy flux . We parameterize the hail backscattering cross sections at L, S, C, and X bands as a function of size between 0.5 and 5.0 cm, matching the range of the observed PSDs. The scattering calculations use the T-matrix method for size parameters below 1.0 and the discrete dipole approximation (DDA) method otherwise. The DDA calculations are done for 48 digital models of realistic hailstones of sizes between 1 and 5 cm. The DDA cross sections are calculated for multiple orientations and averaged assuming a fully random orientation distribution to provide a single value per hailstone. The T-matrix reflectivity assuming solid ice spheres presents negligible differences to DDA results for size parameters below 1.0. Therefore, T matrix was used to fill in the gaps left by the DDA calculations. The results are mapped to the same size bins of the observed PSDs, allowing the calculation of the radar reflectivity. This is then correlated to , allowing a potential improvement of past retrieval methods of from Z in multiple wavelengths.

Effect of microstructural heterogeneity on corrosion fatigue crack growth behavior of 60-mm thick-plate TC4 titanium alloy NG-GTAW welded joint

The microstructure, mechanical properties and corrosion fatigue crack growth behavior of 60-mm thick-plate TC4 titanium alloy narrow gap gas-tungsten arc welding (NG-GTAW) welded joints applied to the shell of the Jiaolong deep submersible were studied, and the effect of microstructural heterogeneity on the corrosion fatigue crack growth behavior was discussed. The results show that significant microstructure evolution occurred in the welded joint with acicular α′ martensite coarsening and a random orientation distribution. The hardness and tensile strength distribution of the welded joints correspond to the difference in the microstructure, with a smaller size and more randomly oriented α′ martensite providing superior strengthening effect. The acicular α′ martensite of the weld metal (WM) has excellent resistance to crack growth compared to the α and β phases of the base metal (BM). The degree of crack serration depends on the size and orientation distribution of the α grains and acicular α′ martensite. As the stress increases, the fatigue resistance of BM may have a more serious deterioration trend compared with WM.

High-temperature deformation behavior and recrystallization mechanism of a near beta titanium alloy Ti-55511 in β phase region

Isothermal compression tests, electron backscatter diffraction (EBSD) observations and quantitative analysis were performed to investigate high-temperature deformation behavior and recrystallization mechanism of Ti-55511 alloy in β phase region. The stress-strain curves, calculated activation energy and microstructure characterization show that dynamic recovery (DRV) and dynamic recrystallization (DRX) are the main soften mechanisms. DRX becomes more significant with decreasing strain rate and increasing deformation temperature and true strain. Recrystallization grains are identified by calculated grain orientation spread (GOS). It is found that quantitative microstructural characteristics, such as DRX volume fraction and DRX grain size, are strongly dependent on process parameters. The discontinuous dynamic recrystallization (DDRX) by grain boundaries bulging and continuous dynamic recrystallization (CDRX) by progressive sub-grains rotation are observed in the deformed microstructure of Ti55511 alloy. The flow softening rate and kinetics exponent of DDRX are higher than those of CDRX. With increasing deformation temperature, DRX mechanism has transformed from DDRX to CDRX, resulting in the unexpected decrease of DRX kinetics exponent nD and flow softening rate. The ηbcc fiber, εbcc fiber and ξbcc fiber are the dominant texture component in deformed Ti–55511 alloy. The weakening of the deformation texture is accompanied by the occurrence of DRX, which can be attributed to the random orientation distribution of DRX grains.

Adiabatic shear banding in FCC metallic single and poly-crystals using a micromorphic crystal plasticity approach

Finite element (FE) simulations are performed for hat-shaped specimens made of face-centered cubic (FCC) metallic single and poly-crystals in order to investigate strain localization phenomena under adiabatic conditions which are related to adiabatic shear band (ASB) formation process. A micromorphic crystal plasticity model is used to overcome the main limitation of classical plasticity models, namely the mesh size dependency in strain localization problems. A thermodynamically consistent formulation of the constitutive equations is proposed for micromorphic thermo-elasto-viscoplasticity of single crystals. The temperature evolution under adiabatic conditions is derived from the competition between plastic power and energy storage. The micromorphic crystal plasticity model is used first to simulate strain localization induced by thermal softening in a metallic single crystal strip loaded in simple shear undergoing single-slip. The FE solution of this boundary-value problem is validated using an analytical solution. Regarding single crystal hat-shaped specimen simulations, five different crystal orientations are considered to study the formation, intensity and orientation of shear bands. In particular, one special crystal orientation is found resistant to shear banding. In addition, the formation of shear bands in hat-shaped polycrystalline aggregates is investigated. The specimens are polycrystalline aggregates with different grain sizes, namely the coarse-grained and fine-grained specimens with random crystal orientation distribution. Furthermore, several realizations of the microstructures are taken into account for statistical considerations. The micromorphic crystal plasticity model incorporates a characteristic length scale, which induces a grain size effect in the simulation of polycrystalline specimens. The grain boundaries act as obstacles against shear band formation. A significant grain size effect, namely the finer the grain size the higher the resulting load, is predicted by the simulations under isothermal conditions. However, the fine-grained specimens are found to fail earlier by shear banding than some coarse-grained samples, the latter being associated with significant dispersion of the results depending on grain orientations. The effect of grain size on the width of the shear band is also analyzed. The temperature-dependent material parameters and shear band widths considered in the paper correspond to Nickel-based superalloy Inconel 718 in a large temperature range. No strain hardening was considered in the hat-shaped specimen test to simplify the interpretation of the results.