Rotation Of Grains Research Articles

Microstructure and texture evolution of TA32 titanium alloy during superplastic deformation

A study based on uniaxial tension tests were conducted in this paper to understand the hot deformation behaviors of TA32 alloy during superplastic deformation at a deformation temperature of 915°C and an initial strain rate of 6.64×10−3s−1. In the test, the maximum fracture elongation reached 1065%, exhibiting good superplastic deformation ability. Electron back-scattered diffraction (EBSD) was used to analyze and study evolution of microstructures and textures in the process of deformation with the following findings: During tensile deformation, the fraction of low-angle grain boundaries (<15°, LAGBs) decreased drastically, while that of high-angle grain boundaries (≥15°, HAGBs) increased from 45% to 96.2%. As the further straining, the role of continuous dynamic recrystallization (CDRX) was weakened, while that of discontinuous dynamic recrystallization (DDRX) was strengthened. Dynamic recrystallization helped refine coarse grains in the initial structures and reduced deformation defects in grains. The superplastic deformation mechanism of TA32 alloy was dominated by boundary sliding of high-angle grains, coordinated by dislocation motion at the softening stage and by grain rotation at the steady deformation stage respectively. In addition, the drawing force contributed to both dynamic recrystallization and rotation of grains.

Reorientation mechanisms of block copolymer/CdSe quantum dot composites under application of an electric field.

Time- and temperature-resolved in situ birefringence measurements were applied to analyze the effect of nanoparticles on the electric field-induced alignment of a microphase separated solution of poly(styrene)-block-poly(isoprene) in toluene. Through the incorporation of isoprene-confined CdSe quantum dots the reorientation behavior is altered. Particle loading lowers the order-disorder transition temperature, and increases the defect density, favoring nucleation and growth as an alignment mechanism over rotation of grains. The temperature dependent alteration in the reorientation mechanism is analyzed via a combination of birefringence and synchrotron SAXS. The detailed understanding of the effect of nanoparticles on the reorientation mechanism is an important prerequisite for optimization of electric-field-induced alignment of block copolymer/nanoparticle composites where the block copolymer guides the nanoparticle self-assembly into anisotropic structures.

B21-O-02In situTEM observation of Cu-doped graphene

We directly observed atomic-scale transformations of Cu-doped graphene by aberration-corrected TEM, operated at 80 kV. Many metals, including Cr, Ti, Pd, Ni, and Al, were experimentally reported to etch graphene [ 1 ]. Cu atoms, however, promoted graphene reconstruction rather than etching in our experiments. They preferentially replaced C atoms in defective graphene areas and modified their structures via repeated C–C rotations, formation and mending of nanopores, resulting in the rotation of grains mediated by contaminants and lattice defects. Furthermore, Cu atoms promoted to connect the neighbouring edges of two graphene layers and created stable nanopores in bilayer graphene. Edge-closed nanopores were predicted to exhibit wide range electronic properties (from semiconductor to metallic), depending on the size and distance from neighbouring nanopores [ 2 ]. We also observed bilayer graphene with other impurity atoms, such as Si and Pt; they, however, etched only one layer and did not create through-pores. Our results suggested that individual Cu atoms can catalyse the reconstruction of carbon atoms and create novel carbon nanostructures.

Acoustic Measurements Aid Mechanical-Damage Characterization in Proppant Packs

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper IPTC 18092, “Mechanical-Damage Characterization in Proppant Packs Using Acoustic Measurements,” by Aderonke Aderibigbe, SPE, Clotilde Chen Valdes, and Zoya Heidari, SPE, Texas A&amp;M University, and Tihana Fuss, Saint-Gobain Proppants, prepared for the 2014 International Petroleum Technology Conference, Kuala Lumpur, 10–12 December. The paper has not been peer reviewed. The strength and conductivity of proppant packs are key parameters for assessing their performance. Mechanical damage of proppants usually is analyzed by crush tests. However, measurements from these tests remain questionable because of discrepancies in procedures and test results. This paper introduces a new technique based on interpretation of acoustic measurements to quantify mechanical damage in propping agents. Introduction Mechanical damage that leads to compaction and crushing is studied for sands by measuring ultrasonic velocities. Elastic properties can be estimated from compressional- and shear-wave velocities calculated from acoustic measurements. The elastic properties of unconsolidated sands usually are studied by use of effective-medium models for granular media. Effective- medium-theory approaches such as the Hertz- Mindlin model are often used to derive the effective elastic moduli of packings of identical and spherical granular materials. This model combines the Hertzian contact model, which is used to estimate the normal (compressional) stiffness for two identical spheres, and the Mindlin contact model, which is used to estimate the tangential (shear) stiffness of the spheres. Work investigating the relationship between the ratio of compressional- to shear-wave velocities and pressure as a result of preconsolidation and sorting in unconsolidated sands showed that the Hertz-Mindlin model overestimated the shear moduli and underestimated the pressure dependence of the moduli and the velocities of the unconsolidated sands when compared with experimental data from ultrasonic measurements. The discrepancy was attributed to the inability of the model to account for rotation of grains and slip at their boundaries. The work applied a modified Hertz-Mindlin model, in which it is assumed that there is no friction between the grains (hence, the tangential stiffness is negligible), to obtain a reasonable match between the model and experimental results. The disparities were attributed to the angularity of the sand grains and the assumption of no slip at the grain contacts. The disparities were modified by introducing an average- angularity parameter and assuming slip at the grain contacts. Previous publications documented applications of effective-medium models for assessment of elastic properties in rocks and unconsolidated sands. However, effective-medium models have not been applied in the assessment of elastic properties of proppant packs.

“And this Reason has a Justification”: Medieval Scientific Argument for the Custom against Eating Legumes on Passover

This article discusses a botanical-agricultural reason for the prohibition against eating legumes on Passover, one presented by Rabbeinu Manoah (Provence, thirteenth century). According to Rabbeinu Manoah, the basis of the prohibition is an ancient agricultural concept whereby through changes in climate and annual rainfall, wheat kernels become legumes. It seems that technical changes in agriculture in the Middle Ages affected the ban. The practice of using a three-field rotation of grains and legumes in European fields increased the mixture of aftergrowths of legumes from the previous years in the grain harvest of the present year. This phenomenon strengthened the ancient concept among contemporary agriculturists that grains had changed into legumes, when, in fact, the legumes were merely aftergrowths.

Smaller critical size and enhanced strength by nano-laminated structure in nickel

Because of a shift in the dominant deformation mechanisms, the strength/hardness of metals increases with decreasing grain size down to a critical value, then decreases with further grain refinement. Here, laminated structure with low-angle grain boundaries was found to have smaller critical size and enhanced strength when compared to the equiaxed grain structure, through a series of large-scale molecular dynamics simulations. Then, the corresponding atomistic mechanisms were investigated by checking and comparing the rotations of grains and equivalent strain partitioning in grain boundary atoms. In laminated structure, grain boundary activies were found to be promoted with decreasing lamellar thickness. More importantly, when compared to the equiaxed grain structure at the same length scale (lamellar thickness/grain size), grain boundary activities were found to be inhibited by the laminated structure, which is the main reason for the enhanced strength and the smaller critical size. Besides grain boundary activities, formation of extended dislocations, formation of deformation twins, partial dislocations interacting with formed TBs, partial dislocation blocking by and even transmission through low-angle grain boundaries were also found to play important roles in plastic deformation of nano-laminated structure. The current findings should provide insights for designing stronger and more stable nanostructured metals.

Effect of Temperature on the Nano/Microstructure and Mechanical Behavior of Nanotwinned Ag Films

In situ and ex situ annealed nanotwinned (NT) Ag thin films have been investigated by TEM and tensile testing to reveal the thermal stability of the twin boundaries, grain boundaries, dislocation densities, and their respective influence of the macroscopic yield stress. The NT Ag films synthesized by magnetron sputtering form both coherent (CTB, Σ3{111}) and incoherent (ITB, Σ3{112}) twin boundaries that are thermally stable up to 473 K (200 °C), i.e., no obvious changes in grain size, twin spacing, and yield stress. In situ TEM observations show the dislocations become mobile at 453 K (180 °C) resulting in dislocation annihilation primarily at twin and grain boundaries. Rotation of grains with low-angle grain boundaries was observed during in situ heating, resulting in the growth of columnar grains above 453 K (180 °C). However, no noticeable changes in the spacings of CTBs were observed during the entire in situ and the ex situ annealing [up to 873 K (600 °C)]. The increase in grain size and concomitant decrease in yield stress following annealing at various temperatures can be described by the Hall-Petch relationship, demonstrating that grain size rather than twin spacing is most sensitive to thermal annealing and plays a dominant role in the deformation of NT Ag films.

A gradient nano/micro-structured surface layer on copper induced by severe plasticity roller burnishing

In order to investigate a gradient nano/micro-structured surface layer on pure copper produced by severe plasticity roller burnishing (SPRB) and grain refinement mechanism, the microstructure characteristics and material properties of sample at various depths from the topmost surface were investigated by SEM, TEM, XRD, OM etc. The experimental results show that the gradient nano/micro-structure was introduced into the surface layer of over 100 μm in thickness. The remarkable increase in hardness near the topmost surface was mainly attributed to the reduced grain size. The equiaxed nano-sized grains were in random orientation and the most of their boundaries were low-angle grain boundaries (LAGBs). The coarse grains are refined into the few micro-sized grains by dislocation activities; deformation twinning was found to be the primary form for the formation of submicron grains; the formation of nanostructure was dominated by dislocation activities accompanied with rotation of grains in local region.

Methodology for estimating the critical resolved shear stress ratios of α-phase Ti using EBSD-based trace analysis

A novel method for calculating the critical resolved shear stress (CRSS) ratios of different deformation system types in polycrystalline non-cubic metals has been developed. The mean CRSS ratios between different deformation systems were calculated for both commercially pure (CP) Ti and Ti–5Al–2.5Sn (wt.%) tensile deformed at ambient temperature and 455°C using an in situ scanning electron microscope-based testing technique combined with electron backscattered diffraction. It was found that the relative activity of the different deformation systems changes as a function of alloying composition and deformation temperature. Prismatic slip was the most active deformation mode for CP Ti. CP Ti exhibited a lower resistance to prismatic slip at both ambient and elevated temperatures compared with Ti–5Al–2.5Sn. For Ti–5Al–2.5Sn, prismatic slip was the most active deformation system at ambient temperature although the basal slip activity significantly increased compared to CP Ti, mostly likely due to an increased c/a ratio resulting in a closer packed basal plane. At 455°C, basal slip exhibited a lower CRSS than prismatic slip for Ti–5Al–2.5Sn. The relative activity of other deformation systems was also affected by alloying and temperature. The statistical resampling technique of bootstrapping was used to generate multiple equivalent data sets from which mean CRSS ratios between different deformation systems, and associated confidence intervals, could be deduced. It was found that the mean CRSS ratios at low and high strains varied slightly for the same testing conditions. Moreover, lesser activated slip systems resulted in relatively larger confidence intervals for the CRSS means. This variability may be attributed to a number of potential factors, including measurement errors, rotations of grains during deformation, local stress state variations, and work hardening. The analysis further suggests that awareness of the intrinsic statistical variability in CRSS ratios should be considered when formulating crystal plasticity constitutive models.

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.

Electric Field Induced Selective Disordering in Lamellar Block Copolymers

External electric fields align nanostructured block copolymers by either rotation of grains or nucleation and growth depending on how strongly the chemically distinct block copolymer components are segregated. In close vicinity to the order-disorder transition, theory and simulations suggest a third mechanism: selective disordering. We present a time-resolved small-angle X-ray scattering study that demonstrates how an electric field can indeed selectively disintegrate ill-aligned lamellae in a lyotropic block copolymer solution, while lamellae with interfaces oriented parallel to the applied field prevail. The present study adds an additional mechanism to the experimentally corroborated suite of mechanistic pathways, by which nanostructured block copolymers can align with an electric field. Our results further unveil the benefit of electric field assisted annealing for mitigating orientational disorder and topological defects in block copolymer mesophases, both in close vicinity to the order-disorder transition and well below it.

Wear mechanisms of structural steels and effect of wear on their mechanical and acoustic properties during tension

Optical and scanning electron microscopy have been used to study the wear mechanisms of structural steels with various structures and strengths, as well as to assess their mechanical and acoustic properties after friction. The prevailing wear mechanisms have been revealed; they are governed by the strength and structure of the steels and involve the refinement and rotation of grains, the formation of parallel rows of microcracks, the strain dissolution of cementite, and martensitic transformation, as well as the formation of seizure sites in the friction contact zone, shear and intergranular pores, and microcracks. The low-carbon steel with a ultrafine-grained structure has demonstrated a high wear resistance. Friction for 3000 h had a weak effect on the mechanical properties of the steels during tension.

On the definition of an average strain tensor for two-dimensional granular material assemblies

The description of deformation of granular assemblies is one of the basic ingredients, in view of developing constitutive relations following a micromechanical line. Ignoring the deformation of particles, the macroscopic strain of a granular specimen results from the relative motions of particles, including both translational and rotational kinematics. This paper investigates to which extend the rotation of grains should be accounted for in the description of macroscopic strains, depending on the size of (that is, the number of particles in) the granular specimen. Some examples of simple two-dimensional granular assemblies of circular grains are considered and discussed to illustrate the general results derived from a micromechanical approach.

Experimental and numerical investigations of the plane strain compression of an oligocrystalline pure copper specimen

To evaluate if the deformation in the bulk of an oligocrystalline sample during a plane strain compression test can be accurately simulated numerically with the aid of a crystal plasticity model a detailed comparison between the experimentally found and numerically predicted deformation is made. In the experiment an incremental plane strain compression (PSC) test is conducted on an oligocrystalline specimen. The incremental PSC test is complemented by electron backscatter diffraction (EBSD) measurements. The experimental results enable qualitative observation of the specimen's microstructure and orientation distribution at intermediate stages of the plastic deformation. This incremental PSC test is numerically simulated with the aid of a crystal plasticity finite element model. The numerical model accounts for the specimen's microstructure and the orientations of the crystals. Mapping the microstructure and the grain orientations on the workpiece in the finite element model provides realistic boundary conditions. Both, the experiment and numerical simulation show that the deformation in the oligocrystalline specimen with only a few grains in the deformation zone considerably differs from that of well known continuum like materials. Qualitative comparisons of the incremental PSC test and its numerical simulation showed that most of the experimentally observed behaviour of the grains and their mutual interaction can be seen in the results of the CPFEM simulation as well. From quantitative comparisons it is concluded that the two-dimensional CPFEM simulation can predict the rotations of grains in the bulk of the three-dimensional microstructure of the oligocrystalline specimen only in a qualitative manner. The lack of quantitative agreement is most probably caused by the fact that this two-dimensional, plane strain CPFEM model does not account for the interaction of the grains in the microstructure beyond the analysed cross sectional surface.

Assessing the Influence of Porosity in the Deformation of Metal–Ceramic Composites

The aim of the paper is to develop the previously formulated model [19, 20] of the polycrystalline composite to include porosity growth at metallic interfaces of metal–ceramic composites (MCCs). Examples of this kind of MCCs are: (1) a two-phase material composed of brittle grains WC joined by the plastic binder Co, which can contain a small degree of porosity introduced during the cooling process [31], (2) TiC–Mo2C hard phase grains surrounded by tough binder phase Ni [12]. This work focuses on the description of the deformation of the MCC material including the modelling of a real material internal structure taking into account porosity growth during the loading. Experimental observations of the WC/Co composite [10] indicate that the majority of the fracture energy of MCC is expended through ductile failure of the plastic binder Co (dimple rupture across the binder or in the binder near the binder/carbide interface). This process is preceded by porosity growth at metallic interfaces and finally leads to inter-granular cracks propagation. This paper presents micromechanical modelling of the MCC response in the case of uniaxial tension of 3-D Representative Volume Element (RVE) with the application of the Finite Element Analysis (FEA). The MCC material includes: elastic grains and inter-granular metallic layers containing technological pores that create its real complex internal structure. The quasi-static deformation process of the material comprises elastic deformation of brittle grains, elasto-plastic deformation of inter-granular layers (of different thickness: 2–4 μm) and additional deformation due to micro-porosity development in the layers. A micro-sample analysis leads to the conclusion that a small amount of technological porosity changes the qualitative behaviour of the MCC including deformation, rotation of grains, roughness, and level of plastic strains.

Constitutive law of dense granular matter

The frictional properties of dense granular matter under steady shear flow are investigated using numerical simulation. Shear flow tends to localize near the driving boundary unless the coefficient of restitution is close to zero and the driving velocity is small. The bulk friction coefficient is independent of shear rate in dense and slow flow, whereas it is an increasing function of shear rate in rapid flow. The coefficient of restitution affects the friction coefficient only in such rapid flow. Contrastingly, in dense and slow regime, the friction coefficient is independent of the coefficient of restitution and mainly determined by the elementary friction coefficient and the rotation of grains. It is found that the mismatch between the vorticity of flow and the angular frequency of grains plays a key role to the frictional properties of sheared granular matter.

Micromorphological characteristics of glacimarine sediments: implications for distinguishing genetic processes of massive diamicts

Glacimarine diamicts are produced by diverse processes, and genetic differentiation is often problematic using macro-sedimentological criteria alone. Micromorphology offers a potentially helpful tool in such investigations. Macroscopically massive diamict samples of known glacimarine origin, from the Polar North Atlantic, Antarctica and north Irish Sea, were prepared for micromorphological analysis to (1) identify microstructures unique to different modes of sedimentation and (2) interpret genetic processes from those structures. The samples comprised examples of debris-flow, iceberg-turbate and suspension settling deposits from late Quaternary glacier-influenced marine environments: tidewater glacier, sub- or pro-ice shelf and continental slopes in front of ice stream termini. Results show two significant features of debris-flow sediments: a bimodal grain fabric of near-horizontal and -vertical grains, and laminated clay and silt coatings on sand and pebble grains. Coatings are best developed in sediments with finer grain-size distributions and in debris-flow sediments which have had relatively long run-out distances on trough-mouth fans, suggesting continuous rotation of grains in a buoyant, turbulent aqueous environment. This is significant because it precludes debris-flow delivery by plug flow. The micromorphology of iceberg turbate has not been described previously. It contains structures similar to those described in tills, so that unambiguous identification of these sediments seems unlikely based on micromorphological criteria alone. Suspension sediments range from fine-grained massive diamicts containing microfossils to more heterogeneous coarser sediments characterised by abrupt textural variations, from ice-distal and ice-proximal glacimarine environments respectively. The ice-proximal sediments contain fine vertical lineations marking the trajectories of dropstones through wet matrix. These dropstone tracks have not been reported in previous studies.

Tension deformation mechanism and fracture characteristics of high nitrogen steel 6Cr21Mn10MoVNbN at 25–800°C

The tensile deformation behaviour and the fracture mechanism of 6Cr21Mn10MoVNbN alloy were studied in the range between 25–800°C using thermal simulator, hardness tester, optical microscopy, scanning electronic microscopy attached with energy dispersive spectroscopy and transmission electronic microscopy. Elongation of the steel was found to decrease as temperature increases, which was due to the growth of the coarse nitrides and carbides and the precipitation of Cr2N at grain boundaries. It was also found that the fracture behaviour and deformation mechanism differed at different temperatures. At a low temperature, such as 225°C, the steel showed features of transgranular fracture and the deformation mechanism of the steel was dislocation slip and deformation twinning. At a high temperature, such as 800°C, the steel demonstrated characteristics of intergranular failure and the major deformation mechanism of the steel was dislocation slip and rotation of grains.

Effects of Surface Roughening on Asperity Flattening

A series of experiments have been conducted using aluminum strips, which manifest a strong tendency toward roughening in forming processes, to investigate the evolvement of surface roughness and its resistance to flattening. The strips are stretched to specific bulk strains and are then compressed by a smooth glass at various pressures. It is found that except for very high bulk strain, the free surface on the transverse specimen roughens more seriously and irregularly than the longitudinal one due to the fierce rotations of grains, which make the grains parallel to the straining direction and eventually mutate the topography into an isotropic one. As the roughened surface is compressed, the transverse specimen may roughen further in the case of large bulk strain. According to the finite element method (FEM) analysis using serrated surfaces, the sharper asperity undergoes more serious straining on its peak and basically such a strain increment is not affected by the degree of flattening. The ability to resist flattening, defined as the “asperity hardness,” varies with asperity angle as well as the enhanced shear strength on the asperity tip, rather than the substrate. A new quantitative description of the asperity hardness is suggested, and the new calculated contact area ratios are in good agreement with the experiments except for very high pressure where additional mechanics such as adiabatic recrystallization might happen. The concept of the additional hardening on asperity peaks can be employed to refine the prediction of asperity contact in the analysis of metal forming.

Modeling economic and agro-environmental dynamics of potato production systems

Crop rotation and other input management practices are of particular interest for their potential impacts on economic and agro-environmental components of potato production. Although crop yield and experimental impacts of rotations of grains, oilseed and legume crops have been published for several experimental studies in Canada there are few models related to the economic and environmental dynamics of potato production. We describe a dynamic model which integrates environmental and economic processes in potato production. The potato rotation model consists of interconnected modules of irrigation and precipitation, soil characteristics, soil erosion, soil water, phosphorus, nitrogen, soil organic matter, farming operations, crop yield and the related calculation of economic return. While not all aspects of crop production have been interlinked, including nitrogen carry-over, this model is the first step in the analysis of experimental data for irrigated potato rotations conducted in southern Manitoba.