Translation Of Grains Research Articles

Anomalous strain-energy-driven macroscale translation of grains during nonisothermal annealing

We report a mode of grain growth, involving the macroscopic translation of grain centers during nonisothermal annealing. Through synchrotron high-energy x-ray diffraction microscopy, we find dissolution of semicoherent precipitates generates dislocations, thereby raising the stored strain energy within grains. The subsequent evolution of grains shows unexpected grain translations over length scales of 10--100 \ensuremath{\mu}m. Phase-field simulations reveal that such translations are not uncommon in strain-energy-driven grain growth, wherein different regions of a grain may grow and shrink simultaneously.

Effects of interparticle friction on the response of 3D cyclically compressed granular material

We numerically study the effect of inter-particle friction coefficient on the response to cyclical pure shear of spherical particles in three dimensions. We focus on the rotations and translations of grains and look at the spatial distribution of these displacements as well as their probability distribution functions. We find that with increasing friction, the shear band becomes thinner and more pronounced. At low friction, the amplitude of particle rotations is homogeneously distributed in the system and is therefore mostly independent from both the affine and non-affine particle translations. In contrast, at high friction, the rotations are strongly localized in the shear zone. This work shows the importance of studying the effects of inter-particle friction on the response of granular materials to cyclic forcing, both for a better understanding of how rotations correlate to translations in sheared granular systems, and due to the relevance of cyclic forcing for most real-world applications in planetary science and industry.

Special Grain Boundaries in Ultrafine-Grained Tungsten.

Field ion microscopy and computer simulation were used for the study of an atomic structure high-angle grain boundary in hard-drawn ultrafine-grained tungsten wire. These boundaries with special misorientations are beyond the scope of the coincident site lattice model. It was demonstrated that the special non-coincident grain boundaries are the plane-matching boundaries, and rigid-body displacements of adjacent nanograins are normal to the <110> misorientation axis. The vectors of rigid-body translations of grains are described by broad asymmetric statistical distribution. Mathematical modeling showed that special incommensurate boundaries with one grain oriented along the {211} plane have comparatively high cohesive energies. The grain-boundary dislocations ½<110> were revealed and studied at the line of local mismatch of {110} atomic planes of adjacent grains.

Motion of Grain Boundaries: Experiments on Bicrystals

Recent research on grain boundary migration is reviewed. Novel in-situ measuring techniques based on orientation contrast imaging and the experimental results obtained on specially grown bicrystals are presented. Particularly, the investigated faceting and migration behavior of low angle grain boundaries under the curvature force in aluminum bicrystals was addressed. In contrast to the pure tilt boundaries, which remained straight/flat and immobile during annealing at elevated temperatures, mixed tilt-twist boundaries readily assumed a curved shape and steadily moved under the capillary force. Computational analysis revealed that this behavior is due to the inclinational anisotropy of grain boundary energy, which in turn depends on boundary geometry. The migration of planar grain boundaries induced by a magnetic field was measured in bismuth and zinc bicrystals. Various structurally different boundaries were investigated. The results revealed that grain boundary mobility essentially depends on the misorientation angle and the inclination of the boundary plane. Stress driven boundary migration in aluminium bicrystals was observed to be coupled to a tangential translation of the grains. The activation enthalpy of high angle boundary migration was found to vary non-monotonously with misorientation angle, whereas for low angle boundaries the migration activation enthalpy was virtually the same. The motion of the mixed tilt-twist boundaries under stress was observed to be accompanied by both the translation of adjacent grains parallel to the boundary plane and their rotation around the boundary plane normal.

Grain boundary motion and grain rotation in aluminum bicrystals: recent experiments and simulations

The results of experimental and computational efforts over recent years to study the motion of geometrically different grain boundaries and grain rotation under various driving forces are briefly reviewed. Novel in-situ measuring techniques based on orientation contrast imaging and applied simulation techniques are described. The experimental results obtained on specially grown aluminum bicrystals are presented and discussed. Particularly, the faceting and migration behavior of low angle grain boundaries under the curvature force is addressed. In contrast to the pure tilt boundaries, which remained flat/faceted and immobile during annealing at elevated temperatures, mixed tilt-twist boundaries readily assumed a curved shape and steadily moved under the capillary force. Computational analysis revealed that this behavior is due to the inclinational anisotropy of grain boundary energy, which in turn depends on boundary geometry. The shape evolution and shrinkage kinetics of cylindrical grains with different tilt and mixed boundaries were studied by molecular dynamics simulations. The mobility of low angle <100> boundaries with misorientation angles higher than 10°, obtained by both the experiments and simulations, was found not to differ from that of the high angle boundaries, but decreases essentially with further decrease of misorientation. The shape evolution of the embedded grains in simulations was found to relate directly to results of the energy computations. Further simulation results revealed that the shrinkage of grains with pure tilt boundaries is accompanied by grain rotation. In contrast, grains with the tilt-twist boundaries composed of dislocations with the mixed edge-screw character do not rotate during their shrinkage. Stress driven boundary migration in aluminium bicrystals was observed to be coupled to a tangential translation of the grains. The activation enthalpy of high angle boundary migration was found to vary non-monotonically with misorientation angle, whereas for low angle boundaries the migration activation enthalpy was virtually the same. The motion of the mixed tilt-twist boundaries under stress was observed to be accompanied by both the translation of adjacent grains parallel to the boundary plane and their rotation around the boundary plane normal.

Grain Boundary Dynamics under an Applied Stress

The response of various structurally different planar grain boundaries in aluminum bicrystals to an applied stress was experimentally investigated. Stress induced boundary migration was observed to be coupled to a tangential translation of the grains for symmetrical and asymmetrical and tilt boundaries with both low and high misorientation angles. The activation enthalpy of high angle boundary migration was found to vary non-monotonously with misorientation angle, whereas for low angle boundaries the migration activation enthalpy was virtually the same. The Σ7 CSL boundaries in bicrystals of different geometry were observed to move under an applied stress, but their migration did not produce shear. These crystallographically equivalent boundaries, however, were found to behave different with respect to migration rate and its temperature dependence. The stress driven migration of the mixed tilt-twist boundary was observed to be accompanied by both the translation of adjacent grains parallel to the boundary plane and their rotation around the boundary plane normal. This behavior was interpreted in terms of the structure of the investigated tilt-twist boundary.

Mechanically Driven Migration of &lt;100&gt; Tilt Grain Boundaries in Al-Bicrystals

The stress induced migration of planar grain boundaries in aluminium bicrystals was measured. Symmetrical &lt;100&gt; tilt grain boundaries with misorientation angles in the range between 5.7° and 17.8° were examined. Boundary migration under a shear stress was observed to be ideally coupled to the lateral translation of grains. The measured ratios of the normal boundary motion to the lateral displacement of grains are in an excellent agreement with the respective boundary geometry. The temperature dependence of grain boundary mobility was measured, and the corresponding activation parameters were determined. The activation enthalpy of boundary migration was found to be independent of misorientation angle in the investigated misorientation range and amounts to H=1.44 eV.

Low angle tilt boundary migration coupled to shear deformation

The shear stress induced migration of planar 〈1 0 0〉 tilt grain boundaries in Al bicrystals was observed to be perfectly coupled to the lateral translation of grains. Boundaries with misorientations of 10.5° and 12.0° on one side of the misorientation range (0–90°) and 81.0° and 81.1° on its other side move in opposite directions under the same applied external stress. The measured ratios of the normal to the lateral motion comply perfectly with the coupling factors of a recently proposed model of boundary motion.

Coupling grain boundary motion to shear deformation

Molecular dynamics (MD) simulations confirm that normal grain boundary (GB) motion must often be coupled to tangential translation of grains and will then produce shear deformation of the lattice traversed by the GB. Conversely, shear stresses applied to a GB can induce its normal motion. Using [0 0 1] symmetrical tilt GBs in copper as a model, the coupling factor β between the GB motion and grain translations has been calculated by MD simulations over the entire misorientation range and a wide range of temperatures. The coupling factor is multivalued, can be positive or negative, and shows an abrupt switch from one branch to another at a tilt angle of about 35°. At high temperatures the response of high-angle GBs to shear changes from coupling to sliding until coupling disappears. No sliding is observed for low-angle GBs up to near the melting point. A geometric model of coupling proposed in this work predicts the misorientation dependence of β in excellent agreement with MD results and relates the multivalued character of β to the point symmetry of the crystal. Two kinds of low-angle GBs with different dislocations occur when the tilt angle is small and again when it approaches 90°. In these limits, the multiplicity of β is explained by different Burgers vectors of the dislocations. The results of this work are summarized as a temperature–misorientation diagram of mechanical responses of GBs. Unsolved problems and future work in this area are discussed.

Superplastic Deformation without Relative Grain Translation?

It is now almost universally accepted that the mechanism by which superplastic deformation occurs involves relative grain translation, by grain boundary sliding, together with dislocation creep and/or diffusional creep. While there has been much debate on the fine points of the process - principally the so-called accommodation mechanisms, the primary role of the relative translation of grains via grain boundary sliding is virtually undisputed. Although this mechanism appears to be consistent with several aspects of microstructural evolution in superplastic deformation, the direct evidence for a dominant grain translation mechanism is mainly drawn from work using surface markers, not withstanding the fact that the extra degree of freedom at the surface of a superplastically deformed sample may make it somewhat unrepresentative of the bulk material. Research carried out recently on Ti-6Al-4V and AA5083+Cr, two well known superplastic materials, using both internal and surface markers throws into some doubt the importance of relative grain translation in superplastic deformation. The behaviour of aligned grain structures - acting as internal markers - is also difficult to reconcile with relative grain translation mechanisms The fact that such microstructures occur in materials which give a high strain rate sensitivity is inconsistent with mechanisms in which grain boundary sliding is a principal, rather than a subservient or even accommodating, mechanism. Evidence from transmission electron microscopy and plastic anisotropy suggest that intragranular slip may have a fundamental role in superplasticity. Certainly, some consideration must be given to mechanisms other than relative grain translation, though these must include the role of grair boundaries in leading to high strain rate sensitivity.

Simple shear experiments on calcite rocks: rheology and microfabric

Ideal simple shearing of Carrara marble and Solnhofen limestone was achieved by inserting calcite rock slices into sandstone cylinders precut at 35°. The experiments covered a wide range in temperatures (25–900°C) at confining pressures of 200 and 250 MPa and at strain rates between 10−3 and 10−5 s−1. Shear strains of up γ = 2.7 were achieved. The stress-strain data for simple shear favourably compare with predictions on the basis of existing flow laws established in coaxial testing. This comparison demonstrates that the simple shear experiments covered most of the previously established flow regimes for calcite rocks. At the highest temperatures Carrara marble exhibited substantial work softening after high shear strains. A complete texture analysis was carried out with the U-stage and a texture goniometer. The following four distinct microfabric regimes were found. (1) Twinning regime: an oblique c-axis maximum deflected from the pole to the flattening plane towards σ1 and against the imposed sense of shear results directly from e-twinning. Extensive twinning (some grains are completely twinned) rapidly rotates the c-axes into near parallelism with σ1. This regime was observed for Carrara marble deformed at temperatures below 700°C and for Solnhofen limestone at room temperature. (2) Intracrystalline slip regime: three c-axis maxima were observed in both rock types at more elevated temperatures and in the absence of twinning. These c-axis orientations result from an alignment of slip planes and slip directions into parallelism with the shear-zone boundary and the shear direction, respectively. This finding supports the idea of an ‘orientation for easy slip’ resulting from simple shearing. In addition to r- and -glide, basal glide is inferred to have been operative. (3) Grain-boundary sliding regime: a great circle distribution of c-axes approximately perpendicular to the direction of finite shortening is observed. This texture is weaker compared to the other regimes. It is interpreted to result from minor contributions of f-slip towards total strain. This regime is restricted to fine-grained Solnhofen limestone deformed at the highest temperatures. (4) Grain-boundary migration regime: only in Carrara marble deformed at the highest temperatures is there substantial grain-boundary migration recrystallization. The texture consists of a dominant c-axis maximum oriented perpendicular to the shear-zone boundary. This single maximum corresponds to an ‘orientation for easy slip’ for basal slip. Grain-boundary migration assists rapid lattice reorientation, and leads to (geometrical) work softening. Quantitative analysis of grain-boundary surface orientations gave the following results. Within the twinning and the intracrystalline slip regimes the characteristic grain shape is elliptical for both rock types. The strain ellipses derived from this analysis closely correspond to the known finite strain at the lowest temperatures. At intermediate temperatures, and especially in Carrara marble, there is a consistent trend for the aspect ratios to indicate too little flattening, and for the angles between the long axis of the strain ellipse and the shear-zone boundary to be too small. In the absence of grain boundary migration recrystallization this finding indicates minor translations of grains with respect to one another (grain-boundary sliding or translations accommodated in the mantle regions of grains). At the highest temperatures, where extensive grain-boundary migration occurs in Carrara marble (grain-boundary migration regime), two preferred orientations of grain boundaries are observed: (1) parallel to the shear zone boundary; and (2) parallel to an oblique direction deflected 20° away from the normal to the shear-zone boundary in the sense of the imposed shear. The same pattern of grain-boundary alignment is observed in Solnhofen limestone deformed in the grain-boundary sliding regime. The geological applications of this experimental work are discussed in terms of calcite tectonites in particular and sheared rocks in general.