Low-angle Dislocation Boundaries Research Articles

Determination of the optimal temperature regime of plastic deformation of micro alloyed automobile wheel steels

It has been established that hot plastic deformation microalloyed steel 10HFTBch type accelerates the excretion of pearlite and causes structure refinement during subsequent cooling due to an increase of start of new phases’ centers. This leads to the formation of a fine-grained dispersed two-phase structure consisting of ferrite with a high dislocation density and pearlite. It has been determined that at the optimum deformation temperature the prevailing mechanism of structure refinement is the fragmentation of austenitic and ferrite grains of a common initial orientation into misoriented subgrains (fragments) with low-angle dislocation boundaries of deformation origin. A thorough thermokinetic analysis of the course of this process made it possible to determine the optimal temperature regime for plastic deformation for this type of steel, which is 850-900 °C. Plastic deformation increases the range of of pearlite’s transformation existence rates by 2.5 times, it was found that after 30% deformation at 950 °C, pearlite transformation begins at a cooling rate of 5 °C/s, and without deformation – at 2 °C/s

Enhanced fatigue endurance limit of Cu through low-angle dislocation boundary

How to remarkably elevate the high-cycle fatigue resistance of metallic materials under cyclic loading is a crucial, yet technically challenging issue for major structural applications. Either traditional strengthening approaches or newly-developed hierarchical nanostructural modifications has been demonstrated to usually display a limited ability of enhancing the 107-cycle fatigue endurance limit. Here, we introduce massive nano-scaled low-angle dislocation boundaries in coarse grained pure Cu by means of simple drawing process, not only enhancing the tensile strength obviously, but also imparting a fatigue limit as high as 130 MPa and a fatigue ratio of 0.35, which is a record for pure Cu. Upon cyclic loading, the extensive activation of strongly confined dislocation slip within homogeneous dislocation cells (with cell size of about 300 nm) effectively mediates cyclic plastic strain with slightly cyclic softening. The elevated stress resistance of DC Cu to cyclic loading is primarily attributed to the high density of built-in low-angle dislocation cells, which are strong and also suppress local surface roughening and crack initiation, thereby contributing to an enhanced fatigue life.

Deformation and metasomatism recorded by single-grain apatite petrochronology

Abstract The timing and processes of ductile deformation and metasomatism can be documented using apatite petrochronology. We integrated microstructural, U-Pb, and geochemical analyses of apatite grains from an exhumed mylonitic shear zone in the St. Barthélémy Massif, Pyrenees, France, to understand how deformation and metasomatism are recorded by U-Pb dates and geochemical patterns. Electron backscatter diffraction (EBSD) analyses documents crystal plastic deformation characterized by low-angle boundaries (<5°) associated with dislocation creep and evidence of multiple slip systems. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb maps indicate that dates in deformed grains reflect, and are governed by, low-angle dislocation boundaries. Apatite rare earth element (REE) and U-Pb behavior is decoupled in high-grade gneiss samples, suggesting REEs record higher-temperature processes than U-Pb isotopic systems. Apatite from (ultra)mylonitic portions of the shear zone showed evidence of metasomatism, and the youngest dates constrain the age of metasomatism. Collectively, these results demonstrate that crystal plastic microstructures and fluid interactions can markedly change apatite isotopic signatures, making single-grain apatite petrochronology a powerful tool for dating and characterizing the latest major deformation and/or fluid events, which are often not captured by higher-temperature chronometers.

Microstructural evolution in Mg-3Gd during accumulative roll-bonding

An Mg-3Gd (wt. %) alloy has been deformed by accumulative roll bonding through four cycles to an equivalent strain of 3.2. The deformed microstructure has been fully characterized by advanced electron microscopy techniques and the mechanical properties are determined by tensile testing. Three characteristic microstructures have been identified and characterized as: cell blocks, twin blocks and nanograins, all with boundary spacings of 100–200 nm. These structures illustrate a structural subdivision by low angle dislocation boundaries and high angle boundaries, which shows a clear resemblance with deformation microstructures in fcc and bcc metals deformed to high strains. The structural evolution has its origin in dislocation and twinning based plasticity which is quantified. The structure-strength relationship of this low-alloyed Mg-3Gd is analysed.

Pt-20Rh dispersion strengthened by ZrO2 - Microstructure and strength

Pt-20Rh dispersion strengthened by ZrO2 particles in different volume fractions has been produced by consolidation of milled powder which has been internally oxidized. The microstructure has been characterized by scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy; tensile properties have been determined at RT and 1000 °C. The Pt-20Rh matrix is subdivided on the submicrometer scale by low angle dislocation boundaries and high angle boundaries. ZrO2 particles with sizes in the range 5–35 nm are uniformly distributed and with increasing volume fraction the flow stress increases and at 0.9% it is doubled both at RT and at 1000 °C. Based on the superposition of particle and matrix strengthening, the flow stress has been calculated and good accord has been found with experiment both at RT and 1000 °C.

Strong pinning of triple junction migration for robust high strain nanostructures

ABSTRACTThe universality of a key recovery mechanism: triple junction migration in high strain nanostructures is revealed herein. This migration is the only means to uniformly coarsen deformed lamellar microstructures. Migration of medium to high angle geometrically necessary boundaries at triple junctions is resisted by strong pinning phenomena. Pinning by low angle dislocation boundaries is the novel mechanism that greatly adds to the solute drag of these higher angle boundaries during migration at triple junctions. Solutes furthermore cause a significant increase in the dislocation density of the low angle boundaries formed during deformation and thus greatly enhance the observed pinning. Boundary pinning by dislocation boundaries and solute drag is analysed for deformed Ni of different purities via in and ex situ electron microscopy. A kinetic model is utilised to obtain activation energies that quantitatively demonstrate the strength of this pinning. A new strategy for achieving robust nanostructured metals is developed based on solute and dislocation pinning of triple junction migration – a universal recovery mechanism in deformed lamellar microstructures.

Strengthening Mechanisms in Ultrafine-Grained and Sub-grained High-Purity Aluminum

This study investigates the effect of grain/sub-grain size, boundary misorientation, and dislocation density on mechanical properties of nanostructured aluminum. A fully recrystallized high-purity aluminum was deformed to different strains from low to ultrahigh strains by a combination of conventional cold rolling and accumulative roll-bonding, followed by annealing for recovery and structural coarsening, to produce sub-grained samples dominated by low-angle boundaries and ultrafine-grained samples dominated by high-angle boundaries. The ultrafine-grained samples showed unusual discontinuous yielding and had a very high strength, which was positively deviated from the extrapolation of the Hall–Petch curve in coarse grains. On the other hand, sub-grained samples showed continuous yielding, and the strength was lower than that of ultrafine-grained samples at the same structural size. It is suggested that in the ultrafine-grained samples, due to lack of dislocation sources in the grains, extremely high stress is required for yielding, which is responsible for the unexpected discontinuous yielding and extra Hall–Petch strengthening. On the other hand, in the sub-grained samples, dislocations in the low-angle dislocation boundaries may act as active dislocation sources, leading to a lower yield stress.

Surface severe plastic deformation induced solute and precipitate redistribution in an Al-Cu-Mg alloy

The precipitate and solute distribution in a surface severely plastically deformed Al-Cu-Mg alloy were revealed by means of transmission electron microscopy and atom probe tomography. Accompanying gradient grain structure, there is a gradient precipitate architecture which evolves from undeformed needle-like S (Al2CuMg) precipitates to shear-deformed and fragmented S precipitates along low angle dislocation boundaries within coarse grains, to reprecipitated Gunier-Preston-Bagaryatsky (GPB) zones and S precipitates in lamellar nanograins, and to completely dissolved and segregated solutes to nanograin boundaries. Dislocation activities induced precipitate dissolution and promoted solute diffusion are two concomitant processes during precipitate and solute redistribution. The variation of precipitate crystallography, including orientation relationship deviation and coherency loss between S precipitates and Al matrix, was attributed to local changes in strain fields around undissolved S precipitates and in nucleation environment for reprecipitated S phase.

The depth-dependent gradient deformation bands in a sliding friction treated Al-Zn-Mg-Cu alloy

Surface sliding friction treatment of Al-Zn-Mg-Cu alloy generally introduces a large number of depth-dependent gradient deformation bands (DBs), which are approximately parallel to {111}Al and penetrated deeply into the coarse grain matrix. The DB segment adjacent to the ultrafine grain (UFG) layer is composed of a string of nanograins and generally shows a bamboo-like morphology. The intersected nanograin boundaries (INBs) and parallel DB boundaries (DBBs) are more often low angle dislocation boundaries. Compared with precipitates in the surrounding matrix, the precipitates at DBBs are much coarser, whereas those within DBs are smaller and scarcer. With increasing depth from the UFG layer, the INBs gradually disappear along with the decrease in the size differentiation between precipitates within DBs and at DBBs. Such deformation banding induced grain refinement and precipitate redistribution were fundamentally attributed to intense dislocation activities and possible dynamic recovery and recrystallization, which can be significantly affected by the gradient shear strain component as well as the randomly encountered coarse dispersoids and precipitates. Additionally, a variant selection phenomenon during mechanically induced precipitate dissolution process was pointed out, and the crystallographic orientation of equivalent variants with respect to the DBB plane was considered to be responsible for this orientation dependent precipitate dissolution. Deformation banding is of great benefit to work hardening, although there are probably some detrimental effects on mechanical properties.

Surface gradient nanostructures in high speed machined 7055 aluminum alloy

The high speed machining induced surface deformation layer in a 7055 aluminum alloy was investigated by means of transmission electron microscopy (TEM) and precession electron diffraction (PED) assisted nanoscale orientation mapping. The gradient nanostructures were composed of equiaxed and lamellar nanograins and ultrafine grains decorated by coarse grain boundary precipitates (GBPs). The presence of low angle dislocation boundaries, the recrystallized nanograins and ultrafine grains showed direct evidence that dislocation activities and dynamic recrystallization are two dominant grain refinement approaches, while the large size and density differences between GBPs and grain interior precipitates (GIPs) unraveled a prominent precipitate redistribution, which can be accomplished via the thermally and mechanically induced precipitate dissolution, solute diffusion and reprecipitation. The quantitative prediction of solute diffusion in the current machining condition agreed well with the TEM observation results. The crystallographic texture of the surface nanostructured layer was proved to be a mixture of brass, cube and weak rotated cube, the severe but diversified thermomechanical effect of high strain, high strain rate and high temperature shear deformation during high speed machining is responsible for texture development.

High temperature dislocation processes in precipitation hardened crystals investigated by a 3D discrete dislocation dynamics

3D discrete dislocation dynamics is employed to investigate motion of general mixed dislocation segments subjected to high temperature loadings in microstructures with impenetrable particles. The implementations of the model first address several benchmark processes including shrinkage of a glissile dislocation loop driven by self-stresses and an annihilation of mutually interacting co-axial prismatic dislocation loops. In particular, we show that models of the microstructure with planar and/or translational symmetry improve efficiency, speed and stability of the calculations. Our simulations then focus on migration of low angle dislocation boundaries in an array of particles while taking into account all mutual dislocation-dislocation interactions and the action of an externally applied stress. The results show for the first time that the migration of tilt dislocation boundaries in crystals with particles can be associated with threshold stresses. The calculated thresholds are in a good agreement with experimental threshold stresses that characterize creep behaviour of precipitation hardened alloys.

Choice of scalar measure for crystal curvature to image dislocation substructure in terms of discrete orientation data

AbstractStarting from Nye’s tensor, alternative characteristics of crystal curvature indicative of dislocation content are considered subject to very low thickness of investigated matter under the free surface and discreteness of orientation sampling. Analysis within the framework of continuum mechanics, undertaken to allow for such conditions peculiar to the electron backscatter diffraction (EBSD) technique, has shown the variable part of orientations expressed in a vector form to be most sensitive to lattice defects when projected to the free surface plane. Hence, as verified with EBSD data on a grain junction in a low deformed IF steel, magnitude of the projected field allows one to map plastic strains inhomogeneous within grains whereas divergence of this field distinctly images and quantifies low-angle dislocation boundaries formed at low strains.

Multi-disclinational description of pentagonal particles with subsurface layer free of twin boundaries

We present the results on the modelling of structural changes in pentagonal small particles (PSPs) during their growth. We prove that after a certain critical size it becomes energetically favourable for a PSP to form a subsurface layer free of twin boundaries (TBs), which are only typical structural elements for smaller size PSPs. In this layer, the low-angle dislocation boundaries (DBs) are formed. Our calculations of the energy stored in the transformed PSP are based on the disclination model of a PSP, in which the TB junctions, as well as TB–DB junctions are treated as wedge disclinations.

Effect of ausforming and cooling condition on the orientation relationship in martensite and bainite of low carbon steels

A method to determine the orientation relationship (OR) in bainitic and martensitic transformations has been employed that does not use reconstruction of prior austenite grains. It is shown that the method keeps accuracy regardless of orientation non-uniformity induced inside austenite grains by hot deformation. The revealed OR behavior for bainite formed under continuous cooling agrees with its temperature dependence previously found in case of isothermal transformation. Ausforming effect on the OR detected in martensite is presumably due to deformation induced subgrains separated by low-angle dislocation boundaries.

Computer simulation of the formation of fragments with medium-angle boundaries in shear bands

The formation and transformation of low-angle dislocation boundaries into medium-angle grain boundaries are analyzed using computer simulation in the particular case of plastic deformation localized in shear bands. The conditions of appearance of fragments with medium-angle boundaries in shear bands are determined. It is shown that the evolution of low-angle boundaries into medium-angle boundaries in a shear band proceeds due to the suppression of active plastic deformation inside a subgrain by the elastic fields of the disclinations that appear in the subgrain junctions as a result of the misfit of plastic rotations along the subgrain boundaries. When these conditions are met, a subgrain behaves like an undeformable inclusion in a surrounding matrix during continuing deformation and undergoes crystallographic rotation. In this case, the subgrain misorientation increases continuously, which leads to the transformation of initial low-angle dislocation boundaries into medium-angle and, in the limit, high-angle boundaries.

Nanostructuring of Ni by Various Modes of Severe Plastic Deformation

Various modes of severe plastic deformation (SPD), such as high-pressure torsion (HPT) at cryogenic temperature, equal channel angular pressing (ECAP) and dynamic channel-angular pressing (DCAP), have been applied for nanostructuring of Ni, and the thermal stability of the structure obtained has been studied. The nanocrystalline structure with average grain sizes of 80 nm and the microhardness of 6200 MPa is produced by HPT in liquid nitrogen. DCAP and ECAP result in the submicrocrystalline structure of a mixed type, with ultra-fine grains separated by high-angle boundaries along with deformation bands and coarse cells with low-angle dislocation boundaries. The thermal stability of the structures obtained by ECAP and DCAP is approximately the same, and it is higher than after the HPT at cryogenic temperature.

Microstructure strengthening mechanisms in different equal channel angular pressed aluminum alloys

Microstructure characterization and identification of the different nature of boundary formation during severe plastic deformation is the basis for a quantitative analysis of material flow stress. In equal channel angular pressing, the microstructure evolves to form low-angle dislocation boundaries and high-angle boundaries whose origin is different. For this reason, boundary misorientation, size and fraction evolve differently with strain. The Hall–Petch relationship in severe plastic deformed aluminum and aluminum alloys has been extensively discussed by Niels Hansen and others in several published works. It appeared that in such cases, the dislocation boundary strengthening contribution is to be taken into account. This paper deals with further insights into the Hansen's and other authors approach to the Hall–Petch relationship. Present approach is based on a detailed microstructure characterization of the different strengthening contributions in severe plastic deformed aluminum alloys. AA1200, AA3103, AA6000 series, and AA2091 were quantitatively characterized by TEM inspections. The calculated alloys yield strengths were compared to measured tensile yield stresses obtaining a quite satisfactory matching. This, ultimately, confirmed the proposed approach and models. Finally, an experimental value of the hardness-to-yield stress, H/σy, was found for all the studied alloys and discussed accordingly.

Effect of Twin Structure on Strain Dependences of Critical Current

ABSTRACTWe suggest a model explaining nonlinear dependences of critical current Jc in YBCO epitaxial films. Two features of YBCO are taken into account: twin domain structure in orthorhombic phase and the anisotropy of uniaxial strain dependence of Tc. Applied strain changes elementary pinning force of the defects located at low-angle dislocation boundaries between differently oriented twin domains. Account of Tc dependence on strain this leads to approximately parabolic strain behavior of Jc. We have obtained analytical expressions for the “initial strain”, which actually describes a natural misbalance between numbers of grain boundaries separating a and b oriented domains, as well as for “strain sensitivity”, which is determined by Tc dependence on uniaxial a and b strains and by the effective redistribution of vortices. Other experimentally observed effects, such as temperature, magnetic field and two-peak strain dependences of Jc, are shown to be described in the framework of suggested model.

A molecular dynamics study of grain boundary free energies, migration mechanisms and mobilities in a bcc Fe–20Cr alloy

Curvature driven migration of a series of 〈110〉 tilt grain boundaries in a bcc Fe–20Cr alloy is simulated using molecular dynamics to investigate the relationship between the atomic migration mechanism and mobility at medium to high temperatures. The boundaries studied include low angle boundaries (LAGB), high angle boundaries (HAGB) and singular boundaries, such as coherent twins. The steady-state boundary shape and curvature are compared with a simple analytical model which incorporates the dependence of absolute mobility and free energy on boundary inclination. The comparison indicates that the 109.5° (11¯2) Σ3 coherent twin boundary will have relatively low energy but high mobility. This result is attributed to a particularly effective repeated shuffle mechanism which occurs on the twinning plane. Two other migration mechanisms are observed, one involving the motion of 〈111〉 glissile dislocations in LAGB and the other involving uncorrelated atomic shuffles in HAGB, sometimes associated with interfacial steps.

Mechanisms of defect formation in ingots of 4H silicon carbide polytype

The methods of optical microscopy and X-ray diffractometry have been used to study the features of defect structure in ingots of the SiC-4H polytype; the ingots have featured different diameters and have been grown by the modified Lely method on seeds with deviations of several degrees from the exact orientation (0001)C in the direction 〈11 $$ \bar 2 $$ 0〉 (off-cut (0001) seeds). The slip bands observed in the crystals are extended along the [11 $$ \bar 2 $$ 0] direction and correspond to the secondary slip system of threading dislocations a/3〈11 $$ \bar 2 $$ 0〉{ $$ \bar 1 $$ 100} for hexagonal close packing (HCP) crystals. Low-angle dislocation boundaries directed along [1 $$ \bar 1 $$ 00] accommodate the disorientation of neighboring domains, which results from their mutual rotation around the [0001] axis. Enlargement of ingots leads to some increase in the dislocation density, mainly due to threading edge dislocations. The average density of micropipes is in the range of 5–20 cm−2 and practically remains unchanged as the ingot size is increased.