Motion Of Grain Boundary Dislocations Research Articles

Re-crystallization mechanism of superplasticity of industrial titanium and nickel alloys

The results of the study are presented, showing that the mechanism of superplastic deformation of industrial titanium and nickel alloys is dynamic recrystallization (fluctuation-structural transition), which leads to a steady segmental (vacancy-segmental) flow of the material when the sample is stretched. The results obtained confirm the fallacy of the concept of the relationship between grain boundary sliding and the motion of grain boundary dislocations during superplastic deformation and the doubtfulness of the physical validity of the velocity sensitivity coefficient as a criterion for superplasticity. Keywords: superplasticity, grain boundary sliding, dynamic recrystallization, fluctuation-structural transition, velocity sensitivity coefficient, polycrystalline superplastic viscosity. Onischenko-ak@uecrus.com

Nanoparticle Alignment: Quantized Grain Boundary States Promote Nanoparticle Alignment During Imperfect Oriented Attachment (Small 29/2020)

In article number 2001423, Selim Elhadj and co-workers offer new insights into the crystallographic alignment of nanoparticles during self-assembly and aggregation. The salient details reveal that grain boundaries between small particles (0–10 nm) rapidly disintegrate via deformation processes due to the quantization of energetically preferred grain boundary structures and the existence of small barriers associated with the motion of grain boundary dislocations to nearby surfaces.

Effect of ultrasound on microstructure evolution of friction stir welded aluminum alloys

The dynamic recrystallization (DRX) in ultrasound affected friction stir welding (UFSW) was investigated using Al–Cu alloy and compared with another joint produced by the same setup but without applying the ultrasound. The effect of the ultrasound on the evolution of recrystallized grains was significantly dependent on the local thermal cycle. For the stir zone (SZ) exposed to high temperatures, both the nucleation rate and growth rate of equiaxed grains during DRX were significantly promoted by the ultrasound. Compared with the SZ without the effect of the ultrasound, the bulging frequency of the local high angle boundaries in the SZ of UFSW joint was increased to 2–4 times, producing a considerable number of fine nuclei. Excess vacancies introduced by the ultrasound were the intrinsic reason for the mentioned phenomena. The sub-grain mobility was radically accelerated due to the enhanced dislocation climb by the excess vacancies and thus the normalized nucleation criterion in the UFSW was reduced, contributing to a larger nucleation rate during DRX. Grain boundary sliding and migration were both facilitated by the ultrasound due to the enhanced movement of grain boundary dislocations and grain boundary self-diffusion, leading to a larger growth rate of equiaxed grains in the SZ of UFSW joint.

Mechanical energy losses in commercial crosslinked low‐density polyethylene in the temperature range between 200 and 400 K

ABSTRACTAging treatments in water and in air and controlled neutron irradiation were performed on commercial crosslinked low‐density polyethylene (XLPE) for promoting different mesostructural arrangements of crystallites and crosslinking degree. Infrared spectroscopy, differential scanning calorimetry, differential thermal analysis, thermogravimetry, and dynamic mechanical analysis were used as characterization techniques. The relaxation peak related to the mobility of the grain boundaries from crystalline zones in XLPE was identified at around 260 K (at 7 Hz), involving an activation energy of 90 ± 4 kJ mol−1. The usual equation for describing the grain boundary mobility in metals involving the movement of grain boundary dislocations was adapted for studying the mobility of the boundaries among the crystalline zones, successfully. In addition, a new mechanical relaxation peak that appears at around 300 K (at 7 Hz), which involves an activation energy of 94 ± 5 kJ mol−1, was found. The driving force controlling this peak was determined as the dragging of the polymer chains at the amorphous zones adjacent to the crystals controlled by the mobility of the crystallites boundaries. The chains movement was done with break away from the physical pinning points. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47605.

Mechanisms of plastic deformation in ultrafine-grained aluminium – In-situ and ex-post studies

The microstructure of a 1050 aluminium alloy produced by hydrostatic extrusion varies in terms of grain boundary characteristics and the dislocation substructure depending on the grain orientation. This leads to a variance of plastic deformation mechanisms under external load. In this paper, the microstructure of as-extruded samples was compared to extruded and deformed in a bulk compression test to follow the reaction of various grains to external strain. In-situ TEM straining experiments were performed to study the variance of mobile dislocation activities depending on the local dislocation substructure in as-extruded material to deduce the operative deformation mechanism. These experiments accompanied with an estimation of strengthening mechanisms allowed to explain the role of different grains in the plastic deformation of ultrafine grained aluminium treated as a heterogenous complex system. It is demonstrated that well-developed ultrafine grains are responsible for providing strength since no intergranular dislocation intersections were reported but the motion of grain boundary dislocations or dislocation annihilation in boundaries. At the same time, relatively large grains with well-developed dislocation substructures accommodate plastic strain by provoking a more complex reaction – unstable dislocation arrays collapse while more advanced structures evolve into new low angle boundaries. The results from in-situ experiments were also used to explain subgrain shape changes observed after bulk deformation.

Sliding and Migration of Tilt Grain Boundaries in a Mg–Zn–Y Alloy

Sliding and migration of tilt grain boundaries in a Mg–Zn–Y alloy have been investigated on the atomic scale using aberration‐corrected scanning transmission electron microscopy. Grain boundary sliding is accommodated by non‐basal dislocations moving along the grain boundary; grain boundary migration is induced by the motion of grain boundary dislocations with synchronized grain boundary diffusion. Simultaneous sliding and migration of tilt boundaries take place in both Mg matrix and long period stacking ordered phases. These results provide evidence for occurrence of grain boundary motion, which may play a role in plasticity of this kind of Mg alloys.

Reversal in the Size Dependence of Grain Rotation.

The conventional belief, based on the Read-Shockley model for the grain rotation mechanism, has been that smaller grains rotate more under stress due to the motion of grain boundary dislocations. However, in our high-pressure synchrotron Laue x-ray microdiffraction experiments, 70nm nickel particles are found to rotate more than any other grain size. We infer that the reversal in the size dependence of the grain rotation arises from the crossover between the grain boundary dislocation-mediated and grain interior dislocation-mediated deformation mechanisms. The dislocation activities in the grain interiors are evidenced by the deformation texture of nickel nanocrystals. This new finding reshapes our view on the mechanism of grain rotation and helps us to better understand the plastic deformation of nanomaterials, particularly of the competing effects of grain boundary and grain interior dislocations.

Modeling the strain-hardening effect and plastic deformation of nanocrystalline FCC metals dispersed with the finest grains

A mechanism-based plasticity model focusing on deformation and failure behavior of nanocrystalline (NC) FCC metals dispersed the finest grains (FGs) is proposed. The stress–strain relationship is derived by invoking the impeding effect of the FGs on the movement of grain boundary dislocations along the grain boundaries and the unique role of nanocracks into the constitutive model. A strain-based cumulative distribution function is also included in the present model to account for the contribution of grain boundaries to the local flow stress due to grain refinement during plastic deformation. It is found that the interaction between dislocations and the FGs contributes to the strain-hardening property and the failure behavior is attributed to the nanocrack evolution. Numerical results show that the proposed model can successfully describe the enhanced strength and ductility of the NC metals in previous experimental literatures. It is further noted that the strength and ductility are sensitive to the volume fraction of the FGs and the density of nanocracks. This quantitative continuum plasticity model can be utilized to optimize the mechanical properties of the nanostructured metals by tuning the material structure parameters.

Phase field crystal modeling of grain rotation with small initial misorientations in nanocrystalline materials

The process of grain rotation with small initial misorientation angles in nanocrystalline materials is investigated by using the phase field crystal (PFC) method. Simulation results reproduce the grain rotation process of nanocrystalline grains and are consistent with the classical coupled motion theory excellently. The grain rotation with very small initial misorientations, which cannot be illustrated via the Frank formula because of the large elastic distortion in the grain, is reasonably described by the strain field analysis. It is also revealed that the significant elastic distortion induced by grain boundaries (GBs) determines the rotational dynamics; moreover, the movement of grain boundary dislocations (GBDs) plays a dominant role in the evolution of elastic distortions which results in the grain rotation.

A novel superplastic mechanism-based constitutive equation and its application in an ultralight two-phase hypereutectic Mg–8.42Li alloy

A superplasticity mechanism based constitutive equation incorporating deformation-induced grain growth and deformation-accelerated grain boundary diffusion was established. The constitutive equation established was applied to the superplastic two-phase hypereutectic Mg–8.42Li alloy. The contribution of net static grain growth to entire grain growth accounts for 51.65% while the contribution of net strain grain growth to entire grain growth accounts for 48.35%. It is shown by calculation that the grain boundary diffusion in the superplastic deformation of fine grained microduplex Mg–8.42Li alloy at low strain rates is not accelerated obviously and deformation-accelerated grain boundary diffusion takes place at high strain rates. It is indicated by normalizing the data of calculation and experiment that the flow mechanism of this alloy at 573K at an initial strain rate of 5×10−4s−1 is modified Mukherjee's grain boundary sliding controlled by the motion of the dislocations in the grain boundaries by a climb-glide process and Gifkins's grain boundary sliding occurring by the motion of grain-boundary dislocations that pile up at triple point.

The role of partial grain boundary dislocations in grain boundary sliding and coupled grain boundary motion

We study the process of grain boundary sliding through the motion of grain boundary dislocations, utilizing molecular dynamics and embedded atom method (EAM) interatomic potentials. For a Σ = 5 [001]{310} symmetrical tilt boundary in bcc Fe, the sliding process was found to occur through the nucleation and glide of partial grain boundary dislocations, with a secondary grain boundary structure playing an important role in the sliding process. While the homogeneous nucleation of these grain boundary dislocations requires shear strain levels higher than 7%, preexisting grain boundary dislocations are shown to glide at applied shear levels of 1.5%. The glide of the dislocations results in coupled motion of the boundary in the directions parallel and perpendicular to itself. Finally, interstitial impurities and vacancies were introduced in the grain boundary to study the effects on the sliding resistance of the boundary. While vacancies and H interstitials act as preferred nucleation sites, C interstitials do not. Both hydrogen and C interstitials stop dislocation glide whereas vacancies do not. A detailed study of the dynamic properties of these dislocations is also presented.

The cyclic fatigue behavior of direct age 718 at 149, 315, 454 and 538 °C

The axial, strain-controlled, cyclic fatigue behavior of direct age alloy 718 is investigated. Ramberg–Osgood (R–O) cyclic stress–cyclic strain relationships were determined at each temperature. The results were used to assess the fatigue life as functions of the R–O alternating plastic strain and R–O alternating stress. As temperature decreased, a departure from Coffin–Manson behavior was observed. The fatigue lives at 149, 315 and 454 °C, plotted as a function of the R–O alternating stress, converge when superimposed and a crossover in the responses is observed. Under fixed alternating stress conditions, the fatigue life below the crossover increases as the temperature increases whilst above, a reversion in the fatigue life temperature dependence is seen. A mechanistic model is proposed which seeks to rationalize the increase in fatigue life, as a function of increasing test temperature, to a thermally activated process that mitigates the generation of substructure defects associated with grain boundary stress concentrations. The model activation energy and activation volume are interpreted to substantiate a mechanism whereby a diffusive flux, away from grain boundary stress concentrations, is enabled by the non-conservative motion of grain boundary dislocations in a grain boundary plane containing obstacles. The model predictions are in reasonable agreement with experimental observations.

Threshold Stress in Creep of Alumina-Silicon Carbide Nanocomposites

Tensile creep and the creep ruptures of an alumina-17 vol% silicon carbide nanocomposite at 1200°C were investigated, with a particular interest in the behaviors at low applied stresses. At stresses of <50 MPa, the creep rates were remarkably decayed and the creep lives were substantially prolonged, which suggested the presence of a threshold stress, below which creep stopped, in the creep of the nanocomposite. The stress was estimated to be in the range of 20–35 MPa. This threshold stress was in agreement with that predicted from another model where the motion of grain-boundary dislocations that were responsible for vacancy nucleation and annihilation was considered to be pinned by hard particles.

Grain Boundary Sliding and Lattice Dislocation Emission in Nanocrystalline Materials under Plastic Deformation

A theoretical model is proposed to describe the physical mechanisms of hardening and softening of nanocrystalline materials during superplastic deformation. According to this model, triple interface junctions are obstacles to glide motion of grain boundary dislocations, which are carriers of grain boundary glide deformation. Transformations of an ensemble of grain boundary dislocations that occur at triple interface junctions bring about the formation of partial dislocations and the local migration of triple junctions. The energy characteristics of these transformations are considered. Pileups of partial dislocations at triple junctions cause hardening and initiate intragrain lattice sliding. When the Burgers vectors of partial dislocations reach a critical value, lattice dislocations are emitted and glide into adjacent grains, thereby smoothing the hardening effect. The local migration of triple interface junctions (caused by grain boundary sliding) and the emission of lattice dislocations bring about softening of a nanocrystalline material. The flow stress is found as a function of the total plastic strain, and the result agrees well with experimental data.

Transformations of grain boundaries in deformed nanocrystalline materials

Theoretical models are suggested which describe transformations of grain boundaries in nanocrystalline materials under plastic deformation. We consider such transformations as decay of low-angle grain boundaries, bowing of high-angle grain boundaries, and emission of partial dislocations by grain boundaries in deformed nanocrystalline materials. In the framework of the suggested model description, lattice dislocations that form a low-angle tilt boundary glide under the action of the forces owing to external (applied) and internal stresses. The balance of the forces causes the critical shear stress at which the low-angle boundary decays. Such decay processes result in the formation of high-density ensembles of mobile lattice dislocations that are capable of inducing plastic flow localization (shear banding) in mechanically loaded nanocrystalline materials. High-angle grain boundaries are modeled as those containing grain boundary dislocations with small Burgers vectors. The movement of grain boundary dislocations under the shear stress action gives rise to bowing of high-angle boundaries. In certain ranges of parameters, grain boundary dislocations undergo splitting transformations followed by emission of partial dislocations from high-angle boundaries into adjacent grain interiors. The models account for experimental data reported in the literature.

Grain Growth During Superplastic Deformation

Significant grain growth occurring during superplastic deformation is related to the micro-mechanism of superplastic flow. Observations performed on the deformed surface of superplastically deformed tensile and shear Pb-62%Sn samples and bi-axially formed AA7475 samples directly indicate that cooperative grain boundary sliding, i.e. sliding of grain groups, is accompanied by cooperative grain boundary migration that can result in an enhanced grain growth. Such a long range correlation in migration of sliding grain boundaries is related to movement of grain boundary dislocations having a step associated with its core. Observed correlation between grain size and strain measured in different regions of a superplastically formed Ti-alloy part and alignment of grain boundaries along shear surfaces support coupling of grain boundary sliding and migration. A model of grain growth considering climb of cellular dislocations, topological defects in a grain array, has been expanded to incorporate gliding and mixed cellular dislocations.

Dynamic internal friction and evaluated activation volume for superplastic deformation in a Zn–5%Al alloy

Abstract The dynamic internal friction (DIF) during superplastic deformation of Zn–5wt.%Al eutectic alloy with an average grain size of 2 μm has been studied at strain rates of 10 −6 –10 −4 s −1 . The activation volumes V a for the superplastic deformation evaluated from the data of DIF are of the order of 10 −27 m 3 , and decrease with the increase of strain rate or applied stress. The results indicate that the elementary process of grain boundary sliding during superplastic deformation is possibly associated with the motion of grain boundary dislocations surmounting the ledges.

High‐Temperature Anelastic and Viscoplastic Deformation of Fine‐Grained Magnesia‐ and Magnesia/Yttria‐Doped Alumina

High‐temperature (1200–1550 K) mechanical‐loss and shear‐modulus measurements were performed on fine‐grained magnesia‐ and magnesia/ytrria‐doped alumina at low frequency (10−4−101 Hz), using a forced‐vibration torsion pendulum. Anelastic and viscoplastic relaxation phenomena were observed, which may be associated with grain‐boundary (GB) sliding and, consequently, may be dependent on the grain size and GB segregation of yttrium. The associated activation enthalpies were 800–1100 kJ/mol. GB precipitation of yttrium aluminum garnet led to enhanced mechanical loss and increased activation enthalpy. Viscoplastic deformation increased sharply as the amplitude of the applied stress increased above a “threshold” value of ∼3 MPa. The mechanical loss and creep rate of identical materials showed qualitatively similar dependence on the grain size and yttrium content. A possible micromechanism of mechanical loss could be the motion of GB dislocations, which is important for the accommodation process of GB sliding.

Analysis of the cooperative grain boundary sliding in terms of cellular dislocations

Abstract Cooperative grain boundary sliding (CGBS) (sliding of grain groups along surfaces formed by sliding grain boundaries), has been considered as a result of rigid shear of grain rows or as sequential shear of grains. The sequential shear of grains has been analysed in terms of movement of cellular dislocations (imperfections in regular grain structure). Localized and extended cellular dislocations are examined. Coupling of CGBS and cooperative grain boundary migration might take place as the result of movement of grain boundary dislocations associated with ledges.

Superplastic deformation carried by grain boundaries

A model of plastic deformation based on grain rolling is proposed for the interpretation of large superplastic strains that may be reached in some metallic as well as ceramic materials in certain ranges of high temperatures and strain rates. The deformation process is modelled by the motion of grain boundary dislocations that glide on the grain facets when sliding takes place and climb on the facets with normal stresses. A constitutive equation for the strain rate is derived for steady state deformation when the emission rate of climbing dislocations is a constant determined by the dislocations climb velocity.