Emission Of Lattice Dislocations Research Articles

Hot Tensile Deformation Mechanism and Fracture Behavior of the ZW31/PMMC Laminate

In this work, a Mg-Zn-Y (ZW31) alloy with good plasticity was introduced into 10 μm 10 vol% SiCp/AZ91 composite materials (PMMCs) via the extrusion compound method, and then the ZW31/PMMC laminate was prepared via multi-pass hot rolling. The hot deformation mechanism and elevated temperature tensile fracture mechanism of ZW31/PMMC laminates were studied using the elevated temperature tensile test. The elevated temperature deformation mechanism is influenced by the strain rate. At low strain rates, grain boundary slip is the primary elevated temperature deformation mechanism of the ZW31/PMMC laminate. However, at high strain rates, the activation of pipeline diffusion is facilitated by the particle deformation zone (PDZ) in the PMMC layer with a high dislocation density, leading to the dominance of dislocation climbing as the main mechanism for elevated temperature deformation of the laminate. Additionally, the implementation of a ZW31/PMMC laminate structure effectively inhibits the initiation and propagation of cavities and microcracks within the laminate layer along the normal direction (ND) while simultaneously blunting crack tips via lattice dislocation emission toward the ZW31 layer. Upon cracking of the PMMC layer, stress concentration occurs in the fracture area of the ZW31 layer, ultimately resulting in necking-induced detachment.

Atomistic modeling of interface strengthening in Al-Si eutectic alloys

Al-Si cast alloys are usually composed of α-Al and Al-Si eutectic. Si flakes and Al matrix generally hold cube-on-cube orientation relationship with the primary interface (111)Al∥(111)Si. Extensive experimental studies demonstrated that Si flakes cannot significantly improve mechanical properties of Al-Si cast alloys. We hypothesize that the weak strengthening effect associated with Si flakes might be attributed to thermomechanical properties of Al-Si interfaces besides their morphologies. To characterize Al-Si interfaces with a large lattice mismatch (> 30%), we proposed the quasi-coincident site lattice (Q-CSL) as reference lattice, and demonstrated that the Q-CSL Al-Si coherent interface has three characteristic coherent structures, one stable and low energy structure and two metastable and high energy structures. The translation vectors for the same type of coherent Q-CSL structures are consistent with three displacement shift complete (DSC) vectors. The two metastable structures can be obtained by shifting the low energy structure with three partial DSC vectors. Semi-coherent interface is composed of the low energy Q-CSL patches and three sets of interface misfit dislocations with Burgers vectors same as the DSC vectors. Atomistic simulations revealed that Al-Si interface exhibits low shear resistance. Ideal shear strength of the Q-CSL coherent interface is 110 MPa and semi-coherent interface is 20 MPa. The low shear resistance is attributed to the glide of interface misfit dislocations. Al-Si interface also exhibits low formation and migration energies of point defects. Owing to low shear strength and low formation and migration energies of point defects, interface sliding or shear readily happen under mechanical loading or during dislocation-interface interactions. Lattice dislocations can cross slip onto or climb along Al-Si interfaces. These reactions decrease the number of accumulated dislocation loops around Si flakes and promote nucleation and emission of lattice dislocations from Al-Si interfaces to matrix, consequently reduce the repulsive force on approaching dislocations and weaken Si flakes strengthening effect. In situ tension and compression tests in a scanning electron microscope reveal relatively weak strengthening effect due to Si flakes, consistent with the computed dislocation interaction with interfaces and shear behavior of interfaces.

Mechanism of Implementation of High Ductility in Ultrafine-Grained Aluminum after Annealing and Subsequent Deformation

A theoretical model that describes the implementation of high ductility in high-pressure torsion (HPT) processed ultrafine-grained aluminum subjected to annealing and additional HPT deformation, is suggested. In the framework of the model, the plastic deformation is realized through the successive emission of lattice dislocations from triple junctions of grain boundaries due to splitting of head dislocations in pile-ups of grain boundary dislocations, which are pressed by an external shear stress against these triple junctions, and their subsequent glide in grain interior and climb along surrounding grain boundaries. The energy characteristics and the critical stresses of dislocation emission have been determined for ultrafine-grained aluminum subjected to annealing with small additional HPT deformation. The theoretical stress–strain curves demonstrate good agreement with experimental data available in the literature.

Crossover from Deformation Twinning to Lattice Dislocation Slip in Metal–Graphene Composites with Bimodal Structures

Theoretical model is suggested, which describes of a new micromechanism of crossover from deformation twinning to lattice dislocation slip in metal–graphene nanocomposite with a bimodal structure. In the framework of the model, the lattice dislocation slip occurs through emission of lattice dislocations from the disclinated grain boundary fragments between a nanocrystalline metal–matrix and large (micrometer-size) grains providing the plastic deformation of bimodal metal–graphene nanocomposite. It is shown that the lattice dislocation emission serves as an effective stress relaxation channel being in competition with nanocrack generation.

Mechanism of Hardening of Ultrafine-Grained Aluminum after Annealing

A theoretical model is proposed that describes the hardening mechanism of ultrafine-grained aluminum obtained by high pressure torsion after low-temperature annealing. Within this model, the hardening is due to the successive transformation of the grain-boundary dislocation structure. In particular, plastic deformation is occurs through the emission of lattice dislocations from triple junctions of grain boundaries containing pile-ups of grain-boundary dislocations, the subsequent sliding of lattice dislocations in the grain body, and the formation of walls of climbing grain-boundary dislocations along opposite grain boundaries. The energy characteristics and critical stresses for emission of lattice dislocations are calculated. Theoretical dependences of the flow stress on the plastic strain, which demonstrate good qualitative and quantitative agreement with experimental data, are constructed.

Strength enhancement induced by grain boundary solute segregations in ultrafine-grained alloys

A model is proposed explaining enhanced strength of ultrafine-grained alloys that contain grain boundary (GB) solute segregations. In the framework of the proposed model these segregations are treated as homogeneous ellipsoidal inclusions and act as the sources of elastic stresses affecting the emission of lattice dislocations from GBs. These segregations pin the ends of lattice dislocation segments at the initial stage of dislocation propagation along GBs, and the unpinning requires a load increase, leading to the enhanced yield strength. We calculate the contribution of GB segregations to the yield strength for the ultrafine-grained 1570 Al alloy. We demonstrate that the maximum yield strength of this alloy is achieved in the case of clustered, nearly spherical Mg segregations with a high Mg concentration and a diameter to thickness ratio of 1.0–1.4, depending on the Mg concentration inside segregations. We also briefly discuss the possible role of GB dislocations in the formation of such concentrated solute segregations as well as the influence of GB segregations on the strengthening of alloys containing nanoscale twins. The results of the calculations agree well with experimental data.

Nanoscale amorphization effect on dislocation emission from an elliptical blunt crack tip in deformed nanocrystalline and ultrafine-grained materials

A theoretical model is established to describe the effect of nanoscale amorphization on the lattice dislocation emission from an elliptical blunt crack tip in deformed nanocrystalline and ultrafine-grained materials. Within the description, the nanoscale amorphization occurs through the splitting transformation of grain boundary disclinations which are produced by grain boundary sliding. The criterion for the first dislocation emission form the elliptical blunt crack is derived. The influence of the nanoscale amorphization and the features of the elliptical blunt crack on the critical stress intensity factors for the dislocation emission are discussed in detail. The obtained results show that the nanoscale amorphization can either enhance or weaken the critical applied SIFs for dislocation emission, depending on the emission angle, the radius of curvature of the elliptical blunt crack and the distance between the nanoscale amorphization and the crack tip. There is a critical crack-junction distance making the dislocation emission most difficult and at this critical crack-junction distance, the stress-release effect of the nanoscale amorphization is strongest.

Механизм упрочнения ультрамелкозернистого алюминия после отжига

A theoretical model is proposed that describes the mechanism of hardening of ultrafine-grained aluminum, obtained by severe plastic torsion deformation, after low-temperature annealing. In the framework of the model, hardening is realized due to the sequential transformation of the grain-boundary dislocation structure. In particular, plastic deformation occurs through the emission of lattice dislocations from triple junctions of grain boundaries containing pile-ups of grain-boundary dislocations, the subsequent sliding of lattice dislocations in the bulk of the grain, and the formation of walls of grain-boundary dislocations climbing along opposite grain boundaries. The energy characteristics and critical stresses for the emission of lattice dislocations are calculated. The theoretical dependences of the flow stress on the plastic deformation are plotted, which show good qualitative and quantitative agreement with experimental data.

Atomistic insight into the dislocation nucleation at crystalline/crystalline and crystalline/amorphous interfaces without full symmetry

Misfit dislocations at bimetal interfaces play a decisive role in determining various deformation behaviors by carrying the shear sliding, serving as a barrier for dislocation transmission and a source of dislocation nucleation. However, when the interface does not possess the distinct feature of misfit dislocations, the nucleation mechanism of lattice dislocations at the interfaces cannot be simply quantified by previously developed atomistic mechanisms based on characteristic misfit dislocations. Using crystalline/crystalline interfaces with a large lattice mismatch and crystalline/amorphous interfaces without local symmetry as prototypes, we show for the first time that the dislocation nucleation at such interfaces is attributable to the localized strain heterogeneities by modifying the volumetric and shear strain components at the atomic level to mechanically respond to different loadings. Using atomic strain tensor analysis, we found that in-plane localized shearing plays a critical role in the emission of lattice dislocations from interfaces, while the corresponding normal components of the volumetric strain tensor will dominate the character of the nucleated lattice dislocation by modifying the atomic excess volume at the interface to overcome the barrier to dislocation nucleation. Further exploration of various crystalline/amorphous interfaces by varying the chemical composition of the amorphous side indicates that chemical heterogeneity may substantially change the strain heterogeneity by forming a different clustered structure at the interface, resulting in the preferred choice of nucleation sites at the boundary regions that can be defined as nano shear traces (NSTs). These results provide a foundation to investigate the effects of strain and chemical heterogeneities in order to provide a realistic explanation of interface mediated deformation mechanisms and an efficient solution to tune interface dominated plasticity.

Effect of nanotwin and dislocation pileup at twin boundary on dislocation emission from a semi-elliptical blunt crack tip in nanocrystalline materials

The effects of the nanotwin and dislocation pileup at the twin boundary on lattice dislocation emission from a surface semi-elliptical blunt crack in deformed nanocrystalline materials (NCMs) are investigated. The nanotwin as a stress source can be described by a wedge disclination quadrupole. Using the complex variable method, complex analytic solution of stress field, the dislocation force and the critical stress intensity factors (SIFs) for the first edge dislocation emission were obtained. Then, through numerical calculation, the influence of twin size, twin orientation, twin position, twin strength and dislocation pileup, the curvature radius of blunt crack tip on the critical stress intensity factors were discussed in detail. The results show that the nanotwin and dislocation pileup has significant influence on the dislocation emission from the blunt crack tip. The presence of nanotwin and dislocation pileup will increase the most probable emission angle, and increase the critical SIFs for dislocation emission, making it harder for the dislocation to emit from the blunt crack tip and thus to decrease the toughness of materials produced by dislocation emission.

Hardening by Annealing and Implementation of High Ductility of Ultra-Fine Grained Aluminum: Experiment and Theory

Abstract The influence of low temperature annealing and subsequent deformation on microstructure, strength and ductility was investigated for the first time for high pressure torsion (HPT) processed commercially pure Al. Extremely high increases in the conventional yield stress (up to 50%) and ultimate tensile strength (up to 30%) were obtained by annealing of the ultrafine grained (UFG) samples in the range 90-200 °C for 1 h. Such increases were accompanied by a sharp drop in ductility up to 1%. Implementation of high ductility at the level of coarse-grained Al, while maintaining high strength of the HPT-processed sample was demonstrated for the first time and achieved by repeating the low temperature annealing followed by subsequent additional HPT deformation. The key role of relaxation of non-equilibrium high-angle grain boundaries (GBs) in the strengthening effect of UFG-Al by annealing is shown. Two theoretical models are suggested to explain the hardening by annealing and the implementation of high ductility in UFG structures. Within the models, plastic deformation occurs through emission of lattice dislocations from triple junctions of GBs containing pile-ups of grain-boundary dislocations, glide of the lattice dislocations across neighboring grains, their accumulation at and climb along the opposite GBs. The energy characteristics and the critical stresses of dislocation emission are determined in two different cases, for UFG Al subjected to annealing only and to annealing with subsequent additional HPT deformation. The calculated theoretical dependences of the flow stress on the plastic deformation value well fit qualitatively and quantitatively to our experimental data.

Grain boundary sliding, triple junction disclinations and strain hardening in ultrafine-grained and nanocrystalline metals

A theoretical model is suggested which describes grain boundary (GB) sliding and its accommodation through dislocation slip in ultrafine-grained and nanocrystalline metals. The initial stage of the accommodating dislocation slip represents emission of lattice dislocations from triple junctions into grain interiors. The lattice dislocations emitted from a triple junction slip across a grain and are absorbed by an opposite GB where they are dissociated into GB dislocations that climb along the GB. In the situation where these GB sliding and accommodating processes are dominant, stress-strain dependences are calculated in ultrafine-grained copper. With the calculated dependences, we found that pronounced strain hardening occurs which is related to the accommodation processes and associated formation of disclinations at triple junctions of GBs. It is theoretically revealed that the special (new) strain hardening mechanism under discussion can play a significant role in enhancing ductility of ultrafine-grained and nanocrystalline metals at comparatively low temperatures.

Effect of cooperative grain boundary sliding and migration on dislocation emission from a branched crack tip in deformed nanocrystalline solids

A theoretical model is established to describe the effect of cooperative grain boundary (GB) sliding and migration on dislocation emission from the tip of branched crack in deformed nanocrystalline solids. The explicit solutions of complex potentials are obtained by means of complex variable method and conformal mapping technique. The critical stress intensity factors (SIFs) for the first lattice dislocation emission from the tip of branched crack are calculated. The effects of the lengths of branched crack and main crack, and the angle between their planes on the critical SIFs for dislocation emission are evaluated in detail. The results indicate that the emission of lattice dislocations from the tip of branched crack is strongly influenced by cooperative GB sliding and migration. When main crack approaches the branched crack, dislocation emission from the tip of branched crack will be suppressed. The main crack tends to propagate while shorter branched crack is prone to be blunted by emitting lattice dislocations from its tip. As a special case, when the planes of main crack and the branched crack are flattened out into one, the present results are in good agreement with previously known results.

Effect of special rotational deformation on the dislocation emission from a branched crack tip in deformed nanocrystalline materials

The problem of the special rotational deformation interacting with an internal crack is studied by developing a theoretical model of deformed nanocrystalline materials. Using the complex variable method of Muskhelishvili and the conformal mapping technique, the expressions of complex potentials and stress fields are obtained analytically. The stress intensity factors (SIFs) near the crack tips and the critical SIFs for the first lattice dislocation emission from the branched crack tip are calculated. The effects of important parameters such as grain size, the length of internal crack, and the angle between the main crack and its branched crack on the critical SIFs for dislocation emission are evaluated in detail. As a result, the special rotational deformation has great influence on the growth of internal crack and the emission of lattice dislocations from the branched crack tip. The disclination quadrupole produced by the special rotational deformation will shield the branched crack tip under a certain condition. Moreover, when the main crack approaches its branched crack, it will stop the emission of lattice dislocations from the branched crack tip.

Dislocation emission from interphase boundaries in deformed nanocomposites

A theoretical model for emission of lattice dislocations from small-angle interphase boundaries characterized by both orientational and dilatational misfit in deformed nanocomposites is proposed. With allowance for the free surface of the material, the forces acting upon the dislocation structures of the interphase boundaries are calculated, through which the dependences of the critical shear stress for dislocation emission on different parameters of the boundary are found. It is shown that the influence of dilatational misfit and proximity of the interphase boundary to the free surface on dislocation emission is insignificant. It is established that the ability of interphase boundaries to emit dislocations is not uniform: emission of certain dislocations is facilitated as compared to ordinary small-angle grain boundaries, while emission of other dislocations may be inhibited.

Effect of special rotational deformation on dislocation emission from interface collinear crack tip in nanocrystalline bi-materials

The work is devoted to investigate the interaction between the special rotational deformation and interface collinear cracks in nanocrystalline bi-materials. As an illustrative example, the effect of the disclination quadrupole produced by the special rotational deformation on the emission of lattice dislocation from a finite interfacial crack tip in nanocrystalline bi-material is explored theoretically using the complex variable method. The complex form expression of dislocation force and the critical stress intensity factors for the first edge dislocation emission under remote mode I loadings and mode II loadings are deduced. And the influences of material properties, grain size, disclination strength, disclination location and orientation, special rotational deformation orientations, and crack length on the critical stress intensity factors are discussed in detail. The results show that the special rotational deformation and the relative shear modulus of the upper the lower half plane have great effect on the lattice dislocation emission from the interface collinear crack tip.

Influence of nanoscale amorphization on emission of dislocations from a finite length crack tip in nanocrystalline materials

The interaction between nanoscale amorphization and crack in deformed nanocrystalline materials is investigated. Effects of nanoscale amorphization characterized by three disclination dipoles on the emission of lattice dislocations from the crack tip are theoretically investigated. The critical stress intensity factors (SIFs) for dislocation emission from the crack tip are calculated. The influences of nanoscale amorphization as well as crack length on the critical SIFs are discussed in detail. The results show that nanoscale amorphization can suppress lattice dislocation emission. The most probable angle of dislocation emission decreases with increment of disclination strength and the shorter length crack drivers easier the dislocation emission from the crack tip.

A coupling crack blunting mechanism in nanocrystalline materials by nano-grain rotation and shear-coupled migration of grain boundaries

A theoretical model has been developed to investigate the effect of a special cooperative process between two main modes of stress-driven nano-grain growth, i.e., nano-grain rotation and shear-coupled migration of grain boundaries, on the emission of lattice dislocations from a semi-infinite crack in nanocrystalline materials. The results obtained show that the above-mentioned collaborative process is highly conducive to the process of dislocation emission, thus, rendering strong crack blunting in nanocrystalline materials.

Strong crack blunting by shear-coupled migration of grain boundaries in nanocrystalline materials

A theoretical model is proposed to illustrate the effect of shear-coupled migration of grain boundaries on the emission of lattice dislocations from a semi-infinite crack tip in nanocrystalline materials. The results obtained show that the shear-coupled migration process is able to considerably enhance the capability of the crack to emit dislocations, thus leading to strong crack blunting. Moreover, the combination of grain boundary migration and dislocation emission can serve as an effective toughening mechanism in nanocrystalline materials. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Effect of cooperative grain boundary sliding and migration on dislocation emitting from a semi-elliptical blunt crack tip in nanocrystalline solids

A theoretical model is established to investigate the interaction between the cooperative grain boundary (GB) sliding and migration and a semi-elliptical blunt crack in deformed nanocrystalline materials. By using the complex variable method, the effect of two disclination dipoles produced by the cooperative GB sliding and migration process on the emission of lattice dislocations from a semi-elliptical blunt crack tip is explored. Closed-form solutions for the stress field and the force acting on the dislocation are obtained in complex form, and the critical stress intensity factors for the first dislocation emission from a blunt crack under mode I and mode II loadings are calculated. Then, the influence of disclination strength, curvature radius of blunt crack tip, crack length, locations and geometry of disclination dipoles, and grain size on the critical stress intensity factors is presented detailedly. It is shown that the cooperative GB sliding and migration and the grain size have significant influence on the dislocation emission from a blunt crack tip.