Grain Boundary Dislocation Pile-ups Research Articles

Structural transformation and energy analysis for pile-up dislocations at triple junction of grain boundary

An energy model for the structure transformation of pile-ups of grain boundary dislocations (GBD) at the triple-junction of the grain boundary of ultrafine-grain materials was proposed. The energy of the pile-up of the GBD in the system was calculated by the energy model, the critical geometric and mechanical conditions for the structure transformation of head dislocation of the pile-up were analyzed, and the influence of the number density of the dislocations and the angle between Burgers vectors of two decomposed dislocations on the transformation mode of head dislocation was discussed. The results show when the GBD is accumulated at triple junction, the head dislocation of the GBD is decomposed into two Burgers vectors of these dislocations unless the angle between the two vectors is less than 90°, and the increase of applied external stress can reduce the energy barrier of the dislocation decomposition. The mechanism that the ultrafine-grained metal material has both high strength and plasticity owing to the structure transformation of the pile-up of the GBD at the triple junction of the grain boundary is revealed.

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.

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.

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.

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.

The effects of the finest grains on the mechanical behaviours of nanocrystalline materials

This article proposes a new constitutive model to account for effects of the finest grains, with sizes ranging from 2 to 4 nm, on the mechanical behaviours of nanocrystalline (NC) materials. In this model, the normal nanograins (ranging from 20 to 100 nm) were treated as though they were composed of a grain interior (GI) and a grain boundary (GB) affected zone (GBAZ). The finest grains were considered to be part of the GBAZ, denoted as super triple junctions (STJs). For the initial plastic deformation stage of the NC materials, a phenomenological constitutive equation was suggested to predict the deformation behaviours of the GBAZ. The formation of GB dislocation (GBD) pileups provides dramatic strain hardening in deformed NC materials and thereby enhances their ductility. Then, the constitutive equations to describe the plastic deformation of the GI and the GBAZ lattice region were established. In this stage, the GBAZ are already saturated with GBD pileups, and GI deformation is the dominant mechanism. Finally, the mechanical model for the NC materials with the finest grains was built using the self-consistent method, and an overall moderate “work hardening,” sustained over a long range of plastic strain, was predicted. The effects of TJs/STJs on the deformation mechanism were quantitatively analysed. The analysis demonstrated that the existence of the finest grains will simultaneously lead to good strength and good ductility.

On the capacity of grain boundary dislocation pileups

It is demonstrated on model examples that inclusion of the interaction between pileups of grain boundary dislocations formed in the vicinity of triple grain-boundary junctions leads to a multifold increase in the capacity of these pileups.

Interaction of wedge crack and grain boundary dislocations

Analysis of the interaction between a triple junction wedge crack and the pile-ups of grain boundary dislocations is used to investigate crack nucleation. Through the numerical solutions of a system of integral equations, the rearrangement of the dislocations and the stress redistribution due to the crack formation are fully taken into account in the analysis. Under remote tension, numerical results are obtained for the stress intensity factors, the dislocation populations on the grain boundaries, as well as the equilibrium crack lengths. The dependence of the crack length and the crack stability on the grain boundary orientations and the pile-up lengths is investigated.

Dislocation Model for Continuous Recrystallisation during Initial Stage of Superplastic Deformation

According to dislocation models, grain boundary sliding (GBS) causes stress concentrations when the sliding is impeded. Relaxation of the stress concentrations can be done by the emission of dislocations from one grain boundary to another. This process could be limited by the rate at which the dislocations are emitted (source control), or traverses the grains (glide or lattice climb control), or are absorbed into the boundaries (grain boundary climb control). The rate at which the grains slide past each other can be controlled by (1) the removal of a pile-up of lattice, the emission of lattice dislocations from grain boundary ledges and by the removal of pile-ups of grain boundary dislocations. These models require the grains to have high angle mobile boundaries such that grain boundary sliding processes can occur and are then accommodated by dislocation movements. For some quasi-single phase aluminum alloys, at the initial stage of superplastic deformation most of grain boundaries have low angles. Since movement of these subgrain boundaries needs very high energy, grain boundary sliding becomes difficult. It is widely reported that continuous recrystallization mechanism dominates the initial stage of superplastic deformation in quasi-single phase aluminum alloys.

A theoretical analysis of crack nucleation due to grain boundary dislocation pile-ups in a random ice microstructure

Abstract The nucleation of cracks is generally recognized as the precursor to the macroscopic fracture of many brittle materials. In this paper, the existence of load-induced cracks in polycrystalline S2 ice is investigated theoretically in terms of the pile-ups of grain boundary dislocations around grain triple points. Stress concentrations due to the elastic or thermal expansion anisotropy of the ice crystals are taken into account. Considering a random microstructure, the numerical results show that: (i) both stable and unstable cracks can be nucleated during loading, (ii) the microstructural stresses due to either type of crystal anisotropy obey a Gaussian distribution, (iii) the sizes of the stable cracks obey a gamma-type distribution with a small mean and a large skewness, (iv) the orientations of the stable cracks are approximately uniformly distributed, and (v) the crack statistics is a function of the microstructure and the loading conditions.

Cavity nucleation at high temperatures involving pile-ups of grain boundary dislocations—effect of solutes and impurities

This paper extends the grain boundary dislocation pile-up model [ Acta metall. 35, 1663 (1987)] to address the problem of solute/impurity enhanced cavitation at high temperature. It is shown that solute/impurity enhanced cavitation is a complex phenomenon. This is because solutes/impurities can alter a host of material parameters such as interfacial and surface energies, boundary diffusivity and creep resistance of the materials. Changes in interfacial and surface energies can alter the thermodynamics of cavity formation, while changes in boundary diffusivity and creep resistance can significantly influence the steady state pile-up strength at the interface. These changes would collectively produce a profound effect on the cavitation behavior of the material. The findings show that solutes/impurities which are strongly surface active, retard boundary diffusivity and decrease the creep resistance of the material are most deleterious; they can reduce the threshold stress for cavitation failure of the material by about an order of magnitude.

Cavity nucleation at high temperatures involving pile-ups of grain boundary dislocations

The current concept of sliding-induced cavitation is first reviewed, with due consideration to the respective role of the remote and internal stresses. The results show that the transient stresses produced by sliding play only a secondary role in cavity nucleation and that at high temperatures, the effect of sliding is eliminated in less than 1 millisecond. It is thus concluded that sliding cannot be the cause for cavity nucleation. Next, a model involving pile-ups of grain boundary dislocations (GBDs) is proposed. Unlike the sliding mechanism, the high strain energy ahead of the pile-up is a steady state phenomenon during secondary creep. It helps to compensate the large capillarity forces in the formation of sub-micron sized cavities, thereby rendering cavity nucleation barrierless. However, a threshold stress exists below which the cavities cannot grow to effect fracture. The present model suggests that cavity nucleation is feasible in single phase metals and alloys at the intersections of cell and grain boundaries. Predominant cavity formation after the onset of steady state creep and an intermediate temperature ductility trough during hot tensile tests are also features of the model. Good agreements are found between the model's predictions and the experiments.