Dislocation Grain Research Articles

Cellular Automaton Modeling of Dynamic Recrystallisation Microstructure Evolution for 316LN Stainless Steel

Based on the theoretical model and physical mechanism of dynamic recrystallization (DRX) in metal materials, the dislocation density change, nucleation and grain growth model during the process of DRX are taken into account. And according to the nucleation driven by dislocation and grain growth kinetic, transformation rules are made. A modeling methodology coupling fundamental metallurgical principles based on amended nucleation rate with the cellular automaton (CA) technique is here derived to simulate the 316LN.The two-dimensional CA model uses quadrilateral element and periodic boundary condition and Von-Neumann neighbor type. The influence of strain, strain rate and deformation temperature on dynamic recrystallization volume fraction and average grain size are analyzed on the basis of established CA model.

Incorporating the grain boundary misorientation effects on slip activity into crystal plasticity

ABSTRACTThe role of grain boundary misorientation angle (GBMA) distribution on slip activity in a high-manganese austenitic steel was investigated through experiments and simulations. Crystal plasticity simulations incorporating the GBMA distribution and the corresponding dislocation–grain boundary interactions were conducted. The computational analysis revealed that the number of active slip systems decreased when GBMA distribution was taken into account owing to the larger volume of grain boundary–dislocation interactions. The current results demonstrate that the dislocation–grain boundary interactions significantly contribute to the overall hardening, and the GBMA distribution constitutes a key parameter dictating the slip activity.

Dislocation Grain Boundary Interactions in Irradiated Metals

Dislocation Grain Boundary Interactions in Irradiated Metals

Damping Mechanism of SiC Particle Reinforced Al Matrix Composites

The low-frequency internal friction behavior of SiC particle reinforced Al matrix composites was studied over a temperature range from 23°C to 550°C at frequencies of 0.1, 1 and 5 Hz. The experimental data were analyzed in terms of the K-G-L dislocation theory and the anelastic relaxation of grain boundary sliding. Two internal friction peaks of the composites were respectively observed over the temperature ranges of 100~250°C and 200~500°C. The dislocation motion is the important damping mechanism of the composites at low temperatures, while the grain boundary relaxation plays a dominant role at high temperatures. The activation energies for dislocation relaxation and grain boundary sliding are 1.2 eV and 1.57 eV, respectively.

Study of nanoindentation mechanical response of nanocrystalline structures using molecular dynamics simulations

Molecular dynamics (MD) simulations are performed to study the nanoindentation onto three different crystal structures including the single crystalline, polycrystalline, and nanotwinned polycrystalline copper. To reveal the effects of crystal structure and twin-lamellae-thickness on the response of nanoindentation, we evaluate the evolution of crystalline structure, dislocation, strain, indentation force, temperature, hardness, and elastic recovery coefficient in the deformation zone. The results of MD simulations show that the hardness, elastic recovery ratio and temperature of those three nanocrystalline copper strongly depend on crystal structure and twin-lamellae-thickness. It is also revealed that as nanoindenter goes deeper, the extent of plastic zone becomes substantially larger. Initial dislocation always nucleates at the beneath of indenter, and the discrete drops of indentation force observed at certain indentation depths, indicates dislocation bursts during the indentation process. In particular, the twining and detwining are dominant over the dislocation nucleation in driving plasticity in nanotwinned polycrystalline during nanoindentation, which are in good agreement with the previous work. Furthermore, we find that plastic deformation has a strong dependence on crystal structure. The plastic deformation of the single crystalline copper relies on the generation, propagation and reaction of dislocations, that of the polycrystalline copper depends on the dislocation–grain boundary (GB) interactions, and that of the nanotwinned polycrystalline copper relies upon the dislocation–twin boundary (TB) interactions as well as twining/detwining. This work not only provides insights into the effects of crystal structure and two-lamellae-thickness on the mechanical properties of copper under nanoindentation, but also shed lights onto the guideline of understanding other FCC nanocrystalline materials.

Compressibility of hierarchic-architectured agglomerates of hydrogen-reduced copper nanopowders

Pure copper nanopowders are hydrogen-reduced in order to eliminate surface oxides and produce hierarchic architectures having inner-sponge structures with partially bonded nano/ultrafine particles and outer irregular-agglomerate boundaries. Due to a decrease in surface area by the particle bonding, the newly designed agglomerates exhibit improved surface stability after reduction, resulting in enhanced oxidation-resistance in the air at room temperature. For a comparative analysis, we also prepare two conventional micropowders having spherical and irregular particles. The compressibility of these three types of powders is analyzed using mechanical compaction. Finite element analyses on the compaction behaviors of the spherical and irregular particles are performed. The mechanical properties and microstructures of the compacts are investigated using microhardness tests, X-ray diffraction, and electron backscatter diffraction technique. Dislocation density, crystallite size, and grain size are correlated with the mechanical and compaction behaviors. From the analyses, three advantages of the hydrogen-reduced copper nanopowder are noted: (1) suppression of oxidation while maintaining nano/ultrafine structure of particles, (2) lower pressure required for high-density compaction than for spherical powders with nano scale, and (3) favorable fabrication of bulk nano/ultrafine structures without cracks or fracture.

Relationship of Microstructure and Mechanical Properties in Er-Containing Aluminum Alloy

This study investigated the mechanical properties and microstructures of Er-containing Al–Mg alloys. The research found that the H114-T sheet of Er-containing Al–Mg alloys showed a relative good comprehensive performance in mechanical properties. With the special rolling and heat treatment process, this H114-T sheet showed different morphology of microstructures with the other sheets in Er-containing Al–Mg alloys. Grains in H114-T sheet performed irregular shape polygon, a number of subgrains appeared in grains, the amount of dislocations in grains decreased. H114-T sheet possessed a lot of Copper texture, this may be one of important factors influenced the mechanical properties.

Correlating the internal length in strain gradient plasticity theory with the microstructure of material

The internal length is the governing parameter in strain gradient theories which among other things have been used successfully to interpret size effects at the microscale. Physically, the internal length is supposed to be related with the microstructure of the material and evolves during the deformation. Based on Taylor hardening law, we propose a power-law relationship to describe the evolution of the variable internal length with strain. Then, the classical Fleck–Hutchinson strain gradient theory is extended with a strain-dependent internal length, and the generalized Fleck–Hutchinson theory is confirmed here, by comparing our model predictions to recent experimental data on tension and torsion of thin wires with varying diameter and grain size. Our work suggests that the internal length is a configuration-dependent parameter, closely related to dislocation characteristics and grain size, as well as sample geometry when this affects either the underlying microstructure or the ductility of the material.

Thermally activated deformation processes in body-centered cubic Cr – How microstructure influences strain-rate sensitivity

The microstructure dependent deformation behavior of chromium, especially in terms of strain-rate sensitivity (SRS), was studied by means of improved nanoindentation methods as a function of temperature. Cr was investigated in the single crystalline (sx) and ultrafine-grained (ufg) condition. With increasing temperature a decreasing SRS was determined for sx-Cr, while for ufg-Cr a significant increase upon overcoming the critical temperature Tc was measured. This is attributed to the increased importance of dislocation–grain boundary interactions, and a change in the dominating deformation behavior.

Mechanism of dislocation channel-induced irradiation assisted stress corrosion crack initiation in austenitic stainless steel

The mechanism by which dislocation channeling induces irradiation assisted stress corrosion cracking was determined using Fe–13Cr15Ni austenitic stainless steel irradiated with protons to a dose of 5dpa and strained at high temperature in both argon and simulated boiling water reactor normal water chemistry environments. Straining induced dislocation channels that were characterized by digital image correlation and confocal microscopy. Dislocation channels were found to be either continuous across the boundary, discontinuous, or discontinuous with slip in the boundary. Discontinuous channels were found to contain the least amount of strain but have the highest propensity for initiating cracks. Discontinuous dislocation channel–grain boundary intersections were shown to have the highest local stress. TEM in-situ straining of irradiated steels and atomistic simulation of dislocation–grain boundary interaction provided supporting evidence that channels that were unable to transfer strain underwent cracking. The inability of channels to relieve stress, by either slip in the adjacent grain or in the grain boundary, resulted in high local stresses and increased susceptibility to stress corrosion cracking initiation.

The effect of different step-size on the visualization of crystallographic defects using SEM/EBSD technique

Crystallographic grains and defects play an important role in many fundamental processes, such as grain growth and recrystallization, damage, and plastic deformation. Due to the importance of these processes, there is considerable interest in characterizing the crystallographic orientation and grain boundary distribution of crystalline materials. In this study, crystallographic defects such as dislocation arrays and grain boundaries and their orientations were investigated in a commercial polycrystalline copper sample using electron backscatter diffraction (EBSD) mapping combined with scanning electron microscopy (SEM). EBSD was used to determine the local orientations at individual points of a regular grid on a planar surface of a specimen. From the orientation differences between neighboring points, the lattice curvature and dislocation density tensor were derived, and the dislocation density distribution accompanying the crystallographic defects was significantly dependent on the SEM/EBSD step size ass...

Microstructure evolution and hardness variation of pack-rolled nanocrystalline nickel

The microstructure evolution and hardness of nanocrystalline nickel during pack rolling at room temperature have been investigated. It was found that the roll-bonding side (R) and non-roll-bonding side (NR) behaved quite differently. The hardness of side R is higher than that of side NR. No obvious work softening was observed in either side R or side NR until the strain reached ~ 0.611. Quantitative X-ray diffraction analysis indicated that the grain size in side NR increases faster than that in side R, a result confirmed by transmission electron microscopy. Texture analysis showed that (2 0 0) preferred orientation first strengthens but then weakens in both sides NR and R, while a strong (2 2 0) preferred orientation emerges, particularly in side R. Further texture analysis suggests that dislocation slip is responsible for the texture discrepancy between side NR and side R. The dislocation activity, grain rotation and grain growth are discussed based on the experimental results.

Micromechanistic origin of irradiation-assisted stress corrosion cracking

A multi-scale study of the micromechanics of dislocation–grain boundary interactions in proton and ion-irradiated stainless steels is presented. Interactions of dislocation channels with grain boundaries result in slip transfer, discontinuous slip without or with slip along the grain boundary. The presence of the irradiation damage enhances the importance of the magnitude of the resolved shear stress on the slip system activated by the grain boundary to transfer slip across it. However, the selected slip system is still determined by the minimization of the grain boundary strain energy density condition. These findings have implications for modelling the mechanical properties of irradiated metals as well as in establishing the mechanism for disrupting the grain boundary oxide, which is a necessary prerequisite for irradiation-assisted stress corrosion cracking.

Microstructure Prediction during Hot Deformation Using New Dynamic Recrystallization Model and Finite Element Analysis

In this study, dynamic recrystallization during nonisothermal hot deformation was numerically simulated by finite element analysis and new physically based dynamic recrystallization model. The dynamic recrystallization model was developed based on mean field approach by assuming grain aggregate as representative volume element. For each grain aggregate, changes of state variables such as dislocation density and grain size were calculated using three sub-models for work hardening, nucleation, and nucleus growth. The developed dynamic recrystallization model was validated by comparing with isothermal hot compression of pure copper. Finally, developed dynamic recrystallization model was combined with finite element method to predict the local changes of microstructure and average grain size during nonisothermal hot compression of pure copper and hot tube extrusion of austenitic stainless steel. The simulation results were in reasonably good agreement with experimentally determined microstructures.

Modeling of dislocation–grain boundary interactions in a strain gradient crystal plasticity framework

This paper focuses on the continuum scale modeling of dislocation---grain boundary interactions and enriches a particular strain gradient crystal plasticity formulation (convex counter-part of Yalcinkaya et al., J Mech Phys Solids 59:1---17, 2011; Int J Solids Struct 49:2625---2636, 2012) by incorporating explicitly the effect of grain boundaries on the plastic slip evolution. Within the framework of continuum thermodynamics, a consistent extension of the model is presented and a potential type non-dissipative grain boundary description in terms of grain boundary Burgers tensor (see e.g. Gurtin, J Mech Phys Solids 56:640---662, 2008) is proposed. A fully coupled finite element solution algorithm is built-up in which both the displacement $${{\varvec{u}}}$$ u and plastic slips $$\gamma ^{\alpha }$$ ? ? are considered as primary variables. For the treatment of grain boundaries within the solution algorithm, an interface element is formulated. The proposed formulation is capable of capturing the effect of misorientation of neighboring grains and the orientation of the grain boundaries on slip evolution in a natural way, as demonstrated by bicrystal specimen examples.

塑性変形の伝ぱに対する粒内転位源と粒界の役割 : 原子モデルによる検討

塑性変形の伝ぱに対する粒内転位源と粒界の役割 : 原子モデルによる検討

A study of the hot and cold deformation of twin-roll cast magnesium alloy AZ31

Recent advances in twin-roll casting (TRC) technology of magnesium have demonstrated the feasibility of producing magnesium sheets in the range of widths needed for automotive applications. However, challenges in the areas of manufacturing, material processing and modelling need to be resolved in order to fully utilize magnesium alloys. Despite the limited formability of magnesium alloys at room temperature due to their hexagonal close-packed crystalline structure, studies have shown that the formability of magnesium alloys can be significantly improved by processing the material at elevated temperatures and by modifying their microstructure to increase ductility. Such improvements can potentially be achieved by processes such as superplastic forming along with manufacturing techniques such as TRC. In this work, we investigate the superplastic behaviour of twin-roll cast AZ31 through mechanical testing, microstructure characterization and computational modelling. Validated by the experimental results, a novel continuum dislocation dynamics-based constitutive model is developed and coupled with viscoplastic self-consistent model to simulate the deformation behaviour. The model integrates the main microstructural features such as dislocation densities, grain shape and grain orientations within a self-consistent viscoplasticity theory with internal variables. Simulations of the deformation process at room temperature show large activity of the basal and prismatic systems at the early stages of deformation and increasing activity of pyramidal systems due to twinning at the later stages. The predicted texture at room temperature is consistent with the experimental results. Using appropriate model parameters at high temperatures, the stress–strain relationship can be described accurately over the range of low strain rates.

Non-planar grain boundary structures in fcc metals and their role in nano-scale deformation mechanisms

This work presents the results of a comparative molecular dynamics study showing that relaxed random grain boundary structures can be significantly non-planar at the nano-scale in fcc metals characterized by low stacking fault values. We studied the relaxed structures of random [1 1 0] tilt boundaries in a polycrystal using interatomic potentials describing Cu and Pd. Grain boundaries presenting non-planar features were observed predominantly for the Cu potential but not for the Pd potential, and we relate these differences to the stacking fault values. We also show that these non-planar structures can have a strong influence on dislocation emission from the grain boundaries as well as on grain boundary strain accommodation processes, such as grain boundary sliding. We studied the loading response in polycrystals of 40 nm grain size to a level of 9% strain and found that the non-planar grain boundaries favour dislocation emission as a deformation mechanism and hinder grain boundary sliding. This has strong implications for the mechanical behaviour of nano-crystalline materials, which is determined by the competition between dislocation activity and grain boundary accommodation of the strain. Thus, the two interatomic potentials for Cu and Pd considered in this work resulted in the same overall stress–strain curve, but significantly different fractions of the strain accommodated by the intergranular versus intragranular deformation mechanisms. Strain localization patterns are also influenced by the non-planarity of the grain boundary structures.

Slip transmission in bcc FeCr polycrystal

Grain boundaries induce heterogeneities in the deformation response of polycrystals. Studying these local variations in response, measured through high resolution strain measurement techniques, is important and can improve our understanding of fatigue damage initiation in the vicinity of grain boundaries and material hardening. In this work, strain fields across grain boundaries were measured using advanced digital image correlation techniques. In conjunction with strain measurements, grain orientations from electron back-scattered diffraction were used to establish the dislocation reactions at each boundary, providing the corresponding residual Burgers vectors due to slip transmission across the interfaces. A close correlation was found between the magnitude of the residual Burgers vector and the local strain change across the boundary. When the residual Burgers vector magnitude (with respect to the lattice spacing) exceeds 1.0, the high strains on one side of the boundary are paired with low strains across the boundary, indicating the difficulties for slip dislocations to penetrate the grain interfaces. When the residual Burgers vector approaches zero, the strain fields vary smoothly across the boundary due to limited resistance to slip transmission. The results suggest that the residual Burgers vector magnitude, which relates to the GB (Grain Boundary) resistance to slip transmission, enables a quantitative analysis of the accumulation of strain at the microstructural level and the development of strain heterogeneities across grain boundaries. The results are presented for FeCr bcc alloy which exhibits single slip per grain making the measurements and dislocation reactions rather straightforward. The work points to the need to incorporate details of slip dislocation–grain boundary interaction (slip transmission) in modeling research.

Effects of Laser Shock Peening on TC11 Titanium Alloy with Different Impacts

The blade, made of TC11 titanium alloy, is prone to result in fatigue failure in the formidable environment in aero-engine. So a higher performance request of the material is brought forward. In this paper, laser shock peening(LSP) as a solution is applied to TC11 titanium alloy and microstructure, residual stress and microhardness with and without LSP were examined and compared via transmission electron microscope(TEM), X ray diffraction(XRD)and microhardness tester. The TEM results indicate that a great high density of dislocations are generated and evolve into the dislocation wall, sub-boundary and grain boundary. The nanocrystallites are formed and become smaller and more uniform with greater impacts. A high compressive residual stress above -540MPa is introduced with an increasing plastically affected layer with different impacts. The microhardness test result shows that LSP can obviously increase the hardness by 20 percent or so, and the affected depth increases with the impact from 600μm to 1200μm.