Grain Boundary Motion Research Articles

Annihilation Mechanism of Low-Angle Grain Boundary in Nanocrystalline Metals

Due to the high density of grain boundaries (GBs), nanocrystalline metals possess superior properties, including enhanced strength, work hardening, and fatigue resistance, in comparison to their conventional counterparts. The expectation of GB migration is critical for grain coarsening and GB annihilation in these materials, significantly affecting the polycrystalline network and mechanical behavior. Here, we perform molecular dynamics (MD) simulations on gold (Au) nanocrystals containing multiple parallelly arranged GBs, with a focus on the investigation of annihilation mechanisms of low-angle grain boundaries (LAGBs). It is observed that the shear-coupled motion of LAGBs, consisting of dislocations, gives rise to their preliminary migration with the reduced separation distance between GBs. With subsequent GB motion, the LAGBs encountered with neighboring GBs, and can be annihilated by various mechanisms, including dislocations interpenetration, dislocations interaction, or dislocations absorption, depending on the specific configuration of the neighboring GB. These findings enhance our understanding of GB interactions and shed light on the controlled fabrication of high-performance nanocrystalline metals.

Twins and grain boundaries-dominated the reverse Bauschinger effect and tension-compression asymmetry

The failure modes and mechanisms of single and polycrystalline aluminum under different stress states are very different. Therefore, molecular dynamics methods were used to study the tension-compression asymmetry and Bauschinger effect of single and polycrystalline aluminum under different loads. We found both single and polycrystalline aluminum exhibit work hardening under tensile and compressive loads. The difference in this hardening phenomenon is manifested in that the characteristics of single and polycrystalline aluminum are brittle fracture and ductile fracture, respectively. Furthermore, we found that the tension-compression asymmetry of polycrystalline aluminum is more obvious than that of single crystal aluminum, and it manifested as a reverse tension-compression asymmetry. The Bauschinger effect is caused by the defects introduced by preloading, and the asymmetry of the Bauschinger effect is caused by different microscopic defects. Dislocations introduced by the pre-stretching of single crystal aluminum degrade its performance under reverse loading. Polycrystalline aluminum has more twinned regions under compressive load, which hinder the movement of dislocations and grain boundaries during reverse stretching, showing the reverse Bauschinger effect. This study has important theoretical guiding significance for a comprehensive and in-depth understanding of the mechanical properties and microstructural evolution mechanism of single and polycrystalline aluminum.

Quantitative experiments on the transition between linear to non-linear segregation of Ag in Cu bicrystals studied by radiotracer grain boundary diffusion

Abstract Non-linear segregation of Ag in Cu grain boundaries (GB) was quantitatively studied by radiotracer GB diffusion measurements on Cu bicrystals with non-special near Σ5 symmetrical [001] tilt boundaries. Bicrystals were used to guarantee a stable GB during the annealing avoiding in this way disturbing effects like GB motion etc. on the measured diffusion penetration profile. The 110mAg radioisotope with a well-defined specific activity was applied. This allowed to determine quantitatively the amount of Ag solute atoms in a section as a function of the penetration depth. The controlled variation of the total amount of diffusing Ag atoms along the Cu GB resulted in a fundamental change of the GB penetration profile shape from almost linear to strongly curved. The curvature of the penetration profiles was unambiguously shown to be caused by non-linear segregation of Ag in Cu GB. The full Ag segregation isotherm was calculated from the corresponding GB diffusion penetration profile.

Large-scale potassium-doped tungsten alloy with superior recrystallization resistance, ductility and strength induced by potassium bubbles

Potassium-doped tungsten (KW) plate with a weight of 25 kg was prepared by hot rolling. Subsequently, part of the KW suffered annealing at 1600, 1800 °C for 0.5 h. For the rolled state, K bubble showed the size and number density of 71 nm, 7.6×1018 m−3 in the grain interior and 98 nm, 1.9×1019m−3 at the grain boundaries. To figure out the influence of K bubbles on recrystallization resistance and mechanical properties, microstructure and tensile performance were examined on the stress relieved and high-temperature annealed KW. The results indicated that the KW displayed excellent recrystallization resistance as the K bubble strings inhibited the transverse movement of grain boundaries. Tensile tests at 100, 150, 200, 250, 300, 500 and 700 °C showed the DBTT was 50–100 °C for the stress relived KW and about 200 °C for the high-temperature annealed KW. Moreover, the KW plate exhibited large ductility as K bubbles in the grain interior can act as the source to emit dislocation and barrier to dislocation movement thereby lead to lots of mobile dislocation. Besides, the high number density of nano-sized K bubbles also improved the KW strength, and the strength contribution of K bubbles calculated by the Orowan mechanism was 245.6 MP at room temperature.

Stability of immiscible nanocrystalline alloys in compositional and thermal fields

Alloying is often employed to stabilize nanocrystalline materials against microstructural coarsening. The stabilization process results from the combined effects of thermodynamically reducing the curvature-dominated driving force of grain-boundary motion via solute segregation and kinetically pinning these same grain boundaries by solute drag and Zener pinning. The competition between these stabilization mechanisms depends not only on the grain-boundary character but can also be affected by imposed compositional and thermal fields that further promote or inhibit grain growth. In this work, we study the origin of the stability of immiscible nanocrystalline alloys in both homogeneous and heterogeneous compositional and thermal fields by using a multi-phase-field formulation for anisotropic grain growth with grain-boundary character-dependent segregation properties. This generalized formulation allows us to model the distribution of mobilities of segregated grain boundaries and the role of grain-boundary heterogeneity on solute-induced stabilization. As an illustration, we compare our model predictions to experimental results of microstructures in platinum-gold nanocrystalline alloys. Our results reveal that increasing the initial concentration of available solute progressively slows the rate of grain growth via both heterogeneous grain-boundary segregation and Zener pinning, while increasing the temperature generally weakens thermodynamic stabilization effects due to entropic contributions. Finally, we demonstrate as a proof-of-concept that spatially-varying compositional and thermal fields can be used to construct dynamically-stable, graded, nanostructured materials. We discuss the implications of using such concepts as alternatives to conventional plastic deformation methods.

Disconnection-Mediated Twin/Twin-Junction Migration in FCC Metals

We present the results of novel, time-resolved, in situ HRTEM observations, molecular dynamics (MD) simulations, and disconnection theory that elucidate the mechanism by which the motion of grain boundaries (GBs) in polycrystalline materials are coupled through disconnection motion/reactions at/adjacent to GB triple junctions (TJs). We focus on TJs composed of a pair of coherent twin boundaries (CTBs) and a {\Sigma}9 GB. As for all GBs, disconnection theory implies that multiple modes/local mechanisms for CTB migration are possible and that the mode selection is affected by the nature of the driving force for migration. While we observe (HRTEM and MD) CTB migration through the motion of pure steps driven by chemical potential jump, other experimental observations (and our simulations) show that stress-driven CTB migration occurs through the motion of disconnections with a non-zero Burgers vector; these are pure-step and twinning-partial CTB migration mechanisms. Our experimental observations and simulations demonstrate that the motion of a GB drags its delimiting TJ and may force the motion of the other GBs meeting at the TJ. Our experiments and simulations focus on two types of TJs composed of a pair of CTBs and a {\Sigma}9 GB; a 107{\deg} TJ readily migrates while a 70{\deg} TJ is immobile (experiment, simulation) in agreement with our disconnection theory even though the intrinsic mobilities of the constituent GBs do not depend on TJ-type. We also demonstrate that disconnections may be formed at TJs (chemical potential jump/stress driven) and at GB/free surface junctions (stress-driven).

• Analysis of factors inhibiting motion of grain boundaries in zinc

The effect of grain boundary edges and facets on mobility of individual grain tilt boundaries [1120] and [1010] in zinc of different purity at different temperatures has experimentally been studied. It was established that in the temperature range (0.82—0.98) Тm, where Тm is the melting temperature, the grain boundary mobility was equally depended on temperature, boundary structure, material purity, and grain boundary features.

Phase field crystal simulation of strain-induced square phase low-angle symmetric tilt grain boundary dislocation reaction

In this paper, the phase field crystal method is used to study the dislocation motion and reaction of the square phase symmetric tilt low-angle grain boundaries, and the dislocation configurations with different misorientation angles are analyzed under the action of applied strain. The geometric phase approach is used to characterize the strain field around the dislocations. The results show that after the solidification relaxation, the interfacial dislocations on both sides of the grain are distributed in parallel but opposite direction. With the increase of misorientation angle between grains, the number of dislocations increases, the spacing between them decreases, and the free energy of the system increases. Imposed by the applied strain, the grain boundary dislocations undergo climbing, launching, and reactive annihilation, with the free energy fluctuating. When the misorientation increases, the dislocation motion mode changes from climbing to climbing-sliping, resulting in more dislocation group configurations, and more reactions between dislocations and dislocation groups. For the dislocation reactions of different configurations, positive shear strain drives dislocations to approach, and negative shear strain drives dislocations to annihilate.

Экспериментальная оценка степени участия границ зёрен и тройных стыков в процессе роста зёрен

The paper analyzes the results obtained by studying the movement of individual grain boundaries and individual triple junctions depending on their crystallography and grain boundary phase transitions and their deviation from classical behavior. Based on statistical data on the appearance of faceting and hysteresis at individual tilt grain boundaries, as well as the nature of their movement, it can be concluded about the possible nature of the heterogeneity of the microstructure in space and in time. And this means that it is possible to obtain isotropic microstructures.

Prismatic edge dislocations in graphite

Dislocations are a central concept in materials science, which dictate the plastic deformation and damage evolution in materials. Layered materials such as graphite admit two general types of interlayer dislocations: basal and prismatic dislocations, of which prismatic dislocations have been relatively less studied. Using density functional theory (DFT) calculations, we have examined different prismatic core structures in graphite and evaluated their structure, energetics and mobility. We find close energetic interplay between bonded and “free-standing” core structures in both zigzag and armchair directions, with a reconstructed stable zigzag core identified. We explore grain boundaries and prismatic dislocation pile-up, identifying metastable structures which may be important in energy storage. The role of interlayer stacking in core structure, dislocation glide and climb is also considered in-depth. Our calculations suggest that the prismatic dislocation core is stable up to high temperatures of approximately 1500 K in bulk graphite. Above this temperature, the breaking of bonds in the dislocation core can facilitate climb, grain-boundary motion, and the annealing of damage through prismatic dislocation glide.

Statistical behaviour of interfaces subjected to curvature flow and torque effects applied to microstructural evolutions

The movement of grain boundaries in pure metals and alloys with a low concentration of dislocations has been historically proved to follow curvature flow behaviour. This mechanism is typically known as grain growth (GG). However, recent 3D in-situ experimental results tend to question this global picture concerning the influence of the curvature on the kinetics of interface migration. This article explains, thanks to 2D anisotropic full-field simulations, how the torque effects can complicate these discussions. It is then illustrated that neglecting torque effects in full-field formulations can have a significant effect on the overall rate of GG, the shape of grains and grain boundaries as well as their local kinetics. The apparent reduced mobility can be much more complex than expected without necessarily questioning the influence of the curvature on the local kinetic equation.

Interaction of Grain Boundaries and Particles in Uncooperative Migration

The ubiquitous motion of grain boundaries is responsible for determining the unique microstructural patterns and stability of polycrystalline materials. By employing the versatile multi‐phase field modeling methodology, the role of inert second‐phase particles on the dynamics of grain boundaries under conditions of uncooperative migration is analyzed. Systematic studies are carried out to understand the influence of various particle shapes on the motion of the grain boundaries with a particular attention put on the interfacial interactions. It is found that the motion of the grain boundaries is impeded by pinning effects exerted by these particles. The ability of the moving boundaries to pass through particles of distinct morphologies is different, with the simulation results showing a big influence of particle alignment and the aspect ratio on the pinning of grain boundaries. The results suggest that elongated and flat‐type spheroid particles are more likely to preserve the grain structure by reducing coarsening significantly.

Effect of nanoscale amorphization on dislocation emission from a semi-elliptical surface crack tip in nanocrystalline materials

An impact analysis model is built to describe the effect of nanoscale amorphization on dislocation emission from a surface semi-elliptical crack tip in nanocrystalline materials. The nanoscale amorphization is formed by the splitting transformation of grain boundary(GB)disclinations caused by the motion of GBs. The analytical solution of the model is obtained by using the complex method, and the influence of nanoscale amorphization, dislocation emission angle, crack length, and curvature radius of surface crack tip on the critical stress intensity factor (SIF) of the first dislocation emission is investigated through numerical analysis. The numerical analysis shows that the impact of nanoscale amorphization on the critical SIF corresponding to dislocation emission depends on the dislocation emission angle, the position and the size of the nanoscale amorphous, the curvature radius, and the length of surface crack. As the curvature radius of surface crack tip and the crack length increase, the normalized critical SIF increases. When the nanoscale amorphization size is small, it has a great impact on the critical SIF for dislocation, but when the size is relatively large, the effect becomes small. The effect of the increasing strength of the nanoscale amorphization on dislocation emission from the surface crack tip is related to the distance between the nanoscale amorphization and the crack tip, and there is a critical crack-junction for which the increase of dislocation strength has little effect on dislocation emission.

The near-surface microstructural evolution and the influence of Si particles during nanoscratching of nanocrystalline Al

Molecular dynamics (MD) simulations are performed to study the near-surface deformation during nanoscratching on nanocrystalline Al. The influence of Si particle on wear behavior of nanocrystalline Al is also investigated by MD simulation. It is shown that the local deformation in nanocrystalline Al workpiece is associated with one or more mechanisms: (1) grain boundary (GB) migration, (2) GB sliding, (3) grain rotation, and (4) dislocations and deformation twin emitting from the indenter-workpiece interface and GBs. Moreover, the coarsening of Al grains is observed as the scratch proceeds. During grain coarsening, the low angle GB dissociates while the high angle GB migrates without dissociation. Interestingly, the GB motion is more active in nanocrystalline Al workpiece than that in Al/Si workpiece. The presence of Si particle changes the stress state of the adjacent Al grains, and therefore enhances the stability of GBs. Meanwhile, the specific wear rate in Al/Si workpiece is significant less than that in nanocrystalline Al workpiece, indicating that the Si particle enhances the wear resistance. In summary, this study sheds light on the interaction mechanism of second-phase and nanocrystalline matrix under sliding wear, which can provide theoretical support for the design of metal matrix composite with excellent wear resistance.

The effect of Er and Zr ratio on microstructure evolution and mechanical properties of 6061 aluminum alloy

In the present study, the 6061 alloy was chosen as base alloy, and micro-alloying effect of different ratios of Er and Zr (2:1, 1:1) on microstructure evolution and mechanical properties was examined after T6 heat treatment. Hot rolling deformation was carried out at 450°C, then a solution aging heat treatment process was adopted at 570°C/0.5h+180°C/5h. The precipitation, recrystallization behaviors were characterized using transmission electron microscopy(TEM), scanning electron microscope(SEM) and electron backscattered diffraction(EBSD) techniques, the mechanical properties were tested using tensile tests. The results shown that recrystallization behavior was inhibited by co-additions of Er and Zr. With the increase of Er and Zr content, the recrystallized grain size and coarse Fe-rich phases at the grain boundaries decreased, the recrystallized grain distributed uniformly, the number of needle-like Mg2Si and granular Al3(Er, Zr) phases gradually increased. In 6061 alloy with Er/Zr ratio of 1, the distribution of precipitates were finer and more uniform, The uniformly distributed Al3(Er, Zr) phases pinned the grain boundaries, which effectively hindered the movement of grain boundaries. The tensile tests exhibited that the properties of 6061 alloy with Er/Zr ratio of 1 reached the highest level, i.e., tensile strength of 344.0MPa, the yield strength of 296.0MPa, and elongation of 17%.

Formation of Stable Schwarz Crystals in Polycrystalline Copper at the Grain Size Limit.

A prototype Schwarz crystal (SC) structure of dividing-space minimal grain boundaries (GBs) constrained by coherent twin boundaries (CTBs) was recently discovered in extremely fine-grained polycrystalline Cu. In this Letter, constraining effects of 3D CTB network on the formation and thermostability of SC are addressed via atomistic simulations. GB migration and evolution of CTB network trigger formation of SC diamond. CTB constraints are critical to generate GBs of zero mean curvature underlying vanishing capillary pressure, and to counterbalance the elastic driving forces of lattice. GB motion can be suppressed at temperatures close to the melting point with GB aperture down to 3nm.

Effects of Gd/Nd ratio on the microstructures and tensile creep behavior of Mg–8Al–Gd–Nd alloys crept at 423K

The specific Nd/Gd mass ratios were designed for optimizing the microstructure of the Mg–8Al–Nd–Gd alloys, and the strengthening mechanism of Al-Gd-Nd phase in the creep process was revealed. The Mg–8Al-1.0Gd-1.0Nd (T6) at Gd/Nd ratio of 1:1 exhibited excellent tensile creep resistance at 423 K/60 MPa, owing to the diffusely distributed Al 2 RE and the fine Al 11 Nd 3 particles, which hindered the motion of dislocations and grain boundary sliding. The stead-state creep rate and creep strain of the alloy were 1.03 × 10 −8 s −1 and 0.312%, respectively, which were superior to that of AZ91 alloy. It could be verified that Gd showed little aging precipitation strengthening effect, while it made the typical rod-like Al 11 Nd 3 evolved into dot-like phase. A TEM thin foil was extracted by focused ion beam (FIB), and the result showed that Nd occupied the Gd sites to form Al 2 (Nd + Gd) solid solution. The TEM bright-field image was taken along the B = [ 1 ¯ 1 ¯ 01 ] Mg zone axis with diffraction condition g = 10 1 ¯ 1 ¯ and Two-beam dark field image with diffraction condition g = 11 2 ¯ 0 , which demonstrated the activation of <a> and <a+c> dislocations during creep deformation.

Microstructure and mechanical properties of a 2 wt% Nb bearing low carbon steel

Design of advanced alloys with simple compositions is a new strategy to improve the recyclability of the alloys. In this study, a 2 wt% Nb bearing low carbon steel (2Nb steel) without addition of Ni, Cr and W was designed and processed by hot forging. The as-hot-forged steel shows a fine grain size microstructure with NbC precipitates formed. Microstructure and tensile properties were characterized at ambient and elevated temperatures. The yield stress and ultimate tensile stress of the as-forged 2Nb steel are 339 MPa and 498 MPa at room temperature, 297 MPa and 430 MPa at 600 °C, respectively. The good high temperature strength was related to the pinning effect of large NbC precipitates present at the grain boundaries and small precipitates formed in the grain interior that hinder the movement of grain boundaries and dislocations, respectively.

High-performance B4C-LaB6 composite ceramics fabricated via hot-pressing sintering with La2O3 as sintering additive

The B4C-LaB6 composite ceramics were fabricated via hot-pressing sintering at 2050 °C and 20 MPa pressure with the mixture of boron carbide (B4C) and 2–5 wt% lanthanum oxides (La2O3) as raw materials. The effects of additive La2O3 content on the microstructures and mechanical properties of composite ceramics were investigated, and reaction mechanisms of La2O3 and B4C at different temperatures were studied in detail. La2CO5, La3BO6 and LaBO3 were formed by the reactions of La2O3 and B4C at different temperatures, and finally LaB6 was formed below 1600 °C. The comprehensive mechanical properties of B4C-LaB6 composite ceramics were optimized by adding 4 wt% La2O3, the flexural strength, fracture toughness and Vickers hardness reached 350 MPa,4.92 MP am1/2 and 39.08 GPa, respectively. The high densification and flexural strength of composite ceramics achieved in the present study were attributed to LaB6 hindering the movement of grain boundary. However, the densification was reduced caused by CO as La2O3 content increased to 5 wt%. The fast channel was formed via B4C reacting with La2O3, which accelerated migration of B4C in the sintering process. The content of La2O3 played an important role in the fracture mode of the composite ceramics, and ultimately affected the fracture toughness of the composite ceramics.

Superhardness Induced by Grain Boundary Vertical Sliding in (001)-textured ZrB[formula omitted] and TiB[formula omitted] Nano Films

Superhardness up to 50 and 60 GPa has been achieved recently in orientationally (001)-textured ZrB2 and TiB2 films composed of nano columnar grains, which almost double those in common TMB2 (TM=Zr, Ti). But the atomic origin, especially the role of shear stresses under indentation on grain boundaries (GBs), remains largely unknown. In this work, we report the identification of the experimentally observed GBs as Σ13 [001] tilt GB by first-principles calculations. The calculated formation energies of the GBs and TM vacancies on the GBs explain well the experimental results. Under shear stresses beneath indentors, the GB motion transforms the accumulated shear stresses parallel to (001) plane into a vertical sliding of the nano grains in [001] direction, while maintaining the GB structure unchanged. This special mode of shear plastic deformation correlates their hardness to the compressive strengths in [001] direction, instead of to the shear strengths on (001) plane as in bulk crystals, with the calculated compressive strengths well accounting for the measured superhardness in these (001)-textured nano ultra-high temperature ceramics.