Kink Boundaries Research Articles

Ripplocation and kink boundaries in graphene/copper nanolaminates: A molecular dynamics study

Over the past decade, four distinct differences have been summarized regarding ripplocation and kink boundaries in layered solids: spontaneous reversibility, atomic sharpness, delamination, and residual stress. However, according to molecular dynamics simulations, these four distinct differences are not completely applicable to the ripplocation and kink boundaries in graphene/copper nanolaminates. Contrary to conventional understanding, ripplocation boundaries can transform into kink boundaries without triggering the collective movement of other ripplocations. In addition, ripplocation boundaries can be irreversible with out-of-plane slip, leading to coherent twins in [110] orientation and stacking faults in [111] orientation. Moreover, delamination only occurs at kink boundaries rather than ripplocation boundaries. Wide-spreading residual stresses are absent for both ripplocation and kink boundaries due to plasticity. These findings broaden the current understanding of ripplocation and kink boundaries in layered solids.

Deformation mechanism and minimum energy path in Silicon–graphite composites with lattice defects

Edge-on atomic layers in layered solids undergo buckling, which creates a structure referred to as a “ripplocation.” A collective set of multiple ripplocations is referred to as a “ripplocation boundary.” In this study, in order to investigate the impact of lattice defects on ripplocation, we investigate the buckling deformation of graphene with lattice defects under confinement. To this end, we conducted confined uniaxial compression simulations by placing graphene sheets with various lattice defects between silicon atomic layers. Different types of lattice defects affect the mechanical properties of graphene, the results show that Young's modulus of graphene with the (1,0)_5 dislocation pair was the maximum at 494.6 GPa, whereas the Young's modulus of the (1,2)_10 dislocation pair was the minimum at 309.5 GPa. In addition, we employed a differential geometry method to study the out-of-plane deformation of a single-layer graphene system. Further, we used the nudged elastic band method for various types of lattice defects in graphene to uncover the minimum transition pathways, activation energy required to move from the (1, 0) dislocation pair to the (1, 1) pair is 44.916 eV. In particular, the results indicate that graphene with lattice defects exhibit kink deformation after buckling compared to that of perfect graphene. Our study not only explores the deformation of ripplocations and kink boundaries in layered solids but also provides a more comprehensive description influencing lattice defects on the nucleation mechanism and mechanical changes of ripplocations in graphene.

Induced kink boundary in Mg-Y-Zn alloy by tube-shape forming

The secondary formability and the effect of shape application on induced kink boundaries are examined using Mg-9at.%Y-6at.%Zn alloy through experimental and numerical studies. A tube-shape is successfully formed without any cracks. This tube-shaped alloy includes kink boundaries associated with the applied shear strain. In the numerical study, a substantial shear strain is created at the contact region, but the shear direction becomes opposite after tube formation. This suggests that de-kinking behavior, such as shrinkage of kink boundaries, has a possibility to occur. In contrast, this formed alloy exhibits higher hardness by a magnitude of 30 Hv than the un-processed alloy. The correlation between kink density and hardness in this tube-shaped alloy is consistent with literature results obtained from several wrought-processed Mg-Y-Zn alloys.

Anomalous Local Lattice Softening around Kink Boundaries in a Mille-Feuille Structured Dilute Mg–Zn–Y Alloy

We have evaluated local elastic properties at kink boundaries in dilute Mg-Zn-Y alloys based on scanning transmission electron microscopy (STEM), first-principles calculations, and geometrical phase analysis (GPA) by measuring atomic-scale strain fields around dislocation cores. STEM observations showed that dislocations constituting kink boundaries are extended into Shockley partial dislocations by accompanying solute-enriched stacking faults (SESF). Using GPA analysis, we found that strain distributions around the partial dislocation cores are asymmetric across the Mg matrix and the SESF, showing local elastic heterogeneity. Comparison with the calculated strain field model, it is indicated that the observed asymmetric strain profiles are essentially caused by a softening of the Mg matrix. This anomalous elastic softening can be interpreted as a change in strain energy of dislocations at the kink boundary, which may contribute to the unique deformation mechanism of kink in dilute Mg alloys.

Numerical Investigation of Kink Strengthening Mechanism due to Kink Band in Long-Period Stacking Ordered Magnesium Alloy

A numerical investigation of the kink strengthening mechanism in long-period stacking ordered magnesium alloy is presented. A higher-order gradient crystal plasticity model is introduced, and the reproducing kernel particle method is adopted for the numerical procedure. The specimen, including a kink band with several kink angles, which is defined as the angle between the inside and outside of the kink band, is considered. A simple shear analysis is conducted to evaluate the kink strengthening due to the kink band. The numerical results suggest two main origins of kink strengthening, namely, orientation and defect strengthening. The former is caused by the spatial distribution of the slip direction due to kink, and the latter is the strengthening induced by crystal defects around the kink boundary. The amplitude of kink strengthening depends on the kink angle, and an optimal kink angle, which maximizes the kink strengthening, might exist. The kink strengthening exhibits a Hall–Petch-like behavior, i.e., the correlation between the inverse of the square root of the kink band width and flow stress is almost linear.

DFT Calculation of High-Angle Kink Boundary in 18R-LPSO Alloy

Kink boundaries formed in Mg-based long period stacking order (LPSO) alloys play a key role in strengthening of these materials. As the kink structure grows, many high-angle kink boundaries are eventually formed which has inclination angle close to 34 degrees. We show that this peculiar structure is a result of irreversible structural transformation and is energetically stable. We also calculate segregation energies of alloying elements Y and Zn to this boundary. Finally, the critical resolved shear stress for the migration of kink boundary is estimated for a pure-Mg kink and that with saturated with segregation. We show that segregated kink boundary requires very high shear stress about 700 MPa for migration.

Orientation Dependence of High Temperature Compressive Behavior of Textured Ti<sub>3</sub>SiC<sub>2</sub>

In order to clarify the orientation dependent deformation behavior of the MAX phase ceramics, compressive deformation behavior was examined in a textured Ti3SiC2 (TSC) at a high temperature of 1200°C. Depending on the relationship between the texture and loading directions, both the deformation behavior and microstructure were strongly influenced, and the resultant basal slip, kink formation and delamination affected the compression behavior of the textured TSC. When the stress was loaded parallel or perpendicular to the basal plane (0TSC and 90TSC), the stress-strain (S-S) curves showed higher peak stresses followed by the reduction in the flow stress. When the stress was loaded 45° to the basal plane (45TSC), the S-S curve showed strain hardening after yielding, but did not show peak stress. Although the strength was higher both in 0TSC and 90TSC than in 45TSC, both 0TSC and 90TSC showed the formation of cracks and delamination, resulting to the large drop in the flow stress. In contrast to 0TSC and 90TSC, although 45TSC did not exhibit the peak stress, it exhibited work hardening due to the kink boundary formation, irrespective of the formation of delamination. It is reasonable to conclude from the deformation behavior and the deformed microstructures that for the TSC, the kink boundary plays an important role for attaining both deformability and strength.

Microstructure Evolution and Local Hardness of Mg–Y–Zn Alloys Processed by ECAE

Mg–9 at%Y–6 at%Zn and Mg–2 at%Y–1 at%Zn alloys were processed by equal-channel-angular extrusion (ECAE) to investigate their microstructure evolution and local hardness. The area fraction of the kink bands in the Mg–9 at%Y–6 at%Zn alloys increased with increasing the number of ECAE passes, resulting in higher hardness. In contrast, the number of kink boundaries in the local region near the indentation was almost constant. The relationship between the microstructure factors of the kink bands and the local hardness is discussed in comparison with the forged alloy. In the Mg–2 at%Y–1 at%Zn alloys, the microstructural evolution of the α-Mg matrix phase and long-period stacking ordered (LPSO) phase by 1-pass ECAE and the increase in local hardness were discussed.

Kink band formation mechanism in α/β Ti-9Cr two-phase alloy

A combination of experimental and numerical studies was carried out to understand the microstructure and formation mechanism of kink bands in α/β two-phase titanium alloys. A deformation kink band was introduced in a Ti-9Cr alloy after uniaxial compression test. The strength changes of the α and β phases inside and outside the kink band were evaluated by nanoindentation tests. A potential hardening effect was revealed in the α phase inside the kink band, while a possible softening effect was located in the β phase at the kink boundaries. Microstructural examination of the kink band was conducted by electron backscatter diffraction measurement. The crystal rotation axis was found to be perpendicular to both the (0001) basal plane of the α phase and one of the (110) planes of the β phase. The crystallographic features were characterized by transmission electron microscopy observation, revealing a prismatic 〈a〉 slip that occurred during the compression to accommodate the rotations around the c-axis. Nanoindentation tests were performed on an undeformed specimen to calibrate the crystal plasticity model. A crystal plasticity finite element model was developed and {110}β-rotation-type kink bands were reproduced in the simulation and confirmed the prismatic slip activity.

Predominant factor for effectively forming kink boundaries in Mg–Y–Zn alloy through wrought-process

The numerical and experimental results reveal that application of large strain to bulk Mg-9at.%Y-6at.%Zn alloy leads to formation of a high density of deformation kink boundaries. Shear strain is a controllable parameter for the effective induction of kink boundaries, from the processing viewpoint. This correlation is also applicable to wrought-processed alloys containing the LPSO phase.

Microstructure and mechanical properties of Mg97Y2Zn1 alloy sheets processed by hot rolling

The microstructure and mechanical properties of Mg97Y2Zn1 alloys during rolling at 450 °C and 500 °C have been investigated in this work. The microstructure of pre-rolled alloy mainly consisted of the α-Mg matrix, block 18R LPSO phase distributed at the grain boundary, and lamellar 14H LPSO phase distributed in grain interior. After rolling, the 18R LPSO phase and 14H LPSO phase were kinks obviously at both temperatures of alloys. The alloy rolled at 500 °C produced serious DRX, and the occurrence of DRX is related to the 14H LPSO phase: (i) the interface between 14H LPSO phase and α-Mg matrix could act as a nuclear site of DRX grain and further refine grain by discontinuous DRX mechanism. (ii) The kink boundary of 14H LPSO phase evolved into a grain boundary through the continuous DRX mechanism and promotes the generation of DRX. Tensile testing indicated that the alloy rolled at 450 °C possessed the higher yield strength, and the alloy rolled at 500 °C had the higher ultimate tensile strength. The dominant deformation mechanism of the 450 °C rolled alloy was a prismatic slip and prismatic kink, and the formation of a strong rolling basal texture resulted in the improvement of yield strength. For the alloy rolled at 500 °C, the formation of rolling texture is mainly related to twinning. In addition, the severe DRX degree with refined DRX grains contributed to the improvement of ultimate strength and ductility for the alloy rolled at 500 °C.

Fabrication of Textured Porous Ti<sub>3</sub>SiC<sub>2</sub> by Slip Casting under High Magnetic Field and Microstructural Evolution through High Temperature Deformation

To clarify the effect of constraint conditions on the kink formation, fabrication process of the texture and porosity controlled Ti3SiC2 polycrystals was investigated and microstructural evolution during high temperature deformation was examined in it under high temperature uniaxial compression tests at 1200°C. Dense textured Ti3SiC2 sintered body was fabricated by slip casting in the high magnetic field of 12 T and following pressureless sintering at 1400°C for 1 h. The porosity of the textured Ti3SiC2 was controlled by dispersing polymethyl methacrylate (PMMA) particles into the textured Ti3SiC2 as a spacer media. The highly textured Ti3SiC2 polycrystals with porosity of 8.4 vol% and 16.7 vol%, respectively, were successfully fabricated by the slip casting in the high magnetic field. After the high temperature uniaxial compression perpendicular to the c-axis of the textured structure, both the porous and dense Ti3SiC2 showed kink formation, which is a common deformation mode for anisotropic layered materials. However, the average rotation angles of the kink boundaries were higher in the porous specimen than in the dense specimen. Since the crystal rotation is necessary for the kink formation, kink bands would be preferably developed in the porous area due to its weaker constraint than in the dense area. It can be concluded from the microstructural analysis that the constrain factor caused by the neighbor grains affects the crystalline rotation, resulting in the kink boundary formation with different rotation angles.

Kink Formation through Creep Deformation and Possibility of Kink Strengthening in Ti<sub>3</sub>SiC<sub>2</sub>-MAX Phase

Kink formation and kink strengthening mechanisms were examined in the polycrystalline Ti3SiC2-MAX phase prepared by a spark-plasma-sintering technique. The creep behavior tested by compression at 1200℃ showed two deformation regions depending on the applied stresses; at lower stresses of <120 MPa, the stress exponent n exhibited ≈1.8, whereas at higher stresses, it exhibited n ≥ 6.0. The creep behavior can be ascribed to grain boundary sliding mechanism for the lower stresses with n ≈ 1.8 and dislocation-related creep mechanisms for the higher stresses with n ≥ 6.0. The kink bands were frequently observed to form in the grains deformed only at the higher stresses when its basal plane inclined by about 10-20° against the compressive axis. This suggests that the kink bands might be formed only when two factors of the large stresses acting on the basal plane and the resultant dislocation activities were satisfied. Nanoindentation tests conducted around the formed kink boundaries showed that the hardness increased linearly with decreasing in the distance from the kink boundaries and showed higher values around the kink boundaries. Since the kink boundaries blocked the slip line caused by the nanoindentation, those become a resistance against the dislocation motion caused by the indentation deformation. This suggests that the kink boundaries would be contributed to improve the mechanical properties of the Ti3SiC2-MAX phase.

Intrinsic kink bands strengthening induced by several wrought-processes in Mg-Y-Zn alloys containing LPSO phase

The kink bands strengthening effect was investigated using Mg-9at.%Y-6at.%Zn alloys produced by caliber rolling or extrusion at intermediate and elevated temperatures (373 K and more than 673 K). The results of microstructural observations showed that deformation kink bands were induced in all the alloys, regardless of the wrought-processing method and the processing temperatures. Micro-Vickers hardness tests showed that the regions including kink bands exhibited a higher hardness than those without kink bands, which reveal intrinsic kink bands strengthening. Interestingly, the trend of this kink bands strengthening effect could be expressed by an empirical relationship, which was divided into two lines associated with the wrought-processing temperature. This tendency did not appear to depend on the wrought-processing method. The alloys produced at the intermediate temperature showed markedly superior hardness, since, in addition to the kink bands strengthening, the residual strains/dislocations around kink boundaries contributed to obstruct dislocation slips. • Kink bands were induced regardless of wrought processing-method and -temperatures. • Intrinsic kink bands strengthening effect could be expressed by number of kink bands. • The alloys produced at the intermediate temperature showed markedly superior hardness. • Residual strains around kink bands contributed to obstruct for dislocation slips.

Fabrication of Textured Porous Ti&lt;sub&gt;3&lt;/sub&gt;SiC&lt;sub&gt;2&lt;/sub&gt; by Slip Casting under High Magnetic Field and Microstructural Evolution through High Temperature Deformation

To clarify the effect of constraint conditions on the kink formation, fabrication process of the texture and porosity controlled Ti3SiC2 polycrystals was investigated and microstructural evolution during high temperature deformation was examined in it under high temperature uniaxial compression tests at 1200℃. Dense textured Ti3SiC2 sintered body was fabricated by slip casting in the high magnetic field of 12 T and following pressureless sintering at 1400℃ for 1 h. The porosity of the textured Ti3SiC2 was controlled by dispersing polymethyl methacrylate (PMMA) particles into the textured Ti3SiC2 as a spacer media. The highly textured Ti3SiC2 polycrystals with porosity of 8.4 vol％ and 16.7 vol％, respectively, were successfully fabricated by the slip casting in the high magnetic field. After the high temperature uniaxial compression perpendicular to the c-axis of the textured structure, both the porous and dense Ti3SiC2 showed kink formation, which is a common deformation mode for anisotropic layered materials. However, the average rotation angles of the kink boundaries were higher in the porous specimen than in the dense specimen. Since the crystal rotation is necessary for the kink formation, kink bands would be preferably developed in the porous area due to its weaker constraint than in the dense area. It can be concluded from the microstructural analysis that the constrain factor caused by the neighbor grains affects the crystalline rotation, resulting in the kink boundary formation with different rotation angles.

Formation of -rotation-type kink boundary in Mg–Zn–Y alloy with long-period stacking ordered structure

The dislocation configuration around the <0001>-rotation-type kink bands/boundaries formed in extruded Mg89Zn4Y7 alloys with a long-period stacking ordered structure was investigated by an intragranular misorientation axis analysis and a g•b study. The g•b analysis suggests that the <0001>-rotation-type kink boundary with small tilt angles comprises prismatic <a> edge dislocations. The tendency of kinking with lattice rotation around the <0001> axis to activate at temperatures higher than 673 K can be explained by the temperature dependence of the critical resolved shear stress of prismatic <a> slips. The kink bands arising from the activation of the prismatic <a> slip system are numerically reproduced by a crystal plasticity finite element analysis. Geometrical softening should be significant despite kink-band formation stimulated by the prismatic slip system.

Recovery features of kink boundaries upon post-annealing of a hot-extruded Mg-Zn-Y alloy

We have investigated a post-annealing effect on microscopic structural changes of kink boundaries (KBs), which have been initially introduced by a hot-extrusion of an Mg-Zn-Y alloy containing long period stacking/order (LPSO) phase, based on scanning transmission electron microscopy (STEM) and atom-probe tomography (APT). STEM observations show that most of the KBs appear to be segmented into several submicron-scale, each of which is composed of arrays of extended basal (a-) dislocations that are geometrically necessary to account for the LPSO crystalline rotations. Atomic-scale STEM/APT observations reveal significant reconstructions of these extended dislocation-core structures, which occur at the solute-enriched face-centered cubic layers in the LPSO structures and forms local hexagonal close-packed Mg region with distinct solute depletions. These recovery-like features indeed enhance the thermal stability of the KBs, and the corresponding driving force can be reasonably described according to disclinations, whose elastic energy is defined by crystal rotations and the relevant dipole distances.

Microstructure and properties of nano-laminated Y3Si2C2 ceramics fabricated via in situ reaction by spark plasma sintering

A nano-laminated Y3Si2C2 ceramic material was successfully synthesized via an in situ reaction between YH2 and SiC using spark plasma sintering technology. A MAX phase-like ternary layered structure of Y3Si2C2 was observed at the atomic-scale by high resolution transmission electron microscopy. The lattice parameters calculated from both X-ray diffraction and selected area electron diffraction patterns are in good agreement with the reported theoretical results. The nano-laminated fracture of kink boundaries, delamination, and slipping were observed at the tip of the Vickers indents. The elastic modulus and Vickers hardness of Y3Si2C2 ceramics (with 5.5 wt% Y2O3) sintered at 1500 °C were 156 and 6.4 GPa, respectively. The corresponding values of thermal and electrical conductivity were 13.7 W·m-1·K-1 and 6.3×105 S·m-1, respectively.

Microstructure Evolution of Commercial Pure Titanium During Interrupted In Situ Tensile Test

Microstructure evolution of commercial pure titanium is investigated by interrupted in situ electron backscatter diffraction (EBSD) measurement during tensile deformation along transverse direction at room temperature. After 24 pct elongation, the split basal texture of initial material is weakened and rotated around 90 deg along normal direction (ND). \({{11}\bar{2}{{2}}}\)-\({{10}\bar{1}{{2}}}\) double twin is the main reason for the change of texture. The basal poles are rotated nearly perpendicular to ND by the primary \({{11}\bar{2}{{2}}}\) twin and back to ND through the reorientation of \({{10}\bar{1}{{2}}}\) secondary twin. Both Schmid factor criterion and displacement gradient accommodation are considered to predict the twin-induced texture evolution during TD tension. Kink bands formed by the accumulation of basal 〈a〉 dislocations are also observed in the deformed grain. The activation of other slip systems can deviate the rotation axis and reduce the rotation angle of kink boundary. Besides, the kink boundary with high basal dislocation density obviously hinders the twin transmission and simultaneously can be taken as a preferential nucleation site for \({{11}\bar{2}{{2}}}\) twin.

Quantitative kink boundaries strengthening effect of Mg-Y-Zn alloy containing LPSO phase

This study represents the quantitative kink-band strengthening effect using uniaxial hot-compressed Mg-9%Y-6%Zn alloy. When room-temperature micro-Vickers indentation was performed on a specific area containing kink boundaries under a low indentation load of 10 gf, the magnitude of increase in hardness due to kink-band strengthening was closely related with the number of kink boundaries beneath the indentation, i.e., the kink-band strengthening effect can be expressed by the number of kink boundaries. It is noted that this increased value is calculated by subtraction from the hardness value of the matrix without kink boundaries. The present estimated kink-band strengthening values via microscopic empirical analysis is in a similar range to those of prior macroscopic analysis obtained from the results of compression tests using bulk specimens.