Kink Boundaries Research Articles

Recovery of Kink Boundaries During Post-Annealing of a Hot-Extruded Mg-Zn-Y Alloy

We have investigated post-annealing induced microscopic structural change 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 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 crystal rotations. Atomic-scale STEM/APT observations reveal significant reconstructions of these extended dislocation-core structures, which occur at the solute-enriched face-centered cubic (fcc) layers in the LPSO structures and forms local hexagonal close-packed (hcp) Mg region with distinct solute depletions. These recovery 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.

Dynamic precipitation behavior and mechanical properties of hot-extruded Mg89Y4Zn2Li5 alloys with different extrusion ratio and speed

Dynamic recrystallization (DRX), dynamic precipitation and mechanical properties of the hot-extruded Mg89Y4Zn2Li5 (at. %) alloys with different extrusion ratio and speed are systematically investigated in this work. The results reveal the formation details of recrystallized grains in the presence of long-period stacking ordered (LPSO) phases during extrusion at 350 °C. In a low extrusion ratio of 4, the recrystallized grains start to form discontinuously along the fluctuant 18R/α-Mg interface and continuously along the 14H kink boundaries with high misorientation angles. As increasing of extrusion ratio from 4 to 16, besides discontinuous and continuous DRX, particle-stimulated nucleation (PSN) of recrystallized grains is accelerated by small 18R and 14H fragments. Moreover, various thin LPSO lamellae including 14H, 18R and 24H as well as γ′ phase are dynamically precipitated inside the DRX grains. Under extrusion with a high ratio of 25, the DRX is mainly produced by PSN mechanism, leading to an almost completely recrystallized microstructure. Also, most of the 18R blocks are broken down to small fragments while the 14H lamellae in the original α-Mg matrix are dissolved at the large deformation strain. In addition, decreasing extrusion speed from 1 to 0.1 mm/s at this extrusion ratio effectively limits the growth of DRX grains and facilitates the dynamic precipitation of thin 14H lamellae within the fine DRX grains. Consequently, the full and fine DRX microstructure, small 18R fragment and deformation kinking, abundant thin 14H lamellae in fine DRX grains result in the excellent strength-ductility balance through hot extrusion with ratio of 25 and speed of 0.1 mm/s.

Ripplocations: A Progress Report

It has long been recognized that when the basal planes of crystalline layered solids were loaded in compression edge-on they failed by the formation of kink bands. And while no good model was ever put forth for how that occurred, it was always implicitly, or explicitly, assumed that basal dislocations, BDs, were the culprit. In 2015, the term ripplocation – best described as an atomic layer ripple - was coined and since then we have been making the case that atomic layers, like in other layered systems – be they playing cards, steel sheets or geological formations – also fail by constrained buckling. In graphite, beyond a critical buckling stress, ripplocation boundaries, RB, whose width is equal to the width of the perturbing load are formed. RBs - precursors of kink boundaries, KBs, and fully recoverable - are defined as the locus of points with the highest curvature in each layer. Once nucleated the RBs very rapidly indeed propagate, wave-like, away from under the perturbation. Once they reach a boundary standing waves, whose crests are RB, are formed. It is the to-and-fro motion of such waves and the concomitant friction between the layers that is responsible for the energy dissipated per cycle per unit volume outlined by stress-strain loops characteristic of layered solids that are cyclically loaded. The driving force for the reversibility of these loops is the elastic energy stored in the RBs and matrix surrounding them. Constraining motion normal to the basal planes results in an increase in buckling stresses and a decrease in the wavelengths of the resulting waves. The results of a folding mechanics model, based on confined buckling – that takes into account frictional, bending and foundation energies – on thin steel sheets not only agree surprisingly well with our experimental results, especially at low confining pressures, but also with previous work. At the atomic level transmission electron microscope, TEM, images on the MAX phases, graphite and a layered silicate all confirm existence of ripplocations.. The existence of bulk ripplocations will require a revisiting and reassessment of our understanding of how layered crystalline solids deform.

Kink Formation Process in Long-Period Stacking Ordered Mg-Zn-Y Alloy

The formation process of the kink bands in a directionally solidified, polycrystalline, long-period stacking ordered (LPSO) Mg85Zn6Y9 phase during compression deformation was investigated. In-situ compression observations by scanning electron microscopy and electron backscattered diffraction analysis revealed that regions with localized crystal orientation rotation appeared before ridge kinks formed as surface relief. In this paper, it is called as pre-kink The observed pre-kinks were composed of two separate regions rotated in opposite directions to each other and distinguished from one another by the three boundaries. A transmission electron microscopy observation revealed that the boundaries of the pre-kink are sub-boundaries consisting of edge dislocation array, and both its ends are terminated within the matrix. These characteristics match the reported that of regular ridge kink boundaries. In addition, while pre-kinks do not possess the ability of complete reversibility, its boundaries are capable of moving elastically.

The role of kink boundaries in the deformation and recrystallisation of polycrystalline ice

Kinking is a common process in materials with a strong visco-plastic anisotropy but its impact on the plastic deformation and recrystallisation of crystal aggregates is still poorly constrained. Here, kinking is studied via constant stress experiments on polycrystalline ice under relatively low temperature (240 K) and high differential stress (13.3 MPa and 2.7 MPa, with 50 MPa confining pressure). EBSD data analysis shows that samples comprise large and small grains, interpreted as remnant and recrystallised. Boundary trace analyses and misorientation data of kinked remnant grains reveal that kink boundaries are best characterised with rotation axes within the basal plane of the ice crystal. Inferred slip directions within the basal plane are variable and include <11–20>, <10-10> and intermediate directions. Shorter grain boundaries with rotation axes outside the basal plane surround kink boundaries and indicate that non-basal dislocations or ripplocations play a role in accommodating local kink deformation. More kink boundaries per grain and a larger boundary misorientation are generally found in remnant grains in the high-stress sample. At an aggregate level, kinking as a key facilitator for the dynamic recrystallisation process is represented by many straight grain boundaries with basal rotation axes in the remnant and recrystallised grain population. The statistically preferred slip direction within the basal plane is <21–30>, consistent with coupled slip on two crystallographic a-axes with uneven contributions.

Formation of the &lt;0001&gt;-Rotation-Type Kink Boundary in an Extruded Mg-Zn-Y Alloy of Long-Period Stacking Ordered Structure

Formation of the &lt;0001&gt;-Rotation-Type Kink Boundary in an Extruded Mg-Zn-Y Alloy of Long-Period Stacking Ordered Structure

Effect of deformation and post-annealing on microstructure and mechanical properties of long-period stacking ordered phase in Mg88Ni5Y7 alloy

Precipitation behavior and its effect on mechanical properties in magnesium-based alloys are necessary to be explored at the atomic scale. Here microstructural properties of as-cast, cold-rolled and post-annealed Mg88Ni5Y7 (at%) alloys containing long-period stacking ordered (LPSO) phase have been investigated by advanced electron microscopy techniques. In comparison, cold-rolling of as-cast alloys leads to form kink boundaries within LPSO phase, and post-annealing of the cold-rolled alloys at different temperatures results in the formation of different types of precipitates at the kink boundaries, which affects the mechanical properties of the LPSO phase. Our findings provide a better understanding of microstructure evolutions of the deformed Mg88Ni5Y7 (at%) alloys subjected to annealing treatment, which is of great significance for the design and optimization of the magnesium-based alloys.

Interactions between kinking and [formula omitted] twinning in a Mg–Zn-Gd alloy containing long period stacking ordered (LPSO) phase

Two interaction modes between kinking and {101‾2} twinning in a Mg–Zn-Gd alloy containing long period stacking ordered (LPSO) phase are thoroughly investigated: (i) when kinking forms prior to twinning, twins concomitantly nuclear at kink boundaries (KBs) and twinning propagation is frequently restricted by two adjacent KBs. By contrast, (ii) when twinning forms prior to kinking, kink deformation locally breaks the twins’ theoretical misorientation to their adjacent matrixes, during which basal slip plays a vital role in accommodating the asynchronous deformation between kinking and twinning.

Ripplocations, kink bands and delamination cracks in carbon cathode materials

A new deformation micromechanism operating in the carbon cathode for aluminum electrolysis termed a ripplocation has been proposed in this paper. The creep deformation of semi-graphitic cathode was measured using a modified Rapoport equipment at 965 °C with cryolite ratio = 4.0. The characteristic of the defect was obtained by analyzing TEM photograph of the carbon cathode with different testing times. The results indicated that basal dislocations, bulk ripplocations, kink bands and delamination cracks appeared in succession in the first two stages of the creep deformation. Ripplocations in the carbon cathode make a layer of carbon atoms to glide relative to each other without damaging the in-plane bonds. Ripplocations could also attract each other and result in kink boundaries. The creep strain of the carbon cathode could be accommodated by kink band and delamination cracks during aluminum electrolysis. A more comprehensive understanding of their micromechanics behaviors is very important and could deeply influence our current knowledge of the deformation mechanism of the carbon cathode for aluminum electrolysis.

Segregation of solute atoms along deformation-induced boundaries in an Mg–Zn–Y alloy containing long period stacking ordered phase

The interfacial segregation plays an important role in affecting the mechanical performance of various materials. Here we study the segregation behaviors along deformation-induced interfaces in the matrix of an Mg97Zn1Y2 (at%) alloy compressed at 473 K using atomic-scale high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging techniques. The kink boundary, twinning-like boundary, and tilt boundary within the matrix grain are detected to be segregated with both solute Zn and Y atoms. The segregation along kink boundary and twinning-like boundary is closely linked to the associated partial dislocations, and these boundaries also could be regarded as the nucleation sites for the formation of long period stacking ordered (LPSO) structures of nanometer scale and stacking faults (SFs) during deformation. For the various tilt boundary, the segregation could be explained based on either the O-lattice theory or partial dislocations. The experimental results may shed some new light on tailoring microstructures for improving mechanical properties and thermal stability of magnesium alloys.

Kink boundaries and their role in dynamic recrystallisation of a Mg-Zn-Y alloy

An as-cast Mg94Zn2Y4 alloy has been subjected to compression and equal channel angular pressing separately; the resultant microstructure was characterised using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and transmission Kikuchi diffraction (TKD). The as-cast Mg94Zn2Y4 alloy contains a long-period stacking ordered (LPSO) phase and a Mg24Y5 phase. After the compression, kink boundaries are observed in the LPSO phase and are composed of straight basal ⟨a⟩ dislocations. The dislocation arrangement in the kink boundaries of the LPSO/Mg mixture (alternate thin layers of LPSO and Mg) is similar to that in the LPSO phase. The rotation axes of the kink boundaries in LPSO/Mg are preferentially located in the (0001) plane, though [0001] rotation axis has occasionally been observed. Double kinking and non-basal slip have also been observed in the LPSO/Mg mixture. Dynamically recrystallised Mg grains are observed in the kink boundaries located within the LPSO/Mg mixture, preferentially in the kink boundaries with high misorientation angles. The kink boundary acts as the nucleation site for dynamic recrystallisation due to the high energy stored.

Effects of ZrC content on the synthesis of MAX phase and mechanical properties of Cf-C-SiC-Ti3SiC2-ZrC composites

In this research synthesis of Ti3SiC2 nano-laminate, effects of Al and ZrC on the amount and morphology of the synthesized MAX phase and mechanical properties of the Cf-C-SiC, Cf-C-SiC-Ti3SiC2 and Cf-C-SiC-Ti3SiC2-ZrC composites, fabricated by LSI method, were investigated. The infiltration process was conducted at 1500 °C for 30 min and then the samples were annealed at 1350 °C. X-ray diffraction (XRD) technique and scanning electron microscopy (SEM) were utilized in order to investigate the phase composition and microstructure of the samples, respectively. The results showed that the sample containing Al, had the largest amount of synthesized MAX phase and also addition of ZrC led to the decrease of intensities of MAX phase peaks. Among the samples, Cf-C-SiC-Ti3Si(Al)C2 had the best mechanical properties compared to the others. Bending strength, interlaminar shear strength and fracture toughness of this sample were 505 MPa, 34 MPa and 19.1 MPa m1/2 respectively. The results confirmed that the mechanical properties were decreased by addition of ZrC. Among ZrC-containing samples, the sample containing 10 vol% ZrC has shown the least decrease properties including the bending strength of 369.11 MPa, interlaminar shear strength of 26 MPa and fracture toughness of 16.9 MPa m1/2. Addition of ZrC phase caused pseudo-plastic behavior appearance in the force-displacement curve and led to fibers pull-out and also displacement enhancement. Microstructural observations confirmed the plate-like morphology of synthesized MAX phases. Furthermore, the distance between layers decreased and MAX phase size increased respectively by addition of Al. Also MAX phase size decreased by increasing the ZrC content. It was confirmed that the MAX phase-containing samples can tolerate various micro-deformation mechanisms including: crack deflection, bending and delamination of lamellae, kink boundary and laminate fracture. These mechanisms led to the toughening of the composites.

Mechanical properties and microstructure of the C-C-SiC, C-C-SiC-Ti3SiC2 and C-C-SiC-Ti3Si(Al)C2 composites

This study is focused on the evaluation of various parameters and their effects on the phase composition and mechanical properties of MAX phase-reinforced C-C-SiC composite, fabricated by liquid silicon infiltration (LSI) method. The results have shown that reduction of TiC particles size, elimination of residual silicon and addition of Al have positive impacts not only on the MAX phase content but also on the mechanical properties of the samples. The bending strength of C-C-SiC-Ti3SiC2 and C-C-SiC-Ti3Si(Al)C2 compared to C-C-SiC composites improved by 59.67% and 103.6%, respectively. Moreover, their interlaminar shear strength enhanced by 30.4% and 41.6% and their fracture toughness increased by 72.27% and 89.1%, respectively. Investigations have clarified that inter-bundle matrix of these composites has more MAX phase proving that it can tolerate various micro-transformation mechanisms such as crack deflection, bending and delamination of lamellae, kink boundary and laminate fracture. These phenomena lead to the toughening of such composites and prohibition of crack propagation. Improved properties of the MAX phase-containing samples reveal the potential of such composites as functional and structural materials.

Configuration of dislocations in low-angle kink boundaries formed in a single crystalline long-period stacking ordered Mg-Zn-Y alloy

The dislocation configuration around low-angle kink boundaries formed in bent single crystalline long-period stacking ordered (LPSO) Mg85Zn6Y9 alloys was investigated. Intragranular misorientation axis (IGMA) analysis through SEM/EBSD observation and g•b analysis through TEM observation were conducted. Various kink bands possessing different lattice rotation axes ranging from <11¯00> to <12¯10>, i.e. along <11¯00>, <uvt0>, and <12¯10> were revealed using IGMA analysis. TEM observation with g•b analysis unveiled that the <11¯00>-rotation kink boundary observed using IGMA analysis consisted of basal <a> edge dislocations with one Burgers vector, while the <12¯10>/<uvt0>-rotation kink boundaries were comprised of basal <a> edge dislocations with more than one Burgers vector. Low-angle kink boundaries were regarded as low-angle tilt boundaries consisting of perfect basal <a> dislocations, regardless of their rotation axes. The formation of various kink bands with <uvt0>-combination-rotation axes ranging from <11¯00> to <12¯10> suggests that the statistically stored basal <a> dislocations may turn into geometrically necessary basal <a> edge dislocations during the initial stage of low-angle kink boundary formation. When a kink boundary is considered as a disclination, IGMA analysis capable of determining the lattice rotation axis and the angle is effective in evaluating the Frank vector of the kink boundary.

Dislocation configuration and solute redistribution of low angle kink boundaries in an extruded Mg–Zn–Y–Zr alloy

The microstructural and chemical features of deformation-induced interfaces are one of key issues in engineering materials because they determine plastic deformation behavior and thus affect mechanical properties of the materials. Using atomic-resolution high-angle annular dark-field scanning transmission electron microscopy, we characterized deformation-induced low angle kink boundaries (LAKBs) in long period stacking ordered (LPSO) structures in an extruded Mg–2.3Zn–6.6Y–0.56Zr (wt%) alloy. We clarified that the LAKB in LPSO phase consists of an array of <a> dislocations, while the LAKB in Mg interlayers sandwiched between LPSO phases is composed of an array of dissociated <c+a> and/or <a> dislocations. Correspondingly, the former and the latter LAKBs are depleted and segregated with Zn/Y/Zr elements, respectively. I2 stacking fault (SF) is meanwhile generated in Mg layers, and its energy is evaluated approximately 0.1–1.6mJm−2. Deformation-induced LAKBs, the resultant redistribution of solute elements, and precipitated I2 SFs, are proposed to be responsible for the high strength of extruded Mg alloys containing LPSO structures.

Extensive dynamic recrystallized grains at kink boundary of 14H LPSO phase in extruded Mg92Gd3Zn1Li4 alloy

14H LPSO phase seriously affected dynamic recrystallization (DRX) behavior of extruded Mg92Gd3Zn1Li4 alloy. After hot extrusion, extensive DRXed grains took place at kink boundaries, while only a few of DRXed grains were formed at original grain boundaries. The formation of DRXed grains near kink boundary was beneficial for mechanical properties.

Evidence for Bulk Ripplocations in Layered Solids.

Plastically anisotropic/layered solids are ubiquitous in nature and understanding how they deform is crucial in geology, nuclear engineering, microelectronics, among other fields. Recently, a new defect termed a ripplocation–best described as an atomic scale ripple–was proposed to explain deformation in two-dimensional solids. Herein, we leverage atomistic simulations of graphite to extend the ripplocation idea to bulk layered solids, and confirm that it is essentially a buckling phenomenon. In contrast to dislocations, bulk ripplocations have no Burgers vector and no polarity. In graphite, ripplocations are attracted to other ripplocations, both within the same, and on adjacent layers, the latter resulting in kink boundaries. Furthermore, we present transmission electron microscopy evidence consistent with the existence of bulk ripplocations in Ti3SiC2. Ripplocations are a topological imperative, as they allow atomic layers to glide relative to each other without breaking the in-plane bonds. A more complete understanding of their mechanics and behavior is critically important, and could profoundly influence our current understanding of how graphite, layered silicates, the MAX phases, and many other plastically anisotropic/layered solids, deform and accommodate strain.

Atomic-scale segregations at the deformation-induced symmetrical boundary in an Mg-Zn-Y alloy

Solute atoms segregation to the interfaces, such as grain boundary or twin boundary, often plays a critical role in modulating the properties of a metallic alloy. Deformation induced segregation to the interfaces has been a subject of significant research, since this is one of the key issues to fully understand the deformation mechanism and microstructure evolution in service of engineering materials. By means of the high-resolution aberration-corrected scanning transmission electron microscopy (STEM), we report the investigations of segregations to symmetrical boundaries, kink boundary (KB) and twin boundary (TB), in the Mg-Zn-Y alloys containing long period stacking ordered (LPSO) phases subjected to a compression at room temperature. We found that Zn atoms preferentially segregate to the deformation-induced symmetrical KBs in the LPSO structures and sandwiched Mg layers, while only a small amount of Y atoms concentrate at KB in LPSO structure. These enriched atoms may be in a random distribution, form nanoscale clusters or in a periodic pattern. Furthermore, solute atoms would rather decorate the segment of coherent TBs than enrich the overlapped TBs. Based on the direct atomic observations, the segregation mechanisms to the featured microstructures are proposed.

Cottrell atmospheres along dislocations in long-period stacking ordered phases in a Mg–Zn–Y alloy

Dislocations and local chemistries around them in plastically deformed long-period stacking ordered phases in a Mg–Zn–Y alloy have been investigated by the combination of atomic resolution electron microscopy imaging and nano-probe energy dispersive X-ray spectroscopy measurements. Zn/Y Cottrell atmospheres were observed along dislocations at kink boundaries and individual dislocations in bulk long-period stacking ordered phase. Evidence for unpinning of dislocations from Cottrell atmospheres was also observed. A binding energy of about 0.05eV is estimated between the dislocations and solute atoms trapped within the atmospheres.

Kink formation and concomitant twin nucleation in Mg–Y

During in situ tensile loading of a Mg–2.5wt%Y alloy, a large kink band was observed. The orientation of the kink band varies continuously with a maximum misorientation angle of 18° with respect to the matrix grain. The formation of the kink band is accompanied by the nucleation of numerous {101¯2} twins at the kink boundaries. A mechanistic model is proposed to account for this unique kink band and concomitant twin nucleation via dissociation of basal dislocations. This mechanism is aided by the presence of Y, which suppresses {101¯2} twinning and simultaneously decreases the stacking fault energy.