Twist Grain Research Articles

Microstructural statistics for low-cycle fatigue crack initiation in α+β titanium alloys: A microstructure based RVE assessment

(0001) twist grain boundaries (BTGB) have been identified as critical microstructure configurations regarding fatigue crack initiation in titanium alloys. In the presently reported study, Ti-Al-V based alloys with different microstructures were tested in the low-cycle fatigue regime. Early cracking occurred at BTGB for all investigated alloys and microstructures. Microstructural statistics collected at crack initiation sites revealed a weak sensitivity to moderate differences in α and β stabilizers content and microstructural features. Criteria for the identification of crack initiation sites were then defined using this dataset. An automatic processing routine was applied on large-scale electron back-scattered diffraction maps to analyze the spatial distribution of BTGB susceptible to cracking. The low associated density implies that large microstructural regions, i.e., typically > 1 mm2, must be considered to include microstructural configurations prone to crack nucleation. It is likely to play a critical role in the high lifetime variability of Ti alloys.

Automated extraction of interfacial dislocations and disconnections from atomistic data

We introduce Interfacial Line Defect Analysis (ILDA), a method for identifying and extracting interfacial dislocations and disconnections with little-to-no input from the user on the nature of the interface. By simply providing the orientations and coherency strains for the crystals in a coherent reference state of the interface, ILDA provides exact Burgers vectors. Alternatively, these orientations and strains can be estimated using local atomic deformation gradients, making ILDA fully automated and providing estimated Burgers vectors. ILDA also determines the step height associated with each defect line segment, in case the associated defect is a disconnection. The heart of the method is the identification of atoms residing at co-incidence sites between the two crystals and the construction of a surface mesh connecting these sites that is used to compose Burgers circuits and insert defect line segments. We demonstrate the performance of ILDA in two test cases: a twist grain boundary and a phase boundary.

Microstructural effects on fatigue crack initiation mechanisms in a near-alpha titanium alloy

The role of different microstructural constituents on crack initiation in two-phase titanium alloys is still an area of great controversy. The present study investigates the effects of primary α volume fraction and concomitant macrozones on two different fatigue crack initiation modes concurrently observed in a near-α titanium alloy. Statistically representative regions were monitored in quasi-in-situ studies by interrupting fatigue testing to detect slip trace formation leading to crack initiation. In addition, high-resolution 2D strain maps were generated to quantify in-plane shear strains of slip traces related to microstructural features. The detailed analysis demonstrates that at 90% of the proof stress basal 〈a〉 slip plays a crucial role in crack initiation, regardless of whether cracking is transgranular or intergranular. In addition, a distinct shift from transgranular to intergranular crack initiation was observed with increasing αp fraction driven by the increased number of αp/αp grain boundaries. The high-resolution 2D strain mapping suggest that these intergranular cracks initiated from a burst of basal 〈a〉 slip starting from (0001) twist grain boundaries but penetrating one side of the grain pair in case of a partial (0001) twist grain boundary. From these observations, a new criterion for intergranular cracking is proposed based on a geometrical grain boundary parameter and the preference of neighbouring αp grain pair well aligned for basal 〈a〉 slip. It was found that while (0001) twist grain boundaries increase with increasing αp fraction, macrozones do not further enhance their frequency. However, macrozones of hard orientated grains did enhance transgranular crack formation by potentially higher local stress and increasing slip length over clustered αp grains with low misorientation reducing the requirement for out-of-plane shear.

Misorientation and Temperature Dependence of Small Angle Twist Grain Boundaries in Silicon: Atomistic Simulation of Directional Growth

In nano-crystalline and multi-crystalline silicon, grain boundaries (GBs) and their properties may dominate the overall material performance. With a hybrid Monte Carlo and molecular dynamics (MC/MD) approach capable of reproducing the natural formation process of silicon (001) small angle twist GBs, the misorientation and temperature dependence of GB properties were examined at the atomic level. The GB structures and energies show various transition characteristics around three critical misorientation angles. Structure–property correlations are established by converting the three critical misorientation angles to the corresponding dislocation spacings, which are equal to 6-, 2-, and 1-times dislocation core radius. Stress fields and elastic strain energies agree well with the Continuum theory, and their effects on the dislocation structures and defect sink are discussed. This work also reports the variations of GB structures and energies are governed by a critical temperature at around 800 K, where the GB energies reach the minimum and the dislocation dissociations are suppressed.

A study of grain boundary effects on the stress-induced martensitic transformation and superelasticity in NiTi alloy via atomistic simulation

The stress-induced martensitic transformations and superelasticity behavior in the NiTi alloy with a single crystal model and a twist grain boundary bicrystal model at different temperatures are studied using molecular dynamics simulations. An atomic tracing method is proposed to identify specific numbers of B19′ martensite variants. Under uniaxial compressive loading, the stress-induced martensitic transformation takes place accompanied by the formation of <011>M type II twins, and the deformation process can be divided into three distinct stages based on microstructure evolution and average atomic total energy. It is found that the twist grain boundary induces an increase in the martensite start temperature, which is consistent with the experimental results. There is no residual B19′ martensite at the end of the unloading process, and the irrecoverable strain mainly results from plastic deformation at the grain boundary through the analysis of atomic local shear strains and has hardly changed with increasing deformation temperature. Remarkably, the grain boundary brings about the acceleration of martensite nucleation and an earlier occurrence of stress plateau. Further simulation results manifest that the presence of the twist grain boundary leads to weakened temperature dependence of martensitic transformation stress and a reduction in the hysteresis loop area.

Chiral nitroxide radical with terminal trifluoromethoxy group

ABSTRACTA new chiral liquid crystalline nitroxide radical with a terminal trifluoromethoxy group exhibits not only chiral nematic and chiral smectic A phases but also a twist grain boundary phase in the cooling process. The layer order induced by the trifluoromethoxy group seems frustrated with the helical order.

Investigations of axioms of twist grain boundary phases (TGBPs) in binary mixture of liquid crystals

In this paper, the investigated mixture of liquid crystals shows a spacious range of twist grain boundary phases (TGBPs). Thermo dynamical, optical and dielectric measurements confirmed the presence of TGBPs in the mixtures of nematic and cholesteric liquid crystals. In the heating cycle, differential scanning calorimetry (DSC) thermogram shows the mixed different TGBPs at the high scanning rate. Cylindrical and cone-like textures with brushes of TGBA* is observed under the optical investigation. The grain boundaries and blocks of smectic phases are twisting two times around an axis. Wide-ranging dielectric investigation of nematic and cholesteric liquid crystal mixture have been performed in the 50 Hz–10 MHz range of frequency for the homogeneous conditions. In the absence of direct bias, TGBA* phase exhibits two dielectric relaxation processes, soft mode (SM) and process in the grain boundaries (PGBs). In the presence of electric field, grain boundaries steadily demolished the phases. Whereas the weak dielectric relaxation processes have been distinguished in the presence of the electric field in the TGBPs.

Statement of Retraction: Twisted Grain Boundary Phases in Binary Mixture of Smectic and Cholesteryl Compounds

This article refers to:Twisted Grain Boundary Phases in Binary Mixture of Smectic and Cholesteryl Compounds

Statement of Retraction: Twisted Grain Boundary Phase in Binary Mixture of Liquid Crystals

Statement of Retraction: Twisted Grain Boundary Phase in Binary Mixture of Liquid Crystals

Dependency of grain boundary dislocation network configuration on generalized stacking fault energy surface in FCC metals

We use a microscopic phase-field (MPF) model, incorporating the generalized stacking fault energy (GSFE) surfaces (i.e., the γ-surface) as inputs, to investigate systematically the evolution of dislocation network configurations at low-angle pure twist grain boundaries (GBs) in six face-centered cubic (FCC) metals (Ag, Cu, Rh, Ir, Pd and Pt) that have different GSFE surfaces. The equilibrium configurations of GB dislocation networks are obtained via the interplay between the crystalline energy and the elastic strain energy during the energy minimization process. It was found that for {111} twist GBs, though the geometrically necessary dislocations (GNDs) of the screw type are observed, they can be either full or partial ones depending on the GSFE surfaces of the above metal systems. The areas of the staking faults regions formed between partial dislocations are quantitatively characterized as a function of the magnitude of the stacking fault energy, the elastic modulus and the GB misorientation angle. A fast-acting model for predicting the geometric characteristics of the GB dislocation networks is developed based directly on the material properties and GB misorientation, which is validated by its applications to another two metal systems (Au and Ni) with the same GB.

Twisted grain boundary leads to high thermoelectric performance in tellurium crystals

A twisted grain boundary is introduced in the tellurium crystal to effectively block phonon propagation while maintaining high electron mobility for superior thermoelectric properties.

On the relationship between shock and particle velocities in single and bicrystal systems of Aluminum: A molecular dynamics study

Molecular dynamics (MD) simulations are used to study shock wave propagation in various symmetric tilt and symmetric twist grain boundaries (STGB & STwGB) in Al for a range of impact velocities (1 to 3.5 km/s) using the open source LAMMPS package. The calculated shock velocity (US) to particle velocity (UP) relationship for Al single crystal (SC) matches well with the results of ab-initio and experiments. However for the shock simulations in bi-crystals (biC), the calculated shock velocity range fall in the same order of magnitude but does not show a linear, monotonic, US vs UP relationship. The interaction of the shock with the bi-crystal boundary, and shock propagation along different crystal directions on either side of the bi-crystal boundary are possible reasons for such behaviour. Common Neighbour Analysis (CNA) shows that there are more structural changes occurring in the bicrystal than in the single crystal. This is because the impact is along a different plane than the 〈0 1 0〉 direction as is the case of a single direction. We report effect of grain boundary on the calculated shock velocity.

Crack initiation mechanisms in Ti-6Al-4V subjected to cold dwell-fatigue, low-cycle fatigue and high-cycle fatigue loadings

Although Ti-6Al-4V is the most commonly used titanium alloy in the aerospace industry, the mechanisms governing crack initiation in the different fatigue regimes are not well understood yet. This situation partly pertains to a competition between multiple crack initiation mechanisms. In particular, applied loading conditions were identified as a key parameter governing the transition between mechanisms. The fatigue behavior of Ti-6Al-4V with a bi-modal microstructure was investigated in the present study using different waveforms, load ratios and frequencies to clarify this feature. A detailed characterization of the main crack initiation sites was carried out to identify microstructural configurations governing crack nucleation in low-cycle dwell-fatigue, low-cycle fatigue and high-cycle fatigue regimes. While lifetimes and fracture surfaces showed a good consistency with data from prior studies, a single microstructural configuration was found involved with the formation of crack initiation facets. All investigated cracks leading to specimen failure were nucleated along (0001) twist grain boundaries exhibiting similar features. Based on this finding, a criterion is proposed to identify candidates for crack initiation. This also demonstrates no major sensitivity of the critical microstructural configurations to environment, free surface, loading conditions and, as shown in previous studies, microstructure and composition. However, conventional fatigue failure results from one, or a few, surface or subsurface crack initiation sites while dwell-fatigue failure involves the formation of multiple internal cracks. This is accompanied by a significant dwell-fatigue life debit at peak stress magnitudes close to the yield strength. Features of microstructural configurations prone to crack nucleation are finally compared with data reported in existing literature to propose a mechanism of crack formation in Ti alloys.

Defect redistribution along grain boundaries in SrTiO3 by externally applied electric fields

During thermal annealing at 1425 °C nominal electric field strengths of 50 V/mm and 150 V/mm were applied along the grain boundary planes of a near 45° (100) twist grain boundary in SrTiO3. Electron microscopy characterization revealed interface expansions near the positive electrode around 0.8 nm for either field strength. While the interface width decreased to roughly 0.4 nm after annealing at 50 V/mm, the higher field strength caused decomposition of the boundary structure close to the negative electrode. Electron energy-loss and X-ray photoelectron spectroscopies demonstrated an increased degree of oxygen sublattice distortion at the negative electrode side, and enhanced concentrations of Ti3+ and Ti2+ compared to bulk for both single crystals and bicrystals annealed with an external electric field, respectively. Oxygen migration due to the applied electric field causes the observed alteration of grain boundary structures. At sufficiently high field strength the agglomeration of anion vacancies may lead to the decomposition of the grain boundary.

Shock-induced sliding of (0 0 1) twist grain boundaries in Cu

Using molecular dynamics simulations, we investigate shock-induced sliding of a set of (0 0 1) twist grain boundaries (GBs) in Cu bicrystal, with shock direction parallel to GB and low shock strength. The GB sliding is produced via transverse particle motion in constituent grains and resisted by GB viscosity. Its utmost magnitude su increases first and then decreases with increasing of the angle between [1 0 0] direction in grain and shock direction. Quantitative acoustic wave equation analysis reveals that transverse particle motion exists because of the anisotropic elastic stiffness and particular grain orientation. For GBs of noteworthy sliding, boundary viscosity η is estimated according to the deceleration of volume element in grain, and decreases piecewisely with increasing misorientation angle. su is jointly determined by η and transverse particle velocity dependent on grain orientation.

Dimer-parity-dependent odd-even effects in photoinduced transitions to cholesteric and twist grain boundary smectic-C^{*} mesophases: Experiments and simulations.

We describe investigations on the influence of the flexible spacer parity and length of the guest photoactive liquid-crystalline dimers in guest-host mixtures exhibiting photoinduced transitions involving isotropic (I), cholesteric (N^{*}), and twist grain boundary smectic-C^{*} (TGBC^{*}) phases. Despite a small concentration (3 wt. %) of the guest molecules, the transition temperatures and their photodriven shift (δT) show a strong odd-even parity (of the dimer) dependent effect, with the even-parity systems having a larger value than their odd-parity counterparts; δT is larger for the N^{*}-TGBC^{*} transition than for the I-N^{*} one. The photocalorimetric measurements corroborate these features in addition to showing that, in comparison with the absence-of-ultraviolet (UV) case, the transition enthalpy (ΔH) of the I-N^{*} transition in the UV-on case is diminished by 33 and 12% for the mixtures with even- and odd-parity dimers, respectively. The duration for relaxation from the isothermal photodriven transition also exhibits general features of an odd-even influence. Molecular dynamics simulations demonstrate the presence of significant conformational heterogeneity and associated shift in the conformational space on photostimulation of the guest molecules. The change in the effective shape and nematic order parameter is more pronounced in the even-parity system.

Enantioselective Synthesis of 3-Aryl-3-hydroxypropanoic Esters as Subunits for Chiral Liquid Crystals.

Chiral liquid crystals (LCs) with their unique optical and mechanical properties are perspective functional soft materials for fundamental science and advanced technological applications. Herein, we introduce the chiral 3-aryl-3-hydroxypropanoic ester moiety as a versatile building block for the preparation of LC compounds. Three chiral subunits differing in the aromatic part were obtained through asymmetric transfer hydrogenation using Ru(II) complexes with ee from 98% to >99%. Chiral LC compounds of diverse topologies were further prepared without deterioration of the ee during the synthesis. The mesomorphic behavior of rod-shaped, bent-shaped flexible dimeric, and polycatenar LCs is consistent with their topology─chiral nematic and smectic phases were identified as well as the rarely observed twist grain boundary A and blue phases. The utilization of synthetic chiral building blocks offers the possibility of fine tuning the intermolecular interactions by subtle changes in the molecular structure as well as the preparation of the corresponding racemic forms. This paves the way for the study of self-organization and the structure-property relationship in chiral soft materials.

High-strength and low-dwell-sensitivity titanium alloy showing high tolerance to microcracking under dwell fatigue condition

Dwell fatigue failure of titanium alloy components has been one of the major threats to aircraft safety for the last 50 years. Numerous studies have focused on the identification of critical microstructural configurations, such as microtextured regions, rogue grain pairs and (0001) twist grain boundaries. In this work, a microstructure with a low primary α phase (αp) content was designed to avoid the known weaknesses in the Ti-5Al-7.5 V alloy to achieve high-strength and low-sensitivity to 2-min dwell loading. Conventional and dwell-fatigue tests confirmed the efficiency of such an approach. The deformation occurred primarily by planar slip in the αp grains, resulting in possible cracking along preexisting slip bands in the cycling process. Fatigue and dwell-fatigue failure was dominated by surface crack initiation at facet matching basal planes. Moreover, numerous internal microcracks were formed along basal slip bands and occasionally along basal twist grain boundaries. Load holds were found to facilitate cracking along prismatic planes, which highlights a mechanism switch between basal-dominant cracking mode and collective basal-prismatic mode. The mechanism switch suggests that the prismatic dislocation activity considerably increases due to the low temperature creep induced by load holding. Thanks to the low αp phase microstructure, the alloy shows remarkable tolerance to microcrack growth under dwell condition. Limited growth of the internal microcracks across transformed β regions was identified as a key feature of high dwell-fatigue resistance of the investigated material.

Stitching‐Induced Structural Corrugation of Twisted Grain Boundaries in CVD‐Grown MoS2 Domains

2D MoS2 films represent a promising direction for electronic and photonic devices, benefiting from their intrinsic semiconducting and ultrathin body. However, grain boundaries (GBs), as a common type of structural defect, are inevitable and significantly impair electrical performance and stability. Understanding the underlying forming mechanisms and influences of GBs is key to scalable MoS2 films with high electrical performance. Here, a phenomenon regarding the formation of twisted GBs and the exotic physical properties near the GB region are reported. A set of microscopies and spectroscopies complemented with theoretical calculations consistently show that the mechanical and electrostatic properties near the GB are distinct from the interior ones. The underlying mechanism is proposed to be that the edge stitching behavior of MoS2 single crystals with a misorientation angle leads to structural corrugation due to lattice deformation, which is responsible for the observed exotic properties. The proposed mechanism is further corroborated by the theoretical calculations of band structure as well as surface potentials for normal and twisted MoS2 monolayers. These results uncover an interesting interplay between the GB style and the physical properties and open new avenues for exploring applications in semiconducting MoS2.

Twist-grain boundary phase characterized by AFM technique

ABSTRACT Twist grain boundary (TGB) phases represent liquid crystalline systems with a regular array of defects. In our research, we studied a compound with a stable TGBC phase and pursued its structure using various experimental techniques. Using AFM microscope, we observed the surface of the smectic film and detected a periodic relief. We found that the displacement amplitude is a few nanometres, with a periodicity of about 500 nm. Such periodicity is in accordance with the periodicity of the TGBC blocks’ rotation estimated by polarising microscopy. The surface modulation is explained by the deformation of the TGBC structure, which is created on TGBC films. A simplified model interpreting the observed smectic surface displacement as the consequence of rotating TGBC blocks inside the sample is proposed. TGBC blocks deform differently depending on their orientation with respect to the force acting by the tip of the AFM microscope cantilever probing the smectic surface.