Lattice Rotation Research Articles

Electron Beam-Assisted Au Nanocrystal Shear and Rotation.

Understanding metastable structural transitions under beam irradiation is essential for the phase engineering of nanomaterials. However, in situ studies of beam-induced structural transitions remain challenging. This work uses an electron beam in aberration-corrected high-angle annular dark-field scanning transmission electron microscopy to irradiate Au nanocrystals at room temperature. The electron beam-induced thermal spike is estimated by electron energy loss spectroscopy and the two-temperature model. The thermal spike drives a transition in the Au lattice from nonclose-packed (311) and (220) planes to close-packed (111) planes through nonrandom lattice rotation. This transition is attributed to the gradient distribution of shear strain at the grain boundaries, with a critical shear strain of approximately 0.2 for the shift from the (220) to the (111) planes. These insights reveal the origins of nanocrystal shear and rotation under electron beam irradiation, providing strategies for precise nanocrystal manipulation using beam-assisted techniques.

An emerging frontier of battery innovation: tackling lattice rotation in single-crystalline cathodes.

Due to a lack of spatially resolved characterization studies on statistical and individual particle microstructure at multiple scales, a knowledge gap exists in understanding the mechanistic link between rapid performance failure and atomic-scale structure degradation in single-crystalline Ni-rich battery cathodes. In a recent publication in Science, Huang et al. developed a multi-crystal rocking curve technique (combining X-ray and electron microscopy to capture both statistical and individual lattice distortions), which enables multiscale observations and further proves that the accumulation of the unrecoverable lattice rotation in cathodes upon repeated cycling exacerbates mechanical failure and electrochemical decay. The elucidation of failure mechanisms in single-crystalline cathodes offers valuable insights into the development of long-lasting and high-energy-density cathodes in next-generation batteries, encompassing strategies to mitigate lattice rotation and enhance lattice structure tolerance against lattice distortion within individual particles.

Predicting the evolution of texture and grain size during deformation and recrystallization of polycrystals using field fluctuations viscoplastic self-consistent crystal plasticity

This paper advances a recent formulation of a field fluctuations viscoplastic self-consistent (FF-VPSC) crystal plasticity model to predict not only the evolution of texture in polycrystalline metals undergoing deformation and recrystallization but also the evolution of grain size. The model considers stress and lattice rotation rate fluctuations inside grains to calculate intragranular misorientation spreads. The spreads are used for modeling of grain fragmentation during deformation and grain nucleation during recrystallization enabling the predictions of concomitant evolution of texture and grain size distributions. The evolutions of textures and grain size distributions are first simulated for commercially pure Cu undergoing severe plastic deformation (SPD) in high pressure torsion to agree well with corresponding experimental data. Next, the evolution of texture and grain size in an aluminum alloy (AA) 5182-O after simple tension and static recrystallization are predicted and compared with experiments. Finally, the predictions of texture and grain size distributions in a magnesium alloy WE43 undergoing a thermo-mechanical loading in the dynamic recrystallization regime are presented and compared with experiments. After validation, the model is coupled with the implicit finite element method (FEM) via a user-material subroutine (UMAT) in Abaqus to facilitate modeling of complex boundary conditions and geometries. The multilevel approach is referred to as FE-FF-VPSC in which every integration point embeds the FF-VPSC constitutive law, considering texture evolution and the directionality of deformation mechanisms operating at the single crystal level. FE-FF-VPSC is applied to simulate a sequence of rolling, recrystallization, and deep drawing of a circular cup of AA6022-T4. The simulated processing sequence demonstrates the versatility of the simulation framework developed in the present paper in not only predicting texture and grain size evolution and phenomena pertaining to behavior of materials but also geometrical shape changes important for the optimization of metal forming processes. To this end, the effects of initial texture and underlying anisotropy on the formation of earing profiles during deep drawing are simulated and discussed.

5D X-Ray Nanodiffraction Reveals Nanoscale Structural Deformation in NMC Cathode Materials

Ni-rich layered oxide cathodes LiNixCoyMn1-x-yO2 (NMC, x>0.5) are promising candidates for next-generation lithium-ion batteries thanks to their high capacity and low cost. The main issue that limits their large-scale commercialization is the capacity degradation, which is exacerbated by the chemical and mechanical instabilities introduced from further increasing the Ni content and the higher delithiation states. The chemical and mechanical instabilities are primarily studied with x-ray absorption (e.g. XANES) and diffraction (e.g. HEXRD) techniques, which provide statistically averaged information on thousands of cathode particles with streamlined data acquisition and data analysis routines.X-ray nanodiffraction complements the aforementioned techniques with spatially resolved information on individual particles. A 5-dimensional dataset (2D in the real space and 3D in the reciprocal space) is acquired for each particle, providing a detailed yet complex perspective on the mechanical instabilities. With results from a few recent studies, we show, below, how different ways of reducing the 5D data space can lead to the discovery of specific nanoscale structure deformation inaccessible to most other methods.For a core-shell NMC polycrystal cathode aimed to enhance the cycling stability, we show that the lattice parameter of the secondary particle matches that expected of the designed compositional gradient. Reversing the compositional gradient results in stress during growth as inferred from the remnant lattice rotation observed at the boundary between neighboring primary particles [1]. For a study on the effect of aging, we show the formation of an oxide layer on the surface of single crystal NMC cathodes. A reduced 5D rocking curve indicates that the oxide layer induces a tensile strain on the order of 10-2. The strain measured with nanodiffraction is about 100 times higher than what was measured with HEXRD, the latter being an underestimation averaged over the entirety of the largely unstrained particles [2].Lattice rotation is a prevalent lattice deformation in battery materials commonly associated with the formation of defects. Compared to the overwhelming scientific interest on the lattice strain, the importance of lattice rotation is often overlooked when it comes to mechanical instabilities. With 5D analysis on the lattice rotation, we show a dominating mode of structural degradation in the form of a twisting motion along the long axis of the nanoparticles [3]. More recently, we were able to further link the unrecoverable parts of lattice rotation to the mechanical failure of single crystal cathodes [4].[1] Tongchao Liu, Lei Yu, Jun Lu, Tao Zhou, Xiaojing Huang, Zhonghou Cai, Alvin Dai, Jihyeon Gim, Yang Ren, Xianghui Xiao, Martin V. Holt, Yong S. Chu, Ilke Arslan, Jianguo Wen & Khalil Amine. Nature Communications volume 12, Article number: 6024 (2021)[2] Lei Yu, Jing Wang, Tao Zhou, Junxiang Liu, Weiyuan Huang, Tianyi Li, Tongchao Liu, Jianguo Wen, Khalil Amine, in preparation.[3] Lei Yu, Alvin Dai, Tao Zhou, Weiyuan Huang, Jing Wang, Tianyi Li, Lu Ma, Xianghui Xiao, Mingyuan Ge, Rachid Amine, Steven Ehrlich, Jianguo Wen, Tongchao Liu, Khalil Amine, under review in Nature Communications.[4] Weiyuan Huang, Tongchao Liu, Lei Yu, Jing Wang, Tao Zhou, Junxiang Liu, Tianyi Li, RachidAmine, Xianghui Xiao, Mingyuan Ge, Lu Ma, Steven N. Ehrlich, Martin V. Holt, Jianguo Wen, Khalil Amine, accepted in Science.

Unraveling dynamic recrystallization mechanisms in Ni-based wrought superalloy GH4698 through comprehensive EBSD analysis

A comprehensive examination of dynamic recrystallization (DRX) mechanisms in wrought superalloy GH4698 has been conducted, leveraging Gleeble thermal simulation experiments and advanced Electron Backscatter Diffraction (EBSD) characterization techniques. Rigorous analysis elucidates four distinct recrystallization processes: Discontinuous Dynamic Recrystallization (DDRX), instigated by strain-induced boundary migration (SIBM); Continuous Dynamic Recrystallization (CDRX), characterized by progressive lattice rotation; Particle Stimulated Nucleation (PSN), driven by the Zener pinning effect of MC-type carbides; and Twin-induced Dynamic Recrystallization (TDRX), facilitated by the interaction between twin boundaries and existing dislocations.

Research on the ultra-low temperature tensile deformation mechanism of Al-Mg-Si alloys in different heat treatment states

The susceptibility of Al-Mg-Si alloy to cracking during room temperature large plastic deformation poses a significant obstacle to its widespread application. Cryogenic temperature forming of aluminum alloys has emerged as a prominent research topic. However, the precise mechanisms underlying the enhancement of aluminum alloy plasticity in different states remain elusive. In order to elucidate this matter, a series of uniaxial tensile experiments were conducted on Al-Mg-Si alloy featuring diverse initial states, under varying temperature conditions. The results show that when the deformation temperature is reduced from 25℃ to −196℃, the tensile strength and elongation of the wrought material are increased by 48 MPa and 5.42 %, and the tensile strength and elongation of the annealed material are increased by 118 MPa and 5.65 %. In contrast, while the T4 state sample exhibited a significant improvement in tensile strength (103 MPa), no substantial enhancement in elongation was observed. This disparity can be primarily attributed to the absence of second-phase precipitation in the as-forged and annealed states. As the temperature decreased, the critical shear stress increased, resulting in increased resistance to deformation. Consequently, an increased degree of lattice rotation and activation of dislocations in various slip systems ensued, thereby improving the deformation uniformity. The formation of initial microvoids predominantly occurred close to grain boundaries. Therefore, compared with room temperature deformation, low temperature deformation caused by the accumulation of dislocations and hole sprouting concentrated at a single location was less likely. As a result, the dislocation distribution was more uniform and fracture was less likely to occur. Conversely, in the T4 state, the aging process resulted in the precipitation of a substantial amount of the soluble second phase (Mg5Si6). Consequently, microvoid nucleation transpired at the phase boundaries, leading to an elevated tensile strength. However, because of the increased dislocation hindrance when the deformation temperature was lowered, no significant enhancement in plasticity was observed.

Grain disintegration and dynamic recrystallization during impact tests of additively manufactured nickel-based alloy 718

The high-temperature dynamic mechanical response of Alloy 718 produced via laser-powder bed fusion (LPBF) was investigated through compressive Split-Hopkinson Pressure Bar (SHPB) tests. Simulating the typical service conditions of Alloy 718, the tests were conducted at temperatures ranging from 298 K to 773 K and at strain rates of 1000 s−1 to 1500 s−1. Phenomenological material constitutive models, such as the modified versions of Johnson-Cook and Hensel-Spittel models, were developed based on the SHPB test results. Analysis of microstructural evolution under impact conditions highlighted that columnar grains with high Schmid factors tend to undergo preferential activation and dislocation pile-up. This process leads to the formation of adiabatic shear bands, grain disintegration, and intense lattice rotation, particularly at higher strain rates. Furthermore, increasing the dynamic deformation temperature facilitates the activation of discontinuous dynamic recrystallization (DRX), with strain accumulation promoting localized grain nucleation along heavily dislocated dendritic boundaries. Recognizing the limitations of phenomenological material constitutive models in accurately representing the underlying microstructural evolution, an artificial neural network (ANN)-based constitutive model employing a three-layer backpropagation learning algorithm was implemented, reducing the Average Absolute Relative Error (AARE) to 0.17%.

Bending properties and deformation micromechanisms of near-α titanium alloy sheets/foils considering initial texture characteristics and size effect

Clarifying the bending deformation behaviour of metal foils is essential for meeting the lightweighting requirements of metal honeycomb structures. This study selected Ti65 alloy specimens with various thicknesses and initial α-phase textures to conduct room-temperature three-point bending experiments. A detailed comparative analysis was performed on the bending deformation behaviour of Ti65 alloy sheets and foils. It reveals that in the foil specimens, the weakening effect of surface layer grains is significant. Under the influence of size effects, the springback angle of Ti65 alloy foils with coarse α-phase grains decreases markedly. Additionally, through various methods, including global Schmid factor analysis, lattice rotation analysis, and crystal plasticity finite element method simulations, the specific relationship between α-phase texture and the bending performance of Ti65 alloy foils was thoroughly investigated. The rolling process influences the bending performance of Ti65 alloy by determining the α-phase texture. Unidirectionally rolled Ti65 alloy products exhibit a transverse α-phase texture, which predisposes them to form transverse texture α-phase macrozones during bending. In contrast, cross-directionally rolled Ti65 alloy foils possess a basal α-phase texture biased toward the transverse direction. This not only helps avoid the formation of macrozones but also ensures coordinated deformation across the foil, resulting in superior bending performance.

The Lattice Strain Distribution in GexSn1-x Micro-Disks Investigated at the Sub 100-nm Scale

Experimental assessment of the strain tensor within a microstructure is challenging, especially for small mechanical deformations acting over submicron length scales. In this work, we fully characterize the spatial strain distribution within a suspended micro-disk laser made of Ge1-xSnx alloy, with fine resolution <200 nm. We employ Scanning X-ray Diffraction Microscopy, a model-free method based on synchrotron radiation, to directly obtain maps of all components of lattice strain and rotation, including the shear strains, finding them on a magnitude ~10-3. We correlate these small elastic deformations to structural defects and the relaxation of the three-dimensional microstructure, demonstrating the potential of an advanced X-ray microscopy technique for microelectronics.

Rotational deformation twins in a HfNbTaTiZr refractory high entropy alloy

Results of the analysis of deformation twins formed in a HfNbTaTiZr refractory high entropy alloy during compression deformation at 20 °C – 600 °C are reported. All studied twins were formed by rotation of the matrix crystal lattice around a 〈110〉 pole and have the common reciprocal twinning plane K2 = {001} and reciprocal twinning direction η2 = <11¯0>. At the same time, the twinning plane K1 and direction η1 depend on the degree of rotation of K2 and η2 around the common 〈110〉 pole. The following rotational twinning modes have been identified in HfNbTaTiZr: {11¯4}<22¯1¯> (38.94° rotation), {11¯5}<55¯2¯> (31.58°), {11¯6}<33¯1¯> (26.54°), {11¯7}<77¯2¯> (22.84°), and {11¯8}<44¯1¯> (20.04°). A rotational mechanism of twinning, with the rotation axis 〈011〉 or 〈001〉, is proposed as an alternative to the commonly accepted shear mechanism.

Multiscale framework-based crystal plasticity modeling and texture evolution of the deformation behavior of AISI 304 stainless steel microtubes manufactured through 3D-FBF technology

The grain size often influences the precision and quality of products manufactured by microforming at the microscale. Macroscale finite element modeling (FEM) cannot accurately predict nonuniform deformation and microstructural evolution at the microscale. In addition, the microscale FEM is challenging for forming processes with complex loading boundary conditions. Thus, in this study, a multiscale framework-based CPFEM is proposed to study the deformation behavior of microtubes manufactured through the 3D-FBF process. The acquired results show that significant nonuniform deformation is caused by greater geometric dimensions and smaller grain sizes, which increase the bending radius of microtubes at the macro level. In addition, a larger offset leads to higher flow stress, larger lattice rotation angles, and consequently, a larger bending radius for the microtube, and grain orientation also influences bending deformation, with easily deformable grain orientations leading to greater stress distribution within the grains.

The deformation mechanism of low symmetric Ti–Pt intermetallic compounds containing high density of planar defects

Intermetallic compounds typically exhibit limited plastic deformation capacity due to challenges in activating dislocation slip and deformation twinning, coupled with a lack of alternative deformation mechanisms. Ti–Pt alloys are a prevalent type of intermetallic compound utilized in high-temperature shape memory alloys and as materials for energy applications in electric fields. However, they often exhibit poor deformation capability. Here, we prepared a low-symmetry intermetallic phase, Ti4Pt3, which demonstrates significant plastic deformation capability. This phase features a high density of parallel planar defects, resulting in an exceptionally large lattice periodicity perpendicular to these defects. Through in-situ scanning electron microscope compression tests, we observed substantial plastic deformation in this new phase. Analysis of the deformed Ti4Pt3 phase revealed that the dense planar defects create uniformly distributed sites of internal stress concentration, enabling a rapid increase in back stress within crystals. This phenomenon leads to notable lattice rotation and localized order-disorder transitions, both crucial mechanisms facilitating plastic deformation and enhancing deformation capacity. Our research underscores the potential of leveraging structural asymmetry to enable unconventional deformation mechanisms, thereby enhancing the plasticity of intermetallic materials.

Deformation mechanism of dislocation slip and kinking behaviour in dynamic compression response of TiZrHfNb alloy fabricated by laser directed energy deposition

ABSTRACT In this study, the dynamic response and deformation mechanism of TiZrHfNb RHEA fabricated by laser directed energy deposition (L-DED) were researched under a dynamic compression test at a nominal strain rate of 3500 s−1. The L-DED TiZrHfNb alloy with columnar grains exhibits exceptional mechanical properties with a uniform dynamic flow stress of 1670 ± 10 MPa and a maximum plastic strain of 0.28 ± 0.03. The dislocation slipping and kinking behaviour dominate uniform plastic deformation. First, the dislocations are primarily confined within dislocation channels due to coplanar slip, and the evolution of them in dynamic compressive deformation process has undergone spacing refinement and crossing. Furthermore, the kinking behaviour induced by the lattice rotation with the T = <011> axis denotes the work hardening effect. Particularly, the second kinking behaviour effectively relieves local stress concentration and delays fracture before the formation of adiabatic shear band (ASB).

Nematic Superconductivity and Its Critical Vestigial Phases in the Quasicrystal.

We propose a general mechanism to realize nematic superconductivity (SC) and reveal its exotic vestigial phases in the quasicrystal (QC). Starting from a Penrose-Hubbard model, our microscopic studies suggest that the Kohn-Luttinger mechanism driven SC in the QC is usually gapless due to violation of Anderson's theorem, rendering that both chiral and nematic SCs are common. The nematic SC in the QC can support novel vestigial phases driven by pairing phase fluctuations above its T_{c}. Our combined renormalization group and MonteCarlo studies provide a phase diagram in which, besides the conventional charge-4e SC, two critical vestigial phases emerge, i.e., the quasinematic (QN) SC and QN metal. In the two QN phases, discrete lattice rotation symmetry is counterintuitively "quasibroken" with power-law decaying orientation correlation. They separate the phase diagram into various phases connected via Berezinskii-Kosterlitz-Thouless (BKT) transitions. These remarkable critical vestigial phases, which resemble the intermediate BKT phase in the q state (q≥5) clock model, are a consequence of the fivefold (or higher) anisotropy field brought about by the unique QC symmetry, which are absent in conventional crystalline materials.

In-situ EBSD study of the active slip systems and substructure evolution in a medium-entropy alloy during tensile deformation

Electron backscatter diffraction (EBSD) coupled with in-situ tensile loading is powerful for investigating the microstructural evolution of alloys. Thermo-mechanically treated f.c.c. medium-entropy alloys (MEAs) typically have high densities of annealing twin boundaries (ATBs), which can not only strengthen but also toughen the MEA via interacting with dislocations. However, the evolution of ATBs and other substructures in plastically-deformed MEA has not yet been revealed. Plastic deformation involving dislocation evolution, active slip systems, lattice rotation, boundary transformation, and grain subdivision in a polycrystalline MEA Ni41.4Co23.3Cr23.3Al3Ti3V6 was studied using in-situ EBSD. The slip was accompanied by heterogeneous lattice rotation among grains and within grains, where inhomogeneous plasticity was accommodated by geometrically-necessary dislocations (GNDs). Both GND and low-angled boundaries (LABs) densities substantially increased with progressive strain, which was mainly concentrated in sites approaching ATBs or grain boundaries (GBs). Located stress, lattice rotation, or curvature caused a loss in the coherence of ATBs, which resulted in integrity loss with increasing strain and promoted a decrease in density by 60 %. Further, lattice rotation incompatibility due to constraints from neighboring grains leads to grain fragmentation into various misorientated volumes, which were separated by LABs or high-angle boundaries (HABs). The grain orientation angle increased with progressive strain and crystallographic 〈111〉 orientation gradually spread toward a tensile direction. Slip systems with maximum Schmid factor were activated first at ε ≥ 3.9 %, which is almost the same with experimental slip traces. Both single slip and double slip occurred during plasticity, where straight slip traces tend to curve due to lattice curvature. Slip transfers are not only controlled by geometric compatibility factor, which can occur between some neighboring grains with low geometric compatibility factor but high Schmid factor.

Asymmetrical plastic deformation during spherical micro-indentation of magnesium

The complex deformation of magnesium (Mg) and its alloys has been the focus of many studies in lightweight technologies. In this paper, spherical micro-indentation tests followed by post-test electron microscopy were carried out on large grain pure Mg to isolate the effects of crystal orientation on the activation of deformation along different slip or twinning systems. Both pre- and post-indentation crystal orientations were measured using electron backscatter diffraction (EBSD). The pre-indentation orientations were mapped into a crystal plasticity finite element (CPFE) model to further analyze the results. It is shown that the resulting deformation twinning and the degree of indentation-induced misorientation were strongly correlated with the crystal orientation in the region of the indentation. Depending on the crystal orientation, multiple waves of basal slip were observed to form asymmetrically around the indents. These slip bands lead to more than 12° lattice rotations that are captured by CPFE modeling. For the first time, it is shown that indentation can lead to significant out-of-plane displacement field that can induce twin nucleation at the interface of far-field (>100 μm) neighbouring grains. CPFE simulations indicate that maintaining far-field strain compatibility leads to the nucleation of twins rather than a slip transfer or slip-induced twinning mechanism.

A modified cyclic elastoplastic constitutive model considering the variational dynamic recovery term of back stress for FGH95 under asymmetrical cyclic loading

The asymmetric cyclic loading process occurs in aero-engine turbine discs. A constitutive model that accurately describes the cyclic elastoplastic behaviour of the material is important for structural design and low cycle fatigue life prediction of turbine discs. In this paper, the low cycle fatigue test of FGH95 was carried out at 620℃ under 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0% and 1.1% strain amplitude with strain ratio equal to 0. The material exhibited cyclic hardening followed by cyclic softening at high strain amplitudes and cyclic softening at low strain amplitudes. The mean stress relaxation rate was similar for each strain amplitude. In addition, the evolution of effective stress and back stress was obtained through the method of internal stress division. Then, the relationship between the change in stress amplitude and the change in internal stress was discussed. The results showed that with cyclic loading, cyclic hardening/softening of FGH95 was affected by the competition mechanism of back and effective stresses. Considering that slip deformation and crystal lattice rotation coexist in plastic deformation, the dynamic recovery term of the Abdel-Karim and Ohno model was used. In order to characterize the different magnitudes of back stress change in materials at different plastic strain intervals, a dynamic recovery term coefficient was introduced to the dynamic recovery term and the critical surface of the back stress. The modified model was used to compare with experimental results. Then, it gives a good description of the material’s mean stress relaxation and strain amplitude variation and gives good agreement on the hysteresis loop.

Crystal plasticity modeling of prior austenite orientation effects on deformation behaviors of martensitic steels under different strain paths

The influence of prior austenite grain (PAG) orientation on the deformation behavior of a low-carbon martensitic steel is investigated using crystal plasticity (CP) modeling with hierarchical representative volume elements (RVEs). The Kurdjumov-Sachs (K-S) relationship is refined for accurate PAG reconstruction in martensitic steel. A robust calibration strategy for CP parameters based on nanoindentation tests is developed by integrating analytical calculations with inverse methods. Virtual polycrystalline aggregates are subsequently generated by manipulating initial PAG orientations. The simulations reveal that assigning cube texture to PAGs enhances strain hardening of martensite under plane strain tension. Moreover, the RVEs with brass and copper orientated PAGs exhibit similar deformation behavior, and a more homogeneous strain distribution is realized in the RVE with PAG cube orientation. The influence of PAG orientation on stress and strain fields is correlated with lattice rotation behavior during deformation. Notably, different strain paths elicit distinct lattice rotation trajectories, wherein uniaxial tension and plane strain tension favor grains reorientation toward hard γ-fiber, whilst equi-biaxial tension drives the crystal matrix to concentrate on 〈001〉 and 〈011〉 directions. These findings provide insights into the intricate relationship between microstructural orientation and deformation behavior in martensitic steels, which is crucial for performance optimization.

A Deterministic Method to Construct a Common Supercell Between Two Similar Crystalline Surfaces.

Here, a deterministic algorithm is proposed, that is capable of constructing a common supercell between two similar crystalline surfaces without scanning all possible cases. Using the complex plane, the 2D lattice is defined as the 2D complex vector. Then, the relationship between two surfaces becomes the eigenvector-eigenvalue relation where an operator corresponds to a transformation matrix. It is shown that this transformation matrix can be directly determined from the lattice parameters and rotation angle of the two given crystalline surfaces with O(log Nmax) time complexity, where Nmax is the maximum index of repetition matrix elements. This process is much faster than the conventional brute force approach ( ). By implementing the method in Python code, experimental 2D heterostructures and their moiré patterns and additionally find new moiré patterns that have not yet been reported are successfully generated. According to the density functional theory (DFT) calculations, some of the new moiré patterns are expected to be as stable as experimentally-observed moiré patterns. Taken together, it is believed that the method can be widely applied as a useful tool for designing new heterostructures with interestingproperties.

Strain path dependent dynamic recrystallization and its resulted material flow and microstructure evolution of a titanium alloy

Changing strain path is an effective method to optimize the hot working process and improve microstructure and mechanical performance of titanium alloys. In this work, dynamic recrystallization (DRX) and its resulted material flow and microstructural behaviors in monotonic torsion (MT) and forward-reverse torsion (FRT) were comparatively explored by experimental and crystal plastic finite element simulation methods. The results indicate that both MT and FRT can induce discontinuous DRX (DDRX) and continuous DRX (CDRX). However, compared to MT where CDRX always prevails, FRT makes DDRX more important since it inhibits CDRX by recovering the lattice rotation during the reversal stage. Moreover, in FRT, the driving force and the grain boundary content for DDRX nucleation and bulging are relatively low at a small total strain. And the recovery of lattice rotation in FRT can also suppress CDRX. Therefore, DRX kinetics is obviously delayed at a smaller strain. As strain increases, both the stored energy for DDRX and the long-range misorientation gradient for CDRX are gradually greater, thus improve DRX kinetics. With these effects by DRX, although FRT exhibits a lower grain refinement degree at a smaller total strain compared to MT, it can make grain refinement degree rapidly rise and even close to that in MT at a greater strain. Also, FRT can obviously weaken the deformation texture by inducing extensive DDRX grains and recovering the lattice rotation of retained grains. In addition, the texture formed in forward stage and DRX delaying can result in a decrease in yield strength and softening rate in reversal stage, respectively, i.e., exhibiting remarkable Bauschinger effect, especially at a higher forward strain. These results can provide a basic guidance for designing the hot working process of titanium alloys to obtaining more refined microstructure and excellent properties of the components.