Low-energy Boundaries Research Articles

Superconducting Boride Nb2IrB2 with Variable and Tunable Stacking Behaviors of Two-Dimensional [Nb-Ir-Nb] Triangular Lattices.

In structures with special geometry lattices, variations in stacking sequences are ubiquitous, yielding many novel structures and functionalities. Despite a wealth of intriguing properties and wide-ranging applications, there remains a considerable gap in understanding the correlation between special geometry lattices and functionalities in borides. Here, we design and synthesize a new superconducting boride Nb2IrB2, with a body-centered orthorhombic structure, consisting of alternating two-dimensional [Nb-Ir-Nb] triple-triangular-lattice-layers and B fragment layers. Advanced aberration-corrected scanning transmission electron microscopy observations show variable stacking configurations between [Nb-Ir-Nb] triple-triangular-lattice layers that can be tuned through synthesis conditions. Density functional theory calculations reveal that the coherent low-energy boundary interface plane of {101} between [11̅1] and [010] domains is responsible for the variable stacking behaviors. Energetically favorable structures are thereby reasonably proposed, based on nanoscale imperfect structure units. These findings provide valuable insights for designing and exploring new structures and functionalities within boride systems involving special geometry lattices.

Slip to twinning to slip transition in polycrystalline BCC-Fe: Effect of grain size

A slip-to-twinning-to-slip transition concerning grain size has not been reported in the literature on BCC-Fe so far. In the present investigation, we report slip-to-twinning and twinning-to-slip transitions at 8 nm and 20 nm grain sizes, respectively. For this purpose, we used molecular dynamics simulations to study ⟨110⟩ quasi-3D polycrystals of BCC-Fe with grain sizes ranging from 3 nm to 102 nm at 10 K. Hall-Petch and inverse Hall-Petch relations were obtained depending on the grain size. Grain boundaries (GBs) were converted to lower energy boundaries through twin-GB interactions. Nucleation of twin boundary through GB decomposition was observed even under the constraints of triple junctions. Various dislocation mechanisms were observed, like delayed slip transfer, slip absorption, dislocations from a twin boundary, and formation of debris/point defects/tubular-type cylindrical rods from the gliding dislocations. Dislocation activity was observed to increase with increasing grain size. Luster and Morris parameter was used to understand dislocation-GB interactions. Intergranular fracture was observed.

Investigating the influence of post-deposition heat treatment (PDHT) on the characteristic changes in wire arc additively manufactured 410 martensitic stainless steel thin-wall structure

Obtaining the desired material characteristics of an as-deposited martensitic stainless steel (MSS) structure fabricated by wire arc additive manufacturing (WAAM) is challenging due to several factors like inhomogeneity, precipitation, phase transition, etc. This investigation elucidates the impact of post-deposition heat treatment (PDHT) on the microstructural and mechanical properties of the WAAM-processed 410 MSS thin-wall structure and compares it with the as-deposited 410 thin-wall structure. The MSS thin-wall structure consists of retained austenite (γ), delta (δ) ferrite, martensite laths and Cr-rich M23C6 type carbide. The M23C6 volume fraction increases by up to 18 %, and the γ-phase fraction reduces by up to 40 % by applying PDHT. The PDHT process significantly changed the texture from {110}<011> to {001}<110>. The PDHT process increases the dislocation density (1.974 × 10−15 m−2) than the as-deposited condition (0.924 × 10−15 m−2). The total low energy boundary (LAGB+CSL) contribution of the as-deposited sample is 19.5 %, whereas the PDHT sample demonstrated a higher fraction of low energy boundaries of 32.4 %. The PDHT sample showed a much smaller distribution of grains with an average grain size of 11 μm compared to the as-deposited condition. The overall hardness increases from 468 HV to 487 HV after PDHT compared to the as-deposited condition with lower inhomogeneity across the build direction. The mean 0.2 % proof stress, ultimate tensile stress, and elongation of the PDHT sample increased by 17 %, 5 %, and 60 %, respectively, compared to the as-deposited sample.

Novel role of thermomechanical grain boundary engineering in the microstructure evolution of austenitic stainless steel

The effect of special grain boundary development via the grain boundary engineering process (GBE) was explored in a bimodal microstructured 316L austenitic stainless steel with an average grain size of 1.92 μm. The GBE process was carried out by the rolling-annealing method via two routes of low and medium applied strain, followed by a short annealing period in a single-step and iterative manner. The grain boundary character distribution was obtained by Electron Backscatter Diffraction (EBSD) analysis, and the intergranular corrosion resistance and the tensile behavior of the samples were examined. Microstructural characterization showed that applying low strain repetitively increased the coincidence site lattice (CSL) and Σ3 boundaries percentage and created large twin-related domains (TRD). The reduced sensitization was attributed to a high proportion of CSLs and large-sized TRDs by applying a low-strain route. In the early stages of the low-strain route, the grain boundary character distribution showed a high number of primary twin nuclei in coarse grains and a continuous network of single twin nuclei in smaller grains, creating a high percentage of Σ3-grain boundaries. The low applied strain and the absence of the necessary driving force for the recrystallization and GBs migration led to the formation of twin nuclei, which would assist with the reduction of microstructure energy. A high percentage of Σ3 boundaries and an increase in the percentage of triple points consisting of low energy boundaries were found to be influential factors in increasing elongation.

Thermally driven evolution of deformation nanotwins into nanolaminated structures with low-angle boundaries in 316L stainless steel

Although twin boundary is a kind of low-energy boundary, the interactions between dislocations and twin boundaries promote the detwinning of twin layers. In this work, we investigated the thermally driven evolution of deformation nanotwins in 316L stainless steel. After high-temperature annealing, deformation nanotwins evolve into nanolaminated structures with low-angle boundaries, which are difficult to form in materials with low stacking fault energies. We explored the roles of dislocations and twin boundaries, as well as their interaction mechanism underlying microstructural evolution.

Grain boundary migration in polycrystalline α-Fe

High energy x-ray diffraction microscopy was used to image the microstructure of α-Fe before and after a 600 °C anneal. These data were used to determine the areas, curvatures, energies, and velocities of approximately 40,000 grain boundaries. The measured grain boundary properties depend on the five macroscopic grain boundary parameters. The velocities are not correlated with the product of the mean boundary curvature and grain boundary energy, usually assumed to be the driving force. The total migration of a boundary consists of both translations approximately normal to the boundary and lateral changes in area. Through the lateral changes in area, low energy boundaries tend to expand in area while high energy boundaries shrink, reducing the average energy through grain boundary replacement. The driving force for this process is not related to curvature and might disrupt the expected curvature – velocity relationship.

Distribution and evolution of low-energy twin boundary density in time−space domain during isothermal compression for Ni80A superalloy

Adjusting the density of low-energy boundary, i.e., Σ3n twin boundary, in a thermal-plastic deformation process is a significant approach to enhance grain boundary-related properties for an alloy. In order to uncover the distribution and evolution of twinboundary density in a current-heating forming process, and their inner mechanisms dependent on grain size and stored-energy, an electrical−thermal−mechanical multi-field coupling and macro-micro multi-scale coupling finite element model was established. Simulation results of isothermal compression processes for Ni80A superalloy show that twin-boundary density keeps constant in the heating and holding stages, while it increases quickly and then intends to decrease in the compressing stage. Its distribution gets more homogeneous under high temperature and medium strain rate. The processing parameter domains corresponding to finer grain size and higher stored-energy, as well as higher dynamic recrystallization degree, contribute to promoting the twinboundary density.

The evolution of grain boundary energy in textured and untextured Ca‐doped alumina during grain growth

AbstractThe role of anisotropic grain boundary energy in grain growth is investigated using textured microstructures that contain a high proportion of special grain boundaries. Textured and untextured Ca‐doped alumina was prepared by slip casting inside and outside a high magnetic field, respectively. At 1600°C, the textured microstructure exhibits faster growth than the untextured microstructure and its population of low‐angle boundaries increases. Atomic force microscopy (AFM) is employed to measure the geometry of thermal grooves to assess the relative grain boundary energy of these systems before and after growth. In the textured microstructure, the grain boundary energy distribution narrows and shifts to a lower average energy. Conversely, the energy distribution broadens for the untextured microstructure as it grows and exhibits abnormal grain growth. Further analysis of the boundary networks neighboring abnormal grains reveals an energy incentive that facilitates their growth. These results suggest that coarsening is not the only dominant grain growth mechanism and that the system can lower its energy effectively by replacing high energy boundaries with those of low energy. The faster growth of lower energy boundaries suggests that isotropic simulations do not adequately account for anisotropic grain growth mechanisms or anisotropic mobility.

Prepositive Synergistic Bulge Design for Improving Aerodynamic Performance of Submerged Inlet

A submerged inlet has good stealth characteristics and a low external drag, but it also has the disadvantage of low internal flow efficiency. In view of this, a new efficiency enhancement method based on the prepositive synergistic bulge of the inlet’s anterior lip is proposed. Taking the submerged inlet of an aircraft as the baseline configuration, a miniature bulge with a square bottom and an outer convex form is designed in front of the inlet’s anterior lip. Through the convex shape of the bulge, part of the low-energy boundary layer airflow is diverted away from the inlet’s entrance, so that the airflow greatly reduces the flow separation after entering the inlet, and the internal flow performance of the entire submerged inlet is improved. Taking the flow field of an aircraft in the classic cruise state as an example, the simulation analysis results show that the flow field characteristics of the entire submerged inlet are obviously improved after adding the synergistic bulge. The total pressure recovery coefficient of the new inlet configuration increased by 1.36%, the total pressure distortion index decreased by 10.86%, and the body drag only increased by 0.37% compared with the baseline case. According to calculations of synergistic bulge inlet configurations with different design parameters, the effect of this configuration is relatively stable, whereby the aspect ratio of the bulge has the greatest impact on the performance, and its value should not be less than 0.75. In addition to the advantages of not requiring additional components or occupying space and being easy to manufacture, the method of adding a synergistic bulge can improve the aerodynamic performance of the baseline inlet under most cruise flight conditions, and its additional drag is small, which gives it a wide applicability range.

Low-Energy Boundaries on Vertical Motion Near the Secondary Body in the CR3BP

Low-Energy Boundaries on Vertical Motion Near the Secondary Body in the CR3BP

Energy dissipation by grain boundary replacement during grain growth

The changes in both the grain boundary area and grain boundary energy that occur during grain growth have been measured in polycrystalline Ni using high energy diffraction microscopy. In addition to the reduction of grain boundary area, the average grain boundary energy decreases as higher energy grain boundaries are replaced by lower energy grain boundaries. This energy dissipation mechanism influences grain boundary migration and might explain the absence of a correlation between grain boundary curvature and migration velocity.Classical studies of isotropic grain growth in polycrystals have been based on the idea that grain boundary (GB) migration is driven by the product of the GB energy and curvature. [1,2] While support for this foundational concept is certainly found in studies of bicrystals [3] and simulations [4], recent experimental evidence contradicts this idea. When measured curvatures and velocities were compared in Ni [5] and SrTiO3, [6] they were found to be uncorrelated. Similar observations were reported for α-Fe, [7] where the measured constant of proportionality between curvature and velocity did not behave in expected ways. The purpose of this letter is to present evidence that, during grain growth, the replacement of higher energy GBs by lower energy boundaries dissipates energy by a mechanism that is not related to curvature. This reduction of the average GB energy represents an additional driving force for grain growth.For a GB network comprised of N triangular mesh elements of area ai, the total free energy (F) is:

Microstructural evolution of pre-twinned Mg alloy with annealing temperature and underlying boundary migration mechanism

This study investigates the variations in the microstructural characteristics of a pre-twinned Mg alloy with the temperature of the subsequent annealing treatment. To this end, a rolled AZ31 alloy is compressed to 3% plastic strain along the rolling direction (RD) to activate {10-12} twinning and is subsequently annealed at 200, 250, 300, 350, and 400 °C. Numerous {10-12} twins are formed throughout the compressed material, leading to the formation of a RD-oriented texture. At an annealing temperature of 200 °C, no microstructural variations occur during annealing. As the annealing temperature increases from 250 to 400 °C, the residual strain energy and remaining twin boundaries of the annealed material decrease owing to the promoted static recovery and the increased area fraction of twin-free grown grains. Consequently, an increase in the annealing temperature results in a gradual microstructural transition from a fully twinned grain structure to a completely twin-free grain structure. The microstructural evolution during annealing is predominantly governed by the movement of high-angle grain boundaries via a strain-induced boundary migration mechanism, and a few twin boundaries migrate above 350 °C because of their lower boundary energy. The boundary migration behavior and resultant microstructural evolution are discussed in detail based on the variations in boundary mobility and driving force for boundary migration with annealing temperature.

Upper Limit of Outer Radiation Belt Electron Acceleration Driven by Whistler‐Mode Chorus Waves

AbstractWe investigate the outer radiation belt electron energy spectra of the highest fluxes observed during storms based on Van Allen Probes measurements, which demonstrate a similar shape despite various levels of geomagnetic activity. Using quasi‐linear diffusion simulations, we reproduce the observed electron acceleration by whistler‐mode chorus waves during the storm of 25 October 2016, when the maximum fluxes were close to the highest upper limit observed during the 2013–2018 period. The electrons below ∼1 MeV reached the upper limit of chorus acceleration within ∼1 day and then remained at a stable level, while the multi‐MeV electrons with sharper energy gradient were subject to the continuous acceleration process. Our results reveal the natural upper limit of electron acceleration by chorus, which strongly depends on the lower energy boundary, and the stable seed population at hundreds of keV (but not the chorus intensity) which is a prerequisite for the relativistic electron acceleration.

Atomistic weak interaction criterion for the specificity of liquid metal embrittlement

Liquid metal embrittlement (LME) occurs in some solid–liquid metal elements’ couples (e.g., Fe-Zn and Al-Ga), called specificity. Although some material parameters like solubility and bonding energy were suggested as controlling factors, none could be attributed satisfactorily. Here we have unveiled the primary factor that governs the specificity of LME. From first-principles calculations compared with a systematic surveillance test result, we found that the grain-boundary (GB) adsorption energy shows near-zero values in all embrittling couples; the interaction between solid and liquid metal atoms is weak when an atom from the liquid state penetrates the grain boundary of the solid. Furthermore, we found that the calculated surface adsorption energy that promotes bond-breaking does not correlate to the specificity. Therefore, we consider that the penetration of a liquid metal atom surrounded by weakly interacting solid metal atoms is necessary before the bond-breaking assisted by surface adsorption occurs at a microcrack tip. This mechanism is also applicable for transgranular cracking along low-energy boundaries and crystal planes. While liquid metal atoms penetrate and diffuse into solid GB macroscopically before cracking, liquid metal’s surface adsorption stronger than GB adsorption should promote the bond-breaking of solid metal. In conclusion, the atomistic penetration precedes the surface-adsorption-assisted bond-breaking and controls the specificity of LME.

High thermal stability of nanocrystalline FeNi2CoMo0.2V0.5 high-entropy alloy by twin boundary and sluggish diffusion

A nanocrystalline (NC) face-centered cubic FeNi2CoMo0.2V0.5 high-entropy alloy was produced by high pressure torsion (HPT). The evolutions of microhardness and microstructure of the NC alloy during subsequent isochronal annealing were investigated systematically by electron back-scattering diffraction (EBSD) and high-resolution transmission electron microscopy (HRTEM). It was found that nano-grains and deformation nano-twin lamella were obtained at outer disk edge after HPT process with a hardness plateau of 450 HV. Isochronal annealing below 600 °C induced an evident hardening without precipitation effect, due to the annihilation of mobile dislocations and sustained deformation twin barriers. Evident recrystallization and grain growth of the NC FeNi2CoMo0.2V0.5 high-entropy alloy occurred during isochronal annealing at temperatures higher than 600 °C. The activation energies of recrystallization and grain growth of the NC FeNi2CoMo0.2V0.5 high-entropy alloy were calculated to be 350 kJ/mol and 272 kJ/mol, respectively, corresponding to a slow defects recovery process and a swift GB migration process. The high thermal stability of the NC FeNi2CoMo0.2V0.5 high-entropy was mainly caused by kinetic sluggish diffusion effect and deformation twin boundaries with thermodynamic low boundary energy, which retarded the movements of dislocations and grain boundary.

Variant selection in additively manufactured alpha-beta titanium alloys

The crystallographic arrangements of the α-phase variants in α-β titanium alloys remains less identified due to the crystallographic complexity involved while being essential to understand the α-β microstructural intricacy. To improve the current understanding, specimens of two columnar-grained α-β Ti alloys (Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo) and two equiaxed-grained α-β Ti alloys (Ti-6Al-4V and Ti-4Al-2V) were fabricated by laser metal powder deposition (LMD). Electron backscatter diffraction (EBSD) analyses were applied to more than 105 α-phase variants in each alloy. The results revealed that the Type 4 α/α variant boundary ([10¯553¯]/63.26∘) is prevalent in the two columnar-grained α-β alloys while the Type 2 α/α variant boundary ([112¯0]/60∘) is common in the two equiaxed-grained α-β alloys. Further EBSD characterisation indicates that α-variant selection tends to be more prevalent in equiaxed prior-β grains, featured by the Category I triple-α-variant clusters, which mostly terminate on dense {101¯1} planes with lower boundary energy. Conversely, columnar prior-β grains show significant Category II triple-α-variant clusters, which mostly terminate on less dense {41¯3¯0} planes with higher boundary energy. Self-accommodation to compensate for the β→α transformation strain is assumed to be the major underlying mechanism. The implications of these findings for understanding the tensile strengths are discussed in conjunction with the Schmid factor of α-variants calculated in columnar- and equiaxed-grained Ti-6Al-4V.

Numerical study of the aerodynamic effects of bio-inspired leading-edge serrations on a heaving wing at a low Reynolds number

A numerical study with large-eddy simulation (LES) method is performed on the aerodynamic effects of bio-inspired leading-edge serrations on a wing in heaving motion at a low Reynolds number of Re=2×104. During the research, rigorous contrastive simulations are performed for the baseline wing and the serrated ones. The baseline wing has a NACA0012 profile and the serrated ones are obtained by stretching/shrinking the leading portion of the baseline sinusoidally. Conclusive results show that leading-edge serrations can help reduce aerodynamic hysteresis and suppress dynamic-stall-induced load oscillations effectively. More importantly, serrated wings can be more efficient in lift production than the baseline, especially for relatively high reduced frequencies. In the currently covered parameter space, the largest lift efficiency improvement achieved by the leading-edge serrations can reach 15.92%, and the least improvement is 2.73%. Along with these good properties, the leading-edge serrations have induced larger drag, which indicates they are undesirable for propulsive motivations. Visualization of the flow fields reveals that a series of counter-rotating streamwise vortex pairs are dynamically induced by the leading-edge serrations. These streamwise vortices can act as momentum transporters and help enhance momentum exchange between low-energy boundary layer and high-energy outer flows, which can affect the flow structures as well as the aerodynamics of a heaving wing greatly. The current results may provide inspirations for improving aerodynamics, economies and stabilities of bionic underwater/aerial micro robots or any other devices which have heaving/flapping parts.

Evolution of precipitation particles with respect to grain boundary energy in Grade 91 steel during creep exposure

ABSTRACT Precipitation particles evolution in ASME Grade 91 steel after 233,823 h of creep exposure at 600 °C was characterised after grain boundary energyclassification in terms of misorientation angle (MOA): Low energy boundary(LEB) with MOA up to 15°, high energy boundary (HEB) with MOA from 15° to 45°, andmedium energy boundary (MEB) with MOA from 45° to 63°. The number density of the Laves phase stronglydepends on the boundary energy classification by MOA: the largest in the HEB,followed in order by the MEB and the LEB. The Z-phase particles is preferentiallylocated in grain boundaries where at least one of adjacent boundaries is theHEB or the MEB. Even though the block boundaries belong tothe MEB in terms of MOA, they are not the preferential site for the Z-phase due to their exceptionally smallboundary energy.

Analysis of pulsed suction flow control behavior based on a nonlinear reduced-order model

The use of pulsed suction is an effective unsteady flow control method. However, compared with steady suction, which simply eliminates low-energy boundary layer, pulsed suction flow control presents unique phenomena and complex control mechanisms that are not explained fully. Thus, in this paper, pulsed suction flow control in a compressor cascade is investigated by numerical simulation. Numerical results show that the control performance depends on the suction frequency, momentum, and location, as any unsteady active flow control method. To further reveal the mechanism and explain the phenomena of pulsed suction, a nonlinear reduced-order model for pulsed suction flow control is established on the basis of the Navier–Stokes equation, Stuart vortex row model, Oseen vortex model, and Stuart–Landau equation. The entrainment degree and maximal Lyapunov exponent are used as indices to evaluate the flow losses and control performance in this model. By solving the equations of this model under different parameters, the model presents similar phenomena as found in the numerical simulations with various suction frequencies, momentum, and locations. Thus, it is innovative that the model with calibrated parameters has the ability to roughly predict control performance of unsteady pulsed suction in an efficient way. Also, based on this reduced-order model, one can explain that, unlike steady suction, chaos control, momentum transfer and lock-in mechanisms are especially relevant with the control physics of pulsed suction.

Structural evolution during the cycling of NiTi shape memory alloys

Abstract The structural evolution in NiTi shape memory alloys subjected to pseudoelastic cycling is examined in the present study. Single crystals with [100] and [111] orientations were subjected to repeated compressive cycles and then studied by transmission electron microscopy (TEM). TEM observations were made at cycle numbers 1, 2, 5, 10, and 20 since the majority of degradation occurs during these initial cycle numbers. Under compression, single crystals with [111] orientations degraded much faster than crystals with [100] orientations. Under tension, single crystals with [100] orientations fractured in the elastic region, and crystals with [111] orientations showed considerable degradation as a function of cycling. Intermittent TEM observations on single crystals oriented along the [111] direction showed an increase in dislocation density on multiple active slip systems as a function of cycling. Single crystals oriented along the [100] orientation show a less dramatic increase in dislocation density as a function of cycling. TEM observations have revealed that dislocation structures formed near martensite plates have a similar periodicity as internal twin modes within the martensite. This observation implies that, although the interface between the martensite and parent phase is a low-energy boundary, the local disruptions due to internal twins create preferential nucleation sites for the formation of lattice defects.