Boundary Mobility Research Articles

The Influence of Electric Fields on Grain Boundary Mobility in Alumina

The Influence of Electric Fields on Grain Boundary Mobility in Alumina

Mechanism of microstructure and texture evolution during hot rolling of pure Mg and Mg-0.2%Ce alloy

ABSTRACT The extruded samples of pure Mg and Mg-0.2%Ce alloy were subjected to a 90% reduction in thickness at 250°C during hot rolling. The microstructure, texture, and mechanical properties of the hot-rolled samples were evaluated. The microstructural evolution shows the presence of shear bands in Mg-0.2%Ce alloy at an angle of 30-35° concerning the normal direction and no such bands are observed in pure Mg. Upon annealing the samples at 400°C for one hour, it is observed that the grain size of Mg-0.2%Ce is smaller than pure Mg indicating that the precipitates of Ce arrest grain boundary mobility. The ex-situ EBSD microstructure analysis results show a higher change in the value of α33 components after annealing for pure Mg compared to Mg-0.2%Ce alloy suggesting the precipitates of Mg12Ce stabilises the pyr. <c + a > dislocations. The basal texture is observed in both samples after hot rolling. The experimental texture was simulated using the visco-plastic self-consistent (VPSC) modelling where it is observed that the initial extruded texture is favourable for the higher activity of pyr. <c + a > dislocation and extension twinning in Mg-0.2% e alloy. Further, the yield locus shows that Mg-0.2%Ce alloy shows relatively lower anisotropy compared to pure Mg. The yield strength and UTS of the pure Mg are higher than the Mg-0.2%Ce alloy which is attributed to the relatively more dislocation-dislocation interaction in pure Mg compared to Mg-0.2%Ce alloy resulting in the lower mean free path of the dislocations in pure Mg.

Family boundaries: identification and dynamics

The modern existence of Russian families is characterized by a variety of lifestyle forms. The changing family landscape, which includes a range of blood and non-blood relatives and partners, expands the understanding of this institution, leading to broader definitions of the family and its boundaries. The purpose of the article is to analyze the dynamics of young people’s representations about family boundaries. The article attempts an interdisciplinary synthesis of theoretical and practical knowledge, namely the application of semiotic analysis in sociological research. The theoretical and methodological basis of the study Yu.M. Lotman. The empirical base consisted of data from three studies conducted using a similar methodology: 2007 (n=213), 2017 (n=132), 2023 (n=113). Using the concept of “significant family member”, the mobility of family boundaries is revealed. The respondent’s partners – young people, common-law spouses – “moved” from the periphery to the core of the family. The greatest changes occurred in the structure of family ties. The concept of S.I. Golod made it possible to theoretically substantiate the transformation of ideas about the family and its boundaries. Yu.M. Lotman's approach to the analysis of the phenomenon of borderliness complemented the ideas about the essence and functions of the boundaries of the family sphere and made it possible to theoretically interpret their dynamics. Interdisciplinary synthesis of sociological and semiotic knowledge when studying the boundaries of the family sphere seems promising.

Stacking fault energy during DRV-DRX competition in high-nitrogen austenitic stainless steel used in orthopedic implants

High-nitrogen (high-N) austenitic stainless steels (ASS) are used in orthopedic implants due to their mechanical properties, human biocompatibility, and affordable cost. However, the high content of (Ni–Nb–N)-rich precipitates can cause allergic reactions and significant challenges in the manufacturing of prostheses. This study investigates the competition between work hardening (WH), dynamic recovery (DRV), and dynamic recrystallization (DRX) during the physical simulation of an ASTM F-1586 steel by thermomechanical processing, using the Kocks-Mecking and Avrami constitutive models. The parameters were obtained through continuous isothermal torsion tests at the temperature range of 900–1200 °C, strain rates between 0.01 and 10 s−1 and a total deformation of 4.0. Determined by compositional-analytical methods, the stacking fault energy (γsfe) was correlated with the stress level (σi) according to the Uesugi and Dai models. Results indicated a high activation energy for hot deformation (Qdef = 587 kJ/mol), affecting the shape of the curves. The γsfe value varied along the curves, delaying the onset of the DRX (σc = 0.928σp). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses revealed a direct competition between work-hardened and recrystallized grains in the early part of the curves, due to WH-DRV synergy. After the peak stress (σp), the progress of DRX was slow, with the Avrami exponent (n) between 1.1 and 1.9, completing only after large deformations (σss = 0.714σp). The moderate γsfe value (69 mJ/m2) and fine precipitates of the Z-phase (CrNbN) (<20 nm) influence the grain boundary mobility during the DRV-DRX competition, delineating the stress-strain curve shape.

Compliant Lattice Modulations Enable Anomalous Elasticity in Ni-Mn-Ga Martensite.

High mobility of twin boundaries in modulated martensites of Ni-Mn-Ga-based ferromagnetic shape memory alloys holds a promise for unique magnetomechanical applications. This feature has not been fully understood so far, and in particular, it has yet not been unveiled what makes the lattice mechanics of modulated Ni-Mn-Ga specifically different from other martensitic alloys. Here, results of dedicated laser-ultrasonic measurements on hierarchically twinned five-layer modulated (10M) crystals fill this gap. Using a combination of transient grating spectroscopy and laser-based resonant ultrasound spectroscopy, it is confirmed that there is a shear elastic instability in the lattice, being significantly stronger than in any other martensitic material and also than what the first-principles calculations for Ni-Mn-Ga predict. The experimental results reveal that the instability is directly related to the lattice modulations. A lattice-scale mechanism of dynamic faulting of the modulation sequence that explains this behavior is proposed; this mechanism can explain the extraordinary mobility of twin boundaries in 10M.

Phase-Field Simulation of Grain Growth in Uranium Silicide Nuclear Fuel

Uranium silicide (U3Si2) is regarded as a viable fuel option for improving the safety of nuclear power plants. In the present work, phase-field simulations were employed to investigate grain growth phenomena, encompassing both isotropic and anisotropic grain growth. In simulations of isotropic grain growth, it is commonly assumed that the energy and mobility of the grain boundaries (GBs) remain constant, represented by average values. The calculated grain growth kinetic rate constant, K, exhibits a close correspondence with the experimental measurements, indicating a strong agreement between the two. In simulations of anisotropic grain growth, the values of GB energy and mobility are correlated with the angular disparity between GBs. The simulation results demonstrated that the growth rate of U3Si2 can be influenced by both the energy anisotropy and mobility anisotropy of GBs. Furthermore, the anisotropy in mobility results in a greater prevalence of low-angle GB distribution in comparison to high-angle GBs. However, the energy anisotropy of GBs does not impact the frequency distribution of the angle difference between GBs.

Boron induced abnormal grain growth in alumina

In this work, hot-pressing of alumina in contact with hexagonal boron nitride or doped with boron carbide was conducted at 1500 °C for 30 min. After hot-pressing, abnormal grain growth induced by boron diffusion from these substances into alumina was detected, as clearly demonstrated with SEM, EDS, EBSD, and Raman spectroscopy. Grain boundary complexion transformations, solute drag, or another mechanism relating to interface-controlled grain boundary mobility are presumed to be the fundamental mechanism responsible for abnormal grain growth observed in this work.

Revealing the high strength and high thermal stability of a nano-lamellar Cu-0.1 at.% Zr alloy

Nano-lamellar structured Cu-0.1 at.% Zr alloys are prepared by the combination of equal channel angular pressing and subsequent cryogenic rolling. The advanced processing route enables a systematic reduction of the lamellae boundary spacing down to 45 nm and a concurrent increase of yield strength up to 626 MPa. Subsequent annealing at temperatures up to 673 K induced recovery processes and uniform coarsening through triple junction motion, avoiding a catastrophic degradation of the microstructure and performance and preserving the yield strength at a level comparable to ultrafine-grained copper. Simultaneously, the tensile uniform elongation experienced a substantial recovery up to 11.9 %. The high thermal stability is mainly attributed to the low mobility of grain boundary and the relaxation of non-equilibrium grain boundaries, both of which are facilitated by the microalloying of Cu with Zr. Based on the theory of dislocation pile-up at boundaries, the Hall-Petch equation is adopted and extended to the lamellar structured Cu-0.1 at.% Zr alloy, revealing a lower-critical bound of 90 nm for lamellae boundary spacing, corresponding to an effective boundary spacing of about 150 nm.

Anharmonic incommensurate structure modulation in Ni-Mn-Ga martensite exhibiting highly mobile twin boundaries

We perform neutron diffraction on a bulk Ni50.0Mn27.5Ga22.5 single crystal to investigate the evolution of its five-layered modulated (10M) martensite structure, from 300 K down to 10 K. Close to martensite transformation, the modulation is nearly commensurate, with q = 0.402 in a modulation vector (q, q, 0). Upon cooling, the shift in diffraction satellites indicates a transition to an incommensurate modulation with increasing q. However, the observed fifth-order diffraction satellites below 260 K and even more complex satellite landscape at 10 K cannot be explained by incommensurability alone. Our analysis reveals that the modulation function is highly anharmonic, encompassing Fourier components up to the eighth order. We have developed a single-parameter description of the evolution of modulation across the entire temperature interval of the 10M phase existence. Surprisingly, the structure evolution from commensurate to incommensurate modulation has no significant effect on twin boundary mobility.

Fracture modes and grain growth of tungsten during extreme cyclic heating

To investigate the fracture mechanisms resulting from the thermomechanical loading in fusion reactors, cyclic heating experiments were carried out on tungsten discs (W). These experiments involved subjecting the samples to an arc-jet plasma gun, which can deliver a heat flux exceeding 20 MW m−2. The W discs had varying thicknesses and pulse durations. To gain a better understanding of the thermal environment, finite element (FE) simulations were performed to model heat transport through exposed specimens and the cooling assembly, and these were compared with experimental measurements. The temperature at the edge of the sample was found to reach a maximum of 1500 °C, and the temperature at the center of the upper surface of the disc fluctuated between a maximum of over 2800 °C and a minimum of 500 °C, depending on the thickness of the disc and the thermal contact between the sample and the cooling structures. Cracks were found to propagate through different modal regimes, starting with ductile growth, followed by a combination of ductile/intergranular growth, intergranular growth, and finally brittle transgranular fracture (cleavage fracture). Steep temperature gradients (∼1.6 × 106 °C/m) were found to induce grain growth, with a grain boundary mobility of 7.5 eV under moderate thermal stresses. Alternatively, crack formation and growth can occur due to residual stresses. Cumulative residual stress has been shown to correlate with the duration of exposure to elevated temperatures, leading to increased hardening, although with noticeable grain growth. Consequently, the strength of the sample is primarily driven by thermal stress rather than grain growth.

A high-strength binary Mg-1.2Ce alloy with ultra-fine grains achieved by conventional one-step extrusion during 300–400 °C

Ultra-fine grained magnesium alloys are usually prepared by severe deformation and medium-low temperature deformation. In this work, it has been found that the binary Mg-1.2Ce alloy with the uniform distributed Mg12Ce precipitates smaller than 3 μm is a good candidate to obtain ultra-fine grains by conventional one-step extrusion at high temperatures. All the three Mg-1.2Ce samples extruded at 300 °C, 350 °C and 400 °C show a bimodal microstructure with high-density dislocations, whose average grain sizes are 1.19 ± 0.92 μm, 1.32 ± 1.30 μm, and 1.44 ± 1.33 μm, respectively. A linear relation of ln(d) = -0.05ln(Z)+1.27 was determined relating the average grain size (d) to the Zener-Hollomon parameter (Z). The alloy extruded at 300 °C exhibits an exceptionally high ultimate tensile strength (UTS) of 412.3 ± 5.3 MPa and yield strength (TYS) of 387.6 ± 3.2 MPa, but a low elongation after fracture (El) of 4.9 ± 0.8 %. With the extrusion temperature increasing, the tensile strength gradually decreases. For the 1.2E-400 sample, the TYS and the UTS drop to 347.2 ± 2.1 MPa and 349.4 ± 2.8 MPa, while the El increases to a more acceptable value of 12.6 ± 1.4 %. The microstructure analysis reveals that Ce atoms segregate along grain boundaries and dislocations in the Mg-1.2Ce alloy, which can limit the gliding of GBs or the slipping and climbing of dislocations. Additionally, dislocations can be pinned by Mg12Ce precipitates, thereby also restricting dislocation and grain boundary mobility. Consequently, the dynamic recrystallization (DRX) process is delayed, leading to the formation of a bimodal microstructure.

Quantification of grain boundary mobilities in natural olivine by annealing experiments and full-field modelling

We investigate olivine grain boundary (GB) migration in natural peridotites experimentally annealed at high pressure and high temperature, and couple the experimental observations to full-field grain growth models to provide the distribution of GB mobilities in natural olivine polycrystals. A stack of four slices of natural mylonitic peridotite (Oman ophiolite) was annealed at 1473 K for 5 h under a confining pressure of 300 MPa of argon in a Paterson press. The three sintered interfaces of the stack are characterized using scanning electron microscopy and electron backscatter diffraction (EBSD) to extract a distribution of apparent (2D) GB displacements. Full-field simulations of numerically sintered interfaces are then used to infer the GB mean mobility and distribution in olivine polycrystal allowing forward modelling exercise to reproduce the experimental GB displacement distribution.This yields widely dispersed mobilities, which can be approximated by a log-normal distribution of 10−15.85±2.58m4.J−1.s−1. Both the average and the dispersion of GB mobilities can be explained by silicon grain boundary diffusion. Finally, we demonstrate that the high variability of GB mobilities in olivine implies a decrease of the mean growth rate with time. This elucidates the difficulties of extrapolating experimental grain growth rates to geological timescales and observed microstructures in peridotites.

Atomic‐Scale Study of Intercrystalline (Mg,Fe)O in Planetary Mantles: Mechanics and Thermodynamics of Grain Boundaries Under Pressure

AbstractPolycrystalline (Mg,Fe)O ferropericlase is the second most abundant mantle constituent of the Earth and possibly of super‐Earth exoplanets. Its mechanical behavior is expected to accommodate substantial plastic deformation in Earth's lower mantle. While bulk properties of ferropericlase have been extensively studied, the thermodynamics of grain boundaries and their role on mechanical response remain largely unexplored. Here, we use density functional theory calculations to investigate mechanical behavior and thermodynamics of the {310}[001] grain boundary—a representative proxy for high‐angle {hk0}[001] tilt grain boundaries—at relevant mantle pressures of the Earth and super‐Earth exoplanets. Our results provide evidence that shear‐coupled migration and grain boundary sliding are the dominant mechanisms of (Mg,Fe)O grain boundary mobility. We show that pressure‐induced structural transformations of grain boundaries can trigger a change in the mechanism and direction of grain boundary motion. Significant mechanical weakening of the grain boundary is observed under multi‐megabar pressures, caused by a change in the grain boundary transition state structure during motion. Our results identify grain boundary weakening in periclase as a potential mechanism for viscosity reductions in the mantle of super‐Earths. We further demonstrate that structural grain boundary transitions control the spin crossover of Fe2+ in the grain boundaries. We model iron partitioning behavior between bulk and grain boundaries and predict equipartitioning to occur in μm size ferropericlase grains. Our findings suggest that the iron spin crossover pressure in ferropericlase may increase several tens of GPa by pressure‐induced structural grain boundary transitions in dynamically active fine‐grained lower mantle regions.

Surface-enhanced p-type transparent conducting CuI−Ga2O3 films with high hole transport performance and stability

CuI is a wide bandgap p-type transparent conductor with promising optoelectronic properties. However, conventional CuI films prepared through the iodination of Cu films exhibit rough surface, poor stability and excessively high hole concentration, hindering its application. Herein, we introduce CuI−Ga2O3 composite films as a substitution to overcome the shortcomings of pure CuI. During the iodination process, Ga2O3 hindered the mobility of CuI grain boundaries, resulting in small grain size and low surface roughness. After the charge re-equilibrium and low-energy carrier filtering at the interface between Ga2O3 and CuI, the hole concentration in the films decreased by an order of magnitude. Due to the interface effect of Ga2O3 at the grain boundaries of CuI, the hole mobility increased by 5 times, and the conductivity improved by 53%. The ultra-wide band gap of Ga2O3 enhanced the transmittance of CuI films in short wavelength region. Ga2O3 served to passivate the grain boundaries of CuI, preventing the oxidation of CuI and the loss of iodine, consequently improving the stability of the films. This work will deepen the understanding of the failure and hole transport mechanisms of CuI films.

Evolution of microstructure and grain boundaries during annealing of high-purity tantalum materials

The microstructure and texture evolution of tantalum (Ta) during annealing were studied in this work, in addition to simulating and analyzing of the grain boundary motions and their interactions using molecular dynamics simulations (MD). The results showed that the evolution of the recrystallized texture in the surface region of Ta is different from that in the core region. Coincident site lattice (CSL) is significantly related to the grain orientation and thus directly affects the recrystallization texture. MD results showed that the grain boundary mobility rate is higher at low angle grain boundaries (LAGB), which implied that the mobility rate is influenced by the tilt angle. Lower mobility rate of high angle grain boundary (HAGB) possibly caused by a large number of stabilized Σ3 grain boundaries (GBs) between them. It is shown that the inhomogeneous distribution of the recrystallization texture during the annealing process can be possibly attributed to the presence of a large number of Σ3 GBs.

In situ study and assessment of the phosphorus-induced solute drag effect on the grain boundary mobility of austenite

The solute drag effect of phosphorus (P) in the single-phase austenite (γ-Fe) region was studied under isothermal annealing conditions at temperatures of 1050 °C, 1150 °C, 1250 °C and 1350 °C. High-temperature laser scanning confocal microscopy (HT-LSCM) was used to observe and quantify in situ grain growth on the surface of three different samples containing 0.026 wt.-% P, 0.044 wt.-% P and 0.102 wt.-% P. The dependence of the arithmetic mean grain sizes on time and temperature were modeled mathematically using a simple ordinary differential equation (ODE) according to normal grain growth (NGG) theory. As no other major effects, i.e., Zener-pinning by precipitates, occurred under the selected experimental conditions, grain growth interference was only considered by grain boundary (GB) segregation of P. Thus, the total grain boundary mobility (M) was directly determined depending on the P content and isothermal annealing temperature. The fitted GB mobility values enabled the determination of an average binding energy value between impurity P atoms and grain boundaries (E0 = -1.1 - -0.6 eV) in the system. Finally, the GB segregation of P in γ-Fe was derived from the observed grain growth kinetics. The results showed reasonable agreement with calculations using segregation enthalpies from the literature.

Mechanical response in tension of twin-induced Mg alloys in wide strain rate regimes

Effect of twin boundary with/without segregation of solute atoms on deformation behavior in tension is examined in Mg–3Al–1Zn (AZ31) alloys from low strain rates to high strain rates. Solute atoms (Al and/or Zn atoms) are segregated at {10 1‾ 2} twin boundaries by static annealing at a temperature of 423 K with a holding time of 150 min. The room-temperature tensile mechanical response of the specimens, which have pre-induced twin boundaries, shows detwinning behavior, regardless of the strain rate between 10−5/s and 100/s. The yield strength of these specimens is also unlikely to be affected by strain rate, because twin boundary mobility, i.e., the shrinkage, is an athermal process. On the other hand, the specimen with segregating of solute atoms at twin boundaries exhibits an increased strength and a decreased strain hardening as compared with those in the specimen having non-segregated twin boundaries. Newly formed deformation twins of several types, i.e., the {10 1‾ 2} twins, {101‾ 1} twins and {101‾ 2}-{101‾ 1} double twins, are the origin of fracture in these specimens.

Thermal deformation behavior and microstructure analysis of the as-cast super austenitic stainless steel Sanicro35

Thermal compression experiments on the super austenitic stainless steel Sanicro35 were carried out using a Gleeble 3800 thermal simulation laboratory machine to investigate its thermal deformation behavior at different deformation temperatures (900 °C–1150 °C) and strain rates (0.001–10 s−1). The microstructure of the large deformation zone of the specimen was investigated using electron backscatter diffraction (EBSD). The results show that the thermal compression rheological stress of the super austenitic stainless steel Sanicro35 decreases with increasing temperature and decreasing strain rate. Dynamic recrystallization (DRX) is the main softening mechanism for this material. The morphology characteristics, recrystallization fraction, dislocation density and twin grain boundary distribution of the microstructure were analyzed by EBSD. With the increase of deformation temperature, the higher grain boundary mobility contributed to the growing of DRX grains. As the strain rate increases, the larger deformation storage energy provides sufficient activation energy for DRX grain nucleation, and the nucleation of DRX grains becomes denser. The twin boundaries are mainly distributed within the DRX grains. The smaller the grain size of DRX, the denser the nucleation of twin boundaries, and the generation of twins can promote the development of DRX. The softening mechanism under most deformation conditions is discontinuous dynamic recrystallization (DDRX). However, at 10 s−1, the high strain rate causes microbands to be generated within the deformed grains, and the microband boundaries evolve toward the high-angle grain boundaries (HAGBs) with increasing temperature, which promotes the occurrence of Continuous dynamic recrystallization (CDRX).

Statistics of grain microstructure evolution under anisotropic grain boundary energies and mobilities using threshold-dynamics

This paper investigates the statistics of two-dimensional grain microstructures during grain growth under anisotropic grain boundary (GB) energies and mobilities. We employ the threshold dynamics method, which allows for unparalleled computational speed, to simulate the full-field curvature motion of grain boundaries in a large polycrystal ensemble. Two sets of numerical experiments are performed to explore the effect of GB anisotropy on the evolution of microstructure features. In the first experiment, we focus on abnormal grain growth and find that GB anisotropy introduces a statistical preference for certain grain orientations. This leads to changes in the overall grain size distribution from the isotropic case. In the second experiment, we examine the development of texture and the growth of twin boundaries for different initial microstructures. We find that texture development and twin growth are more pronounced when the initial microstructure has a dominant fraction of high-angle grain boundaries. Our results suggest effective GB engineering strategies for improving material properties.

Ultra-high strength ZTA ceramic foams with large amounts of intragranular structures resulting from dual-phase sol

A creative way to overcome the general problem of high-porosity ceramic foams with low intensity has been proposed here. For the first time, zirconia toughened alumina (ZTA) ceramic foams with a large number of intragranular structures have been prepared by particle-stabilized foams using boehmite sol and ZrO2 sol as raw materials. The grain boundary mobility of alumina in ceramic foams is much higher than that of zirconia due to the extremely high sintering activity of the boehmite nanoparticles, and then a typical intragranular structure of alumina grains wrapping around zirconia grains is formed. Based on the strengthening effects of intragranular zirconia grains, the as-prepared ZTA foams have excellent mechanical properties, with ultra-high compressive strength of 10.3–88.0 MPa and high porosity of 91.0–79.1 %. This paper provides a novel strategy for the preparation of high-performance ceramic foams.