Grain Boundary Effects Research Articles

Edge dislocations, alloy composition, and grain boundaries effects on the mechanical properties in NiCo binary alloy.

In this research, we investigated the mechanical properties of NiCo binary alloy both with and without grain boundaries, across various alloy compositions. We investigated the effects of edge dislocation, alloy compositions, and grain boundaries on the mechanical properties of FCC using molecular dynamics (MD) simulations. By analyzing the influence of the grain boundaries with different alloy compositions on several mechanical properties, we were able to gain a deeper understanding of how these characteristics were modified. The elastic moduli increased with increasing grain boundaries for a specific chemical composition, indicating that becomes more rigid. The anisotropy factor analysis showed that have a natural tendency toward anisotropy, which is further influenced by the presence of grain boundaries. Grain boundaries have a significant effect on strength and ductility across a wide range of alloy compositions. These results indicate that a deeper comprehension of these implications can aid in designing improved binary alloy with superior mechanical characteristics.

In situ phase engineering during additive manufacturing enables high-performance soft-magnetic medium-entropy alloys.

Additive manufacturing (AM) shows promise as a method for producing soft-magnetic multicomponent alloys for use in electric motors and sustainable electromobility applications. However, the simultaneous achievement of a high saturation magnetic flux density (Bs) and a low coercivity (Hc) in AM soft-magnetic materials remains challenging. Herein, we present an approach that integrates an elemental powder mixture of Fe45Co30Ni25 with Fe2O3 nano-oxides, which is then subjected to laser powder bed fusion (LPBF) followed by high-temperature annealing to achieve an FCC-structured Fe45Co30Ni25 MEA/FeO composite. The FeO nanoparticles, a byproduct of the reaction between Fe powders and Fe2O3 nano-oxides, serve as nucleation sites for the formation of a single FCC phase in the MEA matrix. The resulting LPBF MEA/FeO composite has a Bs of 2.05 T and an exceedingly low Hc of 115 A m-1, compared to those of the BCC/FCC dual phase MEA and other state-of-the-art additively manufactured soft-magnetic alloys. In situ Lorentz transmission electron microscope (TEM) revealed that the low Hc of the FCC-structured MEA/FeO composite originates from the reduced pinning effect of grain boundaries in the FCC phase on domain wall movement compared with those in the FCC/BCC dual phase.

Effect of grain boundary engineering on corrosion behavior and mechanical properties of GH3535 alloy in LiCl-KCl molten salt

This study investigated the effect of grain boundary engineering (GBE) on the corrosion behavior and high-temperature mechanical properties of GH3535 alloy in 45LiCl-55KCl wt.% molten salt at 550°C. After corrosion for 300 h, a triple-layered product was formed on the solid solution specimen (Non-GBE), consisting of discontinuous NiCr2O4 outer-layer, Ni3Fe middle-layer, and NiCr2O4 inner-layer. For the GBE specimen, quite milder corrosion occurred on it that its surface still kept original polishing scratches. The real mass loss of the Non-GBE alloy (6.85 mg/cm2) is one order of magnitude higher than that of GBE (0.65 mg/cm2). The beneficial effect of GBE on improving alloy's corrosion resistance is owing to: surface carbide dissolution, discontinuous random high angle grain boundary and low dislocation density. High proportion of Σ3n grain boundaries and less carbide precipitation ensure stable high-temperature deformation performance of GBE sample in molten salt.

Optimizing oxygen ion conductivity by modulating crystalline homogeneity

Keeping the stoichiometry at the optimal value required for electrical conduction throughout polycrystalline ceramics is crucial for various polycrystalline oxygen ion conductors. Using the photoluminescence spectrum of Eu3+ as a structural probe, the dependence between composition uniformity and conductivity was confirmed. We found that, for apatite La9.67×0.99Eu9.67×0.01Si6O26.505 (LSO) oxygen ion conductors synthesized by glycine nitrate combustion method, higher glycine content enhances the La/Si ratio uniformity, thereby improving the bulk conductivity. Surprisingly, thermal diffusion even at 1550°C cannot achieve uniform distribution when the precursor materials are not sufficiently homogeneous. Slight non-uniformity in the La/Si ratio is beneficial for impurities aggregation at high-density grain boundary regions, thereby alleviating the blocking effect of grain boundary on oxygen ion conduction. Both the bulk and total conductivities are higher than the best reported values in polycrystalline apatite by optimizing the La/Si ratio uniformity. The insight gained in this work is beneficial for enhancing conductivity by modulating crystalline homogeneity, and it facilitates further enhancement of ionic conductivity through cations doping for various polycrystalline oxygen ion conductors.

Enhanced ductility and thermal stability of TZM alloys via nanoscale second phase

In this study, molybdenum alloy with the nanoscale second phase dispersion distribution (NM-TZM) was prepared by powder metallurgy. Despite maintaining a high yield strength of 893 MPa, the elongation of the as-forged MN-TZM alloy has been increased to 25.3 %. In addition, the recrystallization start temperature of NM-TZM alloy was increased by 100 °C, reaching 1400 °C. The nanoindentation results indicate that the NM-TZM alloy still exhibits a high hardness of 4.40 GPa following annealing at 1400 °C. The addition of nanoscale second phase can improve the ductility and high temperature stability of the alloy by coupling with dislocations and grain boundaries. A new model for the synergistic effect of grain boundaries and intragrains to improve ductility is proposed, which provides insight into the reason behind the high ductility of NM-TZM.

Effect of grain size and grain boundary on the energy storage performance of polycrystalline ferroelectrics

Dielectric capacitors based on polycrystalline ferroelectrics have attracted much attention due to their significant power density and fast charge–discharge speed. The energy storage performance of polycrystalline ferroelectrics is highly dependent on the grain size and grain boundary. Here, the effect of grain size and grain boundary on the domain structures and polarization–electric field (P–E) hysteresis loops of polycrystalline ferroelectrics are investigated by using a phase-field model based on the time dependent Ginzburg–Landau (TDGL) equation. It is found that the depolarization field in the grain boundary induces the vortex domain when the grain size is reduced or the grain boundary thickness increases in certain extent, resulting in slender P–E loops, which contributes to an improvement in the energy storage efficiency and density. However, as the grain size further decreases or the grain boundary thickness further increases, the energy storage density decreases, which is attributed to the concurrent reduction in both the remnant and saturation polarizations. This study provides a considerable insight for optimizing the energy storage performance by carefully adjusting the grain size and grain boundary thickness in polycrystalline ferroelectrics.

Structural, dielectric, impedance, ferroelectric, piezoelectric, energy harvesting, and optical properties of Ca1-xZnx(Fe0.5V0.5)O3 ceramics

The present work mainly describes the fabrication and characterization (structural, micro-structural, dielectric, piezoelectric, optical, and energy-harvesting) of Ca1-xZnx(Fe0.5V0.5)O3 with x = 0, 0.05, 0.10, and 0.15 complex perovskite. All four compounds are prepared by using the solid-state reaction method (calcination temperature = 1100 °C (6 h) and sintering temperature = 1150 °C (7 h)). The room temperature XRD data analysis confirms all four compounds' single-phase orthorhombic structures. Different modes of vibrations are confirmed by Raman spectroscopy. The grain size increases from 1.09 μm to 2.53 μm with increased zinc concentration. The variation of dielectric constant with frequency has been explained based on Maxwell-Wagner polarization. The effects of grain and grain boundary have been confirmed by impedance spectroscopy, which contributes to the conduction and relaxation mechanism of the compounds. The d33, g33, and energy harvesting performance gradually increase with an increase in zinc concentration, confirming the enhancement of the compounds' piezoelectric properties. The decrease in the band gap from 3.58 eV (x = 0) to 3.00 eV (x = 0.15) confirms the enhancement of optical properties.

Investigation of electrical, dielectric and multiferroic properties of 0.7Bi0.95Sm0.05Fe1-xGaxO3-0.3BaTiO3 composite ceramic system for high temperature piezoelectric applications

Lead-free composite ceramics, 0.7Bi0.95Sm0.05Fe1-xGaxO3-0.3BaTiO3 (BSFGx-BT), were made using a solid-state reaction method for x = 2.5, 5, 10, 15, and 20 %. We have investigated the effects of Samarium (Sm) and Gallium (Ga) co-doping on the crystal structure, topography, spectroscopy, electrical and multiferroic characteristics of the 0.7BiFeO3-0.3BaTiO3 (0.7BFO-0.3BTO) system for piezoelectric applications. The structural results indicate that the composite ceramics exhibited rhombohedra R and tetragonal T phases characterized by R3c and P4mm space groups, respectively which was also further confirmed by X-ray Rietveld refinement pattern analysis. In dielectric behaviour study high Curie temperature of 550 °C was obtained with high temperature stability. Furthermore, the study investigated the effects of grain and grain boundaries on capacitive and resistive properties. The ceramics displayed the characteristics of the negative temperature coefficient of resistance (NTCR). It was observed that as the Ga concentration increased, the grain resistance (Rb) also increased, reaching its maximum value for x =10 %. Additionally, at higher temperatures, this composite exhibited lower electrical resistivity. Importantly, the ceramics showed the co-existence of ferromagnetic and ferroelectric behaviour with the enhanced values of remanent magnetization (Mr)= 0.043 emu/g, coercivity (Hc) = 5730 Oe and remanent polarization (Pr) = 6.07 µC/cm2, coercive field (Ec) = 6.78 kV/cm for the composition x =10 %. These findings suggest that Sm & Ga incorporated 0.7BFO-0.3BTO composite ceramics can show promising high-temperature piezoelectric applications.

An atomic insight into effect of grain boundary on diffusion behavior of Cu/Al dissimilar materials undergoing ultrasonic welding

An atomic insight into effect of grain boundary on diffusion behavior of Cu/Al dissimilar materials undergoing ultrasonic welding

Phase-Field Simulation of Spinodal Decomposition in U-50Zr Metallic Nuclear Fuel.

During the γ phase-δ phase transition, U-50Zr fuel experiences spinodal decomposition, which has a significant effect on fuel properties. However, little is known about the spinodal decomposition of U-50Zr. The spinodal decomposition behavior in U-50Zr is studied in this research using the phase-field approach. The mechanism of spinodal decomposition from a thermodynamic perspective is studied, and the effects of temperature and grain boundary (GB) on spinodal decomposition are analyzed. It is found that the concentration of U atoms in the U-rich phase formed during spinodal decomposition is as high as 90%. The U-rich phase first appears at the GB position, and precipitation phases appear inside the grain later. Ostwald ripening occurs when larger precipitation phases on the GB absorb isolated smaller precipitation phases inside the grain. The coarsening rate of precipitation phases and the time it takes for spinodal decomposition to achieve equilibrium are both influenced by temperature.

Improving mechanical properties of Mg–Zn-Nd-Zr alloy by low alloying with Yb and corresponding strengthening mechanisms

This study was aim to explore the possibility of adding Yb element to Mg–6Zn-3Nd-0.5Zr alloy to develop a commercial high-strength Mg alloy under the condition of low rare earth content. A systematic investigation was conducted and it was shown that the phase compositions were changed by the Yb addition (0–2 wt%). The Mg–Zn phases disappeared while a new phase, (Nd,Yb)Zn2, were formed. The volume fractions of precipitates increased with increasing Yb content. Due to the extra precipitation through the process of extrusion, the volume fractions of precipitates of as-extruded alloys were slightly higher than that of the as-cast alloys. All as-extruded alloys displayed the same strong fiber texture with the (10 1‾ 0) perpendicular and the [1‾ 100] parallel to the extrusion direction. The Yb addition improved the DRX degree, but has little effect on grain orientations, and adjusted the texture intensities by influencing DRX process. When the Yb content is 1.0 wt%, the alloy showed best mechanical properties, with the yield strength and ultimate tensile strength of 400 MPa and 407 MPa. The high strength is the result of the combined effects of excellent grain boundary and texture strengthening.

Study of multiferroic properties of Mn substituted Bi0.8Sm0.1Dy0.1Fe1-xMnxO3 ceramics

Abstract Transition metal Mn substituted various multiferroic Bi0.8Sm0.1Dy0.1Fe1-xMnxO3 (x = 0.00, 0.01, 0.03 and 0.05) ceramics samples were fabricated by the solid-state reaction technique. The distorted rhombohedral perovskite structure with some phase impurity of all samples is confirmed by X-ray diffraction analysis. Field Emission Scanning Electron Microscope and Energy Dispersive X-ray spectroscopy are used for microstructural and quantitative analysis, respectively. Densities are decreased and the average grain size is increased with Mn contents. The initial permeability and relative quality factor (RQF) have found to be increased with Mn contents in the samples. The peaks in RQF shifts toward the lower frequency region with increasing Mn contents can be attributed by Snoek’s relation. All the samples show paramagnetic behavior at room temperature and magnetization slightly increases with Mn contents. The frequency dependent dielectric constant is investigated and found to consequence of space charge polarization. The dispersion behavior in dielectric constants can be explained by the Maxwell-Wagner model. The dielectric constants and dielectric loss decrease with increasing of Mn contents for all the samples. The value of electric modulus is observed to increases with the increasing of frequency and this dispersion phenomena is due to short range mobility of charge carriers in conduction. The effect of grain and grain boundaries on electric properties of samples are investigated by complex impedance analysis. The ac conductivity increases with frequency due to small polaron hopping and it can be attributed according to Jonscher’s power law.

Effect of grain boundaries on the helium degradation mechanisms of alloy 800H: A molecular dynamics study

Alloy 800H is currently used as structural material in light-water cooled nuclear reactors, and it is also considered as candidate materials for various advanced reactor designs. Under operation, the exposure of alloy 800H to neutron irradiation results in the formation of helium (He), mainly from Ni by the transmutation reaction (n, α). In this work, we present a molecular dynamics (MD) simulation study of the behavior and effects of He on the microstructure and mechanical properties of alloy 800H. Our results show that the population of clusters made of 5 to 9 He atoms is nearly constant throughout the simulation time (10 ns), while the population of larger clusters increases as the simulation time increases. The growth of clusters is controlled by either the dissociation and diffusion of smaller clusters towards nearby larger clusters or the merging of larger clusters initially located nearby to each other. A significant accumulation of He is observed at the grain boundaries (GB), while a depletion zone is found at the neighboring regions. As a result, the density of He cluster is significantly higher at the GBs as compared to the intra-granular regions. The nucleation and growth of He clusters also results in the formation of Frenkel pairs (FP), whose associated self-interstitial atoms (SIA) agglomerate into interstitial clusters in the alloy 800H matrix. As a consequence, dislocation segments, mostly of the Shockley type, are generated in the microstructure, and often located next to He clusters. The combination of the aforementioned defect structures and the high density of He clusters at the GBs results in a substantial degradation of the mechanical properties of alloy 800H single crystal and bicrystals.

Investigate the effect of co-doping on the grain size and diffuse phase transition of barium titanate ceramics

An investigation examined the impact of co-doping BaTiO3 ceramics with La3+ and Nd3+ on their microstructural, dielectric, and phase transition properties. The synthesis of BaTiO3 with co-doping of La3+ and Nd3+, using the general formula Ba1-x(La1/3, Nd1/3)xTiO3 (BLNdTx) with varying concentrations of x (0%, 2%, 4%, and 8%), is achieved by the solid-state reaction technique. A temperature-dependent dielectric permittivity investigation was conducted at four distinct frequencies (1 kHz, 10 kHz, 100 kHz, 500 kHz, and 1 MHz) within the 30–200 °C temperature range. The findings indicate that the samples show a diffuse phase transition and a noticeable divergence from the typical Curie-Weiss equation. The diffuseness parameters γ for phase transition rose from 1.15 to 1.75 as x grew from 0 to 8%, respectively. The concurrent impact of surface phenomena, mechanical stress phenomena, and the external effect of grain boundaries might explain the substantial size reduction. An in-depth understanding of the grain size effect and its underlying mechanism would be advantageous for advancing and practically using BaTiO3-based ceramics and other ferroelectrics.

Cr-doped Li-based [formula omitted]-(Beta) type hexaferrites: XRD, FTIR, XPS and dielectric evaluations for high-frequency applications

The rapid growth of electronic technology and communications have led to increase demand for high performance microwave absorbing materials. Ferrites are frequently used to absorb microwaves and protect against electromagnetic contamination. In the present study, β-type hexaferrite samples with different chemical compositions of LiFe11-xCrxO17 (x = 0.0, 0.1, 0.3, and 0.5) have been manufactured through the sol-gel auto-combustion technique. The effect of substituting Cr3+ on its structural, electrical and dielectric characteristics was examined. The XRD evaluation helped to develop a single-phase structure for all samples. The substitution of Cr3+ has changed the structural characteristics. FTIR analysis was used to investigate the vibrational properties of Cr-doped β-type hexaferrite. Two absorption bands in the infrared spectroscopy were used to indicate the tetrahedral and octahedral sites. XPS results revealed the incorporation of every metal ion related to their particular electronic states. The dielectric studies were determined in terms of frequency (1–6 GHz) and every sample behavior was described using the Maxwel-Wagner and Koop theories. Impedance analysis was utilized to study the effects of relaxation time and grain boundaries on the electrical features of the produced nanomaterials. Impedance Cole-Cole plots prove that all samples exhibit non-Debye-type multiple relaxation processes. It was observed that among all the synthesized samples, LiFe10.9Cr0.1O17 has a minimum reflection loss of −37.80 dB at 1.22 GHz. Cr-doped β-type hexaferrite results suggested using and designing hexaferrite-based nanocomposite for high-frequency absorbers in the microwave range.

Strain-rate and size dependence of gradient lamellar nickel investigated by in-situ micropillar compression

Strain rate plays a nonnegligible role in the plastic deformation behavior of metallic materials with heterogeneous nanostructure, while little research focuses on the topic. In-situ compression tests at various strain rates were conducted on the micropillars located at different depths from the gradient lamellar Ni. These tests intended to reveal the strain-rate dependence of micropillars with different lamellar thicknesses and illustrated the competitive relationship between strain rate and intrinsic grain size on the plastic deformation behavior of metallic materials. In addition, due to the strain rate sensitivity related to not only intrinsic size but also external size, single Ni micropillars with different diameters were also compressed at various strain rates in order to clarify the competitive relationship between intrinsic size and external size on the strain rate sensitivity. It is found that lamellar thickness rather than strain rate has more effect on plastic deformation, and external size exhibits a more obvious impact on the strain rate sensitivity. The plastic deformation behaviors of different micropillars are mainly explained by transitions in the effect of lamellar grain boundaries on the dislocation motion at various strain rates and over different lamellar thicknesses, resulting in the change of dislocation multiplication and annihilation rate.

Influence of forging pretreatments on microstructure evolution and surface roughness of Al 6061 alloy

Achieving ultra-smooth surfaces is the goal of aluminum optical manufacturing. Under certain processing conditions, improving the microstructure of aluminum and understanding its relationship with surface roughness requires systematic study. The grain structure and various types of second-phase particles are of paramount importance. This study analyzed the microstructure of 6061 alloy after undergoing severe plastic deformation under various processing conditions followed by T6 homogenization heat treatment. Utilizing a white light interferometer, a comparative analysis of the surface roughness was conducted on specimens that underwent single-point diamond turning to achieve a mirror finish. The assessment of surface roughness on machined surfaces is solely based on white light interferometry. The analysis and discussion focus on the effects of phases (causing scratches and voids), the grains and grain boundaries. Experimental findings signify: the grain size, grain boundary and residual second phase can both influence the surface quality, the increase in deformation temperature and accumulated strain both facilitate the dissolution and fragmentation of the secondary phases. However, they also contribute to some extent to grain growth, resulting in a minimum secondary phase area fraction of 0.87% and grain sizes reaching 147.8 μm. Subsequent heat treatments, while effective in reducing the negative impact of the phases, reveal noticeable step-like structures affecting the quality of surface roughness, with the lowest obtained Ra value being 0.8 nm. A proposed pretreatment method in cleaner ingot processing with lower alloy element content addresses the trade-off between reducing phases and controlling grain growth, aiming to achieve improved surface roughness, promoting the application of polycrystalline aluminum alloys in the field of optics manufacturing.

Effect of samarium on microstructure and mechanical properties of dilute Mg-Al-Ca alloys

To facilitate the industrial application of wrought Mg alloys, this study explores the impact of the rare earth (RE) element Sm on the microstructure and mechanical properties of hot-rolled Mg-1Al-0.3Ca alloy. The results indicate that the average grain size and basal texture intensity of the hot-rolled Mg-1Al-0.7Sm-0.3Ca alloy are significantly reduced compared to the hot-rolled Mg-1Al-0.3Ca alloy. This reduction can be attributed to the pinning effect of grain boundaries and grain refinement facilitated by the presence of the fine Al2Sm phase. Additionally, the addition of Sm leads to an increase in yield strength and ultimate tensile strength, along with a decrease in elongation. This can be attributed to the combined effects of the strengthening mechanism provided by a significant number of Al2Sm particles and the stress concentration occurring at the sharp corners of these particles. Significantly, this study proposes the substitution of expensive RE elements with more cost-effective Sm in the design of Mg alloys for low-alloy systems. The excellent mechanical properties of the Mg-1Al-0.7Sm-0.3Ca alloy provide a reference for the future development of high-performance Mg alloys.

In-situ analysis of the effect of residual fcc phase and special grain boundaries on the deformation dynamics in pure cobalt

Polycrystalline hcp metals – an important class of engineering materials – typically exhibit complex plasticity because of a limited number of slip systems. Among these metals, deformation is even more complicated in cobalt as it commonly contains residual fcc phase due to the incomplete martensitic fcc→hcp transformation upon cooling. In this work, we employ a combination of in-situ (acoustic emission, AE) and ex-situ (scanning electron microscopy, SEM) techniques in order to examine deformation dynamics in pure polycrystalline cobalt varying in grain size and the content of residual fcc phase prepared using systematic thermal treatment and cycling. We reveal that the presence of the fcc phase and special ∼71∘ grain boundaries between different hcp martensite variants brings about higher deformability and strength. The fcc phase provides additional slip systems and also accommodates deformation via the stress-induced fcc→hcp transformation during loading. On the other hand, special boundaries enhance structural integrity and suppress the formation of critical defects. Both these non-trivial effects can dominate over the influence of grain size, being a traditional microstructural variable. The ex-situ SEM experiments further reveal that the stress-induced fcc→hcp transformation is sluggish and only partial even at high strains, and it does not give rise to detectable AE signals, unlike in other materials exhibiting martensitic transformation. In turn, these insights into cobalt plasticity provide new avenues for the microstructure and performance optimization towards the desired applications through the modern concept of grain boundary engineering.

Effect of grain boundary on the ignition and flame propagation mechanisms of TC11 alloy in oxygen-enriched environment

Grain boundaries (GBs) were known as a kind of common defects, but their impact on metal combustion was unclear. This study discovered that when the TC11 alloy's grain size reduced, the critical temperature and oxygen pressure for ignition dropped and the burning velocity accelerated. A modified Frank-Kamenetskii model demonstrated a well match to the relationship between grain size and ignition temperature, and the GBs showed a lower activation energy for ignition than the intragrain. The primary processes via which GBs influenced the kinetics of combustion were the peritectic reaction between Ti and O and the intergranular diffusion of alloying elements.