Effect Of Grain Boundary Diffusion Research Articles

Coercivity enhancement and microstructure optimization of Nd-Fe-Co-B magnets by Dy70Al10Cu20 grain boundary diffusion

The effect of grain boundary diffusion on the magnetic properties and microstructure of Nd-Fe-Co-B sintered magnets is investigated using Dy70Al10Cu20 alloy as the diffusion source. After the secondary annealing of the Nd-Fe-Co-B sintered magnet (Hcj = 7.94 kOe), its coercivity is substantially increased to 13.10 kOe. Following post-diffusion annealing, the coercivity of the sintered magnet significantly increased to 20.86 kOe. Microstructure analysis revealed that after the second annealing, the main phase grains in the magnet were connected but lacked thin intergranular phases. The distribution of Co was uneven, with an abundance of soft magnetic phases rich in Co, which was not conducive to improving coercivity. After diffusion, Dy formed a shell structure with high magnetic crystal anisotropy on the outer side of the main phase grains, leading to a significant increase in coercivity. Al and Cu entered the interior of the magnet, reducing the melting point of the grain boundary phase and promoting the uniform distribution of Co through the flow of the liquid phase. A large amount of thin continuous intergranular phase is generated inside the magnet, which helps reduce the exchange coupling between hard magnetic grains. The experimental results indicate that the Dy-Al-Cu alloy can effectively enhance the grain boundary structure of Nd-Fe-Co-B magnets, reduce the abundance of Co-rich phases, and improve the coercivity of the magnet.

Diffusion in a bicrystal: effect of water uptake through grain boundary on the degradation of halide perovskites

In this work, we investigate the diffusion in a bicrystal strip with a constant concentration of solute atoms/molecules on free surfaces to mimic the diffusion of water molecules in a bicrystal of halide perovskites. The effect of grain boundary diffusion is incorporated in the analysis, and the equation of mass transport in the grain boundary is derived without a time-derivative term. Using the equation of mass transport in the grain boundary, a closed-form solution of the spatiotemporal evolution of the concentration of solute atoms/molecules in the bicrystal is derived. Numerical analysis of the uptake of water in a methylammonium lead iodide (MAPbI3) bicrystal is performed. The degree of degradation of the MAPbI3 bicrystal due to the uptake of water is defined as the ratio of the diffusion length of water in one of the crystal in the bicrystal to the half width of the bicrystal. The numerical results reveal that the degree of degradation of the MAPbI3 bicrystal increases with the increase of the diffusion time and there exists effect of grain size on the degree of degradation of the MAPbI3 bicrystal. The time to reach the same degree of degradation due to the uptake of water is proportional to the width of the MAPbI3 bicrystal of the same thickness, in accord with the experimental results reported in literature. This result points to the need of producing halide perovskite films of large grain sizes in order to improve the structural stability and performance of perovskite-based photovoltaic cells by limiting the structural degradation induced by water diffusion.

Rapid grain boundary diffusion in foraminifera tests biases paleotemperature records

The oxygen isotopic compositions of fossil foraminifera tests constitute a continuous proxy record of deep-ocean and sea-surface temperatures spanning the last 120 million years. Here, by incubating foraminifera tests in 18O-enriched artificial seawater analogues, we demonstrate that the oxygen isotopic composition of optically translucent, i.e., glassy, fossil foraminifera calcite tests can be measurably altered at low temperatures through rapid oxygen grain-boundary diffusion without any visible ultrastructural changes. Oxygen grain boundary diffusion occurs sufficiently fast in foraminifera tests that, under normal upper oceanic sediment conditions, their grain boundaries will be in oxygen isotopic equilibrium with the surrounding pore fluids on a time scale of <100 years, resulting in a notable but correctable bias of the paleotemperature record. When applied to paleotemperatures from 38,400 foraminifera tests used in paleoclimate reconstructions, grain boundary diffusion can be shown to bias prior paleotemperature estimates by as much as +0.86 to −0.46 °C. The process is general and grain boundary diffusion corrections can be applied to other polycrystalline biocarbonates composed of small nanocrystallites (<100 nm), such as those produced by corals, brachiopods, belemnites, and molluscs, the fossils of which are all highly susceptible to the effects of grain boundary diffusion.

Effect of grain boundary diffusion of terbium on mode of mechanical fracture of sintered NdFeB magnets

Grain boundary diffusion process (GBDP) is a widely used method of increasing the coercivity of sintered NdFeB magnets. In this study, the effects of the GBDP on the bending strength and microhardness of sintered NdFeB magnets and the fracture mode were investigated. Results show that the bending strength of magnets is reduced by pickling and heat treatment and greatly recovers after heavy rare earth element (Tb) grain boundary diffusion. The pickling and the heat treatment cause a slight decrease in microhardness. Compared with the recovery of the bending strength, the hardness decreases after the GBDP. The fracture mode of bended magnets changes from intergranular to transgranular. This study helps in further improving the mechanical and magnetic properties of sintered NdFeB magnets.

Effect of grain boundary diffusion in TbF3-Al mixed electrophoresis on properties of sintered Nd-Fe-B magnets

The grain boundary diffusion process of heavy rare-earth elements and low-melting metal can improve the coercivity (Hcj) of sintered Nd-Fe-B magnet significantly. In this work, the effect of TbF3-Al diffusion source on the magnetic properties and microstructure of magnets were investigated. Ratio of TbF3 and Al adjusted by parameters of electrophoretic deposition. The Hcj of the TbF3-Al diffusion magnet increased by 40.9% compared to original magnet. The temperature stability of TbF3-Al diffusion magnet was also improved. Microstructural investigations indicate that Al can promote the formation of Tb-rich thin grain boundary phase and increase the isolation between the main phases. In addition, Al can promote the diffusion of TbF3 into the magnet.

Magnetic properties and Tb diffusion of sintered Nd–Fe–B magnets by suspension plasma spraying

For the first time, a TbF3 coating as a diffusion source was prepared on the surface of sintered Nd–Fe–B magnets by suspension plasma spraying. A sintered Nd–Fe–B magnet with a high coercivity and low remanence loss was obtained. The uniform and dense coating favored grain-boundary diffusion of heavy rare-earth elements, which improved the coercivity of Nd–Fe–B magnets. The effect of grain-boundary diffusion and TbF3 coating amount on the microstructure and magnetic properties of the magnet were studied. Optimal magnetic properties were obtained for a heat treatment of 900 °C for 10 h, when the TbF3 coating amount was 0.2132 mg/mm2. The coercivity reached 23.72 kOe with an increase of 60.16% compared with the original magnet, whereas the remanence did not decrease significantly. Microstructure analysis showed that a Tb-layer core–shell structure formed around the Nd2Fe14B grains after diffusion. The high magnetocrystalline anisotropy of the Tb core–shell structure formation improved the Nd–Fe–B magnet coercivity significantly. Therefore, the method of plasma spray grain-boundary diffusion is efficient and feasible.

Effect of Grain Boundary Diffusion of Tb on the Mode of Mechanical Fracture of Sintered Ndfeb Magnets

Effect of Grain Boundary Diffusion of Tb on the Mode of Mechanical Fracture of Sintered Ndfeb Magnets

Micro-macroscopic modeling of a lithium-ion battery by considering grain boundaries of active materials

Many important properties of electrode materials are profoundly sensitive to deviations from the crystalline perfection. Among them are grain boundaries, which play an important role on battery performance by altering the distributions of ions inside the particles and changing the corresponding stress level. This paper explores the mechanical and microstructural aspects of battery behavior by developing a cell-level model that incorporates grain boundary diffusion in the electrode particles for the anode and cathode. The developed model is compared against a grainless model at various operating conditions to understand how grain boundaries influence capacity and stress generation. The results from the model revealed that the location of the maximum stress might not necessarily occur at the separator interface at any given time as the particles with heterogeneous diffusion paths displayed gradual changes in flux profile, which has just been believed in practice. The grain structure geometry variability showed that less diffusive particles at the rear side endure more stress than high diffusive frontal particles. Also, the results show an appreciable effect of grain boundary diffusion on the voltage profile of the cell for the tested parameters and, overall, a significant reduction in the maximum induced stress in the cell. Stress behavior between the anode and cathode differ significantly and show that the effective diffusivity of a particle might matter much more significantly than its location in the cell. With heterogeneous particle shape modeling capabilities, this approach can be next-generation battery model that improves understanding of battery materials and electrodes.

Effects of rapid heating and uniaxial loading on the phase transformation and mechanical properties of direct partial remelted butt joint of AISI D2 tool steel

Tool steels are widely used in various industries, such as automotive, shipbuilding, manufacturing, and marine, for rolling mills, blanking metals, and punching dies, given their distinctive hardness, resistance to abrasion, and deformation. However, joining tool steels using the conventional fusion welding process is difficult because of the formation of dendritic structures and hot cracking, which affect the strength of joining. Therefore, this study proposed the direct partial remelting process (DPRMP) for the butt joining of AISI D2 tool steel. Moreover, different uniaxial loads of 0, 0.5, 2.5, and 5 N were applied during the joining process to investigate the microstructural evolution and mechanical properties of DPRMP samples. Results showed that the ferrite phase in the base material was changed to the austenite phase with globular shape in the joined samples through DPRMP. In addition, increasing the uniaxial load from 0 N to 5 N reduced the average grain size, increased the shape factor, and caused joining defects, such as voids, to disappear. Furthermore, the tensile strength of the DPRMP samples improved by increasing uniaxial loading from 0 N to 5 N due to the effect of grain boundary diffusion and the decrease in eutectic carbide at the joint interface. The highest tensile strength was achieved in DPRMP-5, with a value of 652 MPa and a welding efficiency of 103%. Moreover, the average hardness value of the fusion zone improved in the DPRMP samples and increased to 400 HV wherein the as-received material presented a value of 226 HV.

Effects of grain boundary diffusion of PrCu alloy on microstructure and coercivity of hot deformed (Nd,Ce)-Fe-B magnets

The technology of using high abundance rare earth element Ce in permanent magnets has attracted many concerns. In this work, the magnetic properties and microstructures of hot deformed (Nd1-xCex)13.8Fe76.1Co4Ga0.5B5.6 magnets and a selective magnet treated with PrCu grain boundary diffusion process (GBDP) were investigated systematically. It is found that Ce is beneficial to improving the plastic deformation ability of NdFeB magnets. The strongest c-axis orientation is achieved in the HD magnets with Ce concentration of x = 0.3, resulting in good comprehensive magnetic properties of Br = 13.00 kGs, Hcj = 10.12 kOe, (BH)max = 38.42 MGOe. Based on that, the GBDP was implemented on the magnets with the best orientation using Pr68Cu32 ribbons and the magnetic properties of Br = 12.87 kGs, Hcj = 15.65 kOe, (BH)max = 37.48 MGOe were obtained. The results of composition distribution and Curie temperature test of the GBDPed magnets illustrate that Pr diffuses into the (Nd,Ce)2(Fe,Co)14B grains to substitute Ce, which not only improves the Curie temperature of the grains, but also enhances the magnetic performance of the magnets. Through GBDP, the thermal stability of magnets has also been improved.

A phase-field investigation of the effect of grain-boundary diffusion on austenite to ferrite transformation

We study the transformation of austenite to ferrite under the influence of Grain-Boundary (GB) diffusion. In order to simulate identical level of penetration of solute through GB as predicted by theory, diffusivity gradient at the boundary of a bulk austenitic domain is calibrated by numerical simulations. Applying a phase-field model for mixed-mode of transformation, it has been found that GB allotriomorph has a constant lengthening rate and the sharp tip of the allotriomorph displaces carbon due to its curvature. The accumulated solute has a significant effect on the thickening behaviour in terms of growth coefficient and exponent. We also discuss the morphological evolution of ‘perturbed’ allotriomorphic ferrite.

Modelling of the effect of grain boundary diffusion on the oxidation of Ni-Cr alloys at high temperature

Grain boundaries in oxide scales have a strong effect on oxidation kinetics when they act as diffusion short circuits. This study proposes a quantitative evaluation of the phenomenon by modelling. Various cases of oxide microstructure evolution are treated using both analytical and numerical resolutions. Results showed that the effect of oxide grain growth on the oxidation kinetics can be analysed considering a transitory stage for which the oxidation kinetics is not purely parabolic. Some guidelines for choosing the appropriate post-treatment method for the analysis and extrapolation of experimental oxidation kinetics are given.

Effect of grain boundary diffusion on electrolyte stability in direct carbon fuel cells with antimony anodes

Direct carbon fuel cells (DCFC) that employ solid oxide electrolytes and molten Sb anodes are promising for the efficient generation of electricity using a range of carbonaceous fuels. The present study examined the etching of yttria-stabilized zirconia (YSZ) and gadolinia-doped ceria (GDC) electrolytes by Sb2O3 produced during fuel-cell operation. Migration of Sb along grain boundaries and electrolyte corrosion were observed for both polycrystalline YSZ and GDC electrolytes; however, corrosion and electrolyte thinning were not observed for a single-crystal YSZ electrolyte, even after long-term operation. These results indicate that Sb migration along grain boundaries plays a significant role in electrolyte corrosion in DCFCs with molten Sb anodes. Several strategies for avoiding this problem are also discussed.

Micro-Macroscopic Modeling of a Lithium-Ion Cell By Considering Grain Boundaries of Active Materials

Many important properties of electrode materials are profoundly sensitive to deviations from the crystalline perfection. Among them are grain boundaries that play an important role on battery performance by changing the ions distribution inside particles and corresponding stress level change. This paper explores the mechanical and microstructural aspect of battery behavior by developing a cell-level model that incorporates grain boundary diffusion in the electrode particles for the anode and cathode electrodes. The developed model is compared against the grainless model at various operating conditions to understand how grain boundaries influence capacity and stress generation. The results show an appreciable effect of grain boundary diffusion on the voltage profile for the cell for the parameters tested and, overall, a significant reduction in the maximum induced stress experienced in the cell. Stress behavior between the anode and cathode differ significantly, and show that the effective diffusivity of a particle might matter much more significantly than its location in the cell.

Modeling of Intermetallic Compounds Growth Between Dissimilar Metals

Abstract A model has been developed to predict growth kinetics of the intermetallic phases (IMCs) formed in a reactive diffusion couple between two metals for the case where multiple IMC phases are observed. The model explicitly accounts for the effect of grain boundary diffusion through the IMC layer, and can thus be used to explore the effect of IMC grain size on the thickening of the reaction layer. The model has been applied to the industrially important case of aluminum to magnesium alloy diffusion couples in which several different IMC phases are possible. It is demonstrated that there is a transition from grain boundary-dominated diffusion to lattice-dominated diffusion at a critical grain size, which is different for each IMC phase. The varying contribution of grain boundary diffusion to the overall thickening kinetics with changing grain size helps explain the large scatter in thickening kinetics reported for diffusion couples produced under different conditions.

Effect of Ca and Sr on the compressive creep behavior of Mg–4Al–RE based magnesium alloys

The compressive creep behavior of Mg–4Al–RE–1.2Ca (AEC4112) alloy and Mg–4Al–RE–1.2Ca–0.2Sr (AECJ411202) alloy is investigated on a special apparatus. The stress exponent n under different stresses and the creep activation energy Qc under different temperatures of AEC4112 alloy and AECJ411202 alloy are estimated by an empirical equation. The results reveal that the compressive creep resistance of Mg–4Al–RE (AE41) alloy is improved significantly by a small amount of Ca and Sr. The steady state compressive creep rates of AEC4112 alloy and AECJ411202 alloy are increased with increasing temperature and applied stress. The steady state compressive creep rates of AEC4112 alloy and AECJ411202 alloy are 2.2817×10−7mm/s and 1.3041×10−7mm/s under 150°C and 100MPa, respectively. The compressive creep resistance is improved by intermetallic compounds Al2Ca, Al2RE and Al4Sr inhibiting the climbing of the dislocation, the sliding and diffusion of the grain boundary. The compressive creep constitutive equations of AEC4112 alloy and AECJ411202 alloy are written as ε̇s1=2.0276×10−10×σ5.662exp(−47447/RT) and ε̇s2=5.8208×10−11×σ6.123exp(−68252/RT) respectively. The compressive creep mechanism of AEC4112 alloy and AECJ411202 alloy is the combined effects of grain boundary diffusion and grain boundary sliding determined by dislocation climbing.

Thermal Stability of Spherical Nanoporous Aggregates and Formation of Hollow Structures by Sintering—A Phase-Field Study

Nanoporous structures are widely used for many applications and hence it is important to investigate their thermal stability. We study the stability of spherical nanoporous aggregates using phase-field simulations that explore systematically the effect of grain boundary diffusion, surface diffusion, and grain boundary mobility on the pathways for microstructural evolution. Our simulations for different combinations of surface and GB diffusivity and GB mobility show four distinct microstructural pathways en route to 100% density: multiple closed pores, hollow shells, hollow shells with a core, and multiple interconnected pores. The microstructures from our simulations are consistent with experimental observations in several different systems. Our results have important implications for rational synthesis of hollow nanostructures or aggregates with open pores, and for controlling the stability of nanoporous aggregates that are widely used for many applications.

Diffusion at Short Circuits: State of the Art

Evidence for solid-state diffusion (the second half of the 19th century). The first measurements of solid state diffusion (W. Roberts-Austen, 1896–1922). The first tracer experiments to determine the solid-state diffusion (G. von Hevesy, 1913–1923). The first evidence of accelerated diffusion in polycrystalline materials (1924–1935). Autoradiographic studies of grain boundary diffusion (50s of 20th century). The first quantitative experimental and theoretical studies of the “short circuiting” diffusion (beginning from 1949, D. Turnbull and R. Hoffman – General Electric Research Lab.): radiotracer serial sectioning method, the Fisher model (1951) for grain boundary diffusion, exact solutions and developments of the Fisher model (1954–1963). The progress in the experimental methods for determination of grain boundary diffusion data and results of measurements for different metallic systems (up to date). The measurements of grain boundary diffusion parameters in the B and C regimes. Grain boundary diffusion and grain boundary segregation. Nonlinear segregation effects. Structural effects of grain boundary diffusion. Diffusion in bicrystals. Diffusion in nanocrystals. Computer simulation of grain boundary diffusion. Mechanisms of grain boundary diffusion.

A simultaneous deformation and diffusion experiment: Quantifying the role of deformation in enhancing metamorphic reactions

The influence of concurrent plastic deformation on a continuous Fe–Mg exchange reaction in ferropericlase/periclase was examined as a function of strain and changing microstructure in a torsion experiment performed at 300 MPa and 1523 K. The measured Fe–Mg interdiffusion coefficient increases non-linearly by a factor of 1.2 to 1.4 (depending on composition) with increasing shear strain (up to a γ of 5.2), strain rate (up to 2.9 × 10 − 4 s − 1 ) and decreasing grain size (from 200 μm to 80 μm). The change in diffusion is attributed to the increase in density of large-angle (> 15°) grain boundaries, whereas both subgrain boundary density and dislocation density do not correlate well with the changing diffusion coefficients. The increased diffusivity due to grain boundary diffusion does not follow any of the proposed equations for combined volume and boundary diffusion, indicating that either the effect of grain boundary diffusion has been underestimated or that there is an additional effect on diffusivity due to intracrystalline deformation (movement of dislocations and/or point defects, grain boundary migration during dynamic recrystallization) even if there is no net stress or strain in the direction of diffusion.

Three-dimensional simulation of intermetallic compound layer growth in a binary alloy system

We present three-dimensional phase-field simulations of the thickening of intermetallic compound (IMC) layers and their microstructural evolution during soldering and aging in a binary alloy system. The focus was on the grain growth in the IMC layer, since the grain boundary (GB) provides a high-diffusivity path and affects the thickening kinetics of the IMC. The radial grain growth of the IMC was analogous to Ostwald ripening when the phase interface was rounded, while it followed normal grain growth with planar phase interface. The thickening mode of the IMC layer was dependent upon the microstructural evolution within the IMC layer with high diffusivity along the GBs. The simulation results successfully confirm literature models on the effect of GB diffusion on IMC thickening.