Grain Boundary Thickness Research Articles

Microstructure of a Dy-free Nd-Fe-B sintered magnet with 2 T coercivity

We investigated the microstructure of the Dy-free high coercivity sintered magnet that was developed by optimizing the chemical composition of Nd-Fe-B sintered magnet with 0.1 at.%Ga doping in order to understand the reasons for the high coercivity of μ0Hc = 2.0 T and the improved squareness of 0.95 compared to a recently developed 0.5 at. %Ga doped magnet. The as-sintered 0.1Ga sample shows a relatively high coercivity of 1.5 T owing to the finer grain size of ∼3.3 μm and the higher Al concentration up to 1.3 at.% compared to post-sinter annealed 0.5Ga magnet that has a grain size of ∼5.5 μm and Al concentration of 0.9 at. %. The post-sinter annealing leads to a substantial coercivity increase from 1.5 to 2.0 T due to the formation of a thick and crystalline intergranular grain boundary (GB) phase containing 30-60 at.% of Nd. The trace Ga addition of 0.1 at.% improved the wettability of the liquids and facilitated the formation of thick GB phase during the post-sinter annealing. The good squareness is mainly attributed to the ferromagnetic nature of the intergranular GB phase as well as the strong crystal alignment to the easy axis. Finite element micromagnetic simulations of the demagnetization processes of the models incorporating experimentally determined GB thickness and chemistry well explain the simultaneous achievement of the high coercivity and good squareness observed in this magnet in contrast to the very poor squareness observed in the post-sinter annealed 0.5Ga sample.

Influence of Grain Boundary Complexion on Deformation Mechanism of High Temperature Bending Creep Process of Cu Bicrystal

Despite of substantial advancement, the effect of grain boundary (GB) complexions on high temperature creep deformation process has not been fully understood. In this paper, we have studied the high temperature bending creep deformation of copper bicrystal with various GB complexions under different loads using molecular dynamics simulation. It has been found that specimen with normal kite GB complexion have better creep resistance properties when subjected to comparatively lower applied load. In case of monolayer Zr segregation, a drastic decrease in creep strength as well as creep plasticity is observed due to inhibition of GB migration. On the other hand, deviation between creep properties for specimen with split-kite GB complexion and split-kite bilayer Zr segregation GB complexion is minimal. Enhanced creep plasticity is observed in case of split-kite bilayer Zr segregation GB complexion, which is due to formation of interpenetrating icosahedral clusters in the necking region. Fracture in specimen with monolayer Zr segregation GB complexion has occurred by means of slip phenomenon at lower deformation load whereas amorphization and necking is observed at higher deformation load. In case of specimen with bilayer Zr segregation GB complexion, it is found that fracture has occurred through amorphization and necking at all deformation loads due to higher GB thickness.

An analysis of the influence of grain boundary strength on microstructure dependent fracture in polycrystalline tungsten

This work examines the influence of changes in grain boundary (GB) strength on microstructure dependent crack propagation in polycrystalline tungsten (W). The property of focus is brittleness index (BI) originally introduced by Evans and Marshall in 1976 and 1979, respectively, used in order to quantify the extent of brittleness of a material. In an earlier work, GBs of Ni-doped polycrystalline W have been characterized for embrittlement using ab-initio quantum mechanical simulations as a function of Ni atomic volume fraction and GB thickness. This work focuses on quantifying the influence of GB strength on microstructure dependent crack propagation. Continuum mechanical GB strength properties and effective W grain properties are derived from ab-initio simulation based stress–strain curves. The crack propagation simulations when GBs are considered of finite width are based on an extended finite element (XFEM) framework. Simulations that consider GBs of infinitesimal width due to mesh resolution issues are based on a combined XFEM-cohesive finite element model that employs cohesive elements at GBs. Analyses of crack propagation through finite width GBs focus on understanding the role of square root of length scale dimension in the original BI formulation. Based on analyses of crack propagation through finite width GBs oriented at angles varying from $$0^{\circ }\,\hbox {to}\,90^{\circ }$$ with respect to advancing crack, a quantitative criterion based on a failure index which predicts crack propagation path in polycrystalline W is proposed and examined.

Multi-phase modelling of intergranular hydrogen segregation/trapping for hydrogen embrittlement

Premature failure in polycrystalline materials due to hydrogen absorption affects a wide range of applications, including clean energy systems, hydrogen storage systems and rocket engines. A good understanding of the diffusion and trapping processes within such materials can inform material choices and component design to reduce the likelihood of such failures. Grain boundary segregation of hydrogen can often lead to intergranular hydrogen embrittlement (IHE). In order to understand the effects of hydrogen on intergranular and transgranular fracture in polycrystalline material it is important to first understand hydrogen diffusion and trapping in the general context of grain boundary segregation engineering (GBSE). Hydrogen diffusion is affected by local microstructural features including intergranular second phase precipitates, grain boundary (GB) thicknesses and geometrically necessary dislocation (GND) density. A multi-scale multi-phase model is presented here that has been developed to study GBSE with respect to hydrogen diffusion and IHE. The results of various multi-scale GBSE models with and without traps (including the effects of microstructure, intergranular precipitate phases and GB thickness) are compared and discussed, and the effects of microstructural parameters such as hydrogen segregation factor and GND trapping density on hydrogen diffusion are investigated.

A Tool for Local Thickness Determination and Grain Boundary Characterization by CTEM and HRTEM Techniques.

A new approach for measurement of local thickness and characterization of grain boundaries is presented. The method is embodied in a software tool that helps to find and set sample orientations useful for high-resolution transmission electron microscopic (HRTEM) examination of grain boundaries in polycrystalline thin films. The novelty is the simultaneous treatment of the two neighboring grains and orienting both grains and the boundary plane simultaneously. The same metric matrix-based formalism is used for all crystal systems. Input into the software tool includes orientation data for the grains in question, which is determined automatically for a large number of grains by the commercial ASTAR program. Grain boundaries suitable for HRTEM examination are automatically identified by our software tool. Individual boundaries are selected manually for detailed HRTEM examination from the automatically identified set. Goniometer settings needed to observe the selected boundary in HRTEM are advised by the software. Operation is demonstrated on examples from cubic and hexagonal crystal systems.

Interfacial reaction of Bi–Ag and Bi–Sb solders on copper substrate with multiple reflow number

Owing to the toxicity of lead, much effort has been put in studies on lead-free solders after environmental legislations prohibited the use of lead in these parts worldwide. Here, Bi–Ag and Bi–Sb alternative solder alloys containing 1·5–5 wt-% Ag and Sb were investigated. The effect of the reflow number and weight percentage of Ag and Sb on the surface properties of solders and interfacial reactions between the solder bulk and the Cu substrate were analysed by optical microscopy and scanning electron microscopy–energy dispersive X-ray. The results show that by increasing the reflow number and wt-% of Ag and Sb in the solder bulk, the thickness of the mechanical grain boundary grooving is increased. However, the thickness of the Cu3Sb intermetallic compound layer at the interface of Bi–5Sb decreased by increasing the reflow number. Moreover, our findings show that the amount of Ag and Sb in solder alloys and the reflow number have different behaviours on the wettability properties. By increasing the reflow number, the wetting angle decreased in Bi–Ag solder alloys, whereas it increased in Bi–Sb solder alloys.

Microstructure and notched tensile fracture of Ti–6Al–4V to Ti–4.5Al–3V–2Fe–2Mo dissimilar welds

Dissimilar welding of Ti–6Al–4V (Ti-6-4) to Ti–4.5A1–3V–2Fe–2Mo (SP-700) alloys was performed using a CO2 laser. The microstructure and notched tensile strength (NTS) of the dissimilar welds were investigated in the as-welded and post-weld heat treatment (PWHT) conditions. Moreover, the results were compared with homogeneous laser welds with the same PWHT. The dilution of SP-700 with the Ti-6-4 alloy caused the formation of fine needle-like α+β structures, resulting in the exhibition of a moderately high fusion zone (FZ) hardness of HV 398. The high FZ hardness (HV 438) for the weld with the PWHT at 482°C was associated with low NTS or high notch brittleness. The fracture appearance of the notched tensile specimen was related to its inherent microstructure. With increasing the PWHT temperature, the thickness of grain boundary α increased, which promoted an intergranular dimple fracture. By contrast, fine shallow dimples were present in the peak-aged weld, which was induced by the refined α+β microstructures in the basket-weave form.

The effect of the microstructure on the DC-bias superposition characteristic of NiCuZn ferrite

The effects of combinational additive on the DC-bias superposition characteristic of NiCuZn ferrite were investigated experimentally. The composite NiCuZn ferrite was prepared by a conventional ceramic method. The sharp decrease of the permeability can be avoided when the magnetic field is small. It is because the permeability is mainly attributed to the χspin with fine grain size. Owing to the thicker nonmagnetic grain boundary, samples with good DC-bias superposition characteristic can be obtained by special combinational additions. Composite addition of Bi2O3, AlBSi glass, and SnO2 can both retard the grain growth and increase the thickness of the nonmagnetic grain boundary δ. Finally, an excellent performance sample was obtained.

Influence of Sintering Aid on the Translucency of Spark Plasma‐Sintered Silicon Nitride Ceramics

Single additives (Y2O3, MgO, and Al2O3) were used as sintering aid of Si3N4. The density, crystal phase, microstructure, transmittance, hardness, and fracture toughness of sintered Si3N4 were investigated. Highly densified sintered bodies were obtained in single Y2O3 and MgO systems, but not in single Al2O3 system. The XRD results indicated that sintered bodies were composed of β‐Si3N4. The SEM images showed that all the sintered bodies had a fine‐grained microstructure with an average diameter of 0.29–0.37 μm. The thickness of grain boundary was changed with additive content. The transmittance (T) and the wavelength (λ) followed the relationship of T∝ exp(−λ−2.3) due to the light scattering. The transmittance was mainly influenced by the refractive index of additives and the thickness of grain boundary phase. The hardness and fracture toughness of sintered ceramics were 12.6–15.5 GPa and 6.2–7.2 MPam1/2, respectively.

Investigation on effects of CuO secondary phase on dielectric properties of CaCu3Ti4O12 ceramics

Two CaCu3Ti4O12 (CCTO) ceramics are separately sintered for 10h at 975 and 1000°C. The correlation between the microstructures and dielectric properties of the ceramics has been explored in this paper. The CuO secondary phase is separated out and locates at grain boundaries of the surface layer when the sintering temperature is raised up to 1000°C. The presence of the CuO phase highly reduces the grain-boundary thickness and helps the grain growth, which together result in the increase of the dielectric constant. The low-frequency dielectric loss decreases due to the density improvement of the CCTO ceramics, and the high-frequency dielectric loss increases due to the reduction of the grain-boundary volume. The dielectric loss as low as 0.074 is obtained from the ceramic pellet sintered at 1000°C.

Solute concentrations and strains in nanograined materials

Taking advantage of both the Gibbs and McLean adsorption isotherms, we develop a Gibbs-approach-based adsorption isotherm for grain boundary (GB) segregation in nanograined (ng) polycrystals. An excess GB thickness is introduced to describe the excess of GB atomic volume in comparison with the atomic volume in lattice. The GB bulk modulus is determined with the excess GB thickness and a universal function. The newly developed adsorption isotherm is able to analyze simultaneously stresses, concentrations and their coupling behaviors in grains and GBs. Numerical calculations and plots are conducted to illustrate the theoretical analysis. The results show that the apparent solute concentration could be greatly enhanced in ng materials, due to a large grain boundary volume fraction and a considerable increase in the lattice concentration that is, in turn, boosted by the concentration-induced stresses.

Effect of Holding Time on the Corrosion Potential of SS304

Stainless steels suffer from classical sensitization when encounter at certain temperature which results in precipitation of chromium carbide along grain boundary with simultaneous depletion of chromium from near grain boundaries. This study focused on corrosion potential test at different sensitization condition. The degree of chromium carbide precipitation was measured based on thickness of grain boundary. With increasing holding time, the thickness of the chromium depletion zone also increases due to diffusion of chromium into solid solution. Corrosion potential test showed that the heated sample at 700°C for 60 minutes resulted in highest corrosion current density and shifted its corrosion potential to more active potential. The potential tend to increase with increasing the degree of chromium carbide precipitation.

Effect of grain boundary on the electromechanical response of ferroelectric polycrystals

The effect of grain boundaries on the electromechanical response of a ferroelectric polycrystal subjected to an electric field or stress is investigated numerically by using a phase field model. The grain boundaries in the phase field model are regarded as dielectrics in which the ferroelectric properties are degraded completely. The phase field simulations show that the presence of dielectric grain boundaries results in a large build-in depolarizing field in grains. The depolarizing field has a significant influence on the coercive field, the switching behaviour of ferroelectric domain under an electric field or stress, and the piezoelectric and dielectric properties of the ferroelectric polycrystal. It is found that both coercive field and remnant polarization decrease with the increase of the thickness of dielectric grain boundary. However, the piezoelectric coefficient and permittivity of the ferroelectric polycrystal become larger when the thickness of dielectric grain boundary increases. The enhancement of dielectric and piezoelectric properties by the dielectric grain boundary suggests a new degree of freedom to tune the electromechanical response of ferroelectric polycrystalline materials.

Influence of ageing temperature and heating rate on the properties and microstructure of β Ti alloy, Ti–6Cr–5Mo–5V–4Al

► Effects of heat treatment on the ageing response of a Ti–6Cr–5Mo–5V–4Al alloy. ► Temperatures of 440–500 °C result in the greatest strengthening. ► Ageing response declines at higher temperatures as α precipitates coarsen. ► Fast heating rates result in coarser, more uneven distributions of precipitates. ► Fast heating rates exhibit rapid loss of hardness due to coarsening. A study of the effects of temperature and heating rate upon the ageing response of a Ti–6Cr–5Mo–5V–4Al alloy was undertaken. The degree of strengthening declined with increasing ageing temperature as the precipitate morphologies became coarser, while the extent and thickness of grain boundary α increased with temperature. Ageing at temperatures between 440 °C and 500 °C result in the greatest age hardening response in times of around 4 h due precipitation of the α phase. At 440 °C the hardness increases from the initial solution treated value of 275 H V to more than 450 H V . The rate of heating to reach the isothermal ageing temperature has a substantial impact on the ageing response. Greater scatter was observed in hardness results for the samples aged using a faster heating rate at 500 °C. For ageing at 560 °C a faster rate of heating to the ageing temperature led to rapid declines in the hardness after the peak, equivalent to around 20% of the peak value. Slower heating rates (5 °C min −1 ) appear to increase the number of nucleation sites for precipitation of the α phase, forming finer, more even distributions of precipitates. Faster heating rates (∼100 °C min −1 ) produce coarser, less evenly distributed precipitates which coarsen more rapidly.

Nanoscaled grain boundaries and pores, microstructure and mechanical properties of translucent Yb:[Lu xY (1− x) O 3] ceramics

Translucent ceramics of Yb:[Lu x Y (1− x) O 3] system doped by ZrO 2 was sintered from nanopowder synthesized by laser evaporation. The relative density of the ceramics was 99.97%, residual pores had sizes from 8 nm to 20 nm, Young modulus was 200 GPa at the applied load of 2000 mN, the microhardness was 12.8 GPa. The grains of ceramics had sizes 1–10 μm, but the thickness of grain boundaries was about 1 nm. The transcrystalline type of the crack propagation was detected in the specially broken ceramics. The results indicated high strength of grain bonds and good perfection of grain boundaries in the studied ceramics but an increased content of pores (higher than 10 −3 vol.%) and stoichiometry deviation (Lu:Y:O = 0.21:0.79:3) from the required one (Lu:Y:O = 0.25:0.75:3).

Electrical transport at low temperatures in dense nanocrystalline Gd-doped ceria

Dense nanocrystalline Ce0.9Gd0.1O2 − δ (nCGO) with a median grain size of ∼ 120 nm was prepared by spark-plasma sintering of nanoscaled powders. The electrical behaviour of nCGO was analysed by impedance spectroscopy and compared with the micro-grained material of the same composition in humidified (H2O or D2O) and dry O2 in the temperature range 25–600 °C. The large volume of grain boundaries in the nanometric material is highly blocking to oxide-ions, the majority charge carriers above 100 °C, such that the impedance response in this range is dominated by the grain-boundary contribution. The much smaller grain-boundary resistance of micrometric CGO is attributable to the larger grain size rather than a different grain-boundary thickness or conductivity. Proton transport dominates the electrical conductivity of nCGO in wet atmospheres below 100 °C, as demonstrated by the presence of a conductive H+/D+ isotope effect. The absence of a measurable electromotive force in a water-based concentration cell with nCGO as separating membrane and a massively higher resistivity of nCGO samples with a blocked lateral surface both strongly indicate that the proton transport is attributable to surface processes associated with chemisorbed and physisorbed water layers rather than grain-bulk or grain-boundary phenomena. The magnitude of the room-temperature surface proton conductivity is 4–5 times greater for nanostructured Ce0.9Gd0.1O2 − δ than the micrometric analogue.

Grain Growth‐Controlled Giant Permittivity in Soft Chemistry CaCu3Ti4O12 Ceramics

We report a dielectric constant of up to 5.4 × 105 at room temperature and 1 kHz for CaCu3Ti4O12 (CCTO) ceramics, derived from multiphase powders (coprecipitation products), made by a “chimie douce” (coprecipitation) method, and then sintered in air. The sintered products are pure‐phase CCTO ceramics. The high dielectric constant is achieved by tuning the size of grains and the thickness of grain boundaries. The grain growth is controlled by varying the concentration of excess CuO in the initial powder (calcined coprecipitation products) between 1 and 3.1 wt%. The dielectric constant of pure CCTO ceramics increases with the initial CuO concentration, reaching its maximum at 2.4 wt% of CuO. A further increase of excess CuO in powders results in a permittivity decrease, accompanied by the formation of CuO as a separate phase in the sintered products. The unusual grain growth behavior is attributed to a eutectic reaction between CuO and TiO2 present in the initial powder.

Grain size dependence of electrical properties of Gd-doped ceria

A Gd-doped ceria powder (GDC, Ce0.8Gd0.2O1.9) prepared from an oxalate precursor was hot-pressed at 1173-1273 K in Ar atmosphere and annealed in air at 1273-1473 K to control the bulk density and grain size. Complex impedance of GDC of the microstructures of 0.29-5.6 μm average grain size and 95-98% theoretical density were measured at 573-1073 K in air in the frequency range from 100 Hz to 2 MHz. The bulk conductivity (σi(b)) was independent of the grain size and the activation energy for the migration of oxide ions was 85-88 kJ/mol. The grain boundary conductivity (σi(g)) was far small as compare with σi(b) (σi(g)=1/40-1/700 of σi(b) at 573 K). The σi(g) was influenced by the average grain size and showed a minimum value at around 3 μm and gradually increased with decreasing grain size. However, the total conductivity decreased for smaller grain size because of the increased thickness of grain boundary of relatively low conductivity. The information of diffusion of oxide ions from the surface into GDC inside was also reported.

Transparent Y2O3:Nd3+ ceramics produced from nanopowders by magnetic pulse compaction and sintering

Transparent Nd-doped cubic Y2O3 ceramics have been prepared by magnetic pulse compaction of nanopowders, followed by sintering. Analysis of the transport of nonequilibrium thermal phonons at liquid helium temperatures has been used to assess the effect of preparation conditions on the microstructure, average thickness, and stability of the grain boundaries in the ceramics. The average grain-boundary thickness in the transparent ceramics is shown to be comparable to the lattice parameter of Y2O3.

Grain boundary partitioning of Ar and He

An experimental procedure has been developed that permits measurement of the partitioning of Ar and He between crystal interiors and the intergranular medium (ITM) that surrounds them in synthetic melt-free polycrystalline diopside aggregates. 37Ar and 4He are introduced into the samples via neutron irradiation. As samples are crystallized under sub-solidus conditions from a pure diopside glass in a piston cylinder apparatus, noble gases diffusively equilibrate between the evolving crystal and intergranular reservoirs. After equilibration, ITM Ar and He is distinguished from that incorporated within the crystals by means of step heating analysis. An apparent equilibrium state (i.e., constant partitioning) is reached after about 20 h in the 1450 °C experiments. Data for longer durations show a systematic trend of decreasing ITM Ar (and He) with decreasing grain boundary (GB) interfacial area as would be predicted for partitioning controlled by the network of planar grain boundaries (as opposed to ITM gases distributed in discrete micro-bubbles or melt). These data yield values of GB-area-normalized partitioning, K ¯ ITM Ar , with units of (Ar/m 3 of solid)/(Ar/m 2 of GB) of 6.8 × 10 3–2.4 × 10 4 m −1. Combined petrographic microscope, SEM, and limited TEM observation showed no evidence that a residual glass phase or grain boundary micro-bubbles dominated the ITM, though they may represent minor components. If a nominal GB thickness ( δ) is assumed, and if the density of crystals and the grain boundaries are assumed equal, then a true grain boundary partition coefficient ( K GB Ar = X crystals Ar / X GB Ar ) , may be determined. For reasonable values of δ, K GB Ar is at least an order of magnitude lower than the Ar partition coefficient between diopside and melt. Helium partitioning data provide a less robust constraint with K ¯ ITM He between 4 × 10 3 and 4 × 10 4 cm −1, similar to the Ar partitioning data. These data suggest that an ITM consisting of nominally melt free, bubble free, tight grain boundaries can constitute a significant but not infinite reservoir, and therefore bulk transport pathway, for noble gases in fine grained portions of the crust and mantle where aqueous or melt fluids are non-wetting and of very low abundance (i.e., <0.1% fluid). Heterogeneities in grain size within dry equilibrated systems will correspond to significant differences in bulk rock noble gas content.