Grain Boundary Composition Research Articles

Effect of tungsten concentration on microstructures of Co-10Ni-6Al-(0,2,4,6)W-6Ti (at%) cobalt-based superalloys

The effects of reducing the W concentration (and concomitantly mass density) of Co-10Ni-6Al-xW-6Ti at% Co-based superalloys, with a γ(f.c.c.) plus γ′(L12) microstructure, is investigated for a series of alloys with W concentrations of 6at% (baseline alloy), 4 and 2at% (W-reduced) and 0at% (W-free). The γ′(L12) solvus temperature decreases strongly (by 46°C per 1at% reduction in W), while the liquidus and solvus temperatures decrease mildly (3–5°C per 1at% W reduction) as measured by differential scanning calorimetry. Scanning electron microscopy was used to image phase formation in all alloys. When aging at 900 or 1000°C for 256h, the W reduction does not result in the appearance of additional phases within the grains beyond the γ(f.c.c.) and γ′(L12) phases present in the original alloy. Grain-boundary precipitates are present for all alloy compositions: W-rich or W-free precipitates after aging at 1000°C, and coarsened γ′(L12) precipitates after aging at 900°C. The composition of grain boundary precipitates were measured with energy-dispersive X-ray spectroscopy. Vickers microhardness values decrease with decreasing W content, due to a reduction in γ′(L12) precipitation and reduced solid-solution strengthening.

Room-Temperature Superplasticity in an Ultrafine-Grained Magnesium Alloy

Superplasticity, a phenomenon of high tensile elongation in polycrystalline materials, is highly effective in fabrication of complex parts by metal forming without any machining. Superplasticity typically occurs only at elevated homologous temperatures, where thermally-activated deformation mechanisms dominate. Here, we report the first observation of room-temperature superplasticity in a magnesium alloy, which challenges the commonly-held view of the poor room-temperature plasticity of magnesium alloys. An ultrafine-grained magnesium-lithium (Mg-8 wt.%Li) alloy produced by severe plastic deformation demonstrated 440% elongation at room temperature (0.35 Tm) with a strain-rate sensitivity of 0.37. These unique properties were associated with enhanced grain-boundary sliding, which was approximately 60% of the total elongation. This enhancement originates from fast grain-boundary diffusion caused by the Li segregation along the grain boundaries and the formation of Li-rich interphases. This discovery introduces a new approach for controlling the room-temperature superplasticity by engineering grain-boundary composition and diffusion, which is of importance in metal forming technology without heating.

Tensorial elastic properties and stability of interface states associated with Σ5(210) grain boundaries in Ni3(Al,Si)

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The effect of the local chemical composition of grain boundaries on the corrosion resistance of a titanium alloy

The influence of the structural-phase state of grain boundaries in a Ti4Al2V (commercial PT3V grade) pseudo-alpha-titanium alloy on its susceptibility to hot-salt intergranular corrosion (IGC) has been studied. It is established that IGC-tested alloy samples exhibit corrosion-induced defects of two types. More extended defects of the first type occur at the V-rich boundaries of coarse grains, while short defects of the second type reside at the grain boundaries with composition close to that of the grain body. The existence of the two types of IGC defects is explained by the classical theory of galvanic microcouples (microcells), according to which the IGC intensity is proportional to the difference of corrosion-active impurity concentrations between the grain boundary and body.

Assessment of segregation kinetics in water-moderated reactors pressure vessel steels under long-term operation

In reactor pressure vessel (RPV) bcc-lattice steels temper embrittlement is developed under the influence of both operating temperature of ∼300 °C and neutron irradiation. Segregation processes in the grain boundaries (GB) begin to play a special role in the assessment of the safe operation of the RPV in case of its lifetime extension up to 60 years or more. The most reliable information on the RPV material condition can be obtained by investigating the surveillance specimens (SS) that are exposed to operational factors simultaneously with the RPV itself. In this paper the GB composition in the specimens with different thermal exposure time at the RPV operating temperature as well as irradiated by fast neutrons (E ≥ 0.5 MeV) to different fluences (20–71)·1022 m−2 was studied by means of Auger electron spectroscopy (AES) including both impurity and main alloying elements content. The data obtained allowed to trace the trend of the operating temperature and radiation-stimulated diffusion influence on the overall segregants level in GB. The revealed differences in the concentration levels of GB segregants in different steels, are due to the different chemical composition of the steels and also due to different grain boundary segregation levels in initial (unexposed) state. The data were used to estimate the RPV steels working capacity for 60 years. The estimation was carried out using both the well-known Langmuir-McLean model and the one specially developed for RPV steels, which takes into account the structure and phase composition of VVER-1000 RPV steels, as well as the long-term influence of operational factors.

Thermodynamic properties of phase-field models for grain boundary segregation

Impurity atoms segregated in grain boundary (GB) regions can dramatically change the physical and chemical properties of the GBs. Such changes often appear to be attributed to the GB energy reduction and/or solute drag effect. Phase-field models have been utilized to clarify both the thermodynamic and kinetic effects of the GB segregation. In this study, we developed phase-field models for GB segregation that are diffuse interface versions of the classical two-phase model of GB segregation. The thermodynamic state at any point in the system is represented as a mixture of a GB phase and a matrix phase. There are two choices for the thermodynamic relation between the GB phase and the matrix phase that constitute the point: the equal composition condition in model I and the equal diffusion potential condition in model II. Most of the previous PFMs for GB segregation appear to be specific cases of model I. We examined the thermodynamic properties of models I and II, and compared them with each other and the classical two-phase model. Although all the models resulted in the same GB composition, the GB energy and its dependency on the composition at the equilibrium state are quite different from each other. In model I, there is a lower bound to the GB energy, which originates from the equal composition condition. The GB energy from model II shows no such lower bound, and it is represented as the vertical distance between the parallel tangent lines on the free energy diagram, as in the classical two-phase model. Nevertheless, the compositional dependence in the model II is quite different from that in the classical two-phase model. This originates from the different choices for the composition-independent parameter in the models: a constant gradient energy coefficient in model II and a constant GB width in the classical two-phase model. Model I is not suitable for simulations of alloys that show a reduction of the GB energy due to GB segregation below a certain limit (in dilute alloys, about half of the GB energy of pure solvents). Model II is a correct choice for such alloys.

Modelling equilibrium grain boundary segregation, grain boundary energy and grain boundary segregation transition by the extended Butler equation

The Butler equation is extended to model equilibrium grain boundary (GB) energy and the equilibrium GB composition of a polycrystal, as a function of the following state parameters: bulk composition, temperature, pressure and the five degrees of freedom of the GB. In the simplest case of an ideal solution and equal atomic sizes of the components, the Butler equation reduces back to the well-known McLean equation of GB segregation. When the components repulse each other in the solid solution, grain boundary segregation transition (GBST) appears below the critical temperature of the bulk solid miscibility gap. The GBST line is a new equilibrium line in equilibrium phase diagrams. This new model is demonstrated for copper (Cu) segregation to the GBs in nickel (Ni) and for the phosphorous (P) segregation to the GBs in bcc iron (Fe). The GBST line appears in the Ni-rich (Fe-rich) corner of the Ni–Cu (Fe–P) phase diagram in coordinates of bulk Cu (P) mole fraction vs temperature at fixed pressure. The mole fraction of the solute (Cu or P), corresponding to the GBST line steadily increases with temperature. At a lower solute content (Cu or P), or at a higher temperature compared to the GBST line, the GB is composed mostly of the solvent atoms (Ni or Fe). Contrariwise, at a higher solute content (Cu or P), or at a lower temperature compared to the GBST line, the GB is composed mostly of the solute atoms (Cu or P). These low-segregation and high-segregation states of the GB are transformed into each other via a reversible first-order GBST. This latter process takes place when the GBST line is crossed by changing the bulk composition or the temperature. The results, theoretically estimated, are in agreement with earlier experimental results.

Do micromagnetic simulations correctly predict hard magnetic hysteresis properties?

Micromagnetic calculations using the finite element technique describe semi-quantitatively the coercivity of novel rare earth permanent magnets in dependence on grain size, grain shape, grain alignment and composition of grain boundaries and grain boundary junctions and allow the quantitative prediction of magnetic hysteretic properties of rare earth free magnets based on densely packed elongated Fe and Co nanoparticles, which depend on crystal anisotropy, aspect ratio and packing density. The nucleation of reversed domains preferentially takes place at grain boundary junctions in granular sintered and melt-spun magnets independently on the grain size. The microstructure and the nanocompostion of the intergranular regions are inhomogeneous and too complex in order to make an exact model for micromagnetic simulations and to allow a quantitative prediction. The incoherent magnetization reversal processes near the end surfaces reduce and determine the coercive field values of Co- and Fe-based nanoparticles.

Quantifying and Correlating the Composition and Conductivity of Grain Boundaries in Ca-doped CeO2 Electrolytes, an AC-STEM EELS Study

Quantifying and Correlating the Composition and Conductivity of Grain Boundaries in Ca-doped CeO2 Electrolytes, an AC-STEM EELS Study

Electrical properties of grain boundaries in interfacially controlled functional ceramics

The mass and charge transport properties as well as structure and composition of grain boundaries in interfacially controlled electroceramic materials are reviewed. Recent advances in the field of donor doped (n-conducting) barium titanate are emphasized, comprising experimental methods such as the characterization of the structure and composition of grain boundaries, the experimental determination of the electrical properties as a function of temperature, oxygen partial pressure and dc bias, the investigation of diffusion along grain boundaries involving conductivity relaxation experiments and 18O - tracer exchange measurements. In addition, phenomenological modeling of transport properties of grain boundaries in donor doped BaTiO3 are outlined in detail, encompassing finite element simulations of diffusion processes as well as modified Schottky barrier models which are suitable for the calculation of the electrical properties as a function of temperature and dc bias.

Microstructure and Composition of Grain Boundary- and Intragranular Oxides in Low Alloy Mn-Cr, Mn-Al-Si and Si-Al-P Hot-rolled Steel Sheets

Abstract During hot rolling, coiling and subsequent cooling of low-alloy steel sheets, internal oxides are formed at the grain boundaries (intergranular) and within the grains (intragranular) as a consequence of oxygen diffusion into the steel and segregating alloy elements. The assembly/microstructure and composition of the oxides found were investigated via transmission electron microscopy (TEM) and electron energy loss (EELS)- or energy-dispersive X-ray (EDX) spectroscopy therein. Our spectroscopic data reveal various simple oxide species, located separately or side by side. Those findings apply to grain boundary oxides and intragranular oxides, for all the steel compositions investigated (Fe-Mn-Cr, Fe-Mn-Al-Si, Fe-Si-Al-P). For example, within the Mn-Cr-alloyed steel we find Mn oxide at all oxide locations (inter- and intragranular), but Cr oxide is not present always. We also discuss the sequences of oxide formation, which follow the thermodynamic rules of oxide formation.

Nanoscale imaging of photocurrent and efficiency in CdTe solar cells.

The local collection characteristics of grain interiors and grain boundaries in thin-film CdTe polycrystalline solar cells are investigated using scanning photocurrent microscopy. The carriers are locally generated by light injected through a small aperture (50-300 nm) of a near-field scanning optical microscope in an illumination mode. Possible influence of rough surface topography on light coupling is examined and eliminated by sculpting smooth wedges on the granular CdTe surface. By varying the wavelength of light, nanoscale spatial variations in external quantum efficiency are mapped. We find that the grain boundaries (GBs) are better current collectors than the grain interiors (GIs). The increased collection efficiency is caused by two distinct effects associated with the material composition of GBs. First, GBs are charged, and the corresponding built-in field facilitates the separation and the extraction of the photogenerated carriers. Second, the GB regions generate more photocurrent at long wavelength corresponding to the band edge, which can be caused by a smaller local band gap. Resolving carrier collection with nanoscale resolution in solar cell materials is crucial for optimizing the polycrystalline device performance through appropriate thermal processing and passivation of defects and surfaces.

Evidence for composition variations and impurity segregation at grain boundaries in high current-density polycrystalline K- and Co-doped BaFe2As2 superconductors

Some polycrystalline forms of the K- and Co-doped BaFe2As2 and SrFe2As2 superconductors now have a critical current density (Jc) within a factor of ∼5 of that required for real applications, even though it is known that some grain boundaries (GBs) block current, thus, raising the question of whether this blocking is intrinsic or extrinsically limited by artefacts amenable to improvement by better processing. Herein, we utilize atom-probe tomography (APT) to study the grain and GB composition in high Jc K- and Co-doped BaFe2As2 polycrystals. We find that all GBs studied show significant compositional variations on the scale of a few coherence lengths (ξ), as well as strong segregation of oxygen impurities, which we believe are largely introduced in the starting materials. Importantly, these findings demonstrate that APT enables quantitative analysis of the highest Jc K-doped BaFe2As2 samples, where analytical transmission electron microscopy (TEM) fails because of the great reactivity of thin TEM samples. The observations of major chemical perturbations at GBs make us cautiously optimistic that there is a large extrinsic component to the GB current blocking, which will be ameliorated by better processing, for which APT will likely be a crucial instrument.

Small Rb+ doping in CaCu3Ti4O12-A possible approach to reduce dielectric loss

Ca1−xRbxCu3Ti4O12 (x=0, 0.01, 0.02 and 0.03) ceramics were synthesized by the sol-gel method. Doping Rb+ reduces dielectric loss, which reaches minimum when x=0.02. By measuring properties of electrical conduction, larger leakage current density and height of grain-boundary Schottky potential barrier (ϕB) were found in the doped samples, and ϕB became maximum when x=0.02. These results are attributed to the increase in the amount of oxygen vacancies and the formation of Cu-rich/Ti-poor grain-boundary layers, and it can be concluded that the dielectric loss in CCTO ceramic can be reduced by manipulating the composition and electrical properties of grain boundary.

Varistor and Magnetic Properties of Nickel Copper Zinc Niobium Ferrite Doped with Bi2O3

Bi2O3 was added into nickel copper zinc niobium ferrite and treated with different thermal processes to change the grain‐boundary chemical composition. The relationship between the grain‐boundary composition and varistor properties were investigated using scanning electron microscopy, transmission electron microscopy, energy dispersion spectroscopy, and X‐ray photoelectric spectroscopy. The experimental results show that Bi2O3 reacts and diffuses into the spinel ferrite grain, forming bismuth iron compounds, causing the spinel ferrite chemical composition near grain boundary becomes iron deficient. The Fe deficiency spinel ferrite near the grain boundary then changes into p‐type conduction. The annealing process after sintering improves the bismuth oxide diffusion and chemical reaction near the grain boundary, which can increase the grain‐boundary resistivity. The n‐type semiconductive grain interior and p‐type spinel ferrite near the grain‐boundary combination can form a double Schottky barrier, leading the specimen to exhibit varistor properties. A multifunctional varistor‐magnetic material with a nonlinear coefficient of 10 and initial permeability of about 225 at 10 MHz can be successfully fabricated by sinteringNi0.2881Cu0.1825Zn0.4802Nb0.0096Fe2.0168O4 ferrites added with 5 mol% Bi2O3 sintered at 950°C, then annealed at 650°C for 1 h.

A novel approach to measure grain boundary segregation in bulk polycrystalline materials in dependence of the boundaries’ five rotational degrees of freedom

We demonstrate a simplified nondestructive 3-D electron backscatter diffraction (EBSD) methodology that enables the measurement of all five degrees of freedom of grain boundaries (GBs) combined with segregation analysis using atom probe tomography (APT). The approach is based on two 2-D EBSD measurements on orthogonal surfaces at a sharp edge of the specimen followed by site-specific GB composition analysis using APT. An example of an asymmetric Σ9 boundary exhibiting GB segregation emphasizes the need for complete GB characterization in this context.

New atom probe approaches to studying segregation in nanocrystalline materials

Atom probe is a technique that is highly suited to the study of nanocrystalline materials. It can provide accurate atomic-scale information about the composition of grain boundaries in three dimensions. In this paper we have analysed the microstructure of a nanocrystalline super-duplex stainless steel prepared by high pressure torsion (HPT). Not all of the grain boundaries in this alloy display obvious segregation, making visualisation of the microstructure challenging. In addition, the grain boundaries present in the atom probe data acquired from this alloy have complex shapes that are curved at the scale of the dataset and the interfacial excess varies considerably over the boundaries, making the accurate characterisation of the distribution of solute challenging using existing analysis techniques. In this paper we present two new data treatment methods that allow the visualisation of boundaries with little or no segregation, the delineation of boundaries for further analysis and the quantitative analysis of Gibbsian interfacial excess at boundaries, including the capability of excess mapping.

A Low-Temperature-Fired Multifunctional Varistor-Magnetic Ferrite Material - CuCr0.2Fe1.8O4

A novel low temperature-fired (950°) multifunctional varistor-magnetic ferrite material can be obtained by adding V2O5 into CuCr0.2Fe1.8O4 ferrite. The relationship between the grain-boundary composition and varistor properties were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersion spectroscopy (EDS), and X-ray photoelectric spectroscopy (XPS). The addition of V2O5 can effectively reduce the sintering temperature of CuCr0.2Fe1.8O4 ferrites to temperatures of lower than 950°. Moreover, the V^(5+) ions dissolved into spinel structure and acted as donor dopant, which resulted in the semi-conductive grain. The copper-rich observation at the grain boundary based on the TEM and EDS results implied that copper oxide would possibly develop at the grain boundary as the acceptor state, forming double Schottky barriers with the n-type semiconductor grains.

The use of a laser-assisted Local Electrode Atom Probe and TEM to examine the microstructure of Zircaloy and precipitate structure following low dose neutron irradiation at nominally 358 °C

Previous research has shown that the irradiation of Zircaloy at very low fluences (⩽7×1022n/m2 (E>1MeV)) results in measurable irradiation hardening, but the defects or solute clusters responsible for such hardening are generally below the resolution limit of electron microscopy. Efforts to analyze Zr-based alloys using conventional atom probe have been largely unsuccessful due to the poor electrical conductivity of Zirconium at cryogenic temperatures. Laser-assisted atom probe methods were introduced in the 1980s as a means for enabling low electrical conductivity specimens to be analyzed. Some recent examples have been reported in the literature where a Local Electrode Atom Probe (LEAP)® was used to successfully analyze non-irradiated Zirconium based alloys, such as Zircaloy-4 and ZIRLO. In this work, the use of a LEAP to examine the microstructure of non-irradiated Zircaloy alloys is evaluated. Optimum conditions for applying the LEAP to non-irradiated Zircaloy-2 and Zircaloy-4 are investigated. Changes in the local composition of precipitates and grain boundaries following neutron irradiation at nominally 358°C to a fluence of 2.9×1025n/m2 in the High Flux Isotope Reactor (HFIR) are determined using Transmission Electron Microscopy. These results are provided as an example of the local changes in microstructure that can influence the evolution of the defect structure.

A Novel Low-Temperature-Fired Multifunctional Varistor-Magnetic Ferrite Materials

A novel low temperature-fired (950°C) multifunctional varistor-magnetic ferrite materials can be obtained by adding V2O5 into CuCr0.2Fe1.8O4 ferrites. The relationship between the grain-boundary composition and varistor properties were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersion spectroscopy (EDS), and X-ray photoelectric spectroscopy (XPS). The addition of V2O5 can effectively reduce the sintering temperature of CuCr0.2Fe1.8O4 ferrites to temperatures of lower than 950°C. Moreover, the V5+ ions occupied the octahedral site of spinel structure and acted as donor dopant, which resulted in the semiconductive grain. The copper-rich observation at the grain boundary based on the TEM and EDS results implied that copper oxide would possibly develop at the grain boundary as the acceptor state, forming double Schottky barriers with the n-type semiconductor grains.