Grain Boundary Composition Research Articles

Grain-boundary structure and segregation behavior in a nickel-base stainless alloy

Atom-probe tomography (APT) is utilized to obtain three-dimensional chemical information concerning grain boundary (GB) segregation in a Ni–Cr–Fe alloy 600 with atomic spatial resolution. Detailed crystallography of GBs is determined using an approach that combines electron backscatter diffraction and focused ion-beam microscopy to establish a GB’s five macroscopic degrees of freedom, followed by an APT GB composition analysis. Characterizations of GB microstructure and microchemistry are performed to improve our understanding of mechanisms controlling intergranular attack and stress-corrosion cracking.

Influence of tempering temperature on both the microstructural evolution and elemental distribution in AISI 4340 steels

In the present study, the influence of tempering temperature on the microstructural evolution and prior austenite grain boundary segregation of AISI 4340 steels was investigated by transmission electron microscope and atom probe. The transmission electron microscopy results showed a variation in the microstructure and the morphology of carbides with a change in tempering temperature. Additionally, the chemical compositions of the prior austenite grain boundaries and carbides were quantified by atom probe tomography. An increase in the tempering temperature led to a decrease in the amount of carbon segregated at the prior austenite grain boundary from 7.9 to 1.3 at.%. It was found that a higher tempering temperature can accelerate the diffusion of carbon from the prior austenite grain boundary into carbide. However, phosphorus atoms were segregated mainly at the prior austenite grain boundary in steel tempered at 400°C (up to 0.18 at.%). It was found that formation of film-like carbide and phosphorus segregation along the prior austenite grain boundary is the main cause of embrittlement in steel tempered at 400°C.

The impact of H/D substitution on the structure, composition and thermal stability of grain boundaries in sub-micron diamond films deposited on silicon

Sub-micron polycrystalline diamond films deposited using H2/CH4 gasses show Raman peaks at ~1130 and ~1470, which have been assigned to the ν1 and ν3 modes of t-PA hydrocarbons within the diamond intergranular space. Here we report the isotopic shift of these peaks in similar diamond films grown from D2/CD4 gas mixtures, as well as the splitting of the 1130cm−1 peak to two peaks at 880 and 855cm−1. We suggest that this unexpected peak splitting is due to an excess of short conjugated t-PA chains and relate this to the lower amount of amorphous carbon in these deuterated films, as evidenced by Raman spectroscopy and secondary ion mass spectrometry (SIMS). This difference in grain boundary amorphous carbon content is also suggested to explain a measured difference in thermal out-diffusion of deuterium from these films, as compared to hydrogen in standard films. A third unreported Raman peak at 820cm−1 is also examined and shown to be related to stressed SiC, seen to relax during high temperature vacuum annealing. The reduction of observed stress is partly assigned to an increase of the sp3/sp2 ratio in the film after annealing.

Grain boundary and interface chemistry of an Nd–Fe–B-based sintered magnet

The compositions of grain boundaries (GBs) and other interfaces surrounding Nd2Fe14B grains in commercial Nd–Fe–B sintered magnets have been investigated by laser-assisted three-dimensional atom probe to understand the mechanism of the coercivity enhancement by post-sinter annealing. While only a slight segregation of Nd and Pr to the GBs was confirmed in the as-sintered sample, a thin Nd-rich amorphous phase layer was observed along the GBs with Cu segregation to the interfaces in the annealed sample. The segregation of Cu to NdOx/Nd2Fe14B interfaces was also found, suggesting that the Nd2Fe14B grains are enveloped by a Cu-enriched layer after the annealing. The concentration of Fe+Co in the thin GB layer was found to be as high as 65at.%, and a model amorphous film processed by sputtering with the same composition as the thin GB layer was found to be ferromagnetic. Ferromagnetic behavior of the thin GB layer suggested that Nd2Fe14B grains are magnetically coupled. The coercivity mechanism of the sintered magnets is discussed based on these new findings.

Direct visualization and characterization of chemical bonding and phase composition of grain boundaries in polycrystalline diamond films by transmission electron microscopy and high resolution electron energy loss spectroscopy

Polycrystalline diamond films are composed of diamond grains, interconnected by grain boundaries of ill defined nature. In this study we characterized the local phase composition, chemical bonding, and structure of the grain boundaries by electron energy loss spectroscopy (EELS) in the plasmonic and core-level ragimes, using a monochromated and high resolution transmission electron microscope. Peaks at 283.5, 286.3, and 287.5 eV in the C(1s) pre-edge structure provide direct evidence for the preferential presence of amorphous carbon and hydrogen bonding at grain boundaries. Plasmonic EELS reveals a high amorphous carbon component in the grain boundaries region.

Direct method for determining the segregation in silver-copper solid solutions not prone to brittle breakage of grain boundaries

A new method is proposed for measuring the chemical composition of grain boundaries (GBs) in copper-based alloys not prone to embrittlement of boundaries. This method is based on embrittlement of copper GBs by bismuth penetrating from the gas phase of bismuth telluride (Bi2Te3). Chemical analysis of the GB surface is performed via Auger-electron spectroscopy. The GB composition is measured in a solid solution of silver (its volume concentration is 1.4 at %) and copper. The segregation annealing temperature is 570°C. The silver concentration across the broken surface of the GB (its thickness is five to ten atomic layers) is 4.7 at %. Therefore, the ratio between silver concentrations within the GB and the grain volume (the enrichment coefficient) is approximately 3.5.

Effect of diffusion annealing on the hysteretic properties of sintered Nd-Fe-B magnets

Effect of diffusion annealing on the formation of dysprosium-saturated grain boundaries in sintered Nd-Fe-B magnets has been studied. As a source of diffusion, we used powders of metallic dysprosium, DyCu, DyGa, and Dy3Co alloys and DyH x hydride. Plates ∼1 mm thick made of sintered magnets were molded into powders with a particle size from 3 to 500 μm and subsequently annealed in a vacuum at 750–850°C for 0.5–10 h. After annealing, the microstructure and the elemental composition of grains and grain boundaries were studied by scanning electron microscopy. Under optimum annealing conditions, Dy atoms were shown to diffuse preferentially through grain boundaries of the magnets. The coercive force H c of Nd-Fe-B plates, which were annealed in contact with the DyGa, Dy3Co and DyH x powders and subsequently annealed at 550°C, increases by 4–6 Oe; no substantial decrease in the remanence takes place. The increase in the coercive force depends of the powder particle size and temperature and time of diffusion annealing.

Effects of solution-grown CdS on Cu(InGa)Se2 grain boundaries

Complete Cu(In,Ga)Se2 (CIGS) solar cells in which the collecting heterojunctions were formed by chemical bath deposition of CdS were analyzed by transmission electron microscopy and energy dispersive spectroscopy. The CIGS was produced at 400 or 580 °C as either single layers or in a two-step bilayer process. The compositions of grain boundaries were found to be the same as the grains before CdS deposition except in the low temperature-deposited bilayer but after CdS was formed the grain boundaries were found to be anion deficient with some evidence of Cu loss from the CIGS and residual Cu found in the CdS in most cases. The results are consistent with n-type doping of the surface of the CIGS and wrapping of the junction around the grains. In bilayer films the grain boundaries were found to be more open containing many voids and this facilitated penetration of the CdS into the boundaries.

Stress Induced by Constrained Sintering of 3YSZ Films Measured by Substrate Creep

3YSZ green layers approximately 10 μm thick were screen printed onto 3YSZ substrates up to 300 μm in thickness. The stress induced by constrained sintering of the film (between 1150° and 1350°C) was measured by monitoring the bending displacement of vertical strips of bilayers using a long‐distance microscope. In order to deduce the stress it was first necessary to measure the creep properties of the substrates by monitoring the bending of horizontal beams under gravity. The creep strain rate of the 3YSZ substrates was linearly dependent on applied stress at the low stresses and strains involved in the present work. The creep viscosity appeared to increase with strain (time), which might be due to changes in grain‐boundary composition, and had higher activation energy at temperatures above approximately 1250°C. The magnitudes of the creep viscosities are in reasonable agreement with other creep data in the literature for 3YSZ. The in‐plane stress induced during constrained sintering of the 3YSZ films had a maximum value of approximately 3 MPa at 1200°C. This behavior is consistent with literature results reported for constrained sintering of bulk alumina. The stress induced by the constraint is of a similar order to the estimated sintering potential.

Wetting and premelting of triple junctions and grain boundaries in the Al–Zn alloys

Phase transitions in grain boundaries (GBs) and GB triple junctions (TJs) can change drastically the properties of polycrystals. The GB and TJ wetting phase transition can occur in the two-phase area of the bulk phase diagram where the liquid and solid phases are in equilibrium. The GB and TJ wetting tie-lines can continue in one-phase area of the bulk phase diagram as a GB or TJ solidus line. This line represents the GB or TJ premelting phase transition. The structure and composition of grain boundaries and GB triple junctions were studied by high-resolution electron microscopy and analytical transmission electron microscopy in the Al–5 at.% Zn polycrystals and by differential scanning calorimetry (DSC) in the Al–7.5 at.% Zn polycrystals. Between bulk solidus and GB or TJ solidus the metastable Zn-rich β m-phase was observed in the GB triple junctions of quenched samples. This phase appears neither in the samples annealed above the bulk solidus nor in those annealed below the GB solidus. Zn-content in this β m-phase corresponds to that of bulk liquidus. This is a structural indication that if the melt wets the GBs or TJs, the GB (or TJ) solidus line appears in the bulk phase diagram, and the liquid-like phase exists in GBs and TJs between bulk solidus and GB (or TJ) solidus lines. The structural observation of this phase is also supported by our data obtained by means of DSC.

Solute segregation to ferrite grain boundaries in nodular cast iron: experiment and prediction

AbstractGrain boundary segregation belongs to the phenomena affecting predominantly mechanical properties of polycrystalline materials. Although extended studies of interfacial phosphorus segregation have been done in low‐alloy steels, no information is available on segregation of this impurity to ferrite grain boundaries in cast irons. In this contribution we describe chemical composition of grain boundaries of the ferrite phase in nodular cast iron. Auger electron spectroscopy (AES) measurements are compared with the prediction of chemical composition based on the recently proposed thermodynamic models (the relationship between the enthalpy of solute segregation and the solid solubility, and the compensation effect). Both these general relationships are applied to predict the segregation of any solute at any grain boundary: this prediction offers a substantial extension of the database on grain boundary segregation. Copyright © 2008 John Wiley & Sons, Ltd.

Influences of Bi 2O 3 on microstructure and magnetic properties of MnZn ferrite

MnZn ferrites were prepared by conventional oxide ceramic process. The effects of Bi 2O 3 on microstructure and magnetic properties of MnZn ferrites were investigated by means of characterizing the fracture surface micrograph, composition of grain boundary, magnetic properties and density by scanning electron microscope (SEM), energy dispersive X-ray spectroscope (EDS), B-H analyzer and Archimedes method, respectively. The results indicate that Bi 2O 3 mainly segregates and concentrates in the grain boundary regions, promotes solid-state reaction and grain growth, reduces porosity and enhances density. Optimum addition of Bi 2O 3 increases the permeability and saturation magnetic induction, meanwhile ensures the well frequency stability of permeability.

The Mechanism of Zr and Hf in Reducing Radiation-Induced Segregation in 316 Stainless Steel

The addition of oversized solutes has the potential to reduce the effects of radiation-induced segregation (RIS) in austenitic alloys. This RIS has been implicated as one of several factors in enhancing stress corrosion cracking (SCC) under irradiation, so oversized solute additions could promote SCC resistance. Either Zr or Hf was added to 316-type stainless steel, at levels between 0.05 and 0.37 at. pct. Samples were irradiated with 3 MeV protons to 3 dpa at 400 °C and analyzed using high-resolution–scanning transmission electron microscopy (HR-STEM) with energy-dispersive X-ray spectroscopy (EDS), to measure the grain-boundary (GB) composition. The Zr additions substantially reduced the amount of RIS, while the Hf was much less effective. Despite similar sizes, first-principles calculations using the Vienna Ab Initio Simulation Package (VASP) demonstrate that solute-vacancy binding for Zr is 1.05 eV vs 0.69 eV for Hf. This difference results in the greater effectiveness of Zr in reducing RIS, as determined by kinetic rate theory calculations, in agreement with experimentally-measured results.

Grain boundary phase observed in Al–5 at.% Zn alloy by using HREM

The nature and behaviour of grain boundary (GB) phases is very important since they can control strength, plasticity, resistivity, grain growth, corrosion resistance, etc, especially in nanocrystalline materials. For nanocrystalline Al-based light alloys, extremely high plasticity has been observed in restricted temperature and concentration intervals close to the solidus line. This phenomenon is not fully understood. It can be explained by formation of GB phases not included in the bulk phase diagram. Therefore, the structure of GB phases, as well as thermodynamic conditions for their existence, has to be carefully studied. In this work the structure and composition of GBs and GB triple junctions in Al–5 at.% Zn polycrystals annealed in the temperature region above and below the bulk solidus line were studied by high-resolution electron microscopy and analytical transmission electron microscopy. Evidence has been obtained that a thin layer of a liquid-like phase exists in GBs and GB triple junctions slightly below the bulk solidus line.

Grain-Boundary Physics in PolycrystallineCuInSe2Revisited: Experiment and Theory

Current studies have attributed the remarkable performance of polycrystalline CuInSe2 (CIS) to anomalous grain-boundary (GB) physics in CIS. The recent theory predicts that GBs in CIS are hole barriers, which prevent GB electrons from recombining. We examine the atomic structure and chemical composition of (112) GBs in Cu(In,Ga)Se2 (CIGS) using high-resolution Z-contrast imaging and nanoprobe x-ray energy-dispersive spectroscopy. We show that the theoretically predicted Cu-vacancy rows are not observed in (112) GBs in CIGS. Our first-principles modeling further reveals that the (112) GBs in CIS do not act as hole barriers. Our results suggest that the superior performance of polycrystalline CIS should not be explained solely by the GB behaviors.

High Thermal Conductivity Aluminum Nitride Substrates Prepared by Aqueous Tape Casting

High thermal conductivity aluminum nitride (AlN) substrates were prepared by aqueous tape casting. The characteristics of surface‐treated AlN powder were studied in aqueous ball‐milling media. The oxygen content of AlN powder with the dispersant was lower than that of AlN powder without the dispersant at the same ball‐milling time. The isoelectric points of the surface‐treated AlN with and without a dispersant were, respectively, at pH∼3.35 and pH∼3.90. The atmosphere had considerable effects on organic additive burnout of aqueous AlN green sheets. The composition of grain boundaries changed with increasing holding time at 1850°C. A translucent AlN substrate with a uniform microstructure and a thermal conductivity of 263 W·(m·K)−1 was obtained by pressureless sintering at 1850°C for 6 h in a nitrogen atmosphere.

Observation of the effect of grain orientation on chromium segregation in longitudinal magnetic media

The nanoscale grain size of current hard disk media demands high-resolution methods for characterization. Therefore, high-resolution transmission electron microscopy (TEM), energy-filtered TEM (EFTEM) imaging, and nanoprobe energy-dispersive spectroscopy were chosen for the examination of Co84−x−yCr16PtxTay longitudinal hard disk media samples. Experiments were conducted to elucidate the relationship between nanoscale variations in grain-boundary composition and the atomic structure of the grain boundaries. Results show that Cr enriches the grain boundaries of the magnetic layer in all examined cases, but the addition of Ta greatly enhances this enrichment at some boundaries. Nanoprobe and EFTEM results show that enrichment varies according to the grain-boundary orientation of the hcp cobalt alloy crystals. Grains with tilt-boundary orientations near 0°, 30°, and 60°, found by measuring the angle between adjacent c-axis directions, show the least amount of segregation. Grains with tilt-boundary orientations near 15°, 45° and 75° show maximum amounts of Cr segregation. This trend exists in both CoCrTa and CoCrTaPt samples, and efforts to interpret the results within the context of hcp coincident site lattice theory are described.

Influence of the de-waxing atmosphere on the properties of AlN ceramics processed from aqueous media

The influence of binder burnout atmosphere (air or N 2) on surface chemistry of thermo-chemically treated AlN powders processed in aqueous media, and on the final properties of AlN ceramics was studied. The surface chemistry after de-waxing was accessed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). X-ray diffraction (XRD), SEM, high-resolution transmission electron microscopy (HR-TEM), were used to identify the phase assemblage and for microstructural analysis. The effects of the residual carbon and oxygen at the surface on the thermal conductivity and sintered density of AlN samples were investigated. The surface C/O ratios were observed to correlate with the sintering behaviour, the composition and distribution of secondary phases, and grain-boundary composition, as well as thermal conductivity of AlN samples. Thermal conductivities of about 140 W/mK were obtained for the aqueous processed AlN samples de-waxed in nitrogen atmosphere and sintered for 2 h at 1750 °C in the presence of 4 wt.% YF 3 + 2 wt.% CaF 2 as sintering additives.

Correlation Between Grain‐Boundary Segregation and Grain‐Boundary Plane Orientation in Nb‐Doped TiO2

Grain‐boundary (GB) segregation has been measured in Nb‐doped TiO2 (rutile) by Auger electron spectroscopy analyses of the two surfaces exposed by inter‐granular fracture. In addition, the orientations of the exposed GB planes on both sides of the fracture surface have been determined. A strong anisotropy of GB composition is found. Measurements on 166 individual GB fracture surfaces show that boundary planes with low and high levels of segregation tend to be clustered along the (110)–(010) and (001)–(011) regions of the stereographic triangle, respectively. The matching halves of GB planes with either high or low segregation also tend to be terminated by planes in the same respective regions of orientation space. High and low levels of segregation occur in regions of GB plane orientation space estimated to correspond to high and low GB energy, respectively. This trend is consistent with qualitative arguments based on Gibbsian adsorption.

Segregation of Calcium to Magnesium Oxide Grain Boundaries

Grain boundary (GB) segregation has been measured in Ca-doped MgO by examining intergranular fracture surfaces with Auger electron spectroscopy. The measurements reveal several interesting features. The composition of any given GB on the fracture surface is almost uniform, except for small variations due to deviations from planarity. There is a strong anisotropy of GB composition, which can amount to as much as a factor of six between low and high segregation GB's. Finally, although the compositions of opposite sides of a GB fracture are uniform, there are sometimes significant differences between the two sides, in agreement with a recently formulated model of GB composition as a function of GB character.