Intergranular Films Research Articles

Nanocomposite powder with three-dimensional network structure for preparing alumina–titania nanocomposite coating with advanced performance

Nanocomposite powder with three-dimensional network structure was prepared by adding nano-dopants (ZrO2 and CeO2) in Al2O3–TiO2 nanocomposite powder and employing plasma jet processing. Alumina–titania nanocomposite coating was fabricated through plasma spraying the nanocomposite powder. The formation mechanism of the nanocomposite powder was investigated, and the strengthening and toughening mechanisms of the nanocomposite coating was revealed. The internal microstructure of the powder particles experienced liquid phase sintering during plasma jet processing. The nanocomposite powder with three-dimensional network structure had a microstructure consisting of amorphous intergranular network thin film rich in Ti, Zr and Ce surrounding the α-Al2O3 colonies. Microhardness, crack growth resistance and bonding strength of the nanocomposite coatings were improved compared with that of Metco 130 coating. The strengthening mechanism was mainly microstructure refinement strengthening. The toughening mechanisms included crack deflection and crack trapping. The three-dimensional network structure may work as reinforcing and toughening units in the nanocomposite coating.

The Influence of Cation Additives on Grain‐Boundary Mobility in Yttrium Aluminum Garnet (YAG)

Pure yttrium aluminum garnet (YAG) nano‐powders, doped with 1.4 at.% of La2O3, ZrO2, MgO, Nb2O5, and SiO2, were vacuum sintered to full density and subjected to grain growth at 1700°C, for up to 15 h. The YAG intrinsic grain‐boundary (GB) mobility, determined from the pure fully dense YAG specimens, was 2.9 × 10−16 [m3·N−1·s−1]. All dopants, except for La2O3, increased the GB mobility compared to pure YAG (La2O3 didn't cause any significant change in YAG's GB mobility). All GBs were found to be free of secondary phases or intergranular films. These findings differ from numerous publications where dopants were found to either inhibit grain growth by solute drag, or to enhance grain growth due to liquid phase sintering. It was found that the GB mobility systematically increased with the decrease in the dopant cation radius. Moreover, it seems that vacancy population plays an important role in determining the GB mobility of YAG.

Influence of composite powders’ microstructure on the microstructure and properties of Al2O3–TiO2 coatings fabricated by plasma spraying

Three kinds of alumina–titania composite powders with different microstructure were plasma sprayed to prepare alumina–titania coatings. The influence of composite powders’ microstructure on the microstructure and properties of alumina–titania coatings was investigated. There was typical lamellar structure and columnar grains formed in the conventional micro-composite coating, which were derived from the coarse-grained microstructure and segregated composition of the micro-composite powder feedstock. Comparing with the micro-composite coating, there was not typical lamellar structure and columnar grains in the coatings prepared from nanocomposite powders, which was mainly attributed to the fine-grained microstructure and homogeneous distribution of chemical composition in the nanocomposite powders. There were two types of microstructures formed in the two coatings prepared from nanocomposite powders. Fully-melted regions with splat structure consisting of γ-Al2O3 dissolved with titania and amorphous phase in two nanocomposite coatings were formed by the fully melted nanocomposite powders during plasma spraying. Partially-melted regions with particulate structure consisting of α-Al2O3 and amorphous phase in the nanocomposite coating was formed by the partially melted or unmelted nanocomposite powder with particulate structure, and partially-melted regions with three-dimensional network structure consisting of amorphous intergranular network thin film rich in Ti, Zr and Ce surrounding the α-Al2O3 colonies in the nanocomposite coating were formed by the partially melted or unmelted nanocomposite powder with three-dimensional network structure. The microhardness, crack growth resistance and sliding wear resistance of the coatings prepared from the nanocomposite powders, especially the nanocomposite coating prepared from the nanocomposite powder with three-dimensional network structure, were enhanced comparing with that of the conventional micro-composite coating. The superior properties of the nanocomposite coatings were attributed to the uniform composition distribution in their nanocomposite powders.

Role of tensile forces in hot tearing formation of cast Al-Si alloy

The instrumented applied rod casting apparatus (ARCA) was developed to investigate the effects of tensile forces in the hot tearing formation of cast Al-Si alloys. The obtained data of tensile forces/temperature was used to identify hot tearing initiation and propagation and the fracture surface of samples was also investigated. The result shows that the applied tensile forces have a complex effect on load onset for the hot tearing initiation and propagation. During the casting solidification, the tensile forces are gradually increased with the increase of solid fraction. Under the action of tensile forces, there will appear hot tearing and crack propagation on the surface of the sample. When the tensile forces exceed the inherent strength of alloys, there will be fractures on the sample. As for the A356 alloy, the critical fracture stress is about 0.1 MPa. The hot tearing surface morphology shows that the remaining intergranular bridge and liquid films are thick enough to allow the formation of dendrite-tip bumps on the fracture surface.

Role of oxygen on the interfacial adsorption sites of Lu and La in β-Si3N4

Molecular dynamics computer simulations show the important role of oxygen replacing nitrogen at surface sites in the prismatic surface of β-Si3N4 on the adsorption of rare-earth ions from the intergranular film. Such adsorption affects the grain morphology, which plays an important role in mechanical properties. The simulations successfully obtain the correct Lu sites Lu1 and Lu2 on the prismatic surface with O replacement at specific combinations of N surface sites. The results match the experimental high angle annular dark field scanning transmission electron microscopy data. Unlike the Lu results, surface oxidation does not have a large effect on La adsorption sites on the prismatic surface, but does have an influence on the La surface occupancy. The simulation results provide the mechanisms for both the experimental variances observed for the adsorption sites of these two rare-earth additives in the intergranular films and can be used to explain the different grain growth behavior seen experimentally.

Creep and microstructure of Nextel™ 720 fiber at elevated temperature in air and in steam

Creep of Nextel™ 720 alumina–mullite fiber tows was investigated at 1100 and 1200°C for tensile stresses of 100–400MPa in air and in steam. Fiber microstructures were characterized after creep by transmission electron microscopy. At low stresses steam increased creep rates by up to an order of magnitude and reduced creep lifetimes. At high stresses creep rates in steam and air were similar. Cavitation was prevalent in steam but not in air. The creep-rupture data obtained at 1200°C were analyzed in terms of a Monkman–Grant (MG) relationship. The MG parameters were independent of the test environment. Results reveal that the MG relationship can be used to predict creep rupture for Nextel™ 720 fibers and composites reinforced with these fibers at 1200°C in air and in steam. In steam the mullite in the Nextel™ 720 fibers decomposed to porous alumina. Decomposition kinetics were linear and had an activation energy of ∼200kJmol−1. Intergranular films were not observed on alumina grain boundaries or alumina–mullite interphase boundaries after creep in steam. Creep mechanisms are discussed.

Grain growth in BaTiO3 ceramics assisted by an intergranular glass film

Abstract This research demonstrates that BaTiO 3 ceramics exhibit increased grain boundary mobility ( M b ) triggered by an inter-granular glass film (IGF) as previously demonstrated in polycrystalline Al 2 O 3 ceramics conversion to single-crystal sapphire upon annealing at high temperatures. These results suggest that secondary abnormal grain growth (SAGG) during sintering of BaTiO 3 ceramics at 1365 °C, a temperature above the eutectic points in the BaTiO 3 –SiO 2 and BaTiO 3 –TiO 2 systems, is another example of faster M b caused by IGF. Higher M b magnified through an interface glass film of several nanometers thickness facilitates a faster grain growth rate that produces exceedingly large grains of ∼2.4 mm in size. The growth mechanism involving twin-plane re-entrant (TPRE) corners may not be the only route responsible for SAGG in BaTiO 3 , particularly when sintered at above eutectic temperatures. How this abnormal grain growth occurs through mechanisms involving liquid-phase-enhanced M b and the {111} twins as a chemically induced growth fault is discussed.

A review of wetting versus adsorption, complexions, and related phenomena: the rosetta stone of wetting

This paper reviews the fundamental concepts and the terminology of wetting. In particular, it focuses on high temperature wetting phenomena of primary interest to materials scientists. We have chosen to split this review into two sections: one related to macroscopic (continuum) definitions and the other to a microscopic (or atomistic) approach, where the role of chemistry and structure of interfaces and free surfaces on wetting phenomena are addressed. A great deal of attention has been placed on thermodynamics. This allows clarification of many important features, including the state of equilibrium between phases, the kinetics of equilibration, triple lines, hysteresis, adsorption (segregation) and the concept of complexions, intergranular films, prewetting, bulk phase transitions versus “interface transitions”, liquid versus solid wetting, and wetting versus dewetting.

Analysis of weld cracking in shotblasting chambers made of Hadfield steel

In fillet welds made of 18Cr–9Ni–7Mn austenitic stainless steel centerline cracks were established along the connection line of two solidification fronts in the weld. On the grain boundaries oxide and carbide films were identified. Hot cracks resulted from a combined activity of a large gap between plates and the resulting high tensile stresses occurring during cooling, free root surface of the weld in the large gap, and intergranular oxide and carbide films in the weld. Since the parent metal was cut with highly oxidating plasma and atmospherically corroded surfaces was not ground prior to welding, remelted surface oxides passed into the weld pool. The increased Cr content identified in intergranular oxide films results from Cr oxidation in the filler material.

On Equilibrium Ga Intergranular Films in Cr2GaC

The presence of equilibrium intergranular films (IGFs) in metallic systems has long been postulated; compelling evidence for this idea, however, is still lacking. Herein differential scanning calorimetry, in the−60°C to 70°C temperature range, on bulk and Cr2GaC powders was carried out. A sharp endothermic trough, roughly 50°C below the melting point of bulk pure Ga, in the powdered samples was taken to be strong evidence for the presence of IGFs in the latter. The troughs for the sintered body were much more diffuse suggesting that in this case, a distribution of grain boundary thicknesses exists. A temperature dependent adsorption energy is postulated to explain the reversible extrusion and resorption of 100 μm thick Ga micro-pillars.

The Relationship between Grain Boundary Energy, Grain Boundary Complexion Transitions, and Grain Size in Ca-Doped Yttria

The thermal groove technique has been used to measure relative grain boundary energies in two 100 ppm Ca-doped yttria samples. The first has a normal grain size distribution and the boundaries have a bilayer of segregated Ca. In the second sample, there is a combination of large grains and small grains. The boundaries around the large grains are known to have an intergranular film. The results show that the relative energies of boundaries in the sample with normal grain growth and the boundaries around small grains far from larger grains in the second sample are similar. Also, boundaries surrounding the largest grains and small grains immediately adjacent to them have the same and significantly lower energies. The results indicate that grain boundaries with an intergranular film have a lower energy than those with bilayer segregation and that the intergranular film extends beyond the periphery of the largest grains, but not throughout the entire sample.

PRECIPITATION BEHAVIOR OF,M讼3C6 AND ITSEFFECTS ON DUCTILITY AND TOUGHNESSOF A NOVEL Cr-Mn-N AUSTENITICHEAT RESISTANT STEEL

Cr-Mn-N austenitic heat resistant steels have bright future in wide applications due to their lower cost, excellent mechanical properties, and outstanding local corrosion resistance after substituting the nickel partly or completely by the Mn, N and C elements. As the content of C or N elements increasing in Cr-Mn-N steels, a large quantity of precipitates form during hot working, resulting in embrittlement of these steels. In the present study, a new Cr-Mn-N austenitic heat resistant steel was aged at temperatures from 600 to 1000 in duration from 10 min up to 6000 min. By OM, SEM and XRD, the microstructure evolution of precipitates and their effects on ductility and toughness of the studied steel were investigated. The results indicated that precipitates formed during aging were mainly Cr-rich M23C6 carbides, whose morphologies changed in a sequence of intergranular films, lamellae in cellular microstructure and intragranular rods or particles with the increase of aging time and/or temperature. The time-temperature-precipitation (TTP) curves for M23C6 carbides were determined, which have a typical C-shaped profile with a nose temperature between 850 and 900 , and an incubation period not more than 1 min. In addition, it was found out that the aging embrittlement of the studied steel is strongly dependent on the morphology of M23C6 carbides. The intergranular film-shaped M23C6 carbides were considered as the main factor to favor cracks rapidly expanding along grain boundaries, finally resulting in brittle intergranular fracture. KEY WORDS Cr-Mn-N austenitic heat resistant steel, M23C6 carbide, cellular microstructure, aging embrittlement

Fatigue Crack Growth Behavior of Silicon Nitride: Roles of Grain Aspect Ratio and Intergranular Film Composition

The role of microstructure in affecting the fatigue crack growth resistance of grain bridging silicon nitride ceramics doped with rare earth (RE = Y, La, Lu) oxide sintering additives was investigated. Three silicon nitride ceramics were prepared using MgO‐RE2O3 and results were compared with a commercial Al2O3‐Y2O3‐doped material. Decreasing stress intensity range (ΔK) fatigue tests were conducted using compact‐tension specimens to measure steady‐state fatigue crack growth rates. Specimens doped with MgO‐RE2O3 additives showed a significantly higher resistance to crack growth than those with Al2O3‐Y2O3 additives and this difference was attributed to the much higher grain aspect ratio for the MgO‐RE2O3‐doped ceramics. When the crack growth data were normalized with respect to the total contribution of toughening by bridging determined from the monotonically loaded R‐curves, the differences in fatigue resistance were greatly reduced with the data overlapping considerably. Finally, all of the MgO‐RE2O3‐doped silicon nitrides displayed similar steady‐state fatigue crack growth behavior suggesting that they are relatively insensitive to the intergranular film.

下部地殻・最上部マントルにおける間隙流体フィルム存在の証拠 : 捕獲岩の結晶表面微細構造と高分解能CT観察から

下部地殻・最上部マントルにおける間隙流体フィルム存在の証拠 : 捕獲岩の結晶表面微細構造と高分解能CT観察から

Effects of sintering aids on the densification of Mo–Si–B alloys

The effects of seven sintering aids (0.5 at.% Ni, Co, Fe, Cr, Zr, Nb, and Pd) on the densification of Mo–Si–B alloys of six different compositions (Mo, Mo–0.2Si, Mo–0.2Si–0.02B, Mo–2.5Si–2.5B, Mo–7Si–5B, and Mo–8.9Si–7.7B at.%) are systematically investigated. It was found that Ni, Co, and Fe are effective in enhancing densification of Mo–Si–B alloys, and Ni is the most effective sintering aid. This study supports a previously proposed hypothesis that activated sintering results from enhanced mass transport in the sintering-aid-induced quasi-liquid intergranular films (a type of grain boundary complexion). The relative effectiveness of these sintering aids can be rationalized by analyzing several key thermodynamic parameters that control the stability of premelting-like grain boundary complexions. Future studies are needed to develop interfacial thermodynamic models and methods for computing “grain boundary complexion (phase) diagrams” for multicomponent systems, which can be a useful component for the “Materials Genome” project that will enable better predictions of the activated sintering and other materials phenomena.

SiAlON Bulk Glasses and Their Role in Silicon Nitride Grain Boundaries: Composition-Structure-Property Relationships

SiAlON glasses are silicates or alumino-silicates, containing Mg, Ca, Y or rare earth (RE) ions as modifiers, in which nitrogen atoms substitute for oxygen atoms in the glass network. These glasses are found as intergranular films and at triple point junctions in silicon nitride ceramics and these grain boundary phases affect their fracture behaviour. This paper provides an overview of the preparation of M-SiAlON glasses and outlines the effects of composition on properties. As nitrogen substitutes for oxygen in SiAlON glasses, increases are observed in glass transition temperatures, viscosities, elastic moduli and microhardness. These property changes are compared with known effects of grain boundary glass chemistry in silicon nitride ceramics. Oxide sintering additives provide conditions for liquid phase sintering, reacting with surface silica on the Si₃N₄ particles and some of the nitride to form SiAlON liquid phases which on cooling remain as intergranular glasses. Thermal expansion mismatch between the grain boundary glass and the silicon nitride causes residual stresses in the material which can be determined from bulk SiAlON glass properties. The tensile residual stresses in the glass phase increase with increasing Y:Al ratio and this correlates with increasing fracture toughness as a result of easier debonding at the glass/β-Si₃N₄ interface.

Structure and lithium ion diffusion in lithium silicate glasses and at their interfaces with lithium lanthanum titanate crystals

Solid state lithium ion electrolytes are important to the development of next generation safer and high power density lithium ion batteries. Lithium containing glasses such as lithium silicate glasses have been widely studied due to their high ionic conductivity. Recently, lithium silicate glasses were introduced in polycrystalline lithium lanthanum titanate (LLT) ceramics as intergranular thin films between the crystalline grains to achieve higher lithium ion conductivities in these solid state electrolytes. In this work, we present investigations of the structure and diffusion behavior of lithium silicate glasses and their interfaces with LLT crystals using molecular dynamics simulations. The short and medium range structures of the lithium silicate glasses were characterized and the ceramic/glass interface models were obtained using MD simulations. Lithium ion diffusion behaviors in the glass and across the glass/ceramic interfaces, as well as the effect of atomic structure on diffusion behaviors, were investigated. It was found that there existed a minor segregation of lithium ions at the glass/crystal interface. The interface lithium ion diffusion energy barrier was found to be dominated by the glass phase.

Identification of light elements in silicon nitride by aberration-corrected scanning transmission electron microscopy

In silicon nitride structural ceramics, the overall mechanical and thermal properties are controlled by the atomic and electronic structures at the interface between the ceramic grains and the amorphous intergranular films (IGFs) formed by various sintering additives. In the last ten years the atomic arrangements of heavy elements (rare-earths) at the Si(3)N(4)/IGF interfaces have been resolved. However, the atomic position of light elements, without which it is not possible to obtain a complete description of the interfaces, has been lacking. This review article details the authors' efforts to identify the atomic arrangement of light elements such as nitrogen and oxygen at the Si(3)N(4)/SiO(2) interface and in bulk Si(3)N(4) using aberration-corrected scanning transmission electron microscopy.

Influence of additions of Zr, Ti–B, Sr, and Si as well as of mold temperature on the hot-tearing susceptibility of an experimental Al–2% Cu–1% Si alloy

This study was undertaken to investigate the effects of chemical composition and mold temperature (MT) on the hot-tearing susceptibility (HTS) of an experimental Al–2% Cu–1% Si alloy using a constrained rod casting mold. The HTS results were then compared with 206 (Al–5 wt% Cu) alloys containing the same additions. In general, the Al–2% Cu–1% Si based alloys exhibited higher resistance to hot-tearing than did the 206-based alloys. It was found that an elevated MT is beneficial in reducing the HTS of the Al–2% Cu–1% Si and 206 alloys in that the HTS value decreased from over 21 to less than 5, as the MT was increased from 250 to 450 °C. Increasing the Si content reduced the HTS of the Al–2% Cu–1% Si alloy considerably; this reduction may be attributed to an increase in the volume fraction of eutectic in the structure. The addition of Sr caused deterioration in the hot-tearing resistance of the base alloy due to the formation of Sr-oxides and an extension of the freezing range of the alloy. The refinement of the grain structure obtained with the Zr–Ti–B addition decreased the severity of hot-tearing as a result of an increase in the number of intergranular liquid films per unit volume and a delay in reaching the coherency point. It was also observed that α-Fe intermetallic particles may impede the propagation of hot-tearing cracks. The Al–2% Cu–1% Si alloy with 1 wt% Si addition was judged to be the best composition in view of its low HTS.

Electrical Conductivity of Rocks and Dominant Charge Carriers: The Paradox of Thermally Activated Positive Holes

In this paper we have focused on fundamental processes that are important for understanding the electrical properties of materials, both single crystal minerals and igneous rocks, both laboratory-grown and from natural environments. The prevailing view in the geophysics community is that the electrical conductivity structure of the Earth’s continental crust over the 5-35 km depth range can best be understood by assuming the presence of intergranular fluids and/or intergranular carbon films. Based on studies of melt-grown MgO, magma-derived sanidine and anorthosite feldspar and upper mantle olivine single crystal we present evidence for the presence of electronic charge carriers, the importance of which has been largely ignored. These charge carriers derive from peroxy defects, which are introduced during cooling, under non-equilibrium conditions, through a redox conversion of pairs of solute OH- arising from the solid state dissolution of H2O. It can be shown that, during reheating, the peroxy defects become thermally activated in a 2-step process. Step 2 leads to the release of defect electrons in the oxygen anion sub lattice. Known as positive holes and symbolized by h•, these electronic charge carriers are associated with energy states at the upper edge of the valence band. They are highly mobile. Chemically equivalent to O– in a matrix of O2– they are highly oxidizing. However, though metastable, the h• can exist in minerals, which crystallized in highly reduced environments. The h• appear to control the electrical conductivity of crustal rocks over much of the 5-35 km depth range. We make the extraordinary and seemingly paradoxial claim that MgO crystals, grown from the melt under the viciously reducing conditions of a carbon arc fusion furnace, contain peroxy defects in their crystal structure, hence oxygen in the valence state 1–. When the peroxy defects break up, they release positive hole charge carriers, formally defect electron in the oxygen anion sublattice, equivalent to O– in a matrix of O2–.These positive holes have two outstanding properties: they are highly mobile and highly oxidizing.