Carlos G. Levi

Phase equilibria and solidification in Ti-Al alloys

High temperature phase equilibria and microstructure evolution during solidification were investigated for Ti-Al alloys in the range 40–55 at.%Al. In situ high temperature X-ray diffraction was used to study the phases present at elevated temperatures. It was found that a hexagonal close-packed α-phase exists close to the melting point for alloys containing 46–50 at.%Al, while leaner alloys (<44%Al) are in a cubic β-phase field at similar temperatures. Examination of dendritic morphologies in arc-button shrinkage cavities revealed the crystallography of the primary solidification phase. These observations were coupled to TEM analysis of the final microstructures to deduce phase sequencing during solidification and solid-state transformations. For alloys in the range 40–49 at. %Al, β was found to be the primary phase in local equilibrium with the liquid, and from 49 to 55 at.%Al, the primary phase was a. α. γ-segregate was first observed at 46 at.%Al and its fraction increased with Al content. Both β and α dendrites transformed in the solid-state to a lath structure consisting of layers of α2 and γ. The γ-segregate did not transform further. A revised phase diagram is proposed, for the composition range studied, incorporating two peritectics L + β → α, and L + α → γ, together with a high temperature α-phase field.

Hydrothermal synthesis of KNbO 3 and NaNbO 3 powders

Orthorhombic KNbO 3 and NaNbO 3 powders were hydrothermally synthesized in KOH and NaOH solutions (6.7-15 M) at 150 and 200 °C. An intermediate hexaniobate species formed first before eventually converting to the perovskite phase. For synthesis in KOH solutions, the stability of the intermediate hexaniobate ion increased with decreasing KOH concentrations and temperatures. This led to significant variations in the induction periods and accounted for the large disparity in the mass of recovered powder for different processing parameters. It is also believed that protons were incorporated in the lattice of the as-synthesized KNbO 3 powders as water molecules and hydroxyl ions.

Microstructure and texture of EB-PVD TBCs grown under different rotation modes

Abstract The role of vapor incidence pattern (VIP) on the microstructure and texture of thermal barrier coatings (TBCs) produced by electron-beam physical vapor deposition (EB-PVD) is examined. Two distinct VIPs are induced by proper design of the substrate rotation mode. One is the sunrise–sunset pattern typical of conventional deposition on the curved face of a rotating cylinder (mode A), and the other is a conical pattern resulting when deposition is done on the cylinder base at an offset distance from the plume axis (mode P). These geometries afford fundamental insight on the processes of microstructure and texture evolution and also have practical implications to the variability of properties that may be expected between the body and platform regions of a turbine airfoil. Comparable deposition rates and thickness uniformity can be achieved by proper selection of the experimental geometry. Both coatings exhibit the typical 〈100〉 texture normal to the substrate, but mode A also yields a preferred in-plane orientation which is absent in mode P. The ensuing differences in column packing and tip shadowing yield lower densities and larger pipe-like inter-columnar voids in mode P. The absence of an in-plane evolutionary selection mechanism in the latter also leads to narrower columns than in mode A.

Microstructure evolution in tial alloys with b additions: Conventional solidification

Solidification microstructures of arc-melted, near-equiatomic TiAl alloys containing boron additions are analyzed and compared with those of binary Ti-Al and Ti-B alloys processed in a similar fashion. With the exception of the boride phase, the matrix of the ternary alloy consists of the same α2 (DO19) and γ (Ll0) intermetallic phases found in the binary Ti-50 at. pct Al alloy. On the other hand, the boride phase, which is TiB (B27) in the binary Ti-B alloys, changes to TiB2 (C32) with the addition of Al. The solidification path of the ternary alloys starts with the formation of primary α (A3) for an alloy lean in boron (∼1 at. pct) and with primary TiB2 for a higher boron concentration (∼5 at. pct). In both cases, the system follows the liquidus surface down to a monovariant line, where both α and TiB2 are solidified concurrently. In the final stage, the α phase gives way to γ, presumably by a peritectic-type reaction similar to the one in the binary Ti-Al system. Upon cooling, the α dendrites order to α2 and later decompose to a lath structure consisting of alternating layers of γ and α2.

The structure of δ-alumina evolved from the melt and the γ → δ transformation

Abstract Electron diffraction patterns are used to analyze the structure of a metastable alumina (δ) found in powders 2 O 3 . The δ phase is believed to form directly from the liquid and also by solid state transformation of the cubic spinel γ. Both phases are based on an f.c.c. packing of anions with the interstitial cations having a higher degree of order in the δ phase. The γ → δ transformation begins with the ordering of tetrahedral cations in domains ~ 1–2 nm in size. The intensity distribution in reciprocal space indicates that the transformation is continuous from the spots of the disordered spinel, through diffuse scattering and finally the appearance of discrete superlattice reflections of the δ-phase. The δ is orthorhombic with crystal axes parallel to those of the parent spinel and unit cell dimensions that are simple multiples of the lattice parameter of γ. Two variants of the structure were observed with lattice parameters a δ = 2 a γ , b δ = 1.5 a γ , c δ = either a γ or 2 a γ . The allowed reflections indicate a space group of P2 1 2 1 2 1 for the structure with c δ = a γ , which is the more commonly observed.

Mechanical Properties of Porous-Matrix Ceramic Composites

The use of porous matrices to enable damage tolerance in ceramic composites has emerged as a new paradigm in high performance materials. This paradigm obviates the need for fiber coatings for the purpose of crack deflection, thereby providing opportunities for lower cost manufacturing relative to that of conventional coated-fiber systems. Furthermore, upon selection of all-oxide constituents, the prospect for meeting the long-term durability requirements of high-temperature components for future gas turbine engine technologies becomes realizable. This article reviews the mechanical properties of this class of all-oxide composite, The properties of interest include the in-plane strength and notch-sensitivity subject to both fiber- and matrix-dominated loadings, and interlaminar strength. Special emphasis is placed on the role of the porous matrix in each of these properties. Finally, the issue of the stability of the matrix microstructure following prolonged exposure at elevated temperature is addressed and its role in maintaining desirable mechanical properties is demonstrated.

Nucleation and growth of Al2O3/metal composites by oxidation of aluminum alloys

The nucleation and growth mechanisms during high temperature oxidation of liquid Al-3% Mg and Al-3% Mg-3% Si alloys were studied with the aim of enhancing our understanding of a new composite fabrication process. The typical oxidation sequence consists of an initial event of rapid but brief oxidation, followed by an incubation period of limited oxide growth after which bulk Al2O3/Al composite forms. A duplex oxide layer, MgO (upper) and MgAl2O4 (lower), forms on the alloy surface during initial oxidation and incubation. The spinel layer remains next to the liquid alloy during bulk oxide growth and is the eventual repository for most of the magnesium in the original alloy. Metal microchannels developed during incubation continuously supply alloy through the composite to the reaction interface. During the growth process, a layered structure exists at the upper extremity of the composite, consisting of MgO at the top surface, MgAl2O4 (probably discontinuous), Al alloy, and finally the bulk Al2O3 composite containing microchannels of the alloy. The bulk oxide growth mechanism appears to involve continuous formation and dissolution of the Mg-rich oxides at the surface, diffusion of oxygen through the underlying liquid metal, and epitaxial growth of Al2O3 on the existing composite body. The roles of Mg and Si in the composite growth process are discussed.

Evolution of boride morphologies in TiAl-B alloys

The solidification of γ-TiAl alloys with relatively low (<2 at. pct) additions of boron is discussed. Binary Ti-Al alloys containing 49 to 52 at. pct Al form primary α-(Ti) dendrites from the melt, which are subsequently surrounded by γ segregate as the system goes through the peritectic reactionL + α →γ. Alloys between 45 and 49 at. pct Al go through a double peritectic cascade, forming primary β-(Ti) surrounded by α-(Ti) and eventually by γ in the interdendritic spaces. Boron additions to these binary alloys do not change the basic solidifi-cation sequence of the matrix but introduce the refractory compound TiB2 in a variety of mor-phologies. The boride develops as highly convoluted flakes in the leaner alloys, but needles, plates, and equiaxed particles gradually appear as the B content increases above ∼1 at. pct. Increasing the solidification rate initially promotes the formation of flakes over plates/needles and ultimately gives way to very fine equiaxed TiB2 particles in the interdendritic spaces of the metallic matrix. Furthermore, the primary phase selection in the 49 to 52 at. pct Al range changes from α-(Ti) to β-(Ti) at supercoolings of the order of 200 K. The different boride morphologies are fully characterized, and their evolution is rationalized in terms of differences in their nucleation and growth behavior and their relationship to the solidification of the inter-metallic matrix.

The high temperature α field in the titanium-aluminum phase diagram

Il est etabli de facon definitive que la phase en equilibre avec un alliage liquide Ti-50% Al est la solution solide hexagonale compacte basee sur la structure du Tiα, plutot que sur une forme cubique centree suggeree par le diagramme de phases. Revision de ce dernier pour y inclure un champ α haute temperature et une seconde reaction peritectique L+α→γ

Low temperature/low pressure hydrothermal synthesis of barium titanate: Powder and heteroepitaxial thin films

Barium titanate powder and heteroepitaxial thin films were successfully produced by hydrothermal routes at ambient pressure and temperatures less than 100 °C. This processing method provides a simple low temperature route for producing epitaxied barium titanate thin films on single-crystal SrTiO 3 substrates and powders which could also be extended to other systems. A dissolution/reprecipitation growth mechanism also was proposed for the formation of barium titanate by this route using previously published aqueous stability diagrams. Repeated hydrothermal treatments improved film thickness and surface coverage at the expense of increased surface roughness.