Vikram Jayaram

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.

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.

Pressureless infiltration of Al-Mg based alloys into Al2O3 preforms: Mechanisms and phenomenology

Al-Mg based alloys have been infiltrated into Al 2 O 3 preforms by pressureless infiltration in nitrogen atmosphere at and above 800°C. The study revealed the twin role of Mg in initiation and in continuation of infiltration. Mg reduces the alumina layer on the Al-alloy surface thereby bringing about the contact between the melt and the preform, which marks the initiation of infiltration. It has been determined that the premature termination of infiltration is related to localised depletion at the infiltration front of Mg, which getters the residual oxygen in the nitrogen thereby making the front free of passivating alumina or spinel. Evaporation of Mg from the front and reaction of Mg with the reinforcement are the principal causes for depletion. Two methods have been devised to avoid/delay termination thereby making it possible to fabricate useful section thicknesses with minimal Mg in the alloy.

Contact damage in TiN coatings on steel

Damage mechanisms beneath Vickers indentations are examined on 5\mu m TiN film deposited on stainless steel substrate as a function of load. Prominent mode of cracking includes surface edge cracks and subsurface inclined cracks. No interfacial delaminations were noted at the TiN/steel interface. The tangential traction, radial stress and shear stress distribution around an axisymmetric indentation field are used to assess the driving force for crack propagation.

Characterization of Al2O3ZrO2 powders produced by electrohydrodynamic atomization

Abstract The roles of supercooling and alloy composition in the microstructure evolution of rapidly solidified powders have been systematically examined across the Al2O3ZrO2 binary system. Powders ranging in diameter from 10 nm to over 100 μm were produced by electrohydrodynamic atomization of Al2O3ZrO2 rods prepared by colloidal techniques. At the largest supercoolings (powders less than 1 μm), single-phase homogeneous structures, both amorphous and crystalline, may be observed across the entire phase diagram. With increasing particle size, segregation gives rise to duplex microstructures which consist of metastable alumina and monoclinic or tetragonal zirconia, or of tetragonal zirconia and glass. Coarse (greater than 10 μm) powders exhibit different morphologies in the primary phase and the eutectic constituent, with varying supercoolings and cooling rates. The trends in phase selection and segregation are examined with the aid of available information on Tg and schematic T0 curves for the different phases.

A thermogravimetric study of the oxidative growth of Al 2 O 3 /Al alloy composites

The oxidation of liquid Al-Mg-Si alloys at 900-1400 °C was studied by thermogravimetric analysis (TGA). The development of a semi-protective surface layer of MgO/MgAl 2 O 4 allows the continuous formation of an Al 2 O 3 -matrix composite containing an interpenetrating network of metal microchannels at 1000-1350 °C. An initial incubation period precedes bulk oxidation, wherein Al 2 O 3 grows from a near-surface alloy layer by reaction of oxygen supplied by the dissolution of the surface oxides and Al supplied from a bulk alloy reservoir through the microchannel network. The typical oxidation rate during bulk growth displays an initial acceleration followed by a parabolic deceleration in a regime apparently limited by Al transport to the near-surface layer. Both regimes may be influenced by the Si content in this layer, which rises due to preferential Al and Mg oxidation. The growth rates increase with temperature to a maximum at ~1300 °C, with a nominal activation energy of 270 kJ/mole for an Al-2.85 wt. % Mg-5.4 wt. % Si alloy in O 2 at furnace temperatures of 1000-1300 °C. An oscillatory rate regime observed at 1000-1075 °C resulted in a banded structure of varying Al 2 O 3 -to-metal volume fraction.

Nature of contact deformation of TiN films on steel

Nanoindentation experiments were carried out on a columnar ∼1.5-m-thick TiN film on steel using a conical indenter with a 5-m tip radius. Microstructural examination of the contact zone indicates that after initial elastic deformation, the deformation mechanism of the TiN is dominated by shear fracture at inter-columnar grain boundaries of the TiN film. A simple model is proposed whereby the applied load is partitioned between a deforming TiN annulus and a central expanding cavit yi n the steel substrate. It is possible to obtain a good fit to the experimental load–displacement curves with only one adjustable parameter, namely the inter-columnar shear fracture stress of the TiN film. The implication of results in the context of the performance of TiN films in service is also discussed.

Soft chemical routes to the synthesis of extended solid solutions of wurtzite ZnO–MO (M=Mg,Co,Ni)

Spray pyrolysis and gel decomposition have been used to generate extended solid solutions of MO (M=Mg,Co and Ni) in the wurtzite modification of ZnO through kinetic limitations. Both routes yield a significantly greater extent of solution of CoO as compared to MgO and NiO. Published lattice stabilities for the wurtzite modifications of MO and the rock-salt form of ZnO are used to develop free energy–composition curves to relate the ease of solution to the driving force for partitioning. Reasons for the absence of single-phase rock-salt-based solid solutions are discussed.

Non-equilibrium phase synthesis in Al2O3–Y2O3 by spray pyrolysis of nitrate precursors

Abstract The phase evolution in the Al 2 O 3 –Y 2 O 3 system has been studied for 4, 10, 15 and 37.5% Y 2 O 3 amorphous powders prepared by spray pyrolysis of nitrate precursor solutions. Two distinct metastable transformation sequences were identified for the amorphous powders upon heat treatment. Crystallisation accompanied by partitioning leads to a mixture of hexagonal YAlO 3 and γ-Al 2 O 3 (spinel structure). Partitionless crystallisation, on the other hand, leads directly to a γ-Al 2 O 3 solid solution in dilute alloys, and garnet at temperatures as low as 800°C in the stoichiometric composition provided segregation is avoided during decomposition. Further heat treatment of a yttria-supersaturated γ-Al 2 O 3 leads to the precipitation of the orthorhombic YAlO 3 that is stable up to temperatures as high as 1600°C, while hexagonal YAlO 3 converts to garnet, Y 3 Al 5 O 12 . A rationalisation of the phase evolution sequence has been attempted on the basis of kinetic considerations, cation coordination and semi-quantitative free energy–composition curves for the various competing phases.

Pressure Consolidation of Amorphous ZrO2-Al2O3 by Plastic Deformation of Powder Particles

Amorphous