Nahum Travitzky

Alumina–Ti aluminide interpenetrating composites: microstructure and mechanical properties

Abstract The microstructure and mechanical properties of dense interpenetrating phase Al2O3–TiAl–Ti3Al composites fabricated by pressure-assisted thermal explosion of TiO2–Al powder blend have been studied. The unusual wavy morphology of aluminide–Al2O3 interface matching the lamellar γ–α2 aluminide structure was formed due to big difference between oxygen solubility in γ and α2. Crack deflection by alumina grains and crack bridging by the more ductile intermetallic may control the fracture toughness of these composites.

Fabrication and properties of Al-infiltrated RBAO-based composites

Abstract The reaction-bonded Al 2 O 3 (RBAO) process is applied to fabricate open porosity Al 2 O 3 -based composites with SiC and Al 2 O 3 particulate inclusions. These are then gas-pressure infiltrated with liquid aluminum. The Al-infiltrated composites exhibit strongly improved mechanical properties, e.g. fracture toughness and bond strength of samples containing 30 vol.% 13 μm diameter Al 2 O 3 platelets are enhanced 1·6 to 5·8 MPa√m and from 85 to 760 MPa, respectively. In all cases, crack bridging by ductile Al ligaments is the main toughening mechanism. Filling of void defects, caused by particulate agglomeration, with Al is especially effective in reducing the strength-controlling flaw size.

Microstructure and properties of metal infiltrated RBSN composites

Abstract Reaction-bonded silicon nitride (RBSN)-metal composites were fabricated using gas-pressure infiltration. Various RBSN types have been infiltrated with molten Al, an Al-Si-Mg alloy, a Ti-Al intermetallic, and Si, resulting in considerable increase in mechanical properties when compared to uninfiltrated RBSN. For example, strength was raised to 510 from 227 M Pa when infiltrated with a Ti-39 wt% Al alloy and the toughness to > 5 from 2·7 M Pa√mwhen pure Al was infiltrated. Si infiltration proved to be most effective in enhancing the wear resistance.

Crystallisation Kinetics of a -Spodumene-Based Glass Ceramic

LZSA (Li2O-ZrO2-SiO2-Al2O3) glass ceramic system has shown high potential to obtain LTCC laminate tapes at low sintering temperature (<1000°C) for several applications, such as screen-printed electronic components. Furthermore, LZSA glass ceramics offer interesting mechanical, chemical, and thermal properties, which make LZSA also a potential candidate for fabricating multilayered structures processed by Laminated Objects Manufacturing (LOM) technology. The crystallization kinetics of an LZSA glass ceramic with a composition of 16.9Li2O⋅5.0ZrO2⋅65.1SiO2⋅8.6Al2O3 was investigated using nonisothermal methods by differential thermal analysis and scanning electronic microscopy. Apparent activation energy for crystallization was found to be in the 274–292 kJ⋅mol−1 range, and an Avrami parameter n of 1 was obtained that is compared very favorably with SEM observations.

Chemical stability of cordierite-ZrO2 composites

Abstract Cordierite-ZrO 2 composites containing unstabilized and Y 2 O 3 -stabilized ZrO 2 were sintered at temperatures between 1200 and 1400°C for up to 48 h. In most cases both types of composites reacted to form zircon and spinel when sintered for more than 24 h. Only when Y 2 O 3 -doped ZrO 2 composites were annealed for up to 48 h, hardly any compositional changes took place due to the formation of an Al 2 O 3 -SiO 2 -Y 2 O 3 reaction barrier between ZrO 2 and the matrix.

Influence of sintering conditions on mechanical and thermal properties of cordierite-ZrO2 composites

Abstract Bend strength, fracture toughness and thermal diffusivity of cordierite composites with 30 vol.% of either unstabilized or Y2O3-stabilized ZrO2 sintered at 1400°C have been determined as a function of sintering time. Depending on sintering time, the phase composition varies from cordierite + monoclinic ZrO2 to spinel + zircon. Cordierite with tetragonal ZrO2 sintered for 1 h exhibits a fracture toughness of ∼4·1 M Pa m , while composites consisting essentially of spinel and zircon with both inter- and intragranular monoclinic ZrO2 particles show the highest fracture toughness ( ∼4·7 M Pa m ). These composites are obtained when cordierite is sintered with unstabilized ZrO2 at 1400°C for >4h. They also exhibit the highest thermal diffusivity (48 × 10−3 cm2/s).

Robocasting of carbon-alumina core-shell composites using co-extrusion

Purpose This study aims to achieve the fabrication of three-dimensional core-shell filament-based lattice structures by means of robocasting combined with co-extrusion. For core and shell materials, colloidal gels composed of submicron carbon and alumina powders were developed, respectively. Simultaneously, the co-extrusion process was also studied by numerical simulation to investigate the feed pressure-dependent wall thickness. Design/methodology/approach Significant differences in the rheological behavior of the carbon and alumina gels were observed because of differences of the particle morphology and surface chemistry of the carbon and alumina powders. Precise control over the cross-sectional diameter of the core and shell green state elements was achieved by alteration of the feed pressures used during co-extrusion. Findings After subsequent thermal treatment in an oxidizing atmosphere (e.g. air), in which the carbon core was oxidized and burned out, lattice structures formed of hollow filaments of predetermined wall thickness were manufactured; additionally, C-Al2O3 core-shell filament lattice structures could be derived after firing in an argon atmosphere. Originality/value Green lattice truss structures with carbon core and alumina shell filaments were successfully manufactured by robotically controlled co-extrusion. As feedstocks carbon and alumina gels with significantly different rheological properties were prepared. During co-extrusion, the core paste exhibited a much higher viscosity than the shell paste, which benefited the co-extrusion process. Simultaneously, the core and shell diameters were exactly controlled by core and shell feed pressures and studied by numerical simulation. The experimentally and numerically derived filament wall thickness showed qualitative agreement with each other; with decreasing core pressure during co-extrusion, the wall thickness increased.