Showing papers by "Robert F. Singer published in 2002"

The investigation of morphometric parameters of aluminium foams using micro-computed tomography

Abstract: The development of different manufacturing techniques for foams, most especially metallic foams have opened a new market of opportunities for multifunctional cellular metallic materials. However, it is yet unclear how to quantitatively characterise the microstructure and internal architecture of foams precisely. In this paper, the potential of using micro-computed tomography in characterising the morphometric parameters of foams are explored. 3D morphometric parameters of different types of closed cell aluminium alloy foam have been characterised, and the effects of the measurement resolution, and the integration time on the measured morphometric parameters have been studied. The model based and model independent morphometric parameters are comparable with measurements using other characterisation techniques. We conclude that the new generations of micro-computed tomography equipment have versatile capability for a non-invasive and -destructive characterisation of foams.

Structural changes of tungsten heating filaments during CVD of diamond

Abstract: Hot tungsten filaments (2000 °C) are used for the activation of the gas phase during hot filament chemical vapor deposition (CVD) of diamond. In the present investigation, technically pure and doped tungsten filaments were subjected to typical process conditions. The changes in the tungsten filaments microstructure were investigated. During the first 60 s, a rapid transformation of W into W2C occurs due to the high reactivity of the metallic surface. Afterwards, the W2C shell grows inward at a linear rate. The activation energy of this process is 30 kJ mol − 1 , which corresponds to the dissolution of methane on the filament surface which is the rate determining step. Once W has transformed completely into W2C, a WC shell starts to grow from the surface. In doped tungsten a parabolic behavior can be found for the growth of WC which occurs when the diffusion of carbon through the growing shell is the rate-determining step. Additionally, the temperatures of the filament and substrates drop considerably. Both filament and substrate temperature drops are discussed in detail with the help of a model describing the power balance. © 2002 Elsevier Science B.V. All rights reserved.

Modeling of Metal Foaming with Lattice Boltzmann Automata

Abstract: The formation and decay of foams produced by gas bubble expansion in a molten metal is numerically simulated with the Lattice Boltzmann Method (LBM) which belongs to the cellular automaton techniques. The present state of the two dimensional model allows the investigation of the foam evolution process comprising bubble expansion, bubble coalescence, drainage, and eventually foam collapse. Examples demonstrate the influence of the surface tension, viscosity and gravity on the foaming process and the resulting cell structure. In addition, the potential of the LBM to solve problems with complex boundary conditions is illustrated by means of a foam developing within the constraints of a mould as well as a foaming droplet exposed to gravity.

Effect of hafnium on the castability of directionally solidified nickel-base superalloys

Abstract: The effect of Hf additions on the solidification characteristics and castability of the Ni-base superalloy IN792 has been studied. A quenching technique was employed to freeze-in the microstructure during solidification. The crack length in tube-shaped demonstration castings was evaluated. Hf additions were found to reduce the hot-tearing susceptibility significantly. Hf also caused a more gradual decrease of the liquid fraction with temperature in the last stages of solidification, which was observed to improve castability in other alloys previously and which can be correlated with lower strains and strain rates in the vulnerable period where hot tears form. It was also found that the γ/γ′ rosette structure, often referred to as γ/γ′ eutectic in the literature, actually forms by a discontinuous precipitation mechanism from the solid. The amount of γ/γ′ observed in the interdendritic area, therefore, does not reflect the amount of remaining liquid in the last stages of solidification.