Sintered materials studied by small-angle neutron scattering

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Abstract

Sintered single-phase materials with open or closed porosity are usually described by a two phase model. During sintering, the decrease of pore volume is accompanied with decreasing internal surface area measured by SANS. Such investigation can be extended to sintered WC–Co composites made from nano-crystalline powder having different kinds of internal surfaces. The deformation of remaining porosity after sintering can also be monitored by the 2D anisotropy of SANS distribution as the sintered body is processed through several steps of thermo-mechanical treatment. The macroscopic deformation of doped tungsten wire is followed by the elongation of the potassium gas filled bubbles and the annealing causes spheroidization with vanishing SANS anisotropy. The SANS method establishes a new description of mechanisms of thermomechanical processing.

Introduction

Before sintering, powder particles or agglomerates of a single-phase solid material (metal, oxide) are surrounded by gas or vacuum. After the sintering, the sintered body can be close to the theoretical density, a lot of remaining pores are, however, always present in the matrix. This residual porosity consists of gas-filled bubbles or empty pores. From the point of view of small-angle neutron scattering (SANS), both the initial single-phase solid material powder and the sintered body should be regarded as a two-phase system [1] having sharp interfaces between the solid material and vacuum during the whole process of sintering.

The area of total pore surface in samples from different stages of sintering process can be determined by SANS with Porod evaluation. It characterizes the quality of sintering, similar to the total pore volume fraction determined from density measurements. Blaschko et al introduced “sintering trajectory” by plotting the square root of total pore surface area as a function of total pore volume and showed how these more or less straight trajectory lines describe the development of sintering of single-phase solid materials [2].

Technically relevant materials – powder mixtures, composites, structure ceramics – are, however, not single-phase solid materials and therefore they include at least two kinds of interfaces inside the body. This makes the application of SANS method more complicated.

During thermomechanical processing after sintering like rolling, swaging and wire drawing, the shape, size and concentration of the residual porosity change. The definitely oriented character of the applied external macroscopic strain results in a uniform orientation of deformed pores in the body. As a consequence of this, a significant anisotropy in the two-dimensional scattering distribution of SANS intensity can be observed.

Section snippets

Sintering trajectory of WC–CO multiphase material by isotropic sans

The nanopowder WC–Co was manufactured by Nanodyne Inc. using a spray conversion process [3] with 25 vol% Co binder phase. Two kinds of interfaces, WC–Co and Co–vacuum, are present in the material during the sintering process,. The third possible interface, the WC–vacuum, is absent because the surface of WC particles is always fully covered by Co. As the contrast factors for the WC–Co and Co–vacuum interfaces have a ratio of 1.7, neither of them is dominant. This prevents a trivial separation

SANS anisotropy of elongated bubbles in doped tungsten

The investigated tungsten alloy is commercially used as incandescent filament in conventional light sources and is produced from doped tungsten powder by sintering. Only small (50–500 nm) K filled bubbles and larger (1–5 μm) empty pores remain in the bulk [4].

The subsequent steps of mechanical working at 1000–1300 K result in more and more elongated bubbles. It is assumed by Moon and Koo [5] that during the mechanical working, the amount of deformation of microscopic bubbles and pores is identical

Analysis of ASANS intensities of doped tungsten

The analysis of scattering maps reveals further details. The circular shape in the central region of samples having a diameter of 1 mm and below suggested that among the majority of very elongated ellipsoids, spheres are also present in the sample and the scattering distribution corresponds to their overall superposition. The reduced anisotropy of 0.4 mm sample after 1650 K annealing shows that the number density of ellipsoids decreases because many ellipsoids undergo breaking up during the

Acknowledgements

The authors thank G. Pepy for the evaluation program and O. Horacsek for helpful discussions. LLB Saclay, France is gratefully acknowledged for using the spectrometers. Special thanks to A. Nagy and Gy. Nagy, GE Lighting Tungsram for the samples. The work was supported by the OTKA, Hungary, contract No. T025747.

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