Sintered materials studied by small-angle neutron scattering
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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