1 - A brittle material toughened: tungsten-fibre-reinforced tungsten

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A brittle material toughened: tungsten-fibre-reinforced tungsten

28 May 2013

Tungsten has a very high melting point but a disadvantage is its brittleness, which under stress makes it fragile and prone to damage.

Microscopic cross-section of the new tungsten compound showing the circular structures of hair-fine tungsten wires, tightly bunched and embedded in the tungsten matrix (photo: IPP, Johann Riesch)

Tungsten is particularly suitable as material for the highly stressed parts of a vessel enclosing a hot fusion plasma, it being the metal with the highest melting point. A disadvantage, however, is its brittleness, which under stress makes it fragile and prone to damage.

A novel, more resilient compound material has now been developed by Max Planck Institute for Plasma Physics (IPP) at Garching. It consists of homogeneous tungsten with coated tungsten wires embedded. A feasibility study has just shown the basic suitability of the new compound.

In its study, IPP looked for structures capable of distributing local tension. Fibre-reinforced ceramics served as models; for example, brittle silicon carbide is made five times as tough when reinforced with silicon carbide fibres. After a few preliminary studies IPP scientist Johann Riesch investigated whether a similar treatment would work with tungsten metal.

The first step was to produce the new material. A tungsten matrix had to be reinforced with coated long fibres consisting of extruded tungsten wire thin as hair. The wires, originally intended as luminous filaments for light bulbs, where supplied by Osram GmbH.

Various materials for coating them were investigated at IPP, including erbium oxide. The completely coated tungsten fibres were then bunched together, either parallel or braided. To fill out the gaps between the wires with tungsten Johann Riesch and his co-workers then developed a new process in conjunction with British partner, Archer Technicoat.

Whereas tungsten workpieces are usually pressed together from metal powder at high temperature and pressure, a more gentle method of producing the compound was found. The tungsten is deposited on the wires from a gaseous mixture by applying a chemical process at moderate temperatures.

The tungsten-fibre-reinforced tungsten was successfully produced via this process and, moreover, the desired result was achieved: the fracture toughness of the new compound had tripled in relation to fibre-less tungsten after the first tests.

The research team discovered that the fibres bridged cracks in the matrix and distributed the locally acting energy in the material. The interfaces between fibres and the tungsten matrix  have to be weak enough to give way when cracks form and be strong enough to transmit the force between the fibres and matrix. In bending tests this could be observed directly by means of X-ray microtomography.

The team then investigated whether the enhanced toughness could be maintained in duty. Samples embrittled by prior thermal treatment were investigated and confirmed once more that, if the matrix fails when stressed, the fibres are able to bridge any cracks and stem them.

Samples are now to be produced under improved process conditions and with optimised interfaces - a prerequisite for large-scale production.

The researchers believe that the new material might also be of interest beyond the field of fusion research.

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