Fusion reactions are the power source of the sun and other stars. This could be a new, so far unexplored, virtually inexhaustible primary energy source. Plasma-wall interactions are the challenge which must be faced when building a fusion power plant, like DEMO. Presently, tungsten is the main candidate as plasma-facing armor material. Advantages of tungsten include a high melting point, a high thermal conductivity, low tritium retention, and a low erosion yield. A problem is oxide volatilization under accidental conditions where the temperature of the first wall can reach 1200 K to 1450 K and air ingress occurs. Therefore smart tungsten alloys are developed. Smart alloys are supposed to preserve properties of tungsten during plasma operation coupled with suppressed tungsten oxide formation in case of an accident.
Thin film tungsten-chromium-yttrium (W-Cr-Y) samples prepared by magnetron sputtering are used as model system. The thin films have a thickness of 3 μm to 7 μm. The mechanisms of oxidation and its dynamics are studied using a thermogravimetric system, x-ray diffraction, secondary ion mass spectroscopy, focused ion beam, and electron microscopy. An elemental composition scan of alloying elements was conducted.
The new material composition featuring W, ~12 wt.% Cr, ~0.4 wt.% Y showed suppression of evaporation by developing a stable Cr2O3-layer, least pores, and least oxidation in comparison to those of any other known tungsten-based alloy. At 1273 K in dry 20 kPa O2 – 80 kPa Ar atmosphere an oxidation rate of 3×10-6 mg2 cm-4 s-1 was measured. By scanning transmission electron microscopy a homogeneous distribution of Y in the tungsten matrix was detected. Y concentration below the detection limit in the surface Cr2O3 layer, and Y-enriched spots with a size of 30 nm at the interface between the tungsten matrix and the Cr2O3 are shown.
At 1473 K in dry 20 kPa O2 – 80 kPa Ar atmosphere the ternary W-Cr-Y alloys exhibited parabolic oxidation behavior. Limitations for currently available smart alloys were found in humid air at 1273 K.
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