On Oxidation Resistance Mechanisms at 1273 K of Tungsten-Based Alloys Containing Chromium and Yttria

Abstract

Tungsten (W) is currently deemed the main candidate for the plasma-facing armor material of the first wall of future fusion reactors, such as DEMO. Advantages of W include a high melting point, high thermal conductivity, low tritium retention, and low erosion yield. However, was an accident to occur, air ingress into the vacuum vessel could occur and the temperature of the first wall could reach 1200 K to 1450 K due to nuclear decay heat. In the absence of cooling, the temperature remains in that range for several weeks. At these temperatures, the radioactive tungsten oxidizes and then volatilizes. Smart W alloys are therefore being developed. Smart alloys are supposed to preserve properties of W during plasma operation while suppressing tungsten oxide formation in case of an accident. This study focuses on investigations of thin film smart alloys produced by magnetron sputtering. These alloys provide an idealistic system with a homogeneous distribution of the elements W, chromium (Cr), and yttrium (Y) on an atomic scale. The recommended composition is W with 12 weight % of Cr and 0.5 weight % of Y. Passivation and a suppression of WO 3 sublimation is shown. For the first time, the mechanisms yielding the improved oxidation resistance are analyzed in detail. A protective Cr 2 O 3 layer forms at the surface. The different stages of the oxidation processes up to the failure of the protective function are analyzed for the first time. Using 18 O as a tracer, it is shown for the first time that the oxide growth occurs at the surface of the protective oxide. The Cr is continuously replenished from the bulk of the sample, including the Cr-rich phase which forms during exposure at 1273 K. A homogenous distribution of yttria within the W-matrix, which is preserved during oxidation, is a peculiarity of the analyzed alloy. Further, an Y-enriched nucleation site is found at the interface between metal and oxide. This nucleation sites are deemed to be crucial for the improved oxidation resistance.

Fusion Materials Development at Forschungszentrum Jülich

“…In Figure 8, one of the main achievements is shown, the production of, and oxidation study on field-assisted sintering (FAST) produced, bulk W-Cr-Y samples. [85] In ref. [85], it is stated that to achieve full passive safety it is required that the alloy maintains the protective oxide layer for several weeks to suppress sublimation.…”

“…[85] In ref. [85], it is stated that to achieve full passive safety it is required that the alloy maintains the protective oxide layer for several weeks to suppress sublimation. Thus, studies for passivation of up to 467 h were performed, as shown in Figure 8.…”

“…One of the crucial aspects identified was also the necessity of quantifying not only the oxidation/mass gain but the direct measurement of the sublimiation of the oxides. [85,86] It was shown that WCrY systems can suppress the oxidation rate significantly compared with that of pure W and have a much better performance than previous systems such W-Cr-Ti. [83,[87][88][89][90][91][92][93] In addition to the properties related to oxidation, the relevant studies also include the preparation for reactor deployment.…”

High Temperature Corrosion and Oxidation of Metals

“…There is an article on oxidation resistance of some critical components of the fusion reactors which is a cutting-edge futuristic topic [1]. Tungsten-base alloys are among main candidates for such applications because of their very high melting point, high thermal conductivity, low tritium retention, and low erosion yield.…”

Materials development for new high heat-flux component mock-ups for DEMO

“…An issue not tackled in this contribution is the formation of radioactive and highly volatile W-oxide (WO 3 ) compounds during accidental air ingress. To suppress the release of Woxides W-based self-passivating alloys can be incorporated into the composite approach [18,19,20,21,22,23,24,25,26,27].…”