Non-stoichiometry in monoclinic zirconia and amorphous zirconia

A combination of materials modelling techniques and targeted experimental investigations have identified the manner in which non-stoichiometry is accommodated within both crystalline and amorphous ZrO2. Not only is excess oxygen possible in both crystalline and amorphous ZrO2, but it is found that there is a high propensity for significant deviations – especially in the amorphous system – forming ZrO2+x. This has clear implications to the behavior and degradation of ZrO2 as a thermal barrier coating in aerospace and energy components, but also as the boundary oxide protecting zirconium alloys in aggressive environments, including within a water cooled nuclear power reactor. The behavior was highlighted through a combination of both Raman spectroscopy and associated atomic scale predictions coupled with thermodynamic analysis of the system. As excess oxygen cannot readily oxidize Zr4+ ions beyond this charge state, the additional oxygen is accommodated instead as a peroxide ion – O22-. This peroxide specie has a distinct covalent bond not expected in the stoichiometric ionic ZrO2 system that is readily observable using Raman spectroscopy. Now that excess oxygen accommodation in ZrO2 has been highlighted, an understanding of how various dopant or alloying elements can impact its behavior can be targeted to improve component reliability. The presence of amorphous phases at grain boundaries is also discussed in terms of potential super-highways for oxygen transport through the oxide system

In-situ TEM investigation of nano-scale helium bubble evolution in tantalum-doped tungsten at 800 °C

The aim of this work is to probe the helium-induced defect production and accumulation in 40 keV He+ irradiated polycrystalline W and its alternative alloy W-5wt.%Ta using transmission electron microscopy (TEM) combined with in-situ helium irradiation at 800°С. A maximum damage level of 1 dpa with a maximum He-to-dpa ratio of 5.5 at%/dpa has been reached in this work for both materials, which corresponds to an ion fluence of 7.33 × 1016 He+/cm2. The presence of radiation-induced dislocation loops was not observed at this temperature. The low density of the incipient bubbles in W has been already detected at 0.004 dpa, which corresponds to a fluence of 3.3 × 1014He+/cm2. The experiments conducted at 800°C have shown that the addition of 5wt.% of tantalum into tungsten may diminish the binding of He ions with vacancies into complexes, which serve as the core of the bubble, thus hindering helium bubble formation below 0.02 dpa and their further growth and population at higher damage levels. By exceeding the damage dose ≥0.3 dpa, a progressive transition from a spherical to a faceted shape of the bubbles has been observed in W but not in the W-5Ta alloy. At 1 dpa, &gt;80% of the bubbles in W were of the faceted type with the facet planes of {110}.</p

Stoichiometry deviation in amorphous zirconium dioxide

Amorphous zirconia (a-ZrO(2)) has been simulated using a synergistic combination of state-of-the-art methods: employing reverse Monte-Carlo, molecular dynamics and density functional theory together. This combination has enabled the complex chemistry of the amorphous system to be efficiently investigated. Notably, the a-ZrO(2) system was observed to accommodate excess oxygen readily – through the formation of neutral peroxide (O(2)(2−)) defects – a result that has implications not only in the a-ZrO(2) system, but also in other systems employing network formers, intermediates and modifiers. The structure of the a-ZrO(2) system was also determined to have edge-sharing characteristics similar to structures reported in the amorphous TeO(2) system and other chalcogenide-containing glasses

Void evolution in tungsten and tungsten-5wt.% tantalum under in-situ proton irradiation at 800 and 1000 °C

We have probed void evolution in polycrystalline W and W-5wt.%Ta material at 800 and 1000 °C, by transmission electron microscopy during in-situ irradiation with a 40 keV proton beam. The presence of radiation-induced dislocation loops was not observed prior to void formation at those elevated temperatures. The damaged W microstructure was characterised by the presence of a population of randomly distributed voids, whose number density reduces when the irradiation temperature increases. Soft impingement of voids becomes noticeable at damage levels ≥0.2 dpa. In contrast, the excess of free vacancies in the W-5wt.%Ta material irradiated at 800 °C only leads to the formation of visible voids in this TEM study (≥2 nm) after post-irradiation annealing of the sample at 1000 °C. Solute Ta atoms also cause a significant increase in the number density of voids when comparing the microstructure of both materials irradiated at 1000 °C, and a gradual progression towards saturation in average void size at ≥0.2 dpa. Moreover, we have detected a progressive transition from a spherical to a faceted shape in a number of voids present in both materials at damage levels ≥0.3 dpa.</p

Monolayer-thick TiO precipitation in V-4Cr-4Ti alloy induced by proton irradiation

We have characterised to atomic resolution the mono-layer thick TiO-type precipitate induced by proton irradiation in V-4Cr-4Ti alloy at a dose of 0.3 dpa and a temperature of 350 °C. Its formation coincides with the coarsening radiation-induced interstitial a/2〈111〉 dislocation loops that are already present at 300 °C. The dislocation network induced by prior cold work is mostly recovered at 300 °C and 0.3 dpa, and is therefore expected to exert a minimal effect on the precipitate formation. This monolayer-thick precipitate constitutes an early stage in the radiation-induced aging process of V-4Cr-4Ti at low temperatures, and can potentially absorb additional light elements in reactor environments.</p