MAX phase materials and self-passivating W-based alloys are potential materials for hightemperature applications, especially for renewable energy technologies where the efficiency is limited by the highest operating temperature of the materials used. An important criterion for the use of such materials at high-temperatures is their resistance to oxidation at these temperatures and environmental conditions, through the formation of a slow-growing, passivating oxide layer. In the present work, the focus is on two such materials: (a) the alumina-forming MAX phase chromium aluminium carbide (Cr2AlC), which exhibits great potential for use in concentrated solar power (CSP) stations, and (b) the chromia-forming tungsten-based alloy W-11.4Cr-0.6Y, which has been developed as a potential material for use in the plasma-facing first wall of future fusion reactors, and shows increased oxidation resistance due to the addition of yttrium (Y), a well-known reactive element. In order to develop these materials further and eventually enable their use in hightemperature applications for energy production, the oxidation behaviour and microstructure of the oxide scale under the expected operating conditions, i.e., at temperatures ≥ 1000 °C under a humid atmosphere, were studied. This involved the use of techniques such as thermogravimetric analysis (TGA) to study oxidation kinetics, as well as microstructural characterisation through X-ray diffraction (XRD), focused ion beam – scanning electron microscopy (FIB-SEM), energy-dispersive X-ray spectroscopy (EDX) and transmission electron microscopy (TEM). Since the formation of a passivating oxide scale is a diffusioncontrolled process, the study of diffusion pathways such as grain boundaries (GBs), as well as interfaces, which determine the strength of oxide scale adherence, can provide crucial information regarding the oxidation mechanism, both with and without the addition of a reactive element. Hence, a major focus of the present work was on using advanced characterisation tools such as atom probe tomography (APT), which can provide threedimensional compositional information at the nanoscale. APT was used to study GBs and interfaces, gain more insight into the transport of cations and anions through the oxide scale and bulk material, and determine the key processes that control the growth and formation of the oxide scale. Since alumina is stable at temperatures greater than 1000 °C, the oxidation behaviour of the MAX phase Cr2AlC was studied at both 1000 °C and 1200 °C. An α-Al2O3 scale is formed at both temperatures, consisting of both inward and outward-growing layers, with a continuous Cr7C3 layer beneath the oxide. The decomposition of Cr2AlC to Cr7C3 is the source of more than 70% of aluminium needed for oxide scale formation, while the underlying MAX phase can only tolerate < 1 at.% depletion of Al. Thus, this MAX phase shows a distinctive oxidation behaviour different from the common high-temperature oxidation of alloys. Al diffusion to the oxide scale proceeds differently through the MAX phase and carbide – it occurs predominantly through GBs in Cr2AlC and through the bulk in Cr7C3. However, the rateii limiting step for oxide scale growth seems to be the ionic diffusion of oxygen through GBs of the large-grained inner alumina layer, resulting in the observed parabolic oxidation kinetics. Importantly, no segregation of Cr, C or any impurities, which could affect ionic transport, was found at these alumina GBs by APT, rendering the results of this work as characteristic of a pure Cr2AlC oxidation, and shows that the addition of reactive elements could potentially further improve the oxidation resistance. The oxidation behaviour of the self-passivating tungsten alloy W-11.4Cr-0.6Y, which is a chromia former, was only studied at 1000 °C, since chromia starts to volatilise at higher temperatures. The main focus of this work was on the effect of Y on oxidation resistance, as well as determining the mechanism by which it acts. The oxidation behaviour of the binary alloy W-11.4Cr and the ternary W-11.4Cr-0.6Y alloy, prepared through identical synthesis routes, were studied. A continuous, passivating chromia layer is only obtained with the addition of Y. Although a porous, complex oxide scale containing mixed oxide layers and WO3 is formed in both cases, the addition of Y results in a less porous oxide scale, which makes it more adherent, and results in a 50-fold reduction in the oxide growth rate, leading to greater oxidation resistance. By performing a two-step oxidation experiment using 18O as a tracer, in combination with APT analysis, it was also shown that Y segregates at chromia GBs and results in the inward growth of oxide, hence providing a possible mechanism for the reactive element effect of Y in chromia-forming W-based alloys.
Anicha Reuban (Wed,) studied this question.