Quasi-crystalline alloys based on the Al-Cu-Fe system have attracted considerable attention due to their unique physical and chemical properties, in particular, high corrosion resistance. However, thermal stability and oxidation resistance at elevated temperatures remain the subject of research aimed at improving operational characteristics. The introduction of additional alloying elements can significantly affect the oxidation process, changing the kinetics, morphology and composition of the resulting oxide layers.
Studies show that at high temperatures, a protective layer of aluminum oxide (Al2O3) is formed on the surface of the Al63Cu25Fe12 alloy, slowing down further oxidation. However, the presence of copper and iron can lead to the formation of less stable oxides, affecting the overall stability of the coating. Alloying with elements such as Cr, Ti, or Zr can contribute to the formation of denser and more adhesively strong oxide layers enriched with the corresponding elements. These elements, having a high affinity for oxygen, are selectively oxidized, concentrating at the grain boundaries and preventing the diffusion of oxygen into the material.
Experimental data indicate that the addition of chromium increases the scale resistance of the Al63Cu25Fe12 alloy by forming a complex oxide layer containing Cr2O3. Titanium and zirconium also demonstrate a positive effect by promoting the formation of a finer-grained structure of oxides and improving their protective properties. The study of the oxidation mechanisms of modified alloys using modern surface analysis methods such as X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) allows us to obtain detailed information on the composition and morphology of oxide layers, as well as to identify the role of alloying elements in the oxidation process.
The high-temperature oxidation of (Al63Cu25Fe12)100−xCex (where x takes the values 1, 2, 3, 4) and Al62Cu24Fe12Zn2 samples was studied. It was found that the Al62Cu24Fe12Zn2 alloy, which has a quasi-crystalline structure, demonstrates increased oxidation resistance at 773 K compared to zinc-free Al63Cu25Fe12. In contrast, the structure of the (Al63Cu25Fe12)100−xCex alloys includes both quasi-crystalline and crystalline phases. It was found that the mass gain during oxidation in air at temperatures of 773 and 1073 K increases proportionally to the increase in the cerium concentration. The data obtained indicate that the introduction of zinc slows down the oxidation process, while the addition of cerium promotes it. Ultimately, the alloy powders containing both cerium and zinc are oxidized to FeAl2O4. The surface area of the zinc-containing powder oxidized at 773 and 1073 K is smaller than that of the ternary alloy oxidized under the same conditions. In contrast, the surface area of the cerium-containing powders oxidized at 1073 K increases with increasing cerium concentration.
In the context of the studies aimed at the application of quasicrystals, the reaction of steam reforming of methanol was studied using a catalyst obtained by leaching of the Al63Cu25Fe12 alloy. An important factor for increasing the catalytic activity is the control over the surface area and topology of the alloy powders by various pretreatment methods. Oxidation in air is one of the simplest ways to increase the surface area of some alloys. It is assumed that aluminum-based quasicrystals have high oxidation resistance due to the formation of a passivating aluminum oxide layer on the surface. However, this protective layer may hinder the production of powders with a developed surface area, which are necessary for use as catalyst precursors. Studies of high-temperature oxidation of aluminum-based quasicrystals showed that the icosahedral phase has a higher oxidation resistance than the crystalline phases. The surface oxide layer on a bulk sample of Al63Cu25Fe12 was studied in detail. It is shown that bulk Al–Cu–Fe quasicrystals exhibit different oxidation behavior than powders, and that the oxidation process is significantly influenced by the copper content.
Author: Michiaki Yamasaki, An Pang Tsai
Institute: National Institute of Materials Science, 1-2-1, Sengen, Tsukuba 305-0047, Japan