Quasicrystalline alloys of the Al-Cu-Fe system, in particular Al63Cu25Fe12, have unique properties such as high hardness, low friction coefficient and corrosion resistance. However, at high temperatures they are susceptible to oxidation, which can degrade their performance characteristics.
The introduction of additional elements such as Cr, Ti or Zr can significantly change the oxidation process. For example, the addition of chromium promotes the formation of a protective oxide film rich in Cr2O3, which prevents further oxidation. Titanium and zirconium, in turn, can form oxides that improve the adhesion of the oxide film to the substrate and reduce the oxidation rate.
A study of high-temperature oxidation of (AlCu25Fe12)100−xCex (x=1, 2, 3, 4) and Al62Cu24Fe12Zn2 alloys was conducted. It was found that the Al62Cu24Fe12Zn2 alloy, which has a quasi-crystalline structure, demonstrates increased oxidation resistance compared to the zinc-free Al63Cu25Fe12 alloy at 773 K. At the same time, the (Al63Cu25Fe12)100−xCex alloys consist of quasi-crystalline and crystalline phases. It was shown that the mass gain during oxidation in air at 773 and 1073 K increases proportionally to the cerium content. The obtained results indicate that the introduction of zinc slows down the oxidation process, while the addition of cerium, on the contrary, promotes it. Powders of cerium- and zinc-containing alloys undergo oxidation with the formation of FeAl2O4. The specific surface area of zinc-containing powder oxidized at 773 and 1073 K is smaller than that of the oxidized powder of the ternary alloy. At the same time, the specific surface area of cerium-containing powders oxidized at 1073 K increases with increasing cerium concentration.
In an attempt to apply quasicrystals, the reaction of steam reforming of methanol on a catalyst prepared from Al63Cu25Fe12 alloy powder by leaching was investigated. In order to enhance the catalytic activity, it is necessary to control the surface area and topology of the alloy powders through various pretreatment processes. Oxidation in air is one of the simplest ways to increase the surface area of some alloys. However, aluminum-based quasicrystals are expected to have high oxidation resistance due to the formation of a passivating aluminum oxide layer on the surface.
Studies of oxidation kinetics show that it depends on temperature, oxygen partial pressure and alloy composition. At low temperatures, oxidation is slow and is controlled by ion diffusion through the oxide film. At high temperatures, the oxidation rate increases and the process can be controlled by the reaction at the metal-oxide interface.
Understanding the oxidation mechanisms and the influence of additional elements allows the development of new quasicrystalline alloys with improved high-temperature resistance.
Author: Michiaki Yamasaki, An Pang Tsai
Institute: National Institute of Materials Science, 1-2-1, Sengen, Tsukuba 305-0047, Japan