In recent years, there has been increased interest in aluminum-based metal alloys doped with transition metals, particularly chromium and iron. These alloys exhibit promising properties such as high strength, low density, and improved corrosion resistance, making them attractive for use in the aviation, automotive, and other industries. Particular attention is paid to the study of phase diagrams and the identification of new intermetallic compounds in the Al–Cr–Fe system, since their structure and composition directly affect the performance characteristics of the alloys.
One of the promising areas is the development of approximants – complex crystalline structures close in atomic structure to quasicrystals. Quasicrystals, which have a unique combination of order and aperiodicity, attract attention with their unusual physical properties, but their fragility limits their practical application. Approximants, in turn, retain many useful properties of quasicrystals, but have greater plasticity and manufacturability.
In this work, new approximants in the Al–Cr–Fe system were synthesized and studied using arc melting and subsequent heat treatment. The crystal structures of the obtained phases, as well as their composition and microstructure, were determined using X-ray diffraction analysis and electron microscopy. Particular attention was paid to studying the oxidation stability of new approximants at high temperatures. It was found that alloying with certain elements can significantly increase oxidation stability by forming protective oxide layers on the alloy surface. The results obtained open up prospects for the development of new heat-resistant and corrosion-resistant materials based on the Al–Cr–Fe system.
Two new orthorhombic approximations of the delta phase, as well as a phase structurally similar to γ-brass, have been identified in the Al–Cr–Fe alloy. Diffraction studies show their isotypicity with known compounds in the Al–Cu–Fe–Cr and Al–Cr systems. The addition of Cr significantly increases oxidation resistance both in aqueous media and at high temperatures.
The Al–Cr–Fe system in the aluminum-rich region is characterized by the presence of an icosahedral phase and several delta-phase approximations, usually coexisting with the face-centered cubic aluminum phase. We have discovered a compositional region where aluminum is not present in its pure form. Here, for the first time in this system, two orthorhombic delta-phase approximations are revealed, isotypic to the intermetallics found in other systems. The γ-AlCrFe phase belongs to the γ-brass type and isostructural to rhombohedral Al8Cr5, which can be considered as an approximation of a quasicrystal. Based on the known atomic coordinates of Al8Cr5 and orthorhombic compounds Al–Cu–Fe–Cr, models for the crystallographic description of these Al–Cr–Fe approximations are developed.
Oxidation in water at room temperature was found to be the most aggressive, resulting in the greatest oxidation of chromium, formation of thick oxide layers, and strong segregation of aluminum. Oxidation in oxygen at 1040 °C is comparable in oxide thickness. Oxidation in air at room temperature and in vacuum represents milder conditions.
Author: V.Demange, F. Machizaud, JM Dubois, JW Anderegg, PA Thiel, DJ Sordelet
Institute: LSG2M-CIM (UMR 7584 CNRS-INPL-UHP), Ecole des Mines, Parc de Saurupt, F-54042 Nancy, France, Department of Chemistry, ISU and Ames Laboratory, Spedding Hall, Ames, IA 50011, USA, Department of Materials Science, ISU and Ames Laboratory, Metals Development, Ames, IA 50011, USA