Aluminum-based alloys doped with copper and iron are attracting attention due to their potential for creating materials with improved mechanical properties. The microstructure and, as a result, the hardness of the Al–Cu–Fe alloy depend significantly on the percentage ratio of the elements, as well as on the heat treatment modes.
The addition of copper promotes the formation of strengthening phases such as Al2Cu (θ-phase), which are dispersed in the aluminum matrix. Increasing the copper content generally leads to an increase in hardness, but exceeding the optimum value can cause the formation of large precipitates that reduce ductility. Iron, in turn, forms intermetallics such as AlFe, which can have both a strengthening and embrittlement effect depending on the size and distribution of the particles.
To study the influence of the component composition on the volume fraction of the icosahedral phase, phase transformations and hardness characteristics, four alloys of the Al–Cu–Fe system with different element contents were prepared and studied: a) Al65Cu20Fe15, b) Al64Cu22.5Fe13.5, c) Al61Cu26Fe13 and d) Al62Cu25.5Fe12.5. The samples were analyzed both immediately after casting and after annealing.
The results showed that alloys a) Al65Cu20Fe15 and b) Al64Cu22.5Fe13.5, which have a higher iron content, are characterized by the maximum content of the λ-Al13Fe4 phase and demonstrate the highest hardness values in the cast state. However, heat treatment of these alloys led to the formation of the smallest amount of the icosahedral phase.
Alloys c) Al61Cu26Fe13 and d) Al62Cu25.5Fe12.5 responded more effectively to annealing, reaching the content of the icosahedral phase up to 87% and 99%, respectively. After heat treatment, an increase in the hardness of the phases was observed compared to the cast state: the hardness of the icosahedral phase increased by 20-25%, and the hardness of λ-Al13Fe4 – by 9%. The icosahedral phase in alloy a) Al65Cu20Fe15 was characterized by the highest strength after annealing, with a hardness index of 844HV.
Combined alloying with copper and iron allows for a synergistic effect, where strengthening is achieved through the combined action of various phases. However, to achieve optimal characteristics, it is necessary to carefully control the chemical composition and heat treatment modes. High-temperature annealing, for example, promotes the dissolution of the strengthening phases, and subsequent quenching and aging lead to their release in a dispersed state, which increases the hardness and strength of the alloy.
The microstructure of the Al–Cu–Fe alloy is also affected by impurities and process parameters such as cooling rate and degree of deformation. In general, optimization of chemical composition and processing modes allows obtaining alloys with the required combination of hardness and ductility for various applications.
Author: M. A. Suárez, R. Esquivel, J. Alcántara, H. Dorantes, J. F. Chávez
Institute: ESIQIE – National Polytechnic Institute, Department of Metallurgical and Materials Engineering, UPALM, Mexico City, 07738, Mexico