The influence of thermomechanical treatment modes on the structure and mechanical properties of nanoquasicrystalline Al–Fe–Cr alloys is studied. It is shown that optimal conditions allow the formation of a structure with a high density of the nanoquasicrystalline phase, which provides a significant increase in strength at elevated temperatures. The influence of alloying elements on structural stability and mechanical properties is studied.
In recent decades, there has been a growing interest in developing materials that can maintain high strength at elevated temperatures. Traditional aluminum alloys, widely used in the aviation and automotive industries, have limitations in operating temperature due to reduced strength caused by grain growth and atom diffusion. In this regard, the development of new types of aluminum alloys with improved high-temperature characteristics is an urgent task.
One of the promising areas is the development of nanoquasicrystalline (NCC) aluminum-based alloys doped with transition metals such as iron and chromium. The quasicrystalline (QC) structure, characterized by an aperiodic arrangement of atoms, provides high hardness and creep resistance. In addition, the nanoscale distribution of the KK phase in the aluminum matrix preserves the ductility of the alloy and prevents brittle fracture.
Alloys of the Al–Fe–Cr system are among the most studied NCC alloys. They have the potential to be used as high-temperature structural materials due to their high melting point, good corrosion resistance and high strength. However, to achieve optimal mechanical properties, it is necessary to control the size and distribution of the CC phase in the matrix.
In this work, Al–Fe–Cr alloys with a nominal composition of Al85Fe10Cr5 obtained by plasma sputtering were used. To improve the uniformity of the structure and grain grinding, the alloys were subjected to mechanical processing (MO) in a planetary ball mill for various times. The powders were then compacted by hot pressing at a temperature of 400 °C and a pressure of 50 MPa. The obtained samples were subjected to various modes of heat treatment (TO), including annealing at different temperatures and times.
The structure of the alloys was studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). RD analysis was performed on a diffractometer using CuKa radiation. TEM studies were performed using an electron microscope equipped with an energy-dispersive spectrometer (EMF) for determining the chemical composition of phases.
The mechanical properties of the alloys were evaluated by the Vickers microhardness method at a load of 100 g. Uniaxial tensile tests were performed at various temperatures on a testing machine with a strain rate of 10-3 s-1.
Mechanical processing is an effective way to grind grains and form a nanocrystalline structure. RD analysis showed that after MO, a significant broadening of diffraction peaks occurs for 20 hours, which indicates a decrease in the size of crystallites and an increase in the density of lattice defects. TEM studies confirmed the formation of a nanocrystalline structure with a grain size of less than 50 nm.
Heat treatment plays an important role in shaping the structure and mechanical properties of NCC alloys. Annealing at elevated temperatures promotes the growth of the CC phase and changes in its morphology. As a result of annealing at 600 °C for 1 hour, the formation of spherical KC particles with a size of 100-200 nm, evenly distributed in the aluminum matrix, is observed.
Microhardness tests have shown that annealing at 600 °C leads to an increase in the hardness of the alloy compared to the state after MO. This is due to the formation of the KK phase, which has a high hardness. However, a further increase in the annealing temperature leads to a decrease in hardness due to the growth of CC particles and a decrease in the density of lattice defects.
Tensile tests at elevated temperatures have shown that alloys subjected to annealing at 600 °C have high strength and ductility. The ultimate tensile strength at 300 °C is about 400 MPa, and the relative elongation is about 10%.
Additional alloying elements, such as manganese and cerium, were introduced to improve the structural stability and mechanical properties of Al–Fe–Cr alloys. The addition of manganese increases the recrystallization temperature of the aluminum matrix and slows down the growth of the CC phase at elevated temperatures. The addition of cerium contributes to the formation of a more uniform microstructure and dispersed hardening of the alloy.
Studies have shown that alloying with manganese and cerium increases the strength and creep resistance of Al-Fe-Cr alloys at elevated temperatures. The ultimate tensile strength at 400 °C for alloyed alloys is about 350 MPa, which is significantly higher than for non-alloyed alloys.
As a result of the conducted studies, it was found that thermomechanical processing allows the formation of a nanoquasicrystalline structure in Al–Fe–Cr alloys. Optimal maintenance modes ensure the formation of spherical CC particles with a size of 100-200 nm, evenly distributed in the aluminum matrix, which leads to a significant increase in strength and ductility at elevated temperatures. Alloying with manganese and cerium increases the structural stability and creep resistance of the alloys.
The results obtained indicate the promising use of NCC alloys based on Al-Fe-Cr as high-temperature structural materials. Further research will focus on optimizing the composition and maintenance modes to achieve maximum mechanical properties and operational reliability of alloys.
Author: Marina Galano, Fernando Audebert, Asunción García Escorial, Ian C. Stone, Brian Cantor
Institute: Department of Materials, University of Oxford, Parks Road, OX1 3PH, Oxford, UK, Advanced Materials Group, Facultad de Ingeniería, Universidad de Buenos Aires. Paseo Colón 850, Buenos Aires 1063, Argentina, CENIM-CSIC, Avda. Gregorio del Amo 8, Madrid 28040, Spain, Vicechancellor’s Office, University of York, Heslington YO10 5DD, York, UK