The effect of copper and chromium alloying on the microstructure, anticorrosive characteristics, and nanomechanical properties of new aluminum alloys of the Al-Ni-Fe system is studied. Scanning electron microscopy (SEM), X-ray phase analysis (XRD), electrochemical impedance spectroscopy (EIS), and nanoindentation have been used to study changes in the phase composition, corrosion resistance, and microhardness of alloys depending on the content of additives. It is shown that the introduction of Cu and Cr leads to the formation of new intermetallic phases, which have a significant effect on the structure of grain boundaries and passivation characteristics. The results of the study demonstrate the possibility of optimizing the composition of alloys to improve their operational properties in aggressive environments.
Aluminum-based alloys have traditionally been widely used in various industries due to their combination of low density, high specific strength and good corrosion resistance. However, further optimization of their mechanical and corrosion properties is necessary to expand the application areas of aluminum alloys, especially in conditions of increased loads and aggressive environments. One of the promising areas is the development of alloys doped with transition metals, such as nickel and iron, which contribute to increasing strength and heat resistance. The introduction of additional alloying elements, such as copper and chromium, can significantly affect the microstructure and properties of alloys, optimizing the balance of their performance characteristics.
Alloys of the Al-Ni-Fe system with different contents of copper and chromium were chosen as the objects of research. The alloys were obtained by induction melting in an argon atmosphere with subsequent casting in a copper mold. The resulting ingots were subjected to homogenizing annealing at a temperature of 500°C for 24 hours to reduce liquation.
Microstructural studies were performed using a Tescan Vega 3 SB scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EMF) for local chemical analysis. The phase composition of the alloys was determined by X-ray phase analysis (XRD) on a Bruker D8 Advance diffractometer using Cu Ka radiation. Corrosion tests were performed by electrochemical impedance spectroscopy (EIS) in 3.5% NaCl solution at room temperature. Nanomechanical properties were determined by the nanoindentation method on a NanoTest Vantage device, which allows measuring microhardness and elastic modulus.
Analysis of the microstructure of the Al-Ni-Fe alloy (Alloy 1) revealed the presence of a eutectic structure consisting of an aluminum matrix and Ni-and Fe-based intermetallic phases. The introduction of copper (Alloy 2) leads to the formation of new copper-rich phases, mainly along grain boundaries. Chromium doping (Alloy 3) leads to grain grinding and the formation of fine precipitates of chromium carbides. The alloy doped with both copper and chromium (Alloy 4) has the most complex microstructure with a combination of eutectic regions, copper-rich phases, and fine chromium precipitates.
EMF analysis of microstructural components allowed us to determine their chemical composition and identify intermetallic phases. The presence of copper in the alloys leads to the formation of Al2Cu and AlNiCu phases, which are distributed mainly along grain boundaries. Chromium forms fine precipitates of carbides dispersed in the aluminum matrix.
X-ray diffraction results confirm the presence of an aluminum matrix (Al) and nickel-and iron-based intermetallic phases, such as Al3Ni and AlFe, in the alloys. Al2Cu and AlNiCu phases were identified in copper-doped alloys. The introduction of chromium leads to the appearance of peaks corresponding to chromium carbides (Cr23C6). Analysis of the ratio of peak intensities of various phases allows us to estimate their relative content in alloys. The introduction of copper leads to an increase in the proportion of phases enriched in copper, and the introduction of chromium leads to an increase in the proportion of chromium carbides.
Studies of the corrosion resistance of alloys by the EIS method have shown that copper and chromium additives have a significant effect on the passivation characteristics. The introduction of copper leads to a decrease in corrosion resistance, which is associated with the formation of galvanic pairs between the aluminum matrix and copper-rich phases. Chromium doping, on the other hand, increases corrosion resistance by forming a passive film on the surface of the alloy enriched in chromium. In an alloy doped with both copper and chromium, there is a compromise between these two effects.
The impedance spectra of the alloys were approximated using an equivalent electrical circuit consisting of electrolyte resistance (Rs), film resistance (Rf), film capacitance (Cf), charge transfer resistance (Rct), and double-layer capacitance (Cdl). The values of these parameters reflect the characteristics of the passive film and the rate of corrosion processes.
Measurements of microhardness and elastic modulus by nanoindentation showed that the introduction of copper and chromium affects the mechanical properties of alloys. The introduction of copper leads to a certain decrease in the microhardness of the aluminum matrix, which is associated with the formation of soft phases enriched in copper. Chromium doping, on the contrary, increases microhardness due to dispersion hardening. In an alloy doped with both copper and chromium, a combination of these two effects is observed.
Analysis of indentation curves (load-depth) makes it possible to estimate the ductility and elasticity of alloys. The introduction of copper leads to an increase in plasticity, and the introduction of chromium leads to an increase in elasticity.
The results of these studies have shown that copper and chromium alloying significantly affects the microstructure, anticorrosive characteristics,and nanomechanical properties of Al-Ni-Fe alloys.
The introduction of copper leads to the formation of new intermetallic phases enriched in copper, mainly along grain boundaries, and a decrease in corrosion resistance.
Chromium alloying contributes to grain grinding, the formation of fine precipitates of chromium carbides, and an increase in corrosion resistance.
Optimization of the copper and chromium content makes it possible to obtain alloys with an improved balance of performance properties, combining high strength and corrosion resistance.
Further research is needed to study in more detail the effect of alloying elements on the corrosion mechanism and to develop optimal alloy compositions for various applications.
Author: Rafał Babilas, Katarzyna Młynarek-Żak, Adrian Radoń, Wojciech Łoński, Mariola Kądziołka-Gaweł, Tymon Warski, Darya Rudomilova, Edyta Wyszkowska, Łukasz Kurpaska
Institute: Department of Engineering Materials and Biomaterials, Silesian University of Technology, Konarskiego 18a St., Gliwice 44-100, Poland, Institute of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice, Poland, Łukasiewicz Research Network, Institute of Non-Ferrous Metals, Sowińskiego 5 St., 44-100 Gliwice, Poland, Technopark Kralupy, University of Chemistry and Technology Prague, 166 28 Prague, Czech Republic, National Centre for Nuclear Research, NOMATEN CoE MAB+, Andrzeja Soltana 7 St., 05-400 Otwock-Świerk, Poland