In this paper, the corrosion properties and hardness of nanostructured Al and Al–Cr alloys obtained by high-energy ball grinding (HESH) are studied. X-ray diffraction and transmission electron microscopy methods were used to determine the microstructure and grain size of powders. Corrosion resistance was evaluated by electrochemical methods in 3.5% NaCl solution. Hardness was measured by microindentation. The results obtained show that nanostructuring significantly affects the corrosion properties and hardness of Al and Al–Cr alloys.
Aluminum and its alloys are widely used in various industries due to their low density, good electrical conductivity and acceptable corrosion resistance. However, their mechanical properties, such as strength and hardness, are often insufficient for some applications. One of the promising ways to improve these properties is to create nanostructured materials.
Nanostructuring, which involves reducing the grain size to the nanometer range, can significantly increase the strength and hardness of metals and alloys. This is achieved by increasing the proportion of grain boundaries that prevent the movement of dislocations, which are the main carriers of plastic deformation. In addition, nanostructuring can also affect the corrosion resistance of materials, since grain boundaries are defective regions of the structure that may be more susceptible to corrosion.
One of the most effective methods for producing nanostructured materials is high-energy ball grinding (HESH). This method makes it possible to obtain nanostructured powders by intense plastic deformation of the initial materials during the collision of balls and powder particles. As a result of HES, grains are crushed, defects in the crystal structure are formed, and nanostructures are formed.
The aim of this work is to study the effect of nanostructuring on the corrosion properties and hardness of Al and Al–Cr alloys obtained by the HESHI method. Al-Cr alloys are of interest due to their potential use as structural materials with high strength and improved corrosion resistance.
To obtain nanostructured Al and Al–Cr alloys, the method of high-energy ball grinding was used. Aluminum powders (99.9% purity) and chromium powders (99.5% purity) were used as starting materials. A mixture of aluminum and chromium powders in various proportions was loaded into a steel container along with steel balls. Grinding was carried out in a planetary mill Fritsch Pulverisette 7 classic at a speed of 600 rpm for various periods of time (up to 20 hours) in an argon atmosphere to prevent oxidation of powders.
The phase composition and microstructure of the obtained powders were studied by X-ray diffraction (XRD) using a Bruker D8 Advance diffractometer using Cu Ka radiation. The grain size was calculated using the Scherrer formula based on the broadening of diffraction peaks. The microstructure of the powders was also studied by transmission electron microscopy (TEM) using a JEOL JEM-2100 microscope.
The corrosion properties of the alloys were evaluated by electrochemical polarization in 3.5% NaCl solution at room temperature. An Autolab PGSTAT302N potentiostat/galvanostat with a three-electrode cell was used for electrochemical measurements. Pressed alloy powder pressed into epoxy resin was used as the working electrode. The platinum electrode was used as a counter electrode, and the silver chloride electrode (Ag/AgCl) was used as a reference electrode. The Tafel polarization curves were recorded at a scanning rate of 1 mV / s in the range of potentials ±250 mV relative to the corrosion potential. The corrosion potential (Ecorr) and corrosion current (Icorr) were determined from the Tafel polarization curves.
The hardness of the obtained powders was measured by microindentation on a Buehler Micromet 5104 microhardness meter using a load of 100 g and a holding time of 15 seconds. For each sample, at least 10 measurements were made, after which the average hardness value was calculated.
The results of X-ray diffraction showed that the HES process involves grain grinding and the formation of nanostructures in Al and Al–Cr alloys. The broadening of the diffraction peaks indicates a decrease in the grain size and an increase in the defect of the crystal structure. The calculation of the grain size using the Scherrer formula showed that after 20 hours of grinding, the grain size in Al and Al–Cr alloys decreases to 20-30 nm.
Transmission electron microscopy confirmed the formation of nanostructures in Al and Al–Cr alloys. TEM images show nanoscale grains with the size corresponding to the XRD data. In addition, TEM images show a high dislocation density and other defects in the crystal structure formed during the HESHI process.
Electrochemical measurements have shown that nanostructuring has a significant effect on the corrosion resistance of Al and Al–Cr alloys. In particular, it was found that nanostructured Al-Cr alloys have a higher corrosion resistance compared to pure aluminum and coarse–grained Al-Cr alloys. This may be due to the formation of a protective oxide film on the surface of the alloy, which prevents the penetration of a corrosive environment. In addition, an increase in the chromium content of the alloy also contributes to an increase in corrosion resistance.
The results of microindentation showed that nanostructuring leads to a significant increase in the hardness of Al and Al–Cr alloys. The hardness of nanostructured Al-Cr alloys is significantly higher than that of pure aluminum and coarse–grained Al-Cr alloys. This is due to an increase in the dislocation density and a decrease in the grain size, which complicates the movement of dislocations and increases the material’s resistance to plastic deformation.
As a result of the conducted studies, it was found that high-energy ball grinding is an effective method for obtaining nanostructured Al and Al–Cr alloys. Nanostructuring leads to a significant increase in the hardness and improvement of the corrosion resistance of alloys. Al–Cr alloys obtained by the HESHI method have increased corrosion resistance compared to pure aluminum and coarse–grained Al-Cr alloys. The results obtained indicate the prospects of using nanostructured Al-Cr alloys as structural materials with high strength and improved corrosion resistance.
Author: R.K. Gupta, D. Fabijanic, R. Zhang, N. Birbilis
Institute: Department of Chemical and Biomolecular Engineering, The University of Akron, Akron, OH 44325, USA, Institute for Frontier Materials, Deakin University, VIC 3216, Australia, Department of Materials Science and Engineering, Monash University, VIC 3800, Australia