The influence of microstructural features on the corrosion properties of aluminium alloys hardened with quasicrystals is discussed in this Diploma work. The quasicrystalline alloy Al-Mn-Cu-Mg-Si-Ti (designated QC) was investigated. AlSi9Cu3 (DIN226 standard designation) was used as a reference.
The two alloys were cast in a steel die with controlled cooling rate. After the preparation of the samples, the microstructure characterisation was carried out. In the quasicrystalline alloy, the microstructure was dominated by a primary phase with 5-fold symmetry, representing the quasicrystalline phase. Open-circuit potential, linear polarisation, impedance spectroscopy and potentiodynamic polarisation measurements were performed on the alloys. After 1 h, the QC alloy reached an open-circuit potential of -571,3±37,3 mV and the DIN226 alloy -544,8±22,6 mV. In the linear polarisation measurements, the corrosion current density was 111.2±7.0 nA/cm2 for alloy QC and 92.9±31.8 nA/cm2 for alloy DIN226. Impedance spectroscopy measurements showed an impedance value of 247,8±15,6 kohm/cm2 for alloy QC and 290,2±66 kohm/cm2 for alloy DIN226. The potentiodynamic polarisation measurements, using Tafel extrapolation, gave a corrosion current density value for the QC alloy of 145,1±18,0 nA/cm2 and for the DIN226 alloy of 145,8±28,2 nA/cm2. Due to the non-repeatability of the results, standard deviations were calculated. Taking these into account, it was concluded that the quasicrystalline alloy has comparably good corrosion properties to the commercially very successful AlSi9Cu3 alloy. In the case of the QC alloy, corrosion attacked mainly the αAl phase, in the vicinity of the Al2Cu intermetallic compound particles and not the quasicrystalline phase, indicating good corrosion resistance of the quasicrystalline phase. Both alloys were characterised by intergranular corrosion progression.
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