Structure Formation and Corrosion Behaviour of Quasicrystalline Al–Ni–Fe Alloys

The formation of quasicrystalline decagonal phase and related crystalline phases was investigated by a combination of optical metallography, powder X-ray diffraction, atomic absorption spectroscopy and differential thermal analysis. Corrosion behaviour of quasicrystal Al–Ni–Fe alloys was studied by gravimetric and potentiodynamic polarization experiments in saline and acidic solutions at room temperature. The decagonal phase exhibits two modifications (AlFe- and AlNi-based) depending on the composition. In Al72Ni13Fe15 alloy it coexists with monoclinic Al5FeNi phase. In Al71.6Ni23Fe5.4 alloy crystalline Al13(Ni,Fe)4, Al3(Ni,Fe)2, and Al3(Ni,Fe) phases are seen adjacent to the quasicrystalline decagonal phase. Stability of quasicrystal phase up to room temperature was shown to be connected with its incomplete decomposition during cooling at a rate of 50 K/min. Al72Ni13Fe15 alloy has more than twice larger volume fraction of this phase compared to that of Al71.6Ni23Fe5.4 alloy. A dependence of microhardness on composition was observed as well, with Al72Ni13Fe15 alloy having substantially higher values. In acidic solutions, Al71.6Ni23Fe5.4 alloy showed the best corrosion performance. In saline solutions, the investigated alloys remained mainly untouched by corrosion. Mass-change kinetics exhibited parabolic growth rate. After a potentiodynamic scan in 3.0 M NaCl solution polarization of Al72Fe15Ni13 and Al71.6Ni23Fe5.4 alloys revealed that stationary potential values became more negative, with anodic process slowed down. The polarization curves showed that both the quasicrystalline alloys turned to passive state in this solution. Key words: decagonal phase, microstructure, corrosion behaviour, stationary potential, electrochemical passivity zone.