Structure, Electrical and Corrosion Properties of Quasicrystalline Al–Cu–Fe–Sc Thin Films

For the first time, quasicrystalline Al66.5Cu20Fe13Sc0.5 films with 200-260 nm thickness cooled at 1012-1014 K/s were produced by modernized method of three-electrode ion-plasma sputtering. Films were deposited on NaCl or glass-ceramic substrates. The coatings have been analyzed with X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Electrical resistivity was measured by four-probe method. Corrosion behavior in 3 %, 5 % and 16 % NaCl aqueous solutions was studied by potentiodynamic method and model tests. The deposits have nanoquasicrystalline icosahedral structure. As Sc added, the size of coherent scattering regions is found to decrease from 3 nm to 1.6 nm. Measurements of electrical resistivity during heating show that the investigated Al–Cu–Fe–Sc film is stable up to 623 K. Additions of Sc to Al–Cu–Fe film reduce thermal stability of quasicrystalline phase. The decrease in the quasicrystal-forming ability of the i-phase is expressed by the reduced transition temperature that is 100 K lower as Sc is added. Post-annealing treatment for 3 h at 873 K reveals that the resultant microstructural size tends to be coarser, and the coherent scattering regions increase by a factor of two. Additions of Sc to Al–Cu–Fe film made it less susceptible to corrosion in saline solutions. Corrosion of the as-sputtered Al–Cu–Fe–Sc film proceeds at the lowest rate, but after annealing it shows highest corrosion rate. With concentration of sodium chloride in saline solutions increasing from 3 % to 16 %, the values of free corrosion potentials are found to shift to more negative values. Model corrosion tests for 1, 2, 3, 4, 8 days with 5 % NaCl solution indicate that all the investigated films remain virtually untouched by corrosion. No marks of pittings typical for as-cast Al–Cu–Fe alloys are observed on the surface affected by saline solution.