Approximant Phases in Quasicrystalline AlCuFe Thin Films

• Quasicrystalline materials exhibit properties that are very different from conventional metallic materials. They are mostly metallic alloys, and show high hardness and stiffness but low electrical and thermal conductivity. The coefficient of friction and surface energy of the quasicrystalline materials are also very low. Approximants are a family of phases that are related to the quasicrystals. These phases share the local atomic arrangement of quasicrystals and have as a result many similar physical properties. Bulk quasicrystals are too brittle for many of the suggested applications, instead the most important area of applications concerns that of surface coatings. In this work, quasicrystalline and approximant phases have been synthesized in thin films. Multilayered Al/Cu/Fe thin films, with a nominal global composition corresponding to the quasicrystalline phase, have been deposited by magnetron sputtering onto Si and Al2O3 substrates. During isothermal annealing at temperatures up to 700 °C homogeneous thin films were formed. It is found that when using Si as substrate a film-substrate reaction occurs already below 390 °C, where Si diffuses into the film. This changes the composition, promoting the formation of the cubic α-approximant phase. Annealing at 600 °C for 4 h the cubic a-approximant phase forms in a polycrystalline state, with a small amount of a second phase, τ7-Al3Fe2Si3. The film is within 1.5 at.% of the ideal composition of the a-approximant phase, and contains 8 at.% Si. Continued annealing for 64 h provides for more diffusion of Si to 12 at.%, which result in an increase of the t7-phase. The rate of Si in-diffusion was observed to decrease with annealing time, and the lattice parameter of the a-phase was continuously decreasing as diffused Si substituted for Al. No degradation of the crystal quality of the remaining α-phase was observed even after as much as 150 h of treatment. When annealing the same Al/Cu/Fe thin film grown on non-reactant Al2O3 substrates the icosahedral Al62.5Cu25Fe12.5 quasicrystalline phase is formed.

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